The pressure-induced structural response of rare earth hafnate and stannate pyrochlore from 0.1-50 GPa

NASA Astrophysics Data System (ADS)

Turner, K. M.; Rittman, D.; Heymach, R.; Turner, M.; Tracy, C.; Mao, W. L.; Ewing, R. C.

2017-12-01

Complex oxides with the pyrochlore (A2B2O7) and defect-fluorite ((A,B)4O7) structure-types undergo structural transformations under high-pressure. These compounds are under consideration for applications including as a proposed waste-form for actinides generated in the nuclear fuel cycle. High-pressure transformations in rare earth hafnates (A2Hf2O7, A=Sm, Eu, Gd, Dy, Y, Yb) and stannates (A2Sn2O7, A=Nd, Gd, Er) were investigated to 50 GPa by in situ Raman spectroscopy and synchrotron x-ray diffraction (XRD). Rare-earth hafnates form the pyrochlore structure for A=La-Tb and the defect-fluorite structure for A=Dy-Lu. Lanthanide stannates form the pyrochlore structure. Raman spectra revealed that at ambient pressure all compositions have pyrochlore-type short-range order. Stannate compositions show a larger degree of pyrochlore-type short-range ordering relative to hafnates. In situ high-pressure synchrotron XRD showed that rare earth hafnates and stannates underwent a pressure-induced phase transition to a cotunnite-like (Pnma) structure that begins between 18-25 GPa in hafnates and between 30-33 GPa in stannates. The phase transition is not complete at 50 GPa, and upon decompression, XRD indicates that all compositions transform to defect-fluorite with an amorphous component. In situ Raman spectroscopy showed that disordering in stannates and hafnates occurs gradually upon compression. Pyrochlore-structured hafnates retain short-range order to a higher pressure (30 GPa vs. <10 GPa) than defect-fluorite-structured hafnates. Hafnates and stannates decompressed from 50 GPa show Raman spectra consistent with weberite-type structures, also reported in irradiated stannates. The second-order Birch-Murnaghan equation of state fit gives a bulk modulus of 250 GPa for hafnate compositions with the pyrochlore structure, and 400 GPa for hafnate compositions with the defect-fluorite structure. Stannates have a lower bulk modulus relative to hafnates (between 80-150 GPa). Stannate and hafnate pyrochlore compositions taken to high pressure show structural transformations consistent with irradiated pyrochlore, and compositionally disordered pyrochlore: a long-range structure best described by defect-fluorite, and a short-range structure best described by weberite.

Effect of structure and thermodynamic stability on the response of lanthanide stannate pyrochlores to ion beam irradiation.

PubMed

Lian, J; Helean, K B; Kennedy, B J; Wang, L M; Navrotsky, A; Ewing, R C

2006-02-09

The lanthanide stannates, Ln2Sn2O7, Ln=La-Lu and Y, have the isometric pyrochlore structure, A2B2O7, and their structural properties have been refined by Rietveld analysis of powder neutron and synchrotron X-ray diffraction data. In this study, the enthalpies of formation of selected stannate pyrochlores, Ln=La, Nd, Sm, Eu, Dy, and Yb, were measured by high-temperature oxide melt solution calorimetry. Their radiation response was determined by 1 MeV Kr2+ ion irradiation combined with in situ TEM observation over the temperature range of 25 to 1000 K. The enthalpy of formation from binary oxides of stannate pyrochlores became more endothermic (from -145 to -40 kJ/mol) as the size of the lanthanide in the A-site decreases. A more exothermic trend of the enthalpy of formation was observed in stannate pyrochlores with larger lanthanide ions, particularly La, possibly as a result of increased covalency in the Sn-O bond. In contrast to lanthanide titanate pyrochlores, Ln2Ti2O7, that are generally susceptible to radiation-induced amorphization and zirconate pyrochlores, Ln2Zr2O7, that are generally resistant to radiation-induced amorphization, the lanthanide stannate pyrochlores show a much greater variation in their response to ion irradiation. La, Nd, and Gd stannates experience the radiation-induced transformation to the aperiodic state, and the critical amorphization temperatures are approximately 960, 700, and 350 K, respectively. Y and Er stannate pyrochlores cannot be amorphized by ion beam irradiation, even at 25 K, and instead disorder to a defect fluorite structure. Comparison of the calorimetric and ion irradiation data for titanate, zirconate, and stannate pyrochlores reveals a strong correlation among subtle changes in crystal structure with changing composition, the energetics of the disordering process, and the temperature above which the material can no longer be amorphized. In summary, as the structure approaches the ideal, ordered pyrochlore structure, radiation-induced amorphization is more easily attained. This is consistent with an increasingly exothermic trend in the enthalpies of formation of pyrochlores from the oxides, that is, the greater the thermochemical stability of the pyrochlore structure, the more likely it will be amorphized upon radiation damage rather than recover to a disordered fluorite structure.

Research for preparation of cation-conducting solids by high-pressure synthesis and other methods

NASA Technical Reports Server (NTRS)

Goodenough, J. B.; Hong, H. Y. P.; Kafalas, J. A.; Dwight, K., Jr.

1975-01-01

It was shown that two body-centered-cubic skeleton structures, the Im3 KSbO3 phase and the defect-pyrochlore phase A(+)B2X6, do exhibit fast Na(+)-ion transport. The placement of anions at the tunnel intersection sites does not impede Na(+)-ion transport in (NaSb)3)(1/6 NaF), and may not in (Na(1+2x)Ta2 5F)(Ox). The activation energies are higher than those found in beta-alumina. There are two possible explanations for the higher activation energy: breathing of the bottleneck (site face or edge) through which the A(+) ions must pass on jumping from one site to another may be easier in a layer structure and/or A(+)-O bonding may be stronger in the cubic structures because the O(2-) ion bonds with two (instead of three) cations of the skeleton. If the former explanation is dominant, a lower activation energy may be achieved by optimizing the lattice parameter. If the latter is dominant, a new structural principle may have to be explored.

The influence of crystal structure on ion-irradiation tolerance in the Sm(x)Yb(2-x)TiO5 series

DOE Office of Scientific and Technical Information (OSTI.GOV)

Aughterson, R. D.; Lumpkin, G. R.; de los Reyes, M.

2016-04-01

his ion-irradiation study covers the four major crystal structure types in the Ln(2)TiO(5) series (Ln = lanthanide), namely orthorhombic Pnma, hexagonal P63/mmc, cubic (pyrochlore-like) Fd-3m and cubic (fluorite-like) Fm-3m. This is the first systematic examination of the complete Ln(2)TiO(5) crystal system and the first reported examination of the hexagonal structure. A series of samples, based on the stoichiometry Sm(x)Yb(2-x)TiO5 (where x = 2, 1.4, 1, 0.6, and 0) have been irradiated using 1 MeV Kr2+ ions and characterised in-situ using a transmission electron microscope. Two quantities are used to define ion-irradiation tolerance: critical dose of amorphisation (D-c), which is themore » irradiating ion dose required for a crystalline to amorphous transition, and the critical temperature (T-c), above which the sample cannot be rendered amorphous by ion irradiation. The structure type plus elements of bonding are correlated to ion-irradiation tolerance. The cubic phases, Yb2TiO5 and Sm0.6Yb1.4TiO5, were found to be the most radiation tolerant, with Tc values of 479 and 697 K respectively. The improved radiation tolerance with a change in symmetry to cubic is consistent with previous studies of similar compounds.« less

Response of Gd 2 Ti 2 O 7 and La 2 Ti 2 O 7 to swift-heavy ion irradiation and annealing

DOE Office of Scientific and Technical Information (OSTI.GOV)

Park, Sulgiye; Lang, Maik; Tracy, Cameron L.

2015-07-01

Swift heavy ion (2 GeV 181Ta) irradiation-induced amorphization and temperature-induced recrystallization of cubic pyrochlore Gd2Ti2O7 (Fd3¯m) are compared with the response of a compositionally-similar material with a monoclinic-layered perovskite-type structure, La2Ti2O7 (P21). The averaged electronic energy loss, dE/dx, was 37 keV/nm and 35 keV/nm in Gd2Ti2O7 and La2Ti2O7, respectively. Systematic analysis of the structural modifications was completed using transmission electron microscopy, synchrotron X-ray diffraction, Raman spectroscopy, and small-angle X-ray scattering. Increasing ion-induced amorphization with increasing ion fluence was evident in the X-ray diffraction patterns of both compositions by a reduction in the intensity of the diffraction maxima concurrent with themore » growth in intensity of a broad diffuse scattering halo. Transmission electron microscopy analysis showed complete amorphization within ion tracks (diameter: ~10 nm) for the perovskite-type material; whereas a concentric, core–shell morphology was evident in the ion tracks of the pyrochlore, with an outer shell of disordered yet still crystalline material with the fluorite structure surrounding an amorphous track core (diameter: ~9 nm). The radiation response of both titanate oxides with the same stoichiometry can be understood in terms of differences in their structures and compositions. While the radiation damage susceptibility of pyrochlore A2B2O7 materials decreases as a function of the cation radius ratio rA/rB, the current study correlates this behavior with the stability field of monoclinic structures, where rLa/rTi > rGd/rTi. Isochronal annealing experiments of the irradiated materials showed complete recrystallization of La2Ti2O7 at 775 °C and of Gd2Ti2O7 at 850 °C. The annealing behavior is discussed in terms of enhanced damage recovery in La2Ti2O7, compared to the pyrochlore compounds Gd2Ti2O7. The difference in the recrystallization behavior may be related to structural constraints, i.e., reconstructing a low symmetry versus a high symmetry phase.« less

Frequency dependent dielectric properties of combustion synthesized Dy2Ti2O7 pyrochlore oxide

NASA Astrophysics Data System (ADS)

Jeyasingh, T.; Saji, S. K.; Kavitha, V. T.; Wariar, P. R. S.

2018-05-01

Nanocrystalline pyrochlore material Dysprosium Titanate (Dy2Ti2O7) has been synthesized through a single step optimized combustion route. The phase purity and phase formation of the combustion product has been characterized using X-Ray diffraction analysis (XRD) and Fourier Transform Infrared Spectroscopy (FT-IR) analysis. X-Ray diffraction analysis (XRD) reveal that Dy2Ti2O7 is highly crystalline in nature with cubic structure in the Fd3m space group. The microstructures and average particle size of the prepared nanopowder were examined by High Resolution Transmission Electron Microscopy (HR-TEM). The optical band gap of the Dy2Ti2O7 nanoparticles is determined from the absorption spectrum, was attributed to direct allowed transitions through optical band gap of 3.98 eV. The frequency dependent dielectric measurements have been carried out on the sintered pellet in the frequency range 1 Hz-10 MHz. The measured value of dielectric constant (ℇ') was ˜ 43 and loss tangent (tan δ) was 4×10-3 at 1 MHz, at room temperature.

A 2 TiO 5 (A = Dy, Gd, Er, Yb) at High Pressure

DOE Office of Scientific and Technical Information (OSTI.GOV)

Park, Sulgiye; Rittman, Dylan R.; Tracy, Cameron L.

The structural evolution of lanthanide A2TiO5 (A = Dy, Gd, Yb, and Er) at high pressure is investigated using synchrotron X-ray diffraction. The effects of A-site cation size and of the initial structure are systematically examined by varying the composition of the isostructural lanthanide titanates, and the structure of dysprosium titanate polymorphs (orthorhombic, hexagonal and cubic), respectively. All samples undergo irreversible high pressure phase transformations, but with different onset pressures depending on the initial structure. While individual phase exhibits different phase transformation histories, all samples commonly experience a sluggish transformation to a defect cotunnite-like (Pnma) phase for a certain pressuremore » range. Orthorhombic Dy2TiO5 and Gd2TiO5 form P21am at pressures below 9 GPa and Pnma above 13 GPa. Pyrochlore-type Dy2TiO5 and Er2TiO5 as well as defect-fluorite-type Yb2TiO5 form Pnma at ~ 21 GPa, followed by Im-3m. Hexagonal Dy2TiO5 forms Pnma directly, although a small amount of remnants of hexagonal Dy2TiO5 is observed even at the highest pressure (~ 55 GPa) reached, indicating a kinetic limitations in the hexagonal Dy2TiO5 phase transformations at high pressure. Decompression of these materials leads to different metastable phases. Most interestingly, a high pressure cubic X-type phase (Im-3m) is confirmed using highresolution transmission electron microscopy on recovered pyrochlore-type Er2TiO5. The kinetic constraints on this metastable phase yield a mixture of both the X-type phase and amorphous domains upon pressure release. This is the first observation of an X-type phase for an A2BO5 composition at high pressure.« less

Crystal structure, stoichiometry, and dielectric relaxation in Bi 3.32Nb 7.09O 22.7 and structurally related ternary phases

NASA Astrophysics Data System (ADS)

Grey, I. E.; Vanderah, T. A.; Mumme, W. G.; Roth, R. S.; Guzman, J.; Nino, J. C.; Levin, I.

2008-03-01

The crystal structure of the phase previously reported to occur at 4:9 Bi 2O 3:Nb 2O 5 has been determined using single-crystal X-ray and powder neutron diffraction ( P6 3/ mmc; a=7.4363(1) Å, c=19.7587(5) Å; Z=2). The structural study combined with phase equilibrium analyses indicate that the actual composition is Bi 3.32Nb 7.09O 22.7. This binary compound is the end-member of a family of four phases which form along a line between it and the pyrochlore phase field in the Bi 2O 3:Fe 2O 3:Nb 2O 5 system. The structures are derived from the parent pyrochlore end-member by chemical twinning, and can also be described as unit-cell intergrowths of the pyrochlore and hexagonal tungsten bronze (HTB) structures. The dielectric properties of the three chemically twinned pyrochlore phases, Bi 3.32Nb 7.09O 22.7, Bi 9.3Fe 1.1Nb 16.9O 57.8 and Bi 5.67FeNb 10O 35, were characterized. All exhibit low-temperature, broad dielectric relaxation similar to that of the Bi-Fe-Nb-O pyrochlore. At 1 MHz and ≈175 K the observed relative permittivites were 345, 240, and 205, respectively, compared to 125 for the Bi-Fe-Nb-O pyrochlore. The higher relative permittivities observed for the chemically twinned pyrochlore derivatives are ascribed to the presence of HTB blocks in their structures: The Bi atoms located in the HTB blocks feature highly asymmetric coordination environments compared to pyrochlore, and the magnitude of the relative permittivity increases with the proportion of Bi located within the HTB portions of the structures.

Opportunities for functional oxides in yttrium oxide-titanium oxide-zirconium oxide system: Applications for novel thermal barrier coatings

NASA Astrophysics Data System (ADS)

Francillon, Wesley

This dissertation is an investigation of materials and processed under consideration for next generation thermal structural oxides with potential applications as thermal barrier coatings; wherein, high temperature stability and mechanical properties affect durability. Two notable next generation materials systems under investigation are pyrochlore and co-doped zirconia oxides. The motivation for this work is based on current limitations of the currently used thermal barrier material of yttria stabilized zirconia (YSZ) deposited by the plasma spray processes. The rapid quenching associated with the plasma spray process, results in a metastable structure that is a non-transformable tetragonal structure in the yttria partially stabilized zirconia system rather than the equilibrium anticipated two phase mixture of cubic and monoclinic phases. It has been shown that this metastable structure offers enhanced toughness and thus durability during thermomechanical cycling from the operating temperatures in excess of 1000C to ambient. However, the metastable oxides are susceptible to partitioning at temperatures greater than 1200C, thus resulting in a transformation of the tetragonal phase oxides. Transformations of the tetragonal prime phase into the parent cubic and tetragonal prime phase result in coating degradation. Several of the emerging oxides are based on rare earth additions to zirconia. However, there is limited information of the high temperature stability of these oxide coatings and more notably these compositions exhibit limited toughness for durable performance. A potential ternary composition based on the YSZ system that offers the ability to tailor the phase structure is based YO1.5-TiO2 -ZrO2. The ternary of YO1.5-TiO2-ZrO 2 has the current TBC composition of seven molar percent yttria stabilized zirconia, pyrochlore phase oxide and zirconia doped with yttria and titania additions (Ti-YSZ). The Ti-YSZ phase field is of interest because at equilibrium it is a single tetragonal phase. Thus, compositions are of single phase tetragonal phase, theoretically, should not undergo high temperature partitioning. Single Tetragonal phase oxides of Ti-YSZ also offer the possibility of enhanced toughness and higher temperature stability akin to those observed in yttria partially stabilized zirconia. Many pyrochlore oxides are under review because they have shown to have lower thermal conductivity than YSZ oxides. This study focused on chemically synthesizing homogeneous starting material compositions in a metastable state (preferably amorphous), following its evolution according to the phase hierarchy under conditions of kinetic constraints. The current equilibrium diagram of YO1.5-TiO2-ZrO 2 is based on theoretical calculations. One of the contributions of this work is the redefined phase fields in YO1.5-TiO2-ZrO 2 based on our experimental results. Investigated compositions were based on tie lines of Y2-xTi2ZrxO7+x/2 and Y2Ti2-yZryO7 representing substitution of Zr4+ for Y3+ and Zr4+ for Ti4+ respectively. More notably, we observed extended metastable phases in pyrochlore and fluorite oxides at low temperature. The significance of this result is that it offers a larger compositional range for investing pyrochlore oxides with associated high temperature phase stability for TBC applications. In tetragonal oxides, our results showed that Ti-YSZ results have slower partitioning kinetics in comparison to YSZ at high temperature. This study also emphasized the deposition of advanced ceramic coatings by plasma spray for tetragonal and pyrochlore systems, compositionally complex functional oxides that may potentially have lower thermal conductivity values compared to current YSZ oxides. Next generation thermal barrier coatings require powders with high chemical purity, chemical homogeneity, controlled particle size/shape and pertinent phase state. Thermal spray offers an avenue to create novel materials and deposits directly from the precursor and compositionally controlled powder feedstock. This study contributed to investigating an unexplored field that offers a variety of opportunities in materials synthesis that would not be possible by conventional methods. Understanding processing-microstructure-property correlations is of considerable importance in thermal spray of functional oxide materials. This thesis demonstrated by radio-frequency thermal spray that the complex pyrochlore oxide Y 2Ti2O7 could be deposited by directly injecting molecularly mixed precursors to form oxide coatings. Structural analysis revealed the metastable fluorite phase; however, with thermal treatments at relatively low temperature of 700°C the pyrochlore phase was obtained. For Ti-YSZ coatings, the tetragonal phase oxides were obtained with unique microstructures, however, the tetragonal prime destabilized at 1200°C. This dissertation explored novel oxide compositions through detailed structural analysis. The approach presented a comprehensive and integrated investigation as it pertains to phase evolution of oxides in powder feedstock to coating characteristics (phase/properties).

High-pressure synthesis and characterizations of the R2Pt2O7 pyrochlores.

NASA Astrophysics Data System (ADS)

Cai, Yunqi; Cui, Qi; Cheng, Jinguang; Dun, Zhiling; Zhou, Haidong; Ma, Jie; Cruz, C. Dela; Yan, Jiaqiang; Li, Xiang; Zhou, Jianshi

Pyrochlore R2B2O7 where R3 + stands for rear-earth ion and B4 + for a nonmagnetic cation such as Sn4 +or Ti4 +consist of an important family of geometrically frustrated magnets, which have been the focus of extensive investigations over last decades. To further enlarge the R2B2O7, we have chosen to stabilize the Pt-based cubic pyrochlores under HPHT conditions for two reasons: (1) Pt4 + is in a low-spin state which ionic radius is located in between Ti4 + (0.605\\x85) and Sn4 + (0.69\\x85), and (2) Pt4 + has a spatially much more extended 5d orbitals and thus enhanced Pt 5d-O 2p hybridizations that might modify the local anisotropic exchange interactions. Such an effect has never been taken into account in the previous studies. In this work, we will present the detailed characterizations on the pyrochlores R2Pt2O7 obtained under HPHT conditions. This work is supported by the National Science Foundation of China (Grant Nos.11304371, 11574377), part of the work was supported by the CEM, and NSF MRSEC, under Grant DMR-1420451, and Grant No. NSF-DMR-1350002.

Nuclear waste disposal—pyrochlore (A2B2O7): Nuclear waste form for the immobilization of plutonium and "minor" actinides

NASA Astrophysics Data System (ADS)

Ewing, Rodney C.; Weber, William J.; Lian, Jie

2004-06-01

During the past half-century, the nuclear fuel cycle has generated approximately 1400 metric tons of plutonium and substantial quantities of the "minor" actinides, such as Np, Am, and Cm. The successful disposition of these actinides has an important impact on the strategy for developing advanced nuclear fuel cycles, weapons proliferation, and the geologic disposal of high-level radioactive waste. During the last decade, there has been substantial interest in the use of the isometric pyrochlore structure-type, A2B2O7, for the immobilization of actinides. Most of the interest has focused on titanate-pyrochlore because of its chemical durability; however, these compositions experience a radiation-induced transition from the crystalline-to-aperiodic state due to radiation damage from the alpha-decay of actinides. Depending on the actinide concentration, the titanate pyrochlore will become amorphous in less than 1000 years of storage. Recently, systematic ion beam irradiations of a variety of pyrochlore compositions has revealed that many zirconate pyrochlores do not become amorphous, but remain crystalline as a defect fluorite structure-type due to disordering of the A- and B-site cations. The zirconate pyrochlores will remain crystalline even to very high doses, greater than 100 displacements per atom. Systematic experimental studies of actinide-doped and ion beam-irradiated pyrochlore, analyses of natural U- and Th-bearing pyrochlore, and simulations of the energetics of the disordering process now provide a rather detailed understanding of the structural and chemical controls on the response of pyrochlore to radiation. These results provide a solid basis for predicting the behavior and durability of pyrochlore used to immobilize plutonium.

Nanostructured KTaTeO6 and Ag-doped KTaTeO6 Defect Pyrochlores: Promising Photocatalysts for Dye Degradation and Water Splitting

NASA Astrophysics Data System (ADS)

Venkataswamy, Perala; Sudhakar Reddy, CH.; Gundeboina, Ravi; Sadanandam, Gullapelli; Veldurthi, Naveen Kumar; Vithal, M.

2018-03-01

In this study, the nanostructured parent KTaTeO6 (KTTO) and Ag-doped KTaTeO6 (ATTO) catalysts with defect pyrochlore structure were prepared by solid-state and ion-exchange methods, respectively. The synthesized materials were characterized by various techniques to determine their chemical composition, morphology and microstructural features. The XRD studies show that both KTTO and ATTO have cubic structure (space group Fd3m) with high crystallinity. The doping of Ag altered the BET surface area of parent KTTO. The nano nature of the samples was studied by TEM images. A considerable red-shift in the absorption edge is observed for ATTO compared to KTTO. Incorporation of Ag+ in the KTTO lattice is clearly identified from EDX, elemental mapping and XPS results. Degradation of methyl violet and solar water splitting reactions were used to access the photocatalytic activity of KTTO and ATTO. The results obtained suggest that compared to KTTO, the ATTO showed higher photocatalytic activity in both cases. The favourable properties such as high surface area, more surface hydroxyl groups, stronger light absorption in visible region and narrower band gap energy were supposed to be the reasons for the high activity observed in ATTO.

Search for solid conductors of Na(+) and K(+) ions: Five new conductors

NASA Technical Reports Server (NTRS)

Singer, J.; Kautz, H.; Fielder, W. L.; Fordyce, J.

1975-01-01

Five conductors of three structure types were discovered which, as solids, can transport Na(+) or K(+) ions with conductivities of approximately .00001/(omega cm) at 300 K. These compounds are: (1) the pyrochlores NaTaWO6 and NaTa2O5F, both with an activation energy for conduction delta E of 21 kJ/mole; (2) the bodycentered cubic form of NaSbO3, with delta E = 42 kJ/mole; and (3) the niobates 2Na2O with 3Nb2O5 and 2K2O with 3Nb2O5, with the alkali ions probably in open layers of the incompletely determined structure; delta E = 17 kJ/mole. On the basis of approximately 40 structure types, some generalizations were made regarding the relation between structure and ionic transport.

Microstructural Evolution of Dy2O3-TiO2 Powder Mixtures during Ball Milling and Post-Milled Annealing

PubMed Central

Huang, Jinhua; Ran, Guang; Lin, Jianxin; Shen, Qiang; Lei, Penghui; Wang, Xina; Li, Ning

2016-01-01

The microstructural evolution of Dy2O3-TiO2 powder mixtures during ball milling and post-milled annealing was investigated using XRD, SEM, TEM, and DSC. At high ball-milling rotation speeds, the mixtures were fined, homogenized, nanocrystallized, and later completely amorphized, and the transformation of Dy2O3 from the cubic to the monoclinic crystal structure was observed. The amorphous transformation resulted from monoclinic Dy2O3, not from cubic Dy2O3. However, at low ball-milling rotation speeds, the mixtures were only fined and homogenized. An intermediate phase with a similar crystal structure to that of cubic Dy2TiO5 was detected in the amorphous mixtures annealed from 800 to 1000 °C, which was a metastable phase that transformed to orthorhombic Dy2TiO5 when the annealing temperature was above 1050 °C. However, at the same annealing temperatures, pyrochlore Dy2Ti2O7 initially formed and subsequently reacted with the remaining Dy2O3 to form orthorhombic Dy2TiO5 in the homogenous mixtures. The evolutionary mechanism of powder mixtures during ball milling and subsequent annealing was analyzed. PMID:28772375

The role of surfaces, chemical interfaces, and disorder on plutonium incorporation in pyrochlores

DOE PAGES

Perriot, Romain; Dholabhai, Pratik P.; Uberuaga, Blas P.

2016-07-27

Pyrochlores, a class of complex oxides with formula A 2B 2O 7, are one of the candidates for nuclear waste encapsulation, due to the natural occurrence of actinide-bearing pyrochlore minerals and laboratory observations of high radiation tolerance. In this work, we use atomistic simulations to determine the role of surfaces, chemical interfaces, and cation disorder on the plutonium immobilization properties of pyrochlores as a function of pyrochlore chemistry. We find that both Pu 3+ and Pu 4+ segregate to the surface for the four low-index pyrochlore surfaces considered, and that the segregation energy varies with the chemistry of the compound.more » We also find that pyrochlore/pyrochlore bicrystals A 2B 2O 7/A 2'B 2'O 7 can be used to immobilize Pu 3+ and Pu 4+ either in the same or separate phases of the compound, depending on the chemistry of the material. Finally, we find that Pu 4+ segregates to the disordered phase of an order/disorder bicrystal, driven by the occurrence of local oxygen-rich environments. However, Pu 3+ is weakly sensitive to the oxygen environment, and therefore only slightly favors the disordered phase. This behavior suggests that, at some concentration, Pu incorporation can destabilize the pyrochlore structure. Together, these results provide new insight into the ability of pyrochlore compounds to encapsulate Pu and suggest new considerations in the development of waste forms based on pyrochlores. Particularly, the phase structure of a multi-phase pyrochlore composite can be used to independently getter decay products based on their valence and size.« less

Review of A 2B 2O 7 Pyrochlore Response to Irradiation and Pressure

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lang, Maik; Zhang, Fuxiang; Zhang, Jiaming

2010-10-01

This article reviews recent research on swift heavy-ion irradiations and high-pressure studies on pyrochlores of the Gd 2Zr 2-xTi xO 7 binary. Applying three complementary analytical techniques (synchrotron X-ray diffraction, Raman spectroscopy and transmission electron microscopy) allowed for the investigation of the response of pyrochlore to irradiation and/or pressure. The chemical composition of pyrochlore has a strong effect on the character and energetics of the type of structural modifications that can be obtained under pressure or irradiation: For Ti-rich pyrochlores, the crystalline-to-amorphous transition is the dominant process. When Zr is substituted for Ti, an order–disorder transformation to the defect-fluorite structuremore » becomes the increasingly dominant process. Except for Gd 2Zr 2O 7, single ion tracks in pyrochlore consist of an amorphous core, surrounded by a crystalline, but disordered, defect-fluorite shell. This shell is surrounded by a defect-rich pyrochlore region. In contrast to similar effects observed when pressure or irradiation are applied separately, the response of the pyrochlore structure is significantly different when it is exposed simultaneously to pressure and irradiation. The combination of relativistic heavy ions with high pressure results in the formation of a new metastable pyrochlore phase. TEM and quantum–mechanical calculations suggest that these novel structural modifications are caused by the formation of nanocrystals and the modified energetics of nanomaterials.« less

Review of A2B2O7 Pyrochlore Response to Irradiation and Pressure

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lang, M.; Zhang, F; Zhang, J

2010-01-01

This article reviews recent research on swift heavy-ion irradiations and high-pressure studies on pyrochlores of the Gd{sub 2}Zr{sub 2-x}Ti{sub x}O{sub 7} binary. Applying three complementary analytical techniques (synchrotron X-ray diffraction, Raman spectroscopy and transmission electron microscopy) allowed for the investigation of the response of pyrochlore to irradiation and/or pressure. The chemical composition of pyrochlore has a strong effect on the character and energetics of the type of structural modifications that can be obtained under pressure or irradiation: For Ti-rich pyrochlores, the crystalline-to-amorphous transition is the dominant process. When Zr is substituted for Ti, an order-disorder transformation to the defect-fluorite structuremore » becomes the increasingly dominant process. Except for Gd{sub 2}Zr{sub 2}O{sub 7}, single ion tracks in pyrochlore consist of an amorphous core, surrounded by a crystalline, but disordered, defect-fluorite shell. This shell is surrounded by a defect-rich pyrochlore region. In contrast to similar effects observed when pressure or irradiation are applied separately, the response of the pyrochlore structure is significantly different when it is exposed simultaneously to pressure and irradiation. The combination of relativistic heavy ions with high pressure results in the formation of a new metastable pyrochlore phase. TEM and quantum-mechanical calculations suggest that these novel structural modifications are caused by the formation of nanocrystals and the modified energetics of nanomaterials.« less

Influence of synthesis conditions on the crystal structure of the powder formed in the “HfO2 - CeO2/Ce2O3” system

NASA Astrophysics Data System (ADS)

Popov, V. V.; Menushenkov, A. P.; Khubbutdinov, R. M.; Yastrebtsev, A. A.; Svetogorov, R. D.; Zubavichus, Ya V.; Trigub, A. L.; Sharapov, A. S.; Pisarev, A. A.; Kurilkin, V. V.; Tsarenko, N. A.; Arzhatkina, L. A.

2017-12-01

Influence of synthesis conditions (type of atmosphere: reduction or oxidation, annealing temperature) on the chemical composition and structure of the compounds formed in the “HfO2 - CeO2/Ce2O3” system has been investigated by X-ray absorption fine structure spectroscopy combined with Raman spectroscopy, X-ray diffraction and thermogravimetric analysis. It was revealed that isothermal annealing of precursor at temperatures less than 1000°C in air leads to formation of Ce0.5Hf0.5O2 powders with cubic fluorite-type structure (space group Fm-3m). Further increase of annealing temperatures above 1000°C causes decomposition of formed crystal structure into two phases: cubic and monoclinic. Annealing in reduction hydrogen atmosphere causes formation of Ce4+ 2x Ce3+ 2-2x Hf2O7+x compounds with intermediate oxidation state of cerium, where value of x depends on the reducing conditions and treatment parameters. Annealing in vacuum at 1400°C strongly reduces the content of Ce4+ in a powder samples and leads to formation of pyrochlore structure (space group Fd-3m) with predominant +3 oxidation state of cerium.

Single crystal structure and SHG of defect pyrochlores CsB{sup V}MoO{sub 6} (B{sup V}=Nb,Ta)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fukina, D.G., E-mail: dianafuk@yandex.ru; Suleimanov, E.V.; Yavetskiy, R.P.

2016-09-15

The crystal structure and non-linear optical properties of CsNbMoO{sub 6} and CsTaMoO{sub 6} defect pyrochlores have been studied. The single crystals of these compounds grown by the flux method possess an octahedral faceting and reach up to 50 µm in size. The crystal structures of CsB{sup V}MoO{sub 6} (B{sup V}=Nb, Ta) were investigated by X-ray diffraction method. Both compounds crystallize in the cubic symmetry with noncentrosymmetric space group F-43m. The second harmonic generation of CsNbMoO{sub 6} and CsTaMoO{sub 6}was found to be 1.6×10{sup −2} and 8.5×10{sup −4} of lithium niobate, correspondingly. It has been determined that distortions of [MO{sub 6}]more » polyhedra (M=Nb, Ta, Mo) as well as polarizability and covalency of Nb–O and Ta–O bonds have a great effect on the second harmonic generation. - Highlights: • CsNbMoO{sub 6} and CsTaMoO{sub 6} homogeneous single crystals have been grown. • The crystal structure of CsTaMoO{sub 6} has been studied. • Nonlinear optical properties of CsNbMoO{sub 6} and CsTaMoO{sub 6} have been found. • The microscopic origin of the second harmonic generation (SHG) response have been identified.« less

Pressure-induced structural modifications of rare-earth hafnate pyrochlore

NASA Astrophysics Data System (ADS)

Turner, Katlyn M.; Rittman, Dylan R.; Heymach, Rachel A.; Tracy, Cameron L.; Turner, Madison L.; Fuentes, Antonio F.; Mao, Wendy L.; Ewing, Rodney C.

2017-06-01

Complex oxides with the pyrochlore (A2B2O7) and defect-fluorite ((A,B)4O7) structure-types undergo structural transformations under high-pressure. Rare-earth hafnates (A2Hf2O7) form the pyrochlore structure for A  =  La-Tb and the defect-fluorite structure for A  =  Dy-Lu. High-pressure transformations in A2Hf2O7 pyrochlore (A  =  Sm, Eu, Gd) and defect-fluorite (A  =  Dy, Y, Yb) were investigated up to ~50 GPa and characterized by in situ Raman spectroscopy and synchrotron x-ray diffraction (XRD). Raman spectra at ambient pressure revealed that all compositions, including the defect-fluorites, have some pyrochlore-type short-range order. In situ high-pressure synchrotron XRD showed that all of the rare earth hafnates investigated undergo a pressure-induced phase transition to a cotunnite-like (orthorhombic) structure that begins between 18 and 25 GPa. The phase transition to the cotunnite-like structure is not complete at 50 GPa, and upon release of pressure, the hafnates transform to defect-fluorite with an amorphous component. For all compositions, in situ Raman spectroscopy showed that disordering occurs gradually with increasing pressure. Pyrochlore-structured hafnates retain their short-range order to a higher pressure (30 GPa vs.  <10 GPa) than defect-fluorite-structured hafnates. Rare earth hafnates quenched from 50 GPa show Raman spectra consistent with weberite-type structures, as also reported for irradiated rare-earth stannates. The second-order Birch-Murnaghan equation of state fit gives a bulk modulus of ~250 GPa for hafnates with the pyrochlore structure, and ~400 GPa for hafnates with the defect-fluorite structure. Dy2Hf2O7 is intermediate in its response, with some pyrochlore-type ordering, based on Raman spectroscopy and the equation of state, with a bulk modulus of ~300 GPa. As predicted based on the similar ionic radius of Zr4+ and Hf4+, rare-earth hafnates show similar behavior to that reported for rare earth zirconates at high pressure.

Pressure-induced structural modifications of rare-earth hafnate pyrochlore.

PubMed

Turner, Katlyn M; Rittman, Dylan R; Heymach, Rachel A; Tracy, Cameron L; Turner, Madison L; Fuentes, Antonio F; Mao, Wendy L; Ewing, Rodney C

2017-06-28

Complex oxides with the pyrochlore (A 2 B 2 O 7 ) and defect-fluorite ((A,B) 4 O 7 ) structure-types undergo structural transformations under high-pressure. Rare-earth hafnates (A 2 Hf 2 O 7 ) form the pyrochlore structure for A  =  La-Tb and the defect-fluorite structure for A  =  Dy-Lu. High-pressure transformations in A 2 Hf 2 O 7 pyrochlore (A  =  Sm, Eu, Gd) and defect-fluorite (A  =  Dy, Y, Yb) were investigated up to ~50 GPa and characterized by in situ Raman spectroscopy and synchrotron x-ray diffraction (XRD). Raman spectra at ambient pressure revealed that all compositions, including the defect-fluorites, have some pyrochlore-type short-range order. In situ high-pressure synchrotron XRD showed that all of the rare earth hafnates investigated undergo a pressure-induced phase transition to a cotunnite-like (orthorhombic) structure that begins between 18 and 25 GPa. The phase transition to the cotunnite-like structure is not complete at 50 GPa, and upon release of pressure, the hafnates transform to defect-fluorite with an amorphous component. For all compositions, in situ Raman spectroscopy showed that disordering occurs gradually with increasing pressure. Pyrochlore-structured hafnates retain their short-range order to a higher pressure (30 GPa vs.  <10 GPa) than defect-fluorite-structured hafnates. Rare earth hafnates quenched from 50 GPa show Raman spectra consistent with weberite-type structures, as also reported for irradiated rare-earth stannates. The second-order Birch-Murnaghan equation of state fit gives a bulk modulus of ~250 GPa for hafnates with the pyrochlore structure, and ~400 GPa for hafnates with the defect-fluorite structure. Dy 2 Hf 2 O 7 is intermediate in its response, with some pyrochlore-type ordering, based on Raman spectroscopy and the equation of state, with a bulk modulus of ~300 GPa. As predicted based on the similar ionic radius of Zr 4+ and Hf 4+ , rare-earth hafnates show similar behavior to that reported for rare earth zirconates at high pressure.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Perriot, Romain; Dholabhai, Pratik P.; Uberuaga, Blas P.

Pyrochlores, a class of complex oxides with formula A 2B 2O 7, are one of the candidates for nuclear waste encapsulation, due to the natural occurrence of actinide-bearing pyrochlore minerals and laboratory observations of high radiation tolerance. In this work, we use atomistic simulations to determine the role of surfaces, chemical interfaces, and cation disorder on the plutonium immobilization properties of pyrochlores as a function of pyrochlore chemistry. We find that both Pu 3+ and Pu 4+ segregate to the surface for the four low-index pyrochlore surfaces considered, and that the segregation energy varies with the chemistry of the compound.more » We also find that pyrochlore/pyrochlore bicrystals A 2B 2O 7/A 2'B 2'O 7 can be used to immobilize Pu 3+ and Pu 4+ either in the same or separate phases of the compound, depending on the chemistry of the material. Finally, we find that Pu 4+ segregates to the disordered phase of an order/disorder bicrystal, driven by the occurrence of local oxygen-rich environments. However, Pu 3+ is weakly sensitive to the oxygen environment, and therefore only slightly favors the disordered phase. This behavior suggests that, at some concentration, Pu incorporation can destabilize the pyrochlore structure. Together, these results provide new insight into the ability of pyrochlore compounds to encapsulate Pu and suggest new considerations in the development of waste forms based on pyrochlores. Particularly, the phase structure of a multi-phase pyrochlore composite can be used to independently getter decay products based on their valence and size.« less

Opposite correlations between cation disordering and amorphization resistance in spinels versus pyrochlores

PubMed Central

Uberuaga, Blas Pedro; Tang, Ming; Jiang, Chao; Valdez, James A.; Smith, Roger; Wang, Yongqiang; Sickafus, Kurt E.

2015-01-01

Understanding and predicting radiation damage evolution in complex materials is crucial for developing next-generation nuclear energy sources. Here, using a combination of ion beam irradiation, transmission electron microscopy and X-ray diffraction, we show that, contrary to the behaviour observed in pyrochlores, the amorphization resistance of spinel compounds correlates directly with the energy to disorder the structure. Using a combination of atomistic simulation techniques, we ascribe this behaviour to structural defects on the cation sublattice that are present in spinel but not in pyrochlore. Specifically, because of these structural defects, there are kinetic pathways for the relaxation of disorder in spinel that are absent in pyrochlore. This leads to a direct correlation between amorphization resistance and disordering energetics in spinel, the opposite of that observed in pyrochlores. These results provide new insight into the origins of amorphization resistance in complex oxides beyond fluorite derivatives. PMID:26510750

Opposite correlations between cation disordering and amorphization resistance in spinels versus pyrochlores

DOE PAGES

Uberuaga, Blas Pedro; Tang, Ming; Jiang, Chao; ...

2015-10-29

Understanding and predicting radiation damage evolution in complex materials is crucial for developing next-generation nuclear energy sources. Here, using a combination of ion beam irradiation, transmission electron microscopy and X-ray diffraction, we show that, contrary to the behaviour observed in pyrochlores, the amorphization resistance of spinel compounds correlates directly with the energy to disorder the structure. Using a combination of atomistic simulation techniques, we ascribe this behaviour to structural defects on the cation sublattice that are present in spinel but not in pyrochlore. Specifically, because of these structural defects, there are kinetic pathways for the relaxation of disorder in spinelmore » that are absent in pyrochlore. This leads to a direct correlation between amorphization resistance and disordering energetics in spinel, the opposite of that observed in pyrochlores. Furthermore, these results provide new insight into the origins of amorphization resistance in complex oxides beyond fluorite derivatives.« less

Opposite correlations between cation disordering and amorphization resistance in spinels versus pyrochlores.

PubMed

Uberuaga, Blas Pedro; Tang, Ming; Jiang, Chao; Valdez, James A; Smith, Roger; Wang, Yongqiang; Sickafus, Kurt E

2015-10-29

Understanding and predicting radiation damage evolution in complex materials is crucial for developing next-generation nuclear energy sources. Here, using a combination of ion beam irradiation, transmission electron microscopy and X-ray diffraction, we show that, contrary to the behaviour observed in pyrochlores, the amorphization resistance of spinel compounds correlates directly with the energy to disorder the structure. Using a combination of atomistic simulation techniques, we ascribe this behaviour to structural defects on the cation sublattice that are present in spinel but not in pyrochlore. Specifically, because of these structural defects, there are kinetic pathways for the relaxation of disorder in spinel that are absent in pyrochlore. This leads to a direct correlation between amorphization resistance and disordering energetics in spinel, the opposite of that observed in pyrochlores. These results provide new insight into the origins of amorphization resistance in complex oxides beyond fluorite derivatives.

Crystal structure of the pyrochlore oxide superconductor KOs{sub 2}O{sub 6}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yamaura, Jun-Ichi; Yonezawa, Shigeki; Muraoka, Yuji

2006-01-15

We report the single-crystal X-ray analysis of the structure of the pyrochlore oxide superconductor KOs{sub 2}O{sub 6}. The structure was identified as the {beta}-pyrochlore structure with space group Fd3-bar m and lattice constant a=10.089(2)A at 300K: the K atom is located at the 8b site, not at the 16d site as in conventional pyrochlore oxides. We found an anomalously large atomic displacement parameter U{sub iso}=0.0735(8)A{sup 2} at 300K for the K cation, which suggests that the K cation weakly bound to an oversized Os{sub 12}O{sub 18} cage exhibits intensive rattling, as recently observed for clathrate compounds. The rattling of Amore » cations is a common feature in the series of {beta}-pyrochlore oxide superconductors AOs{sub 2}O{sub 6} (A=Cs, Rb and K), and is greatest for the smallest K cation.« less

Electronic excitation induced amorphization in titanate pyrochlores: an ab initio molecular dynamics study

PubMed Central

Xiao, H. Y.; Weber, W. J.; Zhang, Y.; Zu, X. T.; Li, S.

2015-01-01

The response of titanate pyrochlores (A2Ti2O7, A = Y, Gd and Sm) to electronic excitation is investigated utilizing an ab initio molecular dynamics method. All the titanate pyrochlores are found to undergo a crystalline-to-amorphous structural transition under a low concentration of electronic excitations. The transition temperature at which structural amorphization starts to occur depends on the concentration of electronic excitations. During the structural transition, O2-like molecules are formed, and this anion disorder further drives cation disorder that leads to an amorphous state. This study provides new insights into the mechanisms of amorphization in titanate pyrochlores under laser, electron and ion irradiations. PMID:25660219

Electronic excitation induced amorphization in titanate pyrochlores: an ab initio molecular dynamics study.

PubMed

Xiao, H Y; Weber, W J; Zhang, Y; Zu, X T; Li, S

2015-02-09

The response of titanate pyrochlores (A2Ti2O7, A = Y, Gd and Sm) to electronic excitation is investigated utilizing an ab initio molecular dynamics method. All the titanate pyrochlores are found to undergo a crystalline-to-amorphous structural transition under a low concentration of electronic excitations. The transition temperature at which structural amorphization starts to occur depends on the concentration of electronic excitations. During the structural transition, O2-like molecules are formed, and this anion disorder further drives cation disorder that leads to an amorphous state. This study provides new insights into the mechanisms of amorphization in titanate pyrochlores under laser, electron and ion irradiations.

Electronic excitation induced amorphization in titanate pyrochlores: an ab initio molecular dynamics study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xiao, Haiyan Y.; Weber, William J.; Zhang, Yanwen

2015-02-09

In this study, the response of titanate pyrochlores (A 2Ti 2O 7, A = Y, Gd and Sm) to electronic excitation is investigated utilizing an ab initio molecular dynamics method. All the titanate pyrochlores are found to undergo a crystalline-to-amorphous structural transition under a low concentration of electronic excitations. The transition temperature at which structural amorphization starts to occur depends on the concentration of electronic excitations. During the structural transition, O 2-like molecules are formed, and this anion disorder further drives cation disorder that leads to an amorphous state. This study provides new insights into the mechanisms of amorphization inmore » titanate pyrochlores under laser, electron and ion irradiations.« less

Pyrochlore-type catalysts for the reforming of hydrocarbon fuels

DOEpatents

Berry, David A [Morgantown, WV; Shekhawat, Dushyant [Morgantown, WV; Haynes, Daniel [Morgantown, WV; Smith, Mark [Morgantown, WV; Spivey, James J [Baton Rouge, LA

2012-03-13

A method of catalytically reforming a reactant gas mixture using a pyrochlore catalyst material comprised of one or more pyrochlores having the composition A.sub.2-w-xA'.sub.wA''.sub.xB.sub.2-y-zB'.sub.yB''.sub.zO.sub.7-.DELTA.. Distribution of catalytically active metals throughout the structure at the B site creates an active and well dispersed metal locked into place in the crystal structure. This greatly reduces the metal sintering that typically occurs on supported catalysts used in reforming reactions, and reduces deactivation by sulfur and carbon. Further, oxygen mobility may also be enhanced by elemental exchange of promoters at sites in the pyrochlore. The pyrochlore catalyst material may be utilized in catalytic reforming reactions for the conversion of hydrocarbon fuels into synthesis gas (H.sub.2+CO) for fuel cells, among other uses.

Pyrochlore catalysts for hydrocarbon fuel reforming

DOEpatents

Berry, David A.; Shekhawat, Dushyant; Haynes, Daniel; Smith, Mark; Spivey, James J.

2012-08-14

A method of catalytically reforming a reactant gas mixture using a pyrochlore catalyst material comprised of one or more pyrochlores having the composition A₂B_2-y-zB'_yB"_zO_7-.DELTA., where y>0 and z.gtoreq.0. Distribution of catalytically active metals throughout the structure at the B site creates an active and well dispersed metal locked into place in the crystal structure. This greatly reduces the metal sintering that typically occurs on supported catalysts used in reforming reactions, and reduces deactivation by sulfur and carbon. Further, oxygen mobility may also be enhanced by elemental exchange of promoters at sites in the pyrochlore. The pyrochlore catalyst material may be utilized in catalytic reforming reactions for the conversion of hydrocarbon fuels into synthesis gas (H₂+CO) for fuel cells, among other uses.

Lanthanide stannate pyrochlores (Ln2Sn2O7; Ln  =  Nd, Gd, Er) at high pressure

NASA Astrophysics Data System (ADS)

Turner, Katlyn M.; Tracy, Cameron L.; Mao, Wendy L.; Ewing, Rodney C.

2017-12-01

Lanthanide stannate pyrochlores (Ln2Sn2O7; Ln  =  Nd, Gd, and Er) were investigated in situ to 50 GPa in order to determine their structural response to compression and compare their response to that of lanthanide titanate, zirconate, and hafnate pyrochlores. The cation radius ratio of A3+/B4+ in pyrochlore oxides (A2B2O7) is thought to be the dominant feature that influences their response on compression. The ionic radius of Sn4+ is intermediate to that of Ti4+, Zr4+, and Hf4+, but the 〈Sn-O〉 bond in stannate pyrochlore is more covalent than the 〈B-O〉 bonds in titanates, zirconate, and hafnates. In stannates, based on in situ Raman spectroscopy, pyrochlore cation and anion sublattices begin to disorder with the onset of compression, first measured at 0.3 GPa. The extent of sublattice disorder versus pressure is greater in stannates with a smaller Ln3+ cation. Stannate pyrochlores (Fd-3m) begin a sluggish transformation to an orthorhombic, cotunnite-like structure at ~28 GPa similar transitions have been observed in titanate, zirconate, and hafnate pyrochlores at varying pressures (18-40 GPa) with cation radius ratio. The extent of the phase transition versus pressure varies directly with the size of the Ln3+ cation. Post-decompression from ~50 GPa, Er2Sn2O7 and Gd2Sn2O7 adopt a pyrochlore structure, rather than the multi-scale defect-fluorite  +  weberite-type structure adopted by Nd2Sn2O7 that is characteristic of titanate, zirconate, and hafnate pyrochlores under similar conditions. Like pyrochlore titanates, zirconates, and hafnates, the bulk modulus, B 0, of stannates varies linearly and inversely with cation radius ratio from 1 1 1 GPa (Nd2Sn2O7) to 251 GPa (Er2Sn2O7). The trends of bulk moduli in stannates in this study are in excellent agreement with previous experimental studies on stannates and suggest that the size of the Ln3+ cation is the primary determining factor of B 0. Additionally, when normalized to r A/r B, the bulk moduli of stannates are comparable to those of zirconates and hafnates, which vary from titanates. Our results suggest that the cation radius ratio strongly influences the bulk moduli of stannates, as well as their overall compression response.

Defect formation energy in pyrochlore: the effect of crystal size

NASA Astrophysics Data System (ADS)

Wang, Jianwei; Ewing, Rodney C.; Becker, Udo

2014-09-01

Defect formation energies of point defects of two pyrochlores Gd2Ti2O7 and Gd2Zr2O7 as a function of crystal size were calculated. Density functional theory with plane-wave basis sets and the projector-augmented wave method were used in the calculations. The results show that the defect formation energies of the two pyrochlores diverge as the size decreases to the nanometer range. For Gd2Ti2O7 pyrochlore, the defect formation energy is higher at nanometers with respect to that of the bulk, while it is lower for Gd2Zr2O7. The lowest defect formation energy for Gd2Zr2O7 is found at 15-20 Å. The different behaviors of the defect formation energies as a function of crystal size are caused by different structural adjustments around the defects as the size decreases. For both pyrochlore compositions at large sizes, the defect structures are similar to those of the bulk. As the size decreases, for Gd2Ti2O7, additional structure distortions appear at the surfaces, which cause the defect formation energy to increase. For Gd2Zr2O7, additional oxygen Frenkel pair defects are introduced, which reduce the defect formation energy. As the size further decreases, increased structure distortions occur at the surfaces, which cause the defect formation energy to increase. Based on a hypothesis that correlates the energetics of defect formation and radiation response for complex oxides, the calculated results suggest that at nanometer range Gd2Ti2O7 pyrochlore is expected to have a lower radiation tolerance, and those of Gd2Zr2O7 pyrochlore to have a higher radiation tolerance. The highest radiation tolerance for Gd2Zr2O7 pyrochlore is expected to be found at ˜2 nanometers.

Thermodynamics of radiation induced amorphization and thermal annealing of Dy 2Sn 2O 7 pyrochlore

DOE PAGES

Chung, Cheng-Kai; Lang, Maik; Xu, Hongwu; ...

2018-06-14

Thermodynamics and annealing behavior of swift heavy ion amorphized Dy 2Sn 2O 7 pyrochlore were studied. Its amorphization enthalpy, defined as the total energetic difference between the irradiation amorphized and undamaged Dy 2Sn 2O 7 states, was determined to be 283.6 ± 6.5 kJ/mol by high temperature oxide melt drop solution calorimetry. It has been an enigma that stannate and some other pyrochlores do not follow the general r A/r B-radiation resistance relation seen in most pyrochlore systems. In this paper, we use the amorphization enthalpy, which reflects all the complex chemical and structural characteristics, as a more effective parametermore » to correlate the radiation damage resistance of pyrochlores with their compositions. It successfully explains the superior radiation damage resistance of the stannate pyrochlores compared with titanate pyrochlores. Differential scanning calorimetry (DSC) reveals a strong exothermic event starting at 978 K, which is attributed to long-range recrystallization based on X-ray diffraction (XRD) analysis, similar to the effect previously observed in Dy 2Ti 2O 7. A second pronounced heat event beginning at ~1148 K, which results from local structural rearrangement, is clearly decoupled from the first event for irradiated Dy 2Sn 2O 7. Both the heat releases measured by DSC on heating to 1023 and 1473 K, and the excess enthalpies of the annealed samples indicate that the recovery to the original, ordered state was not fully achieved up to even 1473 K, despite XRD showing the apparent restoration of crystalline pyrochlore structure. The remaining metastability may be attributed to local disorder in the form of weberite-like short-range domains in the recrystallized material. Intriguingly, the second event for different pyrochlores generally starts at similar temperatures while the onset of the long range recrystallization is compositionally dependent. Finally, the amorphization and thermal annealing behavior observed in irradiated Dy 2Sn 2O 7 may provide insights into the general mechanisms of radiation damage and recovery of pyrochlores relevant to their nuclear applications.« less

Nature of the chemical bond and prediction of radiation tolerance in pyrochlore and defect fluorite compounds

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lumpkin, Gregory R.; Institute of Materials and Engineering Science, Australian Nuclear Science and Technology Organisation, Private Mail Bag 1, Menai, NSW 2234; Pruneda, Miguel

2007-04-15

The radiation tolerance of synthetic pyrochlore and defect fluorite compounds has been studied using ion irradiation. We show that the results can be quantified in terms of the critical temperature for amorphization, structural parameters, classical Pauling electronegativity difference, and disorder energies. Our results demonstrate that radiation tolerance is correlated with a change in the structure from pyrochlore to defect fluorite, a smaller unit cell dimension, and lower cation-anion disorder energy. Radiation tolerance is promoted by an increase in the Pauling cation-anion electronegativity difference or, in other words, an increase in the ionicity of the chemical bonds. A further analysis ofmore » the data indicates that, of the two possible cation sites in ideal pyrochlore, the smaller B-site cation appears to play the major role in bonding. This result is supported by ab initio calculations of the structure and bonding, showing a correlation between the Mulliken overlap populations of the B-site cation and the critical temperature. - Graphical abstract: Three-dimensional representation of the predicted critical amorphization temperature in pyrochlores.« less

A new triclinic modification of the pyrochlore-type KOs{sub 2}O{sub 6} superconductor

DOE Office of Scientific and Technical Information (OSTI.GOV)

Katrych, S.; Gu, Q.F.; Bukowski, Z.

2009-03-15

A new modification of KOs{sub 2}O{sub 6}, the representative of a new structural type (Pearson symbol aP18, a=5.5668(1) A, b=6.4519(2) A, c=7.2356(2) A, {alpha}=65.377(3){sup o}, {beta}=70.572(3){sup o}, {gamma}=75.613(2){sup o} space group P-1, no. 2 was synthesized employing high pressure technique. Its structure was determined by single-crystal X-ray diffraction. The structure can be described as two OsO{sub 6} octahedral chains relating to each other through inversion and forming big voids with K atoms inside. Quantum chemical calculations were performed on the novel compound and structurally related cubic compound. High-pressure X-ray study showed that cubic KOs{sub 2}O{sub 6} phase was stable upmore » to 32.5(2) GPa at room temperature. - Graphical abstract: A new modification of KOs{sub 2}O{sub 6}, the representative of a new structural type (Pearson symbol aP18, a=5.5668(1) A, b=6.4519(2) A, c=7.2356(2) A, {alpha}=65.377(3){sup o}, {beta}=70.572(3){sup o}, {gamma}=75.613(2){sup o} space group P-1, no. 2 was synthesized employing high pressure technique. The structure can be described as two OsO{sub 6} octahedral chains relating to each other through inversion and forming big voids with K atoms inside.« less

Fast ion conductivity in strained defect-fluorite structure created by ion tracks in Gd 2Ti 2O 7

DOE PAGES

Aidhy, Dilpuneet S.; Sachan, Ritesh; Zarkadoula, Eva; ...

2015-11-10

In this research, the structure and ion-conducting properties of the defect-fluorite ring structure formed around amorphous ion-tracks by swift heavy ion irradiation of Gd 2Ti 2O 7 pyrochlore are investigated. High angle annular dark field imaging complemented with ion-track molecular dynamics simulations show that the atoms in the ring structure are disordered, and have relatively larger cation-cation interspacing than in the bulk pyrochlore, illustrating the presence of tensile strain in the ring region. Density functional theory calculations show that the non-equilibrium defect-fluorite structure can be stabilized by tensile strain. The pyrochlore to defect-fluorite structure transformation in the ring region ismore » predicted to be induced by recrystallization during a melt-quench process and stabilized by tensile strain. Static pair-potential calculations show that planar tensile strain lowers oxygen vacancy migration barriers in pyrochlores, in agreement with recent studies on fluorite and perovskite materials. From these results, it is suggested that strain engineering could be simultaneously used to stabilize the defect-fluorite structure and gain control over its high ion-conducting properties.« less

Transmission electron microscopic study of pyrochlore to defect-fluorite transition in rare-earth pyrohafnates

DOE Office of Scientific and Technical Information (OSTI.GOV)

Karthik, Chinnathambi, E-mail: Karthikchinnathambi@boisestate.edu; Center for Advanced Energy Studies, 995 University Blvd, Idaho Falls, ID 83415; Anderson, Thomas J.

2012-10-15

A structural transition in rare earth pyrohafnates, Ln{sub 2}Hf{sub 2}O{sub 7} (Ln=Y, La, Pr, Nd, Tb, Dy, Yb and Lu), has been identified. Neutron diffraction showed that the structure transforms from well-ordered pyrochloric to fully fluoritic through the lanthanide series from La to Lu with a corresponding increase in the position parameter x of the 48f (Fd3{sup Macron }m) oxygen site from 0.330 to 0.375. As evidenced by the selected area electron diffraction, La{sub 2}Hf{sub 2}O{sub 7}, Pr{sub 2}Hf{sub 2}O{sub 7} and Nd{sub 2}Hf{sub 2}O{sub 7} exhibited a well-ordered pyrocholoric structure with the presence of intense superlattice spots, which becamemore » weak and diffuse (in Dy{sub 2}Hf{sub 2}O{sub 7} and Tb{sub 2}Hf{sub 2}O{sub 7}) before disappearing completely as the series progressed towards the Lu end. High resolution electron microscopic studies showed the breakdown of the pyrochlore ordering in the form of antiphase domains resulting in diffused smoke-like superlattice spots in the case of Dy{sub 2}Hf{sub 2}O{sub 7} and Tb{sub 2}Hf{sub 2}O{sub 7}. - Graphical abstract: Transmission electron microscopic studies showed the ordered pyrochlore to defect fluorite transition in rare-earth pyrohafnates to occur via the formation of anti-phase domains to start with. Highlights: Black-Right-Pointing-Pointer Pyrochlore to fluorite structural transition in rare earth pyrohafnates. Black-Right-Pointing-Pointer La{sub 2}Hf{sub 2}O{sub 7}, Pr{sub 2}Hf{sub 2}O{sub 7} and Nd{sub 2}Hf{sub 2}O{sub 7} showed well ordered pyrochlore structure. Black-Right-Pointing-Pointer Short range ordering in Dy{sub 2}Hf{sub 2}O{sub 7} and Tb{sub 2}Hf{sub 2}O{sub 7}. Black-Right-Pointing-Pointer Break down of pyrochlore ordering due to antiphase boundaries. Black-Right-Pointing-Pointer Rest of the series showed fluoritic structure.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Uberuaga, Blas Pedro

In this study, complex oxides such as pyrochlores have a myriad of potential technological applications, including as fast ion conductors and radiation-tolerant nuclear waste forms. They are also of interest for their catalytic and spin ice properties. Many of these functional properties are enabled by the atomic structure of the cation sublattices. Pyrochlores (A2B2O7) contain two different cations (A and B), typically a 3+ rare earth and a 4+ transition metal such as Hf, Zr, or Ti. The large variety of chemistries that can form pyrochlores leads to a rich space in which to search for exotic new materials. Furthermore,more » how cations order or disorder on their respective sublattices for a given chemical composition influences the functional properties of the oxide. For example, oxygen ionic conductivity is directly correlated with the level of cation disorder — the swapping of A and B cations 1. Further, the resistance of these materials against amorphization has also been connected with the ability of the cations to disorder 2, 3. These correlations between cation structure and functionality have spurred great interest in the structure of the cation sublattice under irradiation, with significant focus on the disordering mechanisms and disordered structure. Previous studies have found that, upon irradiation, pyrochlores often undergo an order-to-disorder transformation, in which the resulting structure is, from a diffraction point of view, indistinguishable from fluorite (AO2) (ref. 3). Shamblin et al. now reveal that the structure of disordered pyrochlore is more complicated than previously thought 4.« less

Effect of C6+ Ion Irradiation on structural and electrical properties of Yb and Eu doped Bi1.5 Zn0.92 Nb1.5 O6.92 pyrochlores

NASA Astrophysics Data System (ADS)

Yumak, Mehmet; Mergen, Ayhan; Qureshi, Anjum; Singh, N. L.

2015-03-01

Pyrochlore general formula of A2B2X7 where A and B are cations and X is an anion Pyrochlore compounds exhibit semiconductor, metallic or ionic conduction properties, depending on the doping, compositions/ substituting variety of cations and oxygen partial pressure. Ion beam irradiation can induce the structural disordering by mixing the cation and anion sublattices, therefore we aim to inevestigate effects of irradiation in pyrochlore compounds. In this study, Eu and Yb-doped Bi1.5Zn0.92Nb1.5O6.92 (Eu-BZN, Yb-BZN) Doping effect and single phase formation of Eu-BZN, Yb-BZN was characterized by X-ray diffraction technique (XRD). Radiation-induced effect of 85 MeV C6+ ions on Eu-BZN, Yb-BZN was studied by XRD, scanning electron microscopy (SEM) and temperature dependent dielectric measurements at different fluences. XRD results revealed that the ion beam-induced structural amorphization processes in Eu-BZN and Yb-BZN structures. Our results suggested that the ion beam irradiation induced the significant change in the temprature depndent dielectric properties of Eu-BZN and Yb-BZN pyrochlores due to the increased oxygen vacancies as a result of cation and anion disordering. Department of Metallurgical and Materials Eng., Marmara University, Istanbul-81040, Turkey.

Phase relations in the system In{sub 2}O{sub 3}-TiO{sub 2}-Fe{sub 2}O{sub 3} at 1100 C in air

DOE Office of Scientific and Technical Information (OSTI.GOV)

Brown, F.; Flores, M.J.R.; Kimizuka, N.

1999-04-01

Phase relations in the system In{sub 2}O{sub 3}-TiO{sub 2}-Fe{sub 2}O{sub 3} at 1100 C in air are determined by means of a classic quenching method. There exist In{sub 2}TiO{sub 5}, Fe{sub 2}TiO{sub 5} having a pseudo-Brookite-type phase and a new phase, In{sub 3}Ti{sub 2}FeO{sub 10} having a solid solution range from In{sub 2}O{sub 3}:TiO{sub 2}:Fe{sub 2}O{sub 3} = 4:6:1 to In{sub 2}O{sub 3}:TiO{sub 2}:Fe{sub 2}O{sub 3} = 0.384:0.464:0.152 (mole ratio) on the line InFeO{sub 3}-In{sub 2}Ti{sub 2}O{sub 7}. The crystal structures of In{sub 3}Ti{sub 2}FeO{sub 10} are pyrochlore related with a{sub m} = 5.9171 (5) {angstrom}, b{sub m} = 3.3696more » (3) {angstrom}, c{sub m} = 6.3885 (6) {angstrom}, and {beta} = 108.02 (1){degree} in a monoclinic crystal system at 1100 C, and a{sub 0} = 5.9089 (5) {angstrom}, b{sub 0} = 3.3679 (3) {angstrom}, and c{sub 0} = 12.130 (1) {angstrom} in an orthorhombic system at 1200 C. The relationship between the lattice constants of these phases and those of the cubic pyrochlore type are approximately as follows: a{sub m} = {minus}{1/4}a{sub p} + ({minus}{1/2})b{sub p} + ({minus}{1/4})c{sub p}, b{sub m} = {minus}{1/4}a{sub p} + (0)b{sub p} + ({1/4})c{sub p}, c{sub m} = {1/4}a{sub p} + ({minus}{1/2})b{sub p} + ({1/4})c{sub p} and {beta} = 109.47{degree} in the monoclinic system, and a{sub 0} = {minus}{1/4}a{sub p} + ({minus}{1/2})b{sub p} + ({minus}{1/4})c{sub p}, b{sub 0} = {minus}{1/4}a{sub p} + (0)b{sub p} + ({1/4})c{sub p}, and c{sub 0} = 2/3a{sub p} + ({minus}2/3)b{sub p} + (2/3)c{sub p} in the orthorhombic system, where a{sub p} = b{sub p} = c{sub p} = 9.90 ({angstrom}) are the lattice constants of In{sub 2}Ti{sub 2}O{sub 7} having the cubic pyrochlore type. All solid solutions of In{sub 3}Ti{sub 2}FeO{sub 10} have incommensurate structures with a periodicity of q {times} b{sup *} (q = 0.281--0.356) along the b{sup *} axis and the stoichiometric phase has q = 1/3. In FeO{sub 3} having a layered structure type is unstable between 750 and 1100 C.« less

Magnetic and charge transport properties of the Na-based Os oxide pyrochlore

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shi, Y.G., E-mail: SHI.Youguo@nims.go.j; International Center for Materials Nanoarchitectonics; JST, Transformative Research-Project on Iron Pnictides

2009-04-15

The Na-based osmium oxide pyrochlore was synthesized for the first time by an ion-exchange method using KOs{sub 2}O{sub 6} as a host. The composition was identified as Na{sub 1.4}Os{sub 2}O{sub 6}.H{sub 2}O by electron probe micro-analysis, thermogravimetric analysis, and structural analysis using synchrotron X-ray diffraction. Na{sub 1.4}Os{sub 2}O{sub 6}.H{sub 2}O crystallizes in a regular pyrochlore structure with some defects (space group: Fd-3m, a=10.16851(1) A). Electrical resistivity, heat capacity, and magnetization measurements clearly showed absence of superconductivity down to 2 K, being in large contrast to what was found for the beta-type pyrochlore superconductor AOs{sub 2}O{sub 6} (A=Cs, Rb, and K).more » The Sommerfeld coefficient is 22 mJ K{sup -2} mol{sup -1}, being the smallest among AOs{sub 2}O{sub 6}. A magnetic anomaly at {approx}57 K and associated magneto-resistance (+3.7% at 2 K in 70 kOe) were found. - Graphical abstract: Crystal structure of the Na-based Os oxide pyrochlore Na{sub 1.4}Os{sub 2}O{sub 6}.H{sub 2}O.« less

Density Functional Theory Study of Leaching Performance of Different Acids on Pyrochlore (100) Surface

NASA Astrophysics Data System (ADS)

Yang, Xiuli; Fang, Qing; Ouyang, Hui

2018-04-01

Pyrochlore leaching using hydrofluoric, sulfuric, and hydrochloric acids has been studied via experimental methods for years, but the interactions between niobium atoms on the pyrochlore surface and different acids have not been investigated. In this work, first-principles calculations based on density functional theory were used to elucidate the leaching performance of these three acids from the viewpoint of geometrical and electronic structures. The calculation results indicate that sulfate, chloride, and fluoride anions influence the geometric structure of pyrochlore (100) to different extents, decreasing in the order: sulfate, fluoride, chloride. Orbitals of O1 and O2 atoms of sulfate hybridized with those of surface niobium atom. Fluorine orbitals hybridized with those of surface niobium atoms. However, no obvious overlap exists between any orbitals of chlorine and surface niobium, revealing that chlorine does not interact chemically with surface niobium atoms.

Density Functional Theory Study of Leaching Performance of Different Acids on Pyrochlore (100) Surface

NASA Astrophysics Data System (ADS)

Yang, Xiuli; Fang, Qing; Ouyang, Hui

2018-06-01

Pyrochlore leaching using hydrofluoric, sulfuric, and hydrochloric acids has been studied via experimental methods for years, but the interactions between niobium atoms on the pyrochlore surface and different acids have not been investigated. In this work, first-principles calculations based on density functional theory were used to elucidate the leaching performance of these three acids from the viewpoint of geometrical and electronic structures. The calculation results indicate that sulfate, chloride, and fluoride anions influence the geometric structure of pyrochlore (100) to different extents, decreasing in the order: sulfate, fluoride, chloride. Orbitals of O1 and O2 atoms of sulfate hybridized with those of surface niobium atom. Fluorine orbitals hybridized with those of surface niobium atoms. However, no obvious overlap exists between any orbitals of chlorine and surface niobium, revealing that chlorine does not interact chemically with surface niobium atoms.

Complex oxides: Intricate disorder

DOE PAGES

Uberuaga, Blas Pedro

2016-02-29

In this study, complex oxides such as pyrochlores have a myriad of potential technological applications, including as fast ion conductors and radiation-tolerant nuclear waste forms. They are also of interest for their catalytic and spin ice properties. Many of these functional properties are enabled by the atomic structure of the cation sublattices. Pyrochlores (A2B2O7) contain two different cations (A and B), typically a 3+ rare earth and a 4+ transition metal such as Hf, Zr, or Ti. The large variety of chemistries that can form pyrochlores leads to a rich space in which to search for exotic new materials. Furthermore,more » how cations order or disorder on their respective sublattices for a given chemical composition influences the functional properties of the oxide. For example, oxygen ionic conductivity is directly correlated with the level of cation disorder — the swapping of A and B cations 1. Further, the resistance of these materials against amorphization has also been connected with the ability of the cations to disorder 2, 3. These correlations between cation structure and functionality have spurred great interest in the structure of the cation sublattice under irradiation, with significant focus on the disordering mechanisms and disordered structure. Previous studies have found that, upon irradiation, pyrochlores often undergo an order-to-disorder transformation, in which the resulting structure is, from a diffraction point of view, indistinguishable from fluorite (AO2) (ref. 3). Shamblin et al. now reveal that the structure of disordered pyrochlore is more complicated than previously thought 4.« less

Quasiparticle Breakdown and Spin Hamiltonian of the Frustrated Quantum Pyrochlore Yb_{2}Ti_{2}O_{7} in a Magnetic Field.

PubMed

Thompson, J D; McClarty, P A; Prabhakaran, D; Cabrera, I; Guidi, T; Coldea, R

2017-08-04

The frustrated pyrochlore magnet Yb_{2}Ti_{2}O_{7} has the remarkable property that it orders magnetically but has no propagating magnons over wide regions of the Brillouin zone. Here we use inelastic neutron scattering to follow how the spectrum evolves in cubic-axis magnetic fields. At high fields we observe, in addition to dispersive magnons, a two-magnon continuum, which grows in intensity upon reducing the field and overlaps with the one-magnon states at intermediate fields leading to strong renormalization of the dispersion relations, and magnon decays. Using heat capacity measurements we find that the low- and high-field regions are smoothly connected with no sharp phase transition, with the spin gap increasing monotonically in field. Through fits to an extensive data set of dispersion relations combined with magnetization measurements, we reevaluate the spin Hamiltonian, finding dominant quantum exchange terms, which we propose are responsible for the anomalously strong fluctuations and quasiparticle breakdown effects observed at low fields.

Comparing photonic band structure calculation methods for diamond and pyrochlore crystals.

PubMed

Vermolen, E C M; Thijssen, J H J; Moroz, A; Megens, M; van Blaaderen, A

2009-04-27

The photonic band diagrams of close-packed colloidal diamond and pyrochlore structures, have been studied using Korringa-Kohn-Rostoker (KKR) and plane-wave calculations. In addition, the occurrence of a band gap has been investigated for the binary Laves structures and their constituent large- and small-sphere substructures. It was recently shown that these Laves structures give the possibility to fabricate the diamond and pyrochlore structures by self-organization. The comparison of the two calculation methods opens the possibility to study the validity and the convergence of the results, which have been an issue for diamond-related structures in the past. The KKR calculations systematically give a lower value for the gap width than the plane-wave calculations. This difference can partly be ascribed to a convergence issue in the plane-wave code when a contact point of two spheres coincides with the grid.

Energy landscape in frustrated systems: Cation hopping in pyrochlores

NASA Astrophysics Data System (ADS)

Brooks Hinojosa, Beverly; Asthagiri, Aravind; Nino, Juan C.

2013-07-01

We investigate the dynamics of the local environment and electronic structure in inherently dipolar frustrated pyrochlore compounds to help identify the fundamental nature of dipolar disorder in pyrochlore systems and determine the necessary and sufficient conditions for dielectric relaxation. We map out the energy landscape associated with cation hopping events in three compounds and correlate the hopping pathway with experimental dielectric response. Comprehensive analysis of the calculations allows us to postulate rules to predict the occurrence of relaxation and cation hopping pathways.

Energetics of defects formation and oxygen migration in pyrochlore compounds from first principles calculations

NASA Astrophysics Data System (ADS)

Li, Yan; Kowalski, Piotr M.

2018-07-01

In order to get better understanding of the selective order-disorder transition in pyrochlore compounds, using ab initio methods we calculated the formation energies of coupled cation anti-site and anion Frenkel pair defects and the energy barriers for the oxygen migration for number of families of A2B2 O7 pyrochlore-type compounds. While these parameters have been previously computed with force field-based methods, the ab initio results provide more reliable values that can be confidently used in subsequent analysis. We found a fairly good correlation between the formation energies of the coupled defects and the stability field of pyrochlores. In line with previous studies, the compounds that crystallize in defect fluorite structure are found to have smaller values of coupled defect formation energies than those crystallizing in the pyrochlore phase, although the correlation is not that sharp as in the case of isolated anion Frenkel pair defect. The investigation of the energy barriers for the oxygen migration shows that it is not a good, sole indicator of the tendency of the order-disorder phase transition in pyrochlores. However, we found that the oxygen migration barrier is reduced in the presence of the cation antisite defect. This points at disordering-induced enhancement of oxygen diffusion in pyrochlore compounds.

Similar local order in disordered fluorite and aperiodic pyrochlore structures

DOE PAGES

Shamblin, Jacob; Tracy, Cameron; Palomares, Raul; ...

2017-10-01

A major challenge to understanding the response of materials to extreme environments (e.g., nuclear fuels/waste forms and fusion materials) is to unravel the processes by which a material can incorporate atomic-scale disorder, and at the same time, remain crystalline. While it has long been known that all condensed matter, even liquids and glasses, possess short-range order, the relation between fully-ordered, disordered, and aperiodic structures over multiple length scales is not well understood. For example, when defects are introduced (via pressure or irradiation) into materials adopting the pyrochlore structure, these complex oxides either disorder over specific crystallographic sites, remaining crystalline, ormore » become aperiodic. Here we present neutron total scattering results characterizing the irradiation response of two pyrochlores, one that is known to disorder (Er2Sn2O7) and the other to amorphize (Dy2Sn2O7) under ion irradiation. The results demonstrate that in both cases, the local pyrochlore structure is transformed into similar short range configurations that are best fit by the orthorhombic weberite structure, even though the two compositions have distinctly different structures, aperiodic vs. disordered-crystalline, at longer length scales. Thus, a material's resistance to amorphization may not depend primarily on local defect formation energies, but rather on the structure's compatibility with meso-scale modulations of the local order in a way that maintains long-range periodicity.« less

Frustration under pressure: Exotic magnetism in new pyrochlore oxides

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wiebe, C. R.; Department of Chemistry, University of Manitoba, Winnipeg, Manitoba R3T 2N2; Department of Physics and Astronomy, McMaster University, Hamilton, Ontario L8S 4M1

2015-04-01

Pyrochlore structures, of chemical formula A{sub 2}B{sub 2}O{sub 7} (A and B are typically trivalent and tetravalent ions, respectively), have been the focus of much activity in the condensed matter community due to the ease of substitution of rare earth and transition metal ions upon the two interpenetrating corner-shared tetrahedral lattices. Over the last few decades, superconductivity, spin liquid states, spin ice states, glassy states in the absence of chemical disorder, and metal-insulator transitions have all been discovered in these materials. Geometric frustration plays a role in the relevant physics of all of these phenomena. In the search for newmore » pyrochlore materials, it is the R{sub A}/R{sub B} cation radius ratio which determines the stability of the lattice over the defect fluorite structure in the lower limit. Under ambient pressure, the pyrochlores are stable for 1.36 ≤ R{sub A}/R{sub B} ≤ 1.71. However, using high pressure synthesis techniques (1-10 GPa of pressure), metastable pyrochlores exist up to R{sub A}/R{sub B} = 2.30. Many of these compounds are stable on a timescale of years after synthesis, and provide a means to greatly enhance exchange, and thus test theories of quantum magnetism and search for new phenomena. Within this article, we review new pyrochlore compounds synthesized via high pressure techniques and show how the ground states are extremely sensitive to chemical pressure.« less

Coulomb spin liquid in anion-disordered pyrochlore Tb 2Hf 2O 7

DOE PAGES

Sibille, Romain; Lhotel, Elsa; Hatnean, Monica Ciomaga; ...

2017-10-12

Here, the charge ordered structure of ions and vacancies characterizing rare-earth pyrochlore oxides serves as a model for the study of geometrically frustrated magnetism. The organization of magnetic ions into networks of corner-sharing tetrahedra gives rise to highly correlated magnetic phases with strong fluctuations, including spin liquids and spin ices. It is an open question how these ground states governed by local rules are affected by disorder. Here we demonstrate in the pyrochlore Tb 2Hf 2O 7, that the vicinity of the disordering transition towards a defective fluorite structure translates into a tunable density of anion Frenkel disorder while cationsmore » remain ordered. Quenched random crystal fields and disordered exchange interactions can therefore be introduced into otherwise perfect pyrochlore lattices of magnetic ions. We show that disorder can play a crucial role in preventing long-range magnetic order at low temperatures, and instead induces a strongly fluctuating Coulomb spin liquid with defect-induced frozen magnetic degrees of freedom.« less

Crystal Structure and Magnetic Properties of New Cubic Quaternary Compounds RT2Sn2Zn18 (R = La, Ce, Pr, and Nd, and T = Co and Fe)

NASA Astrophysics Data System (ADS)

Isikawa, Yosikazu; Mizushima, Toshio; Ejiri, Jun-ichi; Kitayama, Shiori; Kumagai, Keigou; Kuwai, Tomohiko; Bordet, Pierre; Lejay, Pascal

2015-07-01

The new cubic quaternary intermetallic compounds RT2Sn2Zn18 (R = La, Ce, Pr, and Nd, and T = Co and Fe) were synthesized by the mixture-metal flux method using Zn and Sn. The crystal structure was investigated by powder X-ray diffraction and with a four-circle X-ray diffractometer using single crystals. The space group of the compounds is Fdbar{3}m (No. 227). The rare-earth atom is at the cubic site which is the center of a cage composed of Zn and Sn atoms. The crystal structure is the same as the CeCr2Al20-type crystal structure except the atoms at the 16c site, i.e., the Zn atoms at the 16c site are completely replaced by Sn atoms, indicating that the compounds are crystallographically new ordered quaternary compounds. The lattice parameter a and the physical properties of the magnetic susceptibility χ, the magnetization M, and the specific heat C of these cubic caged compounds were investigated. LaCo2Sn2Zn18 and LaFe2Sn2Zn18 are enhanced Pauli paramagnets that originate from the Co and Fe itinerant 3d electrons. CeCo2Sn2Zn18 and CeFe2Sn2Zn18 are also enhanced Pauli paramagnets that originate from both the 3d electrons and Ce 4f electrons. PrCo2Sn2Zn18 and PrFe2Sn2Zn18 are nonmagnetic materials with huge values of C divided by temperature, which indicates that the ground state of Pr ions is a non-Kramers' doublet. NdCo2Sn2Zn18 and NdFe2Sn2Zn18 are magnetic materials with the Néel temperatures of 1.0 and 3.8 K, respectively. All eight compounds have large magnetic moments of Co/Fe in the paramagnetic temperature region, and thus their magnetic moments are inferred to be magnetically frustrating owing to the pyrochlore lattice in the low-temperature region.

Experimental Insights into Ground-State Selection of Quantum XY Pyrochlores

NASA Astrophysics Data System (ADS)

Hallas, Alannah M.; Gaudet, Jonathan; Gaulin, Bruce D.

2018-03-01

Extensive experimental investigations of the magnetic structures and excitations in the XY pyrochlores have been carried out over the past decade. Three families of XY pyrochlores have emerged: Yb2B2O7, Er2B2O7, and, most recently, [Formula: see text]Co2F7. In each case, the magnetic cation (either Yb, Er, or Co) exhibits XY anisotropy within the local pyrochlore coordinates, a consequence of crystal field effects. Materials in these families display rich phase behavior and are candidates for exotic ground states, such as quantum spin ice, and exotic ground-state selection via order-by-disorder mechanisms. In this review, we present an experimental summary of the ground-state properties of the XY pyrochlores, including evidence that they are strongly influenced by phase competition. We empirically demonstrate the signatures for phase competition in a frustrated magnet: multiple heat capacity anomalies, suppressed TN or TC, sample- and pressure-dependent ground states, and unconventional spin dynamics.

Ion beam irradiation of lanthanum and thorium-doped yttrium titanates

NASA Astrophysics Data System (ADS)

Lian, J.; Zhang, F. X.; Peters, M. T.; Wang, L. M.; Ewing, R. C.

2007-05-01

Y2Ti2O7 pyrochlores doped with La have been sintered at 1373 K for 12 h with the designed compositions of the (LaxY1-x)2Ti2O7 system (x = 0, 0.08, 0.5, and 1), and the phase compositions were analyzed by X-ray diffraction. Limited amounts of La were incorporated into yttrium titanate pyrochlore structure for La-doped samples; while, the end member composition of La2Ti2O7 formed a layered perovskite structure. Ion beam-induced amorphization occurred for all compositions in the (LaxY1-x)2Ti2O7 binary under 1 MeV Kr2+ irradiation at room temperature, and the critical amorphization dose decreased with increasing amounts of La3+. The critical amorphization temperatures for Y2Ti2O7, (La0.162Y0.838)2Ti2O7 and La2Ti2O7 were determined to be ∼780, 890 and 920 K, respectively. Th4+ and Fe3+-doped yttrium titanate pyrochlores were synthesized at 1373 K by sintering Y2Ti2O7 with (ThO2 + Fe2O3). Pyrochlore structures and the chemical compositions were primarily identified by the X-ray diffraction and energy dispersive X-ray (EDX) measurements. The lattice parameter and the critical amorphization dose (1 MeV Kr2+ at room temperature) increase for yttrium titanate pyrochlores with the addition of Th. The increasing 'resistance' to amorphization with less La and greater Th and Fe contents for (Y1-xLax)2Ti2O7 and Y2Ti2O7-Fe2O3-ThO2 systems, respectively, are consistent with the changes in the average ionic radius ratio at the A-sites and B-sites. These results suggest that the addition of lanthanides and actinides (e.g., Th, U, or Pu) will affect the structural stability, as well as the radiation response behavior of the pyrochlore structure-type.

Spin-orbit-driven magnetic structure and excitation in the 5d pyrochlore Cd 2Os 2O 7

DOE PAGES

Calder, Stuart A; Vale, James G.; Bogdanov, Nikolay; ...

2016-06-07

Here, much consideration has been given to the role of spin-orbit coupling (SOC) in 5d oxides, particularly on the formation of novel electronic states and manifested metal-insulator transitions (MITs). SOC plays a dominant role in 5d 5 iridates (Ir 4+), undergoing MITs both concurrent (pyrochlores) and separated (perovskites) from the onset of magnetic order. However, the role of SOC for other 5d configurations is less clear. For example, 5d 3 (Os 5+) systems are expected to have an orbital singlet with reduced effective SOC. The pyrochlore Cd 2Os 2O 7 nonetheless exhibits a MIT entwined with magnetic order phenomenologically similarmore » to pyrochlore iridates. Here, we resolve the magnetic structure in Cd 2Os 2O 7 with neutron diffraction and then via resonant inelastic X-ray scattering determine the salient electronic and magnetic energy scales controlling the MIT. In particular, SOC plays a subtle role in creating the electronic ground state but drives the magnetic order and emergence of a multiple spin-flip magnetic excitation.« less

Epitaxial crystals of Bi₂Pt₂O₇ pyrochlore through the transformation of δ–Bi₂O₃ fluorite

DOE PAGES

Gutiérrez–Llorente, Araceli; Joress, Howie; Woll, Arthur; ...

2015-03-01

Bi₂Pt₂O₇ pyrochlore is thought to be one of the most promising oxide catalysts for application in fuel cell technology. Unfortunately, direct film growth of Bi₂Pt₂O₇ has not yet been achieved, owing to the difficulty of oxidizing platinum metal in the precursor material to Pt⁴⁺. In this work, in order to induce oxidation of the platinum, we annealed pulsed laser deposited films consisting of epitaxial δ–Bi₂O₃ and co-deposited, comparatively disordered platinum. We present synchrotron x-ray diffraction results that show the nonuniform annealed films contain the first epitaxial crystals of Bi₂Pt₂O₇. We also visualized the pyrochlore structure by scanning transmission electron microscopy,more » and observed ordered cation vacancies in the epitaxial crystals formed in a bismuth-rich film but not in those formed in a platinum-rich film. The similarity between the δ–Bi₂O₃ and Bi₂Pt₂O₇ structures appears to facilitate the pyrochlore formation. These results provide the only route to date for the formation of epitaxial Bi₂Pt₂O₇.« less

Crystal chemistry of pyrochlore from the Mesozoic Panda Hill carbonatite deposit, western Tanzania

NASA Astrophysics Data System (ADS)

Boniface, Nelson

2017-02-01

The Mesozoic Panda Hill carbonatite deposit in western Tanzania hosts pyrochlore, an ore and source of niobium. This study was conducted to establish the contents of radioactive elements (uranium and thorium) in pyrochlore along with the concentration of niobium in the ore. The pyrochlore is mainly hosted in sövite and is structurally controlled by NW-SE (SW dipping) or NE-SW (NW dipping) magmatic flow bands with dip angles of between 60° and 90°. Higher concentrations of pyrochlore are associated with magnetite, apatite and/or phlogopite rich flow bands. Electron microprobe analyses on single crystals of pyrochlore yield very low UO2 concentrations that range between 0 and 0.09 wt% (equivalent to 0 atoms per formula unit: a.p.f.u.) and ThO2 between 0.55 and 1.05 wt% (equivalent to 0.1 a.p.f.u.). The analyses reveal high concentrations of Nb2O5 (ranging between 57.13 and 65.50 wt%, equivalent to a.p.f.u. ranging between 1.33 and 1.43) and therefore the Panda Hill Nb-oxide is classified as pyrochlore sensu stricto. These data point to a non radioactive pyrochlore and a deposit rich in Nb at Panda Hill. The Panda Hill pyrochlore has low concentrations of REEs as displayed by La2O3 that range between 0.10 and 0.49 wt% (equivalent to a.p.f.u. ranging between 0 and 0.01) and Ce2O3 ranging between 0.86 and 1.80 wt% (equivalent to a.p.f.u. ranging between 0.02 and 0.03), Pr2O3 concentrations range between 0 and 0.23 wt% (equivalent to 0 a.p.f.u.), and Y2O3 is 0 wt% (equivalent to 0 a.p.f.u.). The abundance of the REEs in pyroclore at the Panda Hill Carbonatite deposit is of no economic significance.

Symmetry and light stuffing of H o 2 T i 2 O 7 ,   E r 2 T i 2 O 7 , and Y b 2 T i 2 O 7 characterized by synchrotron x-ray diffraction

DOE Office of Scientific and Technical Information (OSTI.GOV)

Baroudi, Kristen; Gaulin, Bruce D.; Lapidus, Saul H.

2015-07-01

The Ho2Ti2O7, Er2Ti2O7 and Yb2Ti2O7 pyrochlores were studied by synchrotron X-ray diffraction to determine whether the (002) peak, forbidden in the pyrochlore space group Fd-3m but observed in single crystal neutron scattering measurements, is present due to a deviation of their pyrochlore structure from Fd-3m symmetry. Synchrotron diffraction measurements on precisely synthesized stoichiometric and non-stoichiometric powders and a crushed floating zone crystal of Ho2Ti2O7 revealed that the (002) reflection is absent in all cases to a sensitivity of approximately one part in 30,000 of the strongest X-ray diffraction peak. This indicates to high sensitivity that the structural space group ofmore » these rare earth titanate pyrochlores is Fd-3m, and that thus the (002) peak observed in the neutron scattering experiments has a non-structural origin. The cell parameters and internal strain for lightly stuffed Ho2+xTi2-xO7 are also presented.« less

Role of composition, bond covalency, and short-range order in the disordering of stannate pyrochlores by swift heavy ion irradiation

NASA Astrophysics Data System (ADS)

Tracy, Cameron L.; Shamblin, Jacob; Park, Sulgiye; Zhang, Fuxiang; Trautmann, Christina; Lang, Maik; Ewing, Rodney C.

2016-08-01

A2S n2O7 (A =Nd ,Sm,Gd,Er,Yb,and Y) materials with the pyrochlore structure were irradiated with 2.2 GeV Au ions to systematically investigate disordering of this system in response to dense electronic excitation. Structural modifications were characterized, over multiple length scales, by transmission electron microscopy, x-ray diffraction, and Raman spectroscopy. Transformations to amorphous and disordered phases were observed, with disordering dominating the structural response of materials with small A -site cation ionic radii. Both the disordered and amorphous phases were found to possess weberite-type local ordering, differing only in that the disordered phase exhibits a long-range, modulated arrangement of weberite-type structural units into an average defect-fluorite structure, while the amorphous phase remains fully aperiodic. Comparison with the behavior of titanate and zirconate pyrochlores showed minimal influence of the high covalency of the Sn-O bond on this phase behavior. An analytical model of damage accumulation was developed to account for simultaneous amorphization and recrystallization of the disordered phase during irradiation.

C60 and U ion irradiation of Gd 2Ti xZr 2-xO 7 pyrochlore

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Jiaming; Toulemonde, Marcel; Lang, Maik

2015-08-01

Gd 2Ti xZr 2-xO 7 (x = 0 to 2) pyrochlore was irradiated by 30 MeV C 60 clusters, which provide an extremely high ionizing energy density. Here, high-resolution transmission electron microscopy revealed a complex ion-track structure in Gd 2Ti 2O 7 and Gd 2TiZrO 7, consisting of an amorphous core and a shell of a disordered, defect-fluorite structure.

Synthesis and characterization of Bi{sub 1.56}Sb{sub 1.48}Co{sub 0.96}O{sub 7} pyrochlore sun-light-responsive photocatalyst

DOE Office of Scientific and Technical Information (OSTI.GOV)

Naceur, Benhadria, E-mail: nacer1974@yahoo.fr; Abdelkader, Elaziouti, E-mail: elaziouti_a@yahoo.com; Dr Moulay Tahar University, Saida

2016-02-15

Graphical abstract: Heterogeneous photo Fenton process with dye sensitized mechanism of RhB by Bi{sub 1.56}Sb{sub 1.48}Co{sub 0.96}O{sub 7} compound. - Highlights: • Bi{sub 1.56}Sb{sub 1.48}Co{sub 0.96}O{sub 7} (BSCO) catalyst was synthesized by improved solid state reaction method. • BSCO/H{sub 2}O{sub 2}/UVA and BSCO/H{sub 2}O{sub 2}/SL catalyst systems exhibit excellent photocatalytic activities for rhodamine B. • The photocatalytic degradation was preceded via heterogeneous photo Fenton mechanism process. • ·OH radicals are the main reactive species for the degradation of RhB. - Abstract: Novel nanostructure pyrochlore Bi{sub 1.56}Sb{sub 1.48}Co{sub 0.96}O{sub 7} was successfully synthesized via solid state reaction method in air. Themore » as-synthesized photocatalyst was characterized by X-ray diffraction, Scanning electron microscopy and UV–vis diffuse reflectance spectroscopy techniques. The results showed that the BSCO was crystallized with the pyrochlore-type structure, cubic crystal system and space group Fd3m. The average particle size and band gap for BSCO were D = 76.29 nm and E{sub g} = 1.50 eV respectively. Under the optimum conditions for discoloration of the dye: initial concentration of 20 mg L{sup −1} RhB, pH 7, 25 °C, 0.5 mL H{sub 2}O{sub 2} and BSCO/dye mass ration of 1 g L{sup −1}, 97.77 and 90.16% of RhB were removed with BSCO/H{sub 2}O{sub 2} photocatalytic system within 60 min of irradiation time under UVA- and SL irradiations respectively. Pseudo-second-order kinetic model gave the best fit, with highest correlation coefficients (R{sup 2} ≥ 0.99). On the base of these results, the mechanism of the enhancement of the discoloration efficiency was discussed. .« less

Synchrotron X-ray diffraction and Raman spectroscopy of Ln{sub 3}NbO{sub 7} (Ln=La, Pr, Nd, Sm-Lu) ceramics obtained by molten-salt synthesis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Siqueira, K.P.F.; Soares, J.C.; Granado, E.

2014-01-15

Ln{sub 3}NbO{sub 7} (Ln=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) ceramics were obtained by molten-salt synthesis and their structures were systematically investigated by synchrotron X-ray diffraction (SXRD), second harmonic generation (SHG) and Raman spectroscopy. It was observed that ceramics with the largest ionic radii (La, Pr, Nd) crystallized into the Pmcn space group, while the ceramics with intermediate ionic radii (Sm-Gd) exhibited a different crystal structure belonging to the Ccmm space group. For this last group of ceramics, this result was corroborated by SHG and Raman scattering and ruled out any possibility formore » the non-centrosymmetric C 222{sub 1} space group, solving a recent controversy in the literature. Finally, according to SXRD, Tb-Lu containing samples exhibited an average defect fluorite structure (Fm3{sup ¯}m space group). Nonetheless, broad scattering at forbidden Bragg reflections indicates the presence of short-range domains with lower symmetry. Vibrational spectroscopy showed the presence of six Raman-active modes, inconsistent with the average cubic fluorite structure, and in line with the existence of lower-symmetry nano-domains immersed in the average fluorite structure of these ceramics. - Graphical abstract: Raman spectrum for Sm{sub 3}NbO{sub 7} ceramics showing their 27 phonon modes adjusted through Lorentzian lines. According to synchrotron X-ray diffraction and Raman scattering, this material belongs to the space group Cmcm. Display Omitted - Highlights: • Ln{sub 3}NbO{sub 7} ceramics were obtained by molten-salt synthesis. • SXRD, SHG and Raman scattering confirmed orthorhombic and cubic structures. • Ccmm instead of C222{sub 1} is the correct structure for Sm–Gd ceramics. • Pmcn space group was confirmed for La-, Pr- and Nd-based ceramics. • For Tb–Lu ceramics, ordered domains of a pyrochlore structure were observed.« less

George E. Valley, Jr. Prize Talk: Quantum Frustrated Magnetism and its Expression in the Ground State Selection of Pyrochlore Magnets

NASA Astrophysics Data System (ADS)

Ross, Kate

In the search for novel quantum states of matter, such as highly entangled Quantum Spin Liquids, ``geometrically frustrated'' magnetic lattices are essential for suppressing conventional magnetic order. In three dimensions, the pyrochlore lattice is the canonical frustrated geometry. Magnetic materials with pyrochlore structures have the potential to realize unusual phases such as ``quantum spin ice'', which is predicted to host emergent magnetic monopoles, electrons, and photons as its fundamental excitations. Even in pyrochlores that form long range ordered phases, this often occurs through unusual routes such as ``order by disorder'', in which the fluctuation spectrum dictates the preferred ordered state. The rare earth-based pyrochlore series R2Ti2O7 provides a fascinating variety of magnetic ground states. I will introduce the general anisotropic interaction Hamiltonian that has been successfully used to describe several materials in this series. Using inelastic neutron scattering, the relevant anisotropic interaction strengths can be extracted quantitatively. I will discuss this approach, and its application to two rare earth pyrochlore materials, Er2Ti2O7 and Yb2Ti<2O7, whose ground state properties have long been enigmatic. From these studies, ErTi2O7 and Yb2Ti2O7 have been suggested to be realizations of "quantum order by disorder" and "quantum spin ice", respectively. This research was supported by NSERC of Canada and the National Science Foundation.

Giant reversible magnetocaloric effect in the pyrochlore Er 2 Mn 2 O 7 due to a cooperative two-sublattice ferromagnetic order

DOE PAGES

Cai, Y. Q.; Jiao, Y. Y.; Cui, Qi; ...

2017-11-29

Most magnetic refrigeration materials showing a large and reversible magnetocaloric effect (MCE) undergo a second-order ferromagnetic (FM) transition involving large-moment magnetic species on one sublattice. Furthermore, a stronger MCE is expected near a cooperative FM order of two or more magnetic species with large magnetic moments residing on different sublattices, but experimental realizations are rare. Here we report on the discovery of large MCE in the cubic pyrochlore Er 2Mn 2O 7 near its second-order FM transition at T c ≈ 34K; under the magnetic field change of 1 and 5 T, the maximum magnetic entropy change –ΔS M ismore » 5.27 and 16.1Jkg –1K –1, and the estimated magnetic refrigerant capacity reaches 68 and 522Jkg –1, respectively. These latter values are among the largest for the known MCE materials. The observed giant and reversible MCE in Er 2Mn 2O 7 is mainly attributed to the large saturation moment of 18.9μ B per formula unit owing to a simultaneous FM ordering of the rear-earth Er 3+ and transition-metal Mn 4+ localized moments. Our results suggest that Er 2Mn 2O 7 pyrochlore is a promising candidate for magnetic refrigeration applications in the temperature range 20–80 K. More importantly, this work provides a new material system for developing high-performance MCE materials that can exhibit a strongly coupled FM transition involving two magnetic sublattices of large local moments in a single-phase material.« less

Giant reversible magnetocaloric effect in the pyrochlore Er 2 Mn 2 O 7 due to a cooperative two-sublattice ferromagnetic order

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cai, Y. Q.; Jiao, Y. Y.; Cui, Qi

Most magnetic refrigeration materials showing a large and reversible magnetocaloric effect (MCE) undergo a second-order ferromagnetic (FM) transition involving large-moment magnetic species on one sublattice. Furthermore, a stronger MCE is expected near a cooperative FM order of two or more magnetic species with large magnetic moments residing on different sublattices, but experimental realizations are rare. Here we report on the discovery of large MCE in the cubic pyrochlore Er 2Mn 2O 7 near its second-order FM transition at T c ≈ 34K; under the magnetic field change of 1 and 5 T, the maximum magnetic entropy change –ΔS M ismore » 5.27 and 16.1Jkg –1K –1, and the estimated magnetic refrigerant capacity reaches 68 and 522Jkg –1, respectively. These latter values are among the largest for the known MCE materials. The observed giant and reversible MCE in Er 2Mn 2O 7 is mainly attributed to the large saturation moment of 18.9μ B per formula unit owing to a simultaneous FM ordering of the rear-earth Er 3+ and transition-metal Mn 4+ localized moments. Our results suggest that Er 2Mn 2O 7 pyrochlore is a promising candidate for magnetic refrigeration applications in the temperature range 20–80 K. More importantly, this work provides a new material system for developing high-performance MCE materials that can exhibit a strongly coupled FM transition involving two magnetic sublattices of large local moments in a single-phase material.« less

Heavy ion irradiation effects of brannerite-type ceramics

NASA Astrophysics Data System (ADS)

Lian, J.; Wang, L. M.; Lumpkin, G. R.; Ewing, R. C.

2002-05-01

Brannerite, UTi 2O 6, occurs in polyphase Ti-based, crystalline ceramics that are under development for plutonium immobilization. In order to investigate radiation effects caused by α-decay events of Pu, a 1 MeV Kr + irradiation on UTi 2O 6, ThTi 2O 6, CeTi 2O 6 and a more complex material, composed of Ca-containing brannerite and pyrochlore, was performed over a temperature range of 25-1020 K. The ion irradiation-induced crystalline-to-amorphous transformation was observed in all brannerite samples. The critical amorphization temperatures of the different brannerite compositions are: 970 K, UTi 2O 6; 990 K, ThTi 2O 6; 1020 K, CeTi 2O 6. The systematic increase in radiation resistance from Ce-, Th- to U-brannerite is related to the difference of mean atomic mass of A-site cation in the structure. As compared with the pyrochlore structure-type, brannerite phases are more susceptible to ion irradiation-induced amorphization. The effects of structure and chemical compositions on radiation resistance of brannerite-type and pyrochlore-type ceramics are discussed.

Atomic displacements in the charge ice pyrochlore Bi2Ti2O6O' studied by neutron total scattering

NASA Astrophysics Data System (ADS)

Shoemaker, Daniel P.; Seshadri, Ram; Hector, Andrew L.; Llobet, Anna; Proffen, Thomas; Fennie, Craig J.

2010-04-01

The oxide pyrochlore Bi2Ti2O6O' is known to be associated with large displacements of Bi and O' atoms from their ideal crystallographic positions. Neutron total scattering, analyzed in both reciprocal and real space, is employed here to understand the nature of these displacements. Rietveld analysis and maximum entropy methods are used to produce an average picture of the structural nonideality. Local structure is modeled via large-box reverse Monte Carlo simulations constrained simultaneously by the Bragg profile and real-space pair distribution function. Direct visualization and statistical analyses of these models show the precise nature of the static Bi and O' displacements. Correlations between neighboring Bi displacements are analyzed using coordinates from the large-box simulations. The framework of continuous symmetry measures has been applied to distributions of O'Bi4 tetrahedra to examine deviations from ideality. Bi displacements from ideal positions appear correlated over local length scales. The results are consistent with the idea that these nonmagnetic lone-pair containing pyrochlore compounds can be regarded as highly structurally frustrated systems.

Crystal Structure and Thermoelectric Properties of β-Pyrochlore-Type Alkali Iron Tungsten Oxides with Cage-Like Structure

NASA Astrophysics Data System (ADS)

Mizuta, Kohei; Ohtaki, Michitaka

2016-03-01

We report the electrical and thermal properties of β-pyrochlore (defect pyrochlore) oxides AFe0.33W1.67O6 ( A = K, Rb, Cs) with a crystal structure having a small cation surrounded by oversized cage-like framework. The thermal conductivity, κ, of CsFe0.33W1.67O6 and RbFe0.33W1.67O6 showed extremely low values as oxides (below 1.0 W/mK) similar to those of ATaWO6 ( A = K, Rb, Cs) which we have already reported. These low κ values are ascribed to a "rattling" motion of the A cations, evidenced by their crystal structure refinement and the Raman spectra. Their electrical conductivity, σ, was in the order of 10-3 S/cm, and the Seebeck coefficient, S, was -500 to -600 μV/K. The electrical conductivity of AFe0.33W1.67O6 ( A = Rb, Cs) was much higher than those of ATaWO6 ( A = Rb, Cs), suggesting that an appropriate selection of the framework composition enables us to have better thermoelectric performance.

Heavy ion irradiation-induced microstructural evolution in pyrochlore Lu{sub 2}Ti{sub 2}O{sub 7} at room temperature and 723 K

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xie, Q.R.; Zhang, J., E-mail: zhangjian@xmu.edu.cn; Dong, X.N.

Polycrystalline pyrochlore Lu{sub 2}Ti{sub 2}O{sub 7} pellets were irradiated with 600 keV Kr{sup 3+} at room temperature and 723 K to a fluence of 4×10{sup 15} ions/cm{sup 2}, corresponding to an average ballistic damage dose of 10 displacements per atom in the peak damage region. Irradiation-induced microstructural evolution was examined by grazing incidence X-ray diffraction, and cross-sectional transmission electron microscopy. Incomplete amorphization was observed in the sample irradiated at room temperature due to the formation of nano-crystal which has the identical structure of pyrochlore, and the formation of nano-crystal is attributed to the mechanism of epitaxial recrystallization. However, an orderedmore » pyrochlore phase to a swelling disordered fluorite phase transformation is occurred for the Lu{sub 2}Ti{sub 2}O{sub 7} sample irradiated at 723 K, which is due to the disordering of metal cations and anion vacancies. - Graphical Abstract: Polycrystalline pyrochlore Lu{sub 2}Ti{sub 2}O{sub 7} pellets were irradiated with 600 keV Kr{sup 3+} to a fluence of 4×10{sup 15} ions/cm{sup 2} at room temperature and 723 K, Incomplete amorphization was observed in the sample irradiated at room temperature due to the formation of nano-crystal. However, an ordered pyrochlore phase to a swelling disordered fluorite phase transformation is occurred for the Lu{sub 2}Ti{sub 2}O{sub 7} sample irradiated at 723 K, which is due to the disordering of metal cations and anion vacancies. - Highlights: Pyrochlore Lu{sub 2}Ti{sub 2}O{sub 7} pellets were irradiated by heavy ions at RT and 723 K. At RT irradiation, ~75% of amorphization was achieved. The nano-crystals were formed in the damage layer at RT irradiation. The formed nano-crystals enhanced the radiation tolerance of Lu{sub 2}Ti{sub 2}O{sub 7}. A pyrochlore to fluorite phase transformation was observed at 723 K irradiation.« less

High-Performance Pyrochlore-Type Yttrium Ruthenate Electrocatalyst for Oxygen Evolution Reaction in Acidic Media

DOE PAGES

Kim, Jaemin; Shih, Pei-Chieh; Tsao, Kai-Chieh; ...

2017-07-27

Development of acid-stable electrocatalysts with low overpotential for oxygen evolution reaction (OER) is a major challenge to produce hydrogen directly from water. We report in this paper a pyrochlore yttrium ruthenate (Y 2Ru 2O 7-δ) electrocatalyst that has significantly enhanced performance toward OER in acid media over the best-known catalysts, with an onset overpotential of 190 mV and high stability in 0.1 M perchloric acid solution. X-ray absorption near-edge structure (XANES) indicates Y 2Ru 2O 7-δ electrocatalyst had a low valence state that favors the high OER activity. Density functional theory (DFT) calculation shows this pyrochlore has lower band centermore » energy for the overlap between Ru 4d and O 2p orbitals and is therefore more stable Ru–O bond than RuO 2, highlighting the effect of yttrium on the enhancement in stability. Finally, the Y 2Ru 2O 7-δ pyrochlore is also free of expensive iridium metal and thus is a cost-effective candidate for practical applications.« less

High-Performance Pyrochlore-Type Yttrium Ruthenate Electrocatalyst for Oxygen Evolution Reaction in Acidic Media

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kim, Jaemin; Shih, Pei-Chieh; Tsao, Kai-Chieh

Development of acid-stable electrocatalysts with low overpotential for oxygen evolution reaction (OER) is a major challenge for the production of hydrogen directly from water. We report in this paper a pyrochlore yttrium ruthenate (Y2Ru2O7-δ) electrocatalyst that has significantly enhanced performance towards OER in acid media over the best-known catalysts, with an onset overpotential of 190 mV and high stability in 0.1-M perchloric acid solution. X-ray absorption near-edge structure (XANES) indicates Y2Ru2O7-δ electrocatalyst had a low valence state that favors the high OER activity. Density functional theory (DFT) calculation shows this pyrochlore has lower band center energy for the overlap betweenmore » Ru 4d and O 2p orbitals and therefore more stable Ru-O bond than RuO2, highlighting the effect of yttrium on the enhancement in stability. The Y2Ru2O7-δ pyrochlore is also free of expensive iridium metal, thus a cost-effective candidate for practical applications.« less

High-Performance Pyrochlore-Type Yttrium Ruthenate Electrocatalyst for Oxygen Evolution Reaction in Acidic Media

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kim, Jaemin; Shih, Pei-Chieh; Tsao, Kai-Chieh

Development of acid-stable electrocatalysts with low overpotential for oxygen evolution reaction (OER) is a major challenge to produce hydrogen directly from water. We report in this paper a pyrochlore yttrium ruthenate (Y 2Ru 2O 7-δ) electrocatalyst that has significantly enhanced performance toward OER in acid media over the best-known catalysts, with an onset overpotential of 190 mV and high stability in 0.1 M perchloric acid solution. X-ray absorption near-edge structure (XANES) indicates Y 2Ru 2O 7-δ electrocatalyst had a low valence state that favors the high OER activity. Density functional theory (DFT) calculation shows this pyrochlore has lower band centermore » energy for the overlap between Ru 4d and O 2p orbitals and is therefore more stable Ru–O bond than RuO 2, highlighting the effect of yttrium on the enhancement in stability. Finally, the Y 2Ru 2O 7-δ pyrochlore is also free of expensive iridium metal and thus is a cost-effective candidate for practical applications.« less

Forging Fast Ion Conducting Nanochannels with Swift Heavy Ions: The Correlated Role of Local Electronic and Atomic Structure

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sachan, Ritesh; Cooper, Valentino R.; Liu, Bin

2016-12-19

Atomically disordered oxides have attracted significant attention in recent years due to the possibility of enhanced ionic conductivity. However, the correlation between atomic disorder, corresponding electronic structure, and the resulting oxygen diffusivity is not well understood. The disordered variants of the ordered pyrochlore structure in gadolinium titanate (Gd 2Ti 2O 7) are seen as a particularly interesting prospect due to intrinsic presence of a vacant oxygen site in the unit atomic structure, which could provide a channel for fast oxygen conduction. In this paper, we provide insights into the subangstrom scale on the disordering-induced variations in the local atomic environmentmore » and its effect on the electronic structure in high-energy ion irradiation-induced disordered nanochannels, which can be utilized as pathways for fast oxygen ion transport. With the help of an atomic plane-by-plane-resolved analyses, the work shows how the presence of various types of TiO x polyhedral that exist in the amorphous and disordered crystalline phase modify the electronic structures relative to the ordered pyrochlore phase in Gd 2Ti 2O 7. Finally, the correlated molecular dynamics simulations on the disordered structures show a remarkable enhancement in oxygen diffusivity as compared with ordered pyrochlore lattice and make that a suitable candidate for applications requiring fast oxygen conduction.« less

Computational and Experimental Studies of the Radiation Response of Gd2Ti2O7 Pyrochlore

DOE Office of Scientific and Technical Information (OSTI.GOV)

Devanathan, Ram; Weber, William J.

2005-12-16

The structure and property changes in Gd2Ti2O7 (polycrystalline pyrochlore) were examined following irradiation with 1 MeV Kr+, 0.6 MeV Bi+ and 4 MeV Au2+ ions over the temperature range 30-950 K. Gd2Ti2O7 readily amorphizes with a low temperature (30 K) critical dose for amorphization of {approx} 0.15 displacements per atom (dpa). The critical temperature above which amorphization does not occur is about 1190 K. Nano-indentation studies reveal that the structural changes were accompanied by decreases of 15% in the Young's modulus. 1 MeV Kr+ irradiation of amorphous Gd2Ti2O7 at 1065 K resulted in ion-beam-assisted recrystallization. These experimental studies were complementedmore » with molecular dynamics simulations of low energy recoils in Gd2Ti2O7 and Gd2Zr2O7 using a Buckingham type potential. The displacement threshold energy surface in both pyrochlores is highly anisotropic. Displacement energies are higher for all sublattices in the titanate pyrochlore compared to the zirconate. Ti sublattice displacements require energies in excess of 100 eV, and result in multiple displacements and defect clusters. The formation of these clusters might impede dynamic defect recovery and facilitate amorphization.« less

Growth, Faraday and inverse Faraday characteristics of Tb₂Ti₂O₇ crystal.

PubMed

Guo, Feiyun; Sun, Yilin; Yang, Xiongsheng; Chen, Xin; Zhao, Bin; Zhuang, Naifeng; Chen, Jianzhong

2016-03-21

Tb₂Ti₂O₇ (TTO) single crystal with dimensions of 20 × 20 × 16 mm³ was grown by the Czochralski method. Rietveld structure refinement of X-ray diffraction (XRD) data confirms that the compound crystallizes in the cubic system with pyrochlore structure. Transmission spectra, Magnetic circular dichroism (MCD) spectra, Faraday and inverse Faraday characteristics of TTO crystal have been measured and analyzed in detail. The results demonstrate that TTO crystal has high transmittance at 700-1400 nm waveband and a larger Verdat constant than that of TGG reported. Magnetic circular dichroism (MCD) spectra showed that the 4f→4f transitions of Tb³⁺ have significant contributions to the magneto-optical activity (MOA). In the time-resolved pump-probe spectroscopy, the rotation signals of the probe beam based on the inverse Faraday effect in magneto-optical crystal were observed at zero time delay, the full width at half maximum of the rotation and ellipticity signals can be as fast as ~500 fs, which indicates that TTO crystal can be a promising material for ultrafast all-optical magnetic switching.

Adiabatic cooling processes in frustrated magnetic systems with pyrochlore structure

NASA Astrophysics Data System (ADS)

Jurčišinová, E.; Jurčišin, M.

2017-11-01

We investigate in detail the process of adiabatic cooling in the framework of the exactly solvable antiferromagnetic spin-1/2 Ising model in the presence of the external magnetic field on an approximate lattice with pyrochlore structure. The behavior of the entropy of the model is studied and exact values of the residual entropies of all ground states are found. The temperature variation of the system under adiabatic (de)magnetization is investigated and the central role of the macroscopically degenerated ground states in cooling processes is explicitly demonstrated. It is shown that the model parameter space of the studied geometrically frustrated system is divided into five disjunct regions with qualitatively different processes of the adiabatic cooling. The effectiveness of the adiabatic (de)magnetization cooling in the studied model is compared to the corresponding processes in paramagnetic salts. It is shown that the processes of the adiabatic cooling in the antiferromagnetic frustrated systems are much more effective especially in nonzero external magnetic fields. It means that the frustrated magnetic materials with pyrochlore structure can be considered as very promising refrigerants mainly in the situations with nonzero final values of the magnetic field.

Adiabatic cooling processes in frustrated magnetic systems with pyrochlore structure.

PubMed

Jurčišinová, E; Jurčišin, M

2017-11-01

We investigate in detail the process of adiabatic cooling in the framework of the exactly solvable antiferromagnetic spin-1/2 Ising model in the presence of the external magnetic field on an approximate lattice with pyrochlore structure. The behavior of the entropy of the model is studied and exact values of the residual entropies of all ground states are found. The temperature variation of the system under adiabatic (de)magnetization is investigated and the central role of the macroscopically degenerated ground states in cooling processes is explicitly demonstrated. It is shown that the model parameter space of the studied geometrically frustrated system is divided into five disjunct regions with qualitatively different processes of the adiabatic cooling. The effectiveness of the adiabatic (de)magnetization cooling in the studied model is compared to the corresponding processes in paramagnetic salts. It is shown that the processes of the adiabatic cooling in the antiferromagnetic frustrated systems are much more effective especially in nonzero external magnetic fields. It means that the frustrated magnetic materials with pyrochlore structure can be considered as very promising refrigerants mainly in the situations with nonzero final values of the magnetic field.

Steam Reforming of CH4 Using Ni- Substituted Pyrochlore Catalysts

NASA Astrophysics Data System (ADS)

Haynes, Daniel J.

The steam reforming of methane (SMR) continues to remain an important industrial reaction for large-scale production of H2 as well as synthesis gas mixtures which can be used for the production of useful chemicals (e.g. methanol). Although SMR is a rather mature technology, traditional nickel based catalysts used industrially are subjected to severe temperatures and reaction conditions, which lead to irreversible activity loss through sintering, support collapse, and carbon formation. Pyrochlore-based mixed oxide have been identified as refractory materials that can be modified through the substitution of catalytic metals and other promoting species into the structure to mitigate these issues causing deactivation. For this study, a lanthanum zirconate pyrochlore catalyst was substituted with Ni to determine whether the oxide structure could effectively stabilize the activity of the catalytic metal during the SMR. The effect of different variables including calcination temperature, a comparison of a substituted versus supported Ni pyrochlore catalyst, Ni weight loading, and Sr promotion have been evaluated to determine the location of the Ni in the structure, and their effect on catalytic behavior. It was revealed that the effect of calcination temperature on a 6wt% Ni substituted pyrochlore produced by the Pechini method demonstrated very little Ni was soluble in the pyrochlore lattice. It was further revealed that by XRD, TEM, and atom probe tomography that, despite the metal loading, Ni exsolves from the structure upon crystallization of the pyrochlore at 700°C, and forms NiO at the surface and grain boundaries. An additional separate La2ZrNiO6 perovskite phase also began to form at higher temperatures (>800°C). Increasing calcination temperature was found to lead to slight sintering of the NiO at the surface, which made the NiO more reducible. Meanwhile decreasing the Ni weight loading was found to produce a lower reduction temperature due to the presence of less lanthanum at the surface. Comparing the XANES and EXAFS profiles for the supported and substituted Ni catalysts showed no detectable difference between the spectra, further confirming the Ni to be present as NiO after substitution. Activity testing found the NiO on the surface to be the active catalytic form. However, the Ni proved to be highly sensitive to sintering and oxidation by steam, especially under high pressures (1.8 MPa), which caused all catalysts to suffer irreversible activity loss. Low reaction pressures (0.23 MPa) elicited an activation effect in which the Ni initially lost activity, but then recovered to near equilibrium yields after nearly 10 hours time on stream. The activation was correlated with an increase in Ni particle size, as observed by XRD. It is speculated from similar behavior reported in the literature that ability to recovery activity was linked to a redox cycling of the Ni particles under reaction conditions, in which a small amount of the Ni was redistributed over the surface through the volatilization with steam, as well as the exsolution of the Sr from the structure which helped stabilize the Ni. A comparison of the supported and substituted forms of Ni showed the substituted form to have a weaker interaction with the pyrochlore surface, which is the likely cause for the rapid sintering and deactivation by steam. It was also found that the substitution of more Sr into the structure produced only strong basic sites, which resulted in a lower degree of activity loss and more stable activity under high pressure conditions (1.8 MPa) compared to the catalyst with less Sr. Carbon formation was not an issue for these materials, due to the rapid sintering and oxidation of the metal in-situ. Fitting the Ni particle size growth with time showed a parabolic relationship with growth proportional to time by the power 0.28. This value is close to results observed in the literature which indicates the Ni agglomerates via atomic migration.

High-pressure Raman study on the superconducting pyrochlore oxide Cd2Re2O7

NASA Astrophysics Data System (ADS)

Matsubayashi, Yasuhito; Hasegawa, Takumi; Ogita, Norio; Yamaura, Jun-ichi; Hiroi, Zenji

2018-05-01

The superconducting pyrochlore oxide Cd2Re2O7 (Tc = 1 K), which is now considered as a candidate of the spin-orbit-coupled metal, shows an inversion-symmetry-breaking structural transition at Ts1 = 200 K . Ts1 decreases with increasing pressure and disappears at around Pc = 4.2 GPa , where at least four high-pressure phases with tiny structural distortions are suggested by means of powder X-ray diffraction [Yamaura PRB 2017]. We have carried out Raman scattering experiments to investigate changes in the crystal symmetry under high pressures up to 4.8 GPa. A structural transition at 1.9-3.0 GPa and the recovery of inversion symmetry above Pc are observed at 12 K.

Magnetic and Structural Properties of A-Site Ordered Chromium Spinel Sulfides: Alternating Antiferromagnetic and Ferromagnetic Interactions in the Breathing Pyrochlore Lattice

NASA Astrophysics Data System (ADS)

Okamoto, Yoshihiko; Mori, Masaki; Katayama, Naoyuki; Miyake, Atsushi; Tokunaga, Masashi; Matsuo, Akira; Kindo, Koichi; Takenaka, Koshi

2018-03-01

We report a comprehensive study on the magnetic and structural properties of the spinel sulfides LiInCr4S8, LiGaCr4S8, and CuInCr4S8, where Li+/Cu+ and Ga3+/In3+ ions form a zinc-blende-type order. On the basis of synchrotron X-ray diffraction and magnetization data obtained using polycrystalline samples, these three sulfides are suggested to be breathing pyrochlore magnets with alternating antiferromagnetic and ferromagnetic interactions on the small and large tetrahedra, respectively. The measured magnetization processes of the three sulfides up to 72 T are significantly different. The magnetization curves of LiInCr4S8 and CuInCr4S8 have large hysteresis loops with different shapes, while there is no hysteresis in that of LiGaCr4S8. Geometrical frustration of the small tetrahedron is likely to give rise to a wide variety of ground states, indicating the rich physics in these antiferromagnetic-ferromagnetic breathing pyrochlore magnets.

The low temperature synthesis, characterization and properties of ferroelectrics

NASA Astrophysics Data System (ADS)

Xu, Jie

2000-10-01

PZT 50:50 xerogels prepared by two different sol-gel routes crystallized in a similar fashion to give a mixture of tetragonal and rhombohedral at high temperature (1000°C). Both the diffraction and EXAFS data suggest that the compositional inhomogeneity of the samples prepared by the two routes is similar. The crystallization of CZT gels is complicated. Crystalline CaCO 3 was always detected in the dry gels regardless of the sample composition and preparation methods. At intermediate temperatures a fluorite related phase was always formed and it transformed to perovskite at higher temperatures. The EXAFS data suggest that perovskite CZT samples prepared using alkoxide sol-gel chemistry may not be random solid solutions. All the solution processed ZrTiO4 materials crystallized in the range 600--700°C. The KTN samples prepared using a conventional alkoxide sol-gel route crystallized completely to perovskite at lower temperatures than those prepared using prehydrolyzed precursors. The EXAFS data for the KTN samples prepared using a conventional alkoxide sol-gel route are consistent with a random distribution of tantalum and niobium in the solid solution. However, materials prepared using the inhomogeneous sol-gel route and by the direct reaction of mixed oxides were shown to be compositionally inhomogeneous. The heterogeneity could not be removed by regrinding and heating the mixed oxide samples several times. K2Ta4-xNbxO11 (x = 0, 2, 4) samples were prepared using alkoxide sol-gel chemistry and their crystallization was examined by powder X-ray diffraction. A Rietveld structure analysis of the pyrochlore formed from a gel with bulk composition K2Ta 2Nb2O11 indicated that it was rich in potassium relative to the bulk sample. On heating to high temperatures tetragonal tungsten bronzes were formed. A Rietveld analysis was also performed for K2Ta 2Nb2O11 with tetragonal tungsten bronze structure. The defect pyrochlores "AgTaO3" and GaTaO 3 were synthesized by ion-exchange using pyrochlore KTaO3 as a starting material. The structures of the pyrochlores were examined using the Rietveld method. The pyrochlore-to-perovskite transformations were also explored.

Non-Fermi-liquid and topological states with strong spin-orbit coupling.

PubMed

Moon, Eun-Gook; Xu, Cenke; Kim, Yong Baek; Balents, Leon

2013-11-15

We argue that a class of strongly spin-orbit-coupled materials, including some pyrochlore iridates and the inverted band gap semiconductor HgTe, may be described by a minimal model consisting of the Luttinger Hamiltonian supplemented by Coulomb interactions, a problem studied by Abrikosov and collaborators. It contains twofold degenerate conduction and valence bands touching quadratically at the zone center. Using modern renormalization group methods, we update and extend Abrikosov's classic work and show that interactions induce a quantum critical non-Fermi-liquid phase, stable provided time-reversal and cubic symmetries are maintained. We determine the universal power-law exponents describing various observables in this Luttinger-Abrikosov-Beneslavskii state, which include conductivity, specific heat, nonlinear susceptibility, and the magnetic Gruneisen number. Furthermore, we determine the phase diagram in the presence of cubic and/or time-reversal symmetry breaking perturbations, which includes a topological insulator and Weyl semimetal phases. Many of these phases possess an extraordinarily large anomalous Hall effect, with the Hall conductivity scaling sublinearly with magnetization σ(xy)∼M0.51.

Effect of CMAS composition on hot corrosion behavior of gadolinium zirconate thermal barrier coating materials

DOE PAGES

Deng, Wenzhuo; Fergus, Jeffrey W.

2017-07-06

The resistance of synthesized pyrochlore-type Gd 2Zr 2O 7 bulk specimens to four calcium-magnesium aluminosilicate (CMAS) compositions at different temperatures was investigated. The reaction products were identified by x-ray diffraction and penetration depths were examined using scanning electron microscopy. A dense reaction layer is comprised mainly of Ca 2Gd 8(SiO 4) 6O 2 and a cubic fluorite phase formed during the CMAS attack, and some unreacted CMAS was found in a transition layer below the reaction layer. The overall infiltration depth changed slightly with temperature, however, the thickness of the reaction layer and the morphology of the transition layer variedmore » distinctly with temperature. The sintered sample underwent the most severe degradation by the CaO-lean CMAS, whereas the effect of CaSO 4 and CaCO 3 was not significant. Furthermore, the Gd content of the ZrO 2-based cubic fluorite phase depends on the temperature and the molar ratio of Ca:Si in the CMAS.« less

Weyl magnons in breathing pyrochlore antiferromagnets

PubMed Central

Li, Fei-Ye; Li, Yao-Dong; Kim, Yong Baek; Balents, Leon; Yu, Yue; Chen, Gang

2016-01-01

Frustrated quantum magnets not only provide exotic ground states and unusual magnetic structures, but also support unconventional excitations in many cases. Using a physically relevant spin model for a breathing pyrochlore lattice, we discuss the presence of topological linear band crossings of magnons in antiferromagnets. These are the analogues of Weyl fermions in electronic systems, which we dub Weyl magnons. The bulk Weyl magnon implies the presence of chiral magnon surface states forming arcs at finite energy. We argue that such antiferromagnets present a unique example, in which Weyl points can be manipulated in situ in the laboratory by applied fields. We discuss their appearance specifically in the breathing pyrochlore lattice, and give some general discussion of conditions to find Weyl magnons, and how they may be probed experimentally. Our work may inspire a re-examination of the magnetic excitations in many magnetically ordered systems. PMID:27650053

Weyl magnons in breathing pyrochlore antiferromagnets

DOE PAGES

Li, Fei-Ye; Li, Yao-Dong; Kim, Yong Baek; ...

2016-09-21

Frustrated quantum magnets not only provide exotic ground states and unusual magnetic structures, but also support unconventional excitations in many cases. Using a physically relevant spin model for a breathing pyrochlore lattice, we discuss the presence of topological linear band crossings of magnons in antiferromagnets. These are the analogues of Weyl fermions in electronic systems, which we dub Weyl magnons. The bulk Weyl magnon implies the presence of chiral magnon surface states forming arcs at finite energy. We argue that such antiferromagnets present a unique example, in which Weyl points can be manipulated in situ in the laboratory by appliedmore » fields. We discuss their appearance specifically in the breathing pyrochlore lattice, and give some general discussion of conditions to find Weyl magnons, and how they may be probed experimentally. Our work may inspire a re-examination of the magnetic excitations in many magnetically ordered systems.« less

Weyl magnons in pyrochlore antiferromagnets with an all-in-all-out order

NASA Astrophysics Data System (ADS)

Jian, Shao-Kai; Nie, Wenxing

2018-03-01

We investigate topological magnon band crossings of pyrochlore antiferromagnets with all-in-all-out (AIAO) magnetic order. By general symmetry analysis and spin-wave theory, we show that pyrochlore materials with AIAO orders can host Weyl magnons under external magnetic fields or uniaxial strains. Under a small magnetic field, the magnon bands of the pyrochlore with AIAO background can feature two opposite-charged Weyl points, which is the minimal number of Weyl points realizable in quantum materials, and has not been experimentally observed so far. We further show that breathing pyrochlores with AIAO orders can exhibit Weyl magnons upon uniaxial strains. These findings apply to any pyrochlore material supporting AIAO orders, irrespective of the forms of interactions. Specifically, we show that the Weyl magnons are robust against direct (positive) Dzyaloshinskii-Moriya interactions. Because of the ubiquitous AIAO orders in pyrochlore magnets including R2Ir2O7 , and experimentally achievable external strain and magnetic field, our predictions provide a promising arena to witness the Weyl magnons in quantum magnets.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Kuo; Hu, Yufei; Wang, Yingxia, E-mail: Wangyx@pku.edu.cn

Isostructural compounds RE{sub 3}Sb{sub 3}Co{sub 2}O{sub 14} (RE=La, Pr, Nd, Sm–Ho) with an ordered pyrochlore structure were synthesized. The structure of La{sub 3}Sb{sub 3}Co{sub 2}O{sub 14} was solved ab initio based on powder XRD data, and refined by combining with high resolution neutron diffraction data. La{sub 3}Sb{sub 3}Co{sub 2}O{sub 14} crystallizes in the space group R-3m with the unit cell parameters a=7.52954(2) Å and c=17.59983(6) Å. The structures of other members in this family are confirmed by Rietveld refinement using powder X-ray diffraction data. The cations (RE, Sb and Co) in RE{sub 3}Sb{sub 3}Co{sub 2}O{sub 14} are orderly distributed, presentingmore » as [RE{sub 3}Co][Sb{sub 3}Co]O{sub 14} formula, and giving rise to two distinctive Kagome lattices constructed by RE{sup 3+} and Sb{sup 5+}, respectively. Co{sup 2+} occupies 8-coordinated and 6-coordinated environments, showing low spin (S=1/2) and high spin (S=3/2) states respectively. The magnetic susceptibility and UV–visual spectroscopy supports the magnetic observation. TDDFT calculation was performed to interpret the electronic states. The compounds [RE{sub 3}Co][Sb{sub 3}Co]O{sub 14} provide a profound example in which the ideal 2D Kagome lattice is derived from the 3D pyrochlore-type structure by an ordered distribution of the metal cations. - Graphical abstract: La{sub 3}Sb{sub 3}Co{sub 2}O{sub 14} crystallizes in a pyrochlore related structure with an ordered distribution of cations, giving rise to two sets of ideal 2D Kagome lattices formed by La{sup 3+} or Sb{sup 5+} respectively. This rhombohedral pyrochlore is a tolerant structure for stable compounds composed by many light rare-earth and d-transition elements. Substituting Zn{sup 2+} or Mg{sup 2+} for Co{sup 2+} will provide a series of compounds useful for studying magnetic interactions in the rare-earth Kagome lattices. - Highlights: • Pyrochlore-type La{sub 3}Sb{sub 3}Co{sub 2}O{sub 14} shows an ordered distribution of cations in its structure. • It presents two sets of ideal 2D Kagome lattices formed by La{sup 3+} or Sb{sup 5+}, respectively. • A family of isostructural compounds RE{sub 3}Sb{sub 3}Co{sub 2}O{sub 14} (RE=Pr, Nd, Sm–Ho) were realized. • Co{sup 2+} presents two spin states with S=1/2, 3/2 due to two coordinated environments.« less

Ceramic Single Phase High-Level Nuclear Waste Forms: Hollandite, Perovskite, and Pyrochlore

NASA Astrophysics Data System (ADS)

Vetter, M.; Wang, J.

2017-12-01

The lack of viable options for the safe, reliable, and long-term storage of nuclear waste is one of the primary roadblocks of nuclear energy's sustainable future. The method being researched is the incorporation and immobilization of harmful radionuclides (Cs, Sr, Actinides, and Lanthanides) into the structure of glasses and ceramics. Borosilicate glasses are the main waste form that is accepted and used by today's nuclear industry, but they aren't the most efficient in terms of waste loading, and durability is still not fully understood. Synroc-phase ceramics (i.e. hollandite, perovskite, pyrochlore, zirconolite) have many attractive qualities that glass waste forms do not: high waste loading, moderate thermal expansion and conductivity, high chemical durability, and high radiation stability. The only downside to ceramics is that they are more complex to process than glass. New compositions can be discovered by using an Artificial Neural Network (ANN) to have more options to optimize the composition, loading for performance by analyzing the non-linear relationships between ionic radii, electronegativity, channel size, and a mineral's ability to incorporate radionuclides into its structure. Cesium can be incorporated into hollandite's A-site, while pyrochlore and perovskite can incorporate actinides and lanthanides into their A-site. The ANN is used to predict new compositions based on hollandite's channel size, as well as the A-O bond distances of pyrochlore and perovskite, and determine which ions can be incorporated. These new compositions will provide more options for more experiments to potentially improve chemical and thermodynamic properties, as well as increased waste loading capabilities.

Zircon-pyrochlore ores of Proterozoic Gremyakha-Vyrmes polyphase massif, Kola Peninsula: source and evolution

NASA Astrophysics Data System (ADS)

Sorokhtina, Natalia; Belyatsky, Boris; Antonov, Anton; Kononkova, Natalia; Lepekhina, Elena; Kogarko, Lia

2017-04-01

The alkaline-ultrabasic Gremyakha-Vyrmes massif occurs within the Central Kola terrane in the northern part of the Fennoscandian Shield and consists of diverse rock complexes: basic-ultrabasic rocks, foidolites, alkaline metasomatic rocks and carbonatites, alkaline granites and granosyenites. Nb-Zr ore deposit is confined to alkaline metasomatic rocks developed over foidolites. The metasomatites are represented by albitites and aegirinites occur as submeridionally orientated zones extending up to 6-8 km and several hundred meters thickness. They are mainly composed of albite and aegirine, but amphibole, annite, microcline, fluorapatite, titanite, ilmenite, pyrochlore group minerals, zircon are present [Sorokhtina et al., 2016]. Carbonatites are developed sporadically and accessory zircon but not the pyrochlore is observed only in contact zones with albitites and aegerinites. In metasomatites, zircon and pyrochlore are main rare metal minerals, which are formed at the latest stages of crystallization. Ca-dominant fluorcalcio- and hydroxycalciopyrochlores are the most abundant, whereas U-dominant pyrochlore, oxyuranobetafite, zero-valent-dominant (Ba, Sr-dominant) pyrochlore, hydro- or kenopyrochlore are rare. The pyrochlore-group minerals form heterogeneous metacrystals containing inclusions of host rock minerals, calcite, ilmenite, zircon, sulfides, and graphite. While pyrochlore is replaced by Si-rich "pyrochlore" (SiO2 is up to 18 wt.%.), cation-deficient hydrated pyrochlore, Fe-Si-Nb, U-Si-Nb, and Al-Si-Nb phases along fracture zones and margins. The early generation zircon is represented by large heterogeneous metacrystals filled with inclusions of various host rock minerals, calcite, ilmenite, thorite, thorianite and sulfides, while the late zircons are empty of inclusions. Zircons are nearly stoichiometric in composition; but intermediate zones are enriched in Pb, Y and Th, and overgrowths are enriched Hf only. According to CL and ion-microprobe analysis zircon has polygenetic nature: some relics inherited from foidolite crystallized at about 800°C, whereas the newly formed - at 600°C [Watson et al., 2006]. The time interval of the magmatic massif formation may be estimated as long as 80-100 Ma only. The basic-ultrabasic rocks and foidolites were intruded consistently at 1982 ± 6 Ma and 1894±12 according to SHRIMP-II U-Pb zircon dating, but the whole-rock Sm-Nd isotope dating has resulted in 1879±99 Ma and reflects the impact of alkaline granite intrusion (1871±9 Ma). The late differentiates from alkaline magma crystallization were the main source of rare metals for zircon-pyrochlore ores of alkaline metasomatites. The metasomatic rocks (aegirinites, albitites) and carbonatites were formed as late as 1910 ± 15 Ma (SHRIMP-II U-Pb zircon, titanite, pyrochlore). While some pyrochlore grains from metasomatites are showed that U-Pb age of ore formation is 1766 ± 24 and 1764 ± 19 respectively. That can be attributed to additional source of rare metals connected with fluids formed during regional metamorphism 1750 m.y. ago [Glebovitskii et al., 2014]. The last probable source of rare-metal material and ore-deposit evolution stage (recrystallization) is established by individual pyrochlore grain Sm-Nd and U-Pb systems and evidences tectono-thermal activity at the Paleozoic plume magmatism, which was followed by structural and chemical mineral changes. The research was done within the framework of the scientific program of Russian Academy of Sciences and state contract K41.2014.014 with Sevzapnedra. References: Watson E. B., Wark D. A., Thomas J. B. Crystallization thermometers for zircon and rutile // Contrib. Mineral. Petrol. 2006. 151, 413-433. Glebovitskii V.A., Bushmin S.A., Belyatsky B.V., Bogomolov E.S., Borozdin A.P., Savva E.V., Lebedeva Y.M. Rb-Sr age of metasomatism and ore formation in the low-temperature shear zones of the Fenno-Karelian craton, Baltic Shield // Petrology. 2014. 22(2). 184-204. Sorokhtina N.V., Kogarko L.N., Shpachenko A.K., Senin V. G. Composition and Conditions of Crystallization of zircon from the rare-metal ores of the Gremyakha-Vyrmes massif, Kola Peninsula // Geochemistry International. 2016. 54 (12). 1035-1048.

Using machine learning to identify factors that govern amorphization of irradiated pyrochlores

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pilania, Ghanshyam; Whittle, Karl R.; Jiang, Chao

Structure–property relationships are a key materials science concept that enables the design of new materials. In the case of materials for application in radiation environments, correlating radiation tolerance with fundamental structural features of a material enables materials discovery. Here, we use a machine learning model to examine the factors that govern amorphization resistance in the complex oxide pyrochlore (A 2B 2O 7) in a regime in which amorphization occurs as a consequence of defect accumulation. We examine the fidelity of predictions based on cation radii and electronegativities, the oxygen positional parameter, and the energetics of disordering and amorphizing the material.more » No one factor alone adequately predicts amorphization resistance. We find that when multiple families of pyrochlores (with different B cations) are considered, radii and electronegativities provide the best prediction, but when the machine learning model is restricted to only the B = Ti pyrochlores, the energetics of disordering and amorphization are critical factors. We discuss how these static quantities provide insight into an inherently kinetic property such as amorphization resistance at finite temperature. Lastly, this work provides new insight into the factors that govern the amorphization susceptibility and highlights the ability of machine learning approaches to generate that insight.« less

Using machine learning to identify factors that govern amorphization of irradiated pyrochlores

DOE PAGES

Pilania, Ghanshyam; Whittle, Karl R.; Jiang, Chao; ...

2017-02-10

Structure–property relationships are a key materials science concept that enables the design of new materials. In the case of materials for application in radiation environments, correlating radiation tolerance with fundamental structural features of a material enables materials discovery. Here, we use a machine learning model to examine the factors that govern amorphization resistance in the complex oxide pyrochlore (A 2B 2O 7) in a regime in which amorphization occurs as a consequence of defect accumulation. We examine the fidelity of predictions based on cation radii and electronegativities, the oxygen positional parameter, and the energetics of disordering and amorphizing the material.more » No one factor alone adequately predicts amorphization resistance. We find that when multiple families of pyrochlores (with different B cations) are considered, radii and electronegativities provide the best prediction, but when the machine learning model is restricted to only the B = Ti pyrochlores, the energetics of disordering and amorphization are critical factors. We discuss how these static quantities provide insight into an inherently kinetic property such as amorphization resistance at finite temperature. Lastly, this work provides new insight into the factors that govern the amorphization susceptibility and highlights the ability of machine learning approaches to generate that insight.« less

Mn 4+ emission in pyrochlore oxides

DOE PAGES

Du, Mao-Hua

2014-08-27

For the existing Mn 4+ activated red phosphors have relatively low emission energies (or long emission wavelengths) and are therefore inefficient for general lighting. Density functional calculations are performed to study Mn 4+ emission in rare-earth hafnate, zirconate, and stannate pyrochlore oxides (RE 2Hf 2O 7, RE 2Zr 2O 7, and RE 2Sn 2O 7). We show how the different sizes of the RE 3+ cation in these pyrochlores affect the local structure of the distorted MnO 6 octahedron, the Mn–O hybridization, and the Mn 4+ emission energy. The Mn 4+ emission energies of many pyrochlores are found to bemore » higher than those currently known for Mn 4+ doped oxides and should be closer to that of Y 2O 3:Eu 3+ (the current commercial red phosphor for fluorescent lighting). The O–Mn–O bond angle distortion in a MnO6 octahedron is shown to play an important role in weakening Mn–O hybridization and consequently increasing the Mn 4+ emission energy. Our result shows that searching for materials that allow significant O–Mn–O bond angle distortion in a MnO 6 octahedron is an effective approach to find new Mn 4+ activated red phosphors with potential to replace the relatively expensive Y 2O 3:Eu 3+ phosphor.« less

Ab initio molecular dynamics simulations of ion-solid interactions in zirconate pyrochlores

DOE PAGES

Xiao, Haiyan Y.; Weber, William J.; Zhang, Yanwen; ...

2015-01-31

In this paper, an ab initio molecular dynamics method is employed to study low energy recoil events in zirconate pyrochlores (A 2Zr 2O 7, A = La, Nd and Sm). It shows that both cations and anions in Nd 2Zr 2O 7 and Sm 2Zr 2O 7 are generally more likely to be displaced than those in La 2Zr 2O 7. The damage end states mainly consist of Frenkel pair defects, and the Frenkel pair formation energies in Nd 2Zr 2O 7 and Sm 2Zr 2O 7 are lower than those in La 2Zr 2O 7. These results suggest thatmore » the order–disorder structural transition more easily occurs in Nd 2Zr 2O 7 and Sm 2Zr 2O 7 resulting in a defect-fluorite structure, which agrees well with experimental observations. Our calculations indicate that oxygen migration from 48f and 8b to 8a sites is dominant under low energy irradiation. A number of new defects, including four types of cation Frenkel pairs and six types of anion Frenkel pairs, are revealed by ab initio molecular dynamics simulations. The present findings may help to advance the fundamental understanding of the irradiation response behavior of zirconate pyrochlores.« less

Probing disorder in isometric pyrochlore and related complex oxides

NASA Astrophysics Data System (ADS)

Shamblin, Jacob; Feygenson, Mikhail; Neuefeind, Joerg; Tracy, Cameron L.; Zhang, Fuxiang; Finkeldei, Sarah; Bosbach, Dirk; Zhou, Haidong; Ewing, Rodney C.; Lang, Maik

2016-05-01

There has been an increased focus on understanding the energetics of structures with unconventional ordering (for example, correlated disorder that is heterogeneous across different length scales). In particular, compounds with the isometric pyrochlore structure, A2B2O7, can adopt a disordered, isometric fluorite-type structure, (A, B)4O7, under extreme conditions. Despite the importance of the disordering process there exists only a limited understanding of the role of local ordering on the energy landscape. We have used neutron total scattering to show that disordered fluorite (induced intrinsically by composition/stoichiometry or at far-from-equilibrium conditions produced by high-energy radiation) consists of a local orthorhombic structural unit that is repeated by a pseudo-translational symmetry, such that orthorhombic and isometric arrays coexist at different length scales. We also show that inversion in isometric spinel occurs by a similar process. This insight provides a new basis for understanding order-to-disorder transformations important for applications such as plutonium immobilization, fast ion conduction, and thermal barrier coatings.

Modifications of structural and physical properties induced by swift heavy ions in Gd2Ti2O7 and Y2Ti2O7 pyrochlores

NASA Astrophysics Data System (ADS)

Sellami, N.; Sattonnay, G.; Grygiel, C.; Monnet, I.; Debelle, A.; Legros, C.; Menut, D.; Miro, S.; Simon, P.; Bechade, J. L.; Thomé, L.

2015-12-01

The structural transformations induced by ionization processes in Gd2Ti2O7 and Y2Ti2O7 pyrochlores irradiated with swift heavy ions have been studied using XRD and Raman experiments. Results show that irradiation induces amorphization and that the phase transformation build-up can be accounted for in the framework of a model involving a single-impact mechanism. The radiation induced amorphization build-up is faster in Gd2Ti2O7 than in Y2Ti2O7. Moreover, a decrease of the thermal conductivity (measured by the laser flash method) is induced by irradiation both in Gd2Ti2O7 and Y2Ti2O7.

Structure and radiation effect of Er-stuffed pyrochlore Er2(Ti2-xErx)O7-x/2 (x = 0-0.667)

NASA Astrophysics Data System (ADS)

Yang, D. Y.; Xu, C. P.; Fu, E. G.; Wen, J.; Liu, C. G.; Zhang, K. Q.; Wang, Y. Q.; Li, Y. H.

2015-08-01

Er-stuffed pyrochlore series Er2(Ti2-xErx)O7-x/2 (x = 0, 0.162, 0.286, 0.424 and 0.667) were synthesized using conventional ceramic processing procedures. The structure of Er2(Ti2-xErx)O7-x/2 is effectively tailored by the Er stuffing level (x). In order to study the radiation effect of Er-stuffed pyrochlores, irradiation experiments were performed with 400 keV Ne2+ ions to fluences ranging from 5 × 1014 to 3.0 × 1015 ions/cm2 at cryogenic condition. Irradiation induced microstructural evolution was examined using a grazing incidence X-ray diffraction technique. It is found that the irradiated layer of Er2(Ti2-xErx)O7-x/2 undergoes significant lattice disordering and swelling at fluences of ⩽1.5 × 1015 ions/cm2 and amorphization at fluences of ⩾1.5 × 1015 ions/cm2. The radiation effect depends strongly on the chemical compositions of the samples. Both the lattice swelling percentage and the amorphous fraction decrease with increasing x. The experimental results are discussed in the context of cation antisite defect. The defect formation energy which varies as a function of x is responsible for the difference in the structural behaviors of Er2(Ti2-xErx)O7-x/2 under 400 keV Ne2+ ion irradiation.

The Pressure-Induced Structural Response of A2Hf2O7 (A=Y, Sm, Eu, Gd, Dy, Yb) Compounds from 0.1-50 GPa

NASA Astrophysics Data System (ADS)

Turner, K. M.; Rittman, D.; Heymach, R.; Turner, M.; Tracy, C.; Mao, W. L.; Ewing, R. C.

2016-12-01

A2B2O7 (A, B= cations) compounds have structures that make their properties conducive to many applications; for example they are a proposed waste-form for actinides generated in the nuclear fuel cycle. This interest in part is due to their structural responses to extreme environments of high P, T, or under intense irradiation. Depending on their cationic radius ratio, ra/rb, A2B2O7 compounds either crystallize as pyrochlore (ra/rb=1.46-1.7) or "defect fluorite" (ra/rb>1.46). The structure types are similar: they are derivatives of ideal fluorite with two cations and 1/8 missing anions. In pyrochlore, the cations and anion vacancy are ordered. In "defect fluorite"-structured oxides, the cations and anion vacancies are random. A2B2O7 compounds rarely amorphize in extreme environments. Rather, they disorder and undergo phase transitions; this resistance to amorphization contributes to the durability of this potential actinide waste-form. Under high-pressure, A2B2O7 compounds are known to disorder or form a cottunite-like phase. Their radius ratio affects their response to extreme environments; "defect fluorite" type compounds tend to disorder, and pyrochlore type compounds tend to form the cottunite-like phase. We have examined six A2Hf2O7 compounds (A=Y, Sm, Eu, Gd, Dy, Yb) in situ to 50 GPa. By keeping the B-site constant (Hf), we examined the effect of a changing radius ratio on the pressure-induced structural response of hafnates. We used symmetric DACs, ruby fluorescence, stainless steel gaskets, and methanol: ethanol (4:1 by volume) pressure medium. We characterized these materials with in situ Raman spectroscopy at Stanford University, and synchrotron X-Ray Diffraction (XRD) at APS 16 BM-D and ALS 12.2.2. The compounds were pyrochlore structured (Sm, Eu, Gd) and "defect-fluorite" structured (Y, Dy, Yb) hafnates . These compounds undergo a slow phase transition to a high-pressure cotunnite-like phase between 18-30 GPa. They undergo disordering of their cation and anionic sites as pressure is increased. The pressure of their phase transitions correlates directly with their radius ratio. Our results are comparable to many high-pressure studies of rare earth zirconates and titanates, but contrast from previous experiments performed on rare earth hafnates, specifically La2Hf2O7.

Disappearance of the metal-insulator transition in iridate pyrochlores on approaching the ideal R2Ir2O7 stoichiometry

NASA Astrophysics Data System (ADS)

Sleight, Arthur W.; Ramirez, Arthur P.

2018-07-01

Recently, rare earth iridates, R2Ir2O7, with the pyrochlore structure have been intensively investigated due to their promise as either topological Mott insulators or Weyl semimetals. Single crystals of such pyrochlores with R = Nd, Sm, Eu, and Dy were prepared hydrothermally in sealed gold tubes at 975 K and show significantly higher electrical resistivities than previously reported for either crystals or polycrystalline samples. Furthermore, none of the present crystals exhibit the metal-insulator transition found for some samples of these phases. Lower resistivities are ascribed to lack of control of x and y in R2-xIr2O7-y in other more commonly used synthesis methods, yielding uncertainty in the Ir oxidation state. We also report resistivity of R2Ru2O7 crystals for R = Yb, Gd, Eu, and Nd, prepared in the same manner. These results suggest that the observed charge transport in hydrothermally grown iridate crystals is that of essentially stoichiometric phases and is consistent a with the existence of Weyl nodes.

Epitaxial thin films of pyrochlore iridate Bi 2+xIr 2-yO 7-δ: structure, defects and transport properties

DOE PAGES

Yang, W. C.; Xie, Y. T.; Zhu, W. K.; ...

2017-08-10

While pyrochlore iridate thin films are theoretically predicted to possess a variety of emergent topological properties, experimental verification of these predictions can be obstructed by the challenge in thin film growth. We report on the pulsed laser deposition and characterization of thin films of a representative pyrochlore compound Bi 2Ir 2O 7. Moreover, the films were epitaxially grown on yttria-stabilized zirconia substrates and have lattice constants that are a few percent larger than that of the bulk single crystals. The film composition shows a strong dependence on the oxygen partial pressure. Density-functional-theory calculations indicate the existence of BiIr antisite defects,more » qualitatively consistent with the high Bi: Ir ratio found in the films. Both Ir and Bi have oxidation states that are lower than their nominal values, suggesting the existence of oxygen deficiency. The iridate thin films show a variety of intriguing transport characteristics, including multiple charge carriers, logarithmic dependence of resistance on temperature, antilocalization corrections to conductance due to spin-orbit interactions, and linear positive magnetoresistance.« less

Epitaxial thin films of pyrochlore iridate Bi 2+xIr 2-yO 7-δ: structure, defects and transport properties

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang, W. C.; Xie, Y. T.; Zhu, W. K.

While pyrochlore iridate thin films are theoretically predicted to possess a variety of emergent topological properties, experimental verification of these predictions can be obstructed by the challenge in thin film growth. We report on the pulsed laser deposition and characterization of thin films of a representative pyrochlore compound Bi 2Ir 2O 7. Moreover, the films were epitaxially grown on yttria-stabilized zirconia substrates and have lattice constants that are a few percent larger than that of the bulk single crystals. The film composition shows a strong dependence on the oxygen partial pressure. Density-functional-theory calculations indicate the existence of BiIr antisite defects,more » qualitatively consistent with the high Bi: Ir ratio found in the films. Both Ir and Bi have oxidation states that are lower than their nominal values, suggesting the existence of oxygen deficiency. The iridate thin films show a variety of intriguing transport characteristics, including multiple charge carriers, logarithmic dependence of resistance on temperature, antilocalization corrections to conductance due to spin-orbit interactions, and linear positive magnetoresistance.« less

μ SR study of NaCaNi2F7 in zero field and applied longitudinal magnetic field

NASA Astrophysics Data System (ADS)

Cai, Yipeng; Wilson, Murray; Hallas, Alannah; Liu, Lian; Frandsen, Benjamin; Dunsiger, Sarah; Krizan, Jason; Cava, Robert; Uemura, Yasutomo; Luke, Graeme

Rich physics of abundant magnetic ground states has been realized in the A2B2X7 geometrically frustrated magnetic pyrochlores. Recently, a new spin-1 Ni2+ pyrochlore, NaCaNi2F7, was synthesized and shown to have spin freezing at 3.6 K with a frustration index of f 36 and antiferromagnetic exchange interactions [1] . This structure has chemical disorder on the A site caused by randomly distributed Ca and Na ions, which causes bond disorder around the magnetic Ni sites. We present Zero Field (ZF) and Longitudinal Field (LF) muon spin rotation (μSR) measurements on this single crystal pyrochlore. Our data shows that the Ni2+ spins start freezing around 4 K giving a static local field of 140 G. The data show no oscillations down to 75 mK which indicates no long range magnetic order. They are well described by the dynamic Gaussian Kubo-Toyabe function with a non-zero hopping rate that is not easily decoupled with an applied longitudinal field, which implies persistent spin dynamics down to 75 mK.

Synthesis, characterization, and catalytic activity of Rh-based lanthanum zirconate pyrochlores for higher alcohol synthesis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Abdelsayed, Victor; Shekhawat, Dushyant; Poston, James A.

2013-05-01

Two lanthanum zirconate pyrochlores (La{sub 2}Zr{sub 2}O{sub 7}; LZ) were prepared by Pechini method and tested for higher alcohols selectivity. In one, Rh was substituted into the pyrochlore lattice (LRZ, 1.7 wt%) while for the second, Rh was supported on an unsubstituted La{sub 2}Zr{sub 2}O{sub 7} (R/LZ, 1.8 wt%). X-ray photoelectron spectroscopy (XPS) and temperature programmed reduction (TPR) results show that the surface reducibility depends on whether the Rh is in (or supported on) the LZ pyrochlore. Rhodium in the LRZ is more reducible than rhodium supported on the R/LZ pyrochlore, likely due to the presence of a perovskite phasemore » (LaRhO{sub 3}; identified by XRD), in which rhodium is more reducible. The formation of the perovskite accompanies that of the pyrochlore. CO hydrogenation results show higher ethanol selectivity for R/LZ than LRZ, possibly due to the strong interaction between Rh and LZ on the R/LZ, forming atomically close Rh{sup +}/Rh{sup 0} sites, which have been suggested to favor ethanol production.« less

Krypton ion irradiation-induced amorphization and nano-crystal formation in pyrochlore Lu2Ti2O7 at room temperature

NASA Astrophysics Data System (ADS)

Xie, Qiu-Rong; Zhang, Jian; Yin, Dong-Min; Guo, Qi-Xun; Li, Ning

2015-12-01

Polycrystalline pyrochlore Lu2Ti2O7 pellets are irradiated with 600-keV Kr3+ ions up to a fluence of 1.45 × 1016 Kr3+/cm2. Irradiation induced structural modifications are examined by using grazing incidence x-ray diffraction (GIXRD) and cross-sectional transmission electron microscopy (TEM). The GIXRD reveals that amorphous fraction increases with the increase of fluences up to 2 × 1015 Kr3+/cm2, and the results are explained with a direct-impact model. However, when the irradiation fluence is higher than 2 × 1015 Kr3+/cm2, the amorphous fraction reaches a saturation of ∼80%. Further TEM observations imply that nano-crystal is formed with a diameter of ∼10 nm within the irradiation layer at a fluence of 4 × 1015 Kr3+/cm2. No full amorphization is achieved even at the highest fluence of 1.45 × 1016 Kr3+/cm2 (∼36 displacement per atom). The high irradiation resistance of pyrochlore Lu2Ti2O7 at higher fluence is explained on the basis of enhanced radiation tolerance of nano-crystal structure. Project sponsored by the National Natural Science Foundation of China (Grant No. 11205128) and the Fundamental Research Funds for the Central Universities, China (Grant No. 2012121034).

A spin-liquid with pinch-line singularities on the pyrochlore lattice.

PubMed

Benton, Owen; Jaubert, L D C; Yan, Han; Shannon, Nic

2016-05-26

The mathematics of gauge theories lies behind many of the most profound advances in physics in the past 200 years, from Maxwell's theory of electromagnetism to Einstein's theory of general relativity. More recently it has become clear that gauge theories also emerge in condensed matter, a prime example being the spin-ice materials which host an emergent electromagnetic gauge field. In spin-ice, the underlying gauge structure is revealed by the presence of pinch-point singularities in neutron-scattering measurements. Here we report the discovery of a spin-liquid where the low-temperature physics is naturally described by the fluctuations of a tensor field with a continuous gauge freedom. This gauge structure underpins an unusual form of spin correlations, giving rise to pinch-line singularities: line-like analogues of the pinch points observed in spin-ice. Remarkably, these features may already have been observed in the pyrochlore material Tb2Ti2O7.

A spin-liquid with pinch-line singularities on the pyrochlore lattice

PubMed Central

Benton, Owen; Jaubert, L.D.C.; Yan, Han; Shannon, Nic

2016-01-01

The mathematics of gauge theories lies behind many of the most profound advances in physics in the past 200 years, from Maxwell's theory of electromagnetism to Einstein's theory of general relativity. More recently it has become clear that gauge theories also emerge in condensed matter, a prime example being the spin-ice materials which host an emergent electromagnetic gauge field. In spin-ice, the underlying gauge structure is revealed by the presence of pinch-point singularities in neutron-scattering measurements. Here we report the discovery of a spin-liquid where the low-temperature physics is naturally described by the fluctuations of a tensor field with a continuous gauge freedom. This gauge structure underpins an unusual form of spin correlations, giving rise to pinch-line singularities: line-like analogues of the pinch points observed in spin-ice. Remarkably, these features may already have been observed in the pyrochlore material Tb2Ti2O7. PMID:27225400

Charge dynamics in the colossal magnetoresistance pyrochlore Tl2Mn2O7

NASA Astrophysics Data System (ADS)

Okamura, H.; Koretsune, T.; Matsunami, M.; Kimura, S.; Nanba, T.; Imai, H.; Shimakawa, Y.; Kubo, Y.

2001-11-01

Optical conductivity data [σ(ω)] of the colossal magnetoresistance (CMR) pyrochlore Tl2Mn2O7 are presented as functions of temperature (T) and external magnetic field (B). Upon cooling and upon applying B near the Curie temperature, where the CMR manifests itself, σ(ω) shows a clear transition from an insulatorlike to a metallic electronic structure as evidenced by the emergence of a pronounced Drude-like component below ~0.2 eV. Analyses on the σ(ω) spectra show that both T- and B-induced evolutions of the electronic structure are very similar to each other, and that they are universally related to the development of macroscopic magnetization (M). In particular, the effective carrier density obtained from σ(ω) scales with M2 over wide ranges of T and B. The contributions to the CMR from the carrier effective mass and scattering time are also evaluated from the data.

Cation displacements and the structures of the superconducting pyrochlore osmates AOs2O6 ( A=K , Rb, and Cs)

NASA Astrophysics Data System (ADS)

Galati, Rosa; Simon, Charles; Henry, Paul F.; Weller, Mark T.

2008-03-01

Variable temperature, 2K

Structural transformations and disordering in zirconolite (CaZrTi2O7) at high pressure.

PubMed

Salamat, Ashkan; McMillan, Paul F; Firth, Steven; Woodhead, Katherine; Hector, Andrew L; Garbarino, Gaston; Stennett, Martin C; Hyatt, Neil C

2013-02-04

There is interest in identifying novel materials for use in radioactive waste applications and studying their behavior under high pressure conditions. The mineral zirconolite (CaZrTi(2)O(7)) exists naturally in trace amounts in diamond-bearing deep-seated metamorphic/igneous environments, and it is also identified as a potential ceramic phase for radionuclide sequestration. However, it has been shown to undergo radiation-induced metamictization resulting in amorphous forms. In this study we probed the high pressure structural properties of this pyrochlore-like structure to study its phase transformations and possible amorphization behavior. Combined synchrotron X-ray diffraction and Raman spectroscopy studies reveal a series of high pressure phase transformations. Starting from the ambient pressure monoclinic structure, an intermediate phase with P2(1)/m symmetry is produced above 15.6 GPa via a first order transformation resulting in a wide coexistence range. Upon compression to above 56 GPa a disordered metastable phase III with a cotunnite-related structure appears that is recoverable to ambient conditions. We examine the similarity between the zirconolite behavior and the structural evolution of analogous pyrochlore systems under pressure.

Metallic 'Ferroelectrity' in the Pyrochlore Cd 2Re 2O 7

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sergienko, I. A.; Keppens, Veerle; McGuire, M. A.

2004-01-01

A class of materials known as 'ferroelectric metals' was discussed theoretically by Anderson and Blount in 1965, but to date no examples of this class have been reported. Here we present measurements of the elastic moduli of Cd{sub 2}Re{sub 2}O{sub 7} through the 200 K cubic-to-tetragonal phase transition. A Landau analysis of the moduli reveals that the transition is consistent with Cd{sub 2}Re{sub 2}O{sub 7} being classified as a 'ferroelectric metal' in the weaker sense described by Anderson and Blount (loss of a center of symmetry). First-principles calculations of the lattice instabilities indicate that the dominant lattice instability corresponds tomore » a two-fold degenerate mode with Eu symmetry, and that motions of the O ions forming the O octahedra dominate the energetics of the transition.« less

Thermal annealing of natural, radiation-damaged pyrochlore

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zietlow, Peter; Beirau, Tobias; Mihailova, Boriana

Abstract Radiation damage in minerals is caused by the α-decay of incorporated radionuclides, such as U and Th and their decay products. The effect of thermal annealing (400–1000 K) on radiation-damaged pyrochlores has been investigated by Raman scattering, X-ray powder diffraction (XRD), and combined differential scanning calorimetry/thermogravimetry (DSC/TG). The analysis of three natural radiation-damaged pyrochlore samples from Miass/Russia [6.4 wt% Th, 23.1·10

Liandratite from Karkonosze pegmatites, Sudetes, Southwestern Poland

NASA Astrophysics Data System (ADS)

Matyszczak, Witold

2018-06-01

The chemical composition of liandratite, U6+(Nb,Ta)2O8, was determined from material collected in Niobium, Yttrium, Fluorine type (NYF) pegmatites of the Karkonosze intrusion (Sudetes, SW Poland). Liandratite occurs mainly as rims, up to 40 µm thick, and fracture infillings in fergusonite-(Y) and other Nb-Ta-Ti minerals. Its formation was related to the fluid-driven alteration of primary minerals by three potential mechanisms: (i) direct replacement of a primary mineral by liandratite; (ii) breakdown of the primary mineral to liandratite and a product with the composition of minerals of the pyrochlore group; (iii) multistage alteration, which involved: removal of A-site cations (mostly Y + REE with the exception of U4+) and formation of phases with the composition of pyrochlore group minerals; then crystallization of U-, Bi-, Pb-rich pyrochlores and their replacement by liandratite. The chemical compositions of liandratite formed by the breakdown of different primary minerals are also different, mainly in their U, Ti and Nb contents. Excess Ti, relative to the U6+Nb2O8 end-member, is incorporated into the structure together with additional U. The Ti content of liandratite, and partially through this the U content, are dependent on the nature of the precursor mineral.

Tuning Frustration in Rare Earth Pyrochlores by Platinum Substitution

NASA Astrophysics Data System (ADS)

Hallas, Alannah; Gaudet, Jonathan; Sharma, Arzoo; Wilson, Murray; Cai, Yipeng; Tachibana, Makoto; Wiebe, Chris; Gaulin, Bruce; Luke, Graeme

A successful mechanism for exploring the rich physics of rare earth pyrochlores, R2B2O7, is to substitute the non-magnetic B-site. Varying the ionic radius of the B-site induces an internal chemical pressure. Some rare earths are robust to substitutions; for example, the holmium-based pyrochlores all exhibit a dipolar spin ice state. In the case of other rare earths such as ytterbium, the ground states are remarkably fragile to chemical pressure. In this talk, I will introduce two materials with a new non-magnetic B-site: platinum. The ionic radius of platinum is comparable to that of titanium, which occupies the B-site in the most well-studied family of pyrochlores. Thus, platinum does not induce a strong chemical pressure on the lattice. Nevertheless, using Gd2Pt2O7 and Er2Pt2O7 as examples, I will show that platinum does affect a dramatic change on the magnetic properties. We trace this effect to platinum's empty eg orbitals, which mediate superexchange pathways not available in other rare earth pyrochlores. In Gd2Pt2O7, this results in a striking 160% enhancement of TN as compared to other Gd-based pyrochlores. In Er2Pt2O7, the ordering temperature is strongly suppressed and the ground state is altered.

Neutron spectroscopic study of crystal field excitations in Tb 2Ti 2O 7 and Tb 2Sn 2O 7

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, J.; Fritsch, Katharina; Hao, Z.

2014-04-01

We present time-of-flight inelastic neutron scattering measurements at low temperature on powder samples of the magnetic pyrochlore oxides Tb 2Ti 2O 7 and Tb 2Sn 2O 7. These two materials possess related, but different ground states, with Tb 2Sn 2O 7 displaying "soft" spin ice order below T N approx 0.87 K, while Tb 2Ti 2O 7 enters a hybrid, glassy-spin ice state below T g approx 0.2 K. Our neutron measurements, performed at T = 1.5 K and 30 K, probe the crystal field states associated with the J = 6 states of Tb 3+ within the appropriate Fd3-barmmore » pyrochlore environment. These crystal field states determine the size and anisotropy of the Tb 3+ magnetic moment in each material's ground state, information that is an essential starting point for any description of the low temperature phase behavior and spin dynamics in Tb 2Ti 2O 7 and Tb 2Sn 2O 7. While these two materials have much in common, the cubic stanate lattice is expanded compared to the cubic titanate lattice. As our measurements show, this translates into a factor of approx 2 increase in the crystal field bandwidth of the 2J +1 = 13 states in Tb 2Ti 2O 7 compared with Tb 2Sn 2O 7. Our results are consistent with previous measurements on crystal field states in Tb 2Sn 2O 7, wherein the ground state doublet corresponds primarily to mJ = {vert_bar}+-5> and the first excited state doublet to mJ = {vert_bar}+-4>. In contrast, our results on Tb 2Ti 2O 7 differ markedly from earlier studies, showing that the ground state doublet corresponds to a significant mixture of mJ = {vert_bar}+-5>, mJ = {vert_bar}+-4> and mJ = {vert_bar}+-2>, while the first excited state doublet corresponds to a mixture of mJ = {vert_bar}+-4>, mJ = {vert_bar}+-5> and mJ = {vert_bar}+-1>. We discuss these results in the context of proposed mechanisms for the failure of Tb 2Ti 2O 7 to develop conventional long range order down to 50 mK.« less

Structure and Thermal Expansion of YSZ and La 2Zr 2O 7 Above 1500°C from Neutron Diffraction on Levitated Samples

DOE PAGES

Ushakov, Sergey V.; Navrotsky, Alexandra; Weber, Richard J. K.; ...

2015-07-28

High-temperature time-of-flight neutron diffraction experiments were performed in this paper on cubic yttria-stabilized zirconia (YSZ, 10 mol% YO 1.5) and lanthanum zirconate (LZ) prepared by laser melting. Three spheroids of each composition were aerodynamically levitated and rotated in argon flow and heated with a CO 2 laser. Unit cell, positional and atomic displacement parameters were obtained by Rietveld analysis. Below ~1650°C the mean thermal expansion coefficient (TEC) for YSZ is higher than for LZ (13 ± 1 vs. 10.3 ± 0.6) × 10 -6/K. From ~1650°C to the onset of melting of LZ at ~2250°C, TEC for YSZ and LZmore » are similar and within (7 ± 2) × 10 -6/K. LZ retains the pyrochlore structure up to the melting temperature with Zr coordination becoming closer to perfectly octahedral. Congruently melting LZ is La deficient. The occurrence of thermal disordering of oxygen sublattice (Bredig transition) in defect fluorite structure was deduced from the rise in YSZ TEC to ~25 × 10 -6/K at 2350°C–2550°C with oxygen displacement parameters (U iso) reaching 0.1 Å 2, similar to behavior observed in UO 2. Acquisition of powder-like high-temperature neutron diffraction data from solid-levitated samples is feasible and possible improvements are outlined. Finally, this methodology should be applicable to a wide range of materials for high-temperature applications.« less

High-pressure behavior of A 2 B 2 O 7 pyrochlore (A=Eu, Dy; B=Ti, Zr)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rittman, Dylan R.; Turner, Katlyn M.; Park, Sulgiye

2017-01-24

In situ high-pressure X-ray diffraction and Raman spectroscopy were used to determine the influence of composition on the high-pressure behavior of A 2B 2O 7 pyrochlore (A = Eu, Dy; B = Ti, Zr) up to ~50 GPa. Based on X-ray diffraction results, all compositions transformed to the high-pressure cotunnite structure. The B-site cation species had a larger effect on the transition pressure than the A-site cation species, with the onset of the phase transformation occurring at ~41 GPa for B = Ti and ~16 GPa B = Zr. But, the A-site cation affected the kinetics of the phase transformation,more » with the transformation for compositions with the smaller ionic radii, i.e., A = Dy, proceeding faster than those with a larger ionic radii, i.e., A = Eu. Our results were consistent with previous work in which the radius-ratio of the A- and B-site cations determined the energetics of disordering, and compositions with more similarly sized A- and B-site cations had a lower defect formation energy. Raman spectra revealed differences in the degree of short-range order of the different compositions. Due to the large phase fraction of cotunnite at high pressure for B = Zr compositions, Raman modes for cotunnite could be observed, with more modes recorded for A = Eu than A = Dy. These additional modes are attributed to increased short-to-medium range ordering in the initially pyrochlore structured Eu 2Zr 2O 7 as compared with the initially defect-fluorite structured Dy 2Zr 2O 7.« less

High-pressure behavior of A 2 B 2 O 7 pyrochlore (A=Eu, Dy; B=Ti, Zr)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rittman, Dylan R.; Turner, Katlyn M.; Park, Sulgiye

2017-01-28

In situ high-pressure X-ray diffraction and Raman spectroscopy were used to determine the influence of composition on the high-pressure behavior of A 2B 2O 7 pyrochlore (A=Eu, Dy; B=Ti, Zr) up to ~50GPa. Based on X-ray diffraction results, all compositions transformed to the high-pressure cotunnite structure. The B-site cation species had a larger effect on the transition pressure than the A-site cation species, with the onset of the phase transformation occurring at ~41 GPa for B=Ti and ~16 GPa B=Zr. However, the A-site cation affected the kinetics of the phase transformation, with the transformation for compositions with the smaller ionicmore » radii, i.e., A=Dy, proceeding faster than those with a larger ionic radii, i.e., A=Eu. These results were consistent with previous work in which the radius-ratio of the A- and B-site cations determined the energetics of disordering, and compositions with more similarly sized A- and B-site cations had a lower defect formation energy. Raman spectra revealed differences in the degree of short-range order of the different compositions. Due to the large phase fraction of cotunnite at high pressure for B=Zr compositions, Raman modes for cotunnite could be observed, with more modes recorded for A=Eu than A=Dy. These additional modes are attributed to increased short-to-medium range ordering in the initially pyrochlore structured Eu 2Zr 2O 7 as compared with the initially defect-fluorite structured Dy 2Zr 2O 7.« less

Crystal-field, exchange interactions and magnetism in pyrochlore ferromagnet R2V2O7 (R3+=Y, Lu)

NASA Astrophysics Data System (ADS)

Ali Biswas, A.; Jana, Y. M.

2013-03-01

The temperature dependence of the observed bulk magnetic susceptibility, magnetization, paramagnetic Curie temperature θCW, magnetic specific heat of ferromagnetic semi-conducting pyrochlore-based vanadate compounds Y2V2O7 and Lu2V2O7, which are the simplest of R2M2O7 pyrochlore series of oxides, are simulated and analyzed, simultaneously and consistently, within the frame work of the appropriate crystal-field (CF) theory and a mean-field approximation by introducing effective anisotropic molecular-field tensors and also taking account of appreciable spin-orbit coupling. The electronic and magnetic properties are correlated to the structural parameters. Ten-fold degenerate 2D term of 3d1 V4+-ions is split into five Kramers doublets with overall CF splitting Δ1≈2 eV and the total splitting of the 2T2g state Δ0≈0.4 eV under combined actions of octahedral CF, trigonal (D3d) distortion at V-site and spin-orbit coupling. The ground doublet is a well-isolated effectively spin s=1/2 state, characterized by the anisotropic g-tensors and directional magnetic moments. The degeneracy of the ground state is lifted by the spin-spin correlations among V4-tetrahedra at T∼170 K, which causes the formation of ferromagnetic clusters in these pyrochlores. The temperature dependence of the calculated directional site-susceptibilities shows that the V4+ ions have a substantial easy-axis single-ion anisotropy along local <111> axis of a given V4-tetrahedron in the magnetic phase where ferromagnetic clusters coexist with paramagnetic phase.

Epitaxial growth of iridate pyrochlore Nd 2Ir 2O 7 films

DOE PAGES

Gallagher, J. C.; Esser, B. D.; Morrow, R.; ...

2016-02-29

Epitaxial films of the pyrochlore Nd 2Ir 2O 7 have been grown on (111)-oriented yttria-stabilized zirconia (YSZ) substrates by off-axis sputtering followed by post-growth annealing. X-ray diffraction (XRD) results demonstrate phase-pure epitaxial growth of the pyrochlore films on YSZ. Scanning transmission electron microscopy (STEM) investigation of an Nd 2Ir 2O 7 film with a short post-annealing provides insight into the mechanism for crystallization of Nd 2Ir 2O 7 during the post-annealing process. STEM images reveal clear pyrochlore ordering of Nd and Ir in the films. As a result, the epitaxial relationship between the YSZ and Nd 2Ir 2O 7 ismore » observed clearly while some interfacial regions show a thin region with polycrystalline Ir nanocrystals.« less

Metal-Insulator Transition in Epitaxial Pyrochlore Iridates Bi2Ir2O7 thin Films

NASA Astrophysics Data System (ADS)

Chu, Jiun-Haw; Liu, Jian; Yi, Di; Rayan-Serrao, C.; Suresha, S.; Marti, Xavi; Riggs, Scott; Shapiro, Max; Ian, Fisher; Ramesh, R.

2013-03-01

Recently there is a surge of interest in searching for topological order in correlated electronic systems such as transition metal oxides. The strong spin-orbit interaction of 5d electrons and the geometric frustration in the crystal lattice make the pyrochlore iridate(A2Ir2O7) an ideal candidate to achieve this goal. Pioneering experiments on bulk polycrystalline and single crystal samples revealed a temperature dependent metal-insulator transition coupled to a long range magnetic order, and the transition temperature can be tuned by either A-site ionic radius or an external pressure. In this talk we present our efforts to understand and control the metal-insulator transition and the underlying electronic structure of pyrochlore iridates via epitaxial Bi2Ir2O7 thin films. Bulk Bi2Ir2O7 is located at the metallic side of the phase diagram. However as the film's thickness decreases the transport evolves from a metallic to a strongly localized character. Resonant X-ray spectroscopy suggests that the density of states near Fermi level is dominated by the Ir Je ff =1/2 states. Intriguingly, the magnetoresistance shows a linear field dependence over a wide range of fields at low temperatures, which is possibly consistent with the existence of Dirac nodes.

Phase Competition in the Palmer-Chalker X Y Pyrochlore Er2Pt2O7

NASA Astrophysics Data System (ADS)

Hallas, A. M.; Gaudet, J.; Butch, N. P.; Xu, Guangyong; Tachibana, M.; Wiebe, C. R.; Luke, G. M.; Gaulin, B. D.

2017-11-01

We report neutron scattering measurements on Er2Pt2O7 , a new addition to the X Y family of frustrated pyrochlore magnets. Symmetry analysis of our elastic scattering data shows that Er2Pt2O7 orders into the k =0 , Γ7 magnetic structure (the Palmer-Chalker state), at TN=0.38 K . This contrasts with its sister X Y pyrochlore antiferromagnets Er2Ti2O7 and Er2Ge2O7 , both of which order into Γ5 magnetic structures at much higher temperatures, TN=1.2 and 1.4 K, respectively. In this temperature range, the magnetic heat capacity of Er2Pt2O7 contains a broad anomaly centered at T*=1.5 K . Our inelastic neutron scattering measurements reveal that this broad heat capacity anomaly sets the temperature scale for strong short-range spin fluctuations. Below TN=0.38 K , Er2Pt2O7 displays a gapped spin-wave spectrum with an intense, flat band of excitations at lower energy and a weak, diffusive band of excitations at higher energy. The flat band is well described by classical spin-wave calculations, but these calculations also predict sharp dispersive branches at higher energy, a striking discrepancy with the experimental data. This, in concert with the strong suppression of TN, is attributable to enhanced quantum fluctuations due to phase competition between the Γ7 and Γ5 states that border each other within a classically predicted phase diagram.

Molecular dynamics simulation of low-energy recoil events in titanate pyrochlores

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dong, Liyuan; Setyawan, Wahyu; Li, Yuhong

2017-01-01

Molecular dynamics simulations of low-energy displacements in titanate pyrochlores have been carried out along three main directions, to determineE dfor A, Ti and O, corresponding defect configurations, and defect formation dynamics.

What can one learn about material structure given a single first-principles calculation?

NASA Astrophysics Data System (ADS)

Rajen, Nicholas; Coh, Sinisa

2018-05-01

We extract a variable X from electron orbitals Ψn k and energies En k in the parent high-symmetry structure of a wide range of complex oxides: perovskites, rutiles, pyrochlores, and cristobalites. Even though calculation was done only in the parent structure, with no distortions, we show that X dictates material's true ground-state structure. We propose using Wannier functions to extract concealed variables such as X both for material structure prediction and for high-throughput approaches.

Structural and magnetic investigations of single-crystalline neodymium zirconate pyrochlore Nd2Zr2O7

NASA Astrophysics Data System (ADS)

Hatnean, M. Ciomaga; Lees, M. R.; Petrenko, O. A.; Keeble, D. S.; Balakrishnan, G.; Gutmann, M. J.; Klekovkina, V. V.; Malkin, B. Z.

2015-05-01

We report structural and magnetic properties studies of large high-quality single crystals of the frustrated magnet Nd2Zr2O7 . Powder x-ray diffraction analysis confirms that Nd2Zr2O7 adopts the pyrochlore structure. Room-temperature x-ray diffraction and time-of-flight neutron-scattering experiments show that the crystals are stoichiometric in composition with no measurable site disorder. The temperature dependence of the magnetic susceptibility shows no magnetic ordering at temperatures down to 0.5 K. Fits to the magnetic susceptibility data using a Curie-Weiss law reveal a ferromagnetic coupling between the Nd moments. Magnetization versus field measurements show a local Ising anisotropy along the <111 > axes of the Nd3 + ions in the ground state. Specific heat versus temperature measurements in zero applied magnetic field indicate the presence of a thermal anomaly below T ˜7 K, but no evidence of magnetic ordering is observed down to 0.5 K. The experimental temperature dependence of the single-crystal bulk dc susceptibility and isothermal magnetization are analyzed using crystal field theory and the crystal field parameters and exchange coupling constants determined.

New insights on ion track morphology in pyrochlores by aberration corrected scanning transmission electron microscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sachan, Ritesh; Zhang, Yanwen; Ou, Xin

Here we demonstrate the enhanced imaging capabilities of an aberration corrected scanning transmission electron microscope to advance the understanding of ion track structure in pyrochlore structured materials (i.e., Gd 2Ti 2O 7 and Gd 2TiZrO 7). Track formation occurs due to the inelastic transfer of energy from incident ions to electrons, and atomic-level details of track morphology as a function of energy-loss are revealed in the present work. A comparison of imaging details obtained by varying collection angles of detectors is discussed in the present work. A quantitative analysis of phase identification using high-angle annular dark field imaging is performedmore » on the ion tracks. Finally, a novel 3-dimensional track reconstruction method is provided that is based on depth dependent imaging of the ion tracks. The technique is used in extracting the atomic-level details of nanoscale features, such as the disordered ion tracks, which are embedded in relatively thicker matrix. Another relevance of the method is shown by measuring the tilt of the ion tracks relative to the electron beam incidence that helps in knowing the structure and geometry of ion tracks quantitatively.« less

New insights on ion track morphology in pyrochlores by aberration corrected scanning transmission electron microscopy

DOE PAGES

Sachan, Ritesh; Zhang, Yanwen; Ou, Xin; ...

2016-12-13

Here we demonstrate the enhanced imaging capabilities of an aberration corrected scanning transmission electron microscope to advance the understanding of ion track structure in pyrochlore structured materials (i.e., Gd 2Ti 2O 7 and Gd 2TiZrO 7). Track formation occurs due to the inelastic transfer of energy from incident ions to electrons, and atomic-level details of track morphology as a function of energy-loss are revealed in the present work. A comparison of imaging details obtained by varying collection angles of detectors is discussed in the present work. A quantitative analysis of phase identification using high-angle annular dark field imaging is performedmore » on the ion tracks. Finally, a novel 3-dimensional track reconstruction method is provided that is based on depth dependent imaging of the ion tracks. The technique is used in extracting the atomic-level details of nanoscale features, such as the disordered ion tracks, which are embedded in relatively thicker matrix. Another relevance of the method is shown by measuring the tilt of the ion tracks relative to the electron beam incidence that helps in knowing the structure and geometry of ion tracks quantitatively.« less

Quadratic Fermi node in a 3D strongly correlated semimetal

PubMed Central

Kondo, Takeshi; Nakayama, M.; Chen, R.; Ishikawa, J. J.; Moon, E.-G.; Yamamoto, T.; Ota, Y.; Malaeb, W.; Kanai, H.; Nakashima, Y.; Ishida, Y.; Yoshida, R.; Yamamoto, H.; Matsunami, M.; Kimura, S.; Inami, N.; Ono, K.; Kumigashira, H.; Nakatsuji, S.; Balents, L.; Shin, S.

2015-01-01

Strong spin–orbit coupling fosters exotic electronic states such as topological insulators and superconductors, but the combination of strong spin–orbit and strong electron–electron interactions is just beginning to be understood. Central to this emerging area are the 5d transition metal iridium oxides. Here, in the pyrochlore iridate Pr2Ir2O7, we identify a non-trivial state with a single-point Fermi node protected by cubic and time-reversal symmetries, using a combination of angle-resolved photoemission spectroscopy and first-principles calculations. Owing to its quadratic dispersion, the unique coincidence of four degenerate states at the Fermi energy, and strong Coulomb interactions, non-Fermi liquid behaviour is predicted, for which we observe some evidence. Our discovery implies that Pr2Ir2O7 is a parent state that can be manipulated to produce other strongly correlated topological phases, such as topological Mott insulator, Weyl semimetal, and quantum spin and anomalous Hall states. PMID:26640114

Quadratic Fermi node in a 3D strongly correlated semimetal

DOE PAGES

Kondo, Takeshi; Nakayama, M.; Chen, R.; ...

2015-12-07

We report that strong spin–orbit coupling fosters exotic electronic states such as topological insulators and superconductors, but the combination of strong spin–orbit and strong electron–electron interactions is just beginning to be understood. Central to this emerging area are the 5d transition metal iridium oxides. Here, in the pyrochlore iridate Pr 2Ir 2O 7, we identify a non-trivial state with a single-point Fermi node protected by cubic and time-reversal symmetries, using a combination of angle-resolved photoemission spectroscopy and first-principles calculations. Owing to its quadratic dispersion, the unique coincidence of four degenerate states at the Fermi energy, and strong Coulomb interactions, non-Fermimore » liquid behaviour is predicted, for which we observe some evidence. Lastly, our discovery implies that Pr 2Ir 2O 7 is a parent state that can be manipulated to produce other strongly correlated topological phases, such as topological Mott insulator, Weyl semimetal, and quantum spin and anomalous Hall states.« less

Zirconia ceramics for excess weapons plutonium waste

NASA Astrophysics Data System (ADS)

Gong, W. L.; Lutze, W.; Ewing, R. C.

2000-01-01

We synthesized a zirconia (ZrO 2)-based single-phase ceramic containing simulated excess weapons plutonium waste. ZrO 2 has large solubility for other metallic oxides. More than 20 binary systems A xO y-ZrO 2 have been reported in the literature, including PuO 2, rare-earth oxides, and oxides of metals contained in weapons plutonium wastes. We show that significant amounts of gadolinium (neutron absorber) and yttrium (additional stabilizer of the cubic modification) can be dissolved in ZrO 2, together with plutonium (simulated by Ce 4+, U 4+ or Th 4+) and impurities (e.g., Ca, Mg, Fe, Si). Sol-gel and powder methods were applied to make homogeneous, single-phase zirconia solid solutions. Pu waste impurities were completely dissolved in the solid solutions. In contrast to other phases, e.g., zirconolite and pyrochlore, zirconia is extremely radiation resistant and does not undergo amorphization. Baddeleyite (ZrO 2) is suggested as the natural analogue to study long-term radiation resistance and chemical durability of zirconia-based waste forms.

Tunable Magnon Weyl Points in Ferromagnetic Pyrochlores.

PubMed

Mook, Alexander; Henk, Jürgen; Mertig, Ingrid

2016-10-07

The dispersion relations of magnons in ferromagnetic pyrochlores with Dzyaloshinskii-Moriya interaction are shown to possess Weyl points, i. e., pairs of topologically nontrivial crossings of two magnon branches with opposite topological charge. As a consequence of their topological nature, their projections onto a surface are connected by magnon arcs, thereby resembling closely Fermi arcs of electronic Weyl semimetals. On top of this, the positions of the Weyl points in reciprocal space can be tuned widely by an external magnetic field: rotated within the surface plane, the Weyl points and magnon arcs are rotated as well; tilting the magnetic field out of plane shifts the Weyl points toward the center Γ[over ¯] of the surface Brillouin zone. The theory is valid for the class of ferromagnetic pyrochlores, i. e., three-dimensional extensions of topological magnon insulators on kagome lattices. In this Letter, we focus on the (111) surface, identify candidates of established ferromagnetic pyrochlores which apply to the considered spin model, and suggest experiments for the detection of the topological features.

Structural, photoluminescence and radioluminescence properties of Eu{sup 3+} doped La{sub 2}Hf{sub 2}O{sub 7} nanoparticles

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wahid, Kareem; Pokhrel, Madhab; Mao, Yuanbing, E-mail: yuanbing.mao@utrgv.edu

This study presents the structural, optical, and radioluminescent characterization of newly synthesized europium-doped lanthanum hafnate (La{sub 2}Hf{sub 2}O{sub 7}:xmol%Eu{sup 3+}, x=0 to 35) nanoparticles (NPs) for use as phosphors and scintillation materials. Samples prepared through a combined co-precipitation and molten salt synthetic process were found to crystalize in the pyrochlore phase, a radiation tolerant structure related to the fluorite structure. These samples exhibit red luminescence under ultraviolet and X-ray excitation. Under these excitations, the optical intensity and quantum yield of the La{sub 2}Hf{sub 2}O{sub 7}:xmol%Eu{sup 3+} NPs depend on the Eu{sup 3+} concentration and are maximized at 5%. It ismore » proposed that there is a trade-off between the quenching due to defect states/cross-relaxation and dopant concentration. An optimal dopant concentration allows the La{sub 2}Hf{sub 2}O{sub 7}:5 mol%Eu{sup 3+} NPs to show the best luminescent properties of all the samples. - Graphical abstract: Incident X-ray and UV photons interact with La{sub 2}Hf{sub 2}O{sub 7}: xmol%Eu{sup 3+}(x=1–35) nanoparticles (NPs) to yield strong red luminescence centered at 612 nm. Colored spheres inside NP diagram represent pyrochlore coordination environment of La{sub 2}Hf{sub 2}O{sub 7}:xmol%Eu{sup 3+}. Blue, red, yellow, green and black spheres represent hafnium(IV) atoms, lanthanum(III)/europium(III) atoms, oxygen atoms at 48f site, oxygen atoms at 8b site and oxygen vacancies, respectively. - Highlights: • La{sub 2}Hf{sub 2}O{sub 7}:xmol%Eu{sup 3+} (x=0–35) nanoparticles with weakly-ordered pyrochlore structures were synthesized. • Optically and X-ray excited emission spectra showed strong luminescence centered at 612 nm. • Photoluminescence quantum yield increases with doping concentration up to 5% and decreases at higher concentrations.« less

2.7 MeV Ar11+ ion irradiation induced structural evolution in Lu2(Ti2-xLux)O7-x/2 pyrochlores

NASA Astrophysics Data System (ADS)

Yang, D. Y.; Liu, C. G.; Zhang, K. Q.; Xia, Y.; Chen, L. J.; Liu, H.; Li, Y. H.

2015-11-01

This paper aims to study the radiation effects of nonstoichiometric pyrochlore series Lu2(Ti2-xLux)O7-x/2 (x = 0-0.667). Polycrystalline Lu2(Ti2-xLux)O7-x/2 samples were irradiated with 2.7 MeV Ar11+ ions up to a fluence of 8 × 1014 ions/cm2. The irradiated samples were characterized using grazing incidence X-ray diffraction technique. The results reveal that Lu2(Ti2-xLux)O7-x/2 samples undergo significant amorphization and lattice swelling upon irradiation. Specifically, the amorphization process is predominantly driven by ballistic nuclear energy deposition of Ar11+ ions at this energy regime, which can be well described by a direct-impact/defect-stimulated model. Both the amorphization fraction and the relative variation of lattice parameter decrease with increasing x, showing a strong dependence on the chemical composition. The results are then discussed in the framework of the structural disorder and recovery ability from damage, applying an atomic layer model.

New dielectric ceramics Pb(Cd)BiM/sup IV/SbO/sub 7/ (M/sup IV/ = Ti, Zr, Sn) with the pyrochlore structure

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lambachri, A.; Monier, M.; Mercurio, J.P.

1988-04-01

Dielectric ceramics have been obtained by natural sintering of pyrochlore phases with general formula Pb(Cd)BiM/sup IV/SbO/sub 7/ (M/sup IV/ = Ti, Zr, Sn). Low frequency dielectric characteristics have been studied with respect to the processing conditions: sintering without additive and in the presence of some low melting compounds (PbO, Pb/sub 5/Ge/sub 3/O/sub 11/, Bi/sub 12/PbO/sub 19/ and Bi/sub 12/CdO/sub 19/). The dielectric constants of these ceramics lie between 30 and 60, the dielectric losses range from 10 to 30.10/sup -4/ and the temperature coefficient of the dielectric constants (20 - 100/sup 0/C) can be tailored by means of additives inmore » the +- 30 ppm K/sup -1/ range.« less

Static magnetic susceptibility, crystal field and exchange interactions in rare earth titanate pyrochlores.

PubMed

Malkin, B Z; Lummen, T T A; van Loosdrecht, P H M; Dhalenne, G; Zakirov, A R

2010-07-14

The experimental temperature dependence (T = 2-300 K) of single crystal bulk and site susceptibilities of rare earth titanate pyrochlores R(2)Ti(2)O(7) (R = Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb) is analyzed in the framework of crystal field theory and a mean field approximation. Analytical expressions for the site and bulk susceptibilities of the pyrochlore lattice are derived taking into account long range dipole-dipole interactions and anisotropic exchange interactions between the nearest neighbor rare earth ions. The sets of crystal field parameters and anisotropic exchange coupling constants have been determined and their variations along the lanthanide series are discussed.

Two-magnon scattering in the 5d all-in-all-out pyrochlore magnet Cd2Os2O7.

PubMed

Nguyen, Thi Minh Hien; Sandilands, Luke J; Sohn, C H; Kim, C H; Wysocki, Aleksander L; Yang, In-Sang; Moon, S J; Ko, Jae-Hyeon; Yamaura, J; Hiroi, Z; Noh, Tae Won

2017-08-15

5d pyrochlore oxides with all-in-all-out magnetic order are prime candidates for realizing strongly correlated, topological phases of matter. Despite significant effort, a full understanding of all-in-all-out magnetism remains elusive as the associated magnetic excitations have proven difficult to access with conventional techniques. Here we report a Raman spectroscopy study of spin dynamics in the all-in-all-out magnetic state of the 5d pyrochlore Cd 2 Os 2 O 7 . Through a comparison between the two-magnon scattering and spin-wave theory, we confirm the large single ion anisotropy in this material and show that the Dzyaloshinskii-Moriya and exchange interactions play a significant role in the spin-wave dispersions. The Raman data also reveal complex spin-charge-lattice coupling and indicate that the metal-insulator transition in Cd 2 Os 2 O 7 is Lifshitz-type. Our work establishes Raman scattering as a simple and powerful method for exploring the spin dynamics in 5d pyrochlore magnets.Pyrochlore 5d transition metal oxides are expected to have interesting forms of magnetic order but are hard to study with conventional probes. Here the authors show that Raman scattering can be used to measure magnetic excitations in Cd 2 Os 2 O 7 and that it exhibits complex spin-charge-lattice coupling.

Room temperature aerobic oxidation of amines by a nanocrystalline ruthenium oxide pyrochlore nafion composite catalyst.

PubMed

Venkatesan, Shanmuganathan; Kumar, Annamalai Senthil; Lee, Jyh-Fu; Chan, Ting-Shan; Zen, Jyh-Myng

2012-05-14

The aerobic oxidation of primary amines to their respective nitriles has been carried out at room temperature using a highly reusable nanocrystalline ruthenium oxide pyrochlore Nafion composite catalyst (see figure). Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Phase equilibria and crystal chemistry of rubidium niobates and rubidium tantalates

NASA Technical Reports Server (NTRS)

Minor, D. B.; Roth, R. S.; Parker, H. S.; Brower, W. S.

1977-01-01

The phase equilibria relations and crystal chemistry of portions of the Rb2O-Nb2O5 and Rb2O-Ta2O5 systems were investigated for structures potentially useful as ionic conductors. A hexagonal tungsten bronze-type (HTB) structure was found in both systems as well as three hexagonal phases with mixed HTB-pyrochlore type structures. Ion exchange experiments between various alkali ions are described for several phases. Unit cell dimensions and X-ray diffraction powder patterns are reported.

Deformation-induced structural transition in body-centred cubic molybdenum

PubMed Central

Wang, S. J.; Wang, H.; Du, K.; Zhang, W.; Sui, M. L.; Mao, S. X.

2014-01-01

Molybdenum is a refractory metal that is stable in a body-centred cubic structure at all temperatures before melting. Plastic deformation via structural transitions has never been reported for pure molybdenum, while transformation coupled with plasticity is well known for many alloys and ceramics. Here we demonstrate a structural transformation accompanied by shear deformation from an original <001>-oriented body-centred cubic structure to a <110>-oriented face-centred cubic lattice, captured at crack tips during the straining of molybdenum inside a transmission electron microscope at room temperature. The face-centred cubic domains then revert into <111>-oriented body-centred cubic domains, equivalent to a lattice rotation of 54.7°, and ~15.4% tensile strain is reached. The face-centred cubic structure appears to be a well-defined metastable state, as evidenced by scanning transmission electron microscopy and nanodiffraction, the Nishiyama–Wassermann and Kurdjumov–Sachs relationships between the face-centred cubic and body-centred cubic structures and molecular dynamics simulations. Our findings reveal a deformation mechanism for elemental metals under high-stress deformation conditions. PMID:24603655

Heat capacity and magnetic properties of fluoride CsFe{sup 2+}Fe{sup 3+}F{sub 6} with defect pyrochlore structure

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gorev, M.V., E-mail: gorev@iph.krasn.ru; Institute of Engineering Physics and Radio Electronics, Siberian State University, 660074 Krasnoyarsk; Flerov, I.N.

2016-05-15

Heat capacity, Mössbauer and Raman spectra as well as magnetic properties of fluoride CsFe{sub 2}F{sub 6} with defect pyrochlore structure were studied. In addition to recently found above room temperature three successive structural transformations Pnma-Imma-I4{sub 1}amd-Fd-3m, phase transition of antiferromagnetic nature with the 13.7 K Neel temperature and a broad heat capacity anomaly with a maximum at about 30 K were observed. The room temperature symmetry Pnma is unchanged at least down to 7 K. Simple model of indirect bond used to estimate the exchange interactions and to propose a magnetic structure model. - Graphical abstract: The ordered arrangement ofmore » Fe{sup 2+} and Fe{sup 3+} ions in high-spin states as well as antiferromagnetic phase transition followed by significant magnetic frustrations were found in pyrocholore-related CsFe{sup 2+}Fe{sup 3+}F{sub 6}. A magnetic structure was proposed using a simple model of indirect bonds. - Highlights: • The Pnma structure in pyrocholore CsFe{sub 2}F{sub 6} is stable down to helium temperature. • Mössbauer spectra confirmed the ordering of Fe{sup 2+} and Fe{sup 3+} ions. • Antiferromagnetic transformation and significant magnetic frustrations are found. • Experimental magnetic entropy agrees with entropy for Fe ions in high-spin state. • Superexchange interactions were calculated and a magnetic structure was proposed.« less

Exchange interactions in two-state systems: rare earth pyrochlores.

PubMed

Curnoe, S H

2018-06-13

The general form of the nearest neighbour exchange interaction for rare earth pyrochlores is derived based on symmetry. Generally, the rare earth angular momentum degeneracy is lifted by the crystal electric field (CEF) into singlets and doublets. When the CEF ground state is a doublet that is well-separated from the first excited state the CEF ground state doublet can be treated as a pseudo-spin of some kind. The general form of the nearest neighbour exchange interaction for pseudo-spins on the pyrochlore lattice is derived for three different types of pseudo-spins. The methodology presented in this paper can be applied to other two-state spin systems with a high space group symmetry.

Exchange interactions in two-state systems: rare earth pyrochlores

NASA Astrophysics Data System (ADS)

Curnoe, S. H.

2018-06-01

The general form of the nearest neighbour exchange interaction for rare earth pyrochlores is derived based on symmetry. Generally, the rare earth angular momentum degeneracy is lifted by the crystal electric field (CEF) into singlets and doublets. When the CEF ground state is a doublet that is well-separated from the first excited state the CEF ground state doublet can be treated as a pseudo-spin of some kind. The general form of the nearest neighbour exchange interaction for pseudo-spins on the pyrochlore lattice is derived for three different types of pseudo-spins. The methodology presented in this paper can be applied to other two-state spin systems with a high space group symmetry.

Thermo-selective Tm(x)Ti(1-x)O(2-x/2) nanoparticles: from Tm-doped anatase TiO2 to a rutile/pyrochlore Tm2Ti2O7 mixture. An experimental and theoretical study with a photocatalytic application.

PubMed

Navas, Javier; Sánchez-Coronilla, Antonio; Aguilar, Teresa; De los Santos, Desireé M; Hernández, Norge C; Alcántara, Rodrigo; Fernández-Lorenzo, Concha; Martín-Calleja, Joaquín

2014-11-07

This is an experimental and theoretical study of thulium doped TiO2 nanoparticles. From an experimental perspective, a method was used to synthesize thulium-doped TiO2 nanoparticles in which Tm(3+) replaces Ti(4+) in the lattice, which to our knowledge has neither been reported nor studied theoretically so far. Different proportions of anatase and rutile phases were obtained at different annealing temperatures, and XRD and Raman spectroscopy also revealed the presence of a pyrochlore phase (Tm2Ti2O7) at 1173 K. Thus, the structure of the Tm-doped nanoparticles was thermally-controlled. Furthermore, XPS showed the presence of Tm(3+) in the samples synthesized, which produces oxygen vacancies to maintain the local neutrality in the lattice. The presence of Tm(3+) in the samples led to changes in the UV-Vis absorption spectra, so they showed photoluminescence properties and new states in the band gap, which produce a new lower energy electronic transition than the main TiO2 one. Periodic DFT calculations were performed to understand the experimentally produced structures. The production of oxygen vacancies was analysed and the changes generated in the structure were fully detailed. The DOS and PDOS analyses confirmed the experimental results obtained using UV-Vis spectroscopy, and showed that the new electronic states in the band gap are due to interactions of the f state of Tm and the p state of O. Likewise, the charge study and the ELF analysis indicate that when Tm is introduced into the TiO2 structure, the Ti-O bond around the oxygen vacancy is strengthened. Finally, an example of a photocatalytic application was developed to show the high efficiency of the samples due to the heterojunction in the interfaces of the phases in the samples, which improved the charge separation and the good charge carrier mobility due to the presence of the pyrochlore phase, as was also shown theoretically.

Strain engineered pyrochlore at high pressure

DOE PAGES

Rittman, Dylan R.; Turner, Katlyn M.; Park, Sulgiye; ...

2017-05-22

Strain engineering is a promising method for next-generation materials processing techniques. Here, we use mechanical milling and annealing followed by compression in diamond anvil cell to tailor the intrinsic and extrinsic strain in pyrochlore, Dy 2Ti 2O 7 and Dy 2Zr 2O 7. Raman spectroscopy, X-ray pair distribution function analysis, and X-ray diffraction were used to characterize atomic order over short-, medium-, and long-range spatial scales, respectively, under ambient conditions. Raman spectroscopy and X-ray diffraction were further employed to interrogate the material in situ at high pressure. High-pressure behavior is found to depend on the species and concentration of defectsmore » in the sample at ambient conditions. Overall, we show that defects can be engineered to lower the phase transformation onset pressure by ~50% in the ordered pyrochlore Dy 2Ti 2O 7, and lower the phase transformation completion pressure by ~20% in the disordered pyrochlore Dy 2Zr 2O 7. Lastly, these improvements are achieved without significantly sacrificing mechanical integrity, as characterized by bulk modulus.« less

Two-magnon scattering in the 5d all-in-all-out pyrochlore magnet Cd 2Os 2O 7

DOE PAGES

Nguyen, Thi Minh Hien; Sandilands, Luke J.; Sohn, C. H.; ...

2017-08-15

5d pyrochlore oxides with all-in-all-out magnetic order are prime candidates for realizing strongly correlated, topological phases of matter. Despite significant effort, a full understanding of all-in-all-out magnetism remains elusive as the associated magnetic excitations have proven difficult to access with conventional techniques. Here we report a Raman spectroscopy study of spin dynamics in the all-in-all-out magnetic state of the 5d pyrochlore Cd 2Os 2O 7. Through a comparison between the two-magnon scattering and spin-wave theory, we confirm the large single ion anisotropy in this material and show that the Dzyaloshinskii–Moriya and exchange interactions play a significant role in the spin-wavemore » dispersions. The Raman data also reveal complex spin–charge–lattice coupling and indicate that the metal–insulator transition in Cd 2Os 2O 7 is Lifshitz-type. In conclusion, our work establishes Raman scattering as a simple and powerful method for exploring the spin dynamics in 5d pyrochlore magnets.« less

Low energy ion-solid interactions and chemistry effects in a series of pyrochlores

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dong, Liyuan; Li, Yuhong; Devanathan, Ram

The effect of chemistry on low energy recoil events was investigated at 10 K for each type of atom in pyrochlores using molecular dynamics simulation. Contour plots of the threshold displacement energy (Ed) in Gd2Zr2O7 have been produced along more than 80 directions for each individual species. The Ed surface for each type of atom in Gd2Zr2O7 is highly anisotropic; Ed of Zr exhibits the largest degree of anisotropy, while that of O8b exhibits the smallest. The recommended values of Ed in Gd2Zr2O7 based on the observed minima are 56, 94 and 25 eV, respectively for Gd, Zr and O.more » The influence of cation radius on Ed in pyrochlores A2B2O7 (with A-site ranging from Lu3+ to La3+ and B-site ranging from Ti4+ to Ce4+) was also investigated along three directions [100], [110] and [111]. The Ed in pyrochlores strongly depended on the atom type, atom mass, knock-on direction, and lattice position. The defects produced after low energy displacement events included cation antisite defects, cation Frenkel pairs, anion Frenkel pairs, various vacancies and interstitials. Ce doping in pyrochlores may affect the radiation response, because it resulted in drastic changes in cation and anion displacement energies and formation of an unusual type of anti-site defect. This work demonstrates links between Ed and amorphization resistance.« less

Steering Charge Kinetics of Tin Niobate Photocatalysts: Key Roles of Phase Structure and Electronic Structure

NASA Astrophysics Data System (ADS)

Huang, Shushu; Wang, Chunyan; Sun, Hao; Wang, Xiaojing; Su, Yiguo

2018-05-01

Tin niobate photocatalysts with the phase structures of froodite (SnNb2O6) and pyrochlore (Sn2Nb2O7) were obtained by a facile solvothermal method in order to explore the impact of phase structure and electronic structure on the charge kinetics and photocatalytic performance. By employing tin niobate as a model compound, the effects of phase structure over electronic structure, photocatalytic activity toward methyl orange solution and hydrogen evolution were systematically investigated. It is found that the variation of phase structure from SnNb2O6 to Sn2Nb2O7 accompanied with modulation of particle size and band edge potentials that has great consequences on photocatalytic performance. In combination with the electrochemical impedance spectroscopy (EIS), transient photocurrent responses, transient absorption spectroscopy (TAS), and the analysis of the charge-carrier dynamics suggested that variation of electronic structure has great impacts on the charge separation and transfer rate of tin niobate photocatalysts and the subsequent photocatalytic performance. Moreover, the results of the X-ray photoelectron spectroscopy (XPS) indicated that the existent of Sn4+ species in Sn2Nb2O7 could result in a decrease in photocatalytic activity. Photocatalytic test demonstrated that the SnNb2O6 (froodite) catalyst possesses a higher photocatalytic activity toward MO degradation and H2 evolution compared with the sample of Sn2Nb2O7 (pyrochlore). On the basis of spin resonance measurement and trapping experiment, it is expected that photogenerated holes, O2 -•, and OH• active species dominate the photodegradation of methyl orange.

Steering Charge Kinetics of Tin Niobate Photocatalysts: Key Roles of Phase Structure and Electronic Structure.

PubMed

Huang, Shushu; Wang, Chunyan; Sun, Hao; Wang, Xiaojing; Su, Yiguo

2018-05-23

Tin niobate photocatalysts with the phase structures of froodite (SnNb 2 O 6 ) and pyrochlore (Sn 2 Nb 2 O 7 ) were obtained by a facile solvothermal method in order to explore the impact of phase structure and electronic structure on the charge kinetics and photocatalytic performance. By employing tin niobate as a model compound, the effects of phase structure over electronic structure, photocatalytic activity toward methyl orange solution and hydrogen evolution were systematically investigated. It is found that the variation of phase structure from SnNb 2 O 6 to Sn 2 Nb 2 O 7 accompanied with modulation of particle size and band edge potentials that has great consequences on photocatalytic performance. In combination with the electrochemical impedance spectroscopy (EIS), transient photocurrent responses, transient absorption spectroscopy (TAS), and the analysis of the charge-carrier dynamics suggested that variation of electronic structure has great impacts on the charge separation and transfer rate of tin niobate photocatalysts and the subsequent photocatalytic performance. Moreover, the results of the X-ray photoelectron spectroscopy (XPS) indicated that the existent of Sn 4+ species in Sn 2 Nb 2 O 7 could result in a decrease in photocatalytic activity. Photocatalytic test demonstrated that the SnNb 2 O 6 (froodite) catalyst possesses a higher photocatalytic activity toward MO degradation and H 2 evolution compared with the sample of Sn 2 Nb 2 O 7 (pyrochlore). On the basis of spin resonance measurement and trapping experiment, it is expected that photogenerated holes, O 2 -• , and OH • active species dominate the photodegradation of methyl orange.

Effects of stuffing on the atomic and electronic structure of the pyrochlore Yb2Ti2O7

NASA Astrophysics Data System (ADS)

Ghosh, Soham S.; Manousakis, Efstratios

2018-06-01

There are reasons to believe that the ground state of the magnetic rare-earth pyrochlore Yb2Ti2O7 is on the boundary between competing ground states. We have carried out ab initio density functional calculations to determine the most stable chemical formula as a function of the oxygen chemical potential and the likely location of the oxygen atoms in the unit cell of the "stuffed" system. We find that it is energetically favorable in the stuffed crystal (with an Yb replacement on a Ti site) to contain oxygen vacancies which dope the Yb 4 f orbitals and qualitatively change the electronic properties of the system. In addition, with the inclusion of the contribution of spin-orbit coupling (SOC) on top of the GGA + U approach, we investigated the electronic structure and the magnetic moments of the most stable stuffed system. In our determined stuffed structure the valence bands as compared to those of the pure system are pushed down and a change in hybridization between the O 2 p orbitals and the metal ion states is found. Our first-principle findings should form a foundation for effective models describing the low-temperature properties of this material whose true ground state remains controversial.

Insights on dramatic radial fluctuations in track formation by energetic ions

DOE PAGES

Sachan, Ritesh; Lang, Maik; Trautmann, Christina; ...

2016-06-02

We discuss the insights on the unexpected dramatic radial variations in the ion tracks formed by energetic ion (2.3 GeV 208Pb) irradiation at a constant electronic energy-loss (~42 keV/nm) in pyrochlore structured Gd 2TiZrO 7. Though previous studies have shown track formation and average track diameter measurements, this work brings further clarity on why quantitative analysis of ion track formation in Gd 2Ti xZr (1-x)O 7 systems can be more complicated than the currently accepted behavior for ion tracks. The ion track profile is usually considered to be diametrically uniform at constant values of the electronic energy-loss. This study showsmore » the diameter variations to be as large as ~40% within an extremely short incremental track length of ~20 nm. Our molecular dynamics simulations show that these fluctuations in diameter of amorphous core and overall track diameter are attributed to (i) the stochastic nature of inelastic energy loss along the track and (ii) the random substitution of Ti atoms by Zr atoms on the B-site in the pyrochlore lattice. Furthermore, the partial substitution of Ti by Zr increases the favorability of the defect-fluorite structure formation over amorphous phase stochastically, by introducing localized inhomogeneity in atomic structure, density and strain.« less

Insights on dramatic radial fluctuations in track formation by energetic ions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sachan, Ritesh; Lang, Maik; Trautmann, Christina

We discuss the insights on the unexpected dramatic radial variations in the ion tracks formed by energetic ion (2.3 GeV 208Pb) irradiation at a constant electronic energy-loss (~42 keV/nm) in pyrochlore structured Gd 2TiZrO 7. Though previous studies have shown track formation and average track diameter measurements, this work brings further clarity on why quantitative analysis of ion track formation in Gd 2Ti xZr (1-x)O 7 systems can be more complicated than the currently accepted behavior for ion tracks. The ion track profile is usually considered to be diametrically uniform at constant values of the electronic energy-loss. This study showsmore » the diameter variations to be as large as ~40% within an extremely short incremental track length of ~20 nm. Our molecular dynamics simulations show that these fluctuations in diameter of amorphous core and overall track diameter are attributed to (i) the stochastic nature of inelastic energy loss along the track and (ii) the random substitution of Ti atoms by Zr atoms on the B-site in the pyrochlore lattice. Furthermore, the partial substitution of Ti by Zr increases the favorability of the defect-fluorite structure formation over amorphous phase stochastically, by introducing localized inhomogeneity in atomic structure, density and strain.« less

New data on carbonatites of the Il'mensky-Vishnevogorsky alkaline complex, the southern Urals, Russia

NASA Astrophysics Data System (ADS)

Nedosekova, I. L.

2007-04-01

Carbonatites that are hosted in metamorphosed ultramafic massifs in the roof of miaskite intrusions of the Il’mensky-Vishnevogorsky alkaline complex are considered. Carbonatites have been revealed in the Buldym, Khaldikha, Spirikha, and Kagan massifs. The geological setting, structure of carbonatite bodies, distribution of accessory rare-metal mineralization, typomorphism of rock-forming minerals, geochemistry, and Sr and Nd isotopic compositions are discussed. Dolomite-calcite carbonatites hosted in ultramafic rocks contain tetraferriphlogopite, richterite, accessory zircon, apatite, magnetite, ilmenite, pyrrhotite, pyrite, and pyrochlore. According to geothermometric data and the composition of rock-forming minerals, the dolomite-calcite carbonatites were formed under K-feldspar-calcite, albite-calcite, and amphibole-dolomite-calcite facies conditions at 575-300°C. The Buldym pyrochlore deposit is related to carbonatites of these facies. In addition, dolomite carbonatites with accessory Nb and REE mineralization (monazite, aeschynite, allanite, REE-pyrochlore, and columbite) are hosted in ultramafic massifs. The dolomite carbonatites were formed under chlorite-sericite-ankerite facies conditions at 300-200°C. The Spirikha REE deposit is related to dolomite carbonatite and alkaline metasomatic rocks. It has been established that carbonatites hosted in ultramafic rocks are characterized by high Sr, Ba, and LREE contents and variable Nb, Zr, Ti, V, and Th contents similar to the geochemical attributes of calcio-and magnesiocarbonatites. The low initial 87Sr/86Sr = 0.7044-0.7045 and ɛNd ranging from 0.65 to -3.3 testify to their derivation from a deep mantle source of EM1 type.

Use of the Primitive Unit Cell in Understanding Subtle Features of the Cubic Closest-Packed Structure

ERIC Educational Resources Information Center

Hawkins, John A.; Rittenhouse, Jeffrey L.; Soper, Linda M.; Rittenhouse, Robert C.

2008-01-01

One of the most important crystal structures adopted by metals is characterized by the "abcabc"...stacking of close-packed layers. This structure is commonly referred to in textbooks as the cubic close-packed (ccp) or face-centered cubic (fcc) structure, since the entire lattice can be generated by replication of a face-centered cubic unit cell…

Investigation of the Structural Stability of Ion-Implanted Gd 2Ti 2-xSn xO 7 Pyrochlore-Type Oxides by Glancing Angle X-ray Absorption Spectroscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Aluri, Esther Rani; Hayes, John R.; Walker, James D.S.

2016-03-24

Rare-earth titanate and stannate pyrochlore-type oxides have been investigated in the past for the sequestration of nuclear waste elements because of their resistance to radiation-induced structural damage. In order to enhance this property, it is necessary to understand the effect of radioactive decay of the incorporated actinide elements on the local chemical environment. In this study, Gd 2Ti 2–xSn xO 7 materials have been implanted with Au– ions to simulate radiation-induced structural damage. Glancing angle X-ray absorption near-edge spectroscopy (GA-XANES), glancing angle X-ray absorption fine structure (GA-EXAFS) analysis, and powder X-ray diffraction have been used to investigate changes in themore » local coordination environment of the metal atoms in the damaged surface layer. Examination of GA-XANES/EXAFS spectra from the implanted Gd 2Ti 2–xSn xO 7 materials collected at various glancing angles allowed for an investigation of how the local coordination environment around the absorbing atoms changed at different depths in the damaged surface layer. This study has shown the usefulness of GA-XANES to the examination of ion-implanted materials and has suggested that Gd 2Ti 2–xSn xO 7 becomes more susceptible to ion-beam-induced structural damage with increasing Sn concentration.« less

Phase transformation and microstructural evolution of nanostructured oxides and nitrides under ion irradiations

NASA Astrophysics Data System (ADS)

Lu, Fengyuan

Material design at the nanometer scale is an effective strategy for developing advanced materails with enhanced radiation tolerance for advanced nuclear energy systems as high densities of surfaces and interfaces of the nanostructured materials may behave as effective sinks for defect recovery. However, nanostructured materials may not be intrinsically radiation tolerant, and the interplay among the factors of crystal size, temperature, chemical composition, surface energy and radiation conditions may eventually determine material radiation behaviors. Therefore, it is necessary to understand the radiation effects of nanostructured materials and the underlying physics for the design of advanced nanostructured nuclear materials. The main objective of this doctoral thesis is to study the behavior of nanostructured oxides and nitrides used as fuel matrix and waste forms under extreme radiation conditions with the focus of phase transformation, microstructural evolution and damage mechanisms. Radiation experiments were performed using energetic ion beam techniques to simulate radiation damage resulting from energetic neutrons, alpha-decay events and fission fragments, and various experimental approaches were employed to characterize materials’ microstructural evolution and phase stability upon intense radiation environments including transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. Thermal annealing experiments indicated that nanostructured ZrO2 phase stability is strongly affected by the grain size. Radiation results on nanostructured ZrO2 indicated that thermodynamically unstable or metastable high temperature phases can be induced by energetic beam irradiation at room temperature. Various phase transformation among different polymorphs of monoclinic, tetragonal and amorphous states can be induced, and different mechanisms are responsible for structural transformations including oxygen vacancies accumulation upon displacive damage, radiation-assistant recrystallization and thermal spike by ionization radiation. The radiation response of nanosized pyrochlores indicated that the radiation tolerance of nanoceramics is highly dependent on the composition and size. Nanosized tantalate pyrochlores KxLnyTa2O 7-v (Ln = Gd, Y, Lu) with the average grain size around 10 - 15 nm are highly sensitive to radiation-induced amorphization. The pyrochlore A to B site ionic radius ratio rA/rB is crucial in determining the radiation tolerance of pyrochlores, and a minimum rA/rB of 1.605 exists for the occurring of radiation induced amorphization. The interplay among chemical compositions, structural deviation and grain size eventually determines the phase stability and structural transformation processes of tantalate pyrochlores under intense radiation environments. ZrN shows extremely high phase stability under both displacive ion irradiation and ionizing swift heavy ion irradiation. However, a contraction in lattice constant up to ~ 1.42 % can be induced in nanocrystalline ZrN irradiated with displacive ion beams. In contrast, the strongly ionizing swift heavy ions cannot induce any lattice contraction. Such lattice contractions may be due to a negative strain field in the ZrN nanograins related to N vacancies built up upon displacive radiation. Ion irradiations also lead to the formation of orthorhombic ZrSi phase at the interface between ZrN and Si substrate, resulting from atom mixing and precipitation upon ion irradiations. The fundamental knowledge provides critical data for assessing and quantifying nanostructured ceramics as fuel matrix and waste forms utilized in the extreme environments of advanced nuclear energy systems. Further possibilities are being pursued in manipulating microstructure at the nano-scale, controlling phase stability and tailoring the physical properties of materials for various important engineering applications.

Insights into dynamic processes of cations in pyrochlores and other complex oxides

DOE PAGES

Uberuaga, Blas Pedro; Perriot, Romain

2015-08-26

Complex oxides are critical components of many key technologies, from solid oxide fuel cells and superionics to inert matrix fuels and nuclear waste forms. In many cases, understanding mass transport is important for predicting performance and, thus, extensive effort has been devoted to understanding mass transport in these materials. However, most work has focused on the behavior of oxygen while cation transport has received relatively little attention, even though cation diffusion is responsible for many phenomena, including sintering, radiation damage evolution, and deformation processes. Here, we use accelerated molecular dynamics simulations to examine the kinetics of cation defects in onemore » class of complex oxides, A₂B₂O₇ pyrochlore. In some pyrochlore chemistries, B cation defects are kinetically unstable, transforming to A cation defects and antisites at rates faster than they can diffuse. When this occurs, transport of B cations occurs through defect processes on the A sublattice. Further, these A cation defects, either interstitials or vacancies, can interact with antisite disorder, reordering the material locally, though this process is much more efficient for interstitials than vacancies. Whether this behavior occurs in a given pyrochlore depends on the A and B chemistry. Pyrochlores with a smaller ratio of cation radii exhibit this complex behavior, while those with larger ratios exhibit direct migration of B interstitials. Similar behavior has been reported in other complex oxides such as spinels and perovskites, suggesting that this coupling of transport between the A and B cation sublattices, while not universal, occurs in many complex oxide.« less

Electrical Characteristics and Preparation of Nanostructured Pb(Zr0.5Ti0.5)O3 Films by Spray Pyrolysis

NASA Astrophysics Data System (ADS)

Koo, Horng-Show; Chen, Mi; Hotta, Yoichi; Kawai, Tomoji

2007-07-01

Nanostructured thin films of Pb(Zr0.5Ti0.5)O3 on Pt (1000 Å)/Ti (100 Å)/SiO2 (2000 Å)/Si substrates are prepared by spray pyrolysis and subsequently rapid thermal annealing. Lead nitrate, zirconium nitrate and titanium isopropoxide are used as starting material with ethylene glycol as solvent. The crystal structure of the as-sprayed films are transformed from the amorphous, pyrochlore and multiple phases of pyrochlore and perovskite to the single phase of perovskite as the annealing temperature is increased up to 500 °C. For the formation of single phase perovskite, excess lead of 10 mol % is required to compensate the loss of lead during the processing of the primitive films. The physical characteristics of the resultant films show the dielectric constant (\\varepsilonr) of 400, remanent polarization (2Pr) of 30.0 μC/cm2 and coercive field (2Ec) of 70.0 kV/cm, respectively.

Multilayer Thermal Barrier Coating (TBC) Architectures Utilizing Rare Earth Doped YSZ and Rare Earth Pyrochlores

NASA Technical Reports Server (NTRS)

Schmitt, Michael P.; Rai, Amarendra K.; Bhattacharya, Rabi; Zhu, Dongming; Wolfe, Douglas E.

2014-01-01

To allow for increased gas turbine efficiencies, new insulating thermal barrier coatings (TBCs) must be developed to protect the underlying metallic components from higher operating temperatures. This work focused on using rare earth doped (Yb and Gd) yttria stabilized zirconia (t' Low-k) and Gd2Zr2O7 pyrochlores (GZO) combined with novel nanolayered and thick layered microstructures to enable operation beyond the 1200 C stability limit of current 7 wt% yttria stabilized zirconia (7YSZ) coatings. It was observed that the layered system can reduce the thermal conductivity by approximately 45 percent with respect to YSZ after 20 hr of testing at 1316 C. The erosion rate of GZO is shown to be an order to magnitude higher than YSZ and t' Low-k, but this can be reduced by almost 57 percent when utilizing a nanolayered structure. Lastly, the thermal instability of the layered system is investigated and thought is given to optimization of layer thickness.

Use of Pom Pons to Illustrate Cubic Crystal Structures.

ERIC Educational Resources Information Center

Cady, Susan G.

1997-01-01

Describes a method that uses olefin pom pons to illustrate cubic crystal structure. Facilitates hands-on examination of different packing arrangements such as hexagonal close-packed and cubic close-packed structures. (JRH)

Stability of the Weyl-semimetal phase on the pyrochlore lattice

NASA Astrophysics Data System (ADS)

Berke, Christoph; Michetti, Paolo; Timm, Carsten

2018-04-01

Motivated by the proposal of a Weyl-semimetal phase in pyrochlore iridates, we consider a Hubbard-type model on the pyrochlore lattice. To shed light on the question as to why such a state has not been observed experimentally, its robustness is analyzed. On the one hand, we study the possible phases when the system is doped. Magnetic frustration favors several phases with magnetic and charge order that do not occur at half filling, including additional Weyl-semimetal states close to quarter filling. On the other hand, we search for density waves that break translational symmetry and destroy the Weyl-semimetal phase close to half filling. The uniform Weyl semimetal is found to be stable, which we attribute to the low density of states close to the Fermi energy.

Field emission of silicon emitter arrays coated with sol-gel (Ba0.65Sr0.35)1-xLaxTiO3 thin films

NASA Astrophysics Data System (ADS)

Lu, H.; Pan, J. S.; Chen, X. F.; Zhu, W. G.

2007-07-01

(Ba0.65Sr0.35)1-xLaxTiO3 (BSLT) thin films with different La concentrations have been deposited on Si field emitter arrays (FEAs) using sol-gel technology for field electron emission applications. The films exhibit the perovskite structure at low La substitution level (x ≤0.5) and the pyrochlore phase at high La concentration (x ≥0.75). The 30-nm-thick BSLT (x =0.25) thin film has higher crystallinity of perovskite structure in the surface region. An x-ray photoelectron spectroscopy study indicates that the oxygen vacancy concentration decreases with La substitution. With respect to the undoped Ba0.65Sr0.35TiO3 thin film, the Fermi level shifts down for the BSLT sample with x =0.1 ascribed to the decreasing oxygen vacancy concentration, and then shifts up for the BSLT sample with x =0.25 attributed to the increasing La substitution level. In highly doped films with an x value over 0.5, it shifts down again associated with the second pyrochlore phase formation. The best enhancement in field emission is found for the BSLT-coated (x =0.25) Si FEAs due to the improved perovskite structure in the surface region and up-moved Fermi level of the coating.

Sintering Characteristics of Multilayered Thermal Barrier Coatings Under Thermal Gradient and Isothermal High Temperature Annealing Conditions

NASA Technical Reports Server (NTRS)

Rai, Amarendra K.; Schmitt, Michael P.; Bhattacharya, Rabi; Zhu, Dongming; Wolfe, Douglas E.

2014-01-01

Pyrochlore oxides have most of the relevant attributes for use as next generation thermal barrier coatings such as phase stability, low sintering kinetics and low thermal conductivity. One of the issues with the pyrochlore oxides is their lower toughness and therefore higher erosion rate compared to the current state-of-the-art TBC material, yttria (6 to 8 wt%) stabilized zirconia (YSZ). In this work, sintering characteristics were investigated for novel multilayered coating consisted of alternating layers of pyrochlore oxide viz Gd2Zr2O7 and t' low k (rare earth oxide doped YSZ). Thermal gradient and isothermal high temperature (1316 C) annealing conditions were used to investigate sintering and cracking in these coatings. The results are then compared with that of relevant monolayered coatings and a baseline YSZ coating.

Self-propagating plus quick pressing synthesis and characterizations of Gd2-xNdxTi1.3Zr0.7O7 (0 ≤ x ≤ 1.4) pyrochlores

NASA Astrophysics Data System (ADS)

He, Zongsheng; Zhang, Kuibao; Peng, Le; Zhao, Wenwen; Xue, Jiali; Zhang, Haibin

2018-06-01

Synroc is recognized as an ideal matrice for the immobilization of high-level radioactive waste (HLW). In this study, the Synroc mineral of pyrochlore was employed as host phase for the immobilization of Nd2O3, which was selected as surrogate of trivalent actinide nuclides. Gd2-xNdxTi1.3Zr0.7O7/Cu composites were rapidly synthesized by self-propagating high-temperature synthesis plus quick pressing (SHS/QP) using CuO as the oxidant and Ti as the reductant. The result shows that the Nd2O3 doped reactions could be ignited as x ≤ 1.4 and Gd2-xNdxTi1.3Zr0.7O7 pyrochlores were successfully prepared with Cu as the secondary phase. The synthesized pyrochlore-based waste form exhibits density of 4.93 g/cm3 and Vickers hardness of 14.90 GPa, as well as promising aqueous durability. The LRGd and LRNd value of the x = 1.4 sample are as low as 3.28 × 10-5 and 2.27 × 10-5 g m-2·d-1 after 42 days leaching.

Effects of sintering temperature on the pyrochlore phase in PZT nanotubes and their transformation to the perovskite phase by coating with PbO multilayers.

PubMed

Han, Jin Kyu; Choi, Yong Chan; Jeon, Do Hyen; Lee, Min Ku; Bu, Sang Don

2014-11-01

We report the phase evolution of Pb(Zr0.52Ti0.48)O3 nanotubes (PZT-NTs), from the pyrochlore to perovskite phase, with an outer diameter of about 420 nm and a wall thickness of about 10 nm. The PZT-NTs were fabricated in pores of porous anodic alumina membrane (PAM) using a spin coating of PZT sol-gel solution and subsequent annealing at 500-700 degrees C in oxygen gas. The pyrochlore phase was found to be formed at 500 degrees C, and also found not to be transformed into the perovskite phase, even though annealing was performed at higher temperatures to 700 degrees C. Elementary distribution analysis of PZT-NTs embedded in PAM reveal that Pb diffusion from nanotubes into pore walls of PAM is one of the main reasons. By employing firstly an additional PbO coating on the pyrochlore nanotubes and then subsequent annealing at 700 degrees C, we have successfully achieved an almost pure perovskite phase in nanotubes. These results suggest that PbO acts as a Pb-compensation agent in the Pb- deficient PZT-NTs. Moreover, our method can be used in the synthesis of all metal-oxide materials, including volatile elements.

Tm-doped TiO2 and Tm2Ti2O7 pyrochlore nanoparticles: enhancing the photocatalytic activity of rutile with a pyrochlore phase.

PubMed

De Los Santos, Desiré M; Navas, Javier; Aguilar, Teresa; Sánchez-Coronilla, Antonio; Fernández-Lorenzo, Concha; Alcántara, Rodrigo; Piñero, Jose Carlos; Blanco, Ginesa; Martín-Calleja, Joaquín

2015-01-01

Tm-doped TiO2 nanoparticles were synthesized using a water-controlled hydrolysis reaction. Analysis was performed in order to determine the influence of the dopant concentration and annealing temperature on the phase, crystallinity, and electronic and optical properties of the resulting material. Various characterization techniques were utilized such as X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy and UV-vis spectroscopy. For the samples annealed at 773 and 973 K, anatase phase TiO2 was obtained, predominantly internally doped with Tm(3+). ICP-AES showed that a doping concentration of up to 5.8 atom % was obtained without reducing the crystallinity of the samples. The presence of Tm(3+) was confirmed by X-ray photoelectron spectroscopy and UV-vis spectroscopy: the incorporation of Tm(3+) was confirmed by the generation of new absorption bands that could be assigned to Tm(3+) transitions. Furthermore, when the samples were annealed at 1173 K, a pyrochlore phase (Tm2Ti2O7) mixed with TiO2 was obtained with a predominant rutile phase. The photodegradation of methylene blue showed that this pyrochlore phase enhanced the photocatalytic activity of the rutile phase.

Relief of frustration in the Heisenberg pyrochlore antiferromagnet Gd2Pt2O7

NASA Astrophysics Data System (ADS)

Hallas, A. M.; Sharma, A. Z.; Cai, Y.; Munsie, T. J.; Wilson, M. N.; Tachibana, M.; Wiebe, C. R.; Luke, G. M.

2016-10-01

The gadolinium pyrochlores Gd2B2O7 are among the best realizations of antiferromagnetically coupled Heisenberg spins on a pyrochlore lattice. We present a magnetic characterization of Gd2Pt2O7 , a unique member of this family. Magnetic susceptibility, heat capacity, and muon spin relaxation measurements show that Gd2Pt2O7 undergoes an antiferromagnetic ordering transition at TN=1.6 K. This transition is strongly first order, as indicated by the sharpness of the heat capacity anomaly, thermal hysteresis in the magnetic susceptibility, and a nondivergent relaxation rate in μ SR . The form of the heat capacity below TN suggests that the ground state is an anisotropic collinear antiferromagnet with an excitation spectrum that is gapped by 0.245(1) meV. The ordering temperature in Gd2Pt2O7,TN=1.6 K, is a substantial 160% increase from other gadolinium pyrochlores, which are all known to order at 1 K or lower. We attribute this enhancement in TN to the B -site cation, platinum. Despite being nonmagnetic, platinum has a filled 5 d t2 g orbital and an empty 5 d eg orbital that can facilitate superexchange. Thus, the magnetic frustration in Gd2Pt2O7 is partially "relieved," thereby promoting magnetic order.

Orange-red emitting Gd2Zr2O7:Sm3+: Structure-property correlation, optical properties and defect spectroscopy

NASA Astrophysics Data System (ADS)

Gupta, Santosh K.; Reghukumar, C.; Sudarshan, K.; Ghosh, P. S.; Pathak, Nimai; Kadam, R. M.

2018-05-01

Local structure analysis of dopant ion, understanding host to dopant energy transfer dynamics and defects characterization in a doped material which plays an important role in the designing a highly efficient opto-electronic material. In this connection a new Sm3+ doped Gd2Zr2O7 pyrochlore material was synthesized using gel-combustion technique and was characterized systematically using X-ray diffraction (XRD), time resolved photoluminescence spectroscopy (TRPLS), positron annihilation lifetime spectroscopy (PALS) and density functional theory (DFT) based ab-initio calculation. Based on DFT site selective energetics calculation and luminescence decay measurement, it was observed that the Sm3+ was distributed at both Gd3+ and Zr4+ site with higher Sm3+ fraction at the Gd3+ site. PALS was used to probe the presence of defects in the phosphor. In this work intense orange-red emission is realized through manipulating the energy transfer from host defect emission (oxygen vacancies) to Sm3+ which allows color emission from green in undoped to orange-red in doped samples. Effect of dopant concentration and annealing temperature was probed using TRPLS and PALS. These all information is highly important for researcher looking to achieve pyrochlore based phosphor materials with high quantum yield.

Morphology, structure, optical, and electrical properties of AgSbO3

NASA Astrophysics Data System (ADS)

Yi, Z. G.; Liu, Y.; Withers, R. L.

2010-07-01

The morphology of defect pyrochlore-type, AgSbO3 microparticle/nanoparticles obtained via solid state reaction evolve from irregular to Fullerene-like polyhedra before finally decomposing into metal-organic framework-5 like particles with increase in sintering temperature. The defect pyrochlore-type AgSbO3 particles are slightly Ag deficient while the valence of the antimony ion is shown to be +5 giving rise to a probable stoichiometry of Ag1-xSbVO3-x/2, with x˜0.01-0.04. A highly structured diffuse intensity distribution observed via electron diffraction is interpreted in terms of correlated displacements of one-dimensional (1D) silver ion chains along ⟨110⟩ directions. A redshifting in the absorption edges in UV-visible absorption spectra is observed for samples prepared at sintering temperatures higher than 1000 °C and attributed to the surface plasma resonance effect associated with small amounts of excess metallic Ag on the Ag1-xSbVO3-x/2 particles. An electrical properties investigation of the silver antimonate samples via dielectric, conductivity, and electric modulus spectroscopy shows a prominent dielectric relaxation associated with grain boundaries. The silver ion conductivity is associated with correlated displacements of 1D silver ion chains along ⟨110⟩ directions.

Compositional Evolution of Pyrochlore-Group Minerals in Carbonatites of the Belaya Zima Pluton, Eastern Sayan

NASA Astrophysics Data System (ADS)

Khromova, E. A.; Doroshkevich, A. G.; Sharygin, V. V.; Izbrodin, L. A.

2017-12-01

Pyrochlore-group minerals are the main concentrators of niobium in carbonatites of the Belaya Zima alkaline pluton. Fluorcalciopyrochlore, kenopyrochlore and hydropyrochlore were identified in chemical composition. Their main characteristics are given: compositional variation, morphology, and zoning. During evolution from early calcite to late ankerite carbonatites, the UO2, TiO2, REE, and Y contents gradually increased. All carbonatite types are suggested to contain initial fluorcalciopyrochlore. However, in calcite-dolomite and ankerite carbonatites, it is partially or completely hydrated due to hydrothermal processes at the late stage of the pluton. This hydration resulted in the appearance of kenopyrochlore and hydropyrochlore due to removal of Ca, Na and F, and input of Ba, H2O, K, Si, Fe, and probably U and REE. At the last stage of the pluton, this hydrated pyrochlore was replaced by Fe-bearing columbite.

Ceramic Top Coats of Plasma-Sprayed Thermal Barrier Coatings: Materials, Processes, and Properties

NASA Astrophysics Data System (ADS)

Bakan, Emine; Vaßen, Robert

2017-08-01

The ceramic top coat has a major influence on the performance of the thermal barrier coating systems (TBCs). Yttria-partially-stabilized zirconia (YSZ) is the top coat material frequently used, and the major deposition processes of the YSZ top coat are atmospheric plasma spraying and electron beam physical vapor deposition. Recently, also new thermal spray processes such as suspension plasma spraying or plasma spray-physical vapor deposition have been intensively investigated for TBC top coat deposition. These new processes and particularly the different coating microstructures that can be deposited with them will be reviewed in this article. Furthermore, the properties and the intrinsic-extrinsic degradation mechanisms of the YSZ will be discussed. Following the TBC deposition processes and standard YSZ material, alternative ceramic materials such as perovskites and hexaaluminates will be summarized, while properties of pyrochlores with regard to their crystal structure will be discussed more in detail. The merits of the pyrochlores such as good CMAS resistance as well as their weaknesses, e.g., low fracture toughness, processability issues, will be outlined.

Symmetry and Structure of Cubic Semiconductor Surfaces.

PubMed

Jenkins, Stephen J

2017-11-07

A systematic stereographic approach to the description of surface symmetry and structure, applied previously to face-centered cubic, body-centered cubic, and hexagonal close-packed metals, is here extended to the surfaces of diamond-structure and zinc-blende-structure semiconductors. A variety of symmetry-structure combinations are categorized and the chiral properties of certain cases emphasized. A general condition for nonpolarity in the surfaces of zincblende materials is also noted.

The role of grain boundaries in hydrogen diffusion in metals at 25 C

NASA Technical Reports Server (NTRS)

Danford, M. D.

1993-01-01

The effect of grain size on hydrogen diffusion at 25 C was examined for 4340 steel (body-centered cubic) and for Inconel 718 (face-centered cubic). It was found that the effect of grain size is important for body-centered cubic structures, but plays a much less important role in face centered cubic structures. Accurate measurements of hydrogen desorption coefficients during hydrogen desorption show that these are not greatly different for both types of structures.

Photophysical and Photocatalytic Properties of BiSnSbO6 under Visible Light Irradiation

PubMed Central

Huang, Panqi

2018-01-01

BiSnSbO6 with strong photocatalytic activity was first fabricated by a high-temperature, solid-state sintering method. The resulting BiSnSbO6 was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), UV-vis diffuse reflectance spectroscopy (DRS) and X-ray photoelectron spectroscopy (XPS). The results showed that BiSnSbO6, with a pyrochlore structure and a cubic crystal system by a space group Fd3m, was well crystallized. The lattice parameter or the band gap of BiSnSbO6 was 10.234594 Å or 2.83 eV. Compared with N-doped TiO2, BiSnSbO6 showed higher photocatalytic activity in the degradation of benzotriazole and rhodamine B. The apparent first-order rate constant for BiSnSbO6 in the degradation of benzotriazole and rhodamine B was 0.0182 min−1 and 0.0147 min−1, respectively. On the basis of the scavenger experiment, during the photocatalytic process, the main active species were arranged in order of increasing photodegradation rate: •OH < •O2− < h+. The removal rate of benzotriazole or rhodamine B was approximately estimated to be 100% with BiSnSbO6 as a photocatalyst after 200 min visible-light irradiation. Plentiful CO2 produced by the experiment indicated that benzotriazole or rhodamine B was continuously mineralized during the photocatalytic process. Finally, the possible photodegradation pathways of benzotriazole and rhodamine B were deduced. PMID:29587420

Novel rattling of K atoms in aluminium-doped defect pyrochlore tungstate

NASA Astrophysics Data System (ADS)

Shoko, Elvis; Kearley, Gordon J.; Peterson, Vanessa K.; Mutka, Hannu; Koza, Michael M.; Yamaura, Jun-ichi; Hiroi, Zenji; Thorogood, Gordon J.

2014-07-01

Rattling dynamics have been identified as fundamental to superconductivity in defect pyrochlore osmates and aluminium vanadium intermetallics, as well as low thermal conductivity in clathrates and filled skutterudites. Combining inelastic neutron scattering (INS) measurements and ab initio molecular dynamics (MD) simulations, we use a new approach to investigate rattling in the Al-doped defect pyrochlore tungstates: AAl0.33W1.67O6 (A = K, Rb, Cs). We find that although all the alkali metals rattle, the rattling of the K atoms is unique, not only among the tungstates but also among the analogous defect osmates, KOs2O6 and RbOs2O6. Detailed analysis of the MD trajectories reveals that two unique features set the K dynamics apart from the rest, namely, (1) quasi one-dimensional local diffusion within a cage, and (2) vibration at a range of frequencies. The local diffusion is driven by strongly anharmonic local potentials around the K atoms exhibiting a double-well structure in the direction of maximum displacement, which is also the direction of local diffusion. On the other hand, vibration at a range of frequencies is a consequence of the strong anisotropy in the local potentials around the K atoms as revealed by directional magnitude spectra. We present evidence to show that it is the smaller size rather than the smaller mass of the K rattler which leads to the unusual dynamics. Finally, we suggest that the occurrence of local diffusion and vibration at a range of frequencies in the dynamics of a single rattler, as found here for the K atoms, may open new possibilities for phonon engineering in thermoelectric materials.

Effects of Doping on Thermal Conductivity of Pyrochlore Oxides for Advanced Thermal Barrier Coatings

NASA Technical Reports Server (NTRS)

Bansal, Narottam P.; Zhu, Dongming; Eslamloo-Grami, Maryam

2006-01-01

Pyrochlore oxides of general composition, A2B2O7, where A is a 3(+) cation (La to Lu) and B is a 4(+) cation (Zr, Hf, Ti, etc.) have high melting point, relatively high coefficient of thermal expansion, and low thermal conductivity which make them suitable for applications as high-temperature thermal barrier coatings. The effect of doping at the A site on the thermal conductivity of a pyrochlore oxide La2Zr2O7, has been investigated. Oxide powders of various compositions La2Zr2O7, La(1.7)Gd(0.3)Zr2O7, La(1.7)Yb(0.3)Zr2O7 and La(1.7)Gd(0.15)Yb(0.15)Zr2O7 were synthesized by the citric acid sol-gel method. These powders were hot pressed into discs and used for thermal conductivity measurements using a steady-state laser heat flux test technique. The rare earth oxide doped pyrochlores La(1.7)Gd(0.3)Zr2O7, La(1.7)Yb(0.3)Zr2O7 and La(1.7)Gd(0.15)Yb(0.15)Zr2O7 had lower thermal conductivity than the un-doped La2Zr2O7. The Gd2O3 and Yb2O3 co-doped composition showed the lowest thermal conductivity.

Electrostatic swelling of bicontinuous cubic lipid phases.

PubMed

Tyler, Arwen I I; Barriga, Hanna M G; Parsons, Edward S; McCarthy, Nicola L C; Ces, Oscar; Law, Robert V; Seddon, John M; Brooks, Nicholas J

2015-04-28

Lipid bicontinuous cubic phases have attracted enormous interest as bio-compatible scaffolds for use in a wide range of applications including membrane protein crystallisation, drug delivery and biosensing. One of the major bottlenecks that has hindered exploitation of these structures is an inability to create targeted highly swollen bicontinuous cubic structures with large and tunable pore sizes. In contrast, cubic structures found in vivo have periodicities approaching the micron scale. We have been able to engineer and control highly swollen bicontinuous cubic phases of spacegroup Im3m containing only lipids by (a) increasing the bilayer stiffness by adding cholesterol and (b) inducing electrostatic repulsion across the water channels by addition of anionic lipids to monoolein. By controlling the composition of the ternary mixtures we have been able to achieve lattice parameters up to 470 Å, which is 5 times that observed in pure monoolein and nearly twice the size of any lipidic cubic phase reported previously. These lattice parameters significantly exceed the predicted maximum swelling for bicontinuous cubic lipid structures, which suggest that thermal fluctuations should destroy such phases for lattice parameters larger than 300 Å.

Temperature and pressure tuneable swollen bicontinuous cubic phases approaching nature's length scales.

PubMed

Barriga, H M G; Tyler, A I I; McCarthy, N L C; Parsons, E S; Ces, O; Law, R V; Seddon, J M; Brooks, N J

2015-01-21

Bicontinuous cubic structures offer enormous potential in applications ranging from protein crystallisation to drug delivery systems and have been observed in cellular membrane structures. One of the current bottlenecks in understanding and exploiting these structures is that cubic scaffolds produced in vitro are considerably smaller in size than those observed in biological systems, differing by almost an order of magnitude in some cases. We have addressed this technological bottleneck and developed a methodology capable of manufacturing highly swollen bicontinuous cubic membranes with length scales approaching those seen in vivo. Crucially, these cubic systems do not require the presence of proteins. We have generated highly swollen Im3m symmetry bicontinuous cubic phases with lattice parameters of up to 480 Å, composed of ternary mixtures of monoolein, cholesterol and negatively charged lipid (DOPS or DOPG) and we have been able to tune their lattice parameters. The swollen cubic phases are highly sensitive to both temperature and pressure; these structural changes are likely to be controlled by a fine balance between lipid headgroup repulsions and lateral pressure in the hydrocarbon chain region.

Effect of Hydrostatic Pressure on the Structural, Electronic and Optical Properties of SnS2 with a Cubic Structure: The DFT Approach

NASA Astrophysics Data System (ADS)

Bakhshayeshi, A.; Taghavi Mendi, R.; Majidiyan Sarmazdeh, M.

2018-02-01

Recently, a cubic structure of polymorphic SnS2 has been synthesized experimentally, which is stable at room temperature. In this paper, we calculated some structural, electronic and optical properties of the cubic SnS2 structure based on the full potential-linearized augmented plane waves method. We also studied the effect of hydrostatic pressure on the physical properties of the cubic SnS2 structure. Structural results show that the compressibility of the cubic SnS2 phase is greater than its trigonal phase and the compressibility decreases with increasing pressure. Investigations of the electronic properties indicate that pressure changes the density of states and the energy band gap increases with increasing pressure. The variation of energy band gap versus pressure is almost linear. We concluded that cubic SnS2 is a semiconductor with an indirect energy band gap, like its trigonal phase. The optical calculations revealed that the dielectric constant decreases with increasing pressure, and the width of the forbidden energy interval increases for electromagnetic wave propagation. Moreover, plasmonic energy and refractive index are changed with increasing pressure.

X-ray scattering study of pyrochlore iridates: Crystal structure, electronic, and magnetic excitations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Clancy, J. P.; Gretarsson, H.; Lee, E. K. H.

2016-07-06

We have investigated the structural, electronic, and magnetic properties of the pyrochlore iridates Eu 2Ir 2O 7 and Pr 2Ir 2O 7 using a combination of resonant elastic x-ray scattering, x-ray powder diffraction, and resonant inelastic x-ray scattering (RIXS). The structural parameters of Eu 2Ir 2O 7 have been examined as a function of temperature and applied pressure, with a particular emphasis on regions of the phase diagram where electronic and magnetic phase transitions have been reported. We find no evidence of crystal symmetry change over the range of temperatures (~6 to 300 K) and pressures (~0.1 to 17 GPa)more » studied. We have also investigated the electronic and magnetic excitations in single-crystal samples of Eu 2Ir 2O 7 and Pr 2Ir 2O 7 using high-resolution Ir L- 3-edge RIXS. In spite of very different ground state properties, we find that these materials exhibit qualitatively similar excitation spectra, with crystal field excitations at ~3-5 eV, spin-orbit excitations at ~ 0.5-1 eV, and broad low-lying excitations below ~0.15 eV. In single-crystal samples of "Eu-rich" Eu 2Ir 2O 7 (found to possess an actual stoichiometry of Eu 2.18Ir 1.82O 7.06) we observe highly damped magnetic excitations at ~45 meV, which display significant momentum dependence. Here, we compare these results with recent dynamical structure factor calculations« less

Evidence for percolation diffusion of cations and reordering in disordered pyrochlore from accelerated molecular dynamics

DOE PAGES

Perriot, Romain; Uberuaga, Blas P.; Zamora, Richard J.; ...

2017-09-20

Diffusion in complex oxides is critical to ionic transport, radiation damage evolution, sintering, and aging. In complex oxides such as pyrochlores, anionic diffusion is dramatically affected by cation disorder. However, little is known about how disorder influences cation transport. Here, we report results from classical and accelerated molecular dynamics simulations of vacancy-mediated cation diffusion in Gd 2Ti 2O 7 pyrochlore, on the microsecond timescale. We find that diffusion is slow at low levels of disorder, while higher disorder allows for fast diffusion, which is then accompanied by antisite annihilation and reordering, and thus a slowing of cation transport. Cation diffusivitymore » is therefore not constant, but decreases as the material reorders. We also show that fast cation diffusion is triggered by the formation of a percolation network of antisites. This is in contrast with observations from other complex oxides and disordered media models, suggesting a fundamentally different relation between disorder and mass transport.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rittman, Dylan R.; Turner, Katlyn M.; Park, Sulgiye

Strain engineering is a promising method for next-generation materials processing techniques. Here, we use mechanical milling and annealing followed by compression in diamond anvil cell to tailor the intrinsic and extrinsic strain in pyrochlore, Dy 2Ti 2O 7 and Dy 2Zr 2O 7. Raman spectroscopy, X-ray pair distribution function analysis, and X-ray diffraction were used to characterize atomic order over short-, medium-, and long-range spatial scales, respectively, under ambient conditions. Raman spectroscopy and X-ray diffraction were further employed to interrogate the material in situ at high pressure. High-pressure behavior is found to depend on the species and concentration of defectsmore » in the sample at ambient conditions. Overall, we show that defects can be engineered to lower the phase transformation onset pressure by ~50% in the ordered pyrochlore Dy 2Ti 2O 7, and lower the phase transformation completion pressure by ~20% in the disordered pyrochlore Dy 2Zr 2O 7. Lastly, these improvements are achieved without significantly sacrificing mechanical integrity, as characterized by bulk modulus.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rittman, Dylan R.; Turner, Katlyn M.; Park, Sulgiye

Strain engineering is a promising method for next-generation materials processing techniques. Here, we use mechanical milling and annealing followed by compression in diamond anvil cell to tailor the intrinsic and extrinsic strain in pyrochlore, Dy 2Ti 2O 7 and Dy 2Zr 2O 7. Raman spectroscopy, X-ray pair distribution function analysis, and X-ray diffraction were used to characterize atomic order over short-, medium-, and long-range spatial scales, respectively, under ambient conditions. Raman spectroscopy and X-ray diffraction were further employed to interrogate the material in situ at high pressure. High-pressure behavior is found to depend on the species and concentration of defectsmore » in the sample at ambient conditions. Overall, we show that defects can be engineered to lower the phase transformation onset pressure by ~50% in the ordered pyrochlore Dy 2Zr 2O 7, and lower the phase transformation completion pressure by ~20% in the disordered pyrochlore Dy 2Zr 2O 7. These improvements are achieved without significantly sacrificing mechanical integrity, as characterized by bulk modulus.« less

Possible observation of highly itinerant quantum magnetic monopoles in the frustrated pyrochlore Yb2Ti2O7

PubMed Central

Tokiwa, Y.; Yamashita, T.; Udagawa, M.; Kittaka, S.; Sakakibara, T; Terazawa, D.; Shimoyama, Y.; Terashima, T.; Yasui, Y.; Shibauchi, T.; Matsuda, Y.

2016-01-01

The low-energy elementary excitations in frustrated quantum magnets have fascinated researchers for decades. In frustrated Ising magnets on a pyrochlore lattice possessing macroscopically degenerate spin-ice ground states, the excitations have been discussed in terms of classical magnetic monopoles, which do not contain quantum fluctuations. Here we report unusual behaviours of magneto-thermal conductivity in the disordered spin-liquid regime of pyrochlore Yb2Ti2O7, which hosts frustrated spin-ice correlations with large quantum fluctuations owing to pseudospin-1/2 of Yb ions. The analysis of the temperature and magnetic field dependencies shows the presence of gapped elementary excitations. We find that the gap energy is largely suppressed from that expected in classical monopoles. Moreover, these excitations propagate a long distance without being scattered, in contrast to the diffusive nature of classical monopoles. These results suggests the emergence of highly itinerant quantum magnetic monopole, which is a heavy quasiparticle that propagates coherently in three-dimensional spin liquids. PMID:26912080

Possible observation of highly itinerant quantum magnetic monopoles in the frustrated pyrochlore Yb2Ti2O7.

PubMed

Tokiwa, Y; Yamashita, T; Udagawa, M; Kittaka, S; Sakakibara, T; Terazawa, D; Shimoyama, Y; Terashima, T; Yasui, Y; Shibauchi, T; Matsuda, Y

2016-02-25

The low-energy elementary excitations in frustrated quantum magnets have fascinated researchers for decades. In frustrated Ising magnets on a pyrochlore lattice possessing macroscopically degenerate spin-ice ground states, the excitations have been discussed in terms of classical magnetic monopoles, which do not contain quantum fluctuations. Here we report unusual behaviours of magneto-thermal conductivity in the disordered spin-liquid regime of pyrochlore Yb2Ti2O7, which hosts frustrated spin-ice correlations with large quantum fluctuations owing to pseudospin-1/2 of Yb ions. The analysis of the temperature and magnetic field dependencies shows the presence of gapped elementary excitations. We find that the gap energy is largely suppressed from that expected in classical monopoles. Moreover, these excitations propagate a long distance without being scattered, in contrast to the diffusive nature of classical monopoles. These results suggests the emergence of highly itinerant quantum magnetic monopole, which is a heavy quasiparticle that propagates coherently in three-dimensional spin liquids.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nguyen, Thi Minh Hien; Sandilands, Luke J.; Sohn, C. H.

5d pyrochlore oxides with all-in-all-out magnetic order are prime candidates for realizing strongly correlated, topological phases of matter. Despite significant effort, a full understanding of all-in-all-out magnetism remains elusive as the associated magnetic excitations have proven difficult to access with conventional techniques. Here we report a Raman spectroscopy study of spin dynamics in the all-in-all-out magnetic state of the 5d pyrochlore Cd 2Os 2O 7. Through a comparison between the two-magnon scattering and spin-wave theory, we confirm the large single ion anisotropy in this material and show that the Dzyaloshinskii–Moriya and exchange interactions play a significant role in the spin-wavemore » dispersions. The Raman data also reveal complex spin–charge–lattice coupling and indicate that the metal–insulator transition in Cd 2Os 2O 7 is Lifshitz-type. In conclusion, our work establishes Raman scattering as a simple and powerful method for exploring the spin dynamics in 5d pyrochlore magnets.« less

Dynamics of Topological Excitations in a Model Quantum Spin Ice

NASA Astrophysics Data System (ADS)

Huang, Chun-Jiong; Deng, Youjin; Wan, Yuan; Meng, Zi Yang

2018-04-01

We study the quantum spin dynamics of a frustrated X X Z model on a pyrochlore lattice by using large-scale quantum Monte Carlo simulation and stochastic analytic continuation. In the low-temperature quantum spin ice regime, we observe signatures of coherent photon and spinon excitations in the dynamic spin structure factor. As the temperature rises to the classical spin ice regime, the photon disappears from the dynamic spin structure factor, whereas the dynamics of the spinon remain coherent in a broad temperature window. Our results provide experimentally relevant, quantitative information for the ongoing pursuit of quantum spin ice materials.

Atomic scale simulations of pyrochlore oxides with a tight-binding variable-charge model: implications for radiation tolerance.

PubMed

Sattonnay, G; Tétot, R

2014-02-05

Atomistic simulations with new interatomic potentials derived from a tight-binding variable-charge model were performed in order to investigate the lattice properties and the defect formation energies in Gd2Ti2O7 and Gd2Zr2O7 pyrochlores. The main objective was to determine the role played by the defect stability on the radiation tolerance of these compounds. Calculations show that the titanate has a more covalent character than the zirconate. Moreover, the properties of oxygen Frenkel pairs, cation antisite defects and cation Frenkel pairs were studied. In Gd2Ti2O7 the cation antisite defect and the Ti-Frenkel pair are not stable: they evolve towards more stable defect configurations during the atomic relaxation process. This phenomenon is driven by a decrease of the Ti coordination number down to five which leads to a local atomic reorganization and strong structural distortions around the defects. These kinds of atomic rearrangements are not observed around defects in Gd2Zr2O7. Therefore, the defect stability in A2B2O7 depends on the ability of B atoms to accommodate high coordination number (higher than six seems impossible for Ti). The accumulation of structural distortions around Ti-defects due to this phenomenon could drive the Gd2Ti2O7 amorphization induced by irradiation.

Thermoelectric properties of p-type cubic and rhombohedral GeTe

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xing, Guangzong; Sun, Jifeng; Li, Yuwei

Here, we investigate the electronic and thermoelectric properties of GeTe in both cubic and rhombohedral phases. We find that cubic GeTe has an electronic structure with a narrow band gap that is unfavorable at high temperature, where the cubic phase is normally stable. However, cubic GeTe has electronic features that may lead to p-type performance superior to the normal rhombohedral phase at lower temperature. This is explained in part by the combination of light and heavy band character that is very effective in obtaining high thermopower and conductivity. In addition, the valence band edge carrier pockets in cubic GeTe possessmore » the largest anisotropy among cubic IV-VI analogs. These effects are stronger than the effect of band convergence in the rhombohedral structure. The results suggest further study of stabilized cubic GeTe as a thermoelectric.« less

Thermoelectric properties of p-type cubic and rhombohedral GeTe

DOE PAGES

Xing, Guangzong; Sun, Jifeng; Li, Yuwei; ...

2018-05-21

Here, we investigate the electronic and thermoelectric properties of GeTe in both cubic and rhombohedral phases. We find that cubic GeTe has an electronic structure with a narrow band gap that is unfavorable at high temperature, where the cubic phase is normally stable. However, cubic GeTe has electronic features that may lead to p-type performance superior to the normal rhombohedral phase at lower temperature. This is explained in part by the combination of light and heavy band character that is very effective in obtaining high thermopower and conductivity. In addition, the valence band edge carrier pockets in cubic GeTe possessmore » the largest anisotropy among cubic IV-VI analogs. These effects are stronger than the effect of band convergence in the rhombohedral structure. The results suggest further study of stabilized cubic GeTe as a thermoelectric.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu, Xiaonan; Lin, Kun; Gao, Qilong

As one class of the most important intermetallic compounds, the binary Laves-phase is well-known for their abundant magnetic properties. Samarium-iron alloy system, SmFe 2, is a prototypical Laves compound that shows strong negative magnetostriction but relatively weak magnetocrystalline anisotropy. SmFe 2 has been identified as a cubic Fdmore » $$ \\overline{3}\\ $$m structure at room temperature, however, the cubic symmetry does not match the spontaneous magnetization along the [111] cubic direction. Here we studied the crystal structure of SmFe 2 by high-resolution synchrotron X-ray powder diffraction and X-ray total scattering methods. SmFe 2 is found to adopt a centrosymmetric trigonal R$$ \\overline{3}\\ $$m structure at room temperature, which transforms to an orthorhombic Imma structure at 200 K. This transition is in agreement with the changes of easy magnetization direction from [111] cubic to [110] cubic direction, and is further evidenced by the inflexion of thermal expansion behavior, the sharp decline of the magnetic susceptibility in the FC-ZFC curve, and the anomaly in the specific heat capacity measurement. The revised structure and phase transformation of SmFe 2 could be useful to understand the magnetostriction and related physical properties of other RM 2-type pseudo-cubic Laves-phase intermetallic compounds.« less

Superhard BC(3) in cubic diamond structure.

PubMed

Zhang, Miao; Liu, Hanyu; Li, Quan; Gao, Bo; Wang, Yanchao; Li, Hongdong; Chen, Changfeng; Ma, Yanming

2015-01-09

We solve the crystal structure of recently synthesized cubic BC(3) using an unbiased swarm structure search, which identifies a highly symmetric BC(3) phase in the cubic diamond structure (d-BC(3)) that contains a distinct B-B bonding network along the body diagonals of a large 64-atom unit cell. Simulated x-ray diffraction and Raman peaks of d-BC(3) are in excellent agreement with experimental data. Calculated stress-strain relations of d-BC(3) demonstrate its intrinsic superhard nature and reveal intriguing sequential bond-breaking modes that produce superior ductility and extended elasticity, which are unique among superhard solids. The present results establish the first boron carbide in the cubic diamond structure with remarkable properties, and these new findings also provide insights for exploring other covalent solids with complex bonding configurations.

Hybrid-exchange density-functional theory study of the electronic structure of MnV2O4 : Exotic orbital ordering in the cubic structure

NASA Astrophysics Data System (ADS)

Wu, Wei

2015-05-01

The electronic structures of cubic and tetragonal MnV2O4 have been studied using hybrid-exchange density-functional theory. The computed electronic structure of the tetragonal phase shows an antiferro-orbital ordering on V sites and a ferrimagnetic ground state (the spins on V and Mn are antialigned). These results are in good agreement with the previous theoretical result obtained from the local-density approximation + U methods [S. Sarkar et al., Phys. Rev. Lett. 102, 216405 (2009), 10.1103/PhysRevLett.102.216405]. Moreover, the electronic structure, especially the projected density of states of the cubic phase, has been predicted with good agreement with the recent soft x-ray spectroscopy experiment. Similar to the tetragonal phase, the spins on V and Mn in the cubic structure favor a ferrimagnetic configuration. Most interesting is that the computed charge densities of the spin-carrying orbitals on V in the cubic phase show an exotic orbital ordering, i.e., a ferro-orbital ordering along [110] but an antiferro-orbital ordering along [1 ¯10 ] .

Orbital Dimer Model for the Spin-Glass State in Y 2 Mo 2 O 7

DOE PAGES

Thygesen, Peter M. M.; Paddison, Joseph A. M.; Zhang, Ronghuan; ...

2017-02-08

The formation of a spin glass generally requires that magnetic exchange interactions are both frustrated and disordered. Consequently, the origin of spin-glass behavior in Y 2Mo 2O 7-in which magnetic Mo 4+ ions occupy a frustrated pyrochlore lattice with minimal compositional disorder-has been a longstanding question. Here, we use neutron and x-ray pair-distribution function (PDF) analysis to develop a disorder model that resolves apparent incompatibilities between previously reported PDF, extended x-rayabsorption fine structure spectroscopy, and NMR studies, and provides a new and physical explanation of the exchange disorder responsible for spin-glass formation. We show that Mo 4+ ions displace accordingmore » to a local "two-in-two-out" rule on each Mo 4 tetrahedron, driven by orbital dimerization of Jahn-Teller active Mo 4+ ions. Long-range orbital order is prevented by the macroscopic degeneracy of dimer coverings permitted by the pyrochlore lattice. Cooperative O 2- displacements yield a distribution of Mo-O-Mo angles, which in turn introduces disorder into magnetic interactions. In conclusion, our study demonstrates experimentally how frustration of atomic displacements can assume the role of compositional disorder in driving a spin-glass transition.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Thygesen, Peter M. M.; Paddison, Joseph A. M.; Zhang, Ronghuan

The formation of a spin glass generally requires that magnetic exchange interactions are both frustrated and disordered. Consequently, the origin of spin-glass behavior in Y 2Mo 2O 7-in which magnetic Mo 4+ ions occupy a frustrated pyrochlore lattice with minimal compositional disorder-has been a longstanding question. Here, we use neutron and x-ray pair-distribution function (PDF) analysis to develop a disorder model that resolves apparent incompatibilities between previously reported PDF, extended x-rayabsorption fine structure spectroscopy, and NMR studies, and provides a new and physical explanation of the exchange disorder responsible for spin-glass formation. We show that Mo 4+ ions displace accordingmore » to a local "two-in-two-out" rule on each Mo 4 tetrahedron, driven by orbital dimerization of Jahn-Teller active Mo 4+ ions. Long-range orbital order is prevented by the macroscopic degeneracy of dimer coverings permitted by the pyrochlore lattice. Cooperative O 2- displacements yield a distribution of Mo-O-Mo angles, which in turn introduces disorder into magnetic interactions. In conclusion, our study demonstrates experimentally how frustration of atomic displacements can assume the role of compositional disorder in driving a spin-glass transition.« less

Orbital Dimer Model for the Spin-Glass State in Y_{2}Mo_{2}O_{7}.

PubMed

Thygesen, Peter M M; Paddison, Joseph A M; Zhang, Ronghuan; Beyer, Kevin A; Chapman, Karena W; Playford, Helen Y; Tucker, Matthew G; Keen, David A; Hayward, Michael A; Goodwin, Andrew L

2017-02-10

The formation of a spin glass generally requires that magnetic exchange interactions are both frustrated and disordered. Consequently, the origin of spin-glass behavior in Y_{2}Mo_{2}O_{7}-in which magnetic Mo^{4+} ions occupy a frustrated pyrochlore lattice with minimal compositional disorder-has been a longstanding question. Here, we use neutron and x-ray pair-distribution function (PDF) analysis to develop a disorder model that resolves apparent incompatibilities between previously reported PDF, extended x-ray-absorption fine structure spectroscopy, and NMR studies, and provides a new and physical explanation of the exchange disorder responsible for spin-glass formation. We show that Mo^{4+} ions displace according to a local "two-in-two-out" rule on each Mo_{4} tetrahedron, driven by orbital dimerization of Jahn-Teller active Mo^{4+} ions. Long-range orbital order is prevented by the macroscopic degeneracy of dimer coverings permitted by the pyrochlore lattice. Cooperative O^{2-} displacements yield a distribution of Mo-O-Mo angles, which in turn introduces disorder into magnetic interactions. Our study demonstrates experimentally how frustration of atomic displacements can assume the role of compositional disorder in driving a spin-glass transition.

Swift heavy ion track formation in Gd2Zr2-xTixO7 pyrochlore: Effect of electronic energy loss

NASA Astrophysics Data System (ADS)

Lang, Maik; Toulemonde, Marcel; Zhang, Jiaming; Zhang, Fuxiang; Tracy, Cameron L.; Lian, Jie; Wang, Zhongwu; Weber, William J.; Severin, Daniel; Bender, Markus; Trautmann, Christina; Ewing, Rodney C.

2014-10-01

The morphology of swift heavy ion tracks in the Gd2Zr2-xTixO7 pyrochlore system has been investigated as a function of the variation in chemical composition and electronic energy loss, dE/dx, over a range of energetic ions: 58Ni, 101Ru, 129Xe, 181Ta, 197Au, 208Pb, and 238U of 11.1 MeV/u specific energy. Bright-field transmission electron microscopy, synchrotron X-ray diffraction, and Raman spectroscopy reveal an increasing degree of amorphization with increasing Ti-content and dE/dx. The size and morphology of individual ion tracks in Gd2Ti2O7 were characterized by high-resolution transmission electron microscopy revealing a core-shell structure with an outer defect-fluorite dominated shell at low dE/dx to predominantly amorphous tracks at high dE/dx. Inelastic thermal-spike calculations have been used together with atomic-scale characterization of ion tracks in Gd2Ti2O7 by high resolution transmission electron microscopy to deduce critical energy densities for the complex core-shell morphologies induced by ions of different dE/dx.

Magnetic dilution and domain selection in the X Y pyrochlore antiferromagnet Er2Ti2O7

NASA Astrophysics Data System (ADS)

Gaudet, J.; Hallas, A. M.; Maharaj, D. D.; Buhariwalla, C. R. C.; Kermarrec, E.; Butch, N. P.; Munsie, T. J. S.; Dabkowska, H. A.; Luke, G. M.; Gaulin, B. D.

2016-08-01

Below TN=1.1 K, the X Y pyrochlore Er2Ti2O7 orders into a k =0 noncollinear, antiferromagnetic structure referred to as the ψ2 state. The magnetic order in Er2Ti2O7 is known to obey conventional three-dimensional (3D) percolation in the presence of magnetic dilution, and in that sense is robust to disorder. Recently, however, two theoretical studies have predicted that the ψ2 structure should be unstable to the formation of a related ψ3 magnetic structure in the presence of magnetic vacancies. To investigate these theories, we have carried out systematic elastic and inelastic neutron scattering studies of three single crystals of Er2 -xYxTi2O7 with x =0 (pure), 0.2 (10 %Y ) and 0.4 (20 % Y ), where magnetic Er3 + is substituted by nonmagnetic Y3 +. We find that the ψ2 ground state of pure Er2Ti2O7 is significantly affected by magnetic dilution. The characteristic domain selection associated with the ψ2 state, and the corresponding energy gap separating ψ2 from ψ3, vanish for Y3 + substitutions between 10 % Y and 20 % Y , far removed from the three-dimensional percolation threshold of ˜60 % Y . The resulting ground state for Er2Ti2O7 with magnetic dilutions from 20 % Y up to the percolation threshold is naturally interpreted as a frozen mosaic of ψ2 and ψ3 domains.

Development of Ceramic Systems for High temperature Coatings

NASA Technical Reports Server (NTRS)

Eslamloo-Grami, Maryame

2003-01-01

Professor Eslamloo-Grami will synthesize ceramic powders of various compositions based on pyrochlore, perovskite, and magnetoplumbite structures by doping with various oxides. Sol-gel and combustion synthesis routes will be used for powder syntheses. The powders will be characterized for particle size, surface area, microstructure, sintering etc. Thermal conductivity of the hot pressed specimens will also be measured at various temperatures. At the end, a project report will be prepared describing in details the experimental methods, results, discussion, and future research.

Structural vs. intrinsic carriers: contrasting effects of cation chemistry and disorder on ionic conductivity in pyrochlores

DOE PAGES

Perriot, Romain; Uberuaga, Blas P.

2015-04-21

We use molecular dynamics simulations to investigate the role of cation disorder on oxygen diffusion in Gd 2Zr 2O 7 (GZO) and Gd 2Ti 2O 7 (GTO) pyrochlores, a class of complex oxides which contain a structural vacancy relative to the basic fluorite structure. The introduction of disorder has distinct effects depending on the chemistry of the material, increasing the mobility of structural carriers by up to four orders of magnitude in GZO. In contrast, in GTO, there is no mobility at zero or low disorder on the ns timescale, but higher disorder liberates the otherwise immobile carriers, allowing diffusionmore » with rates comparable to GZO for the fully disordered material. Here, we show that the cation disorder enhances the diffusivity by both increasing the concentration of mobile structural carriers and their individual mobility. The disorder also influences the diffusion in materials containing intrinsic carriers, such as additional vacancies VO or oxygen interstitials OI. And while in ordered GZO and GTO the contribution of the intrinsic carriers dominates the overall diffusion of oxygen, OI in GZO contributes along with structural carriers, and the total diffusion rate can be calculated by assuming simple additive contributions from the two sources. Although the disorder in the materials with intrinsic defects usually enhances the diffusivity as in the defect-free case, in low concentrations, cation antisites AB or BA, where A = Gd and B = Zr or Ti, can act as traps for fast intrinsic defects. The trapping results in a lowering of the diffusivity, and causes a non-monotonic behavior of the diffusivity with disorder. Conversely, in the case of slow intrinsic defects, the main effect of the disorder is to liberate the structural carriers, resulting in an increase of the diffusivity regardless of the defect trapping.« less

Unified structure theory of icosahedral quasicrystals: Evidence from neutron powder diffraction patterns that AlCrFeMnSi, AlCuLiMg, and TiNiFeSi icosahedral quasicrystals are twins of cubic crystals containing about 820 or 1012 atoms in a primitive unit cube

PubMed Central

Pauling, Linus

1988-01-01

A unified structure theory of icosahedral quasicrystals, combining the twinned-cubic-crystal theory and the Penrose-tiling-six-dimensional-projection theory, is described. Values of the primitive-cubic lattice constant for several quasicrystals are evaluated from x-ray and neutron diffraction data. The fact that the low-angle diffraction maxima can be indexed with cubic unit cells provides additional support for the twinned-cubic-crystal theory of icosahedral quasicrystals. PMID:16593990

X-Ray Diffraction (XRD) Characterization Methods for Sigma=3 Twin Defects in Cubic Semiconductor (100) Wafers

NASA Technical Reports Server (NTRS)

Park, Yeonjoon (Inventor); Kim, Hyun Jung (Inventor); Skuza, Jonathan R. (Inventor); Lee, Kunik (Inventor); Choi, Sang Hyouk (Inventor); King, Glen C. (Inventor)

2017-01-01

An X-ray defraction (XRD) characterization method for sigma=3 twin defects in cubic semiconductor (100) wafers includes a concentration measurement method and a wafer mapping method for any cubic tetrahedral semiconductor wafers including GaAs (100) wafers and Si (100) wafers. The methods use the cubic semiconductor's (004) pole figure in order to detect sigma=3/{111} twin defects. The XRD methods are applicable to any (100) wafers of tetrahedral cubic semiconductors in the diamond structure (Si, Ge, C) and cubic zinc-blend structure (InP, InGaAs, CdTe, ZnSe, and so on) with various growth methods such as Liquid Encapsulated Czochralski (LEC) growth, Molecular Beam Epitaxy (MBE), Organometallic Vapor Phase Epitaxy (OMVPE), Czochralski growth and Metal Organic Chemical Vapor Deposition (MOCVD) growth.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Conn, Charlotte E.; Darmanin, Connie; Sagnella, Sharon M.

The dopamine D2 long (D2L) receptor and bacteriorhodopsin (bR), which are integral membraneproteins, have been incorporated within bicontinuous cubic mesophases formed by the lipids anandamide and H-farnesoyl monoethanolamide, which have been specifically investigated by us for use as in mesocrystallization media. We show that the incorporated membraneprotein affects the structure of the cubic phases with the particular effect observed dependent on the geometry of the underlying cubic phase. The results are complementary to those obtained in Part 1 of this series, where we demonstrated that the structural effects observed depend on the structure of the membraneprotein. Importantly protein concentrations commonlymore » used for crystallization can destroy the cubic phase matrix, particularly where there is a large discrepancy between the hydrophilic and the hydrophobic spans of the membraneprotein, and the hydrophilic and hydrophobic domain sizes of the cubic phase.« less

Niobium (columbium) and tantalum resources of Brazil

USGS Publications Warehouse

White, Max Gregg

1975-01-01

Most of the niobium resources of Brazil occur as pyrochlore in carbonatites within syenitic intrusives of Late Cretaceous to early Tertiary age in western Minas Gerais and southeastern Goils. Minor amounts of it are produced together with tantalum from columbite-tantalite concentrates from pegmatites and placers adjacent to them, in the Sao Joao del Rei district in south-central Minas Gerais. All the niobium and tantalum produced in Brazil is exported. The only pyrochlore mined is from the Barreiro carbonatite deposit near Araxa in Minas Gerais where concentrates and ferroniobium are produced. Exploration work for pyrochlore and other mineral resources are being undertaken on other carbonatites, particularly at Catalao I in southeast Goias and at Tapira and Serra Negra in western Minas Gerais. Annual production and export from the Barreiro deposit are about 8,000 metric tons of pyrochlore concentrate containing about 60 percent Nb205 and about 2,700 metric tons of ferroniobium with 63 percent Nb2O5. The annual production capacity of the Barreiro plant is 18,000 tons of concentrate and 4,000 tons of ferroniobium. Ore reserves of the Barreiro deposit in all categories are 380 million tons with percent Nb2O5. Annual production of tantalite-columbite from the Sao Joao del Rei district, most of which is exported to the United States, is about 290 tons, of which about 79 percent is tantalite and about percent is columbite. Reserves of tantalite-columbite in the Sao Joao del Rei district are about 43,000 tons of proved and 73,000 tons of probable ore.

Superhard BC 3 in cubic diamond structure

DOE PAGES

Zhang, Miao; Liu, Hanyu; Li, Quan; ...

2015-01-06

We solve the crystal structure of recently synthesized cubic BC 3 using an unbiased swarm structure search, which identifies a highly symmetric BC 3 phase in the cubic diamond structure (d–BC3) that contains a distinct B-B bonding network along the body diagonals of a large 64-atom unit cell. Simulated x-ray diffraction and Raman peaks of d–BC 3 are in excellent agreement with experimental data. Calculated stress-strain relations of d–BC 3 demonstrate its intrinsic superhard nature and reveal intriguing sequential bond-breaking modes that produce superior ductility and extended elasticity, which are unique among superhard solids. Here, the present results establish themore » first boron carbide in the cubic diamond structure with remarkable properties, and these new findings also provide insights for exploring other covalent solids with complex bonding configurations.« less

Topological Magnon Bands and Unconventional Superconductivity in Pyrochlore Iridate Thin Films.

PubMed

Laurell, Pontus; Fiete, Gregory A

2017-04-28

We theoretically study the magnetic properties of pyrochlore iridate bilayer and trilayer thin films grown along the [111] direction using a strong coupling approach. We find the ground state magnetic configurations on a mean field level and carry out a spin-wave analysis about them. In the trilayer case the ground state is found to be the all-in-all-out (AIAO) state, whereas the bilayer has a deformed AIAO state. For all parameters of the spin-orbit coupled Hamiltonian we study, the lowest magnon band in the trilayer case has a nonzero Chern number. In the bilayer case we also find a parameter range with nonzero Chern numbers. We calculate the magnon Hall response for both geometries, finding a striking sign change as a function of temperature. Using a slave-boson mean-field theory we study the doping of the trilayer system and discover an unconventional time-reversal symmetry broken d+id superconducting state. Our study complements prior work in the weak coupling limit and suggests that the [111] grown thin film pyrochlore iridates are a promising candidate for topological properties and unconventional orders.

Disorder-induced transition from grain boundary to bulk dominated ionic diffusion in pyrochlores

DOE PAGES

Perriot, Romain; Dholabhai, Pratik P.; Uberuaga, Blas P.

2017-05-04

In this paper, we use molecular dynamics simulations to investigate the role of grain boundaries (GBs) on ionic diffusion in pyrochlores, as a function of the GB type, chemistry of the compound, and level of cation disorder. We observe that the presence of GBs promotes oxygen transport in ordered and low-disordered systems, as the GBs are found to have a higher concentration of mobile carriers with higher mobilities than in the bulk. Thus, in ordered samples, the ionic diffusion is 2D, localized along the grain boundary. When cation disorder is introduced, bulk carriers begin to contribute to the overall diffusion,more » while the GB contribution is only slightly enhanced. In highly disordered samples, the diffusive behavior at the GBs is bulk-like, and the two contributions (bulk vs. GB) can no longer be distinguished. There is thus a transition from 2D/GB dominated oxygen diffusivity to 3D/bulk dominated diffusivity versus disorder in pyrochlores. Finally, these results provide new insights into the possibility of using internal interfaces to enhance ionic conductivity in nanostructured complex oxides.« less

New type of quantum criticality in the pyrochlore iridates

DOE PAGES

Savary, Lucile; Moon, Eun -Gook; Balents, Leon

2014-11-13

Magnetic fluctuations and electrons couple in intriguing ways in the vicinity of zero-temperature phase transitions—quantum critical points—in conducting materials. Quantum criticality is implicated in non-Fermi liquid behavior of diverse materials and in the formation of unconventional superconductors. Here, we uncover an entirely new type of quantum critical point describing the onset of antiferromagnetism in a nodal semimetal engendered by the combination of strong spin-orbit coupling and electron correlations, and which is predicted to occur in the iridium oxide pyrochlores. We formulate and solve a field theory for this quantum critical point by renormalization group techniques and show that electrons andmore » antiferromagnetic fluctuations are strongly coupled and that both these excitations are modified in an essential way. This quantum critical point has many novel features, including strong emergent spatial anisotropy, a vital role for Coulomb interactions, and highly unconventional critical exponents. Our theory motivates and informs experiments on pyrochlore iridates and constitutes a singular realistic example of a nontrivial quantum critical point with gapless fermions in three dimensions.« less

Spin-1/2 Heisenberg antiferromagnet on the pyrochlore lattice: An exact diagonalization study

NASA Astrophysics Data System (ADS)

Chandra, V. Ravi; Sahoo, Jyotisman

2018-04-01

We present exact diagonalization calculations for the spin-1/2 nearest-neighbor antiferromagnet on the pyrochlore lattice. We study a section of the lattice in the [111] direction and analyze the Hamiltonian of the breathing pyrochlore system with two coupling constants J1 and J2 for tetrahedra of different orientations and investigate the evolution of the system from the limit of disconnected tetrahedra (J2=0 ) to a correlated state at J1=J2 . We evaluate the low-energy spectrum, two and four spin correlations, and spin chirality correlations for a system size of up to 36 sites. The model shows a fast decay of spin correlations and we confirm the presence of several singlet excitations below the lowest magnetic excitation. We find chirality correlations near J1=J2 to be small at the length scales available at this system size. Evaluation of dimer-dimer correlations and analysis of the nature of the entanglement of the tetrahedral unit shows that the triplet sector of the tetrahedron contributes significantly to the ground-state entanglement at J1=J2 .

Structured fluids as microreactors for flavor formation by the Maillard reaction.

PubMed

Vauthey, S; Milo, C; Frossard, P; Garti, N; Leser, M E; Watzke, H J

2000-10-01

Thermal reactions of cysteine/furfural and cysteine/ribose mixtures were studied in model systems to gain more insight into the influence of structured fluids such as L(2) microemulsions and cubic phases on the generation of aroma compounds. Formation of 2-furfurylthiol from cysteine/furfural was particularly efficient in L(2) microemulsions and cubic phases compared to aqueous systems. The reaction led to the formation of two new sulfur compounds, which were identified as 2-(2-furyl)thiazolidine and, tentatively, N-(2-mercaptovinyl)-2-(2-furyl)thiazolidine. Similarly, generation of 2-furfurylthiol and 2-methyl-3-furanthiol from cysteine/ribose mixtures was strongly enhanced in structured fluids. The cubic phase was shown to be even more efficient in flavor generation than the L(2) microemulsion. It was denoted "cubic catalyst" or "cubic selective microreactor". The obtained results are interpreted in terms of a surface and curvature control of the reactions defined by the structural properties of the formed surfactant associates.

Optimization of pyrochlore catalysts for the dry reforming of methane

NASA Astrophysics Data System (ADS)

Polo Garzon, Felipe

The conversion of methane into syngas (a mixture of CO and H2), which can be further converted into a variety of chemicals and particularly liquid fuels, is of growing importance given recent increases in methane production world-wide. Furthermore, since using CO2 as the co-feed offers many environmental advantages, dry reforming of methane (DRM, CH4 + CO2 [special character omitted] 2CO + 2H 2) has received renewed attention. In recent years, experimentalists have shown that the Rh-substituted lanthanum zirconate pyrochlore (LRhZ) material is catalytically active for DRM, exhibits long-term thermal stability and resists deactivation; however, previous to this doctoral work, a detailed understanding of the reaction mechanism on pyrochlore catalyst surfaces was still scarce, making it difficult to optimize this material. In this work, initial computational efforts employing density functional theory (DFT) showed the plane (111) of the LRhZ crystal structure as the one catalytically active for DRM. In addition, the primary reaction pathway was identified, along with two rate determining steps (RDSs), the CH2 oxygenation step and the CHO dehydrogenation step, which lie on the CH 4 dehydrogenation/oxygenation path. The mechanistic understanding of DRM over LRhZ was further developed using steady-state isotopic transient kinetic analysis (SSITKA). Reversible adsorption of CO2 on the surface was observed, along with short surface residence times (< 0.6 s) at 650 and 800 °C, and increasing turnover frequencies with temperature. Comparisons between isotopic responses supported the DFT-derived reaction mechanism. Furthermore, isotopic transient kinetics confirmed that all metal atoms (Rh, Zr and La) on the surface are involved in the reaction mechanism, as previously pointed by DFT calculations. A DFT-based microkinetic model that predicts the reaction performance at different conditions was built. The model was validated against experimental data, showing remarkable agreement, which further confirmed the reliability of the DFT data. Computational analysis of one of the RDSs (the CHO dehydrogenation step) suggested Pd as an effective co-dopant to reduce the activation barrier of this step. This bimetallic Rh-Pd-substituted lanthanum zirconate pyrochlore (Rh-Pd-LZ) was synthesized, characterized and tested. The Rh-Pd-LZ catalyst successfully increased conversions at high temperatures while providing H 2 to CO ratios close to unity; thus fostering DRM and inhibiting the competing reaction, the reverse water gas shift reaction (RWGS, CO2 + H2 [special character omitted] CO + H2O). The Rh-Pd-LZ catalyst outperformed the initial catalyst, the LRhZ, at high temperatures.

First determination of volume changes and enthalpies of the high-pressure decomposition reaction of the structure H methane hydrate to the cubic structure I methane hydrate and fluid methane.

PubMed

Ogienko, Andrey G; Tkacz, Marek; Manakov, Andrey Yu; Lipkowski, Janusz

2007-11-08

Pressure-temperature (P-T) conditions of the decomposition reaction of the structure H high-pressure methane hydrate to the cubic structure I methane hydrate and fluid methane were studied with a piston-cylinder apparatus at room temperature. For the first time, volume changes accompanying this reaction were determined. With the use of the Clausius-Clapeyron equation the enthalpies of the decomposition reaction of the structure H high-pressure methane hydrate to the cubic structure I methane hydrate and fluid methane have been calculated.

Weak lensing probe of cubic Galileon model

NASA Astrophysics Data System (ADS)

Dinda, Bikash R.

2018-06-01

The cubic Galileon model containing the lowest non-trivial order action of the full Galileon action can produce the stable late-time cosmic acceleration. This model can have a significant role in the growth of structures. The signatures of the cubic Galileon model in the structure formation can be probed by the weak lensing statistics. Weak lensing convergence statistics is one of the strongest probes to the structure formation and hence it can probe the dark energy or modified theories of gravity models. In this work, we investigate the detectability of the cubic Galileon model from the ΛCDM model or from the canonical quintessence model through the convergence power spectrum and bi-spectrum.

Advances in Understanding of Swift Heavy-Ion Tracks in Complex Ceramics

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lang, Maik; Devanathan, Ram; Toulemonde, Marcel

2015-02-01

Tracks produced by swift heavy ions in ceramics are of interest for fundamental science as well as for applications covering different fields such as nanotechnology or fission-track dating of minerals. In the case of pyrochlores with general formula A2B2O7, the track structure and radiation sensitivity shows a clear dependence on the composition. Ion irradiated Gd2Zr2O7, e.g., retains its crystallinity while amorphous tracks are produced in Gd2Ti2O7. Tracks in Ti-containing compositions have a complex morphology consisting of an amorphous core surrounded by a shell of a disordered, defect-fluorite phase. The size of the amorphous core decreases with decreasing energy loss andmore » with increasing Zr content, while the shell thickness seems to be similar over a wide range of energy loss values. The large data set and the complex track structure has made pyrochlore an interesting model system for a general theoretical description of track formation including thermal spike calculations (providing the spatial and temporal evolution of temperature around the ion trajectory) and molecular dynamics (MD) simulations (describing the response of the atomic system).Recent MD advances consider the sudden temperature increase by inserting data from the thermal spike. The combination allows the reproduction of the core-shell track characteristic and sheds light on the early stages of track formation including recrystallization of the molten material produced by the thermal spike.« less

Existence of Fe{sup 4+} ions in Co{sub 2.25}Fe{sub 0.75}O{sub 4} spinel ferrite confirmed from SXRD and XANES spectroscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Panda, Manas Ranjan, E-mail: manasranjan056@gmail.com; Bhowmik, R. N.; Sinha, A. K.

2015-06-24

The Co{sub 2.25}Fe{sub 0.75}O{sub 4} ferrite composition has been prepared by chemical co-precipitation route. The as-prepared sample after annealing at 900°C in air formed single phase cubic spinel structure. Synchrotron X-ray diffraction and X-ray absorption near edge structure (XANES) measurements were used to study charge states of the cations in octahedral and tetrahedral sites of the cubic spinel structure. Raman spectra indicated normal cubic spinel structure. XANES data suggested the existence of Fe{sup 4+} ions in the spinel structure.

Fermi surfaces of the pyrite-type cubic AuSb2 compared with split Fermi surfaces of the ullmannite-type cubic chiral NiSbS and PdBiSe

NASA Astrophysics Data System (ADS)

Nishimura, K.; Kakihana, M.; Nakamura, A.; Aoki, D.; Harima, H.; Hedo, M.; Nakama, T.; Ōnuki, Y.

2018-05-01

We grew high-quality single crystals of AuSb2 with the pyrite (FeS2)-type cubic structure by the Bridgman method and studied the Fermi surface properties by the de Haas-van Alphen (dHvA) experiment and the full potential LAPW band calculation. The Fermi surfaces of AuSb2 are found to be similar to those of NiSbS and PdBiSe with the ullmannite (NiSbS)-type cubic chiral structure because the crystal structures are similar each other and the number of valence electrons is the same between two different compounds. Note that each Fermi surface splits into two Fermi surfaces in NiSbS and PdBiSe, reflecting the non-centrosymmetric crystal structure.

An Exceptionally Narrow Band-Gap (∼4 eV) Silicate Predicted in the Cubic Perovskite Structure: BaSiO3.

PubMed

Hiramatsu, Hidenori; Yusa, Hitoshi; Igarashi, Ryo; Ohishi, Yasuo; Kamiya, Toshio; Hosono, Hideo

2017-09-05

The electronic structures of 35 A 2+ B 4+ O 3 ternary cubic perovskite oxides, including their hypothetical chemical compositions, were calculated by a hybrid functional method with the expectation that peculiar electronic structures and unique carrier transport properties suitable for semiconductor applications would be hidden in high-symmetry cubic perovskite oxides. We found unique electronic structures of Si-based oxides (A = Mg, Ca, Sr, and Ba, and B = Si). In particular, the unreported cubic BaSiO 3 has a very narrow band gap (4.1 eV) compared with conventional nontransition-metal silicates (e.g., ∼9 eV for SiO 2 and the calculated value of 7.3 eV for orthorhombic BaSiO 3 ) and a small electron effective mass (0.3m 0 , where m 0 is the free electron rest mass). The narrow band gap is ascribed to the nonbonding state of Si 3s and the weakened Madelung potential. The existence of the predicted cubic perovskite structure of BaSiO 3 was experimentally verified by applying a high pressure of 141 GPa. The present finding indicates that it could be possible to develop a new transparent oxide semiconductor of earth abundant silicates if the symmetry of its crystal structure is appropriately chosen. Cubic BaSiO 3 is a candidate for high-performance oxide semiconductors if this phase can be stabilized at room temperature and ambient pressure.

Size-dependent characteristics of ultra-fine oxygen-enriched nanoparticles in austenitic steels

DOE Office of Scientific and Technical Information (OSTI.GOV)

Miao, Yinbin; Mo, Kun; Zhou, Zhangjian

2016-11-01

Here, a coordinated investigation of the elemental composition and morphology of ultra-fine-scale nanoparticles as a function of size within a variety of austenitic oxide dispersion-strengthened (ODS) steels is reported. Atom probe tomography was utilized to evaluate the elemental composition of these nanoparticles. Meanwhile, the crystal structures and orientation relationships were determined by high resolution transmission electron microscopy. The nanoparticles with sufficient size (>4 nm) to maintain a Y2Ti2-xO7-2x stoichiometry were found to have a pyrochlore structure, whereas smaller YxTiyOz nanoparticles lacked a well-defined structure. The size-dependent characteristics of the nanoparticles in austenitic ODS steels differ from those in ferritic/martensitic ODSmore » steels.« less

Atomic-scale evidence for displacive disorder in bismuth zinc niobate pyrochlore.

PubMed

Jia, Chun-Lin; Jin, Lei; Chen, Yue-Hua; Urban, Knut W; Wang, Hong

2018-05-30

Pyrochlores characterized by the chemical formula A 2 B 2 O 7 form an extended class of materials with interesting physical and chemical properties. The compound Bi 1.5 ZnNb 1.5 O 7 is prototypical. Its excellent dielectric properties make it attractive, e.g. for capacitors, tunable microwave devices and electric-energy storage equipment. Bi 1.5 ZnNb 1.5 O 7 shows an intriguing frequency-dispersive dielectric relaxation at 50 K ≤ T ≤ 250 K, which has been studied intensively but is still not fully understood. In this first study on a pyrochlore by atomic-resolution transmission electron microscopy we observe the Bi atoms on A sites since, due to their low nuclear charge, the contribution of Zn atoms to the contrast of these sites is negligible. We find in our [1¯00]and [112] oriented images that the position of the atomic intensity maxima do not coincide with the projected Wyckoff positions of the basic pyrochlore lattice. This supplies atomic-scale evidence for displacive disorder on split A-type sites. The Bi atoms are sessile, only occasionally we observe in time sequences of images jumps between individual split-site positions. The apertaining jump rate of the order of 0.1-1 Hz is by ten orders of magnitude lower than the values derived in the literature from Arrhenius plots of the low-temperature dielectric relaxation data. It is argued that these jumps are radiation induced. Therefore our observations are ruling out a contribution of Bi-atom jumps to low-temperature dielectric A sites-related relaxation. It is suggested that this relaxation is mediated by jumps of Zn atoms. Copyright © 2018. Published by Elsevier B.V.

Electrocatalysts for oxygen electrodes in fuel cells and water electrolyzers for space applications

NASA Technical Reports Server (NTRS)

Prakash, Jai; Tryk, Donald; Yeager, Ernest

1989-01-01

In most instances separate electrocatalysts are needed to promote the reduction of O2 in the fuel cell mode and to generate O2 in the energy storage-water electrolysis mode in aqueous electrochemical systems operating at low and moderate temperatures (T greater than or equal to 200 C). Interesting exceptions are the lead and bismuth ruthenate pyrochlores in alkaline electrolytes. These catalysts on high area carbon supports have high catalytic activity for both O2 reduction and generation (1,2). Rotating ring-disk electrode measurements provide evidence that the O2 reduction proceeds by a parallel four-electron pathway. The ruthenates can also be used as self-supported catalysts to avoid the problems associated with carbon oxidation, but the electrode performance so far achieved in the research at Case Western Reserve University (CWRU) is considerably less. At the potentials involved in the anodic mode the ruthenate pyrochlores have substantial equilibrium solubility in concentrated alkaline electrolyte. This results in the loss of catalyst into the bulk solution and a decline in catalytic activity. Furthermore, the hydrogen generation counter electrode may become contaminated with reduction products from the pyrochlores (lead, ruthenium). A possible approach to this problem is to immobilize the pyrochlore catalyst within an ionic-conducting solid polymer, which would replace the fluid electrolyte within the porous gas diffusion O2 electrode. For bulk alkaline electrolyte, an anion-exchange polymer is needed with a transference number close to unity for the Oh(-) ion. Preliminary short-term measurements with lead ruthenates using a commercially available partially-fluorinated anion-exchange membrane as an overlayer on the porous gas-fed electrode indicate lower anodic polarization and virtually unchanged cathodic polarization.

Manifolds of magnetic ordered states and excitations in the almost Heisenberg pyrochlore antiferromagnet MgCr2O4

NASA Astrophysics Data System (ADS)

Gao, S.; Guratinder, K.; Stuhr, U.; White, J. S.; Mansson, M.; Roessli, B.; Fennell, T.; Tsurkan, V.; Loidl, A.; Ciomaga Hatnean, M.; Balakrishnan, G.; Raymond, S.; Chapon, L.; Garlea, V. O.; Savici, A. T.; Cervellino, A.; Bombardi, A.; Chernyshov, D.; Rüegg, Ch.; Haraldsen, J. T.; Zaharko, O.

2018-04-01

In spinels A Cr2O4(A =Mg, Zn), realization of the classical pyrochlore Heisenberg antiferromagnet model is complicated by a strong spin-lattice coupling: the extensive degeneracy of the ground state is lifted by a magneto-structural transition at TN=12.5 K. We study the resulting low-temperature low-symmetry crystal structure by synchrotron x-ray diffraction. The consistent features of x-ray low-temperature patterns are explained by the tetragonal model of Ehrenberg et al. [Pow. Diff. 17, 230 (2002), 10.1154/1.1479738], while other features depend on sample or cooling protocol. A complex, partially ordered magnetic state is studied by neutron diffraction and spherical neutron polarimetry. Multiple magnetic domains of configuration arms of the propagation vectors k1=(1/2 1/2 0 ) ,k2=(1 0 1/2 ) appear. The ordered moment reaches 1.94(3) μB/Cr3 + for k1 and 2.08(3) μB/Cr3 + for k2, if equal amount of the k1 and k2 phases is assumed. The magnetic arrangements have the dominant components along the [110] and [1 -10 ] diagonals and a smaller c component. We use inelastic neutron scattering to investigate the spin excitations, which comprise a mixture of dispersive spin waves propagating from the magnetic Bragg peaks and resonance modes centered at equal energy steps of 4.5 meV. We interpret these as acoustic and optical spin wave branches, but show that the neutron scattering cross sections of transitions within a unit of two corner-sharing tetrahedra match the observed intensity distribution of the resonances. The distinctive fingerprint of clusterlike excitations in the optical spin wave branches suggests that propagating excitations are localized by the complex crystal structure and magnetic orders.

Cubic and orthorhombic structures of aluminum hydride Al H3 predicted by a first-principles study

NASA Astrophysics Data System (ADS)

Ke, Xuezhi; Kuwabara, Akihide; Tanaka, Isao

2005-05-01

The most stable structure of aluminum hydride AlH3 is believed to be a hexagonal symmetry. However, using the density functional theory, we have identified two more stable structures for the AlH3 with the cubic and orthorhombic symmetries. Based on the quasiharmonic approximation, the cubic and orthorhombic AlH3 are almost degenerate when the zero-point energies are included. The geometric and electronic structures, the phonon, and the thermodynamic properties for the hexagonal, cubic, and orthorhombic AlH3 have been studied by means of density functional theory and direct ab initio force constant approach. The calculated electronic structures, phonon density of states, and thermodynamic functions [including S(T) and H(T)-H(0) ] for the three hydrides are similar. The results show that these three hydrides have negative enthalpies of formation, but positive free energies of formation. This conclusion is the same as that made by Wolverton for the hexagonal AlH3 [Phys. Rev. B 69, 144109 (2004)]. The thermodynamic properties indicate that the orthorhombic and cubic AlH3 should be more difficult to dissociate than the hexagonal AlH3 .

Magnetic cooling close to a quantum phase transition—The case of Er{sub 2}Ti{sub 2}O{sub 7}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wolf, B.; Tutsch, U.; Dörschug, S.

Magnetic cooling, first introduced in the late twenties of last century, has regained considerable interest recently as a cost-efficient and easy-to-handle alternative to {sup 3}He-based refrigeration techniques. Especially, adiabatic demagnetization of paramagnets—the standard materials for magnetic refrigeration—has become indispensable for the present space applications. To match the growing demand for increasing the efficiency in these applications, a new concept for magnetic cooling based on many-body effects around a quantum-critical-point has been introduced and successfully tested [B. Wolf et al., Proc. Natl. Acad. Sci. U.S.A. 108, 6862 (2011)]. By extending this concept to three-dimensional magnetic systems, we present here the magnetothermalmore » response of the cubic pyrochlore material Er{sub 2}Ti{sub 2}O{sub 7} in the vicinity of its B-induced quantum-critical point which is located around 1.5 T. We discuss performance characteristics such as the range of operation, the efficiency, and the hold time. These figures are compared with those of state-of-the-art paramagnetic coolants and with other quantum-critical systems which differ by the dimensionality of the magnetic interactions and the degree of frustration.« less

Comparison of the Supercooled Spin Liquid States in the Pyrochlore Magnets Dy2Ti2O7 and Ho2Ti2O7

NASA Astrophysics Data System (ADS)

Eyal, Anna; Eyvazov, Azar B.; Dusad, Ritika; Munsie, Timothy J. S.; Luke, Graeme M.; Davis, J. C. Séamus

Despite a well-ordered crystal structure and strong magnetic interactions between the Dy or Ho ions, no long-range magnetic order has been detected in the pyrochlore titanates Ho2Ti2O7 and Dy2Ti2O7. The low temperature state in these materials is governed by spin-ice rules. These constrain the Ising like spins in the materials, yet does not result in a global broken symmetry state. To explore the actual magnetic phases, we simultaneously measure the time- and frequency-dependent magnetization dynamics of Dy2Ti2O7 and Ho2Ti2O7 using toroidal, boundary-free magnetization transport techniques. We demonstrate a distinctive behavior of the magnetic susceptibility of both compounds, that is indistinguishable in form from the permittivity of supercooled dipolar liquids. Moreover, we show that the microscopic magnetic relaxation times for both materials increase along a super-Arrhenius trajectory also characteristic of supercooled glass-forming liquids. Both materials therefore exhibit characteristics of a supercooled spin liquid. Strongly-correlated dynamics of loops of spins is suggested as a possible mechanism which could account for these findings. Potential connections to many-body spin localization will also be discussed.

Structural and mechanical properties of lanthanide doped La1/3Nb0.8Ta0.2O3 thin films prepared by sol-gel method

NASA Astrophysics Data System (ADS)

Brunckova, Helena; Medvecky, Lubomir; Kovalcikova, Alexandra; Fides, Martin; Mudra, Erika; Durisin, Juraj; Skvarla, Jiri; Kanuchova, Maria

2017-04-01

Transparent Eu and Nd doped lanthanum niobate tantalate La1/3Nb0.8Ta0.2O3 (LNT) thin films (˜150 nm) were prepared by sol-gel/spin-coating process on Pt/SiO2/Si substrates and annealing at 1100 °C. The x-ray diffraction analysis of films confirmed formation of the perovskite La1/3NbO3 and La1/3TaO3 phases with traces of pyrochlore LaNbO4. Eu and Nd doped LNT films were smoother with roughness 17.1 and 25.4 nm in comparison with LNT (43.3 nm). In all films was observed heterogeneous microstructure with the perovskite spherical and pyrochlore needle-like particles. The mechanical properties of films were characterized for the first time by conventional and continuous stiffness (CSM) nanoindentation. The Eu and Nd doped LNT film modulus (E) and hardness (H) were higher than LNT (˜99.8 and 4.4 GPa) determined by conventional nanoindentation. It was measured the significant effect of substrate on properties of Eu or Nd films (H ˜ 5.9 or 4.9 GPa and E ˜ 107.3 or 104.1 GPa) by CSM nanoindentation.

Fragile singlet ground-state magnetism in the pyrochlore osmates R 2 Os 2 O 7 ( R = Y and Ho)

DOE PAGES

Zhao, Z. Y.; Calder, S.; Aczel, A. A.; ...

2016-04-25

The singlet ground state magnetism in pyrochlore osmates Y 2Os 2O 7 and Ho 2Os 2O 7 is studied by DC and AC susceptibility, specific heat, and neutron powder diffraction measurements. Despite the expected non-magnetic singlet in the strong spin-orbit coupling (SOC) limit for Os 4+ (5d 4), Y 2Os 2O 7 exhibits a spin-glass (SG) ground state below 4 K with weak magnetism, suggesting possible proximity to a quantum phase transition between the non-magnetic state in the strong SOC limit and the magnetic state in the strong superexchange limit. Ho 2Os 2O 7 has the same structural distortion asmore » occurs in Y 2Os 2O 7. However, the Os sublattice in Ho 2Os 2O 7 shows long- range magnetic ordering below 36 K. We find that the sharp difference of the magnetic ground state between Y 2Os 2O 7 and Ho 2Os 2O 7 signals the singlet ground state magnetism in R 2 Os 2 O 7 is fragile and can be disturbed by the weak 4f—5d interactions.« less

Fragile singlet ground-state magnetism in the pyrochlore osmates R 2 Os 2 O 7 ( R = Y and Ho)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhao, Z. Y.; Calder, S.; Aczel, A. A.

The singlet ground state magnetism in pyrochlore osmates Y 2Os 2O 7 and Ho 2Os 2O 7 is studied by DC and AC susceptibility, specific heat, and neutron powder diffraction measurements. Despite the expected non-magnetic singlet in the strong spin-orbit coupling (SOC) limit for Os 4+ (5d 4), Y 2Os 2O 7 exhibits a spin-glass (SG) ground state below 4 K with weak magnetism, suggesting possible proximity to a quantum phase transition between the non-magnetic state in the strong SOC limit and the magnetic state in the strong superexchange limit. Ho 2Os 2O 7 has the same structural distortion asmore » occurs in Y 2Os 2O 7. However, the Os sublattice in Ho 2Os 2O 7 shows long- range magnetic ordering below 36 K. We find that the sharp difference of the magnetic ground state between Y 2Os 2O 7 and Ho 2Os 2O 7 signals the singlet ground state magnetism in R 2 Os 2 O 7 is fragile and can be disturbed by the weak 4f—5d interactions.« less

Crystal field excitations from Yb3 + ions at defective sites in highly stuffed Yb2Ti2O7

NASA Astrophysics Data System (ADS)

Sala, G.; Maharaj, D. D.; Stone, M. B.; Dabkowska, H. A.; Gaulin, B. D.

2018-06-01

The pyrochlore magnet Yb2Ti2O7 has been proposed as a quantum spin ice candidate, a spin liquid state expected to display emergent quantum electrodynamics with gauge photons among its elementary excitations. However, Yb2Ti2O7 's ground state is known to be very sensitive to its precise stoichiometry. Powder samples, produced by solid-state synthesis at relatively low temperatures, tend to be stoichiometric, while single crystals grown from the melt tend to display weak "stuffing" wherein ˜2 % of the Yb3 +, normally at the A site of the A2B2O7 pyrochlore structure, reside as well at the B site. In such samples Yb3 + ions should exist in defective environments at low levels and be subjected to crystalline electric fields very different from those at the stoichiometric A sites. Neutron scattering measurements of Yb3 + in four compositions of Yb2 +xTi2 -xO7 -y show the spectroscopic signatures for these defective Yb3 + ions and explicitly demonstrate that the spin anisotropy of the Yb3 + moment changes from X Y -like for stoichiometric Yb3 + to Ising-like for "stuffed" B site Yb3 + or for A site Yb3 + in the presence of oxygen vacancies.

Nature of the transition between a ferromagnetic metal and a spin-glass insulator in pyrochlore molybdates.

PubMed

Hanasaki, N; Watanabe, K; Ohtsuka, T; Kézsmárki, I; Iguchi, S; Miyasaka, S; Tokura, Y

2007-08-24

The metal-insulator transition has been investigated for pyrochlore molybdates R(2)Mo(2)O(7) with nonmagnetic rare-earth ions R. The dynamical scaling analysis of ac susceptibility reveals that the geometrical frustration causes the atomic spin-glass state. The reentrant spin-glass phase exists below the ferromagnetic transition. The electronic specific heat is enhanced as compared to the band calculation result, perhaps due to the orbital fluctuation in the half-metallic ferromagnetic state. The large specific heat is rather reduced upon the transition, likely because the short-range antiferromagnetic fluctuation shrinks the Fermi surface.

Isotope effect on superconductivity and Raman phonons of Pyrochlore Cd2Re2O7

NASA Astrophysics Data System (ADS)

Razavi, F. S.; Hajialamdari, M.; Reedyk, M.; Kremer, R. K.

2018-06-01

Cd2Re2O7 is the only α-Pyrochlore exhibiting superconductivity with a transition temperature (Tc) of ∼ 1 K. In this study, we present the effect of oxygen isotope (18O) as well as combined 18O and cadmium isotope (116Cd) substitution on the superconductivity and Raman scattering spectrum of Cd2Re2O7. The change of Tc and the energy gap Δ(T) are reported using various techniques including point contact spectroscopy. The shift in Raman phonon frequencies upon isotope substitution will be compared with measurement of the isotope effect on the superconducting transition temperature.

Control of La-doped Pb(Zr,Ti)O3 crystalline orientation and its influence on the properties of ferroelectric random access memory

NASA Astrophysics Data System (ADS)

Wang, Wensheng; Nomura, Kenji; Yamaguchi, Hideshi; Nakamura, Ko; Eshita, Takashi; Ozawa, Soichiro; Takai, Kazuaki; Mihara, Satoru; Hikosaka, Yukinobu; Hamada, Makoto; Kataoka, Yuji

2017-10-01

We investigated the crystallization mechanisms of sputter-deposited La-doped Pb(Zr,Ti)O3 (PLZT) on a Pt/Ti metal stack in the postdeposition annealing (PDA) at 600 °C in O2-mixed Ar ambient. As-deposited amorphous PLZT generally transforms to a perovskite phase over 550 °C through a metastable pyrochlore phase during the PDA. We found that the O2 content of the PDA ambient crucially affects the pyrochlore-perovskite transformation (PPT) speed. While an O2 content much higher than 2% of the PDA ambient suppresses PPT, an O2 content much lower than 2% enhances PPT. An O2 content around of 2% of the PDA suppresses PPT near the surface of PLZT and simultaneously keeps PPT fast in the inner regions of PLZT in the pyrochlore phase because of the O2 diffusion limit from the PLZT surface, eventually resulting in almost only the growth of highly {111} oriented columnar PLZT on Pt, which reveals better electric properties than those obtained by the PDA with the ambient of O2 contents much higher or lower than 2%.

Electrocatalysis for oxygen electrodes in fuel cells and water electrolyzers for space applications

NASA Technical Reports Server (NTRS)

Prakash, Jai; Tryk, Donald; Yeager, Ernest

1989-01-01

In most instances separate electrocatalysts are needed to promote the reduction of O2 in the fuel cell mode and to generate O2 in the energy storage-water electrolysis mode in aqueous electrochemical systems operating at low and moderate temperatures (T greater than or equal to 200 C). Interesting exceptions are the lead and bismuth ruthenate pyrochlores in alkaline electrolytes. These catalysts on high area carbon supports have high catalytic activity for both O2 reduction and generation. Rotating ring-disk electrode measurements provide evidence that the O2 reduction proceeds by a parallel four-electron pathway. The ruthenates can also be used as self-supported catalysts to avoid the problems associated with carbon oxidation, but the electrode performance so far achieved in the research at Case Western Reserve University (CWRU) is considerably less. At the potentials involved in the anodic mode the ruthenate pyrochlores have substantial equilibrium solubility in concentrated alkaline electrolyte. This results in the loss of catalyst into the bulk solution and a decline in catalytic activity. Furthermore, the hydrogen generation counter electrode may become contaminated with reduction products from the pyrochlores (lead, ruthenium).

Electron interactions, spin-orbit coupling, intersite correlations in pyrochlore iridates: a comparison of single-site and cluster calculations

NASA Astrophysics Data System (ADS)

Wang, Runzhi; Go, Ara; Millis, Andrew

Pyrochlore iridates (R2 Ir2O7) are studied using density functional theory plus single-site and cluster dynamical mean-field theory (DFT+DMFT). The calculations include spin-orbit coupling. Significant differences between the single-site and cluster calculations are found. The single-site approximation fails to account for the properties of the paramagnetic insulator phase, in particular predicting a larger gap than found in experiments, while cluster calculations yield gaps consistent with transport data. A ground-state phase diagram is computed. Paramagnetic metal, metallic all-in/all-out (AIAO) and insulating AIAO phases are found. Tilted Weyl cones are observed in the AIAO metallic phase for a relatively wide range of interaction strength. Our paramagnetic calculations predict almost identical behaviors for the Y and Eu compound, conflicting with the strong material dependence reported in experiments. Inclusion of magnetic order restores the material difference. The physical origin of the difference is discussed. The results indicate that intersite effects, most likely of antiferromagnetic origin, play an important role in studying the physics of pyrochlore iridates. This work is supported by DOE-ER046169.

Catalysts for ultrahigh current density oxygen cathodes for space fuel cell applications

NASA Technical Reports Server (NTRS)

Tryk, Donald A.; Yeager, E.

1992-01-01

The objective was to identify promising electrocatalyst/support systems for oxygen cathodes capable of operating at ultrahigh current densities in alkaline fuel cells. Such cells will require operation at relatively high temperatures and O2 pressures. A number of materials were prepared, including Pb-Ru and Pb-Ir pyrochlores, RuO2 and Pt-doped RuO2, lithiated NiO and La-Ni perovskites. Several of these materials were prepared using techniques that had not been previously used to prepare them. Particularly interesting was the use of the alkaline solution technique to prepare Pt-doped and Pb-Ru pyrochlores in high area form. Also interesting was the use of the fusion (melt) method for preparing the Pb-Ru pyrochlore. Several of the materials were also deposited with platinum. Well-crystallized Pb2Ru2O(7-y) was used to fabricate very high performance O2 cathodes with good stability in room temperature KOH. This material was also found to be stable over a useful potential range at approx. 140 C in concentrated KOH. For some of the samples, fabrication of the gas-fed electrodes could not be fully optimized during this project period. Future work may be directed at this problem. Pyrochlores that were not well-crystallized were found to be unstable in alkaline solution. Very good O2 reduction performance and stability were observed with Pb2RuO(7-y) in a carbon-based gas-fed electrode with an anion-conducting membrane placed on the electrolyte side of the electrode. The performance came within a factor of about two of that observed without carbon. High area platinum and gold supported on several conductive metal oxide supports were examined. Only small improvements in O2 reduction performance at room temperature were observed for Pb2Ru2O(7-y) as a support because of the high intrinsic activity of the pyrochlore. In contrast, a large improvement was observed for Li-doped NiO as a support for Pt. Very poor performance was observed for Au deposited on Li-NiO at approx. 150 C. Nearly reversible behavior was observed for the O2/OH(-) couple for Li-doped NiO at approx. 200 C. The temperature dependence for the O2 reduction was examined.

Exotic behavior and crystal structures of calcium under pressure

PubMed Central

Oganov, Artem R.; Ma, Yanming; Xu, Ying; Errea, Ion; Bergara, Aitor; Lyakhov, Andriy O.

2010-01-01

Experimental studies established that calcium undergoes several counterintuitive transitions under pressure: fcc → bcc → simple cubic → Ca-IV → Ca-V, and becomes a good superconductor in the simple cubic and higher-pressure phases. Here, using ab initio evolutionary simulations, we explore the behavior of Ca under pressure and find a number of new phases. Our structural sequence differs from the traditional picture for Ca, but is similar to that for Sr. The β-tin (I41/amd) structure, rather than simple cubic, is predicted to be the theoretical ground state at 0 K and 33–71 GPa. This structure can be represented as a large distortion of the simple cubic structure, just as the higher-pressure phases stable between 71 and 134 GPa. The structure of Ca-V, stable above 134 GPa, is a complex host-guest structure. According to our calculations, the predicted phases are superconductors with Tc increasing under pressure and reaching approximately 20 K at 120 GPa, in good agreement with experiment. PMID:20382865

Highly defective oxides as sinter resistant thermal barrier coating

DOEpatents

Subramanian, Ramesh

2005-08-16

A thermal barrier coating material formed of a highly defective cubic matrix structure having a concentration of a stabilizer sufficiently high that the oxygen vacancies created by the stabilizer interact within the matrix to form multi-vacancies, thereby improving the sintering resistance of the material. The concentration of stabilizer within the cubic matrix structure is greater than that concentration of stabilizer necessary to give the matrix a peak ionic conductivity value. The concentration of stabilizer may be at least 30 wt. %. Embodiments include a cubic matrix of zirconia stabilized by at least 30-50 wt. % yttria, and a cubic matrix of hafnia stabilized by at least 30-50 wt. % gadolinia.

Mechanical and Thermophysical Properties of Cubic Rock-Salt AlN Under High Pressure

NASA Astrophysics Data System (ADS)

Lebga, Noudjoud; Daoud, Salah; Sun, Xiao-Wei; Bioud, Nadhira; Latreche, Abdelhakim

2018-03-01

Density functional theory, density functional perturbation theory, and the Debye model have been used to investigate the structural, elastic, sound velocity, and thermodynamic properties of AlN with cubic rock-salt structure under high pressure, yielding the equilibrium structural parameters, equation of state, and elastic constants of this interesting material. The isotropic shear modulus, Pugh ratio, and Poisson's ratio were also investigated carefully. In addition, the longitudinal, transverse, and average elastic wave velocities, phonon contribution to the thermal conductivity, and interesting thermodynamic properties were predicted and analyzed in detail. The results demonstrate that the behavior of the elastic wave velocities under increasing hydrostatic pressure explains the hardening of the corresponding phonons. Based on the elastic stability criteria under pressure, it is found that AlN with cubic rock-salt structure is mechanically stable, even at pressures up to 100 GPa. Analysis of the Pugh ratio and Poisson's ratio revealed that AlN with cubic rock-salt structure behaves in brittle manner.

Effect of the introduction of oxide ion vacancies into cubic fluorite-type rare earth oxides on the NO decomposition catalysis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Masui, Toshiyuki; Nagai, Ryosuke; Imanaka, Nobuhito, E-mail: imanaka@chem.eng.osaka-u.ac.jp

2014-12-15

Cubic fluorite-type solid solutions based on Pr{sub 6}O{sub 11} and CeO{sub 2} were synthesized and oxide anion vacancies were intentionally introduced into the cubic fluorite-type lattice through the charge compensating mechanism by Mg{sup 2+} and/or Ca{sup 2+} doping into their lattices. The oxide anion vacancies bring about positive effect on NO decomposition catalysis. The reason for the increase in the catalytic activity was attributed to defect fluorite-type structures close to the C-type cubic one, because C-type cubic rare earth oxides, in which one-quarter of the oxygen atoms in the fluorite-type structure are removed, show high NO decomposition activity. In particular,more » the positive effect of the formation of oxide anion vacancies was significant for Pr{sub 6}O{sub 11} and its solid solutions, because the molar volume of Pr{sub 6}O{sub 11} is larger than that of CeO{sub 2}, and Pr{sub 6}O{sub 11} contains Pr{sup 3+} as well as Pr{sup 4+} and thereby a small amount of oxide anion vacancies exist inherently in the lattice. - Graphical abstract: Oxide anion vacancies intentionally introduced into the cubic fluorite-type lattice bring about positive effect on NO decomposition catalysis. - Highlights: • Cubic fluorite-type solid solutions were synthesized. • Oxide anion vacancies were intentionally introduced into the cubic fluorite-type lattice. • The oxide anion vacancies bring about positive effect on NO decomposition catalysis. • The activity was enhanced by making the structure close to the C-type cubic one.« less

Many-Body Theory of Pyrochlore Iridates and Related Materials

NASA Astrophysics Data System (ADS)

Wang, Runzhi

In this thesis we focus on two problems. First we propose a numerical method for generating optimized Wannier functions with desired properties. Second we perform the state of the art density functional plus dynamical mean-field calculations in pyrochlore iridates, to investigate the physics induced by the cooperation of spin-orbit coupling and electron correlation. We begin with the introduction for maximally localized Wannier functions and other related extensions. Then we describe the current research in the field of spin-orbit coupling and its interplay with correlation effects, followed by a brief introduction of the `hot' materials of iridates. Before the end of the introduction, we discuss the numerical methods employed in our work, including the density functional theory; dynamical mean-field theory and its combination with the exact diagonalization impurity solver. Then we propose our approach for constructing an optimized set of Wannier functions, which is a generalization of the functionality of the classic maximal localization method put forward by Marzari and Vanderbilt. Our work is motivated by the requirement of the effective description of the local subspace of the Hamiltonian by the beyond density functional theory methods. In extensions of density functional theory such as dynamical mean-field theory, one may want highly accurate description of particular local orbitals, including correct centers and symmetries; while the basis for the remaining degrees of freedom is unimportant. Therefore, we develop the selectively localized Wannier function approach which allows for a greater localization in the selected subset of Wannier functions and at the same time allows us to fix the centers and ensure the point symmetries. Applications in real materials are presented to demonstrate the power of our approach. Next we move to the investigation of pyrochlore iridates, focussing on the metal-insulator transition and material dependence in these compounds. We perform combined density functional plus dynamical mean-field calculations in Lu2Ir2O7, Y2Ir2O 7, Eu2Ir2O7, with spin-orbit coupling included and both single-site and cluster approximations appiled. A broad range of Weyl metal is predicted as the intervening phase in the metal-insulator transition. By comparing to experiments, we find that the single-site approximation fails to predict the gap values and substantial difference between the Y and Eu-compound, demonstrating the inadequacy of this approximation and indicating the key role played by the intersite effects. Finally, we provide a more accurate description of the vicinity of the metal-insulator and topological transitions implied by density functional plus cluster dynamical mean-field calculations of pyrochlore iridates. We find definitive evidence of the Weyl semimetal phase, the electronic structure of which can be approximately described as ``Weyl rings" with an extremely flat dispersion of one of the Weyl bands. This Weyl semimetal phase is further investigated by the k • p analysis fitting to the numerical results. We find that this unusual structure leads to interesting behavior in the optical conductivity including a Hall effect in the interband component, and to an enhanced susceptibility.

Structures having enhanced biaxial texture and method of fabricating same

DOEpatents

Goyal, Amit; Budai, John D.; Kroeger, Donald M.; Norton, David P.; Specht, Eliot D.; Christen, David K.

1998-01-01

A biaxially textured article includes a rolled and annealed, biaxially textured substrate of a metal having a face-centered cubic, body-centered cubic, or hexagonal close-packed crystalline structure; and an epitaxial superconductor or other device epitaxially deposited thereon.

Structures having enhanced biaxial texture and method of fabricating same

DOEpatents

Goyal, Amit; Budai, John D.; Kroeger, Donald M.; Norton, David P.; Specht, Eliot D.; Christen, David K.

1999-01-01

A biaxially textured article includes a rolled and annealed, biaxially textured substrate of a metal having a face-centered cubic, body-centered cubic, or hexagonal close-packed crystalline structure; and an epitaxial superconductor or other device epitaxially deposited thereon.

Ion irradiation of ternary pyrochlore oxides.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lumpkin, G. R.; Smith, K. L.; Blackford, M. G.

2009-05-01

Polycrystalline synthetic samples of Y{sub 2}Ti{sub 2-x}Sn{sub x}O{sub 7} with x = 0.4, 0.8, 1.2, and 1.6, together with Nd{sub 2}Zr{sub 2}O{sub 7}, Nd{sub 2}Zr{sub 1.2}Ti{sub 0.8}O{sub 7}, and La{sub 1.6}Y{sub 0.4}Hf{sub 2}O{sub 7}, were irradiated in situ in the intermediate voltage electron microscope (IVEM)-Tandem Facility at Argonne National Laboratory using 1.0 MeV Kr ions at temperatures of 50 to 650 K. Determination of the critical amorphization fluence (F{sub c}) as a function of temperature has revealed a dramatic increase in radiation tolerance with increasing Sn content on the pyrochlore B site. Nonlinear least-squares analysis of the fluence-temperature curves gavemore » critical temperatures (T{sub c}) of 666 {+-} 4, 335 {+-} 12, and 251 {+-} 51 K for the Y{sub 2}Ti{sub 2-x}Sn{sub x}O{sub 7} samples with x = 0.4, 0.8, and 1.2, respectively. The sample with x = 1.6 appears to disorder to a defect fluorite structure at a fluence below 1.25 x 10{sup 15} ions cm{sup -2} and remains crystalline to 5 x 10{sup 15} ions cm{sup -2} at 50 K. Additionally, the critical fluence-temperature response curves were determined for Nd{sub 2}Zr{sub 1.2}Ti{sub 0.8}O{sub 7} and La{sub 1.6}Y{sub 0.4}Hf{sub 2}O{sub 7}, and we obtained T{sub c} values of 685 {+-} 53 K and 473 {+-} 52 K, respectively, for these pyrochlores. Nd{sub 2}Zr{sub 2}O{sub 7} did not become amorphous after a fluence of 2.5 x 10{sup 15} ions cm{sup -2} at 50 K, but there is evidence that it may amorphize at a higher fluence, with an estimated T{sub c} of 135 K. The observed T{sub c} results are discussed with respect to the predicted T{sub c} values based upon a previously published empirical model (Lumpkin, G. R.; Pruneda, M.; Rios, S.; Smith, K. L.; Trachenko, K.; Whittle, K. R.; Zaluzec, N. J. J. Solid State Chem. 2007, 180, 1512). In the Y{sub 2}Ti{sub 2-x}Sn{sub x}O{sub 7} pyrochlores, T{sub c} appears to be linear with respect to composition, and is linear with respect to r{sub A}/r{sub B} and x(48f) for all samples investigated herein.« less

Structures having enhanced biaxial texture and method of fabricating same

DOEpatents

Goyal, A.; Budai, J.D.; Kroeger, D.M.; Norton, D.P.; Specht, E.D.; Christen, D.K.

1999-04-27

A biaxially textured article includes a rolled and annealed, biaxially textured substrate of a metal having a face-centered cubic, body-centered cubic, or hexagonal close-packed crystalline structure; and an epitaxial superconductor or other device epitaxially deposited thereon. 11 figs.

An Easily Constructed Model of a Coordination Polyhedron that Represents the Cubic Closest-Packed Structure.

ERIC Educational Resources Information Center

Yamana, Shukichi

1987-01-01

Illustrates the 18 steps to the development of a model of a coordination polyhedron that represents the cubic closest-packed structure. Uses a sealed, empty envelope in developing the model in teaching about stereochemistry. (TW)

Structures having enhanced biaxial texture and method of fabricating same

DOEpatents

Goyal, A.; Budai, J.D.; Kroeger, D.M.; Norton, D.P.; Specht, E.D.; Christen, D.K.

1998-04-21

A biaxially textured article includes a rolled and annealed, biaxially textured substrate of a metal having a face-centered cubic, body-centered cubic, or hexagonal close-packed crystalline structure; and an epitaxial superconductor or other device epitaxially deposited thereon. 11 figs.

Structures having enhanced biaxial texture and method of fabricating same

DOEpatents

Goyal, A.; Budai, J.D.; Kroeger, D.M.; Norton, D.P.; Specht, E.D.; Christen, D.K.

1998-04-14

A biaxially textured article includes a rolled and annealed, biaxially textured substrate of a metal having a face-centered cubic, body-centered cubic, or hexagonal close-packed crystalline structure; and an epitaxial superconductor or other device epitaxially deposited thereon. 11 figs.

Epitaxial Growth of Cubic Crystalline Semiconductor Alloys on Basal Plane of Trigonal or Hexagonal Crystal

NASA Technical Reports Server (NTRS)

Park, Yeonjoon (Inventor); Choi, Sang H. (Inventor); King, Glen C. (Inventor)

2011-01-01

Hetero-epitaxial semiconductor materials comprising cubic crystalline semiconductor alloys grown on the basal plane of trigonal and hexagonal substrates, in which misfit dislocations are reduced by approximate lattice matching of the cubic crystal structure to underlying trigonal or hexagonal substrate structure, enabling the development of alloyed semiconductor layers of greater thickness, resulting in a new class of semiconductor materials and corresponding devices, including improved hetero-bipolar and high-electron mobility transistors, and high-mobility thermoelectric devices.

Influence of Sn doping in BaSnxTi1-xO3 ceramics on microstructural and dielectric properties

NASA Astrophysics Data System (ADS)

Ansari, Mohd. Azaj; Sreenivas, K.

2018-05-01

BaSnxTi1-x O3 solid solutions with varying Sn content (x = 0.00, 0.05, 0.15, 0.25) prepared by solid state reaction method have been studied for their structural and dielectric properties. X-ray diffraction and Raman spectroscopic analysis show composition induced modifications in the crystallographic structure, and with increasing Sn content the structure changes from tetragonal to cubic structure. The tetragonal distortion decreases with increasing Sn, and the structure becomes purely cubic for x =0.25. Changes in the structure are reflected in the temperature dependent dielectric properties. For increasing Sn content the peak dielectric constant is found to increase and the phase transition temperature (Tc) decreases to lower temperature. The purely cubic structure with x=0.25 shows a diffused phased transition.

Ab Initio High Pressure and Temperature Investigation on Cubic PbMoO3 Perovskite

NASA Astrophysics Data System (ADS)

Dar, Sajad Ahmad; Srivastava, Vipul; Sakalle, Umesh Kumar

2017-12-01

A combined high pressure and temperature investigation on recently reported cubic perovskite PbMoO3 have been performed within the most accurate density functional theory (DFT). The structure was found stable in cubic paramagnetic phase. The DFT calculated analytical and experimental lattice constant were found in good agreement. The analytical tolerance factor as well as the elastic properties further verifies the cubic stability for PbMoO3. The spin polarized electronic band structure and density of states presented metallic nature with symmetry in up and down states. The insignificant magnetic moment also confirms the paramagnetic nature for the compound. The high pressure elastic and mechanical study up to 35 GPa reveal the structural stability of the material in this pressure range. The compound was found to establish a ductile nature. The electrical conductivity obtained from the band structure results show a decreasing trend with increasing temperature. The temperature dependence of thermodynamic parameters such as specific heat ( C v), thermal expansion ( α) has also been evaluated.

Graded Index Silicon Geranium on Lattice Matched Silicon Geranium Semiconductor Alloy

NASA Technical Reports Server (NTRS)

Park, Yeonjoon (Inventor); Choi, Sang H. (Inventor); King, Glen C. (Inventor); Elliott, James R., Jr. (Inventor); Stoakley, Diane M. (Inventor)

2009-01-01

A lattice matched silicon germanium (SiGe) semiconductive alloy is formed when a {111} crystal plane of a cubic diamond structure SiGe is grown on the {0001} C-plane of a single crystalline Al2O3 substrate such that a <110> orientation of the cubic diamond structure SiGe is aligned with a <1,0,-1,0> orientation of the {0001} C-plane. A lattice match between the substrate and the SiGe is achieved by using a SiGe composition that is 0.7223 atomic percent silicon and 0.2777 atomic percent germanium. A layer of Si(1-x), ,Ge(x) is formed on the cubic diamond structure SiGe. The value of X (i) defines an atomic percent of germanium satisfying 0.2277

First-principles investigation on the mechanism of photocatalytic properties for cubic and orthorhombic KNbO3

NASA Astrophysics Data System (ADS)

Xu, Yong-Qiang; Wu, Shao-Yi; Ding, Chang-Chun; Wu, Li-Na; Zhang, Gao-Jun

2018-03-01

The geometric structures, band structures, density of states and optical absorption spectra are studied for cubic and orthorhombic KNbO3 (C- and O-KNO) crystals by using first-principles calculations. Based on the above calculation results, the mechanisms of photocatalytic properties for both crystals are further theoretically investigated to deepen the understandings of their photocatalytic activity from the electronic level. Calculations for the effective masses of electron and hole are carried out to make comparison in photocatalytic performance between cubic and orthorhombic phases. Optical absorption in cubic phase is found to be stronger than that in orthorhombic phase. C-KNO has smaller electron effective mass, higher mobility of photogenerated electrons, lower electron-hole recombination rate and better light absorption capacity than O-KNO. So, the photocatalytic activity of cubic phase can be higher than orthorhombic one. The present work may be beneficial to explore the series of perovskite photocatalysts.

Cubic phase stabilization in nanoparticles of hafnia-zirconia oxides: Particle-size and annealing environment effects

NASA Astrophysics Data System (ADS)

Lu, Chih-Hsin; Raitano, Joan M.; Khalid, Syed; Zhang, Lihua; Chan, Siu-Wai

2008-06-01

Amorphous hafnia (HfO2-y), zirconia (ZrO2-y), and hafnia-zirconia (xHfO2-y-(1-x)(ZrO2-y)) nanoparticles were prepared by combining aqueous solutions of hexamethylenetetramine (HMT) with hafnium dichloride oxide (HfOCl2ṡ8H2O), zirconium dichloride oxide (ZrOCl2ṡ8H2O), or a mixture of these two salts at room temperature. For pure hafnia, transmission electron microscopy showed that the lower cation concentration (0.01M) resulted in the precipitation of smaller amorphous nanoparticles relative to higher concentrations (0.015M-0.04M). Consequently, the lower concentration preparation route coupled with a reducing environment (H2:N2=9:91) during annealing at temperatures between 650 and 850°C allowed for nanoparticles with a cubic structure to be prepared as determined by x-ray diffraction. The structurally cubic hafnia nanoparticles were 6nm or less in diameter and equiaxed. Using the same method (0.01M total metal cation concentration and reducing environment during annealing), nanoparticles of cubic structure were prepared across the entire hafnia-zirconia compositional spectrum, with a critical particle size for the cubic structure of about 6nm. Nanoparticles of tetragonal and monoclinic structure were prepared by increasing the annealing temperature and/or using a less reducing environment. The unique role of HMT in sample preparation is discussed as well.

The Crystal Structure of Micro- and Nanopowders of ZnS Studied by EPR of Mn2+ and XRD.

PubMed

Nosenko, Valentyna; Vorona, Igor; Grachev, Valentyn; Ishchenko, Stanislav; Baran, Nikolai; Becherikov, Yurii; Zhuk, Anton; Polishchuk, Yuliya; Kladko, Vasyl; Selishchev, Alexander

2016-12-01

The crystal structure of micro- and nanopowders of ZnS doped with different impurities was analyzed by the electron paramagnetic resonance (EPR) of Mn 2+ and XRD methods. The powders of ZnS:Cu, ZnS:Mn, ZnS:Co, and ZnS:Eu with the particle sizes of 5-7 μm, 50-200 nm, 7-10 μm, and 5-7 nm, respectively, were studied. Manganese was incorporated in the crystal lattice of all the samples as uncontrolled impurity or by doping. The Mn 2+ ions were used as EPR structural probes. It is found that the ZnS:Cu has the cubic structure, the ZnS:Mn has the hexagonal structure with a rhombic distortion, the ZnS:Co is the mixture of the cubic and hexagonal phases in the ratio of 1:10, and the ZnS:Eu has the cubic structure and a distorted cubic structure with stacking defects in the ratio 3:1. The EPR technique is shown to be a powerful tool in the determination of the crystal structure for mixed-polytype ZnS powders and powders with small nanoparticles. It allows observation of the stacking defects, which is revealed in the XRD spectra.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sandoval-Paz, M.G., E-mail: myrnasandoval@udec.cl; Rodríguez, C.A.; Porcile-Saavedra, P.F.

Copper (I) selenide thin films with orthorhombic and cubic structure were deposited on glass substrates by using the chemical bath deposition technique. The effects of the solution pH on the films growth and subsequently the structural, optical and electrical properties of the films were studied. Films with orthorhombic structure were obtained from baths wherein both metal complex and hydroxide coexist; while films with cubic structure were obtained from baths where the metal hydroxide there is no present. The structural modifications are accompanied by changes in bandgap energy, morphology and electrical resistivity of the films. - Graphical abstract: “Study of themore » crystallographic phase change on copper (I) selenide thin films prepared through chemical bath deposition by varying the pH of the solution” by M. G. Sandoval-Paz, C. A. Rodríguez, P. F. Porcile-Saavedra, C. Trejo-Cruz. Display Omitted - Highlights: • Copper (I) selenide thin films were obtained by chemical bath deposition. • Orthorhombic to cubic phase change was induced by varying the reaction solution pH. • Orthorhombic phase is obtained mainly from a hydroxides cluster mechanism. • Cubic phase is obtained mainly from an ion by ion mechanism. • Structural, optical and electrical properties are presented as a function of pH.« less

Synthesis of ordered mesoporous carbon monoliths with bicontinuous cubic pore structure of Ia3d symmetry.

PubMed

Yang, Haifeng; Shi, Qihui; Liu, Xiaoying; Xie, Songhai; Jiang, Decheng; Zhang, Fuqiang; Yu, Chengzhong; Tu, Bo; Zhao, Dongyuan

2002-12-07

Large-diameter-sized mesoporous carbon monoliths with bicontinuous cubic structure of Ia3d symmetry have been synthesized by using mesoporous silica monoliths as hard templates; such carbon monoliths show potential application of advanced electrodes and electrochemical double layer capacitors.

Crystal structure of hexagonal MnAl(4).

PubMed

Pauling, L

1987-06-01

A structure is proposed for the hexagonal form of MnAl(4), with a(H) = 28.4 A and c(H) = 12.43 A, on the basis of a high-resolution electron micrograph and comparison with crystals of known structures. The proposed structure involves seven 104-atom complexes of 20 Friauf polyhedra, sharing some atoms with one another. It is closely related to the 23.36-A cubic structure of MnAl(4) and to the 14.19-A cubic structure of Mg(32)(Al,Zn)(49).

Long-range dynamical magnetic order and spin tunneling in the cooperative paramagnetic states of the pyrochlore analogous spinel antiferromagnets CdYb2X4 (X =S or Se)

NASA Astrophysics Data System (ADS)

Dalmas de Réotier, P.; Marin, C.; Yaouanc, A.; Ritter, C.; Maisuradze, A.; Roessli, B.; Bertin, A.; Baker, P. J.; Amato, A.

2017-10-01

Magnetic systems with spins sitting on a lattice of corner sharing regular tetrahedra have been particularly prolific for the discovery of new magnetic states for the last two decades. The pyrochlore compounds have offered the playground for these studies, while little attention has been comparatively devoted to other compounds where the rare earth R occupies the same sublattice, e.g., the spinel chalcogenides Cd R2X4 (X =S or Se ). Here, we report measurements performed on powder samples of this series with R =Yb using specific heat, magnetic susceptibility, neutron diffraction, and muon-spin-relaxation measurements. The two compounds are found to be magnetically similar. They long-range order into structures described by the Γ5 irreducible representation. The magnitude of the magnetic moment at low temperature is 0.77 (1) and 0.62 (1) μB for X =S and Se , respectively. Persistent spin dynamics is present in the ordered states. The spontaneous field at the muon site is anomalously small, suggesting magnetic moment fragmentation. A double spin-flip tunneling relaxation mechanism is suggested in the cooperative paramagnetic state up to 10 K. The magnetic space groups into which magnetic moments of systems of corner-sharing regular tetrahedra order are provided for a number of insulating compounds characterized by null propagation wave vectors.

Study of crystal-field interaction in magnetic frustrated lead pyrochlore Gd2Pb2O7

NASA Astrophysics Data System (ADS)

Swarnakar, D.; Jana, Y. M.

2018-05-01

A fine tuning between the crystal field and the molecular field to adopt unique ground state in frustrated magnetic R2M2O7 pyrochlores structures is made by the variation of chemical pressure at R-site caused by substitution of nonmagnetic cation of M-site. Existence of larger cation at M-site increases the lattice parameter or nearest-neighbor bond distance between magnetic R-spins, and causes subtle changes to the local oxygen environment surrounding each R-ion, thereby reduces the chemical pressure at R-site which leads to a dramatic change in the crystal-field and molecular field. To investigate the effect of chemical pressure, the experimental results of powder magnetic susceptibility and isothermal magnetization of the frustrated compound Gd2Pb2O7 containing largest cation, e.g. lead (Pb), at M4+-sites were simulated and analyzed employing a D3d crystal-field (CF) and anisotropic molecular field at R-sites in the self- consistent mean-field approach. The second-ordered axial parameter B20 and total CF splitting of the ground multiplet Gd-ion in Gd2Pb2O7 are 477 cm-1 and 4.8 cm-1 respectively which are the lowest among their isomorphous counterparts, implying reduced effect of the crystal-field at Gd site in Gd2Pb2O7.

Calculations of and evidence for chain packing stress in inverse lyotropic bicontinuous cubic phases.

PubMed

Shearman, Gemma C; Khoo, Bee J; Motherwell, Mary-Lynn; Brakke, Kenneth A; Ces, Oscar; Conn, Charlotte E; Seddon, John M; Templer, Richard H

2007-06-19

Inverse bicontinuous cubic lyotropic phases are a complex solution to the dilemma faced by all self-assembled water-amphiphile systems: how to satisfy the incompatible requirements for uniform interfacial curvature and uniform molecular packing. The solution reached in this case is for the water-amphiphile interfaces to deform hyperbolically onto triply periodic minimal surfaces. We have previously suggested that although the molecular packing in these structures is rather uniform the relative phase behavior of the gyroid, double diamond, and primitive inverse bicontinuous cubic phases can be understood in terms of subtle differences in packing frustration. In this work, we have calculated the packing frustration for these cubics under the constraint that their interfaces have constant mean curvature. We find that the relative packing stress does indeed differ between phases. The gyroid cubic has the least packing stress, and at low water volume fraction, the primitive cubic has the greatest packing stress. However, at very high water volume fraction, the double diamond cubic becomes the structure with the greatest packing stress. We have tested the model in two ways. For a system with a double diamond cubic phase in excess water, the addition of a hydrophobe may release packing frustration and preferentially stabilize the primitive cubic, since this has previously been shown to have lower curvature elastic energy. We have confirmed this prediction by adding the long chain alkane tricosane to 1-monoolein in excess water. The model also predicts that if one were able to hydrate the double diamond cubic to high water volume fractions, one should destabilize the phase with respect to the primitive cubic. We have found that such highly swollen metastable bicontinuous cubic phases can be formed within onion vesicles. Data from monoelaidin in excess water display a well-defined transition, with the primitive cubic appearing above a water volume fraction of 0.75. Both of these results lend support to the proposition that differences in the packing frustration between inverse bicontinuous cubic phases play a pivotal role in their relative phase stability.

From Quasicrystals to Crystals with Interpenetrating Icosahedra in Ca–Au–Al: In Situ Variable-Temperature Transformation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pham, Joyce; Meng, Fanqiang; Lynn, Matthew J.

The irreversible transformation from an icosahedral quasicrystal (i-QC) CaAu 4.39Al 1.61 to its cubic 2/1 crystalline approximant (CA) Ca 13Au 56.31(3)Al 21.69 (CaAu 4.33(1)Al1.67, Pamore » $$\\bar{3}$$ (No. 205); Pearson symbol: cP728; a = 23.8934(4)), starting at ~570 °C and complete by ~650 °C, is discovered from in situ, high-energy, variable-temperature powder X-ray diffraction (PXRD), thereby providing direct experimental evidence for the relationship between QCs and their associated CAs. The new cubic phase crystallizes in a Tsai-type approximant structure under the broader classification of polar intermetallic compounds, in which atoms of different electronegativities, viz., electronegative Au + Al vs electropositive Ca, are arranged in concentric shells. From a structural chemical perspective, the outermost shell of this cubic approximant may be described as interpenetrating and edge-sharing icosahedra, a perspective that is obtained by splitting the traditional structural description of this shell as a 92-atom rhombic triacontahedron into an 80-vertex cage of primarily Au [Au 59.86(2)Al 17.14⟂ 3.00] and an icosahedral shell of only Al [Al 10.5⟂ 1.5]. Following the proposal that the cubic 2/1 CA approximates the structure of the i-QC and on the basis of the observed transformation, an atomic site analysis of the 2/1 CA, which shows a preference to maximize the number of heteroatomic Au–Al nearest neighbor contacts over homoatomic Al–Al contacts, implies a similar outcome for the i-QC structure. Analysis of the most intense reflections in the diffraction pattern of the cubic 2/1 CA that changed during the phase transformation shows correlations with icosahedral symmetry, and the stability of this cubic phase is assessed using valence electron counts. Finally, according to electronic structure calculations, a cubic 1/1 CA, “Ca 24Au 88Al 64” (CaAu 3.67Al 2.67) is proposed.« less

From Quasicrystals to Crystals with Interpenetrating Icosahedra in Ca–Au–Al: In Situ Variable-Temperature Transformation

DOE PAGES

Pham, Joyce; Meng, Fanqiang; Lynn, Matthew J.; ...

2017-12-29

The irreversible transformation from an icosahedral quasicrystal (i-QC) CaAu 4.39Al 1.61 to its cubic 2/1 crystalline approximant (CA) Ca 13Au 56.31(3)Al 21.69 (CaAu 4.33(1)Al1.67, Pamore » $$\\bar{3}$$ (No. 205); Pearson symbol: cP728; a = 23.8934(4)), starting at ~570 °C and complete by ~650 °C, is discovered from in situ, high-energy, variable-temperature powder X-ray diffraction (PXRD), thereby providing direct experimental evidence for the relationship between QCs and their associated CAs. The new cubic phase crystallizes in a Tsai-type approximant structure under the broader classification of polar intermetallic compounds, in which atoms of different electronegativities, viz., electronegative Au + Al vs electropositive Ca, are arranged in concentric shells. From a structural chemical perspective, the outermost shell of this cubic approximant may be described as interpenetrating and edge-sharing icosahedra, a perspective that is obtained by splitting the traditional structural description of this shell as a 92-atom rhombic triacontahedron into an 80-vertex cage of primarily Au [Au 59.86(2)Al 17.14⟂ 3.00] and an icosahedral shell of only Al [Al 10.5⟂ 1.5]. Following the proposal that the cubic 2/1 CA approximates the structure of the i-QC and on the basis of the observed transformation, an atomic site analysis of the 2/1 CA, which shows a preference to maximize the number of heteroatomic Au–Al nearest neighbor contacts over homoatomic Al–Al contacts, implies a similar outcome for the i-QC structure. Analysis of the most intense reflections in the diffraction pattern of the cubic 2/1 CA that changed during the phase transformation shows correlations with icosahedral symmetry, and the stability of this cubic phase is assessed using valence electron counts. Finally, according to electronic structure calculations, a cubic 1/1 CA, “Ca 24Au 88Al 64” (CaAu 3.67Al 2.67) is proposed.« less

Order by disorder and gaugelike degeneracy in a quantum pyrochlore antiferromagnet.

PubMed

Henley, Christopher L

2006-02-03

The (three-dimensional) pyrochlore lattice antiferromagnet with Heisenberg spins of large spin length S is a highly frustrated model with a macroscopic degeneracy of classical ground states. The zero-point energy of (harmonic-order) spin-wave fluctuations distinguishes a subset of these states. I derive an approximate but illuminating effective Hamiltonian, acting within the subspace of Ising spin configurations representing the collinear ground states. It consists of products of Ising spins around loops, i.e., has the form of a Z2 lattice gauge theory. The remaining ground-state entropy is still infinite but not extensive, being O(L) for system size O(L3). All these ground states have unit cells bigger than those considered previously.

Photocatalytic water oxidation by a pyrochlore oxide upon irradiation with visible light: rhodium substitution into yttrium titanate.

PubMed

Kiss, Borbala; Didier, Christophe; Johnson, Timothy; Manning, Troy D; Dyer, Matthew S; Cowan, Alexander J; Claridge, John B; Darwent, James R; Rosseinsky, Matthew J

2014-12-22

A stable visible-light-driven photocatalyst (λ≥450 nm) for water oxidation is reported. Rhodium substitution into the pyrochlore Y2 Ti2 O7 is demonstrated by monitoring Vegard's law evolution of the unit-cell parameters with changing rhodium content, to a maximum content of 3 % dopant. Substitution renders the solid solutions visible-light active. The overall rate of oxygen evolution is comparable to WO3 but with superior light-harvesting and surface-area-normalized turnover rates, making Y2 Ti1.94 Rh0.06 O7 an excellent candidate for use in a Z-scheme water-splitting system. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Effect of salt and surfactant concentration on the structure of polyacrylate gel/surfactant complexes.

PubMed

Nilsson, Peter; Unga, Johan; Hansson, Per

2007-09-20

Small-angle X-ray scattering was used to elucidate the structure of crosslinked polyacrylate gel/dodecyltrimethylammonium bromide complexes equilibrated in solutions of varying concentrations of surfactant and sodium bromide (NaBr). Samples were swollen with no ordering (micelle free), or they were collapsed with either several distinct peaks (cubic Pm3n) or one broad correlation peak (disordered micellar). The main factor determining the structure of the collapsed complexes was found to be the NaBr concentration, with the cubic structure existing up to approximately 150 mM NaBr and above which only the disordered micellar structure was found. Increasing the salt concentration decreases the polyion mediated attractive forces holding the micelles together causing swelling of the gel. At sufficiently high salt concentration the micelle-micelle distance in the gel becomes too large for the cubic structure to be retained, and it melts into a disordered micellar structure. As most samples were above the critical micelle concentration, the bulk of the surfactant was in the form of micelles in the solution and the surfactant concentration thereby had only a minor influence on the structure. However, in the region around 150 mM NaBr, increasing the surfactant concentration, at constant NaBr concentration, was found to change the structure from disordered micellar to ordered cubic and back to disordered again.

Polymorphic Transitions in Cerium-Substituted Zirconolite (CaZrTi 2O 7)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Clark, Braeden M.; Sundaram, S. K.; Misture, Scott T.

Compounds with the formulae CaZr 1–xCe xTi 2O 7 with x = 0.1–0.5 were synthesized by solid state reaction. Cerium was used as a surrogate for actinide elements. A transition from the 2M polymorph to the 4M polymorph (expanded unit cell due to cation ordering) in zirconolite was observed with increasing cerium content. The presence of both tri- and tetravalent Ce, contrary to formulation, was confirmed using X-ray absorption near edge spectroscopy, suggesting substitution on both Ca and Zr sites. Sintering was carried out via spark plasma sintering, during which the perovskite phase (Ca 0.4Ce 0.4TiO 3) was stabilized duemore » to the reducing conditions of this technique. Scanning electron microscopy and energy dispersive spectrometry revealed that the 2M polymorph was dilute in Ce content in comparison to the 4M-zirconolite. High temperature X-ray diffraction was used to detail the kinetics of perovskite to zirconolite transition. It was found that CaCeTi 2O 7 (cubic pyrochlore) formed as an intermediate phase during the transition. Lastly, our results show that a transition from 2M- to 4M-zirconolite occurs with increasing Ce content and can be controlled by adjusting the P O2 and the heat treatment temperature.« less

Polymorphic Transitions in Cerium-Substituted Zirconolite (CaZrTi 2O 7)

DOE PAGES

Clark, Braeden M.; Sundaram, S. K.; Misture, Scott T.

2017-07-19

Compounds with the formulae CaZr 1–xCe xTi 2O 7 with x = 0.1–0.5 were synthesized by solid state reaction. Cerium was used as a surrogate for actinide elements. A transition from the 2M polymorph to the 4M polymorph (expanded unit cell due to cation ordering) in zirconolite was observed with increasing cerium content. The presence of both tri- and tetravalent Ce, contrary to formulation, was confirmed using X-ray absorption near edge spectroscopy, suggesting substitution on both Ca and Zr sites. Sintering was carried out via spark plasma sintering, during which the perovskite phase (Ca 0.4Ce 0.4TiO 3) was stabilized duemore » to the reducing conditions of this technique. Scanning electron microscopy and energy dispersive spectrometry revealed that the 2M polymorph was dilute in Ce content in comparison to the 4M-zirconolite. High temperature X-ray diffraction was used to detail the kinetics of perovskite to zirconolite transition. It was found that CaCeTi 2O 7 (cubic pyrochlore) formed as an intermediate phase during the transition. Lastly, our results show that a transition from 2M- to 4M-zirconolite occurs with increasing Ce content and can be controlled by adjusting the P O2 and the heat treatment temperature.« less

Structural investigation of the substituted pyrochlore AgSbO3 through total scattering techniques.

PubMed

Laurita, Geneva; Page, Katharine; Sleight, A W; Subramanian, M A

2013-10-07

Polycrystalline samples of the pyrochlore series Ag(1-x)M(n)(x)SbO(3+x[(n-1)/2]) (M = Na, K, and Tl) have been structurally analyzed through total scattering techniques. The upper limits of x obtained were 0.05 for Na, 0.16 for K, and 0.17 for Tl. The Ag(+) cation occupies a site with inversion symmetry on a 3-fold axis. When the smaller Na(+) cation substitutes for Ag(+), it is displaced by about 0.6 Å perpendicular to the 3-fold axis to achieve some shorter Na-O bond distances. When the larger Tl(+) cation substitutes for Ag(+), it is displaced by about 1.14 Å along the 3-fold axis and achieves an environment typical of a lone pair cation. Some of the Tl(3+) from the precursor remains unreduced, leading to a formula of Ag(0.772(1))Tl(+)(0.13(2))Tl(3+)(0.036(1))SbO(3.036(1)). The position of the K(+) dopant was effectively modeled assuming that K(+) occupied the same site as Ag(+). The expansion of the lattice caused by substitution of the larger K(+) and Tl(+) cations results in longer Ag-O bond lengths, which would reduce the overlap of the Ag 4d and O 2p orbitals that compose the valence band maximum. Substitution of the smaller Na(+) results in a decrease in the Ag-O bond distance, thus increasing the overlap of the Ag 4d and O 2p orbitals. This will have a direct influence on the band composition and observed properties of this material of interest.

Phase Composition and Disorder in La2(Sn,Ti)2O7 Ceramics: New Insights from NMR Crystallography.

PubMed

Fernandes, Arantxa; McKay, David; Sneddon, Scott; Dawson, Daniel M; Lawson, Sebastian; Veazey, Richard; Whittle, Karl R; Ashbrook, Sharon E

2016-09-15

An NMR crystallographic approach, involving the combination of 119 Sn NMR spectroscopy, XRD, and DFT calculations, is demonstrated for the characterization of La 2 Sn 2- x Ti x O 7 ceramics. A phase change from pyrochlore (La 2 Sn 2 O 7 ) to a layered perovskite phase (La 2 Ti 2 O 7 ) is predicted (by radius ratio rules) to occur when x ≈ 0.95. However, the sensitivity of NMR spectroscopy to the local environment is able to reveal a significant two-phase region is present, extending from x = 1.8 to ∼0.2, with limited solid solution at the two extremes, in broad agreement with powder XRD measurements. DFT calculations reveal that there is preferential site substitution of Sn in La 2 Ti 2 O 7 , with calculated shifts for Sn substitution onto Ti1 and Ti2 sites (in the "bulk" perovskite layers) in better agreement with experiment than those for Ti3 and Ti4 ("edge" sites). Substitution onto these two sites also produces structural models with lower relative enthalpy. As the Sn content decreases, there is a further preference for substitution onto Sn2. In contrast, the relative intensities of the spectral resonances suggest that Ti substitution into the pyrochlore phase is random, although only a limited solid solution is observed (up to ∼7% Ti). DFT calculations predict very similar 119 Sn shifts for Sn substitution into the two proposed models of La 2 Ti 2 O 7 (monoclinic ( P 2 1 ) and orthorhombic ( Pna 2 1 )), indicating it is not possible to distinguish between them. However, the relative energy of the Sn-substituted orthorhombic phase was higher than that of substituted monoclinic cells, suggesting that the latter is the more likely structure.

Linear complexions: Confined chemical and structural states at dislocations

NASA Astrophysics Data System (ADS)

Kuzmina, M.; Herbig, M.; Ponge, D.; Sandlöbes, S.; Raabe, D.

2015-09-01

For 5000 years, metals have been mankind’s most essential materials owing to their ductility and strength. Linear defects called dislocations carry atomic shear steps, enabling their formability. We report chemical and structural states confined at dislocations. In a body-centered cubic Fe-9 atomic percent Mn alloy, we found Mn segregation at dislocation cores during heating, followed by formation of face-centered cubic regions but no further growth. The regions are in equilibrium with the matrix and remain confined to the dislocation cores with coherent interfaces. The phenomenon resembles interface-stabilized structural states called complexions. A cubic meter of strained alloy contains up to a light year of dislocation length, suggesting that linear complexions could provide opportunities to nanostructure alloys via segregation and confined structural states.

The Symmetry and Packing Fraction of the Body Centered Tetragonal Structure

ERIC Educational Resources Information Center

Dunlap, Richard A.

2012-01-01

It is shown that for different ratios of lattice parameters, "c/a," the body centered tetragonal structure may be view as body centered tetragonal, body centered cubic, face centered cubic or hexagonal. This illustrates that the apparent symmetry of a lattice depends on the choice of the conventional unit cell.

How Cubic Can Ice Be?

DOE PAGES

Amaya, Andrew J.; Pathak, Harshad; Modak, Viraj P.; ...

2017-06-28

Using an X-ray laser, we investigated the crystal structure of ice formed by homogeneous ice nucleation in deeply supercooled water nanodrops (r ≈ 10 nm) at ~225 K. The nanodrops were formed by condensation of vapor in a supersonic nozzle, and the ice was probed within 100 μs of freezing using femtosecond wide-angle X-ray scattering at the Linac Coherent Light Source free-electron X-ray laser. The X-ray diffraction spectra indicate that this ice has a metastable, predominantly cubic structure; the shape of the first ice diffraction peak suggests stacking-disordered ice with a cubicity value, χ, in the range of 0.78 ±more » 0.05. The cubicity value determined here is higher than those determined in experiments with micron-sized drops but comparable to those found in molecular dynamics simulations. Lastly, the high cubicity is most likely caused by the extremely low freezing temperatures and by the rapid freezing, which occurs on a ~1 μs time scale in single nanodroplets.« less

Non-noble catalysts and catalyst supports for phosphoric acid fuel cells

NASA Technical Reports Server (NTRS)

Mcalister, A. J.

1981-01-01

Tungsten carbide, which is active for hydrogen oxidation, is CO tolerant and has a hexagonal structure is discussed. Titanium carbide is inactive and has a cubic structure. Four different samples of the cubic alloys W sub x-1Ti sub XC sub 1-y were found to be active and CO tolerant. When the activities of these cubic alloys are weighted by the reciprocal of the square to those of highly forms of WC. They offer important insight into the nature of the active sites on W-C anode catalysts for use in phosphoric acid fuel cells.

On the structure of critical energy levels for the cubic focusing NLS on star graphs

NASA Astrophysics Data System (ADS)

Adami, Riccardo; Cacciapuoti, Claudio; Finco, Domenico; Noja, Diego

2012-05-01

We provide information on a non-trivial structure of phase space of the cubic nonlinear Schrödinger (NLS) on a three-edge star graph. We prove that, in contrast to the case of the standard NLS on the line, the energy associated with the cubic focusing Schrödinger equation on the three-edge star graph with a free (Kirchhoff) vertex does not attain a minimum value on any sphere of constant L2-norm. We moreover show that the only stationary state with prescribed L2-norm is indeed a saddle point.

Crystal structure of hexagonal MnAl4

PubMed Central

Pauling, Linus

1987-01-01

A structure is proposed for the hexagonal form of MnAl4, with aH = 28.4 Å and cH = 12.43 Å, on the basis of a high-resolution electron micrograph and comparison with crystals of known structures. The proposed structure involves seven 104-atom complexes of 20 Friauf polyhedra, sharing some atoms with one another. It is closely related to the 23.36-Å cubic structure of MnAl4 and to the 14.19-Å cubic structure of Mg32(Al,Zn)49. Images PMID:16593837

The structural and Raman spectral studies on Ni0.5Cu0.5Fe2O4 ferrite

NASA Astrophysics Data System (ADS)

Somani, M.; Saleem, M.

2018-05-01

Spinel ferrite Ni0.5Cu0.5Fe2O4 has been successfully prepared via solid state reaction. The crystal structure studies using XRD technique revealed cubic structure of the sample. The XRD spectra was further refined via Retvield Refinement and all the parameters regarding structure were obtained which confirmed cubic structure. The assigned space group was found to be Fd-3m. Particle size was calculated to be 56 nm. The Raman Spectra revealed five active Raman modes which confirmed spinel structure.

Using the Plan View to Teach Basic Crystallography in General Chemistry

ERIC Educational Resources Information Center

Cushman, Cody V.; Linford, Matthew R.

2015-01-01

The plan view is used in crystallography and materials science to show the positions of atoms in crystal structures. However, it is not widely used in teaching general chemistry. In this contribution, we introduce the plan view, and show these views for the simple cubic, body-centered cubic, face-centered cubic, hexagonal close packed, CsCl, NaCl,…

Icosahedral quasicrystals of intermetallic compounds are icosahedral twins of cubic crystals of three kinds, consisting of large (about 5000 atoms) icosahedral complexes in either a cubic body-centered or a cubic face-centered arrangement or smaller (about 1350 atoms) icosahedral complexes in the β-tungsten arrangement

PubMed Central

Pauling, Linus

1989-01-01

The twofold-axis electron-diffraction photographs of icosahedral quasicrystals are of three kinds, reflecting three different structures of the cubic crystals that by icosahedral twinning form the quasicrystals. The first kind, represented by Al13Cu4Fe3, contains two very large icosahedral complexes, each of about 4680 atoms, in the body-centered arrangement, with six smaller icosahedral complexes (104 atoms each) in the principal interstices. The second kind, represented by Al5Mn, contains four of the very large complexes in the face-centered arrangement (cubic close packing), with four of the smaller clusters in the interstices. The third kind, represented by Al6CuLi3, contains eight icosahedral complexes, each of about 1350 atoms, in the β-W arrangement. The supporting evidence for these cubic structures is discussed as well as other evidence showing that the simple quasicrystal theory, which states that quasicrystals do not involve any translational identity operations, has to be modified. Images PMID:16594078

Absence of magnetic long-range order in Y2CrSbO7 : Bond-disorder-induced magnetic frustration in a ferromagnetic pyrochlore

NASA Astrophysics Data System (ADS)

Shen, L.; Greaves, C.; Riyat, R.; Hansen, T. C.; Blackburn, E.

2017-09-01

The consequences of random nonmagnetic-ion dilution for the pyrochlore family Y2(M 1 -xN x)2O7 (M = magnetic ion, N = nonmagnetic ion) have been investigated. As a first step, we experimentally examine the magnetic properties of Y2CrSbO7 (x =0.5 ), in which the magnetic sites (Cr3 +) are percolative. Although the effective Cr-Cr spin exchange is ferromagnetic, as evidenced by a positive Curie-Weiss temperature, ΘCW ≃19.5 K , our high-resolution neutron powder diffraction measurements detect no sign of magnetic long-range order down to 2 K. In order to understand our observations, we construct a lattice model to numerically study the bond disorder introduced by the ionic size mismatch between M and N , which reveals that the bond disorder percolates at xb ≃0.23 , explaining the absence of magnetic long-range order. This model could be applied to a series of frustrated magnets with a pyrochlore sublattice, for example, the spinel compound Zn (Cr1 -xGax )2O4 , wherein a Néel to spin glass phase transition occurs between x =0.2 and 0.25 [Lee et al., Phys. Rev. B 77, 014405 (2008), 10.1103/PhysRevB.77.014405]. Our study stresses the non-negligible role of bond disorder on magnetic frustration, even in ferromagnets.

Catalysts for ultrahigh current density oxygen cathodes for space fuel cell applications

NASA Technical Reports Server (NTRS)

Tryk, D.; Yeager, E.; Shingler, M.; Aldred, W.; Wang, C.

1990-01-01

The objective of this research was to identify promising electrocatalyst/support systems for the oxygen cathode in alkaline fuel cells operating at relatively high temperatures, O2 pressures and current densities. A number of materials were prepared, including Pb-Ru and Pb-Ir pyrochlores, RuO2 and Pt-doped RuO2, and lithiated NiO. Several of these were prepared using techniques that had not been previously used to prepare them. Particularly interesting is the use of the alkaline solution technique to prepare the Pt-doped Pb-Ru pyrochlore in high area form. Well-crystallized Pb(2)Ru(2)O(7-y) was used to fabricate high performance O2 cathodes with relatively good stability in room temperature KOH. This material was also found to be stable over a useful potential range at approximately 140 C in concentrated KOH. Other pyrochlores were found to be either unstable (amorphous samples) or the fabrication of the gas-fed electrodes could not be fully optimized during this project period. Future work may be directed at this problem. High area platinum supported on conductive metal oxide supports produced mixed results: small improvements in O2 reduction performance for Pb(2)Ru(2)O(7-y) but a large improvement for Li-doped NiO at room temperature. Nearly reversible behavior was observed for the O2/OH couple for Li-doped NiO at approximately 200 C.

Cubic liquid crystalline nanoparticles: optimization and evaluation for ocular delivery of tropicamide.

PubMed

Verma, Purnima; Ahuja, Munish

2016-10-01

The purpose of this study was to investigate the potential of cubic liquid crystalline nanoparticles for ocular delivery of tropicamide. Ultrasound-assisted fragmentation of cubic liquid crystalline bulk phases resulted in cubic liquid crystalline nanoparticles employing Pluronic F127 as dispersant. The effects of process variables such as sonication time, sonication amplitude, sonication depth, and pre-mixing time on particle size and polydispersity index was investigated using central composite design. The morphology of tropicamide-loaded nanoparticles was found to be nearly cubical in shape by transmission electron microscopy observation. Further, small angle X-ray scattering experiment confirmed the presence of D and P phase cubic structures in coexistence. The optimized tropicamide-loaded cubic nanoparticles showed in vitro corneal permeation of tropicamide across isolated porcine cornea comparable to its commercial preparation, Tropicacyl®. Ocular tolerance was evaluated by Hen's egg-chorioallantoic membrane test and histological studies. The results of in vivo mydriatic response study demonstrated a remarkably higher area under mydriatic response curve (AUC 0→1440 min ) values of cubic nanoparticles over Tropicacyl® indicating better therapeutic value of cubic nanoparticles. Furthermore, tropicamide-loaded cubic nanoparticles exhibited prolonged mydriatic effect on rabbits as compared to commercial conventional aqueous ophthalmic solution.

The stability of a crystal with diamond structure for patchy particles with tetrahedral symmetry.

PubMed

Noya, Eva G; Vega, Carlos; Doye, Jonathan P K; Louis, Ard A

2010-06-21

The phase diagram of model anisotropic particles with four attractive patches in a tetrahedral arrangement has been computed at two different values of the range of the potential, with the aim of investigating the conditions under which a diamond crystal can be formed. We find that the diamond phase is never stable for our longer-ranged potential. At low temperatures and pressures, the fluid freezes into a body-centered-cubic solid that can be viewed as two interpenetrating diamond lattices with a weak interaction between the two sublattices. Upon compression, an orientationally ordered face-centered-cubic crystal becomes more stable than the body-centered-cubic crystal, and at higher temperatures, a plastic face-centered-cubic phase is stabilized by the increased entropy due to orientational disorder. A similar phase diagram is found for the shorter-ranged potential, but at low temperatures and pressures, we also find a region over which the diamond phase is thermodynamically favored over the body-centered-cubic phase. The higher vibrational entropy of the diamond structure with respect to the body-centered-cubic solid explains why it is stable even though the enthalpy of the latter phase is lower. Some preliminary studies on the growth of the diamond structure starting from a crystal seed were performed. Even though the diamond phase is never thermodynamically stable for the longer-ranged model, direct coexistence simulations of the interface between the fluid and the body-centered-cubic crystal and between the fluid and the diamond crystal show that at sufficiently low pressures, it is quite probable that in both cases the solid grows into a diamond crystal, albeit involving some defects. These results highlight the importance of kinetic effects in the formation of diamond crystals in systems of patchy particles.

Linear complexions: Confined chemical and structural states at dislocations.

PubMed

Kuzmina, M; Herbig, M; Ponge, D; Sandlöbes, S; Raabe, D

2015-09-04

For 5000 years, metals have been mankind's most essential materials owing to their ductility and strength. Linear defects called dislocations carry atomic shear steps, enabling their formability. We report chemical and structural states confined at dislocations. In a body-centered cubic Fe-9 atomic percent Mn alloy, we found Mn segregation at dislocation cores during heating, followed by formation of face-centered cubic regions but no further growth. The regions are in equilibrium with the matrix and remain confined to the dislocation cores with coherent interfaces. The phenomenon resembles interface-stabilized structural states called complexions. A cubic meter of strained alloy contains up to a light year of dislocation length, suggesting that linear complexions could provide opportunities to nanostructure alloys via segregation and confined structural states. Copyright © 2015, American Association for the Advancement of Science.

Chemical expansion affected oxygen vacancy stability in different oxide structures from first principles calculations

DOE PAGES

Aidhy, Dilpuneet S.; Liu, Bin; Zhang, Yanwen; ...

2015-01-21

We study the chemical expansion for neutral and charged oxygen vacancies in fluorite, rocksalt, perovskite and pyrochlores materials using first principles calculations. We show that the neutral oxygen vacancy leads to lattice expansion whereas the charged vacancy leads to lattice contraction. In addition, we show that there is a window of strain within which an oxygen vacancy is stable; beyond that range, the vacancy can become unstable. Using CeO 2|ZrO 2 interface structure as an example, we show that the concentration of oxygen vacancies can be manipulated via strain, and the vacancies can be preferentially stabilized. Furthermore, these results couldmore » serve as guiding principles in predicting oxygen vacancy stability in strained systems and in the design of vacancy stabilized materials.« less

Cluster-Glass Phase in Pyrochlore X Y Antiferromagnets with Quenched Disorder

NASA Astrophysics Data System (ADS)

Andrade, Eric C.; Hoyos, José A.; Rachel, Stephan; Vojta, Matthias

2018-03-01

We study the impact of quenched disorder (random exchange couplings or site dilution) on easy-plane pyrochlore antiferromagnets. In the clean system, order by disorder selects a magnetically ordered state from a classically degenerate manifold. In the presence of randomness, however, different orders can be chosen locally depending on details of the disorder configuration. Using a combination of analytical considerations and classical Monte Carlo simulations, we argue that any long-range-ordered magnetic state is destroyed beyond a critical level of randomness where the system breaks into magnetic domains due to random exchange anisotropies, becoming, therefore, a glass of spin clusters, in accordance with the available experimental data. These random anisotropies originate from off-diagonal exchange couplings in the microscopic Hamiltonian, establishing their relevance to other magnets with strong spin-orbit coupling.

Partial Oxidation of Hydrocarbons in a Segmented Bed Using Oxide-based Catalysts and Oxygen-conducting Supports

NASA Astrophysics Data System (ADS)

Smith, Mark W.

Two objectives for the catalytic reforming of hydrocarbons to produce synthesis gas are investigated herein: (1) the effect of oxygen-conducting supports with partially substituted mixed-metal oxide catalysts, and (2) a segmented bed approach using different catalyst configurations. Excess carbon deposition was the primary cause of catalyst deactivation, and was the focus of the experiments for both objectives. The formation and characterization of deposited carbon was examined after reaction for one of the selected catalysts to determine the quantity and location of the carbon on the catalyst surface leading to deactivation. A nickel-substituted barium hexaaluminate (BNHA), with the formula BaAl 11.6Ni0.4O18.8, and a Rh-substituted lanthanum zirconate pyrochlore (LCZR) with the formula La1.89Ca0.11 Zr1.89Rh0.11, were combined with two different doped ceria supports. These supports were gadolinium-doped ceria (GDC) and zirconium-doped ceria (ZDC). The active catalyst phases were combined with the supports in different ratios using different synthesis techniques. The catalysts were characterized using several different techniques and were tested under partial oxidation (POX) of n-tetradecane (TD), a diesel fuel surrogate. It was found that the presence of GDC and ZDC reduced the formation of carbon for both catalysts; the optimal ratio of catalyst to support was different for the hexaaluminate and the pyrochlore; a loading of 20 wt% of the pyrochlore with ZDC produced the most stable performance in the presence of common fuel contaminants (>50 h); and, the incipient wetness impregnation synthesis method of applying the active catalyst to the support produced more stable product yields than the catalyst prepared by a solid-state mixing technique. Different hexaaluminate and pyrochlore catalysts were used in different configurations in a segmented bed approach. The first strategy was to promote the indirect reforming mechanism by placing a combustion catalyst in the reactor inlet, followed by a reforming catalyst. This approach demonstrated that BNHA can be used in the reactor inlet to promote combustion with 1 wt% Rh-substituted pyrochlore in the reactor outlet, but the combustion catalyst should fill less than 50% of the reactor. The second approach placed specific catalysts in regions of the reactor that have conditions in which they are less likely to deactivate. This showed the most benefit in the use of a sulfur-tolerant noble metal catalyst in the reactor outlet. The carbon formation study was conducted on a 2 wt% Rh-substituted pyrochlore. POX of TD for various run times, followed by temperature programmed oxidation, revealed two different types of carbon deposits in the catalyst bed: carbon that burned off at relatively low temperature (LTC), and carbon that burned off at higher temperatures (HTC). The LTC reached a steady state level within two hours of reaction, and was determined not to lead to catalyst deactivation. The HTC continued to accumulate with time on stream. A mathematical expression was developed to predict the rate of formation of the HTC for a given set of reaction conditions (O/C = 1.25). This expression was modified from data from a test under different reaction conditions (O/C = 1.1) for one length of time, and was found to predict the carbon formation for a different run time within 3%.

First-principles study of the structural properties of Ge

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chang, K.J.; Cohen, M.L.

1986-12-15

With the use of an ab initio pseudopotential method, the structural properties of Ge are investigated at normal and high pressures. The pressure-induced structural phase transitions from cubic diamond to ..beta..-Sn, to simple hexagonal (sh), and to double hexagonal close packed (dhcp) are examined. With the possible exception of the dhcp structure, the calculated transition pressures, transition volumes, and axial ratios are in good agreement with experimental results. We find that sh Ge has characteristics similar to those of sh Si; the bonds between hexagonal layers are stronger than intralayer bonds and the transverse phonon modes become soft near themore » transitions from the sh to ..beta..-Sn and the sh to hcp structures. At normal pressures, we compare the crystal energies for the cubic diamond, hexagonal 2H, and hexagonal 4H structures. Because of the similar sp/sup 3/ bonds in these structures, the structural energy differences are less than about 14 meV, and the 2H and 4H phases are metastable with respect to the cubic diamond structure. The equation of state is also presented and compared with experiment.« less

Comparative study of the pentamodal property of four potential pentamode microstructures

NASA Astrophysics Data System (ADS)

Huang, Yan; Lu, Xuegang; Liang, Gongying; Xu, Zhuo

2017-03-01

In this paper, a numerical comparative study is presented on the pentamodal property of four potential pentamode microstructures (three based on simple cubic and one on body-centered cubic structures) based on phonon band calculations. The finite-element method is employed to calculate the band structures, and the two essential factors of the ratio of bulk modulus B to shear modulus G and the single-mode band gap (SBG) are analyzed to quantitatively evaluate the pentamodal property. The results show that all four structures possess a higher B/G ratio than traditional materials. One of the simple cubic structures exhibits the incomplete SBG, while the three other structures exhibit complete SBG to decouple the compression and shear waves in all propagation directions. Further parametric analyses are presented investigating the effects of geometrical and material parameters on the pentamodal property of these structures. This study provides guidelines for the future design of novel pentamode microstructures possessing a high B/G ratio and a low-frequency broadband SBG.

Structure, thermal expansion coefficient and phase stability of La2(Zr0.7Ce0.3)2O7 studied by molecular dynamic simulation and experiment

NASA Astrophysics Data System (ADS)

Che, JunWei; Liu, XiangYang; Wang, XueZhi; Liang, GongYing

2018-04-01

This paper presents structure, thermal expansion coefficient and phase stability of La2(Zr0.7Ce0.3)2O7 (LZ7C3) ceramic by both theoretical and experimental results. It was found out that LZ7C3 powders had a pyrochlore structure after being heat-treated at temperatures higher than 1473 K or higher according to XRD and TEM results. The calculated average thermal expansion coefficient (TEC) was 7.12 × 10-6 K-1, which is a little smaller than experiment result, but changes of calculated average TECs of LZ, YSZ and LZ7C3 had the same trend with experimental results. Finally, the radial distribution function (RDF) was calculated to study the phase stability of LZ7C3.

Unusual Features of Crystal Structures of Some Simple Copper Compounds

ERIC Educational Resources Information Center

Douglas, Bodie

2009-01-01

Some simple copper compounds have unusual crystal structures. Cu[subscript 3]N is cubic with N atoms at centers of octahedra formed by 6 Cu atoms. Cu[subscript 2]O (cuprite) is also cubic; O atoms are in tetrahedra formed by 4 Cu atoms. These tetrahedra are linked by sharing vertices forming two independent networks without linkages between them.…

Substrate Structures For Growth Of Highly Oriented And/Or Epitaxial Layers Thereon

DOEpatents

Arendt, Paul N.; Foltyn, Stephen R.; Groves, James R.; Jia, Quanxi

2005-07-26

A composite substrate structure including a substrate, a layer of a crystalline metal oxide or crystalline metal oxynitride material upon the substrate, a layer of an oriented cubic oxide material having a rock-salt-like structure upon the crystalline metal oxide or crystalline metal oxynitride material layer is provided together with additional layers such as one or more layers of a buffer material upon the oriented cubic oxide material layer. Jc's of 2.3×106 A/cm2 have been demonstrated with projected Ic's of 320 Amperes across a sample 1 cm wide for a superconducting article including a flexible polycrystalline metallic substrate, an inert oxide material layer upon the surface of the flexible polycrystalline metallic substrate, a layer of a crystalline metal oxide or crystalline metal oxynitride material upon the layer of the inert oxide material, a layer of an oriented cubic oxide material having a rock-salt-like structure upon the crystalline metal oxide or crystalline metal oxynitride material layer, a layer of a buffer material upon the oriented cubic oxide material layer, and, a top-layer of a high temperature superconducting material upon the layer of a buffer material.

The fcc - bcc structural transition: I. A band theoretical study for Li, K, Rb, Ca, Sr, and the transition metals Ti and V

NASA Astrophysics Data System (ADS)

Sliwko, V. L.; Mohn, P.; Schwarz, K.; Blaha, P.

1996-02-01

Employing a high-precision band structure method (FP LAPW - full potential linearized augmented plane wave) we calculate the total energy variation along the tetragonal distortion path connecting the body centred cubic (bcc) and the face centred cubic (fcc) structures. The total energy along this Bain transformation is calculated, varying c/a and volume, providing a first-principles energy surface which has two minima as a function of c/a. These are shallow and occur for the sp metals at the two cubic structures, while Ti (V) has a minimum at fcc (bcc) but a saddle point (i.e. a minimum in volume and a maximum with respect to c/a) at the other cubic structure. These features can be analysed in terms of an interplay between the Madelung contribution and the band energies. Our total energy results allow us to calculate the elastic constants 0953-8984/8/7/006/img1 and 0953-8984/8/7/006/img2 and to study the influence of pressure on the phase stability. These energy surfaces will be used in part II of this paper to investigate finite-temperature effects by mapping them to a Landau - Ginzburg expansion.

Single-crystalline cubic structured InP nanosprings

NASA Astrophysics Data System (ADS)

Shen, G. Z.; Bando, Y.; Zhi, C. Y.; Yuan, X. L.; Sekiguchi, T.; Golberg, D.

2006-06-01

Cubic structured nanosprings, InP nanosprings, have been synthesized via a simple thermochemical process using InP and ZnS as the source materials. Each InP nanospring is formed by rolling up a single InP nanobelt with the growth direction along the ⟨111⟩ orientation. The formation of these novel nanostructures is mainly attributed to the minimization of the electrostatic energy due to the polar charges on the ±(002) side surfaces of cubic InP. Cathodoluminescence properties were also studied, which reveal that the InP nanosprings have three emission bands centered at ˜736, ˜920, and ˜980nm.

Stabilization of cubic Li7La3Hf2O12 by Al-doping

NASA Astrophysics Data System (ADS)

Baklanova, Yana V.; Tyutyunnik, Alexander P.; Tarakina, Nadezda V.; Fortes, A. Dominic; Maksimova, Lidiya G.; Korona, Daniil V.; Denisova, Tatyana A.

2018-07-01

In this paper we report on the stabilization of cubic Li7La3Hf2O12 by Al3+ doping and present a detailed crystal structure study and lithium ion conductivity measurements of the obtained compound. Polycrystalline Al-doped Li7La3Hf2O12 was prepared by a modified solid state method. The compound consists of micrometer size grains encapsulated by a glassy phase, which helps preventing the volatilization of lithium during annealing. Al-doped Li7La3Hf2O12 crystallizes in the garnet-related structure with a cubic unit cell (sp. gr. Ia 3 bar d (230)). A structural refinement using X-ray and neutron powder diffraction data showed that the Al3+ ions occupy only tetrahedral Li+ sites in the structure. The presence of overextended leading edges of the peaks on the XRD and NPD data is described by the introduction of an additional phase with rhombohedral distortion that occurs through a stretching of the cubic phase along the body diagonal. The activation energy as well as the total conductivity at room temperature are close to values obtained for un-doped cubic Li7La3Zr2O12 and Li7La3Hf2O12 garnets, which make Al-doped Li7La3Hf2O12 a potential candidate for the application as solid electrolyte in solid-state rechargeable lithium-ion batteries.

Pyrochlore-rich titanate ceramics for the immobilization of plutonium: redox effects on phase equilibria in cerium- and thorium- substituted analogs

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ryerson, F J; Ebbinghaus, B

2000-05-25

Three compositions representing plutonium-free analogs of a proposed Ca-Ti-Gd-Hf-U-PU oxide ceramic for the immobilization of plutonium were equilibrated at 1 atm, 1350 C over a range of oxygen fugacities between air and that equivalent to the iron-wuestite buffer. The cerium analog replaces Pu on a mole-per-mole basic with Ce; the thorium analog replaces Pu with Th. A third material has 10 wt% Al{sub 2}O{sub 3} added to the cerium analog to encourage the formation of a Hf-analog of, CaHfTi{sub 2}O{sub 7}, zirconolite, which is referred to as hafnolite. The predominant phase produced in each formulation under all conditions is pyrochlore,more » A{sub 2}T{sub 2}O{sub 7}, where the T site is filled by Ti, and Ca, the lanthanides, Hf, U and Pu are accommodated on the A-site. Other lanthanide and uranium-bearing phases encountered include brannerite (UTi{sub 2}O{sub 6}), hafnolite (CaHfTi{sub 2}O{sub 7}), perovskite (CaTiO{sub 3}) and a calcium-lanthanide aluminotitanate with nominal stoichiometry (Ca,Ln)Ti{sub 2}Al{sub 9}O{sub 19}, where Ln is a lanthanide. The phase compositions show progressive shifts with decreasing oxygen fugacity. All of the phases observed have previously been identified in titanate-based high-level radioactive waste ceramics and demonstrate the flexibility of these ceramics to variations in processing parameters. The main variation is an increase in the uranium concentrations of pyrochlore and brannerite which must be accommodated by variations in modal abundance. Pyrochlore compositions are consistent with existing spectroscopic data suggesting that uranium is predominantly pentavalent in samples synthesized in air. A simple model based on ideal stoichiometry suggests the U{sup +4}/{Sigma}U varies linearly with log fO{sub 2} and that all of the uranium is quadravalent at the iron-wuestite buffer.« less

Long-range magnetic order in the Heisenberg pyrochlore antiferromagnets G d2G e2O7 and G d2P t2O7 synthesized under high pressure

NASA Astrophysics Data System (ADS)

Li, X.; Cai, Y. Q.; Cui, Q.; Lin, C. J.; Dun, Z. L.; Matsubayashi, K.; Uwatoko, Y.; Sato, Y.; Kawae, T.; Lv, S. J.; Jin, C. Q.; Zhou, J.-S.; Goodenough, J. B.; Zhou, H. D.; Cheng, J.-G.

2016-12-01

G d2S n2O7 and G d2T i2O7 have been regarded as good experimental realizations of the classical Heisenberg pyrochlore antiferromagnet with dipolar interaction. The former was found to adopt the Palmer-Chalker state via a single, first-order transition at TN≈1 K , while the latter enters a distinct, partially ordered state through two successive transitions at TN 1≈1 K and TN 2= 0.75 K . To shed more light on their distinct magnetic ground states, we have synthesized two more gadolinium-based pyrochlore oxides, G d2G e2O7 and G d2P t2O7 , under high-pressure conditions and performed detailed characterizations via x-ray powder diffraction, dc and ac magnetic susceptibility, and specific heat measurements down to 100 mK. We found that both compounds enter a long-range antiferromagnetically ordered state through a single, first-order transition at TN= 1.4 K for G d2G e2O7 and TN= 1.56 K for G d2P t2O7 , with the specific heat anomaly similar to that of G d2S n2O7 rather than G d2T i2O7 . Interestingly, the low-temperature magnetic specific heat values of both G d2G e2O7 and G d2P t2O7 were found to follow nicely the T3 dependence as expected for a three-dimensional antiferromagnet with gapless spin-wave excitations. We have rationalized the enhancement of TN in terms of the reduced Gd-Gd distances for the chemically pressurized G d2G e2O7 and the addition of extra superexchange pathways through the empty Pt -eg orbitals for G d2P t2O7 . Our current study has expanded the family of gadolinium-based pyrochlores and permits us to achieve a better understanding of their distinct magnetic properties in a more comprehensive perspective.

Time-Reversal Symmetry Breaking and Consequent Physical Responses Induced by All-In-All-Out Type Magnetic Order on the Pyrochlore Lattice

NASA Astrophysics Data System (ADS)

Arima, Taka-Hisa

2014-03-01

Pyrochlore-type 5d transition-metal oxide compounds Cd2Os2O7 and R2Ir2O7 (R =rare earth) undergo a metal-insulator transition accompanied by a magnetic transition. Recently, the magnetic structures of Cd2Os2O7 and Eu2Ir2O7 were investigated by means of resonant x-ray magnetic scattering. The x-ray data indicated the all-in/all-out type magnetic order. The all-in/all-out order breaks the time-reversal symmetry, while the spontaneous magnetization is essentially absent. The magnetic order can be viewed as ferroic magnetic octupolar order. The magnetic order is expected to provide several unique physical properties like quadratic magnetization. linear magneto-capacitance, linear magneto-resistance, linear magneto-mechanical coupling and so on. The symmetry breaking results in two non-equivalent domains, ``all-in/all-out'' and ``all-out/all-in.'' Interestingly, some theoretical works predict that a peculiar metallic state would appear on the domain wall. The observation and control of the domain distribution are essential for studying verious exotic physical responses. We have developed an x-ray technique for domain imaging and started studying the effects of external stimuli on the domain distribution. This work was performed in collaboration with S. Tardif, S. Takeshita, H. Ohsumi, D. Uematsu, H. Sagayama, J. J. Ishikawa, S. Nakatsuji, J. Yamaura, and Z. Hiroi.

Icosahedral quasicrystals as twins of cubic crystals containing large icosahedral clusters of atoms: The 1012-atom primitive cubic structure of Al(6)CuLi(3), the C-phase Al(37)Cu(3)Li(21)Mg(3), and GaMg(2)Zn(3).

PubMed

Pauling, L

1988-06-01

Single-grain precession x-ray diffraction photographs of Al(6)CuLi(3) have been successfully indexed on the basis of icosahedral twinning of cubic crystals with a 1012-atom primitive cubic unit with edge 25.70 A, giving support to the proposal that the so-called icosahedral quasicrystals are twins of crystals containing eight large icosahedral clusters in the beta-W arrangement. In this compound two of the clusters consist of 104 atoms and six consist of 136 atoms, with 24 atoms shared. The same structure is assigned to the C-phase, Al(37)Cu(3)Li(21)Mg(3), and to GaMg(2)Zn(3). A theory of icosahedral quasicrystals and amorphous metals is described.

Icosahedral quasicrystals as twins of cubic crystals containing large icosahedral clusters of atoms: The 1012-atom primitive cubic structure of Al6CuLi3, the C-phase Al37Cu3Li21Mg3, and GaMg2Zn3

PubMed Central

Pauling, Linus

1988-01-01

Single-grain precession x-ray diffraction photographs of Al6CuLi3 have been successfully indexed on the basis of icosahedral twinning of cubic crystals with a 1012-atom primitive cubic unit with edge 25.70 Å, giving support to the proposal that the so-called icosahedral quasicrystals are twins of crystals containing eight large icosahedral clusters in the β-W arrangement. In this compound two of the clusters consist of 104 atoms and six consist of 136 atoms, with 24 atoms shared. The same structure is assigned to the C-phase, Al37Cu3Li21Mg3, and to GaMg2Zn3. A theory of icosahedral quasicrystals and amorphous metals is described. PMID:16593929

Nonsymmorphic cubic Dirac point and crossed nodal rings across the ferroelectric phase transition in LiOsO3

NASA Astrophysics Data System (ADS)

Yu, Wing Chi; Zhou, Xiaoting; Chuang, Feng-Chuan; Yang, Shengyuan A.; Lin, Hsin; Bansil, Arun

2018-05-01

Crystalline symmetries can generate exotic band-crossing features, which can lead to unconventional fermionic excitations with interesting physical properties. We show how a cubic Dirac point—a fourfold-degenerate band-crossing point with cubic dispersion in a plane and a linear dispersion in the third direction—can be stabilized through the presence of a nonsymmorphic glide mirror symmetry in the space group of the crystal. Notably, the cubic Dirac point in our case appears on a threefold axis, even though it has been believed previously that such a point can only appear on a sixfold axis. We show that a cubic Dirac point involving a threefold axis can be realized close to the Fermi level in the nonferroelectric phase of LiOsO3. Upon lowering temperature, LiOsO3 has been shown experimentally to undergo a structural phase transition from the nonferroelectric phase to the ferroelectric phase with spontaneously broken inversion symmetry. Remarkably, we find that the broken symmetry transforms the cubic Dirac point into three mutually crossed nodal rings. There also exist several linear Dirac points in the low-energy band structure of LiOsO3, each of which is transformed into a single nodal ring across the phase transition.

Electronic structure and electron-phonon coupling in TiH$$_2$$

DOE PAGES

Shanavas, Kavungal Veedu; Lindsay, Lucas R.; Parker, David S.

2016-06-15

Calculations using first principles methods and strong coupling theory are carried out to understand the electronic structure and superconductivity in cubic and tetragonal TiHmore » $$_2$$. A large electronic density of states at the Fermi level in the cubic phase arises from Ti-$$t_{2g}$$ states and leads to a structural instability against tetragonal distortion at low temperatures. However, constraining the in-plane lattice constants diminishes the energy gain associated with the tetragonal distortion, allowing the cubic phase to be stable at low temperatures. Furthermore, calculated phonon dispersions show decoupled acoustic and optic modes arising from Ti and H vibrations, respectively and frequencies of optic modes to be rather high. The cubic phase has a large electron-phonon coupling parameter $$\\lambda$$ and critical temperature of several K. Contribution of the hydrogen sublattice to $$\\lambda$$ is found to be small in this material, which we understand from strong coupling theory to be due to the small H-$s$ DOS at the Fermi level and high energy of hydrogen modes at the tetrahedral sites.« less

Anisotropic Exchange within Decoupled Tetrahedra in the Quantum Breathing Pyrochlore Ba 3 Yb 2 Zn 5 O 11

DOE PAGES

Rau, J. G.; Wu, L. S.; May, A. F.; ...

2016-06-24

Tmore » he low energy spin excitation spectrum of the breathing pyrochlore Ba 3 Yb 2 Zn 5 O 11 has been investigated with inelastic neutron scattering. Several nearly resolution limited modes with no observable dispersion are observed at 250 mK while, at elevated temperatures, transitions between excited levels become visible. o gain deeper insight, a theoretical model of isolated Yb 3+ tetrahedra parametrized by four anisotropic exchange constants is constructed. he model reproduces the inelastic neutron scattering data, specific heat, and magnetic susceptibility with high fidelity. he fitted exchange parameters reveal a Heisenberg antiferromagnet with a very large Dzyaloshinskii-Moriya interaction. Ultimately, using this model, we predict the appearance of an unusual octupolar paramagnet at low temperatures and speculate on the development of inter-tetrahedron correlations.« less

Electrocatalysis for oxygen electrodes in fuel cells and water electrolyzers for space applications

NASA Technical Reports Server (NTRS)

Prakash, Jai; Tryk, Donald; Yeager, Ernest

1990-01-01

The lead ruthenate pyrochlore Pb2Ru2O6.5, in both high- and low-area forms, has been characterized using thermogravimetric analysis, X-ray photoelectron spectroscopy, X-ray diffraction, cyclic voltammetry, and O2 reduction and generation kinetic-mechanistic studies. Mechanisms are proposed. Compounds in which part of the Ru is substituted with Ir have also been prepared. They exhibit somewhat better performance for O2 reduction in porous, gas-fed electrodes than the unsubstituted compound. The anodic corrosion resistance of pyrochlore-based porous electrodes was improved by using two different anionically conducting polymer overlayers, which slow down the diffusion of ruthenate and plumbate out of the electrode. The O2 generation performance was improved with both types of electrodes. With a hydrogel overlayer, the O2 reduction performance was also improved.

A three-dimensional single-cell-resolution whole-brain atlas using CUBIC-X expansion microscopy and tissue clearing.

PubMed

Murakami, Tatsuya C; Mano, Tomoyuki; Saikawa, Shu; Horiguchi, Shuhei A; Shigeta, Daichi; Baba, Kousuke; Sekiya, Hiroshi; Shimizu, Yoshihiro; Tanaka, Kenji F; Kiyonari, Hiroshi; Iino, Masamitsu; Mochizuki, Hideki; Tainaka, Kazuki; Ueda, Hiroki R

2018-04-01

A three-dimensional single-cell-resolution mammalian brain atlas will accelerate systems-level identification and analysis of cellular circuits underlying various brain functions. However, its construction requires efficient subcellular-resolution imaging throughout the entire brain. To address this challenge, we developed a fluorescent-protein-compatible, whole-organ clearing and homogeneous expansion protocol based on an aqueous chemical solution (CUBIC-X). The expanded, well-cleared brain enabled us to construct a point-based mouse brain atlas with single-cell annotation (CUBIC-Atlas). CUBIC-Atlas reflects inhomogeneous whole-brain development, revealing a significant decrease in the cerebral visual and somatosensory cortical areas during postnatal development. Probabilistic activity mapping of pharmacologically stimulated Arc-dVenus reporter mouse brains onto CUBIC-Atlas revealed the existence of distinct functional structures in the hippocampal dentate gyrus. CUBIC-Atlas is shareable by an open-source web-based viewer, providing a new platform for whole-brain cell profiling.

Real-space investigation of short-range magnetic correlations in fluoride pyrochlores NaCaCo 2F 7 and NaSrCo 2F 7 with magnetic pair distribution function analysis

DOE PAGES

Frandsen, Benjamin A.; Billinge, Simon J. L.; Ross, Kathryn A.; ...

2017-12-29

Here, we present time-of-flight neutron total scattering and polarized neutron scattering measurements of the magnetically frustrated compounds NaCaCo 2F 7 and NaSrCo 2F 7, which belong to a class of recently discovered pyrochlore compounds based on transition metals and fluorine. The magnetic pair distribution function (mPDF) technique is used to analyze and model the total scattering data in real space. We find that a previously-proposed model of short-range XY-like correlations with a length scale of 10-15 Å, combined with nearest-neighbor collinear antiferromagnetic correlations, accurately describes the mPDF data at low temperature, confirming the magnetic ground state in these materials. Thismore » model is further verified by the polarized neutron scattering data. From an analysis of the temperature dependence of the mPDF and polarized neutron scattering data, we find that short-range correlations persist on the nearest-neighbor length scale up to 200 K, approximately two orders of magnitude higher than the spin freezing temperatures of these compounds. These results highlight the opportunity presented by these new pyrochlore compounds to study the effects of geometric frustration at relatively high temperatures, while also advancing the mPDF technique and providing a novel opportunity to investigate a genuinely short-range-ordered magnetic ground state directly in real space.« less

Real-space investigation of short-range magnetic correlations in fluoride pyrochlores NaCaCo 2F 7 and NaSrCo 2F 7 with magnetic pair distribution function analysis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Frandsen, Benjamin A.; Billinge, Simon J. L.; Ross, Kathryn A.

Here, we present time-of-flight neutron total scattering and polarized neutron scattering measurements of the magnetically frustrated compounds NaCaCo 2F 7 and NaSrCo 2F 7, which belong to a class of recently discovered pyrochlore compounds based on transition metals and fluorine. The magnetic pair distribution function (mPDF) technique is used to analyze and model the total scattering data in real space. We find that a previously-proposed model of short-range XY-like correlations with a length scale of 10-15 Å, combined with nearest-neighbor collinear antiferromagnetic correlations, accurately describes the mPDF data at low temperature, confirming the magnetic ground state in these materials. Thismore » model is further verified by the polarized neutron scattering data. From an analysis of the temperature dependence of the mPDF and polarized neutron scattering data, we find that short-range correlations persist on the nearest-neighbor length scale up to 200 K, approximately two orders of magnitude higher than the spin freezing temperatures of these compounds. These results highlight the opportunity presented by these new pyrochlore compounds to study the effects of geometric frustration at relatively high temperatures, while also advancing the mPDF technique and providing a novel opportunity to investigate a genuinely short-range-ordered magnetic ground state directly in real space.« less

Real-space investigation of short-range magnetic correlations in fluoride pyrochlores NaCaCo2F7 and NaSrCo2F7 with magnetic pair distribution function analysis

NASA Astrophysics Data System (ADS)

Frandsen, Benjamin A.; Ross, Kate A.; Krizan, Jason W.; Nilsen, Gøran J.; Wildes, Andrew R.; Cava, Robert J.; Birgeneau, Robert J.; Billinge, Simon J. L.

2017-12-01

We present time-of-flight neutron total scattering and polarized neutron scattering measurements of the magnetically frustrated compounds NaCaCo2F7 and NaSrCo2F7 , which belong to a class of recently discovered pyrochlore compounds based on transition metals and fluorine. The magnetic pair distribution function (mPDF) technique is used to analyze and model the total scattering data in real space. We find that a previously proposed model of short-range XY-like correlations with a length scale of 10-15 Å, combined with nearest-neighbor collinear antiferromagnetic correlations, accurately describes the mPDF data at low temperature, confirming the magnetic ground state in these materials. This model is further verified by the polarized neutron scattering data. From an analysis of the temperature dependence of the mPDF and polarized neutron scattering data, we find that short-range correlations persist on the nearest-neighbor length scale up to 200 K, approximately two orders of magnitude higher than the spin freezing temperatures of these compounds. These results highlight the opportunity presented by these new pyrochlore compounds to study the effects of geometric frustration at relatively high temperatures, while also advancing the mPDF technique and providing an opportunity to investigate a genuinely short-range-ordered magnetic ground state directly in real space.

Topological Characterization of Carbon Graphite and Crystal Cubic Carbon Structures.

PubMed

Siddiqui, Wei Gao Muhammad Kamran; Naeem, Muhammad; Rehman, Najma Abdul

2017-09-07

Graph theory is used for modeling, designing, analysis and understanding chemical structures or chemical networks and their properties. The molecular graph is a graph consisting of atoms called vertices and the chemical bond between atoms called edges. In this article, we study the chemical graphs of carbon graphite and crystal structure of cubic carbon. Moreover, we compute and give closed formulas of degree based additive topological indices, namely hyper-Zagreb index, first multiple and second multiple Zagreb indices, and first and second Zagreb polynomials.

The Coulombic Lattice Potential of Ionic Compounds: The Cubic Perovskites.

ERIC Educational Resources Information Center

Francisco, E.; And Others

1988-01-01

Presents coulombic models representing the particles of a system by point charges interacting through Coulomb's law to explain coulombic lattice potential. Uses rubidium manganese trifluoride as an example of cubic perovskite structure. Discusses the effects on cluster properties. (CW)

The diagram of phase-field crystal structures: an influence of model parameters in a two-mode approximation

NASA Astrophysics Data System (ADS)

Ankudinov, V.; Galenko, P. K.

2017-04-01

Effect of phase-field crystal model (PFC-model) parameters on the structure diagram is analyzed. The PFC-model is taken in a two-mode approximation and the construction of structure diagram follows from the free energy minimization and Maxwell thermodynamic rule. The diagram of structure’s coexistence for three dimensional crystal structures [Body-Centered-Cubic (BCC), Face-Centered-Cubic (FCC) and homogeneous structures] are constructed. An influence of the model parameters, including the stability parameters, are discussed. A question about the structure diagram construction using the two-mode PFC-model with the application to real materials is established.

A theoretical study of the stability of anionic defects in cubic ZrO 2 at extreme conditions

DOE PAGES

Samanta, Amit

2016-02-19

Using first principles density functional theory calculations, we present a study of the structure, mobility, and the thermodynamic stability of anionic defects in the high-temperature cubic phase of ZrO 2. Our results suggest that the local structure of an oxygen interstitial depends on the charge state and the cubic symmetry of the anionic sublattice is unstable at 0 K. In addition, the oxygen interstitials and the vacancies exhibit symmetry breaking transitions to low-energy structures with tetragonal distortion of the oxygen sublattice at 0 K. However, the vibrational entropy stabilizes the defect structures with cubic symmetry at 2600–2980 K. The formationmore » free energies of the anionic defects and Gibbs free energy changes associated with different defect reactions are calculated by including the vibrational free energy contributions and the effect of pressure on these defect structures. By analyzing the defect chemistry, we obtain the defect concentrations at finite temperature and pressure conditions using the zero temperature ab initio results as input and find that at low oxygen partial pressures, neutral oxygen vacancies are most dominant and at high oxygen partial pressures, doubly charged anionic defects are dominant. As a result, the relevance of the results to the thermal protective coating capabilities of zirconium-based ceramic composites is elucidated.« less

Reply to “Structural and magnetic behavior of the cubic oxyfluoride SrFeO{sub 2}F studied by neutron diffraction”

DOE Office of Scientific and Technical Information (OSTI.GOV)

Clemens, Oliver, E-mail: oliver.clemens@kit.edu; Karlsruher Institut für Technologie, Institut für Nanotechnologie, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen; Berry, Frank J.

2015-03-15

In this article we comment on the results published by Thompson et al. (, J. Solid State Chem. 219 (2014) 173–178) on the crystal structure of SrFeO{sub 2}F, who claim the compound to crystallize in the cubic space group Pm-3m. We give a more detailed explanation of the determination of our previously reported structural model with Imma symmetry (Clemens et al., J. Solid State Chem. 206 (2013) 158–169), with addition of variable temperature XRD measurements with high counting time to provide unambiguous evidence for the Imma model being correct for our sample. - Graphical abstract: The crystal structure of SrFeO{submore » 2}F is discussed with regards to previous reports. - Highlights: • SrFeO{sub 2}F was synthesized by polymer based fluorination of SrFeO{sub 3}. • Evaluation of the diffraction data shows a pseudocubic cell metric. • Superstructure reflections at low d-spacings indicate deviation from cubic symmetry. • The phase transition temperature from orthorhombic to cubic was determined using variable temperature X-ray diffraction. • Results published by Thompson et al. are critically discussed with respect to those observations.« less

Deliberate exotic magnetism via frustration and topology

NASA Astrophysics Data System (ADS)

Nisoli, Cristiano; Kapaklis, Vassilios; Schiffer, Peter

2017-03-01

Introduced originally to mimic the unusual, frustrated behaviour of spin ice pyrochlores, artificial spin ice can be realized in odd, dedicated geometries that open the door to new manifestations of a higher level of frustration.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Linaburg, Matthew R.; McClure, Eric T.; Majher, Jackson D.

The structures of the lead halide perovskites CsPbCl3 and CsPbBr3 have been determined from X-ray powder diffraction data to be orthorhombic with Pnma space group symmetry. Their structures are distorted from the cubic structure of their hybrid analogs, CH3NH3PbX3 (X = Cl, Br), by tilts of the octahedra (Glazer tilt system a–b+a–). Substitution of the smaller Rb+ for Cs+ increases the octahedral tilting distortion and eventually destabilizes the perovskite structure altogether. To understand this behavior, bond valence parameters appropriate for use in chloride and bromide perovskites have been determined for Cs+, Rb+, and Pb2+. As the tolerance factor decreases, themore » band gap increases, by 0.15 eV in Cs1–xRbxPbCl3 and 0.20 eV in Cs1–xRbxPbBr3, upon going from x = 0 to x = 0.6. The band gap shows a linear dependence on tolerance factor, particularly for the Cs1–xRbxPbBr3 system. Comparison with the cubic perovskites CH3NH3PbCl3 and CH3NH3PbBr3 shows that the band gaps of the methylammonium perovskites are anomalously large for APbX3 perovskites with a cubic structure. This comparison suggests that the local symmetry of CH3NH3PbCl3 and CH3NH3PbBr3 deviate significantly from the cubic symmetry of the average structure.« less

Measurements of leakage from Lake Michigan through three control structures near Chicago, Illinois, April-October 1993

USGS Publications Warehouse

Oberg, K.A.; Schmidt, A.R.

1994-01-01

A total of 213 measurements of leakage were made at three control structures near Chicago, Ill.--the Chicago River Controlling Works (CRCW), Thomas J. O'Brien Lock and Dam (O'Brien), and Wilmette Pumping Station (Wilmette)--using acoustic Doppler current profilers (ADCP's) and dye-dilution techniques. The CRCW consists of the Chicago Lock and two sets of sluice gates connected by a network of harbor walls. Leakage measurements were made in April, May, July, September, and October 1993 using an ADCP. The mean and standard deviation of leakage measured by the ADCP for the Chicago Lock river gate were 133 and 39 cubic feet per second, respectively. The mean and standard deviation of the leakage measurements at CRCW were 204 and 70 cubic feet per second, respectively. The mean and standard deviation of leakage measurements at O'Brien on September 17, 1993, were 21 and 10 cubic feet per second, respectively. The mean and standard deviation leakage measured at Wilmette using the ADCP were 59 and 8 cubic feet per second, respectively, in April 1993. After the pump bays at Wilmette were sealed in July 1993, the leakage dropped to less than 15 cubic feet per second in September 1993. Discharge estimated by dye-dilution at the Chicago Lock on July 15, 1993, was 160 cubic feet per second, or within 8 percent of the discharge measured with the ADCP. (USGS)

Interaction of anions with lipid cubic phase membranes, an electrochemical impedance study.

PubMed

Meynaq, Mohammad Yaser Khani; Lindholm-Sethson, Britta; Tesfalidet, Solomon

2018-05-29

Electrochemical impedance spectroscopy is useful to monitor anionic interactions with a Lipid Cubic Phase, as previously demonstrated for cationic interaction (Khani Meynaq et al., 2016). It was expected that the smaller hydrophilic anions, acetate and chloride, would interact differently than the large tryptophan anion with its hydrophobic tail. The impedance measurements enabled estimation of resistances and capacitances of a freestanding lipid cubic phase membrane at exposure to 4 and 40 mM solutions of NaCl, NaOAc and NaTrp. Small-angle X-ray scattering was used for cubic phase identification and to track structural changes within the cubic phase when exposed to the different electrolytes. The membrane resistance increases at exposure to the electrolytes in the order Cl -  < OAc -  < Trp - . The membrane resistance decreases with time at exposure to the hydrophilic anions and increases with time at Trp - exposure. The membrane capacitances were lower for NaTrp compared to NaCl and NaOAc at the corresponding concentrations which is consistent with the results from SAXRD. It is concluded that Trp - ions do not enter the aqueous channels of the cubic phase but are strongly adsorbed to the membrane/electrolyte interface leading to large alteration of the lipid phase structure and a high membrane resistance. Copyright © 2018 Elsevier Inc. All rights reserved.

Synthesis of cubic Ia-3d mesoporous silica in anionic surfactant templating system with the aid of acetate.

PubMed

Deng, Shao-Xin; Xu, Xue-Yan; He, Wen-Chao; Wang, Jin-Gui; Chen, Tie-Hong

2014-08-01

Mesoporous silica with three-dimensional (3D) bicontinuous cubic Ia-3d structure and fascinating caterpillar-like morphology was synthesized by using anionic surfactant N-lauroylsarcosine sodium (Sar-Na) as the template and 3-amionpropyltrimethoxysilane (APS) as the co-structure-directing agent (CSDA) with the aid of acetate. A phase transformation from high interfacial curvature 2D hexagonal to low interfacial curvature 3D cubic Ia-3d occurred in the presence of a proper amount of acetate. Other species of salts (excluding acetate) had the ability to induce the caterpillar-like morphology, but failed to induce the cubic Ia-3d mesostructure. Furthermore, [3-(2-aminoethyl)-aminopropyl]trimethoxysilane (DAPS) was also used as the CSDA to synthesize Ia-3d mesostructured silica under the aid of sodium acetate. After extraction of the anionic surfactants, amino and di-amine functionalized 3D bicontinuous cubic Ia-3d mesoporous silicas were obtained and used as supports to immobilize Pd nanoparticles for supported catalysts. The catalytic activity of the catalysts was tested by catalytic hydrogenation of allyl alcohol. Copyright © 2013 Elsevier Inc. All rights reserved.

Direct Visualisation of the Structural Transformation between the Lyotropic Liquid Crystalline Lamellar and Bicontinuous Cubic Mesophase.

PubMed

Tran, Nhiem; Zhai, Jiali; Conn, Charlotte E; Mulet, Xavier; Waddington, Lynne J; Drummond, Calum J

2018-05-29

The transition between the lyotropic liquid crystalline lamellar and the bicontinuous cubic mesophase drives multiple fundamental cellular processes involving changes in cell membrane topology including endocytosis and membrane budding. While several theoretical models have been proposed to explain this dynamic transformation, experimental validation of these models has been challenging due to the short lived nature of the intermediates present during the phase transition. Herein, we report the direct observation of a lamellar to bicontinuous cubic phase transition in nanoscale dispersions using a combination of cryogenic transmission electron microscopy and static small angle X-ray scattering. The results represent the first experimental confirmation of a theoretical model which proposed that the bicontinuous cubic phase originates from the centre of a lamellar vesicle, then propagates outward via the formation of inter-lamellar attachments and stalks. The observation was possible due to the precise control of the lipid composition to place the dispersion systems at the phase boundary of a lamellar and a cubic phase, allowing for the creation of long-lived structural intermediates. By surveying the nanoparticles using cryogenic transmission electron microscopy, a complete phase transition sequence was established.

Design and performance of tapered cubic anvil used for achieving higher pressure and larger sample cell

NASA Astrophysics Data System (ADS)

Han, Qi-Gang; Yang, Wen-Ke; Zhu, Pin-Wen; Ban, Qing-Chu; Yan, Ni; Zhang, Qiang

2013-07-01

In order to increase the maximum cell pressure of the cubic high pressure apparatus, we have developed a new structure of tungsten carbide cubic anvil (tapered cubic anvil), based on the principle of massive support and lateral support. Our results indicated that the tapered cubic anvil has some advantages. First, tapered cubic anvil can push the transfer rate of pressure well into the range above 36.37% compare to the conventional anvil. Second, the rate of failure crack decreases about 11.20% after the modification of the conventional anvil. Third, the limit of static high-pressure in the sample cell can be extended to 13 GPa, which can increase the maximum cell pressure about 73.3% than that of the conventional anvil. Fourth, the volume of sample cell compressed by tapered cubic anvils can be achieved to 14.13 mm3 (3 mm diameter × 2 mm long), which is three and six orders of magnitude larger than that of double-stage apparatus and diamond anvil cell, respectively. This work represents a relatively simple method for achieving higher pressures and larger sample cell.

Magneto-structural correlation in Co0.8Cu0.2Cr2O4 cubic spinel

NASA Astrophysics Data System (ADS)

Kumar, Ram; Rayaprol, S.; Siruguri, V.; Xiao, Y.; Ji, W.; Pal, D.

2018-05-01

Neutron and X-ray diffraction, magnetic susceptibility, and specific heat measurements have been used to investigate the magneto-structural phase transitions in 20% Cu substituted multiferroic CoCr2O4 spinel. The Jahn-Teller active Cu2+ ion in the tetrahedral A-site of the spinel configuration induces the Jahn-Teller distortion slightly above the Néel temperature. In this compound, we observe a Jahn-Teller distortion of the crystal structure at 90 K. It was further observed that the high temperature cubic (Fd 3 ‾ m) structure coexists with the low temperature orthorhombic (Fddd) structure till the lowest temperature of measurement.

Magnetostructural phase transformations in Tb 1-x Mn 2

DOE PAGES

Zou, Junding; Paudyal, Durga; Liu, Jing; ...

2015-01-16

Magnetism and phase transformations in non-stoichiometric Tb 1-xMn 2 (x = 0.056, 0.039) have been studied as functions of temperature and magnetic field using magnetization, heat capacity, and X-ray powder diffraction measurements. Lowering the temperature, the compounds sequentially order ferrimagnetically and antiferromagnetically, and finally, exhibit spin reorientation transitions. Moreover, these structural distortions from room temperature cubic to low temperature rhombohedral structures occur at T N, and are accompanied by large volume changes reaching ~-1.27% and -1.42%, respectively. First principles electronic structure calculations confirm the phase transformation from the ferrimagnetic cubic structure to the antiferromagnetic rhombohedral structure in TbMn 2.

Effects of Nonhydrostatic Stress on Structural and Optoelectronic Properties of Methylammonium Lead Bromide Perovskite

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Rong; Cai, Weizhao; Bi, Tiange

We report synchrotron X-ray diffraction, photoconductivity, and photoluminescence investigations of methylammonium-lead-bromide (MAPbBr3) under various stress conditions, supported by density-functional-theory (DFT) calculations. The properties of MAPbBr3 show substantial dependence on the hydrostatic conditions. While nonhydrostatic compression of MAPbBr3 leads to amorphization above 2.4 GPa, under quasi-hydrostatic (Ar) and hydrostatic (He) pressure, the sample remains in crystalline phases. A sequence of phase transitions between two cubic phases and orthorhombic Pnma phase is observed when using Ar, or no pressure-transmitting-medium (PTM). In helium-PTM only transitions between the two cubic structures and a new isostructural phase transition with a large volume collapse to amore » third cubic-phase at 2.7 GPa was observed. The photoluminescence measurements indicate a pressure-induced band gap-narrowing in the cubic phase I, and a blue-shift in the orthorhombic structure. DFT calculations illustrate that the dynamics of the organic molecules and the inorganic lattice, coupled via the N–H···Br hydrogen-bonding interactions, affect the Pb–Br distance and the bandgap evolution under pressure.« less

Oriented conductive oxide electrodes on SiO2/Si and glass

DOEpatents

Jia, Quanxi; Arendt, Paul N.

2001-01-01

A thin film structure is provided including a silicon substrate with a layer of silicon dioxide on a surface thereof, and a layer of cubic oxide material deposited upon the layer of silicon dioxide by ion-beam-assisted-deposition, said layer of cubic oxide material characterized as biaxially oriented. Preferably, the cubic oxide material is yttria-stabilized zirconia. Additional thin layers of biaxially oriented ruthenium oxide or lanthanum strontium cobalt oxide are deposited upon the layer of yttria-stabilized zirconia. An intermediate layer of cerium oxide is employed between the yttria-stabilized zirconia layer and the lanthanum strontium cobalt oxide layer. Also, a layer of barium strontium titanium oxide can be upon the layer of biaxially oriented ruthenium oxide or lanthanum strontium cobalt oxide. Also, a method of forming such thin film structures, including a low temperature deposition of a layer of a biaxially oriented cubic oxide material upon the silicon dioxide surface of a silicon dioxide/silicon substrate is provided.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Amaya, Andrew J.; Pathak, Harshad; Modak, Viraj P.

Using an X-ray laser, we investigated the crystal structure of ice formed by homogeneous ice nucleation in deeply supercooled water nanodrops (r ≈ 10 nm) at ~225 K. The nanodrops were formed by condensation of vapor in a supersonic nozzle, and the ice was probed within 100 μs of freezing using femtosecond wide-angle X-ray scattering at the Linac Coherent Light Source free-electron X-ray laser. The X-ray diffraction spectra indicate that this ice has a metastable, predominantly cubic structure; the shape of the first ice diffraction peak suggests stacking-disordered ice with a cubicity value, χ, in the range of 0.78 ±more » 0.05. The cubicity value determined here is higher than those determined in experiments with micron-sized drops but comparable to those found in molecular dynamics simulations. Lastly, the high cubicity is most likely caused by the extremely low freezing temperatures and by the rapid freezing, which occurs on a ~1 μs time scale in single nanodroplets.« less

On the electron density localization in elemental cubic ceramic and FCC transition metals by means of a localized electrons detector.

PubMed

Aray, Yosslen; Paredes, Ricardo; Álvarez, Luis Javier; Martiz, Alejandro

2017-06-14

The electron density localization in insulator and semiconductor elemental cubic materials with diamond structure, carbon, silicon, germanium, and tin, and good metallic conductors with face centered cubic structure such as α-Co, Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au, was studied using a localized electrons detector defined in the local moment representation. Our results clearly show an opposite pattern of the electron density localization for the cubic ceramic and transition metal materials. It was found that, for the elemental ceramic materials, the zone of low electron localization is very small and is mainly localized on the atomic basin edges. On the contrary, for the transition metals, there are low-valued localized electrons detector isocontours defining a zone of highly delocalized electrons that extends throughout the material. We have found that the best conductors are those in which the electron density at this low-value zone is the lowest.

Ab initio study of the structural, vibrational and thermal properties of Ge2Sb2Te5

NASA Astrophysics Data System (ADS)

Odhiambo, Henry; Othieno, Herick

2015-05-01

The structural, vibrational and thermal properties of hexagonal as well as cubic Ge2Sb2Te5 (GST) have been calculated from first principles. The relative stability of the possible stacking sequences of hexagonal GST has been confirmed to depend on the choice for the exchange-correlation (XC) energy functional. It is apparent that without the inclusion of the Te 4d orbitals in the valence states, the lattice parameters can be underestimated by as much as 3.9% compared to experiment and all-electron calculations. From phonon dispersion curves, it has been confirmed that the hexagonal phase is, indeed, stable whereas the cubic phase is metastable. In particular, calculations based on the quasi-harmonic approximation (QHA) reveal an extra heat capacity beyond the Dulong-Petit limit at high temperatures for both hexagonal and cubic GST. Moreover, cubic GST exhibits a residual entropy at 0 K, in agreement with experimental studies which attribute this phenomenon to substitutional disorder on the Sb/Ge/v sublattice.

Fabrication of stable, wide-bandgap thin films of Mg, Zn and O

DOEpatents

Katiyar, Ram S.; Bhattacharya, Pijush; Das, Rasmi R.

2006-07-25

A stable, wide-bandgap (approximately 6 eV) ZnO/MgO multilayer thin film is fabricated using pulsed-laser deposition on c-plane Al₂O₃ substrates. Layers of ZnO alternate with layers of MgO. The thickness of MgO is a constant of approximately 1 nm; the thicknesses of ZnO layers vary from approximately 0.75 to 2.5 nm. Abrupt structural transitions from hexagonal to cubic phase follow a decrease in the thickness of ZnO sublayers within this range. The band gap of the thin films is also influenced by the crystalline structure of multilayer stacks. Thin films with hexagonal and cubic structure have band-gap values of 3.5 and 6 eV, respectively. In the hexagonal phase, Mg content of the films is approximately 40%; in the cubic phase Mg content is approximately 60%. The thin films are stable and their structural and optical properties are unaffected by annealing at 750.degree. C.

Ionic and Optical Properties of Methylammonium Lead Iodide Perovskite across the Tetragonal-Cubic Structural Phase Transition

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hoque, Md Nadim Ferdous; Islam, Nazifah; Li, Zhen

Practical hybrid perovskite solar cells (PSCs) must endure temperatures above the tetragonal-cubic structural phase transition of methylammonium lead iodide (MAPbI3). However, the ionic and optical properties of MAPbI3 in such a temperature range, and particularly, dramatic changes in these properties resulting from a structural phase transition, are not well studied. Herein, we report a striking contrast at approximately 45 degrees C in the ionic/electrical properties of MAPbl3 owing to a change of the ion activation energy from 0.7 to 0.5 eV, whereas the optical properties exhibit no particular transition except for the steady increase of the bandgap with temperature. Thesemore » observations can be explained by the 'continuous' nature of perovskite phase transition. We speculate that the critical temperature at which the ionic/electrical properties change, although related to crystal symmetry variation, is not necessarily the same temperature as when tetragonal-cubic structural phase transition occurs.« less

Interfaces between hexagonal and cubic oxides and their structure alternatives

DOE PAGES

Zhou, Hua; Wu, Lijun; Wang, Hui-Qiong; ...

2017-11-14

Multi-layer structure of functional materials often involves the integration of different crystalline phases. The film growth orientation thus frequently exhibits a transformation, owing to multiple possibilities caused by incompatible in-plane structural symmetry. Nevertheless, the detailed mechanism of the transformation has not yet been fully explored. Here we thoroughly probe the heteroepitaxially grown hexagonal zinc oxide (ZnO) films on cubic (001)-magnesium oxide (MgO) substrates using advanced scanning transition electron microscopy, X-ray diffraction and first principles calculations, revealing two distinct interface models of (001) ZnO/(001) MgO and (100) ZnO/(001) MgO. Here we have found that the structure alternatives are controlled thermodynamically bymore » the nucleation, while kinetically by the enhanced Zn adsorption and O diffusion upon the phase transformation. Finally, this work not only provides a guideline for the interface fabrication with distinct crystalline phases but also shows how polar and non-polar hexagonal ZnO films might be manipulated on the same cubic substrate.« less

Chemical pressure effects on magnetism in the quantum spin liquid candidates Yb2X2O7 (X =Sn, Ti, Ge)

NASA Astrophysics Data System (ADS)

Dun, Z. L.; Lee, M.; Choi, E. S.; Hallas, A. M.; Wiebe, C. R.; Gardner, J. S.; Arrighi, E.; Freitas, R. S.; Arevalo-Lopez, A. M.; Attfield, J. P.; Zhou, H. D.; Cheng, J. G.

2014-02-01

The linear and nonlinear ac susceptibility measurements of Yb-pyrochlores, Yb2X2O7 (X =Sn, Ti, and Ge), show transitions with a ferromagnetic nature at 0.13 and 0.25 K for Yb2Sn2O7 and Yb2Ti2O7, respectively, and an antiferromagnetic ordering at 0.62 K for Yb2Ge2O7. These systematical results (i) provided information about the nature of the unconventional magnetic ground state in Yb2Ti2O7; (ii) realized a distinct antiferromagnetic ordering state in Yb2Ge2O7; and (iii) demonstrated that the application of chemical pressure through the series of Yb-pyrochlores can efficiently perturb the fragile quantum spin fluctuations of the Yb3+ ions and lead to very different magnetic ground states.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Perriot, Romain; Uberuaga, Blas P.; Zamora, Richard J.

Diffusion in complex oxides is critical to ionic transport, radiation damage evolution, sintering, and aging. In complex oxides such as pyrochlores, anionic diffusion is dramatically affected by cation disorder. However, little is known about how disorder influences cation transport. Here, we report results from classical and accelerated molecular dynamics simulations of vacancy-mediated cation diffusion in Gd 2Ti 2O 7 pyrochlore, on the microsecond timescale. We find that diffusion is slow at low levels of disorder, while higher disorder allows for fast diffusion, which is then accompanied by antisite annihilation and reordering, and thus a slowing of cation transport. Cation diffusivitymore » is therefore not constant, but decreases as the material reorders. We also show that fast cation diffusion is triggered by the formation of a percolation network of antisites. This is in contrast with observations from other complex oxides and disordered media models, suggesting a fundamentally different relation between disorder and mass transport.« less

Quantum Spin Ice under a [111] Magnetic Field: From Pyrochlore to Kagome

NASA Astrophysics Data System (ADS)

Bojesen, Troels Arnfred; Onoda, Shigeki

2017-12-01

Quantum spin ice, modeled for magnetic rare-earth pyrochlores, has attracted great interest for hosting a U(1) quantum spin liquid, which involves spin-ice monopoles as gapped deconfined spinons, as well as gapless excitations analogous to photons. However, the global phase diagram under a [111] magnetic field remains open. Here we uncover by means of unbiased quantum Monte Carlo simulations that a supersolid of monopoles, showing both a superfluidity and a partial ionization, intervenes the kagome spin ice and a fully ionized monopole insulator, in contrast to classical spin ice where a direct discontinuous phase transition takes place. We also show that on cooling, kagome spin ice evolves towards a valence-bond solid similar to what appears in the associated kagome lattice model [S. V. Isakov et al., Phys. Rev. Lett. 97, 147202 (2006), 10.1103/PhysRevLett.97.147202]. Possible relevance to experiments is discussed.

Lipidic cubic phase injector facilitates membrane protein serial femtosecond crystallography.

PubMed

Weierstall, Uwe; James, Daniel; Wang, Chong; White, Thomas A; Wang, Dingjie; Liu, Wei; Spence, John C H; Bruce Doak, R; Nelson, Garrett; Fromme, Petra; Fromme, Raimund; Grotjohann, Ingo; Kupitz, Christopher; Zatsepin, Nadia A; Liu, Haiguang; Basu, Shibom; Wacker, Daniel; Han, Gye Won; Katritch, Vsevolod; Boutet, Sébastien; Messerschmidt, Marc; Williams, Garth J; Koglin, Jason E; Marvin Seibert, M; Klinker, Markus; Gati, Cornelius; Shoeman, Robert L; Barty, Anton; Chapman, Henry N; Kirian, Richard A; Beyerlein, Kenneth R; Stevens, Raymond C; Li, Dianfan; Shah, Syed T A; Howe, Nicole; Caffrey, Martin; Cherezov, Vadim

2014-01-01

Lipidic cubic phase (LCP) crystallization has proven successful for high-resolution structure determination of challenging membrane proteins. Here we present a technique for extruding gel-like LCP with embedded membrane protein microcrystals, providing a continuously renewed source of material for serial femtosecond crystallography. Data collected from sub-10-μm-sized crystals produced with less than 0.5 mg of purified protein yield structural insights regarding cyclopamine binding to the Smoothened receptor.

Modeling antigen-antibody nanoparticle bioconjugates and their polymorphs

NASA Astrophysics Data System (ADS)

Desgranges, Caroline; Delhommelle, Jerome

2018-03-01

The integration of nanomaterials with biomolecules has recently led to the development of new ways of designing biosensors, and through their assembly, to new hybrid structures for novel and exciting applications. In this work, we develop a coarse-grained model for nanoparticles grafted with antibody molecules and their binding with antigens. In particular, we isolate two possible states for antigen-antibody pairs during the binding process, termed as recognition and anchoring states. Using molecular simulation, we calculate the thermodynamic and structural features of three possible crystal structures or polymorphs, the body-centered cubic, simple cubic, and face-centered cubic phases, and of the melt. This leads us to determine the domain of stability of the three solid phases. In particular, the role played by the switching process between anchoring and recognition states during melting is identified, shedding light on the complex microscopic mechanisms in these systems.

Computational study of Li2OHCl as a possible solid state battery material

NASA Astrophysics Data System (ADS)

Howard, Jason; Holzwarth, N. A. W.

Preparations of Li2OHCl have recently been experimentally studied as solid state Li ion electrolytes. A disordered cubic phase is known to be stable at temperatures T >35o C. Following previous ideas, first principles supercells are constructed with up to 320 atoms to model the cubic phase. First principles molecular dynamics simulations of the cubic phase show Li ion diffusion occuring on the t =10-12 s time scale, at temperatures as low as T = 400 K. The structure of the lower temperature phase (T <35o C) is not known in detail. A reasonable model of this structure is developed by using the tetragonal ideal structure found by first principles simulations and a model Hamiltonian to account for alternative orientations of the OH groups. Supported by NSF Grant DMR-1507942. Thanks to Zachary D. Hood of GaTech and ORNL for introducing these materials to us.

Spatial localization in heterogeneous systems

NASA Astrophysics Data System (ADS)

Kao, Hsien-Ching; Beaume, Cédric; Knobloch, Edgar

2014-01-01

We study spatial localization in the generalized Swift-Hohenberg equation with either quadratic-cubic or cubic-quintic nonlinearity subject to spatially heterogeneous forcing. Different types of forcing (sinusoidal or Gaussian) with different spatial scales are considered and the corresponding localized snaking structures are computed. The results indicate that spatial heterogeneity exerts a significant influence on the location of spatially localized structures in both parameter space and physical space, and on their stability properties. The results are expected to assist in the interpretation of experiments on localized structures where departures from spatial homogeneity are generally unavoidable.

Self-assembly of three-dimensional open structures using patchy colloidal particles.

PubMed

Rocklin, D Zeb; Mao, Xiaoming

2014-10-14

Open structures can display a number of unusual properties, including a negative Poisson's ratio, negative thermal expansion, and holographic elasticity, and have many interesting applications in engineering. However, it is a grand challenge to self-assemble open structures at the colloidal scale, where short-range interactions and low coordination number can leave them mechanically unstable. In this paper we discuss the self-assembly of three-dimensional open structures using triblock Janus particles, which have two large attractive patches that can form multiple bonds, separated by a band with purely hard-sphere repulsion. Such surface patterning leads to open structures that are stabilized by orientational entropy (in an order-by-disorder effect) and selected over close-packed structures by vibrational entropy. For different patch sizes the particles can form into either tetrahedral or octahedral structural motifs which then compose open lattices, including the pyrochlore, the hexagonal tetrastack and the perovskite lattices. Using an analytic theory, we examine the phase diagrams of these possible open and close-packed structures for triblock Janus particles and characterize the mechanical properties of these structures. Our theory leads to rational designs of particles for the self-assembly of three-dimensional colloidal structures that are possible using current experimental techniques.

Structure Evolution of BaTiO3 on Co Doping: X-ray diffraction and Raman study

NASA Astrophysics Data System (ADS)

Mansuri, Amantulla; Mishra, Ashutosh

2016-10-01

In the present study, we have synthesize polycrystalline samples of BaTi1-xCoxO3 (x = 0, 0.05 and 0.1) with standard solid state reaction technique. The obtained samples are characterized by X-ray diffraction (XRD) and Raman spectroscopy. The detail structural analysis has been performed by Rietveld refinement using Fullprof program. The structural analysis reveal the samples are chemical pure and crystallize in tetragonal phase with space group Pm3m. We observe an increase in lattice parameters which results due to substitution of Co2+ with large ionic radii (0.9) for smaller ionic radii (0.6) Ti4+. Moreover peak at 45.5° shift to 45° on Co doping, which is due to structure phase transition from tetragonal to cubic. Raman study infers that the intensity of characteristic peaks decreases and linewidth increases with Co doping. The bands linked with the tetragonal structure (307 cm1) decreased due to the tetragonal-towards-cubic phase transition with Co doping. Our structural study reveals the expansion of BTO unit cell and tetragonal-to-cubic phase transformation takes place, results from different characterization techniques are conclusive and show structural evolution with Co doping.

Buffer layers for coated conductors

DOEpatents

Stan, Liliana [Los Alamos, NM; Jia, Quanxi [Los Alamos, NM; Foltyn, Stephen R [Los Alamos, NM

2011-08-23

A composite structure is provided including a base substrate, an IBAD oriented material upon the base substrate, and a cubic metal oxide material selected from the group consisting of rare earth zirconates and rare earth hafnates upon the IBAD oriented material. Additionally, an article is provided including a base substrate, an IBAD oriented material upon the base substrate, a cubic metal oxide material selected from the group consisting of rare earth zirconates and rare earth hafnates upon the IBAD oriented material, and a thick film upon the cubic metal oxide material. Finally, a superconducting article is provided including a base substrate, an IBAD oriented material upon the base substrate, a cubic metal oxide material selected from the group consisting of rare earth zirconates and rare earth hafnates upon the IBAD oriented material, and an yttrium barium copper oxide material upon the cubic metal oxide material.

The three dimensionality of cell membranes: lamellar to cubic membrane transition as investigated by electron microscopy.

PubMed

Chong, Ketpin; Deng, Yuru

2012-01-01

Biological membranes are generally perceived as phospholipid bilayer structures that delineate in a lamellar form the cell surface and intracellular organelles. However, much more complex and highly convoluted membrane organizations are ubiquitously present in many cell types under certain types of stress, states of disease, or in the course of viral infections. Their occurrence under pathological conditions make such three-dimensionally (3D) folded and highly ordered membranes attractive biomarkers. They have also stimulated great biomedical interest in understanding the molecular basis of their formation. Currently, the analysis of such membrane arrangements, which include tubulo-reticular structures (TRS) or cubic membranes of various subtypes, is restricted to electron microscopic methods, including tomography. Preservation of membrane structures during sample preparation is the key to understand their true 3D nature. This chapter discusses methods for appropriate sample preparations to successfully examine and analyze well-preserved highly ordered membranes by electron microscopy. Processing methods and analysis conditions for green algae (Zygnema sp.) and amoeba (Chaos carolinense), mammalian cells in culture and primary tissue cells are described. We also discuss methods to identify cubic membranes by transmission electron microscopy (TEM) with the aid of a direct template matching method and by computer simulation. A 3D analysis of cubic cell membrane topology by electron tomography is described as well as scanning electron microscopy (SEM) to investigate surface contours of isolated mitochondria with cubic membrane arrangement. Copyright © 2012 Elsevier Inc. All rights reserved.

Formation of highly structured cubic micellar lipid nanoparticles of soy phosphatidylcholine and glycerol dioleate and their degradation by triacylglycerol lipase.

PubMed

Wadsäter, Maria; Barauskas, Justas; Nylander, Tommy; Tiberg, Fredrik

2014-05-28

Lipid nanoparticles of reversed internal phase structures, such as cubic micellar (I2) structure show good drug loading ability of peptides and proteins as well as some small molecules. Due to their controllable small size and inner morphology, such nanoparticles are suitable for drug delivery using several different administration routes, including intravenous, intramuscular, and subcutaneous injection. A very interesting system in this regard, is the two component soy phosphatidylcholine (SPC)/glycerol dioleate (GDO) system, which depending on the ratio of the lipid components form a range of reversed liquid crystalline phases. For a 50/50 (w/w) ratio in excess water, these lipids have been shown to form a reversed cubic micellar (I2) phase of the Fd3m structure. Here, we demonstrate that this SPC/GDO phase, in the presence of small quantities (5-10 wt %) of Polysorbate 80 (P80), can be dispersed into nanoparticles, still with well-defined Fd3m structure. The resulting nanoparticle dispersion has a narrow size distribution and exhibit good long-term stability. In pharmaceutical applications, biodegradation pathways of the drug delivery vehicles and their components are important considerations. In the second part of the study we show how the structure of the particles evolves during exposure to a triacylglycerol lipase (TGL) under physiological-like temperature and pH. TGL catalyzes the lipolytic degradation of acylglycerides, such as GDO, to monoglycerides, glycerol, and free fatty acids. During the degradation, the interior phase of the particles is shown to undergo continuous phase transitions from the reversed I2 structure to structures of less negative curvature (2D hexagonal, bicontinuous cubic, and sponge), ultimately resulting in the formation of multilamellar vesicles.

Novel Functionally Graded Thermal Barrier Coatings in Coal-Fired Power Plant Turbines

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Jing

This project presents a detailed investigation of a novel functionally graded coating material, pyrochlore oxide, for thermal barrier coating (TBC) in gas turbines used in coal-fired power plants. Thermal barrier coatings are refractory materials deposited on gas turbine components, which provide thermal protection for metallic components at operating conditions. The ultimate goal of this research is to develop a manufacturing process to produce the novel low thermal conductivity and high thermal stability pyrochlore oxide based coatings with improved high-temperature durability. The current standard TBC, yttria stabilized zirconia (YSZ), has service temperatures limited to <1200°C, due to sintering and phase transitionmore » at higher temperatures. In contrast, pyrochlore oxide, e.g., lanthanum zirconate (La 2Zr 2O 7, LZ), has demonstrated lower thermal conductivity and better thermal stability, which are crucial to high temperature applications, such as gas turbines used in coal-fired power plants. Indiana University – Purdue University Indianapolis (IUPUI) has collaborated with Praxair Surface Technologies (PST), and Changwon National University in South Korea to perform the proposed research. The research findings are critical to the extension of current TBCs to a broader range of high-temperature materials and applications. Several tasks were originally proposed and accomplished, with additional new opportunities identified during the course of the project. In this report, a description of the project tasks, the main findings and conclusions are given. A list of publications and presentations resulted from this research is listed in the Appendix at the end of the report.« less

Spiraling elliptic Laguerre-Gaussian soliton in isotropic nonlocal competing cubic-quintic nonlinear media

NASA Astrophysics Data System (ADS)

Wang, Qing; Li, JingZhen; Xie, WeiXin

2018-06-01

This paper introduce a kind of spiraling elliptic Laguerre-Gaussian (SELG) soliton which has complicated structures in its profile and phase, and find that it can be formed in nonlocal cubic, quantic and competing cubic-quintic nonlinear media, respectively. The different-order SELG solitons with the same ellipticity have the same rotation period, cross-term phase coefficient, critical power and different critical orbital angular momentums (OAM). However, with the increase of ellipticity, the rotation period, cross-term phase coefficient, critical power and OAM are all increased. In particular, there are bistable SELG solitons stemmed by the competing effect between self-focusing cubic and self-defocusing quintic nonlinearities.

Formation of met-cars and face-centered cubic structures. Thermodynamically or kinetically controlled

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wei, S.; Guo, B.C.; Deng, H.T.

1994-05-18

On the basis of a series of experimental studies from our laboratory, it is well established that metallocarbohedrenes, or Met-Cars for short, are a stable class of cluster materials. To account for their exceptional stability, we initially proposed a pentagonal dodecahedron structure. This cage-like structure is consistent with all the experimental findings. In general, there are two possible structures that can be developed in these metal-carbon systems, i.e., Met-Cars and cubes. Since only one structural pattern is generally observed for one particular cluster system, it has been suggested that their thermodynamical stabilities might be responsible for the selective formation ofmore » specific structures, e.g., Met-Cars or fcc structures. Herein, we present new experimental results on the system of Nb[sub m]C[sub n] under various conditions. It is shown that the experimental conditions are extremely critical for the formation of either Met-Cars or cubic structures, as predicted by Reddy and Khanma. Moreover, the new data show that the cubic structures do not develop on top of Met-Cars, but rather, they grow independently. The experiments were performed by using both time-of-flight and quadrupole mass spectrometer techniques coupled with a laser vaporization source. 23 refs., 1 fig.« less

The effects of different heat treatment annealing on structural properties of LaFe11.5Si1.5 compound

NASA Astrophysics Data System (ADS)

Norizan, Yang Nurhidayah Asnida; Din, Muhammad Faiz Md; Zamri, Wan Fathul Hakim W.; Hashim, Fakroul Ridzuan; Jusoh, Mohd Taufik; Rahman, Mohd Rashid Abdul

2018-02-01

The cubic NaZn13-type LaFe13-xSix based compounds have been studied systematically and has become one of the most interesting systems for exploring large MCE. Its magnetic properties are strongly doping dependent and provides many of advantage compare to other as magnetic materials for magnetic refrigerator application. In other to produce high quality of cubic NaZn13-type structure, the structural properties of LaFe11.5Si1.5 compounds annealed at different temperature have been investigated. The LaFe11.5Si1.5 compounds was prepared by arc melting and annealed at two different heat treatment which are 1323 K for 14 days and 1523 K for 4 hour. The powder X-ray diffraction (XRD) shows that a short time and high temperature annealing process has benefits for the formation of the NaZn13-type phase compared to a long time and low temperature annealing process. This is shown by the weight fraction of cubic NaZn13- type structure increases from 80% for low temperature annealing to 83% for high temperature annealing. At the same time, high temperature annealing increase the main structure and decrease the impurity (α-Fe and LaFeSi). Furthermore, it can be clearly seen in the Rietveld refinement results that the lattice parameter is increase at the high temperature annealing because of more cubic NaZn13 is formed at higher temperature.

Architecture for coated conductors

DOEpatents

Foltyn, Stephen R.; Arendt, Paul N.; Wang, Haiyan; Stan, Liliana

2010-06-01

Articles are provided including a base substrate having a layer of an oriented cubic oxide material with a rock-salt-like structure layer thereon, and, a layer of epitaxial titanium nitride upon the layer of an oriented cubic oxide material having a rock-salt-like structure. Such articles can further include thin films of high temperature superconductive oxides such as YBCO upon the layer of epitaxial titanium nitride or upon a intermediate buffer layer upon the layer of epitaxial titanium nitride.

Crystal Model Kits for Use in the General Chemistry Laboratory.

ERIC Educational Resources Information Center

Kildahl, Nicholas J.; And Others

1986-01-01

Dynamic crystal model kits are described. Laboratory experiments in which students use these kits to build models have been extremely successful in providing them with an understanding of the three-dimensional structures of the common cubic unit cells as well as hexagonal and cubic closest-packing of spheres. (JN)

Rhombohedral Super Hetero Epitaxy of Cubic SiGe on Trigonal c-plane Sapphire

NASA Technical Reports Server (NTRS)

Choi, Sang H.; Duzik, Adam J.

2017-01-01

New rhombohedral super-hetero-epitaxy technology was developed at NASA. This epitaxy technology enables the growth of unprecedented cubic-trigonal hybrid single crystal structures with lattice match on sapphire (Al2O3) substrates, hence with little strain and very few defects at the interface.

Hierarchical Na-doped cubic ZrO{sub 2} synthesis by a simple hydrothermal route and its application in biodiesel production

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lara-García, Hugo A.; Romero-Ibarra, Issis C.; Pfeiffer, Heriberto, E-mail: pfeiffer@iim.unam.mx

Hierarchical growth of cubic ZrO{sub 2} phase was successfully synthesized via a simple hydrothermal process in the presence of different surfactants (cationic, non-ionic and anionic) and sodium hydroxide. The structural and microstructural characterizations of different ZrO{sub 2} powders were performed using various techniques, such as X-ray diffraction, transmission electron microscopy, N{sub 2} adsorption–desorption, scanning electron microscopy and infrared. Results indicated that sodium addition stabilized the cubic ZrO{sub 2} phase by a Na-doping process, independently of the surfactant used. In contrast, microstructural characteristics varied as a function of the surfactant and sodium presence. In addition, water vapor (H{sub 2}O) and carbonmore » dioxide (CO{sub 2}) sorption properties were evaluated on ZrO{sub 2} samples. Results evidenced that sample surface reactivity changed as a function of the sodium content. Finally, this surface reactivity was evaluated on the biodiesel transesterification reaction using the different synthesized samples, obtaining yields of 93%. - Graphical abstract: Hierarchical growth of cubic Na-ZrO{sub 2} phase was synthesized by hydrothermal processes in the presence of surfactants and sodium. Sodium addition stabilized the cubic phase by a Na-doping process, while the microstructural characteristics varied with surfactants. Finally, this surface reactivity was evaluated on the biodiesel transesterification reaction. - Highlights: • Cubic-ZrO{sub 2} phase was synthesized via a simple hydrothermal process. • ZrO{sub 2} structure and microstructures changed as a function of the surfactant. • Cubic-ZrO{sub 2} phase was evaluated on the biodiesel transesterification reaction.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Miao, Yinbin; Mo, Kun; Zhou, Zhangjian

Here the high temperature tensile performance of an oxide dispersion-strengthened (ODS) 310 steel is reported upon. The microstructure of the steel was examined through both transmission electron microscopy (TEM) and synchrotron scattering. In situ synchrotron tensile investigation was performed at a variety of temperatures, from room temperature up to 800°C. Pyrochlore structure yttrium titanate and sodium chloride structure titanium nitride phases were identified in the steel along with an austenite matrix and marginal residual α’-martensite. The inclusion phases strengthen the steel by taking extra load through particle-dislocation interaction during plastic deformation or dislocation creep procedures. As temperature rises, the loadmore » partitioning effect of conventional precipitate phases starts to diminish, whereas those ultra-fine oxygen-enriched nanoparticles continue to bear a considerable amount of extra load. Introduction of oxygen-enriched nanoparticles in austenitic steel proves to improve the high temperature performance, making austenitic ODS steels promising for advanced nuclear applications.« less

Structural, electronic and elastic properties of heavy fermion YbRh2 Laves phase compound

NASA Astrophysics Data System (ADS)

Pawar, Harsha; Shugani, Mani; Aynyas, Mahendra; Sanyal, Sankar P.

2018-05-01

The structural, electronic and elastic properties of YbRh2 Laves phase intermetallic compound which crystallize in cubic (MgCu2-type) structure have been investigated using ab-initio full potential linearized augmented plane wave (FP- LAPW) method with LDA and LDA+U approximation. The calculated ground state properties such as lattice parameter (a0), bulk modulus (B) and its pressure derivative (B') are in good agreement with available experimental and theoretical data. The electronic properties are analyzed from band structures and density of states. Elastic constants are predicted first time for this compound which obeys the stability criteria for cubic system.

The crystal structure and morphology of NiO-YSZ composite that prepared from local zircon concentrate of Bangka Island

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rahmawati, F., E-mail: fitria@mipa.uns.ac.id; Apriyani, K.; Heraldy, E.

2016-03-29

In order to increase the economic value of local zircon concentrate from Bangka Island, NiO-YSZ was synthesized from Zirconia, ZrO{sub 2} that was prepared from local zircon concentrate. The NiO-YSZ composite was synthesized by solid state reaction method. XRD analysis equipped with Le Bail refinement was carried out to analyze the crystal structure and cell parameters of the prepared materials. The result showed that zirconia was crystallized in tetragonal structure with a space group of P42/NMC. Yttria-Stabilized-Zirconia (YSZ) was prepared by doping 8% mol yttrium oxide into zirconia and then sintered at 1250°C for 3 hours. Doping of 8% molmore » Yttria allowed phase transformation of zirconia from tetragonal into the cubic structure. Meanwhile, the composite of NiO-YSZ consists of two crystalline phases, i.e. the NiO with cubic structure and the YSZ with cubic structure. SEM analysis of the prepared materials shows that the addition of NiO into YSZ allows the morphology to become more roughness with larger grain size.« less

Structure evolution upon chemical and physical pressure in (Sr{sub 1−x}Ba{sub x}){sub 2}FeSbO{sub 6}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tiittanen, T.; Karppinen, M., E-mail: maarit.karppinen@aalto.fi

Here we demonstrate the gradual structural transformation from the monoclinic I2/m to tetragonal I4/m, cubic Fm-3m and hexagonal P6{sub 3}/mmc structure upon the isovalent larger-for-smaller A-site cation substitution in the B-site ordered double-perovskite system (Sr{sub 1−x}Ba{sub x}){sub 2}FeSbO{sub 6}. This is the same transformation sequence previously observed up to Fm-3m upon heating the parent Sr{sub 2}FeSbO{sub 6} phase to high temperatures. High-pressure treatment, on the other hand, transforms the hexagonal P6{sub 3}/mmc structure of the other end member Ba{sub 2}FeSbO{sub 6} back to the cubic Fm-3m structure. Hence we may conclude that chemical pressure, physical pressure and decreasing temperature allmore » work towards the same direction in the (Sr{sub 1−x}Ba{sub x}){sub 2}FeSbO{sub 6} system. Also shown is that with increasing Ba-for-Sr substitution level, i.e. with decreasing chemical pressure effect, the degree-of-order among the B-site cations, Fe and Sb, decreases. - Graphical abstract: In the (Sr{sub 1−x}Ba{sub x}){sub 2}FeSbO{sub 6} double-perovskite system the gradual structural transformation from the monoclinic I2/m to tetragonal I4/m, cubic Fm-3m and hexagonal P6{sub 3}/mmc structure is seen upon the isovalent larger-for-smaller A-site cation substitution. High-pressure treatment under 4 GPa extends stability of the cubic Fm-3m structure within a wider substitution range of x. - Highlights: • Gradual structural transitions upon A-cation substitution in (Sr{sub 1−x}Ba{sub x}){sub 2}FeSbO{sub 6.} • With increasing x structure changes from I2/m to I4/m, Fm-3m and P6{sub 3}/mmc. • Degree of B-site order decreases with increasing x and A-site cation radius. • High-pressure treatment extends cubic Fm-3m phase stability for wider x range. • High-pressure treatment affects bond lengths mostly around the A-cation.« less

Atomistic simulation of cubic and tetragonal phases of U-Mo alloy: Structure and thermodynamic properties

NASA Astrophysics Data System (ADS)

Starikov, S. V.; Kolotova, L. N.; Kuksin, A. Yu.; Smirnova, D. E.; Tseplyaev, V. I.

2018-02-01

We studied structure and thermodynamic properties of cubic and tetragonal phases of pure uranium and U-Mo alloys using atomistic simulations: molecular dynamics and density functional theory. The main attention was paid to the metastable γ0 -phase that is formed in U-Mo alloys at low temperature. Structure of γ0 -phase is similar to body-centered tetragonal (bct) lattice with displacement of a central atom in the basic cell along [ 001 ] direction. Such displacements have opposite orientations for part of the neighbouring basic cells. In this case, such ordering of the displacements can be designated as antiferro-displacement. Formation of such complex structure may be interpreted through forming of short U-U bonds. At heating, the tetragonal structure transforms into cubic γs -phase, still showing ordering of central atom displacements. With rise in temperature, γs -phase transforms to γ-phase with a quasi body-centered cubic (q-bcc) lattice. The local positions of uranium atoms in γ-phase correspond to γs -phase, however, orientations of the central atom displacements become disordered. Transition from γ0 to γ can be considered as antiferro-to paraelastic transition of order-disorder type. This approach to the structure description of uranium alloy allows to explain a number of unusual features found in the experiments: anisotropy of lattice at low temperature; remarkably high self-diffusion mobility in γ-phase; decreasing of electrical resistivity at heating for some alloys. In addition, important part of this work is the development of new interatomic potential for U-Mo system made with taking into account details of studied structures.

Hydrothermal recrystallization of transition metal nitroprussides. Formation of the most stable phases

NASA Astrophysics Data System (ADS)

Echevarría, F.; Reguera, L.; González M, M.; Galicia, J.; Ávila, M.; Reguera, E.

2018-02-01

Hydrothermal recrystallization appears to be an appropriate treatment to explore the structural diversity of porous coordination polymers. In this contribution, such a post-synthesis treatment is applied to divalent transition metal nitroprussides, T[Fe(CN)5NO]•xH2O with T =Mn, Fe, Co, Ni, Cu, Zn, Cd. This family of compounds forms an interesting series of nanoporous coordination polymers with a wide structural diversity, related to the synthesis route used and the solid hydration degree (x). The effect of a hydrothermal recrystallization of previously prepared fine powders using the precipitation method, on their crystal structure and related properties is herein discussed. In this series of coordination polymers, for Fe, Co, Ni the precipitated powders are obtained as cubic phase, with a high porosity related to presence of systematic vacancies for building unit [Fe(CN)5NO]. For Fe and Co a structural transition, from cubic to orthorhombic, was observed, which is associated to formation of a most compact structure. The crystal structure for the new orthorhombic phases was refined from the collected powder HR-XRD patterns. For Ni, the cubic phase remains stable even for large heating time, which is ascribed to the high polarizing power of this metal. The high porosity for the cubic phase allows an easy accommodation for the local deformations around the Ni atom coordination sphere. The structural information from XRD was complemented with CO2 and H2 adsorption and TG data, IR and UV-vis spectra, and magnetic measurements. The magnetic data, through the presence of spin-orbit coupling for Fe and Co in the two phases, provide fine details on the coordination environment for the metal linked at the N ends of the CN group.

Electronic properties and structural phase transition in A4 [M4O4] (A=Li, Na, K and Rb; M=Ag and Cu): A first principles study

NASA Astrophysics Data System (ADS)

Umamaheswari, R.; Yogeswari, M.; Kalpana, G.

2013-02-01

Self-consistent scalar relativistic band structure calculations for AMO (A=Li, Na, K and Rb; M=Ag and Cu) compounds have been performed using the tight-binding linear muffin-tin orbital (TB-LMTO) method within the local density approximation (LDA). At ambient conditions, these compounds are found to crystallize in tetragonal KAgO-type structure with two different space group I-4m2 and I4/mmm. Nowadays, hypothetical structures are being considered to look for new functional materials. AMO compounds have stoichiometry similar to eight-electron half-Heusler materials of type I-I-VI which crystallizes in cubic (C1b) MgAgAs-type structure with space group F-43m. For all these compounds, by interchanging the positions of atoms in the hypothetical cubic structure, three phases (α, β and γ) are formed. The energy-volume relation for these compounds in tetragonal KAgO-type structure and cubic α, β and γ phases of related structure have been obtained. Under ambient conditions these compounds are more stable in tetragonal KAgO-type (I4/mmm) structure. The total energies calculated within the atomic sphere approximation (ASA) were used to determine the ground state properties such as equilibrium lattice parameters, c/a ratio, bulk modulus, cohesive energy and are compared with the available experimental results. The results of the electronic band structure calculations at ambient condition show that LiCuO and NaMO are indirect band gap semiconductors whereas KMO and RbMO are direct band gap semiconductors. At high pressure the band gap decreases and the phenomenon of band overlap metallization occur. Also these compounds undergo structural phase transition from tetragonal I-4m2 phase to cubic α-phase and transition pressures were calculated.

CUBIC pathology: three-dimensional imaging for pathological diagnosis.

PubMed

Nojima, Satoshi; Susaki, Etsuo A; Yoshida, Kyotaro; Takemoto, Hiroyoshi; Tsujimura, Naoto; Iijima, Shohei; Takachi, Ko; Nakahara, Yujiro; Tahara, Shinichiro; Ohshima, Kenji; Kurashige, Masako; Hori, Yumiko; Wada, Naoki; Ikeda, Jun-Ichiro; Kumanogoh, Atsushi; Morii, Eiichi; Ueda, Hiroki R

2017-08-24

The examination of hematoxylin and eosin (H&E)-stained tissues on glass slides by conventional light microscopy is the foundation for histopathological diagnosis. However, this conventional method has some limitations in x-y axes due to its relatively narrow range of observation area and in z-axis due to its two-dimensionality. In this study, we applied a CUBIC pipeline, which is the most powerful tissue-clearing and three-dimensional (3D)-imaging technique, to clinical pathology. CUBIC was applicable to 3D imaging of both normal and abnormal patient-derived, human lung and lymph node tissues. Notably, the combination of deparaffinization and CUBIC enabled 3D imaging of specimens derived from paraffin-embedded tissue blocks, allowing quantitative evaluation of nuclear and structural atypia of an archival malignant lymphoma tissue. Furthermore, to examine whether CUBIC can be applied to practical use in pathological diagnosis, we performed a histopathological screening of a lymph node metastasis based on CUBIC, which successfully improved the sensitivity in detecting minor metastatic carcinoma nodules in lymph nodes. Collectively, our results indicate that CUBIC significantly contributes to retrospective and prospective clinicopathological diagnosis, which might lead to the establishment of a novel field of medical science based on 3D histopathology.

First principle investigation of structural and optical properties of cubic titanium dioxide

NASA Astrophysics Data System (ADS)

Dash, Debashish; Chaudhury, Saurabh; Tripathy, Susanta K.

2018-05-01

This paper presents an analysis of structural and optical properties of cubic titanium dioxide (TiO2) using Orthogonalzed Linear Combinations of Atomic Orbitals (OLCAO) basis set under the framework of Density Functional Theory (DFT). The structural property, specially the lattice constant `a' and the optical properties such as refractive index, extinction coefficient, and reflectivity are investigated and discussed in the energy range of 0-16 eV. Further, the results have compared with previous theoretical as well as with experimental results. It was found that DFT based simulation results are approximation to experimental results.

Low resistance barrier layer for isolating, adhering, and passivating copper metal in semiconductor fabrication

DOEpatents

Weihs, Timothy P.; Barbee, Jr., Troy W.

2002-01-01

Cubic or metastable cubic refractory metal carbides act as barrier layers to isolate, adhere, and passivate copper in semiconductor fabrication. One or more barrier layers of the metal carbide are deposited in conjunction with copper metallizations to form a multilayer characterized by a cubic crystal structure with a strong (100) texture. Suitable barrier layer materials include refractory transition metal carbides such as vanadium carbide (VC), niobium carbide (NbC), tantalum carbide (TaC), chromium carbide (Cr.sub.3 C.sub.2), tungsten carbide (WC), and molybdenum carbide (MoC).

A spatio-spectral polarization analysis of 1 µm-pumped bulk supercontinuum in a cubic crystal (YAG)

NASA Astrophysics Data System (ADS)

Choudhuri, Aradhana; Chatterjee, Gourab; Zheng, Jiaan; Hartl, Ingmar; Ruehl, Axel; Dwayne Miller, R. J.

2018-06-01

We present the first systematic study of the spatio-spectral polarization properties of a supercontinuum generated in a cubic crystal, yttrium-aluminum garnet (YAG), including a full spectral analysis of the white light core and surrounding ring structure. We observe no depolarization of the supercontinuum, and no spatial dependence of polarization ratios for any wavelength. We discuss the discrepancy of YAG's polarization behavior in the context of well-established results in literature reporting self-induced depolarization in other cubic crystals.

DFT applied to the study of carbon-doped zinc-blende (cubic) GaN

NASA Astrophysics Data System (ADS)

Espitia R, M. J.; Ortega-López, C.; Rodríguez Martínez, J. A.

2016-08-01

Employing first principles within the framework of density functional theory, the structural properties, electronic structure, and magnetism of C-doped zincblende (cubic) GaN were investigated. The calculations were carried out using the pseudopotential method, employed exactly as implemented in Quantum ESPRESSO code. For GaC0.0625N0.9375 concentration, a metallic behavior was found. This metallic property comes from the hybridization and polarization of C-2p states and their neighboring N-2p and G-4p states.

Rietveld analysis of the cubic crystal structure of Na-stabilized zirconia

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fagherazzi, G.; Canton, P.; Benedetti, A.

Using x-ray Rietveld analysis the fcc (fluorite-type) structure of a Na-containing nanocrystalline zirconia powder (9.5 nm estimated of crystallite size) obtained by precipitation and subsequent calcination has been confirmed. The result shows that using conventional x-ray diffraction techniques the cubic crystallographic form of ZrO{sub 2} from the tetragonal one in nanosized powders. These conclusions are supported by the findings of independent Raman scattering experiments. {copyright} {ital 1997 Materials Research Society.}

Synroc-D Type Ceramics Produced by Hot Isostatic Pressing and Cold Crucible Melting for Immobilisation of (Al, U) Rich Nuclear Waste

DOE Office of Scientific and Technical Information (OSTI.GOV)

Vance, Eric R.; La Robina, Michael; Li, Huijun

2007-07-01

A synroc-D ceramic consisting mostly of spinel, hollandite, pyrochlore-structured CaUTi{sub 2}O{sub 7}, UO{sub 2}, and Ti-rich regions shows promise for immobilisation of a HLW containing mainly Al and U, together with fission products. Ceramics with virtually zero porosities and waste loadings of 50-60 wt% on an oxide basis were prepared by cold crucible melting (CCM) at {approx}1500 deg. C, and also by subsolidus hot isostatic pressing (HIP) at 1100 deg. C to prevent volatile losses. PCT leaching test values for Cs were < 13 g/L, with all other normalised elemental extractions being well below 1 g/L. (authors)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tumurugoti, P.; Clark, B.M.; Edwards, D.J.

Hollandite-rich multiphase waste form compositions processed by melt-solidification and spark plasma sintering (SPS) were characterized, compared, and validated for nuclear waste incorporation. Phase identification by x-ray diffraction (XRD) and electron back-scattered diffraction (EBSD) confirmed hollandite as the major phase present in these samples along with perovskite, pyrochlore and zirconolite. Distribution of selected elements observed by wavelength dispersive spectroscopy (WDS) maps indicated that Cs formed a secondary phase during SPS processing, which was considered undesirable. On the other hand, Cs partitioned into the hollandite phase in melt-processed samples. Further analysis of hollandite structure in melt-processed composition by selected area electron diffractionmore » (SAED) revealed ordered arrangement of tunnel ions (Ba/Cs) and vacancies, suggesting efficient Cs incorporation into the lattice.« less

Ising versus XY anisotropy in frustrated R(2)Ti(2)O(7) compounds as "Seen" by Polarized Neutrons.

PubMed

Cao, H; Gukasov, A; Mirebeau, I; Bonville, P; Decorse, C; Dhalenne, G

2009-07-31

We studied the field induced magnetic order in R(2)Ti(2)O(7) pyrochlore compounds with either uniaxial (R=Ho, Tb) or planar (R=Er, Yb) anisotropy, by polarized neutron diffraction. The determination of the local susceptibility tensor {chi(parallel to),chi(perpendicular)} provides a universal description of the field induced structures in the paramagnetic phase (2-270 K), whatever the field value (1-7 T) and direction. Comparison of the thermal variations of chi(parallel to) and chi(perpendicular) with calculations using the rare earth crystal field shows that exchange and dipolar interactions must be taken into account. We determine the molecular field tensor in each case and show that it can be strongly anisotropic.

Super-hard cubic BN layer formation by nitrogen ion implantation

NASA Astrophysics Data System (ADS)

Komarov, F. F.; Pilko, V. V.; Yakushev, V. A.; Tishkov, V. S.

1994-11-01

Microcrystalline and amorphous boron thin films were implanted with nitrogen ions at energies from 25 to 125 keV and with doses from 2 × 10 17 to 1 × 10 18 at.cm 2 at temperatures below 200°C. The structure of boron nitride phases after ion implantation, formation of phases and phase transformations were investigated by TEM and TED methods. The cubic boron nitride phase is revealed. The microhardness of the formed films was satisfactorily explained in terms of chemical compound formation by polyenergetic ion implantation. The influence of the copper impurity on the formation of the cubic boron nitride phase is demonstrated. It has also been shown that low concentrations of copper promote cubic BN boundary formation.

76 FR 77271 - Competitive Product Postal Price Changes

Federal Register 2010, 2011, 2012, 2013, 2014

2011-12-12

... Rate packaging, and Commercial Plus Cubic pricing. Changes to the price structure include the following... Price Cubic pricing is reduced from 250,000 to 150,000 pieces, and can use soft packaging; and (3) Open and Distribute pricing for specified trays and flat tubs is to be introduced in January. \\2\\ If...

Geometrical aspects of the frustration in the cubic phases of lyotropic liquid crystals.

PubMed Central

Anderson, D M; Gruner, S M; Leibler, S

1988-01-01

Bicontinuous cubic phases, composed of bilayers arranged in the geometries of periodic minimal surfaces, are found in a variety of different lipid/water systems. It has been suggested recently that these cubic structures arrive as the result of competition between two free-energy terms: the curvature energy of each monolayer and the stretching energy of the lipid chains. This scenario, closely analogous to the one that explains the origin of the hexagonal phases, is investigated here by means of simple geometrical calculations. It is first assumed that the lipid bilayer is of constant thickness and the distribution of the (local) mean curvature of the phospholipid-water interfaces is calculated. Then, assuming the mean curvature of these interfaces is constant, the distribution of the bilayer's thickness is calculated. Both calculations quantify the fact that the two energy terms are frustrated and cannot be satisfied simultaneously. However, the amount of the frustration can be smaller for the cubic phase than for the lamellar and hexagonal structures. Therefore, this phase can appear in the phase diagram between the other two, as observed in many recent experiments. PMID:3399497

The role of the cubic structure in freezing of a supercooled water droplet on an ice substrate

NASA Astrophysics Data System (ADS)

Takahashi, T.; Kobayashi, T.

1983-12-01

The possibility of the formation of a metastable cubic (diamond) structure and its role in freezing of a supercooled water droplet on an ice substrate are discussed in terms of two-dimensional nucleation. The mode of stacking sequence of new layers formed by two-dimensional nucleation is divided into single and multi-nucleation according to the degree of supercooling and to the size of the supercooled droplet. In the case of single nucleation a frozen droplet develops into a complete hexagonal single crystal or an optically single crystal (containing discontinuous stacking faults). In the case of multi-nucleation attention is paid to the size effect and the stacking direction of the nucleus to calculate the waiting time in the nucleation. Then the frozen droplets are crystallographically divided into three categories: completely single crystals, optically single crystals (containing a small cubic structure, i.e. stacking faults) and polycrystals with a misorientation of 70.53° between the c-axes.

BDA: A novel method for identifying defects in body-centered cubic crystals.

PubMed

Möller, Johannes J; Bitzek, Erik

2016-01-01

The accurate and fast identification of crystallographic defects plays a key role for the analysis of atomistic simulation output data. For face-centered cubic (fcc) metals, most existing structure analysis tools allow for the direct distinction of common defects, such as stacking faults or certain low-index surfaces. For body-centered cubic (bcc) metals, on the other hand, a robust way to identify such defects is currently not easily available. We therefore introduce a new method for analyzing atomistic configurations of bcc metals, the BCC Defect Analysis (BDA). It uses existing structure analysis algorithms and combines their results to uniquely distinguish between typical defects in bcc metals. In essence, the BDA method offers the following features:•Identification of typical defect structures in bcc metals.•Reduction of erroneously identified defects by iterative comparison to the defects in the atom's neighborhood.•Availability as ready-to-use Python script for the widespread visualization tool OVITO [http://ovito.org].

Structural phase transition, Néel temperature enhancement, and persistent magneto-dielectric coupling in Cr-substituted Mn3O4

NASA Astrophysics Data System (ADS)

Dwivedi, G. D.; Kumar, Abhishek; Yang, K. S.; Chen, B. Y.; Liu, K. W.; Chatterjee, Sandip; Yang, H. D.; Chou, H.

2016-05-01

Structural phase transition and Néel temperature (TN) enhancement were observed in Cr-substituted Mn3O4 spinels. Structural, magnetic, and dielectric properties of (Mn1-xCrx)3O4 (where x = 0.00, 0.10, 0.20, 0.25, 0.30, 0.40, and 0.50) were investigated. Cr-substitution induces room temperature structural phase transition from tetragonally distorted I41/amd (x = 0.00) to cubic Fd 3 ¯ m (x = 0.50). TN is found to increase from 43 K (x = 0.00) to 58 K (x = 0.50) with Cr-substitution. The spin ordering-induced dielectric anomaly near TN ensures that magneto-dielectric coupling persists in the cubic x = 0.50 system. X-ray absorption spectra reveal that Cr exists in a trivalent oxidation state and prefers the octahedral (Oh)-site, replacing Mn3+. Due to a reduction in the Jahn-Teller active Mn3+ cation and an increase in the smaller Cr3+ cation, the system begins to release the geometrical frustration by lowering its degeneracy. Consequently, a phase transition, from distorted tetragonal structure to the more symmetric cubic phase, occurs.

New lipid family that forms inverted cubic phases in equilibrium with excess water: molecular structure-aqueous phase structure relationship for lipids with 5,9,13,17-tetramethyloctadecyl and 5,9,13,17-tetramethyloctadecanoyl chains.

PubMed

Yamashita, Jun; Shiono, Manzo; Hato, Masakatsu

2008-10-02

With a view to discovering a new family of lipids that form inverted cubic phases, the aqueous phase behavior of a series of lipids with isoprenoid-type hydrophobic chains has been examined over a temperature range from -40 to 65 degrees C by using optical microscopy, DSC (differential scanning calorimetry), and SAXS (small-angle X-ray scattering) techniques. The lipids examined are those with 5,9,13,17-tetramethyloctadecyl and 5,9,13,17-tetramethyloctadecanoyl chains linked to a series of headgroups, that is, erythritol, pentaerythritol, xylose, and glucose. All of the lipid/water systems displayed a "water + liquid crystalline phase" two-phase coexistence state when sufficiently diluted. The aqueous phase structures of the most diluted liquid crystalline phases in equilibrium with excess water depend both on the lipid molecular structure and on the temperature. Given an isoprenoid chain, the preferred phase consistently follows a phase sequence of an H II (an inverted hexagonal phase) to a Q II (an inverted bicontinuous cubic phase) to an L alpha (a lamellar phase) as A* (cross-section area of the headgroup) increases. For a given lipid/water system, the phase sequence observed as the temperature increases is L alpha to Q II to H II. The present study allowed us to find four cubic phase-forming lipid species, PEOC 18+4 [mono- O-(5,9,13,17-tetramethyloctadecyl)pentaerythritol], beta-XylOC 18+4 [1- O-(5,9,13,17-tetramethyloctadecyl)-beta- d-xylopyranoside], EROCOC 17+4 [1- O-(5,9,13,17-tetramethyloctadecanoyl)erythritol], and PEOCOC 17+4 [mono- O-(5,9,13,17-tetramethyloctadecanoyl)pentaerythritol]. The values of T K (hydrated solid-liquid crystalline phase transition temperature) of the cubic phase-forming lipids are all below 0 degrees C. Quantitative analyses of the lipid molecular structure-aqueous phase structure relationship in terms of the experimentally evaluated "surfactant parameter" allow us to rationally select an optimum combination of hydrophilic/hydrophobic part of a lipid molecule that will form a desired phase in a desired temperature range.

Structural, Electronic and Elastic Properties of Heavy Fermion YbTM2 (TM= Ir and Pt) Laves Phase Compounds

NASA Astrophysics Data System (ADS)

Pawar, H.; Shugani, M.; Aynyas, M.; Sanyal, S. P.

2018-02-01

The structural, electronic and elastic properties of YbTM2 (TM = Ir and Pt) Laves phase intermetallic compounds which crystallize in cubic (MgCu2-type) structure, have been investigated using ab-initio full potential linearized augmented plane wave (FP-LAPW) method with LDA and LDA+U approximation. The calculated ground state properties such as lattice parameter (a0), bulk modulus (B) and its pressure derivative (B‧) are in good agreement with available experimental and theoretical data. The electronic properties are analyzed from band structures and density of states. Elastic constants are predicted first time for these compounds which obey the stability criteria for cubic system.

A cubic extended interior penalty function for structural optimization

NASA Technical Reports Server (NTRS)

Prasad, B.; Haftka, R. T.

1979-01-01

This paper describes an optimization procedure for the minimum weight design of complex structures. The procedure is based on a new cubic extended interior penalty function (CEIPF) used with the sequence of unconstrained minimization technique (SUMT) and Newton's method. The Hessian matrix of the penalty function is approximated using only constraints and their derivatives. The CEIPF is designed to minimize the error in the approximation of the Hessian matrix, and as a result the number of structural analyses required is small and independent of the number of design variables. Three example problems are reported. The number of structural analyses is reduced by as much as 50 per cent below previously reported results.

Instability of the layered orthorhombic post-perovskite phase of SrTiO3 and other candidate orthorhombic phases under pressure

NASA Astrophysics Data System (ADS)

Bhandari, Churna; Lambrecht, Walter R. L.

2018-06-01

While the tetragonal antiferro-electrically distorted (AFD) phase with space group I 4 / mcm is well known for SrTiO3 to occur below 105 K, there are also some hints in the literature of an orthorhombic phase, either at the lower temperature or at high pressure. A previously proposed orthorhombic layered structure of SrTiO3, known as the post-perovskite or CaIrO3 structure with space group Cmcm is shown to have significantly higher energy than the cubic or tetragonal phase and to have its minimum volume at larger volume than cubic perovskite. The Cmcm structure is thus ruled out. We also study an alternative Pnma phase obtained by two octahedral rotations about different axes. This phase is found to have slightly lower energy than the I 4 / mcm phase in spite of the fact that its parent, in-phase tilted P 4 / mbm phase is not found to occur. Our calculated enthalpies of formation show that the I 4 / mcm phase occurs at slightly higher volume than the cubic phase and has a negative transition pressure relative to the cubic phase, which suggests that it does not correspond to the high-pressure tetragonal phase. The enthalpy of the Pnma phase is almost indistinguishable from the I 4 / mcm phase. Alternative ferro-electric tetragonal and orthorhombic structures previously suggested in literature are discussed.

The incorporation of plutonium in lanthanum zirconate pyrochlore

NASA Astrophysics Data System (ADS)

Gregg, Daniel J.; Zhang, Yingjie; Middleburgh, Simon C.; Conradson, Steven D.; Triani, Gerry; Lumpkin, Gregory R.; Vance, Eric R.

2013-11-01

The incorporation of plutonium (Pu) within lanthanum zirconate pyrochlore was investigated using air, argon, and N2-3.5%H2 sintering atmospheres together with Ca2+ and Sr2+ incorporation for charge compensation. The samples have been characterised in the first instance by X-ray diffraction (XRD), scanning electron microscopy (SEM) and diffuse reflectance spectroscopy (DRS). The results show Pu can be exchanged for La3+ on the A-site with and without charge compensation and for Zr4+ on the B-site. DRS measurements were made over the wavenumber range of 4000-19,000 cm-1 and the Pu in all air- and argon-sintered samples was found to be present as Pu4+ while that in samples sintered in N2-3.5%H2 was present as Pu3+. The Pu valence was confirmed for three of the samples using X-ray near-edge absorption spectroscopy (XANES). Pu valences >4+ were not observed in any of the samples.

Corrosion testing of candidates for the alkaline fuel cell cathode

NASA Technical Reports Server (NTRS)

Singer, Joseph; Fielder, William L.

1989-01-01

Current/voltage data was obtained for specially made corrosion electrodes of some oxides and of gold materials for the purpose of developing a screening test of catalysts and supports for use at the cathode of the alkaline fuel cell. The data consists of measurements of current at fixed potentials and cyclic voltammograms. These data will have to be correlated with longtime performance data in order to fully evaluate this approach to corrosion screening. Corrosion test screening of candidates for the oxygen reduction electrode of the alkaline fuel cell was applied to two substances, the pyrochlore Pb2Ru2O6.5 and the spinel NiCo2O4. The substrate gold screen and a sample of the IFC Orbiter Pt-Au performance electrode were included as blanks. The pyrochlore data indicate relative stability, although nothing yet can be said about long term stability. The spinel was plainly unstable. For this type of testing to be validated, comparisons will have to be made with long term performance tests.

Emergent Topological Phenomena in Thin Films of Pyrochlore Iridates

NASA Astrophysics Data System (ADS)

Yang, Bohm-Jung; Nagaosa, Naoto

2014-06-01

Because of the recent development of thin film and artificial superstructure growth techniques, it is possible to control the dimensionality of the system, smoothly between two and three dimensions. In this Letter we unveil the dimensional crossover of emergent topological phenomena in correlated topological materials. In particular, by focusing on the thin film of pyrochlore iridate antiferromagnets grown along the [111] direction, we demonstrate that the thin film can have a giant anomalous Hall conductance, proportional to the thickness of the film, even though there is no Hall effect in 3D bulk material. Moreover, in the case of ultrathin films, a quantized anomalous Hall conductance can be observed, despite the fact that the system is an antiferromagnet. In addition, we uncover the emergence of a new topological phase, the nontrivial topological properties of which are hidden in the bulk insulator and manifest only in thin films. This shows that the thin film of correlated topological materials is a new platform to search for unexplored novel topological phenomena.

Stabilization of the cubic phase of HfO2 by Y addition in films grown by metal organic chemical vapor deposition

NASA Astrophysics Data System (ADS)

Rauwel, E.; Dubourdieu, C.; Holländer, B.; Rochat, N.; Ducroquet, F.; Rossell, M. D.; Van Tendeloo, G.; Pelissier, B.

2006-07-01

Addition of yttrium in HfO2 thin films prepared on silicon by metal organic chemical vapor deposition is investigated in a wide compositional range (2.0-99.5at.%). The cubic structure of HfO2 is stabilized for 6.5at.%. The permittivity is maximum for yttrium content of 6.5-10at.%; in this range, the effective permittivity, which results from the contribution of both the cubic phase and silicate phase, is of 22. These films exhibit low leakage current density (5×10-7A /cm2 at -1V for a 6.4nm film). The cubic phase is stable upon postdeposition high temperature annealing at 900°C under NH3.

Negative thermal expansion materials related to cubic zirconium tungstate

NASA Astrophysics Data System (ADS)

Lind, Cora

2001-12-01

A non-hydrolytic sol-gel method for the preparation of ZrW2O 8 was developed. A new trigonal polymorph was discovered, which is structurally related to trigonal ZrMO2O8 and MnRe2O 8 as evidenced by powder x-ray diffraction and EXAFS studies. Seeding of the starting mixtures with cubic ZrW2O8 promoted crystallization of the cubic phase instead of trigonal material. Dehydration of ZrW2O7(OH)2·2H 2O gave cubic ZrW2O8 at 650°C, and a modification of this route led to the discovery of the new NTE materials cubic ZrMo 2O8 and HfMo2O8. These compounds crystallize in the same temperature range as the more stable trigonal AMo2O 8 polymorphs. To facilitate preparation of phase pure cubic molybdates, the influence of precursor chemistry on the crystallization behavior was investigated. The synthesis was extended to the solid solution system ZrxHf 1-xMoyW2-yO8 (0 ≤ x ≤ 1, 0 ≤ y ≤ 2). All compounds showed negative thermal expansion between 77 and 573 K. High-pressure in situ diffraction experiments were conducted on several AM2O8 polymorphs. With the exception of monoclinic ZrMo2O8, all materials underwent at least one pressure induced phase transition. Quasi-hydrostatic experiments on cubic AMo 2O8 led to a reversible transition to a new high-pressure structure, while low-pressure amorphization was observed under non-hydrostatic conditions. Isothermal kinetic studies of the cubic to trigonal transformation for ZrMo2O8 were carried out on four samples. Apparent activation energies of 170--290 kJ/mol were obtained using an Avrami model in combination with an Arrhenius analysis. This corresponds to 5% conversion levels after one year at temperatures between 220 and 315°C. Ex situ studies showed that the conversion at lower temperatures was considerably slower than what would be expected from extrapolation of the kinetic data. Drop solution calorimetry was carried out on several polymorphs of ZrMo 2O8, HfMo2O8 and ZrW2O 8. Only monoclinic ZrMo2O8 was enthalpically stabilized with respect to the binary oxides. For all other polymorphs, the differences in enthalpies of formation from the binary oxides for each AM2O 8 system (A = Zr, Hf; M = Mo, W) were small. Attempts to synthesize new materials MIIRe2O 8 (M = Mg, Zn, Mn, Co) with the cubic ZrW2O8 structure from a hydrate precursor were not successful.

Ionic Graphitization of Ultrathin Films of Ionic Compounds.

PubMed

Kvashnin, A G; Pashkin, E Y; Yakobson, B I; Sorokin, P B

2016-07-21

On the basis of ab initio density functional calculations, we performed a comprehensive investigation of the general graphitization tendency in rocksalt-type structures. In this paper, we determine the critical slab thickness for a range of ionic cubic crystal systems, below which a spontaneous conversion from a cubic to a layered graphitic-like structure occurs. This conversion is driven by surface energy reduction. Using only fundamental parameters of the compounds such as the Allen electronegativity and ionic radius of the metal atom, we also develop an analytical relation to estimate the critical number of layers.

First-principles phase stability at high temperatures and pressure in Nb 90Zr 10 alloy

DOE PAGES

Landa, A.; Soderlind, P.

2016-08-18

The phase stability of Nb 90Zr 10 alloy at high temperatures and compression is explored by means of first-principles electronic-structure calculations. Utilizing the self-consistent ab initio lattice dynamics (SCAILD) approach in conjunction with density-functional theory, we show that pressure-induced mechanical instability of the body-centered cubic phase, which results in formation of a rhombohedral phase at around 50 GPa, will prevail significant heating. As a result, the body-centered cubic structure will recover before melting at ~1800 K.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chezhina, N.V., E-mail: chezhina@nc2490.spb.edu; Korolev, D.A.; Zhuk, N.A.

On the basis of the results of magnetic susceptibility and ESR studies of the Bi{sub 3}Nb{sub 1−x}Fe{sub x}O{sub 7−δ} solid solutions iron atoms in the solid solutions of cubic modification of bismuth niobate were found to exist as Fe(III) monomers and exchange bound Fe(III)-O-Fe(III) dimers with antiferro- and ferromagnetic type of superexchange. The exchange parameters and the distribution of monomers and dimers in the solid solutions were calculated as a function of paramagnetic atom content. - Graphical abstract: The study of the Bi{sub 3}Nb{sub 1−x}Fe{sub x}O{sub 7−δ} solid solutions showed that the introduction of iron atoms into the structure ofmore » Bi{sub 3}NbO{sub 7} stabilizes the cubic structure of bismuth niobate making the phase transition tetragonal ↔ cubic structure irreversible. In the Bi{sub 3}Nb{sub 1−x}Fe{sub x}O{sub 7−δ} solid solutions we observe the formation of dimers with antiferro- and ferromagnetic exchange. Such clusters are partially retained even at the infinite dilution of the solid solution, which testifies for their rigidity. A sufficiently high parameter of ferromagnetic exchange in a dimer (+53 cm{sup −1}) seems to result from iron atoms being located in the vicinity of oxygen vacancy. - Highlights: • The reversible transition cubic – tetragonal modifications in Bi{sub 3}NbO{sub 7} becomes irreversible. • Only cubic modification of Bi{sub 3}Nb{sub 1-x}Fe{sub x}O{sub 7-δ} is stable due to clusters of Fe atoms. • These clusters are sufficiently strong and retained even at the infinite dilution. • The calculations of magnetic susceptibility give the distribution of the clusters and single atoms.« less

The effect of relativity on stability of Copernicium phases, their electronic structure and mechanical properties

NASA Astrophysics Data System (ADS)

Čenčariková, Hana; Legut, Dominik

2018-05-01

The phase stability of the various crystalline structures of the super-heavy element Copernicium was determined based on the first-principles calculations with different levels of the relativistic effects. We utilized the Darwin term, mass-velocity, and spin-orbit interaction with the single electron framework of the density functional theory while treating the exchange and correlation effects using local density approximations. It is found that the spin-orbit coupling is the key component to stabilize the body-centered cubic (bcc) structure over the hexagonal closed packed (hcp) structure, which is in accord with Sol. Stat. Comm. 152 (2012) 530, but in contrast to Atta-Fynn and Ray (2015) [11], Gaston et al. (2007) [10], Papaconstantopoulos (2015) [9]. It seems that the main role here is the correct description of the semi-core relativistic 6p1/2 orbitals. The all other investigated structures, i.e. face-centered cubic (fcc) , simple cubic (sc) as well as rhombohedral (rh) structures are higher in energy. The criteria of mechanical stability were investigated based on the calculated elastic constants, identifying the phase instability of fcc and rh structures, but surprisingly confirm the stability of the energetically higher sc structure. In addition, the pressure-induced structural transition between two stable sc and bcc phases has been detected. The ground-state bcc structure exhibits the highest elastic anisotropy from single elements of the Periodic table. At last, we support the experimental findings that Copernicium is a metal.

Exploring the thermal expansion of fluorides and oxyfluorides with rhenium trioxide-type structures: From negative to positive thermal expansion

NASA Astrophysics Data System (ADS)

Greve, Benjamin K.

This thesis explores the thermal expansion and high pressure behavior of some materials with the ReO3 structure type. This structure is simple and has, in principle, all of the features necessary for negative thermal expansion (NTE) arising from the transverse thermal motion of the bridging anions and the coupled rotation of rigid units; however, ReO 3 itself only exhibits mild NTE across a narrow temperature range at low temperatures. ReO3 is metallic because of a delocalized d-electron, and this may contribute to the lack of NTE in this material. The materials examined in this thesis are all based on d 0 metal ions so that the observed thermal expansion behavior should arise from vibrational, rather than electronic, effects. In Chapter 2, the thermal expansion of scandium fluoride, ScF3 , is examined using a combination of in situ synchrotron X-ray and neutron variable temperature diffraction. ScF3 retains the cubic ReO3 structure across the entire temperature range examined (10 - 1600 K) and exhibits pronounced negative thermal expansion at low temperatures. The magnitude of NTE in this material is comparable to that of cubic ZrW2O8, which is perhaps the most widely studied NTE material, at room temperature and below. This is the first report of NTE in an ReO3 type structure across a wide temperature range. Chapter 3 presents a comparison between titanium oxyfluoride, TiOF 2, and a vacancy-containing titanium hydroxyoxyfluoride, Ti x(O/OH/F)3. TiOF2 was originally reported to adopt the cubic ReO3 structure type under ambient conditions, therefore the initial goal for this study was to examine the thermal expansion of this material and determine if it displayed interesting behavior such as NTE. During the course of the study, it was discovered that the original synthetic method resulted in Tix(O/OH/F)3, which does adopt the cubic ReO3 structure type. The chemical composition of the hydroxyoxyfluoride is highly dependent upon synthesis conditions and subsequent heat treatments. This material readily pyrohydrolyizes at low temperatures (≈350 K). It was also observed that TiOF does not adopt the cubic ReO 3 structure; at room temperature it adopts a rhombohedrally distorted variant of the ReO3 structure. Positive thermal expansion was observed for TiOF2 from 120 K through decomposition into TiO2. At ≈400 K, TiOF2 undergoes a structural phase transition from rhombohedral to cubic symmetry. High pressure diffraction studies revealed a cubic to rhombohedral phase transition for Tix(O/OH/F) 3 between 0.5-1 GPa. No phase transitions were observed for TiOF 2 on compression. In Chapter 4, an in situ variable pressure-temperature diffraction experiment examining the effects of pressure on the coefficients of thermal expansion (CTE) for ScF3 and TaO2F is presented. In the manufacture and use of composites, which is a possible application for low and NTE materials, stresses may be experienced. Pressure was observed to have a negligible effect on cubic ScF3's CTE; however, for TaO 2F the application of modest pressures, such as those that might be experienced in the manufacture or use of composites, has a major effect on its CTE. This effect is associated with a pressure-induced phase transition from cubic to rhombohedral symmetry upon compression. TaO2F was prepared from the direct reaction of Ta2O5 with TaF 5 and from the digestion of Ta2O5 in hot hydrofluoric acid. The effects of pressure on the two samples of TaO2F were qualitatively similar. The slightly different properties for the samples are likely due to differences in their thermal history leading to differing arrangements of oxide and fluoride in these disordered materials. In Chapter 5, the local structures of TiOF2 and TaO2 F are examined using pair distribution functions (PDFs) obtained from X-ray total scattering experiments. In these materials, the anions (O/F) are disordered over the available anion positions. While traditional X-ray diffraction provides detailed information about the average structures of these materials, it is not sufficient to fully understand their thermal expansion. Fits of simple structural models to the low r portions of PDFs for these materials indicate the presence of geometrically distinct M -X-M (M = Ti, Ta; X = O, F) linkages, and a simple analysis of the TaO2F variable temperature PDFs indicates that these distinct links respond differently to temperature.

Fundamental aspects of the structural and electrolyte properties of Li2OHCl from simulations and experiment

NASA Astrophysics Data System (ADS)

Howard, Jason; Hood, Zachary D.; Holzwarth, N. A. W.

2017-12-01

Solid-state electrolytes that are compatible with high-capacity electrodes are expected to enable the next generation of batteries. As a promising example, Li2OHCl was reported to have good ionic conductivity and to be compatible with a lithium metal anode even at temperatures above 100 ∘C . In this work, we explore the fundamental properties of Li2OHCl by comparing simulations and experiments. Using calculations based on density functional theory, including both static and dynamic contributions through the quasiharmonic approximation, we model a tetragonal ground state, which is not observed experimentally. An ordered orthorhombic low-temperature phase was also simulated, agreeing with experimental structural analysis of the pristine electrolyte at room temperature. In addition, comparison of the ordered structures with simulations of the disordered cubic phase provide insight into the mechanisms associated with the experimentally observed abrupt increase in ionic conductivity as the system changes from its ordered orthorhombic to its disordered cubic phase. A large Haven ratio for the disordered cubic phase is inferred from the computed tracer diffusion coefficient and measured ionic conductivity, suggesting highly correlated motions of the mobile Li ions in the cubic phase of Li2OHCl . We find that the OH bond orientations participate in gating the Li ion motions which might partially explain the predicted Li-Li correlations.

75 FR 81592 - National Energy Technology Laboratory; Notice of Intent To Grant Exclusive License

Federal Register 2010, 2011, 2012, 2013, 2014

2010-12-28

..., entitled ``Method for designing a reforming and/or combustion catalyst system'' and ``Pyrochlore-type catalysts for the reforming of hydrocarbon fuels,'' respectively, to Pyrochem Catalyst [[Page 81593... filing written objections. Pyrochem Catalyst Corporation, a new small business, has applied for an...

Study of structural properties of cubic InN films on GaAs(001) substrates by molecular beam epitaxy and migration enhanced epitaxy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Casallas-Moreno, Y. L.; Perez-Caro, M.; Gallardo-Hernandez, S.

InN epitaxial films with cubic phase were grown by rf-plasma-assisted molecular beam epitaxy (RF-MBE) on GaAs(001) substrates employing two methods: migration-enhanced epitaxy (MEE) and conventional MBE technique. The films were synthesized at different growth temperatures ranging from 490 to 550 Degree-Sign C, and different In beam fluxes (BEP{sub In}) ranging from 5.9 Multiplication-Sign 10{sup -7} to 9.7 Multiplication-Sign 10{sup -7} Torr. We found the optimum conditions for the nucleation of the cubic phase of the InN using a buffer composed of several thin layers, according to reflection high-energy electron diffraction (RHEED) patterns. Crystallographic analysis by high resolution X-ray diffraction (HR-XRD)more » and RHEED confirmed the growth of c-InN by the two methods. We achieved with the MEE method a higher crystal quality and higher cubic phase purity. The ratio of cubic to hexagonal components in InN films was estimated from the ratio of the integrated X-ray diffraction intensities of the cubic (002) and hexagonal (1011) planes measured by X-ray reciprocal space mapping (RSM). For MEE samples, the cubic phase of InN increases employing higher In beam fluxes and higher growth temperatures. We have obtained a cubic purity phase of 96.4% for a film grown at 510 Degree-Sign C by MEE.« less

Pharmacokinetics and enhanced oral bioavailability in beagle dogs of cyclosporine A encapsulated in glyceryl monooleate/poloxamer 407 cubic nanoparticles

PubMed Central

Lai, Jie; Lu, Yi; Yin, Zongning; Hu, Fuqiang; Wu, Wei

2010-01-01

Efforts to improve the oral bioavailability of cyclosporine A (CyA) remains a challenge in the field of drug delivery. In this study, glyceryl monooleate (GMO)/poloxamer 407 cubic nanoparticles were evaluated as potential vehicles to improve the oral bioavailability of CyA. Cubic nanoparticles were prepared via the fragmentation of a bulk GMO/poloxamer 407 cubic phase gel by sonication and homogenization. The cubic inner structure formed was verified using Cryo-TEM. The mean diameters of the nanoparticles were about 180 nm, and the entrapment efficiency of these particles for CyA was over 85%. The in vitro release of CyA from these nanoparticles was less than 5% at 12 h. The results of a pharmacokinetic study in beagle dogs showed improved absorption of CyA from cubic nanoparticles as compared to microemulsion-based Neoral®; higher Cmax (1371.18 ± 37.34 vs 969.68 ± 176.3 ng mL−1), higher AUC0–t (7757.21 ± 1093.64 vs 4739.52 ± 806.30 ng h mL−1) and AUC0–∞ (9004.77 ± 1090.38 vs 5462.31 ± 930.76 ng h mL−1). The relative oral bioavailability of CyA cubic nanoparticles calculated on the basis of AUC0–∞ was about 178% as compared to Neoral®. The enhanced bioavailability of CyA is likely due to facilitated absorption by cubic nanoparticles rather than improved release. PMID:20161984

Structure and phase formation behavior and dielectric and magnetic properties of lead iron tantalate-lead zirconate titanate multiferroic ceramics

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wongmaneerung, R., E-mail: re_nok@yahoo.com; Tipakontitikul, R.; Jantaratana, P.

2016-03-15

Highlights: • The multiferroic ceramics consisted of PFT and PZT. • Crystal structure changed from cubic to mixedcubic and tetragonal with increasing PZT content. • Dielectric showed the samples underwent a typical relaxor ferroelectric behavior. • Magnetic properties showed very interesting behavior with square saturated magnetic hysteresis loops. - Abstract: Multiferroic (1 − x)Pb(Fe{sub 0.5}Ta{sub 0.5})O{sub 3}–xPb(Zr{sub 0.53}Ti{sub 0.47})O{sub 3} (or PFT–PZT) ceramics were synthesized by solid-state reaction method. The crystal structure and phase formation of the ceramics were examined by X-ray diffraction (XRD). The local structure surrounding Fe and Ti absorbing atoms was investigated by synchrotron X-ray Absorption Near-Edgemore » Structure (XANES) measurement. Dielectric properties were studied as a function of frequency and temperature using a LCR meter. A vibrating sample magnetometer (VSM) was used to determine the magnetic hysteresis loops. XRD study indicated that the crystal structure of the sample changed from pure cubic to mixed cubic and tetragonal with increasing PZT content. XANES measurements showed that the local structure surrounding Fe and Ti ions was similar. Dielectric study showed that the samples underwent a typical relaxor ferroelectric behavior while the magnetic properties showed very interesting behavior with square saturated magnetic hysteresis loops.« less

Synthesis of Mn doped ZnS nanocrystals: Crystallographic and morphological study

NASA Astrophysics Data System (ADS)

Shaikh, Azharuddin Z.; Shirsath, Narendra B.; Sonawane, Prabhakar S.

2018-05-01

The influence of doping concentration on the physical properties of ZnS nanocrystals synthesized using coprecipitation method at room temperature is reported in this paper. In particular, we have studied the structural properties of Zn1-xMnxS where (x=0.01, 0.03, 0.05) by X-ray diffraction. X-ray peak broadening analysis used to calculate the crystalline sizes, lattice parameters, number of unit cell per particle and volume of unit cell. Crystalline ZnS with a cubic structure is confirmed by XRD results. The grain size of pure and Mn doped samples were found in the range of 7nm to 9nm. All the physical parameters of cubic ZnS nanocrystals were calculated are similar with standard values. The scanning electron microscope (SEM) which revealed that the synthesized nanocrystals are well-crystalline and possessing cubic phase.

Low temperature formation of higher-k cubic phase HfO{sub 2} by atomic layer deposition on GeO{sub x}/Ge structures fabricated by in-situ thermal oxidation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, R., E-mail: zhang@mosfet.t.u-tokyo.ac.jp; Department of Information Science and Electronic Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027; Huang, P.-C.

2016-02-01

We have demonstrated a low temperature formation (300 °C) of higher-k HfO{sub 2} using atomic layer deposition (ALD) on an in-situ thermal oxidation GeO{sub x} interfacial layer. It is found that the cubic phase is dominant in the HfO{sub 2} film with an epitaxial-like growth behavior. The maximum permittivity of 42 is obtained for an ALD HfO{sub 2} film on a 1-nm-thick GeO{sub x} form by the in-situ thermal oxidation. It is suggested from physical analyses that the crystallization of cubic phase HfO{sub 2} can be induced by the formation of six-fold crystalline GeO{sub x} structures in the underlying GeO{sub x}more » interfacial layer.« less

Cubic and Rhombohedral Heterobimetallic Networks Constructed from Uranium, Transition Metals, and Phosphonoacetate. New Methods for Constructing Porous Materials

DOE Office of Scientific and Technical Information (OSTI.GOV)

Alsobrook, Andera N.; Hauser, B. G.; Hupp, Joseph T.

2010-11-01

Four heterobimetallic U(VI)/M(II) (M = Mn, Co, Cd) carboxyphosphonates have been synthesized. M 2[(UO 2) 6(PO 3CH 2CO 2) 3O 3(OH)(H 2O) 2]·16H 2O (M = Mn(II), Co(II), and Cd(II)) adopt cubic three-dimensional network structures with large cavities approximately 16 Å in diameter that are filled with co-crystallized water molecules. [Cd 3(UO 2) 6(PO 3CH 2CO 2) 6(H 2O) 13]·6H 2 O forms a rhombohedral channel structure with hydrated Cd(II) within the channels. The cubic compound (Co) displays differential gas absorption with a surface area for CO 2 uptake of 40 m 2 g -1 at 273 K, and nomore » uptake of N 2 at 77 K.« less

Structural and optical properties of NiFe2O4 synthesized via green technology

NASA Astrophysics Data System (ADS)

Patel, S.; Saleem, M.; Varshney, Dinesh

2018-05-01

The nanoparticles of NiFe2O4 were successfully synthesized via green technology using banana peel extract as the catalyst as well as the medium for reaction technique is reported. Analysis of X-ray diffraction spectrum revealed the cubic structure for the prepared spinel ferrite samples crystallized into cubic spinel structure with the space group Fd3m. The Retvield refinement was carried out which obeyed the results obtained from the XRD spectrum analysis of the sample. Raman spectrum provided confirmation for the spinel structure formation and five active Raman modes were observed. Since the optical band-gap value shows inverse response to the crystallite size, The UV-Vis spectrum study confirmed dual but reduced band-gap value.

Polytype transition of N-face GaN:Mg from wurtzite to zinc-blende

NASA Astrophysics Data System (ADS)

Monroy, E.; Hermann, M.; Sarigiannidou, E.; Andreev, T.; Holliger, P.; Monnoye, S.; Mank, H.; Daudin, B.; Eickhoff, M.

2004-10-01

We have investigated the polytype conversion of a GaN film from N-face wurtzite (2H) to zinc-blende (3C) structure due to Mg doping during growth by plasma-assisted molecular-beam epitaxy. Structural analysis by high-resolution transmission electron microscopy and high-resolution x-ray diffraction measurement revealed alignment of the cubic phase with the [111] axis perpendicular to the substrate surface. The optical characteristics of GaN:Mg layers are shown to be very sensitive to the presence of the cubic polytype. For low Mg doping, photoluminescence is dominated by a phonon-replicated donor-acceptor pair at ˜3.25eV, related to the shallow Mg acceptor level, accompanied by a narrow excitonic emission. For high Mg doping, the photoluminescence spectra are also dominated by a line around 3.25eV, but this emission displays the behavior of excitonic luminescence from cubic GaN. A cubic-related donor-acceptor transition at ˜3.16eV is also observed, together with a broad blue band around 2.9eV, previously reported in heavily Mg-doped 3C-GaN(001).

Structure, rheology and shear alignment of Pluronic block copolymer mixtures.

PubMed

Newby, Gemma E; Hamley, Ian W; King, Stephen M; Martin, Christopher M; Terrill, Nicholas J

2009-01-01

The structure and flow behaviour of binary mixtures of Pluronic block copolymers P85 and P123 is investigated by small-angle scattering, rheometry and mobility tests. Micelle dimensions are probed by dynamic light scattering. The micelle hydrodynamic radius for the 50/50 mixture is larger than that for either P85 or P123 alone, due to the formation of mixed micelles with a higher association number. The phase diagram for 50/50 mixtures contains regions of cubic and hexagonal phases similar to those for the parent homopolymers, however the region of stability of the cubic phase is enhanced at low temperature and concentrations above 40 wt%. This is ascribed to favourable packing of the mixed micelles containing core blocks with two different chain lengths, but similar corona chain lengths. The shear flow alignment of face-centred cubic and hexagonal phases is probed by in situ small-angle X-ray or neutron scattering with simultaneous rheology. The hexagonal phase can be aligned using steady shear in a Couette geometry, however the high modulus cubic phase cannot be aligned well in this way. This requires the application of oscillatory shear or compression.

Properties of tetrahedral clusters and medium range order in GaN during rapid solidification

NASA Astrophysics Data System (ADS)

Gao, Tinghong; Li, Yidan; Yao, Zhenzhen; Hu, Xuechen; Xie, Quan

2017-12-01

The solidification process of liquid gallium nitride has been studied by molecular dynamics simulation using the Stillinger-Weber potential at cooling rate of 10 K/ps. The structural properties of gallium nitride during the rapid cooling process were investigated in detail by the radial distribution functions, Voronoi polyhedron index and the visualization technology. The amorphous structures were formed with many medium range order structures at 200 K. The <4 0 0 0> polyhedron as the main polyhedron was more stable than other polyhedron in GaN during the quenching process. The cubic and hexahedral medium range order structures were formed by the close link between <4 0 0 0> polyhedron. The cubic crystal structures grew up through the crystalline surface by a layer-by-layer method to become more stable structures during the quenching process.

Type I antiferromagnetic order in Ba 2LuReO 6: Exploring the role of structural distortions in double perovskites containing 5d 2 ions

DOE PAGES

Xiong, Jie; Yan, Jiaqiang; Aczel, Adam A.; ...

2017-12-02

The structural, electrical, and magnetic properties of the double perovskite Ba 2LuReO 6 have been examined in this paper. It is an insulator whose temperature dependent conductivity is consistent with variable range hopping electrical transport. A transition to an antiferromagnet state with type I order occurs below T N = 31 K. High resolution time-of-flight neutron powder diffraction measurements show that it retains the cubic double perovskite structure down to 10 K. High intensity, low resolution neutron powder diffraction measurements confirm the antiferromagnetic order and indicate that cubic symmetry is still observed at 1.5 K. The small ordered moment ofmore » 0.34(4)μ B per Re is comparable to estimates of moments on 5d 2 ions in other antiferromagnetically ordered cubic double perovskites. Finally, comparisons with related double perovskites containing 5d 2 ions, such as Os 6+ and Re 5+, reveal that subtle changes in structure or electron configuration of the diamagnetic octahedral cations can have a large impact on the magnetic ground state, the size of the ordered moment, and the Néel temperature.« less

Experimental exploration of novel semimetal state in strong anisotropic Pyrochlore iridate Nd2lr2O7 under high magnetic field

NASA Astrophysics Data System (ADS)

Tian, Z. M.; Kohama, Y.; Tomita, T.; Ishikawa, J.; Mairo, H.; Kindo, K.; Nakatsuji, S.

2016-02-01

We report the anisotropic magnetotransport of Nd2Ir2O7 single crystal under high magnetic field (B) up to 50 T with B along various directions. Only for B // [001] direction, a novel semimetal state is realized under high magnetic field evidenced by a field-induced insulating-semimetalic phase transition with critical field BMI∼12 T related to the destruction of all-in-all-out (AIAO) state. In contrast, abnormal magnetotransport hysteresis behavior is observed for B // [111] direction below the metal-insulator transition temperature (TMI), and magnetotransport reveals the Ir spin structure keep in the AIAO state under high magnetic field with temperature just below TMI, in prospect to realize Weyl semimetal state.

Monitoring a Silent Phase Transition in CH 3NH 3PbI 3 Solar Cells via Operando X-ray Diffraction

DOE PAGES

Schelhas, Laura T.; Christians, Jeffrey A.; Berry, Joseph J.; ...

2016-10-13

The relatively modest temperature of the tetragonal-to-cubic phase transition in CH 3NH 3PbI 3 perovskite is likely to occur during real world operation of CH 3NH 3PbI 3 solar cells. In this work, we simultaneously monitor the structural phase transition of the active layer along with solar cell performance as a function of the device operating temperature. The tetragonal to cubic phase transition is observed in the working device to occur reversibly at temperatures between 60.5 and 65.4 degrees C. In these operando measurements, no discontinuity in the device performance is observed, indicating electronic behavior that is insensitive to themore » structural phase transition. Here, this decoupling of device performance from the change in long-range order across the phase transition suggests that the optoelectronic properties are primarily determined by the local structure in CH 3NH 3PbI 3. That is, while the average crystal structure as probed by X-ray diffraction shows a transition from tetragonal to cubic, the local structure generally remains well characterized by uncorrelated, dynamic octahedral rotations that order at elevated temperatures but are unchanged locally.« less

Monitoring a Silent Phase Transition in CH 3NH 3PbI 3 Solar Cells via Operando X-ray Diffraction

DOE Office of Scientific and Technical Information (OSTI.GOV)

Schelhas, Laura T.; Christians, Jeffrey A.; Berry, Joseph J.

The relatively modest temperature of the tetragonal-to-cubic phase transition in CH 3NH 3PbI 3 perovskite is likely to occur during real world operation of CH 3NH 3PbI 3 solar cells. In this work, we simultaneously monitor the structural phase transition of the active layer along with solar cell performance as a function of the device operating temperature. The tetragonal to cubic phase transition is observed in the working device to occur reversibly at temperatures between 60.5 and 65.4 degrees C. In these operando measurements, no discontinuity in the device performance is observed, indicating electronic behavior that is insensitive to themore » structural phase transition. Here, this decoupling of device performance from the change in long-range order across the phase transition suggests that the optoelectronic properties are primarily determined by the local structure in CH 3NH 3PbI 3. That is, while the average crystal structure as probed by X-ray diffraction shows a transition from tetragonal to cubic, the local structure generally remains well characterized by uncorrelated, dynamic octahedral rotations that order at elevated temperatures but are unchanged locally.« less

An evaluation of concrete recycling and reuse practices

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nakhjiri, K.S.; MacKinney, J.

1997-02-01

Nuclear facilities operated by the Department of Energy (DOE), Department of Defense (DOD), and NRC licensees contain many concrete structures that are contaminated with radioactivity. Dismantling these structures will result in significant quantities of waste materials, both contaminated and uncontaminated. Bartlett estimates the total volume of waste from demolition of concrete structures to be on the order of 4 million cubic meters, but that only 20,000 cubic meters would be contaminated with radioactivity. Other studies suggest that as much as 5% of the concrete in these facilities would be contaminated with radioactivity. While the actual quantity of contaminated material shouldmore » be fixed with greater precision, the fact that so much uncontaminated concrete exists (over 95% of the total 4 million cubic meters) suggests that a program that recycles concrete could produce substantial savings for both government agencies (DOE, DOD) and private companies (NRC licensees). This paper presents a fundamental discussion of (1) various methods of processing concrete, (2) demolition methods, especially those compatible with recycling efforts, and (3) state-of-the-art concrete dismantlement techniques.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xiong, Jie; Yan, Jiaqiang; Aczel, Adam A.

The structural, electrical, and magnetic properties of the double perovskite Ba 2LuReO 6 have been examined in this paper. It is an insulator whose temperature dependent conductivity is consistent with variable range hopping electrical transport. A transition to an antiferromagnet state with type I order occurs below T N = 31 K. High resolution time-of-flight neutron powder diffraction measurements show that it retains the cubic double perovskite structure down to 10 K. High intensity, low resolution neutron powder diffraction measurements confirm the antiferromagnetic order and indicate that cubic symmetry is still observed at 1.5 K. The small ordered moment ofmore » 0.34(4)μ B per Re is comparable to estimates of moments on 5d 2 ions in other antiferromagnetically ordered cubic double perovskites. Finally, comparisons with related double perovskites containing 5d 2 ions, such as Os 6+ and Re 5+, reveal that subtle changes in structure or electron configuration of the diamagnetic octahedral cations can have a large impact on the magnetic ground state, the size of the ordered moment, and the Néel temperature.« less

High-entropy alloys in hexagonal close-packed structure

DOE PAGES

Gao, Michael C.; Zhang, B.; Guo, S. M.; ...

2015-08-28

The microstructures and properties of high-entropy alloys (HEAs) based on the face-centered cubic and body-centered cubic structures have been studied extensively in the literature, but reports on HEAs in the hexagonal close-packed (HCP) structure are very limited. Using an efficient strategy in combining phase diagram inspection, CALPHAD modeling, and ab initio molecular dynamics simulations, a variety of new compositions are suggested that may hold great potentials in forming single-phase HCP HEAs that comprise rare earth elements and transition metals, respectively. Lastly, experimental verification was carried out on CoFeReRu and CoReRuV using X-ray diffraction, scanning electron microscopy, and energy dispersion spectroscopy.

Bismuth doping strategies in GeTe nanowires to promote high-temperature phase transition from rhombohedral to face-centered cubic structure

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Jie; Huang, Rong; Wei, Fenfen

2014-11-17

The phase transition of Bi-doped (∼3 at. %) GeTe nanowires from a rhombohedral (R) to a face-centered cubic (C) structure was observed in in situ high-temperature X-ray diffraction. The promotion of high-temperature R-C phase transition by a doping approach was revealed. Ab initio energy calculations of doped GeTe at various Bi doping concentrations were performed to interpret the promoted temperature-induced phase transitions. Those results indicated that the total energy differences between R and C structures of doped GeTe decreased as Bi doping concentrations increased, which facilitated R-C phase transitions.

Polarization Change in Face-Centered Cubic Opal Films

NASA Astrophysics Data System (ADS)

Wolff, Christian; Romanov, Sergei; Küchenmeister, Jens; Peschel, Ulf; Busch, Kurt

2011-10-01

Artificial opals are a popular platform for investigating fundamental properties of Photonic Crystals (PhC). In this work, we provide a theoretical analysis of polarization-resolved transmission experiments through thin opal films. Despite the full cubic symmetry of the PhC, this system provides a very efficient mechanism for manipulating the polarization state of light. Based on band structure calculations and Bloch mode analysis, we find that this effect closely resembles classical birefringence. Due to the cubic symmetry, however, a description using tensorial quantities is not possible. This indicates fundamental limitations of effective material models for Photonic Crystals and demonstrates the importance of accurately modelling the microscopic geometry of such systems.

The competitive growth of cubic domains in Ti(1-x)AlxN films studied by diffraction anomalous near-edge structure spectroscopy.

PubMed

Pinot, Y; Tuilier, M-H; Pac, M-J; Rousselot, C; Thiaudière, D

2015-11-01

Titanium and aluminium nitride films deposited by magnetron sputtering generally grow as columnar domains made of oriented nanocrystallites with cubic or hexagonal symmetry depending on Al content, which are embedded in more disordered grain boundaries. The substitution of Al atoms for Ti in the cubic lattice of the films improves their resistance to wear and oxidation, allowing their use as protective coatings. Ti K-edge X-ray absorption spectroscopy, which probes both crystallized and more disordered grain boundaries, and X-ray diffraction anomalous fine structure, which is sensitive to short- and long-range order within a given crystallized domain, are carried out on a set of Ti(1-x)AlxN films deposited by magnetron sputtering on Si substrates. Attention is paid to the shape of the pre-edge region, which is sensitive to the symmetry of the site occupied by Ti atoms, either octahedral in face-centred-cubic Ti-rich (TiN, Ti0.54Al0.46N) samples or tetrahedral in hexagonal-close-packed Al-rich (Ti0.32Al0.68N) films. In order to obain information on the titanium environment in the well crystallized areas, subtraction of the smooth part of the energy-dependent structure factor for the Bragg reflections is applied to the pre-edge region of the diffraction anomalous data in order to restore their spectroscopic appearance. A flat pre-edge is related to the typical octahedral environment of Ti atoms for cubic reflections. The difference observed between pre-edge spectra associated with face-centred-cubic 200 and 111 Bragg reflections of Ti0.54Al0.46N is assigned to Ti enrichment of 111 large well ordered domains compared with the more disordered 200 ones. The sharp peak observed in the spectrum recorded from the hexagonal 002 peak of Ti0.32Al0.68N can be regarded as a standard for the pure tetrahedral Ti environment in hexagonal-close-packed nitride.

Hollow Polyimide Microspheres

NASA Technical Reports Server (NTRS)

Weiser, Erik S. (Inventor); St.Clair, Terry L. (Inventor); Echigo, Yoshiaki (Inventor); Kaneshiro, Hisayasu (Inventor)

2001-01-01

A shaped article composed of an aromatic polyimide has a hollow, essentially spherical structure and a particle size of about 100 to about 1500 micrometers, a density of about 1 to about 6 pounds/cubic foot and a volume change of 1 to about 20% by a pressure treatment of 30 psi for 10 minutes at room temperature. A syntactic foam, made of a multiplicity of the shaped articles which are bonded together by a matrix resin to form an integral composite structure, has a density of about 3 to about 30 pounds/cubic feet and a compression strength of about 100 to about 1400 pounds/sq inch.

Method for improving performance of high temperature superconductors within a magnetic field

DOEpatents

Wang, Haiyan; Foltyn, Stephen R.; Maiorov, Boris A.; Civale, Leonardo

2010-01-05

The present invention provides articles including a base substrate including a layer of an oriented cubic oxide material having a rock-salt-like structure layer thereon; and, a buffer layer upon the oriented cubic oxide material having a rock-salt-like structure layer, the buffer layer having an outwardly facing surface with a surface morphology including particulate outgrowths of from 10 nm to 500 run in size at the surface, such particulate outgrowths serving as flux pinning centers whereby the article maintains higher performance within magnetic fields than similar articles without the necessary density of such outgrowths.

Elastic instability of the nano-structured state as an intrinsic probe to study the early formation stages of sol gel derived (Pb1-xCax)TiO3 thin films

NASA Astrophysics Data System (ADS)

Bretos, I.; Ricote, J.; Jiménez-Riobóo, R. J.; Pardo, L.; Calzada, M. L.

2007-12-01

A novel method to investigate the early formation stages of polycrystalline (Pb1-xCax)TiO3 (PCT) perovskite films by means of traditional Brillouin and micro-Brillouin spectroscopy (BS, mBS) is described in the present work. The films were prepared by chemical solution deposition (CSD) onto oxidized (100)Si substrates and treated at temperatures between 350 650 °C by rapid thermal processing (RTP). The elastic instability observed by Brillouin spectroscopy at the nano-structured state of the PCT films was used here to determine their crystallization temperatures. Coexistence of different nanocrystalline phases (e.g., pyrochlore, perovskite) in the films could also be detected by this technique. The reliability of these results is demonstrated by complementary information obtained by X-ray diffraction (XRD) and scanning force microscopy (SFM). The effects of the annealing temperature and of the Ca2+ content on the crystallization process of these films are also discussed.

Nature of the tensor order in Cd 2 Re 2 O 7

DOE Office of Scientific and Technical Information (OSTI.GOV)

Di Matteo, S.; Norman, M. R.

The pyrochlore metal Cd 2Re 2O 7 has been recently investigated by second-harmonic generation (SHG) reflectivity. In this paper, we develop a general formalism that allows for the identification of the relevant tensor components of the SHG from azimuthal scans. We demonstrate that the secondary order parameter identified by SHG at the structural phase transition is the x 2 - y 2 component of the axial toroidal quadrupole. This differs from the 3z 2 - r 2 symmetry of the atomic displacements associated with the I4m2 crystal structure that was previously thought to be its origin. Within the same formalism,more » we suggest that the primary order parameter detected in the SHG experiment is the 3z 2 - r 2 component of the magnetic quadrupole. We discuss the general mechanism driving the phase transition in our proposed framework, and suggest experiments, particularly resonant x-ray scattering ones, that could clarify this issue.« less

Study of the Structural Stability in Intermetallics Using Displacive Transformation Paths

NASA Astrophysics Data System (ADS)

Sob, M.; Wang, L. G.; Vitek, V.

1997-03-01

Relative structural stability of TiAl, FeAl, NiAl and NiTi is studied by investigating displacive phase transformation paths. These include the well known tetragonal (Bain's) and trigonal deformation paths which correspond to large homogeneous straining, and also more complex paths that include the shuffling of atomic planes. The results of full-potential APW total energy calculations show that all higher-energy cubic structures studied are locally unstable with respect to some deformation modes. There may or may not be symmetry-dictated energy extrema corresponding to cubic lattices depending on the atomic ordering. However, other energy extrema that are not imposed by symmetry requirements occur along the transformation paths. Configurations corresponding to energy minima may represent metastable structures that can play an important role in interfaces and other extended defects.

Iron Oxide Nanospheres and Nanocubes for Magnetic Hyperthermia Therapy: A Comparative Study

NASA Astrophysics Data System (ADS)

Nemati, Z.; Das, R.; Alonso, J.; Clements, E.; Phan, M. H.; Srikanth, H.

2017-06-01

Improving the heating capacity of magnetic nanoparticles (MNPs) for hyperthermia therapy is an important but challenging task. Through a comparative study of the inductive heating properties of spherical and cubic Fe3O4 MNPs with two distinct average volumes (˜7000 nm3 and 80,000 nm3), we demonstrate that, for small size (˜7000 nm3), the cubic MNPs heat better compared with the spherical MNPs. However, the opposite trend is observed for larger size (˜80,000 nm3). The improvement in heating efficiency in cubic small-sized MNPs (˜7000 nm3) can be attributed to enhanced anisotropy and the formation of chain-like aggregates, whereas the decrease of the heating efficiency in cubic large-sized MNPs (˜80,000 nm3) has been attributed to stronger aggregation of particles. Physical motion is shown to contribute more to the heating efficiency in case of spherical than cubic MNPs, when dispersed in water. These findings are of crucial importance in understanding the role of shape anisotropy and optimizing the heating response of magnetic nano-structures for advanced hyperthermia.

Unusually large unit cell of lipid bicontinuous cubic phase: towards nature's length scales

NASA Astrophysics Data System (ADS)

Kim, Hojun; Leal, Cecilia

Lipid bicontinuous cubic phases are of great interest for drug delivery, protein crystallization, biosensing, and templates for directing hard material assembly. Structural modulations of lipid mesophases regarding phase identity and unit cell size are often necessary to augment loading and gain pore size control. One important example is the need for unit cells large enough to guide the crystallization of bigger proteins without distortion of the templating phase. In nature, bicontinuous cubic constructs achieve unit cell dimensions as high as 300 nm. However, the largest unit cell of lipid mesophases synthesized in the lab is an order of magnitude lower. In fact, it has been predicted theoretically that lipid bicontinuous cubic phases of unit cell dimensions exceeding 30 nm could not exist, as high membrane fluctuations would damp liquid crystalline order. Here we report non-equilibrium assembly methods of synthesizing metastable bicontinuous cubic phases with unit cell dimensions as high as 70 nm. The phases are stable for very long periods and become increasingly ordered as time goes by without changes to unit cell dimensions. We acknowledge the funding source as a NIH.

Diamond cubic phase of monoolein and water as an amphiphilic matrix for electrophoresis of oligonucleotides.

PubMed

Carlsson, Nils; Winge, Ann-Sofie; Engström, Sven; Akerman, Björn

2005-10-06

We used a cubic liquid crystal formed by the nonionic monoglyceride monoolein and water as a porous matrix for the electrophoresis of oligonucleotides. The diamond cubic phase is thermodynamically stable when in contact with a water-rich phase, which we exploit to run the electrophoresis in the useful submarine mode. Oligonucleotides are separated according to size and secondary structure by migration through the space-filling aqueous nanometer pores of the regular liquid crystal, but the comparatively slow migration means the cubic phase will not be a replacement for the conventional DNA gels. However, our demonstration that the cubic phase can be used in submarine electrophoresis opens up the possibility for a new matrix for electrophoresis of amphiphilic molecules. From this perspective, the results on the oligonucleotides show that water-soluble particles of nanometer size, typical for the hydrophilic parts of membrane-bound proteins, may be a useful separation motif. A charged contamination in the commercial sample of monoolein, most likely oleic acid that arises from its hydrolysis, restricts useful buffer conditions to a pH below 5.6.

Freezing, melting and structure of ice in a hydrophilic nanopore.

PubMed

Moore, Emily B; de la Llave, Ezequiel; Welke, Kai; Scherlis, Damian A; Molinero, Valeria

2010-04-28

The nucleation, growth, structure and melting of ice in 3 nm diameter hydrophilic nanopores are studied through molecular dynamics simulations with the mW water model. The melting temperature of water in the pore was T(m)(pore) = 223 K, 51 K lower than the melting point of bulk water in the model and in excellent agreement with experimental determinations for 3 nm silica pores. Liquid and ice coexist in equilibrium at the melting point and down to temperatures as low as 180 K. Liquid water is located at the interface of the pore wall, increasing from one monolayer at the freezing temperature, T(f)(pore) = 195 K, to two monolayers a few degrees below T(m)(pore). Crystallization of ice in the pore occurs through homogeneous nucleation. At the freezing temperature, the critical nucleus contains approximately 75 to 100 molecules, with a radius of gyration similar to the radius of the pore. The critical nuclei contain features of both cubic and hexagonal ice, although stacking of hexagonal and cubic layers is not defined until the nuclei reach approximately 150 molecules. The structure of the confined ice is rich in stacking faults, in agreement with the interpretation of X-ray and neutron diffraction experiments. Though the presence of cubic layers is twice as prevalent as hexagonal ones, the crystals should not be considered defective Ic as sequences with more than three adjacent cubic (or hexagonal) layers are extremely rare in the confined ice.

Characterization of cubic ceria?zirconia powders by X-ray diffraction and vibrational and electronic spectroscopy

NASA Astrophysics Data System (ADS)

Sánchez Escribano, Vicente; Fernández López, Enrique; Panizza, Marta; Resini, Carlo; Gallardo Amores, José Manuel; Busca, Guido

2003-10-01

The X-ray diffraction (XRD) patterns and the Infrared, Raman and UV-visible spectra of CeO 2ZrO 2 powders prepared by co-precipitation are presented. Raman spectra provide evidence for the largely predominant cubic structure of the powders with CeO 2 molar composition higher than 25%. Also skeletal IR spectra allow to distinguish cubic from tetragonal phases which are instead not easily distinguished on the basis of the XRD patterns. All mixed oxides including pure ceria are strong UV absorbers although also absorb in the violet visible region. By carefully selecting their composition and treatment temperature, the onset of the radiation that they cut off can be chosen in the 425-475 nm interval. Although they are likely metastable, the cubic phases are still pure even after heating at 1173 K for 4 h.

The exchange interactions and the state of manganese atoms in the solid solutions in Bi{sub 3}NbO{sub 7} of cubic and tetragonal modifications

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chezhina, N.V., E-mail: chezhina@nc2490.spb.edu; Zhuk, N.A.; Korolev, D.A.

2016-01-15

The comparative analysis of magnetic behavior of manganese-containing solid solutions Bi{sub 3}Nb{sub 1−x}Mn{sub x}O{sub 7−δ} (x=0.01−0.10) of cubic and tetragonal modifications was performed. Based on the results of magnetic susceptibility studies paramagnetic manganese atoms in solid solutions of cubic and tetragonal modifications were found to be in the form of Mn(III), Mn(IV) monomers and exchange-coupled dimers of Mn(III)–O–Mn(III), Mn(IV)–O–Mn(IV), Mn(III)–O–Mn(IV). The exchange parameters and the distribution of monomers and dimers in solid solutions as a function of the content of paramagnetic atoms were calculated. - Graphical abstract: Structural transition of cubic to tetragonal Bi{sub 3}NbO{sub 7−δ}.

Micro and nano-structured green gallium indium nitride/gallium nitride light-emitting diodes

NASA Astrophysics Data System (ADS)

Stark, Christoph J. M.

Light-emitting diodes (LEDs) are commonly designed and studied based on bulk material properties. In this thesis different approaches based on patterns in the nano and micrometer length scale range are used to tackle low efficiency in the green spectral region, which is known as “green gap”. Since light generation and extraction are governed by microscopic processes, it is instructive to study LEDs with lateral mesa sizes scaled to the nanometer range. Besides the well-known case of the quantum size effect along the growth direction, a continuous lateral scaling could reveal the mechanisms behind the purported absence of a green gap in nanowire LEDs and the role of their extraction enhancement. Furthermore the possibility to modulate strain and piezoelectric polarization by post growth patterning is of practical interest, because the internal electric fields in conventional wurtzite GaN LEDs cause performance problems. A possible alternative is cubic phase GaN, which is free of built-in polarization fields. LEDs on cubic GaN could show the link between strong polarization fields and efficiency roll-off at high current densities, also known as droop. An additional problem for all nitride-based LEDs is efficient light extraction. For a planar GaN LED only roughly 8% of the generated light can be extracted. Novel lightextraction structures with extraction-favoring geometry can yield significant increase in light output power. To investigate the effect of scaling the mesa dimension, micro and nano-sized LED arrays of variable structure size were fabricated. The nano-LEDs were patterned by electron beam lithography and dry etching. They contained up to 100 parallel nano-stripe LEDs connected to one common contact area. The mesa width was varied over 1 μm, 200 nm, and 50 nm. These LEDs were characterized electrically and optically, and the peak emission wavelength was found to depend on the lateral structure size. An electroluminescence (EL) wavelength shift of 3 nm towards smaller values was observed when the stripe width was reduced from 1 μm to 50 nm. At the same time a strong fourfold enhancement of the light emission from the patterned region over the unpatterned area was observed. Micro-patterned LEDs showed non-linear scaling of the light output power, and an enhancement of 39 % was achieved for structures with an area fill ratio of 0.5 over an LED with square mesa. Growth of cubic GaN and cubic GaInN/GaN LEDs was shown by M-OVPE in Vshaped grooves formed by the {111} planes of etched silicon. SEM images of the GaN layer in small ( 0.5 μm) regions show a contrast change where the phase boundary between cubic and wurtzite GaN is expected to occur. The growth parameter space is explored for optimal conditions while minimizing the alloying problem for GaN growth on Si. The cubic GaN phase is confirmed by electron back-scatter diffraction (EBSD) in the V-groove center, whereas wurtzite GaN is found near the groove edges. Luminescence of undoped GaN and GaInN/GaN multi-quantum well structures was studied by cathodoluminescence (CL). The undoped cubic GaN structure showed strong band-edge luminescence at 385 nm (3.22 eV) at 78 K, whereas for the MQW device strong emission at 498 nm is observed, even at room temperature. Full cubic LED structures were grown, and wavelength-stable electroluminescence at 489 nm was demonstrated. LEDs with integrated light extraction structures are grown on free-standing GaN substrates with different off-cut angles. The devices with different off-cut show pronounced features at the top surface that also penetrate the active region. For a 2.24° off-cut, these features resemble fish scales, where the feature sizes are in the μm-range. The 2.24° off-cut LED shows a 3.6-fold increased light output power compared to a LED on virtually on-axis substrate with 0.06° off-cut. The enhancement found in the fish scale LEDs is attributed to increased light scattering, effectively reducing the fraction of trapped light. These results show the potential of structures on the micro and nanometer scale for LED device performance and the progress on cubic GaN could open alternative ways to understand the droop problem.

Evaluation of pulmonary function and respiratory symptoms in pyrochlore mine workers

PubMed Central

Borges, Ritta de Cássia Canedo Oliveira; Barros, José Cerqueira; Oliveira, Fabrício Borges; Brunherotti, Marisa Andrade; Quemelo, Paulo Roberto Veiga

2016-01-01

ABSTRACT Objective: To identify respiratory symptoms and evaluate lung function in mine workers. Methods: This was a cross-sectional observational study involving production sector workers of a pyrochlore mining company. The subjects completed the British Medical Research Council questionnaire, which is designed to evaluate respiratory symptoms, occupational exposure factors, and smoking status. In addition, they underwent pulmonary function tests with a portable spirometer. Results: The study involved 147 workers (all male). The mean age was 41.37 ± 8.71 years, and the mean duration of occupational exposure was 12.26 ± 7.09 years. We found that 33 (22.44%) of the workers had respiratory symptoms and that 26 (17.69%) showed abnormalities in the spirometry results. However, we found that the spirometry results did not correlate significantly with the presence of respiratory symptoms or with the duration of occupational exposure. Conclusions: The frequencies of respiratory symptoms and spirometric changes were low when compared with those reported in other studies involving occupational exposure to dust. No significant associations were observed between respiratory symptoms and spirometry results. PMID:27832236

DOE Office of Scientific and Technical Information (OSTI.GOV)

Perriot, Romain; Dholabhai, Pratik P.; Uberuaga, Blas P.

In this paper, we use molecular dynamics simulations to investigate the role of grain boundaries (GBs) on ionic diffusion in pyrochlores, as a function of the GB type, chemistry of the compound, and level of cation disorder. We observe that the presence of GBs promotes oxygen transport in ordered and low-disordered systems, as the GBs are found to have a higher concentration of mobile carriers with higher mobilities than in the bulk. Thus, in ordered samples, the ionic diffusion is 2D, localized along the grain boundary. When cation disorder is introduced, bulk carriers begin to contribute to the overall diffusion,more » while the GB contribution is only slightly enhanced. In highly disordered samples, the diffusive behavior at the GBs is bulk-like, and the two contributions (bulk vs. GB) can no longer be distinguished. There is thus a transition from 2D/GB dominated oxygen diffusivity to 3D/bulk dominated diffusivity versus disorder in pyrochlores. Finally, these results provide new insights into the possibility of using internal interfaces to enhance ionic conductivity in nanostructured complex oxides.« less

Computational Design of Flat-Band Material.

PubMed

Hase, I; Yanagisawa, T; Kawashima, K

2018-02-26

Quantum mechanics states that hopping integral between local orbitals makes the energy band dispersive. However, in some special cases, there are bands with no dispersion due to quantum interference. These bands are called as flat band. Many models having flat band have been proposed, and many interesting physical properties are predicted. However, no real compound having flat band has been found yet despite the 25 years of vigorous researches. We have found that some pyrochlore oxides have quasi-flat band just below the Fermi level by first principles calculation. Moreover, their valence bands are well described by a tight-binding model of pyrochlore lattice with isotropic nearest neighbor hopping integral. This model belongs to a class of Mielke model, whose ground state is known to be ferromagnetic with appropriate carrier doping and on-site repulsive Coulomb interaction. We have also performed a spin-polarized band calculation for the hole-doped system from first principles and found that the ground state is ferromagnetic for some doping region. Interestingly, these compounds do not include magnetic element, such as transition metal and rare-earth elements.

Computational Design of Flat-Band Material

NASA Astrophysics Data System (ADS)

Hase, I.; Yanagisawa, T.; Kawashima, K.

2018-02-01

Quantum mechanics states that hopping integral between local orbitals makes the energy band dispersive. However, in some special cases, there are bands with no dispersion due to quantum interference. These bands are called as flat band. Many models having flat band have been proposed, and many interesting physical properties are predicted. However, no real compound having flat band has been found yet despite the 25 years of vigorous researches. We have found that some pyrochlore oxides have quasi-flat band just below the Fermi level by first principles calculation. Moreover, their valence bands are well described by a tight-binding model of pyrochlore lattice with isotropic nearest neighbor hopping integral. This model belongs to a class of Mielke model, whose ground state is known to be ferromagnetic with appropriate carrier doping and on-site repulsive Coulomb interaction. We have also performed a spin-polarized band calculation for the hole-doped system from first principles and found that the ground state is ferromagnetic for some doping region. Interestingly, these compounds do not include magnetic element, such as transition metal and rare-earth elements.

Quantum spin ices and magnetic states from dipolar-octupolar doublets on the pyrochlore lattice

NASA Astrophysics Data System (ADS)

Chen, Gang

We consider a class of electron systems in which dipolar-octupolar Kramers doublets arise on the pyrochlore lattice. In the localized limit, the Kramers doublets are described by the effective spin 1/2 pseudospins. The most general nearest-neighbor exchange model between these pseudospins is the XYZ model. In additional to dipolar ordered and octupolar ordered magnetic states, we show that this XYZ model exhibits two distinct quantum spin ice (QSI) phases, that we dub dipolar QSI and octupolar QSI. These two QSIs are distinct symmetry enriched U(1) quantum spin liquids, enriched by the lattice symmetry. Moreover, the XYZ model is absent from the notorious sign problem for a quantum Monte Carlo simulation in a large parameter space. We discuss the potential relevance to real material systems such as Dy2Ti2O7, Nd2Zr2O7, Nd2Hf2O7, Nd2Ir2O7, Nd2Sn2O7 and Ce2Sn2O7. chggst@gmail.com, Refs: Y-P Huang, G Chen, M Hermele, Phys. Rev. Lett. 112, 167203 (2014).

The Hardest Superconducting Metal Nitride

DOE PAGES

Wang, Shanmin; Antonio, Daniel; Yu, Xiaohui; ...

2015-09-03

Transition–metal (TM) nitrides are a class of compounds with a wide range of properties and applications. Hard superconducting nitrides are of particular interest for electronic applications under working conditions such as coating and high stress (e.g., electromechanical systems). However, most of the known TM nitrides crystallize in the rock–salt structure, a structure that is unfavorable to resist shear strain, and they exhibit relatively low indentation hardness, typically in the range of 10–20 GPa. Here, we report high–pressure synthesis of hexagonal δ–MoN and cubic γ–MoN through an ion–exchange reaction at 3.5 GPa. The final products are in the bulk form withmore » crystallite sizes of 50 – 80 μm. Based on indentation testing on single crystals, hexagonal δ–MoN exhibits excellent hardness of ~30 GPa, which is 30% higher than cubic γ–MoN (~23 GPa) and is so far the hardest among the known metal nitrides. The hardness enhancement in hexagonal phase is attributed to extended covalently bonded Mo–N network than that in cubic phase. The measured superconducting transition temperatures for δ–MoN and cubic γ–MoN are 13.8 and 5.5 K, respectively, in good agreement with previous measurements.« less

Water-surface elevation and discharge measurement data for the Red River of the North and its tributaries near Fargo, North Dakota, water years 2014–15

USGS Publications Warehouse

Damschen, William C.; Galloway, Joel M.

2016-08-25

The U.S. Geological Survey, in cooperation with the Fargo Diversion Board of Authority, collected water-surface elevations during a range of discharges needed for calibration of hydrologic and hydraulic models for specific reaches of interest in water years 2014–15. These water-surface elevation and discharge measurement data were collected for design planning of diversion structures on the Red River of the North and Wild Rice River and the aqueduct/diversion structures on the Sheyenne and Maple Rivers. The Red River of the North and Sheyenne River reaches were surveyed six times, and discharges ranged from 276 to 6,540 cubic feet per second and from 166 to 2,040 cubic feet per second, respectively. The Wild Rice River reach also was surveyed six times during 2014 and 2015, and discharges ranged from 13 to 1,550 cubic feet per second. The Maple River reach was surveyed four times, and discharges ranged from 16.4 to 633 cubic feet per second. Water-surface elevation differences from upstream to downstream in the reaches ranged from 0.33 feet in the Red River of the North reach to 9.4 feet in the Maple River reach.

The Hardest Superconducting Metal Nitride

NASA Astrophysics Data System (ADS)

Wang, Shanmin; Antonio, Daniel; Yu, Xiaohui; Zhang, Jianzhong; Cornelius, Andrew L.; He, Duanwei; Zhao, Yusheng

2015-09-01

Transition-metal (TM) nitrides are a class of compounds with a wide range of properties and applications. Hard superconducting nitrides are of particular interest for electronic applications under working conditions such as coating and high stress (e.g., electromechanical systems). However, most of the known TM nitrides crystallize in the rock-salt structure, a structure that is unfavorable to resist shear strain, and they exhibit relatively low indentation hardness, typically in the range of 10-20 GPa. Here, we report high-pressure synthesis of hexagonal δ-MoN and cubic γ-MoN through an ion-exchange reaction at 3.5 GPa. The final products are in the bulk form with crystallite sizes of 50 - 80 μm. Based on indentation testing on single crystals, hexagonal δ-MoN exhibits excellent hardness of ~30 GPa, which is 30% higher than cubic γ-MoN (~23 GPa) and is so far the hardest among the known metal nitrides. The hardness enhancement in hexagonal phase is attributed to extended covalently bonded Mo-N network than that in cubic phase. The measured superconducting transition temperatures for δ-MoN and cubic γ-MoN are 13.8 and 5.5 K, respectively, in good agreement with previous measurements.

Docosapentaenoic acid (DPA) is a critical determinant of cubic membrane formation in amoeba Chaos mitochondria.

PubMed

Deng, Yuru; Almsherqi, Zakaria A; Shui, Guanghou; Wenk, Markus R; Kohlwein, Sepp D

2009-09-01

Very long-chain polyunsaturated fatty acids (VLC-PUFAs), such as docosahexaenoic acid (DHA) and docosapentaenoic acid (DPA), have recently made it to the realm of "magical molecules" based on their multiple presumably beneficial effects in biological systems, making these PUFAs particularly interesting in biomedicine. Their specific biological functions, however, remain enigmatic. Here we provide evidence derived from studies in the amoeba Chaos that indicates a structural role for omega-6 DPA in cell membrane organization, which may help to explain the multiple diverse effects of VLC-PUFA in healthy and diseased states. Amoeba Chaos mitochondria undergo a remarkable and reversible morphological transition into cubic morphology on starvation. This morphological transition is reflected in major changes in fatty acid and lipid composition, as determined by gas liquid chromatography and mass spectrometry, in particular by a drastic increase in C22:5 modified phosphatidylcholine plasmalogen, phosphatidylethanolamine plasmalogen, and phosphatidylinositol species. Liposomes produced in vitro from lipids of starved amoeba cells show a high propensity to form hexagonal tubular and cubic morphologies. Addition of omega-6 DPA, but not of omega-3 DPA, to the cell culture also induced mitochondrial membrane transformation into cubic morphology in fed cells, demonstrating for the first time an important structural role of omega-6 DPA-containing lipids in cell membrane organization.

Origin and chemical composition of the amorphous material from the intergrain pores of self-assembled cubic ZnS:Mn nanocrystals

NASA Astrophysics Data System (ADS)

Stefan, Mariana; Vlaicu, Ioana Dorina; Nistor, Leona Cristina; Ghica, Daniela; Nistor, Sergiu Vasile

2017-12-01

We have shown in previous investigations that the low temperature collective magnetism observed in mesoporous cubic ZnS:Mn nanocrystalline powders prepared by colloidal synthesis, with nominal doping concentrations above 0.2 at.%, is due to the formation of Mn2+ clusters with distributed antiferromagnetic coupling localized in an amorphous phase found between the cubic ZnS:Mn nanocrystals. Here we investigate the composition, origin and thermal annealing behavior of this amorphous phase in such a mesoporous ZnS:Mn sample doped with 5 at.% Mn nominal concentration. Correlated analytical transmission electron microscopy, multifrequency electron paramagnetic resonance and Fourier transform infrared spectroscopy data show that the amorphous nanomaterial consists of unreacted precursor hydrated zinc and manganese acetates trapped inside the pores and on the surface of the cubic ZnS nanocrystals. The decomposition of the acetates under isochronal annealing up to 270 °C, where the mesoporous structure is still preserved, lead to changes in the nature and strength of the magnetic interactions between the aggregated Mn2+ ions. These results strongly suggest the possibility to modulate the magnetic properties of such transition metal ions doped II-VI mesoporous structures by varying the synthesis conditions and/or by post-synthesis thermochemical treatments.

Effect of Lipid-Based Nanostructure on Protein Encapsulation within the Membrane Bilayer Mimetic Lipidic Cubic Phase Using Transmembrane and Lipo-proteins from the Beta-Barrel Assembly Machinery.

PubMed

van 't Hag, Leonie; Shen, Hsin-Hui; Lin, Tsung-Wu; Gras, Sally L; Drummond, Calum J; Conn, Charlotte E

2016-11-29

A fundamental understanding of the effect of amphiphilic protein encapsulation on the nanostructure of the bicontinuous cubic phase is crucial to progressing biomedical and biological applications of these hybrid protein-lipid materials, including as drug delivery vehicles, as biosensors, biofuel cells and for in meso crystallization. The relationship between the lipid nanomaterial and the encapsulated protein, however, remains poorly understood. In this study, we investigated the effect of incorporating the five transmembrane and lipo-proteins which make up the β-barrel assembly machinery from Gram-negative bacteria within a series of bicontinuous cubic phases. The transmembrane β-barrel BamA caused an increase in lattice parameter of the cubic phase upon encapsulation. By contrast, the mainly hydrophilic lipo-proteins BamB-E caused the cubic phase lattice parameters to decrease, despite their large size relative to the diameter of the cubic phase water channels. Analysis of the primary amino acid sequence was used to rationalize this effect, based on specific interactions between aromatic amino acids within the proteins and the polar-apolar interface. Other factors that were found to have an effect were lateral bilayer pressure and rigidity within the lipid bilayer, water channel diameter, and size and structure of the lipo-proteins. The data presented suggest that hydrophilic bioactive molecules can be selectively encapsulated within the cubic phase by using a lipid anchor or aromatic amino acids, for drug delivery or biosensing applications.

Nanodiamonds do not provide unique evidence for a Younger Dryas impact

PubMed Central

Tian, H.; Schryvers, D.; Claeys, Ph.

2011-01-01

Microstructural, δ13C isotope and C/N ratio investigations were conducted on excavated material from the black Younger Dryas boundary in Lommel, Belgium, aiming for a characterisation of the carbon content and structures. Cubic diamond nanoparticles are found in large numbers. The larger ones with diameters around or above 10 nm often exhibit single or multiple twins. The smaller ones around 5 nm in diameter are mostly defect-free. Also larger flake-like particles, around 100 nm in lateral dimension, with a cubic diamond structure are observed as well as large carbon onion structures. The combination of these characteristics does not yield unique evidence for an exogenic impact related to the investigated layer. PMID:21173270

Cubic structure and canted antiferromagnetism of CaMn7O12 doped with trivalent cations (Fe, Al, Cr)

NASA Astrophysics Data System (ADS)

Motin Seikh, Md.; Caignaert, V.; Lebedev, O. I.; Raveau, B.

2014-02-01

In this study, we show the dramatic effect of the doping of the octahedral sites with M3+ cations (Fe3+, Al3+ and Cr3+) upon the structure and magnetism of the rhombohedral double perovskite CaMn7O12. In the oxides CaMn7-xMxO12, charge ordering between Mn3+ and Mn4+ octahedral sites is destroyed leading to the cubic structure (Im-3), whereas the initial magnetic properties (TN~90 K) have disappeared leading to canted antiferromagnetism (TN≈50-70 K) for small x values (x ~0.2-1). A spin glass like behaviour is also observed for larger values (x~1) in the case of Fe substitution.

A unified picture of the crystal structures of metals

NASA Astrophysics Data System (ADS)

Söderlind, Per; Eriksson, Olle; Johansson, Börje; Wills, J. M.; Boring, A. M.

1995-04-01

THE crystal structures of the light actinides have intrigued physicists and chemists for several decades1. Simple metals and transition metals have close-packed, high-symmetry structures, such as body-centred cubic, face-centred cubic and hexagonal close packing. In contrast, the structures of the light actinides are very loosely packed and of low symmetry-tetragonal, orthorhombic and monoclinic. To understand these differences, we have performed total-energy calculations, as a function of volume, for both high-and low-symmetry structures of a simple metal (aluminium), a non-magnetic transition metal (niobium), a ferromagnetic transition metal (iron) and a light actinide (uranium). We find that the crystal structure of all of these metals is determined by the balance between electrostatic (Madelung) interactions, which favour high symmetry, and a Peierls distortion of the crystal lattice, which favours low symmetry. We show that simple metals and transition metals can adopt low-symmetry structures on expansion of the lattice; and we predict that, conversely, the light actinides will undergo transitions to structures of higher symmetry on compression.

Phase degradation in BxGa1-xN films grown at low temperature by metalorganic vapor phase epitaxy

NASA Astrophysics Data System (ADS)

Gunning, Brendan P.; Moseley, Michael W.; Koleske, Daniel D.; Allerman, Andrew A.; Lee, Stephen R.

2017-04-01

Using metalorganic vapor phase epitaxy, a comprehensive study of BxGa1-xN growth on GaN and AlN templates is described. BGaN growth at high-temperature and high-pressure results in rough surfaces and poor boron incorporation efficiency, while growth at low-temperature and low-pressure (750-900 °C and 20 Torr) using nitrogen carrier gas results in improved surface morphology and boron incorporation up to 7.4% as determined by nuclear reaction analysis. However, further structural analysis by transmission electron microscopy and x-ray pole figures points to severe degradation of the high boron composition films, into a twinned cubic structure with a high density of stacking faults and little or no room temperature photoluminescence emission. Films with <1% triethylboron (TEB) flow show more intense, narrower x-ray diffraction peaks, near-band-edge photoluminescence emission at 362 nm, and primarily wurtzite-phase structure in the x-ray pole figures. For films with >1% TEB flow, the crystal structure becomes dominated by the cubic phase. Only when the TEB flow is zero (pure GaN), does the cubic phase entirely disappear from the x-ray pole figure, suggesting that under these growth conditions even very low boron compositions lead to mixed crystalline phases.

Impact of Cubic Pin Finned Surface Structure Geometry upon Spray Cooling Heat Transfer

NASA Technical Reports Server (NTRS)

Silk, Eric A.; Kim, Jungho; Kiger, Ken

2005-01-01

Experiments were conducted to study the effects of enhanced surface structures on heat flux using spray cooling. The surface enhancements consisted of cubic pin fins machined on the top surface of copper heater blocks. The structure height, pitch, and width were parametrically vaned. Each copper block had a projected cross-sectional area of 2.0 sq cm. Measurements were also obtained on a heater block with a flat surface for baseline comparison purposes. A 2 x 2 nozzle array was used with PF-5060 as the working fluid. Thermal performance data were obtained under nominally degassed (chamber pressure of 41.4 kPa) and gassy conditions (chamber with N2 gas at 100.7 kPa) with a bulk fluid temperature of 20.5 C. Results for both the degassed and gassy cases show that structure width and separation distance have a dominant effect upon the heat transfer for the size ranges used. Cubic pin fin height had little impact upon heat flux. The maximum critical heat flux (CHF) attained for any of the surfaces was 121 W/sq cm, giving an enhancement of 51% relative to the flat surface case under nominally degassed conditions. The gassy case had a maximum CHF of 149 W/sq cm, giving an enhancement of 38% relative to the flat surface case.

Stepwise Construction of Heterobimetallic Cages by an Extended Molecular Library Approach.

PubMed

Hardy, Matthias; Struch, Niklas; Topić, Filip; Schnakenburg, Gregor; Rissanen, Kari; Lützen, Arne

2018-04-02

Two novel heterobimetallic complexes, a trigonal-bipyramidal and a cubic one, have been synthesized and characterized using the same C 3 -symmetric metalloligand, prepared by a simple subcomponent self-assembly strategy. Adopting the molecular library approach, we chose a mononuclear, preorganized iron(II) complex as the metalloligand capable of self-assembly into a trigonal-bipyramidal or a cubic aggregate upon coordination to cis-protected C 2 -symmetric palladium(II) or unprotected tetravalent palladium(II) ions, respectively. The trigonal-bipyramidal complex was characterized by NMR and UV-vis spectroscopy, electrospray ionization mass spectrometry (ESI-MS), and single-crystal X-ray diffraction. The cubic structure was characterized by NMR and UV-vis spectroscopy and ESI-MS.

Icosahedral and decagonal quasicrystals of intermetallic compounds are multiple twins of cubic or orthorhombic crystals composed of very large atomic complexes with icosahedral point-group symmetry in cubic close packing or body-centered packing: Structure of decagonal Al6Pd

PubMed Central

Pauling, Linus

1989-01-01

A doubly icosahedral complex involves roughly spherical clusters of atoms with icosahedral point-group symmetry, which are themselves, in parallel orientation, icosahedrally packed. These complexes may form cubic crystallites; three structures of this sort have been identified. Analysis of electron diffraction photographs of the decagonal quasicrystal Al6Pd has led to its description as involving pentagonal twinning of an orthorhombic crystal with a = 51.6 Å, b = 37.6 Å, and c = 33.24 Å, with about 4202 atoms in the unit, comprising two 1980-atom doubly icosahedral complexes, each involving icosahedral packing of 45 44-atom icosahedral complexes (at 0 0 0 and 1/2 1/2 1/2) and 242 interstitial atoms. The complexes and clusters are oriented with one of their fivefold axes in the c-axis direction. Images PMID:16594092

Icosahedral and decagonal quasicrystals of intermetallic compounds are multiple twins of cubic or orthorhombic crystals composed of very large atomic complexes with icosahedral point-group symmetry in cubic close packing or body-centered packing: Structure of decagonal Al(6)Pd.

PubMed

Pauling, L

1989-12-01

A doubly icosahedral complex involves roughly spherical clusters of atoms with icosahedral point-group symmetry, which are themselves, in parallel orientation, icosahedrally packed. These complexes may form cubic crystallites; three structures of this sort have been identified. Analysis of electron diffraction photographs of the decagonal quasicrystal Al(6)Pd has led to its description as involving pentagonal twinning of an orthorhombic crystal with a = 51.6 A, b = 37.6 A, and c = 33.24 A, with about 4202 atoms in the unit, comprising two 1980-atom doubly icosahedral complexes, each involving icosahedral packing of 45 44-atom icosahedral complexes (at 0 0 0 and 1/2 1/2 1/2) and 242 interstitial atoms. The complexes and clusters are oriented with one of their fivefold axes in the c-axis direction.

Programming Hierarchical Self-Assembly of Patchy Particles into Colloidal Crystals via Colloidal Molecules.

PubMed

Morphew, Daniel; Shaw, James; Avins, Christopher; Chakrabarti, Dwaipayan

2018-03-27

Colloidal self-assembly is a promising bottom-up route to a wide variety of three-dimensional structures, from clusters to crystals. Programming hierarchical self-assembly of colloidal building blocks, which can give rise to structures ordered at multiple levels to rival biological complexity, poses a multiscale design problem. Here we explore a generic design principle that exploits a hierarchy of interaction strengths and employ this design principle in computer simulations to demonstrate the hierarchical self-assembly of triblock patchy colloidal particles into two distinct colloidal crystals. We obtain cubic diamond and body-centered cubic crystals via distinct clusters of uniform size and shape, namely, tetrahedra and octahedra, respectively. Such a conceptual design framework has the potential to reliably encode hierarchical self-assembly of colloidal particles into a high level of sophistication. Moreover, the design framework underpins a bottom-up route to cubic diamond colloidal crystals, which have remained elusive despite being much sought after for their attractive photonic applications.

Humidity sensing behavior of tin-loaded 3-D cubic mesoporous silica

NASA Astrophysics Data System (ADS)

Poonia, Ekta; Dahiya, Manjeet S.; Tomer, Vijay K.; Kumar, Krishan; Kumar, Sunil; Duhan, Surender

2018-07-01

The present scientific investigation deals with template synthesis of 3D-cubic mesoporous KIT-6 with in-situ loading of SnO2 to obtain a material with enhanced number of surface active sites. The structural insights have been reported through analysis of XRD, TEM, FESEM, N2 sorption and mid-IR absorption data. X-ray diffraction confirmed 3D-cubic mesoporous structure of silica with Ia 3 bar d symmetry and existence of anatase SnO2 species. A decrease in surface area on loading of SnO2 nanoparticles is revealed via analysis of N2 adsorption-desorption isotherms. Rapid response time of 15 s and super rapid recovery time of 2 s (with response > 100) have been exhibited by sensor based on sample containing 1 wt% of SnO2. Further investigation on sensing performance of nanocomposite with 1 wt% of SnO2 confirmed its ohmic behavior (with negligible V-I hysteresis), excellent cycle stability, outstanding long term stability and very low hysteresis (1.4% at 53% RH).

Computational prediction of body-centered cubic carbon in an all- s p 3 six-member ring configuration

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Zhen -Zhen; Lian, Chao -Sheng; Xu, Jing

2015-06-11

Recent shock compression experiments produced clear evidence of a new carbon phase, but a full structural identification has remained elusive. Here we establish by ab initio calculations a body-centered cubic carbon phase in Ia3¯d(O 10 h) symmetry, which contains twelve atoms in its primitive cell, thus termed BC12, and comprises all-sp 3 six-membered rings. This structural configuration places BC12 carbon in the same bonding type as cubic diamond, and its stability is verified by phonon mode analysis. Simulated x-ray diffraction patterns provide an excellent match to the previously unexplained distinct diffraction peak found in shock compression experiments. Electronic band andmore » density of states calculations reveal that BC12 is a semiconductor with a direct band gap of ~2.97eV. Lastly, these results provide a solid foundation for further exploration of this new carbon allotrope.« less

In situ synthesis and characterization of uranium carbide using high temperature neutron diffraction

NASA Astrophysics Data System (ADS)

Reiche, H. Matthias; Vogel, Sven C.; Tang, Ming

2016-04-01

We investigated the formation of UCx from UO2+x and graphite in situ using neutron diffraction at high temperatures with particular focus on resolving the conflicting reports on the crystal structure of non-quenchable cubic UC2. The agents were UO2 nanopowder, which closely imitates nano grains observed in spent reactor fuels, and graphite powder. In situ neutron diffraction revealed the onset of the UO2 + 2C → UC + CO2 reaction at 1440 °C, with its completion at 1500 °C. Upon further heating, carbon diffuses into the uranium carbide forming C2 groups at the octahedral sites. This resulting high temperature cubic UC2 phase is similar to the NaCl-type structure as proposed by Bowman et al. Our novel experimental data provide insights into the mechanism and kinetics of formation of UC as well as characteristics of the high temperature cubic UC2 phase which agree with proposed rotational rehybridization found from simulations by Wen et al.

Single Sublattice Endotaxial Phase Separation Driven by Charge Frustration in a Complex Oxide

PubMed Central

2013-01-01

Complex transition-metal oxides are important functional materials in areas such as energy and information storage. The cubic ABO3 perovskite is an archetypal example of this class, formed by the occupation of small octahedral B-sites within an AO3 network defined by larger A cations. We show that introduction of chemically mismatched octahedral cations into a cubic perovskite oxide parent phase modifies structure and composition beyond the unit cell length scale on the B sublattice alone. This affords an endotaxial nanocomposite of two cubic perovskite phases with distinct properties. These locally B-site cation-ordered and -disordered phases share a single AO3 network and have enhanced stability against the formation of a competing hexagonal structure over the single-phase parent. Synergic integration of the distinct properties of these phases by the coherent interfaces of the composite produces solid oxide fuel cell cathode performance superior to that expected from the component phases in isolation. PMID:23750709

Temperature dependence of the radiation tolerance of nanocrystalline pyrochlores A 2Ti 2O 7 (A = Gd, Ho and Lu)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wen, J.; Sun, C.; Dholabhai, P. P.

A potentially enhanced radiation resistance of nanocrystalline materials, as a consequence of the high density of interfaces and surfaces, has attracted much attention both to understand the fundamental role of these defect sinks and to develop them for high-radiation environments. Here, irradiation response of nanocrystalline A 2Ti 2O 7 (A = Gd, Ho and Lu) pyrochlore powders with grain sizes of 20–30 nm was investigated by 1-MeV Kr 2+ ion bombardment. In situ transmission electron microscopy (TEM) revealed that the critical amorphization fluence for each nanocrystalline compound at room temperature was greater than that for their coarse-grained counterparts, indicating anmore » enhanced amorphization resistance. The effect of temperature on the irradiation response of one of these compounds, nanocrystalline Lu 2Ti 2O 7, was further examined by performing ion irradiation at an elevated temperature range of 480–600 K. The critical amorphization temperature (T c) was found to be noticeably higher in nanocrystalline Lu 2Ti 2O 7 (610 K) than its coarse-grained counterpart (480 K), revealing that nanocrystalline Lu 2Ti 2O 7 is less resistant to amorphization compared to its coarse-grained phase under high temperatures. We interpret these results with the aid of atomistic simulations. Molecular statics calculations find that cation antisite defects are less energetically costly to form near surfaces than in the bulk, suggesting that the nanocrystalline form of these materials is generally less susceptible to amorphization than coarse-grained counterparts at low temperatures where defect kinetics are negligible. In contrast, at high temperatures, the annealing efficiency of antisite defects by cation interstitials is significantly reduced due to the sink properties of the surfaces in the nanocrystalline pyrochlore, which contributes to the observed higher amorphization temperature in the nano-grained phase than in coarse-grained counterpart. Altogether, these results provide new insight into the behavior of nanocrystalline materials under irradiation.« less

Bubble Formation and Lattice Parameter Changes Resulting from He Irradiation of Defect-Fluorite Gd2Zr2O7

DOE Office of Scientific and Technical Information (OSTI.GOV)

Taylor, Caitlin A.; Patel, Maulik K.; Aguiar, Jeffery A.

2016-08-15

Pyrochlores have long been considered as potential candidates for advanced ceramic waste-forms for the immobilization of radioactive waste nuclides. This work provides evidence that Gd2Zr2O7, often considered the most radiation tolerant pyrochlore, could be susceptible to radiation damage in the form of bubble nucleation at the highest He doses expected over geological time. Ion irradiations were utilized to experimentally simulate the radiation damage and He accumulation produced by ..alpha..-decay. Samples were pre-damaged using 7 MeV Au3+ to induce the pyrochlore to defect-fluorite phase transformation, which would occur due to ..alpha..-recoil damage within several hundred years of storage in a Gd2Zr2O7more » waste-form. These samples were then implanted to various He concentrations in order to study the long-term effects of He accumulation. Helium bubbles 1-3 nm in diameter were observed in TEM at a concentration of 4.6 at.% He. Some bubbles remained isolated, while others formed chains 10-30 nm in length parallel to the surface. GIXRD measurements showed lattice swelling after irradiating pristine Gd2Zr2O7 with 7 MeV Au3+ to a fluence of 2.2 x 1015 Au/cm2. An increase in lattice swelling was also measured after 2.2 x 1015 Au/cm2 + 2 x 1015 He/cm2 and 2.2 x 1015 Au/cm2 + 2 x 1016 He/cm2. A decrease in lattice swelling was measured after irradiation with 2.2 x 1015 Au/cm2 + 2 x 1017 He/cm2, the fluence where bubbles and bubble chains were observed in TEM. Bubble chains are thought to form in order to reduce lattice strain normal to the surface, which is produced by the Au and He irradiation damage.« less

Temperature dependence of the radiation tolerance of nanocrystalline pyrochlores A 2Ti 2O 7 (A = Gd, Ho and Lu)

DOE PAGES

Wen, J.; Sun, C.; Dholabhai, P. P.; ...

2016-03-21

A potentially enhanced radiation resistance of nanocrystalline materials, as a consequence of the high density of interfaces and surfaces, has attracted much attention both to understand the fundamental role of these defect sinks and to develop them for high-radiation environments. Here, irradiation response of nanocrystalline A 2Ti 2O 7 (A = Gd, Ho and Lu) pyrochlore powders with grain sizes of 20–30 nm was investigated by 1-MeV Kr 2+ ion bombardment. In situ transmission electron microscopy (TEM) revealed that the critical amorphization fluence for each nanocrystalline compound at room temperature was greater than that for their coarse-grained counterparts, indicating anmore » enhanced amorphization resistance. The effect of temperature on the irradiation response of one of these compounds, nanocrystalline Lu 2Ti 2O 7, was further examined by performing ion irradiation at an elevated temperature range of 480–600 K. The critical amorphization temperature (T c) was found to be noticeably higher in nanocrystalline Lu 2Ti 2O 7 (610 K) than its coarse-grained counterpart (480 K), revealing that nanocrystalline Lu 2Ti 2O 7 is less resistant to amorphization compared to its coarse-grained phase under high temperatures. We interpret these results with the aid of atomistic simulations. Molecular statics calculations find that cation antisite defects are less energetically costly to form near surfaces than in the bulk, suggesting that the nanocrystalline form of these materials is generally less susceptible to amorphization than coarse-grained counterparts at low temperatures where defect kinetics are negligible. In contrast, at high temperatures, the annealing efficiency of antisite defects by cation interstitials is significantly reduced due to the sink properties of the surfaces in the nanocrystalline pyrochlore, which contributes to the observed higher amorphization temperature in the nano-grained phase than in coarse-grained counterpart. Altogether, these results provide new insight into the behavior of nanocrystalline materials under irradiation.« less

Lattice-Matched Semiconductor Layers on Single Crystalline Sapphire Substrate

NASA Technical Reports Server (NTRS)

Choi, Sang; King, Glen; Park, Yeonjoon

2009-01-01

SiGe is an important semiconductor alloy for high-speed field effect transistors (FETs), high-temperature thermoelectric devices, photovoltaic solar cells, and photon detectors. The growth of SiGe layer is difficult because SiGe alloys have different lattice constants from those of the common Si wafers, which leads to a high density of defects, including dislocations, micro-twins, cracks, and delaminations. This innovation utilizes newly developed rhombohedral epitaxy of cubic semiconductors on trigonal substrates in order to solve the lattice mismatch problem of SiGe by using trigonal single crystals like sapphire (Al2O3) as substrate to give a unique growth-orientation to the SiGe layer, which is automatically controlled at the interface upon sapphire (0001). This technology is different from previous silicon on insulator (SOI) or SGOI (SiGe on insulator) technologies that use amorphous SiO2 as the growth plane. A cubic semiconductor crystal is a special case of a rhombohedron with the inter-planar angle, alpha = 90 deg. With a mathematical transformation, all rhombohedrons can be described by trigonal crystal lattice structures. Therefore, all cubic lattice constants and crystal planes (hkl) s can be transformed into those of trigonal crystal parameters. These unique alignments enable a new opportunity of perfect lattice matching conditions, which can eliminate misfit dislocations. Previously, these atomic alignments were thought to be impossible or very difficult. With the invention of a new x-ray diffraction measurement method here, growth of cubic semiconductors on trigonal crystals became possible. This epitaxy and lattice-matching condition can be applied not only to SiGe (111)/sapphire (0001) substrate relations, but also to other crystal structures and other materials, including similar crystal structures which have pointgroup rotational symmetries by 120 because the cubic (111) direction has 120 rotational symmetry. The use of slightly miscut (less than plus or minus 10 deg.) sapphire (0001) substrate can be used to improve epitaxial relationships better by providing attractive atomic steps in the epitaxial process.

Magnetism and the spin state in cubic perovskite CaCo O3 synthesized under high pressure

NASA Astrophysics Data System (ADS)

Xia, Hailiang; Dai, Jianhong; Xu, Yuanji; Yin, Yunyu; Wang, Xiao; Liu, Zhehong; Liu, Min; McGuire, Michael A.; Li, Xiang; Li, Zongyao; Jin, Changqing; Yang, Yifeng; Zhou, Jianshi; Long, Youwen

2017-07-01

Cubic SrCo O3 with an intermediate spin state can only be stabilized by high pressure and high temperature (HPHT) treatment. It is metallic and ferromagnetic with the highest Curie temperature of the transition-metal perovskites. The chemical substitution by Ca on Sr sites would normally lower crystal symmetry from cubic to orthorhombic as seen in the perovskite family of Ca M O3 (M =M4 + of transition metals, G e4 + , S n4 + , and Z r4 + ) at room temperature. This structural change narrows the bandwidth, so as to further enhance the Curie temperature as the crossover to the localized electronic state is approached. We report a successful synthesis of the perovskite CaCo O3 with a HPHT treatment. Surprisingly, CaCo O3 crystallizes in a simple cubic structure that remains stable down to 20 K, the lowest temperature in the structural study. The new perovskite has been thoroughly characterized by a suite of measurements including transport, magnetization, specific heat, thermal conductivity, and thermoelectric power. Metallic CaCo O3 undergoes two successive magnetic transitions at 86 K and 54 K as temperature decreases. The magnetization at 5 K is compatible with the intermediate spin state t4e1 of C o4 + at the octahedral site. The thermal expansion of the Co-O bond length indicates that the population of high spin state t3e2 increases for T >100 K . The shortest Co-O bond length in cubic CaCo O3 is responsible for delocalizing electrons in the π*-band and itinerant-electron ferromagnetism at T <54 K . A comprehensive comparison between SrCo O3 and CaCo O3 and the justification of their physical properties by first-principles calculation have also been made in this report. Partially filled π* and σ* bands would make CaCo O3 suitable to study the Hund's coupling effect in a metal.

Synthesis of Cubic-Shaped Pt Particles with (100) Preferential Orientation by a Quick, One-Step and Clean Electrochemical Method.

PubMed

Liu, Jie; Fan, Xiayue; Liu, Xiaorui; Song, Zhishuang; Deng, Yida; Han, Xiaopeng; Hu, Wenbin; Zhong, Cheng

2017-06-07

A new approach has been developed for in situ preparing cubic-shaped Pt particles with (100) preferential orientation on the surface of the conductive support by using a quick, one-step, and clean electrochemical method with periodic square-wave potential. The whole electrochemical deposition process is very quick (only 6 min is required to produce cubic Pt particles), without the use of particular capping agents. The shape and the surface structure of deposited Pt particles can be controlled by the lower and upper potential limits of the square-wave potential. For a frequency of 5 Hz and an upper potential limit of 1.0 V (vs saturated calomel electrode), as the lower potential limit decreases to the H adsorption potential region, the Pt deposits are changed from nearly spherical particles to cubic-shaped (100)-oriented Pt particles. High-resolution transmission electron microscopy and selected-area electron diffraction reveal that the formed cubic Pt particles are single-crystalline and enclosed by (100) facets. Cubic Pt particles exhibit characteristic H adsorption/desorption peaks corresponding to the (100) preferential orientation. Ge irreversible adsorption indicates that the fraction of wide Pt(100) surface domains is 47.8%. The electrocatalytic activities of different Pt particles are investigated by ammonia electro-oxidation, which is particularly sensitive to the amount of Pt(100) sites, especially larger (100) domains. The specific activity of cubic Pt particles is 3.6 times as high as that of polycrystalline spherical Pt particles, again confirming the (100) preferential orientation of Pt cubes. The formation of cubic-shaped Pt particles is related with the preferential electrochemical deposition and dissolution processes of Pt, which are coupled with the periodic desorption and adsorption processes of O-containing species and H adatoms.

Strong competition between orbital ordering and itinerancy in a frustrated spinel vanadate

DOE PAGES

Ma, Jie; Lee, Jun Hee; Hahn, Steven E.; ...

2015-01-26

In this study, the crossover from localized to itinerant electron regimes in the geometrically frustrated spinel system Mn 1-xCo xV 2O 4 is explored by neutron-scattering measurements, first-principles calculations, and spin models. At low Co doping, the orbital ordering (OO) of the localized V 3+ spins suppresses magnetic frustration by triggering a tetragonal distortion. At high Co doping levels, however, electronic itinerancy melts the OO and lessens the structural and magnetic anisotropies, thus increasing the amount of geometric frustration for the V-site pyrochlore lattice. Contrary to the predicted paramagentism induced by chemical pressure, the measured noncollinear spin states in themore » Co-rich region of the phase diagram provide a unique platform where localized spins and electronic itinerancy compete in a geometrically frustrated spinel.« less

Method of Fabricating Double Sided Si(Ge)/Sapphire/III-Nitride Hybrid Structure

NASA Technical Reports Server (NTRS)

Choi, Sang Hyouk (Inventor); Park, Yeonjoon (Inventor)

2017-01-01

One aspect of the present invention is a double sided hybrid crystal structure including a trigonal Sapphire wafer containing a (0001) C-plane and having front and rear sides. The Sapphire wafer is substantially transparent to light in the visible and infrared spectra, and also provides insulation with respect to electromagnetic radio frequency noise. A layer of crystalline Si material having a cubic diamond structure aligned with the cubic <111> direction on the (0001) C-plane and strained as rhombohedron to thereby enable continuous integration of a selected (SiGe) device onto the rear side of the Sapphire wafer. The double sided hybrid crystal structure further includes an integrated III-Nitride crystalline layer on the front side of the Sapphire wafer that enables continuous integration of a selected III-Nitride device on the front side of the Sapphire wafer.

A structural analysis of small vapor-deposited 'multiply twinned' gold particles

NASA Technical Reports Server (NTRS)

Yang, C. Y.; Heinemann, K.; Yacaman, M. J.; Poppa, H.

1979-01-01

High resolution selected zone dark field, Bragg reflection imaging and weak beam dark field techniques of transmission electron microscopy were used to determine the structure of small gold particles vapor deposited on NaCl substrates. Attention was focused on the analysis of those particles in the 50-150 A range that have pentagonal or hexagonal bright field profiles. These particles have been previously described as multiply twinned crystallites composed of face-centered cubic tetrahedra. The experimental evidence of the present studies can be interpreted on the assumption that the particle structure is a regular icosahedron or decahedron for the hexagonal or the pentagonal particles respectively. The icosahedron is a multiply twinned rhombohedral crystal and the decahedron is a multiply twinned body-centered orthorhombic crystal, each of which constitutes a slight distortion from the face-centered cubic structure.

Double Sided Si(Ge)/Sapphire/III-Nitride Hybrid Structure

NASA Technical Reports Server (NTRS)

Park, Yeonjoon (Inventor); Choi, Sang Hyouk (Inventor)

2016-01-01

One aspect of the present invention is a double sided hybrid crystal structure including a trigonal Sapphire wafer containing a (0001) C-plane and having front and rear sides. The Sapphire wafer is substantially transparent to light in the visible and infrared spectra, and also provides insulation with respect to electromagnetic radio frequency noise. A layer of crystalline Si material having a cubic diamond structure aligned with the cubic <111> direction on the (0001) C-plane and strained as rhombohedron to thereby enable continuous integration of a selected (SiGe) device onto the rear side of the Sapphire wafer. The double sided hybrid crystal structure further includes an integrated III-Nitride crystalline layer on the front side of the Sapphire wafer that enables continuous integration of a selected III-Nitride device on the front side of the Sapphire wafer.

Investigation of La and Al substitution on the spontaneous polarization and lattice dynamics of the Pb(1-x)LaxTi(1-x)AlxO3 ceramics

NASA Astrophysics Data System (ADS)

Yadav, Arun Kumar; Verma, Anita; Kumar, Sunil; Srihari, Velaga; Sinha, A. K.; Reddy, V. Raghavendra; Liu, Shun Wei; Biring, Sajal; Sen, Somaditya

2018-03-01

The phase purity and crystal structure of Pb(1-x)LaxTi(1-x)AlxO3 (0 ≤ x ≤ 0.25) samples (synthesized via the sol-gel process) were confirmed using synchrotron x-ray powder diffraction (XRD) (wavelength, λ = 0.44573 Å). Rietveld analyses of powder x-ray diffraction data confirmed the tetragonal structure for compositions with x ≤ 0.18 and cubic structure for the sample with x = 0.25. Temperature-dependent XRD was performed to investigate the structural change from tetragonal to cubic structure phase transition. Raman spectroscopy at room temperature also confirmed this phase transition with compositions. Field emission scanning electron microscopy (FESEM) provided information about the surface morphology while an energy dispersive x-ray spectrometer attached with FESEM confirmed the chemical compositions of samples. Temperature and frequency dependent dielectric studies showed that the tetragonal to cubic phase transition decreased from 680 K to 175 K with an increase in the x from 0.03 to 0.25, respectively. This is correlated with the structural studies. Electric field dependent spontaneous polarization showed a proper ferroelectric loop for 0.06 ≤ x ≤ 0.18 belonging to a tetragonal phase, while for x ≥ 0.25, the spontaneous polarization vanishes. Bipolar strain versus electric field revealed a butterfly loop for 0.06 ≤ x ≤ 0.18 compositions. Energy storage efficiency initially increases nominally with substitution but beyond x = 0.18 enhances considerably.

A bicontinuous tetrahedral structure in a liquid-crystalline lipid

NASA Astrophysics Data System (ADS)

Longley, William; McIntosh, Thomas J.

1983-06-01

The structure of most lipid-water phases can be visualized as an ordered distribution of two liquid media, water and hydrocarbons, separated by a continuous surface covered by the polar groups of the lipid molecules1. In the cubic phases in particular, rod-like elements are linked into three-dimensional networks1,2. Two of these phases (space groups Ia3d and Pn3m) contain two such three-dimensional networks mutually inter-woven and unconnected. Under the constraints of energy minimization3, the interface between the components in certain of these `porous fluids' may well resemble one of the periodic minimal surface structures of the type described mathematically by Schwarz4,5. A structure of this sort has been proposed for the viscous isotropic (cubic) form of glycerol monooleate (GMO) by Larsson et al.6 who suggested that the X-ray diagrams of Lindblom et al.7 indicated a body-centred crystal structure in which lipid bilayers might be arranged as in Schwarz's octahedral surface4. We have now found that at high water contents, a primitive cubic lattice better fits the X-ray evidence with the material in the crystal arranged in a tetrahedral way. The lipid appears to form a single bilayer, continuous in three dimensions, separating two continuous interlinked networks of water. Each of the water networks has the symmetry of the diamond crystal structure and the bilayer lies in the space between them following a surface resembling Schwarz's tetrahedral surface4.

Review of high pressure phases of calcium by first-principles calculations

NASA Astrophysics Data System (ADS)

Ishikawa, T.; Nagara, H.; Suzuki, N.; Tsuchiya, J.; Tsuchiya, T.

2010-03-01

We review high pressure phases of calcium which have obtained by recent experimental and first-principles studies. In this study, we investigated the face-centered cubic (fcc) structure, the body-centered cubic (bcc) structure, the simple cubic (sc) structure, a tetragonal P43212 [Ishikawa T et al. 2008 Phys. Rev. B 77 020101(R)], an orthorhombic Cmca [Ishikawa T et al. 2008 Phys. Rev. B 77 020101(R)], an orthorhombic Cmcm [Teweldeberhan A M and Bonev S A 2008 Phys. Rev. B 78 140101(R)], an orthorhombic Pnma [Yao Y et al. 2008 Phys. Rev. B 78 054506] and a tetragonal I4/mcm(00) [Arapan S et al. 2008 Proc. Natl. Acad. Sci. USA 105 20627]. We compared the enthalpies among the structures up to 200 GPa and theoretically determined the phase diagram of calcium. The sequence of the structural transitions is fcc (0- 3.5 GPa) → bcc (3.5 - 35.7 GPa) → Cmcm (35.7- 52GPa) → P43212 (52-109 GPa) → Cmca (109-117.4GPa) → Pnma (117.4-134.6GPa) → I4/mcm(00) (134.6 GPa -). The sc phase is experimentally observed in the pressure range from 32 to 113 GPa but, in our calculation, there is no pressure region where the sc phase is the most stable. In addition, we found that the enthalpy of the hexagonal close-packed (hcp) structure is lower than that of I4/mcm(00) above 495 GPa.

The determination of temperature stability of silver nanotubes by the molecular dynamics simulation

NASA Astrophysics Data System (ADS)

Filatov, O.; Soldatenko, S.; Soldatenko, O.

2018-04-01

Molecular dynamics simulation using the embedded-atom method is applied to study thermal stability of silver nanotubes and its coefficient of linear thermal expansion. The correspondence of face centered cubic structure potential for this task is tested. Three types of nanotubes are modelled: scrolled from graphene-like plane, scrolled from plane with cubic structure and cut from cylinder. It is established that only the last two of them are stable. The last one describes in details. There is critical temperature when free ends of the nanotube close but the interior surface retains. At higher temperatures, the interior surface collapses and the nanotube is unstable.

Studies of the metastable decay of met-cars. The vanadium and niobium systems

NASA Astrophysics Data System (ADS)

Purnell, J.; Wei, S.; Castleman, A. W., Jr.

1994-10-01

The decay fractions and metastable decomposition channels were measured for V(m)C(n)(+) and Nb(m)C(n)(+) clusters using a reflectron time-of-flight mass spectrometer. These measurements for the V(m)C(n)(+) clusters reveal stable (8, 12) and (8, 13) met-car species. However, under the present experimental conditions, the Nb(m)C(n)(+) clusters display dominantly cubic structures, but the decay fraction and metastable decomposition measurements also show the presence of a stable (8, 12) species in the parent cluster spectrum. This evidence indicates that cubic structures and met-cars are being formed simultaneously for the niobium system.

Semiconducting cubic titanium nitride in the Th 3 P 4 structure

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bhadram, Venkata S.; Liu, Hanyu; Xu, Enshi

We report the discovery of a long-sought-after phase of titanium nitride with stoichiometry Ti 3 N 4 using diamond anvil cell experiments combined with in situ high-resolution x-ray diffraction and Raman spectroscopy techniques, supported by ab initio calculations. Ti 3 N 4 crystallizes in the cubic Th 3 P 4 structure [space group I ¯ 4 3 d (220)] from a mixture of TiN and N 2 above ≈ 75 GPa and ≈ 2400 K. The density ( ≈ 5.22 g/cc) and bulk modulus ( K 0 = 290 GPa) of cubic- Ti 3 N 4 ( c - Timore » 3 N 4 ) at 1 atm, estimated from the pressure-volume equation of state, are comparable to rocksalt TiN. Ab initio calculations based on the GW approximation and using hybrid functionals indicate that c - Ti 3 N 4 is a semiconductor with a direct band gap between 0.8 and 0.9 eV, which is larger than the previously predicted values. The c - Ti 3 N 4 phase is not recoverable to ambient pressure due to dynamic instabilities, but recovery of Ti 3 N 4 in the defect rocksalt (or related) structure may be feasible.« less

3-D phononic crystals with ultra-wide band gaps

PubMed Central

Lu, Yan; Yang, Yang; Guest, James K.; Srivastava, Ankit

2017-01-01

In this paper gradient based topology optimization (TO) is used to discover 3-D phononic structures that exhibit ultra-wide normalized all-angle all-mode band gaps. The challenging computational task of repeated 3-D phononic band-structure evaluations is accomplished by a combination of a fast mixed variational eigenvalue solver and distributed Graphic Processing Unit (GPU) parallel computations. The TO algorithm utilizes the material distribution-based approach and a gradient-based optimizer. The design sensitivity for the mixed variational eigenvalue problem is derived using the adjoint method and is implemented through highly efficient vectorization techniques. We present optimized results for two-material simple cubic (SC), body centered cubic (BCC), and face centered cubic (FCC) crystal structures and show that in each of these cases different initial designs converge to single inclusion network topologies within their corresponding primitive cells. The optimized results show that large phononic stop bands for bulk wave propagation can be achieved at lower than close packed spherical configurations leading to lighter unit cells. For tungsten carbide - epoxy crystals we identify all angle all mode normalized stop bands exceeding 100%, which is larger than what is possible with only spherical inclusions. PMID:28233812

3-D phononic crystals with ultra-wide band gaps.

PubMed

Lu, Yan; Yang, Yang; Guest, James K; Srivastava, Ankit

2017-02-24

In this paper gradient based topology optimization (TO) is used to discover 3-D phononic structures that exhibit ultra-wide normalized all-angle all-mode band gaps. The challenging computational task of repeated 3-D phononic band-structure evaluations is accomplished by a combination of a fast mixed variational eigenvalue solver and distributed Graphic Processing Unit (GPU) parallel computations. The TO algorithm utilizes the material distribution-based approach and a gradient-based optimizer. The design sensitivity for the mixed variational eigenvalue problem is derived using the adjoint method and is implemented through highly efficient vectorization techniques. We present optimized results for two-material simple cubic (SC), body centered cubic (BCC), and face centered cubic (FCC) crystal structures and show that in each of these cases different initial designs converge to single inclusion network topologies within their corresponding primitive cells. The optimized results show that large phononic stop bands for bulk wave propagation can be achieved at lower than close packed spherical configurations leading to lighter unit cells. For tungsten carbide - epoxy crystals we identify all angle all mode normalized stop bands exceeding 100%, which is larger than what is possible with only spherical inclusions.

Thermal expansion and phase transitions of α-AlF{sub 3}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Morelock, Cody R.; Hancock, Justin C.; Wilkinson, Angus P., E-mail: angus.wilkinson@chemistry.gatech.edu

ReO{sub 3}-type materials are of interest for their potential low or negative thermal expansion. Many metal trifluorides MF{sub 3} adopt the cubic form of this structure at elevated temperatures, which rhombohedrally distorts upon cooling. The rhombohedral form displays strong positive volume thermal expansion, but cubic MF{sub 3} display much lower and sometimes negative thermal expansion. The expansion behavior of α-AlF{sub 3} was characterized via synchrotron powder diffraction between 323 and 1177 K. α-AlF{sub 3} is rhombohedral at ambient conditions and displays strongly anisotropic thermal expansion. The volume coefficient of thermal expansion (CTE), α{sub V}, at 500 K is ∼86 ppmmore » K{sup −1}, but the linear CTE along the c-axis, α{sub c}, is close to zero. α-AlF{sub 3} becomes cubic on heating to ∼713 K and continues to show positive thermal expansion above the phase transition (α{sub V}(900 K) ∼25 ppm K{sup −1}). - Graphical abstract: α-AlF{sub 3} has a rhombohedrally distorted ReO{sub 3}-type structure at ambient conditions and displays strongly positive volume thermal expansion that is highly anisotropic; the material becomes cubic on heating above ∼713 K and continues to show positive thermal expansion. - Highlights: • ReO{sub 3}-type α-AlF{sub 3} displays strongly anisotropic thermal expansion below 713 K. • α-AlF{sub 3} is cubic above 713 K and maintains positive (isotropic) thermal expansion. • The volume CTE changes from ∼86 to ∼25 ppm K{sup −1} on heating from 500 to 900 K. • The PTE of cubic α-AlF{sub 3} may be due to the presence of local octahedral tilts.« less

Local and average structures of BaTiO 3-Bi(Zn 1/2Ti 1/2)O 3

DOE Office of Scientific and Technical Information (OSTI.GOV)

Usher, Tedi-Marie; Iamsasri, Thanakorn; Forrester, Jennifer S.

The complex crystallographic structures of (1-x)BaTiO 3-xBi(Zn 1/2Ti 1/2)O 3 (BT-xBZT) are examined using high resolution synchrotron X-ray diffraction, neutron diffraction, and neutron pair distribution function (PDF) analyses. The short-range structures are characterized from the PDFs, and a combined analysis of the X-ray and neutron diffraction patterns is used to determine the long-range structures. Our results demonstrate that the structure appears different when averaged over different length scales. In all compositions, the local structures determined from the PDFs show local tetragonal distortions (i.e., c/a > 1). But, a box-car fitting analysis of the PDFs reveals variations at different length scales.more » For 0.80BT-0.20BZT and 0.90BT-0.10BZT, the tetragonal distortions decrease at longer atom-atom distances (e.g., 30 vs. 5 ). When the longest distances are evaluated (r > 40 ), the lattice parameters approach cubic. Neutron and X-ray diffraction yield further information about the long-range structure. Compositions 0.80BT-0.20BZT and 0.90BT-0.10BZT appear cubic by Bragg diffraction (no peak splitting), consistent with the PDFs at long distances. However, these patterns cannot be adequately fit using a cubic lattice model; modeling their structures with the P4mm space group allows for a better fit to the patterns because the space group allows for c-axis atomic displacements that occur at the local scale. Furthermore, for the compositions 0.92BT-0.08BZT and 0.94BT-0.06BZT, strong tetragonal distortions are observed at the local scale and a less-distorted tetragonal structure is observed at longer length scales. In Rietveld refinements, the latter is modeled using a tetragonal phase. Since the peak overlap in these two-phase compositions limits the ability to model the local-scale structures as tetragonal, it is approximated in the refinements as a cubic phase. These results demonstrate that alloying BT with BZT results in increased disorder and disrupts the long-range ferroelectric symmetry present in BT, while the large tetragonal distortion present in BZT persists at the local scale.« less

Local and average structures of BaTiO 3-Bi(Zn 1/2Ti 1/2)O 3

DOE PAGES

Usher, Tedi-Marie; Iamsasri, Thanakorn; Forrester, Jennifer S.; ...

2016-11-11

The complex crystallographic structures of (1-x)BaTiO 3-xBi(Zn 1/2Ti 1/2)O 3 (BT-xBZT) are examined using high resolution synchrotron X-ray diffraction, neutron diffraction, and neutron pair distribution function (PDF) analyses. The short-range structures are characterized from the PDFs, and a combined analysis of the X-ray and neutron diffraction patterns is used to determine the long-range structures. Our results demonstrate that the structure appears different when averaged over different length scales. In all compositions, the local structures determined from the PDFs show local tetragonal distortions (i.e., c/a > 1). But, a box-car fitting analysis of the PDFs reveals variations at different length scales.more » For 0.80BT-0.20BZT and 0.90BT-0.10BZT, the tetragonal distortions decrease at longer atom-atom distances (e.g., 30 vs. 5 ). When the longest distances are evaluated (r > 40 ), the lattice parameters approach cubic. Neutron and X-ray diffraction yield further information about the long-range structure. Compositions 0.80BT-0.20BZT and 0.90BT-0.10BZT appear cubic by Bragg diffraction (no peak splitting), consistent with the PDFs at long distances. However, these patterns cannot be adequately fit using a cubic lattice model; modeling their structures with the P4mm space group allows for a better fit to the patterns because the space group allows for c-axis atomic displacements that occur at the local scale. Furthermore, for the compositions 0.92BT-0.08BZT and 0.94BT-0.06BZT, strong tetragonal distortions are observed at the local scale and a less-distorted tetragonal structure is observed at longer length scales. In Rietveld refinements, the latter is modeled using a tetragonal phase. Since the peak overlap in these two-phase compositions limits the ability to model the local-scale structures as tetragonal, it is approximated in the refinements as a cubic phase. These results demonstrate that alloying BT with BZT results in increased disorder and disrupts the long-range ferroelectric symmetry present in BT, while the large tetragonal distortion present in BZT persists at the local scale.« less

Crystal Structure of Garnet-Related Li-Ion Conductor Li7–3xGaxLa3Zr2O12: Fast Li-Ion Conduction Caused by a Different Cubic Modification?

PubMed Central

2016-01-01

Li-oxide garnets such as Li7La3Zr2O12 (LLZO) are among the most promising candidates for solid-state electrolytes to be used in next-generation Li-ion batteries. The garnet-structured cubic modification of LLZO, showing space group Ia-3d, has to be stabilized with supervalent cations. LLZO stabilized with Ga3+ shows superior properties compared to LLZO stabilized with similar cations; however, the reason for this behavior is still unknown. In this study, a comprehensive structural characterization of Ga-stabilized LLZO is performed by means of single-crystal X-ray diffraction. Coarse-grained samples with crystal sizes of several hundred micrometers are obtained by solid-state reaction. Single-crystal X-ray diffraction results show that Li7–3xGaxLa3Zr2O12 with x > 0.07 crystallizes in the acentric cubic space group I-43d. This is the first definite record of this cubic modification for LLZO materials and might explain the superior electrochemical performance of Ga-stabilized LLZO compared to its Al-stabilized counterpart. The phase transition seems to be caused by the site preference of Ga3+. 7Li NMR spectroscopy indicates an additional Li-ion diffusion process for LLZO with space group I-43d compared to space group Ia-3d. Despite all efforts undertaken to reveal structure–property relationships for this class of materials, this study highlights the potential for new discoveries. PMID:27019548

Process for making a ceramic composition for immobilization of actinides

DOEpatents

Ebbinghaus, Bartley B.; Van Konynenburg, Richard A.; Vance, Eric R.; Stewart, Martin W.; Walls, Philip A.; Brummond, William Allen; Armantrout, Guy A.; Herman, Connie Cicero; Hobson, Beverly F.; Herman, David Thomas; Curtis, Paul G.; Farmer, Joseph

2001-01-01

Disclosed is a process for making a ceramic composition for the immobilization of actinides, particularly uranium and plutonium. The ceramic is a titanate material comprising pyrochlore, brannerite and rutile. The process comprises oxidizing the actinides, milling the oxides to a powder, blending them with ceramic precursors, cold pressing the blend and sintering the pressed material.

Self-assembled multicompartment liquid crystalline lipid carriers for protein, peptide, and nucleic acid drug delivery.

PubMed

Angelova, Angelina; Angelov, Borislav; Mutafchieva, Rada; Lesieur, Sylviane; Couvreur, Patrick

2011-02-15

Lipids and lipopolymers self-assembled into biocompatible nano- and mesostructured functional materials offer many potential applications in medicine and diagnostics. In this Account, we demonstrate how high-resolution structural investigations of bicontinuous cubic templates made from lyotropic thermosensitive liquid-crystalline (LC) materials have initiated the development of innovative lipidopolymeric self-assembled nanocarriers. Such structures have tunable nanochannel sizes, morphologies, and hierarchical inner organizations and provide potential vehicles for the predictable loading and release of therapeutic proteins, peptides, or nucleic acids. This Account shows that structural studies of swelling of bicontinuous cubic lipid/water phases are essential for overcoming the nanoscale constraints for encapsulation of large therapeutic molecules in multicompartment lipid carriers. For the systems described here, we have employed time-resolved small-angle X-ray scattering (SAXS) and high-resolution freeze-fracture electronic microscopy (FF-EM) to study the morphology and the dynamic topological transitions of these nanostructured multicomponent amphiphilic assemblies. Quasi-elastic light scattering and circular dichroism spectroscopy can provide additional information at the nanoscale about the behavior of lipid/protein self-assemblies under conditions that approximate physiological hydration. We wanted to generalize these findings to control the stability and the hydration of the water nanochannels in liquid-crystalline lipid nanovehicles and confine therapeutic biomolecules within these structures. Therefore we analyzed the influence of amphiphilic and soluble additives (e.g. poly(ethylene glycol)monooleate (MO-PEG), octyl glucoside (OG), proteins) on the nanochannels' size in a diamond (D)-type bicontinuous cubic phase of the lipid glycerol monooleate (MO). At body temperature, we can stabilize long-living swollen states, corresponding to a diamond cubic phase with large water channels. Time-resolved X-ray diffraction (XRD) scans allowed us to detect metastable intermediate and coexisting structures and monitor the temperature-induced phase sequences of mixed systems containing glycerol monooleate, a soluble protein macromolecule, and an interfacial curvature modulating agent. These observed states correspond to the stages of the growth of the nanofluidic channel network. With the application of a thermal stimulus, the system becomes progressively more ordered into a double-diamond cubic lattice formed by a bicontinuous lipid membrane. High-resolution freeze-fracture electronic microscopy indicates that nanodomains are induced by the inclusion of proteins into nanopockets of the supramolecular cubosomic assemblies. These results contribute to the understanding of the structure and dynamics of functionalized self-assembled lipid nanosystems during stimuli-triggered LC phase transformations.

Clapeyron slope reversal in the melting curve of AuGa2 at 5.5 GPa.

PubMed

Geballe, Z M; Raju, S V; Godwal, B K; Jeanloz, R

2013-10-16

We use x-ray diffraction in a resistively heated diamond anvil cell to extend the melting curve of AuGa2 beyond its minimum at 5.5 GPa and 720 K, and to constrain the high-temperature phase boundaries between cubic (fluorite structure), orthorhombic (cottunite structure) and monoclinic phases. We document a large change in Clapeyron slope that coincides with the transitions from cubic to lower symmetry phases, showing that a structural transition is the direct cause of the change in slope. In addition, moderate (~30 K) to large (90 K) hysteresis is detected between melting and freezing, from which we infer that at high pressures, AuGa2 crystals can remain in a metastable state at more than 5% above the thermodynamic melting temperature.

Theoretical investigation of the structural, electronic, dynamical and thermal properties of YSn3 and YPb3

NASA Astrophysics Data System (ADS)

Kılıçarslan, Aynur; Salmankurt, Bahadır; Duman, Sıtkı

2017-02-01

We have performed an ab initio study of the structural, electronic, dynamical and thermal properties of the cubic AuCu3-type YSn3 and YPb3 by using the density functional theory, plane-wave pseudopotential method and a linear response scheme, within the generalized gradient approximation. An analysis of the electronic density of states at the Fermi level is found to be governed by the p states of Sn and Pb atoms with some contributions from the d states of Y atoms. The obtained phonon figures indicate that these material are dynamically stable in the cubic structure. Due to the metallic behavior of the compounds, the calculated zone-center phonon modes are triply degenerate. Also the thermal properties have been examined.

Study of liquid crystal space groups using controlled tilting with cryogenic transmission electron microscopy.

PubMed

Sagalowicz, Laurent; Acquistapace, Simone; Watzke, Heribert J; Michel, Martin

2007-11-20

We developed a method that enables differentiation between liquid crystalline-phase particles corresponding to different space groups. It consists of controlled tilting of the specimen to observe different orientations of the same particle using cryogenic transmission electron microscopy. This leads to the visualization of lattice planes (or reflections) that are present for a given structure and absent for the other one(s) and that give information on liquid crystalline structures and their space groups. In particular, we show that we can unambiguously distinguish among particles having the inverted micellar cubic (space group Fd(3)m, 227), the inverted bicontinuous gyroid (space group Ia(3)d, 230), the inverted bicontinuous diamond (space group Pn(3)m, 224), and the inverted bicontinuous primitive cubic structure (space group Im(3)m, 229).

Structure, properties, and possible mechanisms of formation of diamond-like phases

NASA Astrophysics Data System (ADS)

Belenkov, E. A.; Greshnyakov, V. A.

2016-10-01

An analysis was performed for relations between the structural parameters and the properties of 36 carbon diamond-like phases consisting of atoms occupying crystallographically equivalent positions. It was found that the crystal lattices of these phases were in stressed states with respect to the cubic diamond lattice. The density of diamond-like phases, their sublimation energies, bulk moduli, hardnesses, and band gaps depend on the deformation parameters Def and Str. The most stable phases must be phases with minimal parameters Def and Str and also with ring parameter Rng that is most close to the corresponding parameter of cubic diamond. The structures and energy characteristics of fullerites, nanotube bundles, and graphene layers of which diamond-like phases can be obtained as a result of polymerization at high pressures have been calculated.

Continuum modeling of three-dimensional truss-like space structures

NASA Technical Reports Server (NTRS)

Nayfeh, A. H.; Hefzy, M. S.

1978-01-01

A mathematical and computational analysis capability has been developed for calculating the effective mechanical properties of three-dimensional periodic truss-like structures. Two models are studied in detail. The first, called the octetruss model, is a three-dimensional extension of a two-dimensional model, and the second is a cubic model. Symmetry considerations are employed as a first step to show that the specific octetruss model has four independent constants and that the cubic model has two. The actual values of these constants are determined by averaging the contributions of each rod element to the overall structure stiffness. The individual rod member contribution to the overall stiffness is obtained by a three-dimensional coordinate transformation. The analysis shows that the effective three-dimensional elastic properties of both models are relatively close to each other.

Structural, mechanical, and magnetic properties of ferrite-austenite mixture in evaporated 304 stainless steel thin films

NASA Astrophysics Data System (ADS)

Merakeb, Noureddine; Messai, Amel; Djelloul, Abdelkader; Ayesh, Ahmad I.

2015-11-01

In this paper, we investigate the structure, composition, magnetic, and mechanical properties of stainless steel thin films formed by thermal evaporation technique. These thin films reveal novel structural and physical properties where they were found to consist of nanocrystals that are ~90 % body-centred cubic crystal structure which holds ferromagnetic properties (α-phase), and ~10 % face-centred cubic crystal structure which is paramagnetic at room temperature (γ-phase). The presence of the above phases was quantified by X-ray diffraction, transmission electron microscopy, and conversion electron Mössbauer spectroscopy. The magnetic properties were evaluated by a superconducting quantum interference device magnetometer, and they confirmed the dual-phase crystal structure of the stainless thin films, where the presence of γ-phase reduced the magnetization of the produced thin films. In addition, the fabricated stainless steel thin films did not contain micro-cracks, and they exhibit a tensile stress of about 1.7 GPa, hardness of 7.5 GPa, and elastic modulus of 104 GPa.

Quasi-Dual-Packed-Kerneled Au49 (2,4-DMBT)27 Nanoclusters and the Influence of Kernel Packing on the Electrochemical Gap.

PubMed

Liao, Lingwen; Zhuang, Shengli; Wang, Pu; Xu, Yanan; Yan, Nan; Dong, Hongwei; Wang, Chengming; Zhao, Yan; Xia, Nan; Li, Jin; Deng, Haiteng; Pei, Yong; Tian, Shi-Kai; Wu, Zhikun

2017-10-02

Although face-centered cubic (fcc), body-centered cubic (bcc), hexagonal close-packed (hcp), and other structured gold nanoclusters have been reported, it was unclear whether gold nanoclusters with mix-packed (fcc and non-fcc) kernels exist, and the correlation between kernel packing and the properties of gold nanoclusters is unknown. A Au 49 (2,4-DMBT) 27 nanocluster with a shell electron count of 22 has now been been synthesized and structurally resolved by single-crystal X-ray crystallography, which revealed that Au 49 (2,4-DMBT) 27 contains a unique Au 34 kernel consisting of one quasi-fcc-structured Au 21 and one non-fcc-structured Au 13 unit (where 2,4-DMBTH=2,4-dimethylbenzenethiol). Further experiments revealed that the kernel packing greatly influences the electrochemical gap (EG) and the fcc structure has a larger EG than the investigated non-fcc structure. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Frolov, T.; Setyawan, W.; Kurtz, R. J.

Evolutionary grand-canonical search predicts novel grain boundary structures and multiple grain boundary phases in elemental body-centered cubic (bcc) metals represented by tungsten, tantalum and molybdenum.

Additional evidence from x-ray powder diffraction patterns that icosahedral quasi-crystals of intermetallic compounds are twinned cubic crystals

PubMed Central

Pauling, Linus

1988-01-01

Analysis of the measured values of Q for the weak peaks (small maxima, usually considered to be background fluctuations, “noise”) on the x-ray powder diffraction curves for 17 rapidly quenched alloys leads directly to the conclusion that they are formed by an 820-atom or 1012-atom primitive cubic structure that by icosahedral twinning produces the so-called icosahedral quasi-crystals. PMID:16593948

Polymorphism of Ag29(BDT)12(TPP)43- cluster: interactions of secondary ligands and their effect on solid state luminescence.

PubMed

Nag, Abhijit; Chakraborty, Papri; Bodiuzzaman, Mohammad; Ahuja, Tripti; Antharjanam, Sudhadevi; Pradeep, Thalappil

2018-05-31

We present the first example of polymorphism (cubic & trigonal) in single crystals of an atomically precise monolayer protected cluster, Ag29(BDT)12(TPP)43-. We demonstrate that C-Hπ interactions of the secondary ligands (TPP) are dominant in a cubic lattice compared to a trigonal lattice, resulting in a greater rigidity of the structure, which in turn, results in a higher luminescence efficiency in it.

Packing of nonoverlapping cubic particles: Computational algorithms and microstructural characteristics

NASA Astrophysics Data System (ADS)

Malmir, Hessam; Sahimi, Muhammad; Tabar, M. Reza Rahimi

2016-12-01

Packing of cubic particles arises in a variety of problems, ranging from biological materials to colloids and the fabrication of new types of porous materials with controlled morphology. The properties of such packings may also be relevant to problems involving suspensions of cubic zeolites, precipitation of salt crystals during CO2 sequestration in rock, and intrusion of fresh water in aquifers by saline water. Not much is known, however, about the structure and statistical descriptors of such packings. We present a detailed simulation and microstructural characterization of packings of nonoverlapping monodisperse cubic particles, following up on our preliminary results [H. Malmir et al., Sci. Rep. 6, 35024 (2016), 10.1038/srep35024]. A modification of the random sequential addition (RSA) algorithm has been developed to generate such packings, and a variety of microstructural descriptors, including the radial distribution function, the face-normal correlation function, two-point probability and cluster functions, the lineal-path function, the pore-size distribution function, and surface-surface and surface-void correlation functions, have been computed, along with the specific surface and mean chord length of the packings. The results indicate the existence of both spatial and orientational long-range order as the the packing density increases. The maximum packing fraction achievable with the RSA method is about 0.57, which represents the limit for a structure similar to liquid crystals.

The influence of coordinated defects on inhomogeneous broadening in cubic lattices

NASA Astrophysics Data System (ADS)

Matheson, P. L.; Sullivan, Francis P.; Evenson, William E.

2016-12-01

The joint probability distribution function (JPDF) of electric field gradient (EFG) tensor components in cubic materials is dominated by coordinated pairings of defects in shells near probe nuclei. The contributions from these inner shell combinations and their surrounding structures contain the essential physics that determine the PAC-relevant quantities derived from them. The JPDF can be used to predict the nature of inhomogeneous broadening (IHB) in perturbed angular correlation (PAC) experiments by modeling the G 2 spectrum and finding expectation values for V zz and η. The ease with which this can be done depends upon the representation of the JPDF. Expanding on an earlier work by Czjzek et al. (Hyperfine Interact. 14, 189-194, 1983), Evenson et al. (Hyperfine Interact. 237, 119, 2016) provide a set of coordinates constructed from the EFG tensor invariants they named W 1 and W 2. Using this parameterization, the JPDF in cubic structures was constructed using a point charge model in which a single trapped defect (TD) is the nearest neighbor to a probe nucleus. Individual defects on nearby lattice sites pair with the TD to provide a locus of points in the W 1- W 2 plane around which an amorphous-like distribution of probability density grows. Interestingly, however, marginal, separable PDFs appear adequate to model IHB relevant cases. We present cases from simulations in cubic materials illustrating the importance of these near-shell coordinations.

Stocking and structure for maximum growth in sugar maple selection stands.

Treesearch

Thomas R. Crow; Carl H. Tubbs; Rodney D. Jacobs; Robert R. Oberg

1981-01-01

The impacts of stocking, structure, and cutting cycle on basal area, cubic foot volume, board foot volume, and diameter growth are considered. Recommendations are provided for maximum growth in uneven-aged sugar maple stands.

Monodisperse mesoporous silica nanoparticles of distinct topology.

PubMed

Luo, Leilei; Liang, Yucang; Erichsen, Egil Sev; Anwander, Reiner

2017-06-01

Monodisperse and uniform high-quality MCM(Mobil Composition of Matter)-48-type CMSNs (Cubic Mesoporous Silica Nanoparticles) are readily prepared by simply optimizing the molar ratio of ethanol and surfactant in the system TEOS-CTAB-NaOH-H 2 O-EtOH (TEOS=tetraethyl orthosilicate, CTAB=cetyltrimethylammonium bromide, EtOH=ethanol). In the absence of ethanol only hexagonal mesoporous silica with ellipsoidal and spherical morphology are obtained. The presence of ethanol drives a mesophase transformation from hexagonal to mixed hexagonal/cubic, further to purely cubic, and finally to a mixed cubic/lamellar. This is accompanied by a morphology evolution involving a mixture of ellipses/spheres, regular rods, uniform spheres, and finally a mixture of spheres/flakes. Preserving the three-dimensional (3D) cubic MCM-48 structure, use of a small amount of ethanol is beneficial to the improvement of the monodispersity of the CMSNs. Moreover, the quality of the CMSNs can also be controlled by changing the surfactant concentration or adjusting the stirring rate. All MSNs were characterized using powder X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and N 2 physisorption, indicating highly long-range ordered pore arrays, high specific surface areas (max. 1173 m 2 g -1 ) as well as high pore volumes (max. 1.14 cm 3 g -1 ). The monodispersity of the CMSNs was verified by statistical particle size distribution from SEM (scanning electron microscopy)/TEM (transmission electron microscopy) images and DLS (dynamic light scattering). The mesophase transformation can be rationalized on the basis of an ethanol-driven change of the surfactant packing structure and charge matching at the surfactant/silicate interface. The corresponding morphology evolution can be elucidated by an ethanol-controlled hydrolysis rate of TEOS and degree of condensation of oligomeric silicate species via a nucleation and growth process. Copyright © 2017 Elsevier Inc. All rights reserved.

Neutron Diffraction Study Oxygen Dissolution Alpha(sub 2)-Ti3Al

NASA Technical Reports Server (NTRS)

Jones, Camille Y.; Luecke, William E.; Copland, Evan

2005-01-01

Rietveld refinements of neutron powder diffraction data on alpha(sub 2)-Ti3Al have been performed to determine the crystal structure as a function of interstitial oxygen (O) concentration for three alloys with a Ti/Al ratio of approximately equal to 2.34 and O concentrations of 0.25%, 3.99% and 7.71%. The structures of the allows are hexagonal in space group P6(sub 3)/mmc where Ti and Al atoms populate unique sites with excess Al at the Ti site and O atoms occupy octahedral interstitial sites surrounded by six Ti sites. The length of the c-axis was found to increase linearly as the O occupancy of the interstitial sites increased; this lattice lengthening effect was much less pronounced along the alpha axis. Correspondingly, the increases in the lengths of Ti-Al and Ti-Ti bonds with a major component of their direction parallel to the c-axis were roughly an order of magnitude greater than the increases in the lengths of Ti-al and Ti-Ti bonds more closely aligned with the alpha-axis. Densities calculated form the lattice parameters and occupancy factors fall in the range (4.118 plus or minus 0.004) grams per cubic centimeter to (4.194 plus or minus 0.004) grams per cubic centimeter, and exhibit a nearly linear increase with oxygen concentration. Measured densities of (4.113 plus or minus 0.001) grams per cubic centimeter, (4.146 plus or minus 0.009) grams per cubic centimeter, and (4.191 plus or minus 0.002) grams per cubic centimeter for these alloys agree with the results of the refinements.

The structure of ice crystallized from supercooled water

NASA Astrophysics Data System (ADS)

Murray, Benjamin

2013-03-01

The freezing of water to ice is fundamentally important to fields as diverse as cloud formation to cryopreservation. Traditionally ice was thought to exist in two well-crystalline forms: stable hexagonal ice and metastable cubic ice. It has recently been shown, using X-ray diffraction data, that ice which crystallizes homogeneously and heterogeneously from supercooled water is neither of these phases. The resulting ice is disordered in one dimension and therefore possesses neither cubic nor hexagonal symmetry and is instead composed of randomly stacked layers of cubic and hexagonal sequences. We refer to this ice as stacking-disordered ice I (ice Isd) . This result is consistent with a number of computational studies of the crystallization of water. Review of the literature reveals that almost all ice that has been identified as cubic ice in previous diffraction studies and generated in a variety of ways was most likely stacking-disordered ice I with varying degrees of stacking disorder, which raises the question of whether cubic ice exists. New data will be presented which shows significant stacking disorder (or stacking faults on the order of 1 in every 100 layers of ice Ih) in droplets which froze heterogeneously as warm as 257 K. The identification of stacking-disordered ice from heterogeneous ice nucleation supports the hypothesis that the structure of ice that initially crystallises from supercooled water is stacking-disordered ice I, independent of nucleation mechanism, but this ice can relax to the stable hexagonal phase subject to the kinetics of recrystallization. The formation and persistence of stacking disordered ice in the Earth's atmosphere will also be discussed. Funded by the European Research Council (FP7, 240449 ICE)