First principles study of LiAlO2: new dense monoclinic phase under high pressure

NASA Astrophysics Data System (ADS)

Liu, Guangtao; Liu, Hanyu

2018-03-01

In this work, we have systematically explored the crystal structures of LiAlO2 at high pressures using crystal structure prediction method in combination with the density functional theory calculations. Besides the reported α, β, γ, δ and ɛ-phases, here we propose a new monoclinic ζ-LiAlO2 (C2/m) structure, which becomes thermodynamically and dynamically stable above 27 GPa. It is found that the cation coordination number increases from 4 to 6 under compression. Consisting of the compact {LiO6} and {AlO6} octahedrons, the newly-discovered ζ-phase possesses a very high density. Further electronic calculations show that LiAlO2 is still an insulator up to 60 GPa, and its bandgap increases upon compression. The present study advances our understanding on the crystal structures and high-pressure phase transitions of LiAlO2 that may trigger applications in multiple areas of industry and provoke more related basic science research.

A phase transition caught in mid-course: independent and concomitant analyses of the monoclinic and triclinic structures of (nBu4N)[Co(orotate)2(bipy)]·3H2O.

PubMed

Castro, Miguel; Falvello, Larry R; Forcén-Vázquez, Elena; Guerra, Pablo; Al-Kenany, Nuha A; Martínez, Gema; Tomás, Milagros

2017-09-01

The preparation and characterization of the n Bu 4 N + salts of two bis-orotate(2-) complexes of cobalt, namely bis(tetra-n-butylammonium) diaquabis(2,4-dioxo-1,2,3,4-tetrahydropyrimidin-1-ide-6-carboxylato-κ 2 N 1 ,O 6 )cobalt(II) 1.8-hydrate, (C 16 H 36 N) 2 [Co(C 5 H 2 N 2 O 4 ) 2 (H 2 O) 2 ]·1.8H 2 O, (1), and tetra-n-butylammonium (2,2'-bipyridine-κ 2 N,N')bis(2,4-dioxo-1,2,3,4-tetrahydropyrimidin-1-ide-6-carboxylato-κ 2 N 1 ,O 6 )cobalt(III) trihydrate, (C 16 H 36 N)[Co(C 5 H 2 N 2 O 4 ) 2 (C 10 H 8 N 2 )]·3H 2 O, (2), are reported. The Co III complex, (2), which is monoclinic at room temperature, presents a conservative single-crystal-to-single-crystal phase transition below 200 K, producing a triclinic twin. The transition, which involves a conformational change in one of the n Bu groups of the cation, is reversible and can be cycled. Both end phases have been characterized structurally and the system was also characterized structurally in a two-phase intermediate state, using single-crystal diffraction techniques, with both the monoclinic and triclinic phases present. Thermal analysis allows a rough estimate of the small energy content, viz. 0.25 kJ mol -1 , for both the monoclinic-to-triclinic transformation and the reverse transition, in agreement with the nature of the structural changes involving only the n Bu 4 N + cation.

Grain size dependent phase stabilities and presence of a monoclinic (Pm) phase in the morphotropic phase boundary region of (1−x)Bi(Mg{sub 1/2}Ti{sub 1/2})O{sub 3}-xPbTiO{sub 3} piezoceramics

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

Upadhyay, Ashutosh; Singh, Akhilesh Kumar, E-mail: akhilesh-bhu@yahoo.com, E-mail: aksingh.mst@itbhu.ac.in

2015-04-14

Results of the room temperature structural studies on (1−x)Bi(Mg{sub 1/2}Ti{sub 1/2})O{sub 3}-xPbTiO{sub 3} ceramics using Rietveld analysis of the powder x-ray diffraction data in the composition range 0.28 ≤ x ≤ 0.45 are presented. The morphotropic phase boundary region exhibits coexistence of monoclinic (space group Pm) and tetragonal (space group P4 mm) phases in the composition range 0.33 ≤ x ≤ 0.40. The structure is nearly single phase monoclinic (space group Pm) in the composition range 0.28 ≤ x ≤ 0.32. The structure for the compositions with x ≥ 0.45 is found to be predominantly tetragonal with space group P4 mm. Rietveld refinement of the structure rules out the coexistence of rhombohedral and tetragonal phases inmore » the morphotropic phase boundary region reported by earlier authors. The Rietveld structure analysis for the sample x = .35 calcined at various temperatures reveals that phase fraction of the coexisting phases in the morphotropic phase boundary region varies with grain size. The structural parameters of the two coexisting phases also change slightly with changing grain size.« less

A phase transition caught in mid-course: independent and concomitant analyses of the monoclinic and triclinic structures of (nBu4N)[Co(orotate)2(bipy)]·3H2O

PubMed Central

Castro, Miguel; Falvello, Larry R.; Forcén-Vázquez, Elena; Al-Kenany, Nuha A.; Martínez, Gema

2017-01-01

The preparation and characterization of the nBu4N+ salts of two bis-orotate(2−) complexes of cobalt, namely bis­(tetra-n-butyl­ammonium) di­aqua­bis­(2,4-dioxo-1,2,3,4-tetra­hydro­pyrimidin-1-ide-6-carboxyl­ato-κ2 N 1,O 6)cobalt(II) 1.8-hydrate, (C16H36N)2[Co(C5H2N2O4)2(H2O)2]·1.8H2O, (1), and tetra-n-butyl­ammonium (2,2′-bi­pyridine-κ2 N,N′)bis­(2,4-dioxo-1,2,3,4-tetra­hydro­pyrimidin-1-ide-6-carbox­yl­ato-κ2 N 1,O 6)cobalt(III) trihydrate, (C16H36N)[Co(C5H2N2O4)2(C10H8N2)]·3H2O, (2), are reported. The CoIII complex, (2), which is monoclinic at room tem­perature, presents a conservative single-crystal-to-single-crystal phase transition below 200 K, producing a triclinic twin. The transition, which involves a conformational change in one of the nBu groups of the cation, is reversible and can be cycled. Both end phases have been characterized structurally and the system was also characterized structurally in a two-phase inter­mediate state, using single-crystal diffraction techniques, with both the monoclinic and triclinic phases present. Thermal analysis allows a rough estimate of the small energy content, viz. 0.25 kJ mol−1, for both the monoclinic-to-triclinic transformation and the reverse transition, in agreement with the nature of the structural changes involving only the nBu4N+ cation. PMID:28872072

Evidence for existence of functional monoclinic phase in sodium niobate based solid solution by powder neutron diffraction

NASA Astrophysics Data System (ADS)

Mishra, S. K.; Jauhari, Mrinal; Mittal, R.; Krishna, P. S. R.; Reddy, V. R.; Chaplot, S. L.

2018-04-01

We have carried out systematic temperature-dependent neutron diffraction measurements in conjunction with dielectric spectroscopy from 6 to 300 K for sodium niobate based compounds (1-x) NaNbO3-xBaTiO3 (NNBTx). The dielectric constant is measured as a function of both temperature and frequency. It shows an anomaly at different temperatures in cooling and heating cycles and exhibits a large thermal hysteresis of ˜150 K for the composition x = 0.03. The dielectric constant is found to be dispersive in nature and suggests a relaxor ferroelectric behavior. In order to explore structural changes as a function of temperature, we analyzed the powder neutron diffraction data for the compositions x = 0.03 and 0.05. Drastic changes are observed in the powder profiles near 2θ ˜ 30.6°, 32.1°, and 34.6° in the diffraction pattern below 200 K during cooling and above 190 K in heating cycles, respectively. The disappearance of superlattice reflection and splitting in main perovskite peaks provide a signature for structural phase transition. We observed stabilization of a monoclinic phase (Cc) at low temperature. This monoclinic phase is believed to provide a flexible polarization rotation and considered to be directly linked to the high performance piezoelectricity in materials. The thermal hysteresis for composition x = 0.03 is larger than that for x = 0.05. This suggests that the addition of BaTiO3 to NaNbO3 suppresses the thermal hysteresis. It is also observed that the structural phase transition temperature decreases upon increasing the dopant concentration.

Imaging metal-like monoclinic phase stabilized by surface coordination effect in vanadium dioxide nanobeam

PubMed Central

Li, Zejun; Wu, Jiajing; Hu, Zhenpeng; Lin, Yue; Chen, Qi; Guo, Yuqiao; Liu, Yuhua; Zhao, Yingcheng; Peng, Jing; Chu, Wangsheng; Wu, Changzheng; Xie, Yi

2017-01-01

In correlated systems, intermediate states usually appear transiently across phase transitions even at the femtosecond scale. It therefore remains an open question how to determine these intermediate states—a critical issue for understanding the origin of their correlated behaviour. Here we report a surface coordination route to successfully stabilize and directly image an intermediate state in the metal-insulator transition of vanadium dioxide. As a prototype metal-insulator transition material, we capture an unusual metal-like monoclinic phase at room temperature that has long been predicted. Coordinate bonding of L-ascorbic acid molecules with vanadium dioxide nanobeams induces charge-carrier density reorganization and stabilizes metallic monoclinic vanadium dioxide, unravelling orbital-selective Mott correlation for gap opening of the vanadium dioxide metal–insulator transition. Our study contributes to completing phase-evolution pathways in the metal-insulator transition process, and we anticipate that coordination chemistry may be a powerful tool for engineering properties of low-dimensional correlated solids. PMID:28613281

Evidence for photo-induced monoclinic metallic VO{sub 2} under high pressure

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

Hsieh, Wen-Pin, E-mail: wphsieh@stanford.edu; Mao, Wendy L.; Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305

2014-01-13

We combine ultrafast pump-probe spectroscopy with a diamond-anvil cell to decouple the insulator-metal electronic transition from the lattice symmetry changing structural transition in the archetypal strongly correlated material vanadium dioxide. Coherent phonon spectroscopy enables tracking of the photo-excited phonon vibrational frequencies of the low temperature, monoclinic (M{sub 1})-insulating phase that transforms into the metallic, tetragonal rutile structured phase at high temperature or via non-thermal photo-excitations. We find that in contrast with ambient pressure experiments where strong photo-excitation promptly induces the electronic transition along with changes in the lattice symmetry, at high pressure, the coherent phonons of the monoclinic (M{sub 1})more » phase are still clearly observed upon the photo-driven phase transition to a metallic state. These results demonstrate the possibility of synthesizing and studying transient phases under extreme conditions.« less

Growth of potassium niobate micro-hexagonal tablets with monoclinic phase and its excellent piezoelectric property

NASA Astrophysics Data System (ADS)

Chen, Zhong; Huang, Jingyun; Wang, Ye; Yang, Yefeng; Wu, Yongjun; Ye, Zhizhen

2012-09-01

Potassium niobate micro-hexagonal tablets were synthesized through hydrothermal reaction with KOH, H2O and Nb2O5 as source materials by using a polycrystalline Al2O3 as substrate. X-ray diffraction, Raman spectra and selected area electron diffraction analysis results indicated that the tablets exhibit monoclinic phase structure and are highly crystallized. Meanwhile, piezoelectric property of the micro-hexagonal tablets was investigated. The as-synthesized tablets exhibit excellent piezoactivities in the experiments, and an effective piezoelectric coefficient of around 80 pm/V was obtained. The tablets have huge potential applications in micro/nano-integrated piezoelectric and optical devices.

Crystal structure and phase transitions of sodium potassium niobate perovskites

NASA Astrophysics Data System (ADS)

Tellier, J.; Malic, B.; Dkhil, B.; Jenko, D.; Cilensek, J.; Kosec, M.

2009-02-01

This paper presents the crystal structure and the phase transitions of K xNa 1- xNbO 3 (0.4 ≤ x ≤ 0.6). X-ray diffraction measurements were used to follow the change of the unit-cell parameters and the symmetry in the temperature range 100-800 K. At room temperature all the compositions exhibited a monoclinic metric of the unit cell with a small monoclinic distortion (90.32° ≤ β ≤ 90.34°). No major change of symmetry was evidenced in the investigated compositional range, which should be characteristic of the morphotropic phase-boundary region. With increasing temperature, the samples underwent first-order monoclinic-tetragonal and tetragonal-cubic transitions. Only the potassium-rich phases were rhombohedral at 100 K.

Structural, microstructural and vibrational analyses of the monoclinic tungstate BiLuWO6

NASA Astrophysics Data System (ADS)

Ait Ahsaine, H.; Taoufyq, A.; Patout, L.; Ezahri, M.; Benlhachemi, A.; Bakiz, B.; Villain, S.; Guinneton, F.; Gavarri, J.-R.

2014-10-01

The bismuth lutetium tungstate phase BiLuWO6 has been prepared using a solid state route with stoichiometric mixtures of oxide precursors. The obtained polycrystalline phase has been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy. In the first step, the crystal structure has been refined using Rietveld method: the crystal cell was resolved using monoclinic system (parameters a, b, c, β) with space group A2/m. SEM images showed the presence of large crystallites with a constant local nominal composition (BiLuW). TEM analyses showed that the actual local structure could be better represented by a superlattice (a, 2b, c, β) associated with space groups P2 or P2/m. The Raman spectroscopy showed the presence of vibrational bands similar to those observed in the compounds BiREWO6 with RE=Y, Gd, Nd. However, these vibrational bands were characterized by large full width at half maximum, probably resulting from the long range Bi/Lu disorder and local WO6 octahedron distortions in the structure.

Boron-tuning transition temperature of vanadium dioxide from rutile to monoclinic phase

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

Zhang, J. J.; He, H. Y.; Xie, Y.

2014-11-21

The effect of the doped boron on the phase transition temperature between the monoclinic phase and the rutile phase of VO{sub 2} has been studied by performing first-principles calculations. It is found that the phase transition temperature decreases linearly with increasing the doping level of B in each system, no matter where the B atom is in the crystal. More importantly, the descent of the transition temperature is predicted to be as large as 83 K/at. % B, indicating that the boron concentration of only 0.5% can cause the phase transition at room temperature. These findings provide a new routinemore » of modulating the phase transition of VO{sub 2} and pave a way for the practicality of VO{sub 2} as an energy-efficient green material.« less

Hydroxyapatite: Vibrational spectra and monoclinic to hexagonal phase transition

NASA Astrophysics Data System (ADS)

Slepko, Alexander; Demkov, Alexander A.

2015-02-01

Fundamental studies of biomaterials are necessary to deepen our understanding of their degradation and to develop cure for related illnesses. Biomineral hydroxyapatite Ca10(PO4)6(OH)2 is the main mineral constituent of mammal bone, and its synthetic analogues are used in biomedical applications. The mineral can be found in either hexagonal or monoclinic form. The transformation between these two phases is poorly understood, but knowing its mechanism may be critical to reversing processes in bone related to aging. Using density functional theory, we investigate the mechanisms of the phase transformation and estimate the transition temperature to be 680 K in fair agreement with the experimental temperature of 470 K. We also report the heat capacity of hydroxyapatite and a peculiarity in its phonon dispersion that might allow for non-destructive measurements of the crystal composition with applications in preventive medical screening for bone mineral loss.

Pressure-induced changes of the structure and properties of monoclinic α -chalcocite Cu2S

NASA Astrophysics Data System (ADS)

Zimmer, D.; Ruiz-Fuertes, J.; Morgenroth, W.; Friedrich, A.; Bayarjargal, L.; Haussühl, E.; Santamaría-Pérez, D.; Frischkorn, S.; Milman, V.; Winkler, B.

2018-04-01

The high-pressure behavior of monoclinic (P 21/c ) α -chalcocite, Cu2S , was investigated at ambient temperature by single-crystal x-ray diffraction, electrical resistance measurements, and optical absorption spectroscopy up to 16 GPa. The experiments were complemented by density-functional-theory-based calculations. Single-crystal x-ray diffraction data show that monoclinic α -chalcocite undergoes two pressure-induced first-order phase transitions at ˜3.1 and ˜7.1 GPa. The crystal structure of the first high-pressure polymorph, HP1, was solved and refined in space group P 21/c with a =10.312 (4 )Å , b =6.737 (3 )Å , c =7.305 (1 )Å , and β =100.17 (2) ∘ at 6.2(3) GPa. The crystal structure of the second high-pressure polymorph, HP2, was solved and refined in space group P 21/c with a =6.731 (4 )Å , b =6.689 (2 )Å , c =6.967 (8 )Å , and β =93.18 (3) ∘ at 7.9(4) GPa. Electrical resistance measurements upon compression and optical absorption experiments upon decompression show that the structural changes in α -chalcocite are accompanied by changes of the electrical and optical properties. Upon pressure release, the band gap Eg of α -chalcocite (1.24 eV at ambient conditions) widens across the first structural phase transition, going from 1.24 eV at 2.2 GPa (α -chalcocite) to 1.35 eV at 2.6 GPa (HP1), and closes significantly across the second phase transition, going from 1.32 eV at 4.4 GPa (HP1) to 0.87 eV at 4.9 GPa (HP2). The electrical resistance shows similar behavior: its highest value is for the first high-pressure polymorph (HP1), and its lowest value is for the second high-pressure polymorph (HP2) of α -chalcocite. These results are interpreted on the basis of calculated electronic band structures.

Structural discrimination via density functional theory and lattice dynamics: Monoclinic Mg2NiH4

NASA Astrophysics Data System (ADS)

Herbst, J. F.; Hector, L. G., Jr.

2009-04-01

Two distinct crystal structures for the monoclinic, low-temperature phase of Mg2NiH4 , which we designate as LTI and LTII, are available in the published literature. We demonstrate that density functional theory and lattice dynamics can easily identify LTII as the preferable structure at two levels of inquiry. First, enthalpies of formation ΔH calculated using three different forms for the exchange-correlation energy functional are in better agreement with experiment for LTII. Second, the phonon spectrum calculated for LTII contains no anomalies while that for LTI exhibits a variety of soft modes. By analyzing the soft modes in LTI as well as those we find for the known CaMgNiH4 structure with Ca replaced by Mg we derive a crystal structure that closely approximates LTII.

Structural phase transitions in yttrium under ultrahigh pressures

NASA Astrophysics Data System (ADS)

Samudrala, Gopi K.; Tsoi, Georgiy M.; Vohra, Yogesh K.

2012-09-01

X-ray diffraction studies were carried out on the rare earth metal yttrium up to 177 GPa in a diamond anvil cell at room temperature. Yttrium was compressed to 37% of its initial volume at the highest pressure. The rare earth crystal structure sequence hcp → Sm type → dhcp → mixed(dhcp + fcc) → distorted fcc (dfcc) is observed in yttrium below 50 GPa. The dfcc (hR24) phase has been observed to persist in the pressure range of 50-95 GPa. A structural transition from dfcc to a low symmetry phase has been observed in yttrium at 99 ± 4 GPa with a volume change of - 2.6%. This low symmetry phase has been identified as a monoclinic C2/m phase, which has also been observed in other rare earth elements under high pressures. The appearance of this low symmetry monoclinic phase in yttrium shows that its electronic structure under extreme conditions resembles that of heavy rare earth metals, with a significant increase in d-band character of the valence electrons and possibly some f-electron states near the Fermi level.

Structural phase transitions in yttrium under ultrahigh pressures.

PubMed

Samudrala, Gopi K; Tsoi, Georgiy M; Vohra, Yogesh K

2012-09-12

X-ray diffraction studies were carried out on the rare earth metal yttrium up to 177 GPa in a diamond anvil cell at room temperature. Yttrium was compressed to 37% of its initial volume at the highest pressure. The rare earth crystal structure sequence hcp → Sm type → dhcp → mixed(dhcp + fcc) → distorted fcc (dfcc) is observed in yttrium below 50 GPa. The dfcc (hR24) phase has been observed to persist in the pressure range of 50-95 GPa. A structural transition from dfcc to a low symmetry phase has been observed in yttrium at 99 ± 4 GPa with a volume change of - 2.6%. This low symmetry phase has been identified as a monoclinic C2/m phase, which has also been observed in other rare earth elements under high pressures. The appearance of this low symmetry monoclinic phase in yttrium shows that its electronic structure under extreme conditions resembles that of heavy rare earth metals, with a significant increase in d-band character of the valence electrons and possibly some f-electron states near the Fermi level.

Structural phase transition in monolayer MoTe2 driven by electrostatic doping

NASA Astrophysics Data System (ADS)

Wang, Ying; Xiao, Jun; Zhu, Hanyu; Li, Yao; Alsaid, Yousif; Fong, King Yan; Zhou, Yao; Wang, Siqi; Shi, Wu; Wang, Yuan; Zettl, Alex; Reed, Evan J.; Zhang, Xiang

2017-10-01

Monolayers of transition-metal dichalcogenides (TMDs) exhibit numerous crystal phases with distinct structures, symmetries and physical properties. Exploring the physics of transitions between these different structural phases in two dimensions may provide a means of switching material properties, with implications for potential applications. Structural phase transitions in TMDs have so far been induced by thermal or chemical means; purely electrostatic control over crystal phases through electrostatic doping was recently proposed as a theoretical possibility, but has not yet been realized. Here we report the experimental demonstration of an electrostatic-doping-driven phase transition between the hexagonal and monoclinic phases of monolayer molybdenum ditelluride (MoTe2). We find that the phase transition shows a hysteretic loop in Raman spectra, and can be reversed by increasing or decreasing the gate voltage. We also combine second-harmonic generation spectroscopy with polarization-resolved Raman spectroscopy to show that the induced monoclinic phase preserves the crystal orientation of the original hexagonal phase. Moreover, this structural phase transition occurs simultaneously across the whole sample. This electrostatic-doping control of structural phase transition opens up new possibilities for developing phase-change devices based on atomically thin membranes.

A comparison study of the Born effective charges and dielectric properties of the cubic, tetragonal, monoclinic, ortho-I, ortho-II and ortho-III phases of zirconia

NASA Astrophysics Data System (ADS)

Zhang, Yan; Chen, Hua-Xin; Duan, Li; Fan, Ji-Bin; Ni, Lei; Ji, Vincent

2018-07-01

Using density-functional perturbation theory, we systematically investigate the Born effective charges and dielectric properties of cubic, tetragonal, monoclinic, ortho-I (Pbca), ortho-II (Pnma) and ortho-III (Pca21) phases of ZrO2. The magnitudes of the Born effective charges of the Zr and oxygen atoms are greater than their nominal ionic valences (+4 for Zr and -2 for oxygen), indicating a strong dynamic charge transfer from Zr atoms to O atoms and a mixed covalent-ionic bonding in six phases of ZrO2. For all six phases of ZrO2, the electronic contributions εij∞ to the static dielectric constant are rather small (range from 5 to 6.5) and neither strongly anisotropic nor strongly dependent on the structural phase, while the ionic contributions εijion to the static dielectric constant are large and not only anisotropic but also dependent on the structural phase. The average dielectric constant εbar0 of the six ZrO2 phases decreases in the sequence of tetragonal, cubic, ortho-II (Pnma), ortho-I (Pbca), ortho-III (Pca21) and monoclinic. So among six phases of ZrO2, the tetragonal and cubic phases are two suitable phases to replace SiO2 as the gate dielectric material in modern integrated-circuit technology. Furthermore, for the tetragonal ZrO2 the best orientation is [100].

Structural phase transitions in the Ag{sub 2}Nb{sub 4}O{sub 11}-Na{sub 2}Nb{sub 4}O{sub 11} solid solution

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

Woodward, David I., E-mail: d.i.woodward@warwick.ac.uk; Lees, Martin R.; Thomas, Pam A.

2012-08-15

The phase transitions between various structural modifications of the natrotantite-structured system xAg{sub 2}Nb{sub 4}O{sub 11}-(1-x)Na{sub 2}Nb{sub 4}O{sub 11} have been investigated and a phase diagram constructed as a function of temperature and composition. This shows three separate phase transition types: (1) paraelectric-ferroelectric, (2) rhombohedral-monoclinic and (3) a phase transition within the ferroelectric rhombohedral zone between space groups R3c and R3. The parent structure for the entire series has space group R3{sup Macron }c. Compositions with x>0.75 are rhombohedral at all temperatures whereas compositions with x<0.75 are all monoclinic at room temperature and below. At x=0.75, rhombohedral and monoclinic phases coexistmore » with the phase boundary below room temperature being virtually temperature-independent. The ferroelectric phase boundary extends into the monoclinic phase field. No evidence was found for the R3-R3c phase boundary extending into the monoclinic phase field and it is concluded that a triple point is formed. - Graphical abstract: Phase diagram for xAg{sub 2}Nb{sub 4}O{sub 11}-(1-x)Na{sub 2}Nb{sub 4}O{sub 11} solid solution showing changes in crystal symmetry as a function of temperature and composition. The crystal structure is depicted. Highlights: Black-Right-Pointing-Triangle Ferroelectric, rhombohedral Ag{sub 2}Nb{sub 4}O{sub 11} in solid solution with monoclinic Na{sub 2}Nb{sub 4}O{sub 11}. Black-Right-Pointing-Triangle Three phase boundaries were studied as a function of composition and temperature. Black-Right-Pointing-Triangle Both rhombohedral and monoclinic variants exhibit ferroelectricity. The parent phase of the series has space group R3{sup Macron }c.« less

Finite element analysis of the tetragonal to monoclinic phase transformation during oxidation of zirconium alloys

NASA Astrophysics Data System (ADS)

Platt, P.; Frankel, P.; Gass, M.; Howells, R.; Preuss, M.

2014-11-01

Corrosion is a key limiting factor in the degradation of zirconium alloys in light water reactors. Developing a mechanistic understanding of the corrosion process offers a route towards improving safety and efficiency as demand increases for higher burn-up of fuel. Oxides formed on zirconium alloys are composed of both monoclinic and meta-stable tetragonal phases, and are subject to a number of potential mechanical degradation mechanisms. The work presented investigates the link between the tetragonal to monoclinic oxide phase transformation and degradation of the protective character of the oxide layer. To achieve this, Abaqus finite element analysis of the oxide phase transformation has been carried out. Study of the change in transformation strain energy shows how relaxation of oxidation induced stress and fast fracture at the metal-oxide interface could destabilise the tetragonal phase. Central to this is the identification of the transformation variant most likely to form, and understanding why twinning of the transformed grain is likely to occur. Development of transformation strain tensors and analysis of the strain components allows some separation of dilatation and shear effects. Maximum principal stress is used as an indication of fracture in the surrounding oxide layer. Study of the stress distributions shows the way oxide fracture is likely to occur and the differing effects of dilatation and shape change. Comparison with literature provides qualitative validation of the finite element simulations.

Phase transition and epitaxies between hydrated orthorhombic and anhydrous monoclinic uric acid crystals

NASA Astrophysics Data System (ADS)

Boistelle, R.; Rinaudo, C.

1981-05-01

Anhydrous monoclinic and hydrated orthorhombic uric acid crystals can be nucleated and grown from pure water solutions either separately or together with epitaxial relationships. When crystals of one modification exist in the solution they can act as nucleation substrate for the crystals of the other modification. In both cases the new phase grows epitaxially on the substrate; the mutual orientations are the same but the contact planes are different. In addition, the anhydrous modification grows into the hydrated one which undergoes a phase transition by a dissolution-recrystallization process. It is likely that the same processes occur in human stones made up of uric acids.

High pressure spectroscopic studies of phase transition in VO2

NASA Astrophysics Data System (ADS)

Basu, Raktima; Mishra, K. K.; Ravindran, T. R.; Dhara, Sandip

2018-04-01

Vanadium dioxide (VO2) exhibits a reversible first-order metal to insulator transition (MIT) at a technologically important temperature of 340K. A structural phase transition (SPT) from monoclinic M1 to rutile tetragonal R is also reported via another two intermediate phases of monoclinic M2 and triclinic T. Metastable monoclinic M2 phase of VO2 was synthesized by Mg doping in the vapour transport process. Raman spectroscopic measurements were carried out at high pressure on V1-xMgxO2 microrods. Two reversible structural phase transitions from monoclinic M2 to triclinic T at 1.6 GPa and T to monoclinic M1 at 3.2 GPa are observed and are explained by structural relaxation of the strained phases.

Optical thermometry through infrared excited green upconversion in monoclinic phase Gd2(MoO4)3:Yb3+/Er3+ phosphor

NASA Astrophysics Data System (ADS)

Xu, Weijiang; Li, Dongyu; Hao, Haoyue; Song, Yinglin; Wang, Yuxiao; Zhang, Xueru

2018-04-01

Monoclinic phase Gd2(MoO4)3: Yb3+/Er3+ phosphor is synthesized via a simple sol-gel method. The XRD result reveals that the phosphor possesses monoclinic structure with space group C2/c(15). Under the excitation of a 980 nm laser, its emission spectra shows remarkably intense green and negligible red emissions, which are all two-photon process. By investigating effect of temperature on green emission of the sample, the competition between the thermal agitation and non-radiative relaxation of 2H11/2 level can be found, which is verified by the measurement of lifetime. In addition, the sensitivity of optical thermometry is studied based on the fluorescence intensity ratio technique through infrared excited green upconversion. The maximum sensitivity is found to be about 0.02574 K-1 at 510.2 K, suggesting that the phosphor can be used as an excellent material for optical temperature sensing.

The creation of modulated monoclinic aperiodic composites in n-alkane/urea compounds

DOE PAGES

Mariette, Céline; Guérin, Laurent; Rabiller, Philippe; ...

2014-09-12

n-Dodecane/urea is a member of the prototype series of n-alkane/urea inclusion compounds. At room temperature, it presents a quasi-one dimensional liquid-like state for the confined guest molecules within the rigid, hexagonal framework of the urea host. At lower temperatures, we report the existence of two other phases. Below T c=248 K there appears a phase with rank four superspace group P6 122(00γ), the one typically observed at room temperature in n-alkane/urea compounds with longer guest molecules. A misfit parameter, defined by the ratio γ=c h/c g (c host/c guest), is found to be 0.632±0.005. Below T c1=123 K, a monoclinicmore » modulated phase is created with a constant shift along c of the guest molecules in adjacent channels. The maximal monoclinic space group for this structure is P12 11(α0γ). We discuss analogies and differences with n-heptane/urea, which also presents a monoclinic, modulated low-temperature phase.« less

Monoclinic MB phase and phase instability in [110] field cooled Pb(Zn1/3Nb2/3)O3-4.5%PbTiO3 single crystals

NASA Astrophysics Data System (ADS)

Yao, Jianjun; Cao, Hu; Ge, Wenwei; Li, Jiefang; Viehland, D.

2009-08-01

We report the finding of a monoclinic MB phase in Pb(Zn1/3Nb2/3)O3-4.5%PbTiO3 single crystals. High precision x-ray diffraction investigations of [110] field cooled crystals have shown a transformation sequence of cubic(C)→tetragonal(T)→orthorhombic(O)→monoclinic(MB), which is different from that previously reported [A.-E. Renault et al., J. Appl. Phys. 97, 044105 (2005)]. Beginning in the zero-field-cooled condition at 383 K, a rhombohedral (R)→MB→O sequence was observed with increasing field. Coexisting MB and O phases were then found upon removal of field, which fully transformed to MB on cooling to room temperature.

(Na{sub x}k{sub 1−x}){sub 2}Ta{sub 4}O{sub 11}(x≈0.93) piezoelectric phase from the transformation of Ta{sub 2}O{sub 5} thin films of monoclinic structure

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

Le Gallic, M., E-mail: Marie.Le-Gallic@grenoble-inp.fr; Roussel, H.

2013-04-15

A phase of trigonal structure has been reported in previous articles to be piezoelectric with a very high longitudinal d{sub 33} coefficient of several thousand pm/V (J. Solid State Chem. 184 (2011) 2023, 2033). It was observed in Ta{sub 2}O{sub 5} thin films and assumed to be the result of a reversible monoclinic–trigonal phase transition. However, new investigations are reported because the cell parameters of this phase are actually very close to those of the natrotantite mineral Na{sub 2}Ta{sub 4}O{sub 11}. From analyses by X-ray energy dispersive spectroscopy (XEDS), the chemical composition of this trigonal phase corresponds to (Na{sub x}K{submore » 1−x}){sub 2}Ta{sub 4}O{sub 11} with x≈0.93. The origin of sodium, potassium, iron and oxygen atoms is found to be due to a pollution coming from alumina crucibles used for thermal treatments. Knowing both atomic structures Ta{sub 2}O{sub 5} and Na{sub 2}Ta{sub 4}O{sub 11} and their structural relationships, observed by high resolution transmission electron microscopy (HRTEM), the mechanism of the reaction of transformation is re-examined and discussed. This mechanism implies that Ta{sup 5+} vacancies likely exist in monoclinic structures of tantalum oxide and that electrical neutrality could be due to incorporated proton H{sup +} instead of O{sup 2−} vacancies or Ta{sup 5+} interstitials. - Graphical abstract: (a, b) TEM images of interfaces between Ta{sub 2}O{sub 5} monoclinic and (Na{sub 0.93}K{sub 0.07}){sub 2}Ta{sub 4}O{sub 11} trigonal phases where it appears that the ratio of periods for the transformation monoclinic-to-trigonal is 3/2 (c, d) corresponding schema of the reaction of transformation, (a, c) viewed along the zone axes [010]{sub o}∥[1{sup ¯}21{sup ¯}]{sub R} and (b, d) along the zone axes [100]{sub o}∥[101{sup ¯}]{sub R}. Highlights: ► The formation of a piezoelectric phase in Ta{sub 2}O{sub 5} thin films, reported in previous articles, is re-examined. ► Its composition is

Monoclinic deformation of calcite crystals at ambient conditions

NASA Astrophysics Data System (ADS)

Przeniosło, R.; Fabrykiewicz, P.; Sosnowska, I.

2016-09-01

High resolution synchrotron radiation powder diffraction shows that the average crystal structure of calcite at ambient conditions is described with the trigonal space group R 3 bar c but there is a systematic hkl-dependent Bragg peak broadening. A modelling of this anisotropic peak broadening with the microstrain model from Stephens (1999) [15] is presented. The observed lattice parameters' correlations can be described by assuming a monoclinic-type deformation of calcite crystallites. A quantitative model of this monoclinic deformation observed at ambient conditions is described with the space group C 2 / c . The monoclinic unit cell suggested at ambient conditions is related with the monoclinic unit cell reported in calcite at high pressure (Merrill and Bassett (1975) [10]).

The orthorhombic to high-P monoclinic phase transition in Mg-Fe Pyroxenes: Can it produce a seismic discontinuity?

NASA Astrophysics Data System (ADS)

Woodland, Alan B.

The orthorhombic to high-P monoclinic phase transition in (Mg,Fe)SiO3 pyroxene with a mantle-relevant composition (XFs = 0.1) is expected to occur at ˜300 km depth [Woodland and Angel, 1997]. However, the divariant nature of the phase transition in the Mg-Fe system leaves the question open as to whether this transition occurs over a narrow enough pressure interval to cause a seismic discontinuity. New experimental results with binary Mg-Fe pyroxenes constrain the divariant loop to be 0.2 GPa wide at the composition of XFs = 0.4 and on the order of 0.15 GPa for a mantle-relevant composition. This implies that the phase transition will be complete over a depth interval of about 5-6 km in the mantle and it is concluded that the divariant loop of the orthorhombic to high-P monoclinic phase transition in (Mg,Fe)SiO3 pyroxene is indeed narrow enough to produce a “jump” in seismic velocities. The experimentally observed metastable behavior of orthopyroxene could further reduce the effective depth interval of this phase transition. The expected location of this phase transition coincides with a small magnitude seismic discontinuity, the “X-discontinuity”, occasionally observed in seismic profiles at ˜300 km depth, and thus provides a viable petrologic explanation for the origin of this discontinuity, if it truly exists.

β-Na2TeO4: Phase Transition from an Orthorhombic to a Monoclinic Form. Reversible CO2 Capture.

PubMed

Galven, Cyrille; Pagnier, Thierry; Rosman, Noël; Le Berre, Françoise; Crosnier-Lopez, Marie-Pierre

2018-06-18

The present work concerns the tellurate Na 2 TeO 4 which has a 1D structure and could then present a CO 2 capture ability. It has been synthesized in a powder form via a solid-state reaction and structurally characterized by thermal X-ray diffraction experiments, Raman spectroscopy, and differential scanning calorimetry. The room temperature structure corresponds to the β-Na 2 TeO 4 orthorhombic form, and we show that it undergoes a reversible structural transition near 420 °C toward a monoclinic system. Ab initio computations were also performed on the room temperature structure, the Raman vibration modes calculated, and a normal mode attribution proposed. In agreement with our expectations, this sodium oxide is able to trap CO 2 by a two-step mechanism: Na + /H + exchange and carbonation of the released sodium as NaHCO 3 . This capture is reversible since CO 2 can be released upon heating by recombination of the mother phase.

Monoclinic crystal structure of α - RuCl 3 and the zigzag antiferromagnetic ground state

DOE PAGES

Johnson, R. D.; Williams, S. C.; Haghighirad, A. A.; ...

2015-12-10

We have proposed the layered honeycomb magnet α - RuCl 3 as a candidate to realize a Kitaev spin model with strongly frustrated, bond-dependent, anisotropic interactions between spin-orbit entangled j eff = 1/2 Ru 3 + magnetic moments. We report a detailed study of the three-dimensional crystal structure using x-ray diffraction on untwinned crystals combined with structural relaxation calculations. We consider several models for the stacking of honeycomb layers and find evidence for a parent crystal structure with a monoclinic unit cell corresponding to a stacking of layers with a unidirectional in-plane offset, with occasional in-plane sliding stacking faults, inmore » contrast with the currently assumed trigonal three-layer stacking periodicity. We also report electronic band-structure calculations for the monoclinic structure, which find support for the applicability of the j eff = 1/2 picture once spin-orbit coupling and electron correlations are included. Of the three nearest-neighbor Ru-Ru bonds that comprise the honeycomb lattice, the monoclinic structure makes the bond parallel to the b axis nonequivalent to the other two, and we propose that the resulting differences in the magnitude of the anisotropic exchange along these bonds could provide a natural mechanism to explain the previously reported spin gap in powder inelastic neutron scattering measurements, in contrast to spin models based on the three-fold symmetric trigonal structure, which predict a gapless spectrum within linear spin wave theory. Our susceptibility measurements on both powders and stacked crystals, as well as magnetic neutron powder diffraction, show a single magnetic transition upon cooling below T N ≈ 13 K. Our analysis of our neutron powder diffraction data provides evidence for zigzag magnetic order in the honeycomb layers with an antiferromagnetic stacking between layers. Magnetization measurements on stacked single crystals in pulsed field up to 60 T show a single transition

Solid State Synthesis and Properties of Monoclinic Celsian

NASA Technical Reports Server (NTRS)

Bansal, Narottam P.

1996-01-01

Monoclinic celsian of Ba(0.75)Sr(0.25)Al2Si2O8 (BSAS-1) and B(0.85)Sr(O.15)Al2Si2O8 (BSAS-2) compositions have been synthesized from metal carbonates and oxides by solid state reaction. A mixture of BaCO3, SrCO3, Al2O3, and SiO2 powders was precalcined at approx. 900-940 C to decompose the carbonates followed by hot pressing at approx. 1300 C. The hot pressed BSAS-1 material was almost fully dense and contained the monoclinic celsian phase, with complete absence of the undesirable hexacelsian as indicated by x-ray diffraction. In contrast, a small fraction of hexacelsian was still present in hot pressed BSAS-2. However, on further heat treatment at 1200 C for 24 h, the hexacelsian phase was completely eliminated. The average linear thermal expansion coefficients of BSAS-1 and BSAS-2 compositions, having the monoclinic celsian phase, were measured to be 5.28 x 10(exp -6)/deg C and 5.15 x 10(exp -6)/deg C, respectively from room temperature to 1200 C. The hot pressed BSAS-1 celsian showed room temperature flexural strength of 131 MPa, elastic modulus of 96 GPa and was stable in air up to temperatures as high as approx. 1500 C.

Thermal, spectroscopic and structural characterization of isostructural phase transition in 4-hydroxybenzaldehyde

NASA Astrophysics Data System (ADS)

Panicker, Lata

2018-05-01

Polycrystalline samples of 4-hydroxybenzaldehyde (4-HOBAL) were investigated using differential scanning calorimeter (DSC), Raman spectroscopy and X-ray powder diffraction. The DSC data indicated that 4-HOBAL on heating undergoes a polymorphic transformation from polymorph I to polymorph II. The polymorph II formed remains metastable at ambient condition and transforms to polymorph I when annealed at ambient temperature for more than seven days. The structural information of polymorphs I and II obtained using its X-ray powder diffraction patterns indicated that 4-HOBAL undergoes an isostructural phase transition from polymorph I (monoclinic, P21/c) to polymorph II (monoclinic, P21/c). Raman data suggest that this structural change is associated with some change in its molecular interactions. Thus, in 4-HOBAL the polymorphic phase transformation (II to I) even though energetically favoured is kinetically hindered.

Crystal structure, electrical transport and phase transition in 2-methoxyanilinium hexachlorido stannate(IV) dehydrate

NASA Astrophysics Data System (ADS)

Karoui, Sahel; Chouaib, Hassen; Kamoun, Slaheddine

2017-04-01

A new organic-inorganic (C7H10NO)2[SnCl6]2H2O compound was synthesized and characterized by X-ray diffraction, thermal analysis, NMR spectroscopy and dielectric measurements. The crystal structure refinement shows that this compound crystallizes at 298 K in the monoclinic system (P21/a space group (Z = 2)). The structure was solved by Patterson method and refined to a final value of R = 0.034 for 2207 independent observed reflections. The cohesion and stability of the atomic arrangement result from the establishment of Nsbnd H⋯Cl, O(W)sbnd H(W)⋯Cl and Nsbnd H⋯O(W) hydrogen bonds between (C7H10NO)+ cations, isolated (SnCl6)2- anions and water molecules. This compound exhibits a phase transition at 305 K which was characterized by differential scanning calorimetry (DSC), X-rays powder diffraction and dielectric measurements. At high frequency, the electrical σTot.(ω,T) conductivity obey to the Jonscher's power law σTot.(ω,T) = σDC(T) + B(T) ωS(T). DC and AC conductivity in (C7H10NO)2[SnCl6]2H2O was investigated revealing that the phase transition from the monoclinic P21/a (phase I) to the monoclinic C2/c (phase II) which occurs at 305 K is characterized by a change of the mechanism of the electric transport: SPT in phase I and CBH in phase II.

Suppression of superconductivity and structural phase transitions under pressure in tetragonal FeS

DOE PAGES

Lai, Xiaofang; Liu, Ying; Lu, Xujie; ...

2016-08-08

Pressure is a powerful tool to study iron-based superconductors. Here, we report systematic high-pressure transport and structural characterizations of the newly discovered superconductor FeS. It is found that superconductor FeS (tetragonal) partly transforms to a hexagonal structure at 0.4 GPa, and then completely transforms to an orthorhombic phase at 7.4 GPa and finally to a monoclinic phase above 9.0 GPa. The superconducting transition temperature of tetragonal FeS was gradually depressed by pressure, different from the case in tetragonal FeSe. With pressure increasing, the S-Fe-S angles only slightly change but the anion height deviates farther from 1.38 Å. This change ofmore » anion height, together with the structural instability under pressure, should be closely related to the suppression of superconductivity. We also observed an anomalous metal-semiconductor transition at 6.0 GPa and an unusual increased resistance with further compression above 9.6 GPa. The former can be ascribed to the tetragonal-orthorhombic structural phase transition, and the latter to the electronic structure changes of the high-pressure monoclinic phase. Lastly, a phase diagram of tetragonal FeS as functions of pressure and temperature was mapped out for the first time, which will shed new light on understanding of the structure and physics of the superconducting FeS.« less

Origin of thickness dependence of structural phase transition temperatures in BiFeO 3 thin films

DOE PAGES

Yang, Yongsoo; Beekman, Christianne; Siemons, Wolter; ...

2016-03-28

In this study, two structural phase transitions are investigated in highly strained BiFeO 3 thin films grown on LaAlO 3 substrates, as a function of film thickness and temperature via synchrotron x-ray diffraction. Both transition temperatures (upon heating: monoclinic MC to monoclinic MA, and MA to tetragonal) decrease as the film becomes thinner. The existence of an interface layer at the film-substrate interface, deduced from half-order peak intensities, contributes to this behavior only for the thinnest samples; at larger thicknesses (above a few nanometers) the temperature dependence can be understood in terms of electrostatic considerations akin to size effects inmore » ferroelectric phase transitions, but observed here for structural phase transitions within the ferroelectric phase and related to the rearrangement rather than the formation of domains. For ultra-thin films, the tetragonal structure is stable at all investigated temperatures (down to 30 K).« less

Giant Polarization and High Temperature Monoclinic Phase in a Lead-Free Perovskite of Bi(Zn 0.5Ti 0.5)O 3-BiFeO 3

DOE PAGES

Pan, Zhao; Chen, Jun; Yu, Runze; ...

2016-09-15

Lead-free piezoelectrics have attracted increasing attention due to the awareness of lead toxicity to the environment. Here, a new Bi-based lead-free perovskite of (1-x)Bi(Zn 0.5Ti 0.5)O 3-xBiFeO 3 has been synthesized via high-pressure and high-temperature method. It exhibits interest-ing properties of giant polarization, morphotropic phase boundary (MPB), and monoclinic phase. In particular, large tetragonality ( c/a = 1.228) and giant spontaneous polariza-tion of 110 μC/cm 2 has been obtained in 0.6Bi(Zn 0.5Ti 0.5)O 3-0.4BiFeO 3, which is much higher than most available lead-free materials and conventional Pb(Zr,Ti)O 3. MPB is clearly identified to be constituted by tetragonal and monoclinic phasesmore » at x = 0.5. Notably, a single monoclinic phase has been observed at x = 0.6, which exhibits an intriguing high temperature property. In conclusion, the present results are helpful to explore new lead-free MPB systems in bismuth-based compounds.« less

Nanoscale monoclinic domains in epitaxial SrRuO{sub 3} thin films deposited by pulsed laser deposition

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

Ghica, C., E-mail: cghica@infim.ro; Negrea, R. F.; Nistor, L. C.

2014-07-14

In this paper, we analyze the structural distortions observed by transmission electron microscopy in thin epitaxial SrRuO{sub 3} layers used as bottom electrodes in multiferroic coatings onto SrTiO{sub 3} substrates for future multiferroic devices. Regardless of the nature and architecture of the multilayer oxides deposited on the top of the SrRuO{sub 3} thin films, selected area electron diffraction patterns systematically revealed the presence of faint diffraction spots appearing in forbidden positions for the SrRuO{sub 3} orthorhombic structure. High-resolution transmission electron microscopy (HRTEM) combined with Geometric Phase Analysis (GPA) evidenced the origin of these forbidden diffraction spots in the presence ofmore » structurally disordered nanometric domains in the SrRuO{sub 3} bottom layers, resulting from a strain-driven phase transformation. The local high compressive strain (−4% ÷ −5%) measured by GPA in the HRTEM images induces a local orthorhombic to monoclinic phase transition by a cooperative rotation of the RuO{sub 6} octahedra. A further confirmation of the origin of the forbidden diffraction spots comes from the simulated diffraction patterns obtained from a monoclinic disordered SrRuO{sub 3} structure.« less

Monoclinic phase transformation and mechanical durability of zirconia ceramic after fatigue and autoclave aging.

PubMed

Mota, Yasmine A; Cotes, Caroline; Carvalho, Rodrigo F; Machado, João P B; Leite, Fabíola P P; Souza, Rodrigo O A; Özcan, Mutlu

2017-10-01

This study evaluated the influence of two aging procedures on the biaxial flexural strength of yttria-stabilized tetragonal zirconia ceramics. Disc-shaped zirconia specimens and (ZE: E.max ZirCAD, Ivoclar; ZT: Zirkon Translucent, Zirkonzahn) (N = 80) (∅:12 mm; thickness:1.2 mm, ISO 6872) were prepared and randomly divided into four groups (n = 10 per group) according to the aging procedures: C: Control, no aging; M: mechanical cycling (2 × 10 6 cycles/3.8 Hz/200 N); AUT: Aging in autoclave at 134°C, 2 bar for 24 h; AUT + M: Autoclave aging followed by mechanical cycling. After aging, the transformed monoclinic zirconia (%) were evaluated using X-ray diffraction and surface roughness was measured using atomic force microscopy. The average grain size was measured by scanning electron microscopy and the specimens were submitted to biaxial flexural strength testing (1 mm/min, 1000 kgf in water). Data (MPa) were statistically analyzed using 2-way analysis of variance and Tukey's test (α = 0.05). Aging procedures significantly affected (p = 0.000) the flexural strength data but the effect of zirconia type was not significant (p = 0.657). AUT ZT (936.4 ± 120.9 b ) and AUT + M ZE (867.2 ± 49.3 b ) groups presented significantly higher values (p < 0.05) of flexural strength than those of the control groups (C ZT : 716.5 ± 185.7 a ; C ZE : 779.9 ± 114 a ) (Tukey's test). The monoclinic phase percentage (%) was higher for AUT ZE (71), AUT ZT (66), AUT + M ZE (71), and AUT + M ZM (66) compared to the C groups (ZE:0; ZT:0). Surface roughness (µm) was higher for AUT ZE (0.09), AUT ZT (0.08), AUT + M ZE (0.09 µm), and AUT + M ZT (0.09 µm) than those of other groups. Regardless of the zirconia type, autoclave aging alone or with mechanical aging increased the flexure strength but also induced higher transformation from tetragonal to monoclinic phase in both zirconia materials tested. © 2016 Wiley

A new interpretation of the Besnus transition in monoclinic pyrrhotite

NASA Astrophysics Data System (ADS)

Gehring, Andreas

2016-04-01

Non-stoichiometric monoclinic 4C pyrrhotite (ideal formula: Fe7S8) is a major magnetic remanence carrier in the Earth's crust and in extraterrestrial materials. Because of its low-temperature magnetic transition around 30 K also known as Besnus transition, this mineral phase is easily detectable in natural samples. Considering the rock magnetic literature, an intrinsic origin of the Besnus transition similar to that of the Verwey transition has generally been assumed. Although the physical properties of pyrrhotite have intensively been studied, the mechanism behind the pronounced change in magnetization at the low-temperature transition is still debated. To address this question we performed magnetization experiments on a natural pyrrhotite crystal (Fe6.6S8) that consists of an epitaxial intergrowth of a commensurate 4C and an incommensurate 5C* superstructure that are different in their defect structure (1,2). The occurrence of two monoclinic superstructures detected by X-ray diffractometry is magnetically confirmed by symmetric inflection points in hysteresis measurements above the transition at about 30 K. The disappearance of the inflection points and the associated change of the hysteresis parameters indicate that the two superstructures become strongly coupled to form a unitary magnetic anisotropy system at the transition. From this it follows that the Besnus transition in monoclinic pyrrhotite is an extrinsic magnetic phenomenon with respect to the 4C superstructure and therefore the physics behind it is in fact different from that of the well-known Verwey transition. Finally, this novel interpretation explains the rock magnetic data for the low-temperature transition that has been reported for monoclinic pyrrhotite. It will also provide deeper understanding of magnetism in monoclinic pyrrhotite, which in turn will enable a more profound insight to the magnetization properties of the Earth's crust. 1.) Charilaou, M., Kind, J., Koulialias, D., Weidler, P

Synthesis of monoclinic IrTe 2 under high pressure and its physical properties

DOE PAGES

Li, X.; Yan, J. -Q.; Singh, D. J.; ...

2015-10-12

In a pressure-temperature (P-T) diagram for synthesizing IrTe 2 compounds, the well-studied trigonal (H) phase with the CdI 2-type structure is stable at low pressures. The superconducting cubic (C) phase can be synthesized under higher temperatures and pressures. A rhombohedral phase with the crystal structure similar to the C phase can be made at ambient pressure; but the phase contains a high concentration of Ir deficiency. Here, we report that a rarely studied monoclinic (M) phase can be stabilized in narrow ranges of pressure and temperature in this P-T diagram. Moreover, the peculiar crystal structure of the M-IrTe 2 eliminatesmore » the tendency to form Ir-Ir dimers found in the H phase. The M phase has been fully characterized by structural determination and measurements of electrical resistivity, thermoelectric power, DC magnetization, and specific heat. These physical properties have been compared with those in the H and C phases of Ir 1-xTe 2. Finally, we present magnetic and transport properties and specific heat of the M-IrTe 2 can be fully justified by calculations with the density-functional theory.« less

Effects of monoclinic symmetry on the properties of biaxial liquid crystals

NASA Astrophysics Data System (ADS)

Solodkov, Nikita V.; Nagaraj, Mamatha; Jones, J. Cliff

2018-04-01

Tilted smectic liquid crystal phases such as the smectic-C phase seen in calamitic liquid crystals are usually treated using the assumption of biaxial orthorhombic symmetry. However, the smectic-C phase has monoclinic symmetry, thereby allowing disassociation of the principal optic and dielectric axes based on symmetry and invariance principles. This is demonstrated here by comparing optical and dielectric measurements for two materials with highly first-order direct transitions from nematic to smectic-C phases. The results show a high difference between the orientations of the principal axes sets, which is interpreted as the existence of two distinct cone angles for optical and dielectric frequencies. Both materials exhibit an increasing degree of monoclinic behavior with decreasing temperature. Due to fast switching speeds, ferroelectric smectic-C* materials are important for fast modulators and LCoS devices, where the dielectric biaxiality influences device operation.

Phase-controlled synthesis of polymorphic tungsten diphosphide with hybridization of monoclinic and orthorhombic phases as a novel electrocatalyst for efficient hydrogen evolution

NASA Astrophysics Data System (ADS)

Pi, Mingyu; Wu, Tianli; Guo, Weimeng; Wang, Xiaodeng; Zhang, Dingke; Wang, Shuxia; Chen, Shijian

2017-05-01

The design and development of high-efficiency and non-noble-metal hydrogen evolution reaction (HER) electrocatalysts for future clean and renewable energy system has excited significant research interests over the recent years. In this communication, the polymorphic tungsten diphosphide (p-WP2) nanoparticles with mixed monoclinic (α-) and orthorhombic (β-) phases are synthesized by phase-controlled phosphidation route via vacuum capsulation and explored as a novel efficient electrocatalyst towards HER. The p-WP2 catalyst delivers superior performance with excellent stability under both acidic and alkaline conditions over its single phases of α-WP2 and β-WP2. This finding demonstrates that a highly efficient hybrid electrocatalyst can be achieved via precise composition controlling and may open up exciting opportunities for their practical applications toward energy conversion.

Linear optical properties of the monoclinic bismuth borate BiB3O6

NASA Astrophysics Data System (ADS)

Hellwig, H.; Liebertz, J.; Bohatý, L.

2000-07-01

New materials for nonlinear optical (NLO) applications are still of great interest. The monoclinic BiB3O6 (BIBO) shows exceptionally large NLO coefficients. In this article we will present the linear optical properties in the wavelength range between 350 and 2400 nm, the phase matching conditions calculated for second harmonic generation, and optical parametric oscillation. Angular bandwidth data are also given. The wide tuning range of phase matched directions together with the monoclinic symmetry allow a broad variety of applications. The laser damage threshold is comparable to high quality lithium triborate.

Modulated structures and associated microstructures in the ferroelectric phase of Ba1-xSrxAl2O4 for 0.7 ≤ x ≤ 1.0

NASA Astrophysics Data System (ADS)

Tsukasaki, Hirofumi; Ishii, Yui; Tanaka, Eri; Kurushima, Kosuke; Mori, Shigeo

2016-01-01

In order to understand the ferroelectric and ferroelastic phases in Ba1-xSrxAl2O4 for 0.7 ≤ x ≤ 1.0, we have investigated the crystal structures and their associated microstructures of the ferroelectric and ferroelastic phases mainly by transmission electron microscopy (TEM) and scanning transmission electron microscopy-high-angle angular dark-field (STEM-HAADF) experiments, combined with powder X-ray diffraction experiments. Electron diffraction experiments showed that the ferroelectric and ferroelastic phases of Ba1-xSrxAl2O4 for 0.7 ≤ x ≤ 1.0 should be characterized as a modulated structure with the modulation vector of \\boldsymbol{{q}} = 0,1/2,0, whose space group should be monoclinic P21. High-resolution TEM experiments revealed that the microstructures in the monoclinic phase can be characterized as twin structures and nanometer-sized planar defects due to the monoclinic structure with the modulated structures, which are responsible for anomalous elastic behaviors and mechanoelectro-optical properties. In addition, subatomic-resolution STEM-HAADF images clearly indicated that the displacement of Al3+ ions involved in the AlO4 tetrahedra should play a crucial role in the formation of the modulated structures and twin structures.

Characterization of a Diamond Ground Y-TZP and Reversion of the Tetragonal to Monoclinic Transformation.

PubMed

Candido, L M; Fais, Lmg; Ferreira, E B; Antonio, S G; Pinelli, Lap

To characterize the surface of an yttria-stabilized zirconia (Y-TZP) ceramic after diamond grinding in terms of its crystalline phase, morphology, mean roughness (Ra), and wettability as well as to determine a thermal treatment to reverse the resulting tetragonal to monoclinic (t-m) transformation. Y-TZP specimens were distributed into different groups according to the actions (or no action) of grinding and irrigation. Grinding was accomplished using a diamond stone at a low speed. The samples were characterized by x-ray diffraction (XRD), scanning electron microscopy, goniometry, and profilometry. In situ high-temperature XRD was used to determine an annealing temperature to reverse the t-m transformation. Ra was submitted to the Kruskal-Wallis test, followed by the Dunn test (α=0.05). The volume fraction of the monoclinic phase and contact angle were submitted to one-way analysis of variance, followed by the Tukey test (α=0.05). Monoclinic zirconia was observed on the surface of samples after dry and wet grinding with a diamond stone. The volume fraction of the monoclinic phase was smaller on the dry ground samples (3.6%±0.3%) than on the wet ground samples (5.6%±0.3%). High-temperature XRD showed reversion of the t-m phase transformation, which started at 700°C and completed at 800°C in a conventional oven. Grinding with a diamond stone partially transformed the crystalline phase on the surface of a Y-TZP ceramic from tetragonal to monoclinic zirconia while simultaneously increasing the surface roughness and wettability. The t-m transformation could be reversed by heat treatment at 800°C or 900°C for 60 minutes or 1000°C for 30 minutes.

Origins of the structural phase transitions in MoTe2 and WTe2

NASA Astrophysics Data System (ADS)

Kim, Hyun-Jung; Kang, Seoung-Hun; Hamada, Ikutaro; Son, Young-Woo

2017-05-01

Layered transition metal dichalcogenides MoTe2 and WTe2 share almost similar lattice constants as well as topological electronic properties except their structural phase transitions. While the former shows a first-order phase transition between monoclinic and orthorhombic structures, the latter does not. Using a recently proposed van der Waals density functional method, we investigate structural stability of the two materials and uncover that the disparate phase transitions originate from delicate differences between their interlayer bonding states near the Fermi energy. By exploiting the relation between the structural phase transitions and the low energy electronic properties, we show that a charge doping can control the transition substantially, thereby suggesting a way to stabilize or to eliminate their topological electronic energy bands.

Monoclinic tridymite in clast-rich impact melt rock from the Chesapeake Bay impact structure

USGS Publications Warehouse

Jackson, John C.; Horton, J. Wright; Chou, I-Ming; Belkin, Harvey E.

2011-01-01

X-ray diffraction and Raman spectroscopy confirm a rare terrestrial occurrence of monoclinic tridymite in clast-rich impact melt rock from the Eyreville B drill core in the Chesapeake Bay impact structure. The monoclinic tridymite occurs with quartz paramorphs after tridymite and K-feldspar in a microcrystalline groundmass of devitrified glass and Fe-rich smectite. Electron-microprobe analyses revealed that the tridymite and quartz paramorphs after tridymite contain different amounts of chemical impurities. Inspection by SEM showed that the tridymite crystal surfaces are smooth, whereas the quartz paramorphs contain irregular tabular voids. These voids may represent microporosity formed by volume decrease in the presence of fluid during transformation from tridymite to quartz, or skeletal growth in the original tridymite. Cristobalite locally rims spherulites within the same drill core interval. The occurrences of tridymite and cristobalite appear to be restricted to the thickest clast-rich impact melt body in the core at 1402.02–1407.49 m depth. Their formation and preservation in an alkali-rich, high-silica melt rock suggest initially high temperatures followed by rapid cooling.

Monoclinic structure of hydroxylpyromorphite Pb10(PO4)6(OH)2 - hydroxylmimetite Pb10(AsO4)6(OH)2 solid solution series

NASA Astrophysics Data System (ADS)

Giera, Alicja; Manecki, Maciej; Borkiewicz, Olaf; Zelek, Sylwia; Rakovan, John; Bajda, Tomasz; Marchlewski, Tomasz

2016-04-01

Seven samples of hydroxyl analogues of pyromorphite-mimetite solid solutions series were synthesized from aqueous solutions at 80° C in a computer-controlled chemistate: 200 mL aqueous solutions of 0.05M Pb(NO3)2 and 0.03M KH2AsO4 and/or KH2PO4 were dosed with a 0.25 mL/min rate to a glass beaker, which initially contained 100 mL of distilled water. Constant pH of 8 was maintained using 2M KOH. The syntheses yielded homogeneous fine-grained white precipitates composition of which was close to theoretical Pb10[(PO4)6-x(AsO4)x](OH)2, where x = 0, 1, 2, 3, 4, 5, 6. High-resolution powder X-ray diffraction data were obtained in transmission geometry at the beamline 11-BM at the Advanced Photon Source (Argonne National Laboratory in Illinois, USA). The structure Rietveld refinements based on starting parameters of either hexagonal hydroxylpyromorphite or monoclinic mimetite-M were performed using GSAS+EXPGUI software. Apatite usually crystallizes in the hexagonal crystal system with the space group P63/m. For the first time, however, the lowering of the hexagonal to monoclinic crystal symmetry was observed in the hydroxyl variety of pyromorphite-mimetite solid solution series. This is indicated by better fitting of the modeled monoclinic structure to the experimental data. The same is not the case for analogous calcium hydroxylapatite series Ca5(PO4)3OH - Ca5(AsO4)3OH (Lee et al. 2009). Systematical linear increase of unit cell parameters is observed with As substitution from a=9.88, b=19.75, and c=7.43 for Pb10(PO4)6(OH)2 to a=10.23, b=20.32, and c=7.51 for Pb10(AsO4)6(OH)2. A strong pseudohexagonal character (γ ≈ 120° and b ≈ 2a) of the analyzed monoclinic phases was established. This work is partially funded by AGH research grant no 11.11.140.319 and partially by Polish NCN grant No 2011/01/M/ST10/06999. Lee Y.J., Stephens P.W., Tang Y., Li W., Philips B.L., Parise J.B., Reeder R.J., 2009. Arsenate substitution in hydroxylapatite: Structural characterization

Structural phase transitions in niobium oxide nanocrystals

NASA Astrophysics Data System (ADS)

Yuvakkumar, R.; Hong, Sun Ig

2015-09-01

Niobium oxide nanocrystals were successfully synthesized employing the green synthesis method. Phase formation, microstructure and compositional properties of 1, 4 and 7 days incubation treated samples after calcinations at 450 °C were examined using X-ray diffraction, Raman, photoluminescence (PL), infrared, X-ray photoelectron spectra and transmission electron microscopic characterizations. It was observed that phase formation of Nb2O5 nanocrystals was dependent upon the incubation period required to form stable metal oxides. The characteristic results clearly revealed that with increasing incubation and aging, the transformation of cubic, orthorhombic and monoclinic phases were observed. The uniform heating at room temperature (32 °C) and the ligation of niobium atoms due to higher phenolic constituents of utilized rambutan during aging processing plays a vital role in structural phase transitions in niobium oxide nanocrystals. The defects over a period of incubation and the intensities of the PL spectra changing over a period of aging were related to the amount of the defects induced by the phase transition.

Oxygen vacancies dependent phase transition of Y2O3 films

NASA Astrophysics Data System (ADS)

Yu, Pengfei; Zhang, Kan; Huang, Hao; Wen, Mao; Li, Quan; Zhang, Wei; Hu, Chaoquan; Zheng, Weitao

2017-07-01

Y2O3 films have great application potential in high-temperature metal matrix composite and nuclear engineering, used as interface diffusion and reaction barrier coating owing to their excellent thermal and chemical stability, high melting point and extremely negative Gibbs formation energy, and thus their structural and mechanical properties at elevated temperature are especially important. Oxygen vacancies exist commonly in yttrium oxide (Y2O3) thin films and act strongly on the phase structure and properties, but oxygen vacancies dependent phase transition at elevated temperature has not been well explored yet. Y2O3 thin films with different oxygen vacancy concentrations have been achieved by reactive sputtering through varying substrate temperature (Ts), in which oxygen vacancies increase monotonously with increasing Ts. For as-deposited Y2O3 films, oxygen vacancies present at high Ts can promote the nucleation of monoclinic phase, meanwhile, high Ts can induce the instability of monoclinic phase. Thus their competition results in forming mixed phases of cubic and monoclinic at high Ts. During vacuum annealing at 1000 °C, a critical oxygen vacancy concentration is observed, below which phase transition from monoclinic to cubic takes place, and above which phase transfer from monoclinic to the oxygen defective phase (ICDD file no. 39-1063), accompanying by stress reversal from compressive to tensile and maintenance of high hardness.

Ab initio prediction of superdense tetragonal and monoclinic polymorphs of carbon

DOE PAGES

Li, Zhen -Zhen; Wang, Jian -Tao; Xu, Li -Fang; ...

2016-11-02

The design and synthesis of three-dimensional denser carbons are one of the hot issues in condensed matter physics because of their fascinating properties. Here we identify by ab initio calculations several tetragonal and monoclinic polymorphs of carbon that adopt the t32, t32*, m32, and m32* structures in P4¯2 1c, P4 32 12, P2 1/c, and C2 symmetry, respectively. These carbon polymorphs have large 32-atom unit cells in all-sp 3 bonding networks comprising five- and six-membered rings that are dynamically stable, as verified by a phonon mode analysis. Electronic band structure calculations show that they are insulators with band gaps inmore » the range of 5.19–5.41 eV, close to the calculated band gap of 5.34 eV for diamond. Remarkably, these carbon phases possess an extremely high atom number density exceeding that of diamond. Lastly, the present results establish different types of carbon phases and offer insights into their outstanding structural and electronic properties.« less

Ferroelectric Self-Poling, Switching, and Monoclinic Domain Configuration in BiFeO 3 Thin Films

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

Beekman, C.; Siemons, W.; Chi, M.

2016-05-23

Self-poling of ferroelectric films, i.e., a preferred, uniform direction of the ferroelectric polarization in as-grown samples is often observed yet poorly understood despite its importance for device applications. The multiferroic perovskite BiFeO 3, which crystallizes in two distinct structural polymorphs depending on applied epitaxial strain, is well known to exhibit self-poling. This study investigates the effect of self-poling on the monoclinic domain configuration and the switching properties of the two polymorphs of BiFeO 3 (R' and T') in thin films grown on LaAlO 3 substrates with slightly different La 0.3Sr 0.7MnO 3 buffer layers. Our study shows that the polarizationmore » state formed during the growth acts as “imprint” on the polarization and that switching the polarization away from this self-poled direction can only be done at the expense of the sample's monoclinic domain configuration. We observed reduction of the monoclinic domain size and found that it was largely reversible; hence, the domain size is restored when the polarization is switched back to its original orientation. This is a direct consequence of the growth taking place in the polar phase (below T c). Finally, switching the polarization away from the preferred configuration, in which defects and domain patterns synergistically minimize the system's energy, leads to a domain state with smaller (and more highly strained and distorted) monoclinic domains.« less

Ab initio study of the structural phase transitions of the double perovskites Sr2MWO6 (M=Zn, Ca, Mg)

NASA Astrophysics Data System (ADS)

Petralanda, U.; Etxebarria, I.

2014-02-01

We study the interplay of structural distortions in double perovskites Sr2MWO6 (M = Zn, Ca, Mg) by means of first-principles calculations and group theoretical analysis. Structure relaxations of the cubic, tetragonal, and monoclinic phases show that the ground states of the three compounds are monoclinic, although the energy difference between the monoclinic and tetragonal structures is very small in the case of Sr2MgWO6. The symmetry analysis of the distortions involved in the experimental and calculated low-temperature structures shows that the amplitude of two primary distortions associated to rigid rotations of the MX6 and WO6 octahedra are dominant, although the amplitude of a third mode related to deformations of the MX6 groups can not be neglected. The energy maps of the space spanned by the three relevant modes are calculated, and the couplings among the modes are evaluated, showing that the role of a hard secondary mode (in the Landau sense) coupled trilinearly to the two primary instabilities is crucial to stabilize the monoclinic ground state. Results suggest that the key role of the trilinear coupling among three modes could be rather common. A phenomenological theory including the effects of the chemical pressure is also developed. We find that the evolution of the stiffness constants in terms of the atomic substitution follows an accurate linear dependence and that the influence of quantum saturation of the order parameters could stabilize the tetragonal phase of Sr2MgWO6.

Assignments of the Raman modes of monoclinic erbium oxide

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

Yan, D.; Wu, P., E-mail: pingwu@sas.ustb.edu.cn; Zhang, S. P.

2013-11-21

As a heavy rare earth oxide, erbium oxide (Er{sub 2}O{sub 3}) has many attractive properties. Monoclinic Er{sub 2}O{sub 3} has useful properties not found in stable cubic Er{sub 2}O{sub 3}, such as unique optical properties and high radiation damage tolerance. In this study, cubic Er{sub 2}O{sub 3} coating and Er{sub 2}O{sub 3} coating with mixed phases were prepared. The Raman scattering spectra of these coatings were investigated by using a confocal micro-Raman spectrometer equipped with 325, 473, 514, 532, 633, and 784 nm lasers. A total of 17 first-order Raman modes of monoclinic Er{sub 2}O{sub 3} were identified and assigned. Themore » modes at 83, 112, 152, 170, 278, 290, 409, 446, 478, 521, 603, and 622 cm{sup −1} are of A{sub g} symmetry, whereas modes at 71, 98, 333, 409, 446, and 468 cm{sup −1} are of B{sub g} symmetry. This research provides basic data necessary for the characterization of monoclinic Er{sub 2}O{sub 3} by Raman spectroscopy.« less

Monoclinic Tungsten Oxide with {100} Facet Orientation and Tuned Electronic Band Structure for Enhanced Photocatalytic Oxidations.

PubMed

Zhang, Ning; Chen, Chen; Mei, Zongwei; Liu, Xiaohe; Qu, Xiaolei; Li, Yunxiang; Li, Siqi; Qi, Weihong; Zhang, Yuanjian; Ye, Jinhua; Roy, Vellaisamy A L; Ma, Renzhi

2016-04-27

Exploring surface-exposed highly active crystal facets for photocatalytic oxidations is promising in utilizing monoclinic WO3 semiconductor. However, the previously reported highly active facets for monoclinic WO3 were mainly toward enhancing photocatalytic reductions. Here we report that the WO3 with {100} facet orientation and tuned surface electronic band structure can effectively enhance photocatalytic oxidation properties. The {100} faceted WO3 single crystals are synthesized via a facile hydrothermal method. The UV-visible diffuse reflectance, X-ray photoelectron spectroscopy valence band spectra, and photoelectrochemical measurements suggest that the {100} faceted WO3 has a much higher energy level of valence band maximum compared with the normal WO3 crystals without preferred orientation of the crystal face. The density functional theory calculations reveal that the shift of O 2p and W 5d states in {100} face induce a unique band structure. In comparison with the normal WO3, the {100} faceted WO3 exhibits an O2 evolution rate about 5.1 times in water splitting, and also shows an acetone evolution rate of 4.2 times as well as CO2 evolution rate of 3.8 times in gaseous degradation of 2-propanol. This study demonstrates an efficient crystal face engineering route to tune the surface electronic band structure for enhanced photocatalytic oxidations.

Monoclinic Cc-phase stabilization in magnetically diluted lead free Na1/2Bi1/2TiO3—Evolution of spin glass like behavior with enhanced ferroelectric and dielectric properties

NASA Astrophysics Data System (ADS)

Thangavelu, Karthik; Asthana, Saket

2015-09-01

The effect of magnetic cation substitution on the phase stabilization, ferroelectric, dielectric and magnetic properties of a lead free Na0.5Bi0.5TiO3 (NBT) system prepared by O2 atmosphere solid state sintering were studied extensively. Cobalt (Co) was chosen as the magnetic cation to substitute at the Ti-site of NBT with optimized 2.5 mol%. Rietveld analysis of x-ray diffraction data favours the monoclinic Cc phase stabilization strongly rather than the parent R3c phase. FE-SEM micrograph supports the single phase characteristics without phase segregation at the grain boundaries. The stabilized Cc space group was explained based on the collective local distortion effects due to spin-orbit stabilization at Co3+ and Co2+ functional centres. The phonon mode changes as observed in the TiO6 octahedral modes also support the Cc phase stabilization. The major Co3+-ion presence was revealed from corresponding crystal field transitions observed through solid state diffuse reflectance spectroscopy. The enhanced spontaneous polarization (Ps) from ≅38 μC cm-2 to 45 μC cm-2 could be due to the easy rotation of polarization vector along the {(1\\bar{1}0)}{{pc}} in Cc phase. An increase in static dielectric response (ɛ) from ɛ ≅ 42 to 60 along with enhanced diffusivity from γ ≅ 1.53 to 1.75 was observed. Magneto-thermal irreversibility and their magnetic field dependent ZFC/FC curves suggest the possibility of a spin glass like behaviour below 50 K. The monoclinic Cc phase stabilization as confirmed from structural studies was well correlated with the observed ferroic properties in magnetically diluted NBT.

Critical Role of Monoclinic Polarization Rotation in High-Performance Perovskite Piezoelectric Materials.

PubMed

Liu, Hui; Chen, Jun; Fan, Longlong; Ren, Yang; Pan, Zhao; Lalitha, K V; Rödel, Jürgen; Xing, Xianran

2017-07-07

High-performance piezoelectric materials constantly attract interest for both technological applications and fundamental research. The understanding of the origin of the high-performance piezoelectric property remains a challenge mainly due to the lack of direct experimental evidence. We perform in situ high-energy x-ray diffraction combined with 2D geometry scattering technology to reveal the underlying mechanism for the perovskite-type lead-based high-performance piezoelectric materials. The direct structural evidence reveals that the electric-field-driven continuous polarization rotation within the monoclinic plane plays a critical role to achieve the giant piezoelectric response. An intrinsic relationship between the crystal structure and piezoelectric performance in perovskite ferroelectrics has been established: A strong tendency of electric-field-driven polarization rotation generates peak piezoelectric performance and vice versa. Furthermore, the monoclinic M_{A} structure is the key feature to superior piezoelectric properties as compared to other structures such as monoclinic M_{B}, rhombohedral, and tetragonal. A high piezoelectric response originates from intrinsic lattice strain, but little from extrinsic domain switching. The present results will facilitate designing high-performance perovskite piezoelectric materials by enhancing the intrinsic lattice contribution with easy and continuous polarization rotation.

Polymorphism of paracetamol: relative stabilities of the monoclinic and orthorhombic phases inferred from topological pressure-temperature and temperature-volume phase diagrams.

PubMed

Espeau, Philippe; Céolin, René; Tamarit, Josep-Lluis; Perrin, Marc-Antoine; Gauchi, Jean-Pierre; Leveiller, Franck

2005-03-01

The thermodynamic relationships between the two known polymorphs of paracetamol have been investigated, and the subsequent pressure-temperature and temperature-volume phase diagrams were constructed using data from crystallographic and calorimetric measurements as a function of the temperature. Irrespective of temperature, monoclinic Form I and orthorhombic Form II are stable phases at ordinary and high pressures, respectively. The I and II phase regions in the pressure-temperature diagram are bordered by the I-II equilibrium curve, for which a negative slope (dp/dT approximately -0.3 MPa x K(-1)) was determined although it was not observed experimentally. This curve goes through the I-II-liquid triple point whose coordinates (p approximately 234 MPa, T approximately 505 K) correspond to the crossing point of the melting curves, for which dp/dT values of +3.75 MPa x K(-1) (I) and +3.14 MPa x K(-1) (II) were calculated from enthalpy and volume changes upon fusion. More generally, this case exemplifies how the stability hierarchy of polymorphs may be inferred from the difference in their sublimation curves, as topologically positioned with respect to each other, using the phase rule and simple inferences resorting to Gibbs equilibrium thermodynamics. Copyright 2004 Wiley-Liss, Inc. and the American Pharmacists Association.

Real-time atomistic observation of structural phase transformations in individual hafnia nanorods

DOE PAGES

Hudak, Bethany M.; Depner, Sean W.; Waetzig, Gregory R.; ...

2017-05-12

High-temperature phases of hafnium dioxide have exceptionally high dielectric constants and large bandgaps, but quenching them to room temperature remains a challenge. Scaling the bulk form to nanocrystals, while successful in stabilizing the tetragonal phase of isomorphous ZrO 2, has produced nanorods with a twinned version of the room temperature monoclinic phase in HfO 2. Here we use in situ heating in a scanning transmission electron microscope to observe the transformation of an HfO 2 nanorod from monoclinic to tetragonal, with a transformation temperature suppressed by over 1000°C from bulk. When the nanorod is annealed, we observe with atomic-scale resolutionmore » the transformation from twinned-monoclinic to tetragonal, starting at a twin boundary and propagating via coherent transformation dislocation; the nanorod is reduced to hafnium on cooling. Unlike the bulk displacive transition, nanoscale size-confinement enables us to manipulate the transformation mechanism, and we observe discrete nucleation events and sigmoidal nucleation and growth kinetics.« less

Insulating phases of vanadium dioxide are Mott-Hubbard insulators

DOE PAGES

Huffman, T. J.; Hendriks, C.; Walter, E. J.; ...

2017-02-15

Here, we present comprehensive broadband optical spectroscopy data on two insulating phases of vanadium dioxide (VO 2): monoclinic M 2 and triclinic. The main result of our work is that the energy gap and the electronic structure are essentially unaltered by the first-order structural phase transition between the M 2 and triclinic phases. Moreover, the optical interband features in the M 2 and triclinic phases are remarkably similar to those observed in the well-studied monoclinic M 1 insulating phase of VO 2. As the energy gap is insensitive to the different lattice structures of the three insulating phases, we rulemore » out vanadium-vanadium pairing (the Peierls component) as the dominant contributor to the opening of the gap. Rather, the energy gap arises primarily from intra-atomic Coulomb correlations.« less

Superconductivity, Magnetoresistance, Magnetic Anomaly and Crystal Structure of New Phases of Topological Insulators Bi2Se3 and Sb2Te3

NASA Astrophysics Data System (ADS)

Kulbachinskii, V. A.; Buga, S. G.; Serebryanaya, N. R.; Perov, N. S.; Kytin, V. G.; Tarelkin, S. A.; Bagramov, R. H.; Eliseev, N. N.; Blank, V. D.

2018-03-01

We synthesized a new metastable phase of Bi2Se3 topological insulator by a rapid quenching after a high-pressure-high-temperature treatment at P≈7.7 GPa; 673structure of metastable phase is monoclinic P21/m type. We observed the zero-field magnetic susceptibility cusp and linear positive magnetoresistance indicating the topological insulator state. The annealing at 673 K during 2 hours resulted in complete reversible transformation into the initial crystalline rhombohedral structure. Also we synthesized bulk polycrystalline samples of metastable at ambient conditions monoclinic (C2/m) phase of Sb2Te3 by rapid quenching after a high-pressure–high-temperature treatment at P=3.7–7.7 GPa; T=873 K and found superconductivity with Tc=2 K. A zero-field magnetic susceptibility cusp and linear positive magnetoresistance indicate a topological insulator state.

SIMS study of oxygen diffusion in monoclinic HfO2

NASA Astrophysics Data System (ADS)

Mueller, Michael P.; De Souza, Roger A.

2018-01-01

The diffusion of oxygen in dense ceramics of monoclinic HfO2 was studied by means of (18O/16O) isotope exchange annealing and subsequent determination of isotope depth profiles by Secondary Ion Mass Spectrometry. Anneals were performed in the temperature range of 573 ≤T /K ≤ 973 at an oxygen partial pressure of p O2=200 mbar . All measured isotope profiles exhibited two features: the first feature, closer to the surface, was attributed mainly to slow oxygen diffusion in an impurity silicate phase; the second feature, deeper in the sample, was attributed to oxygen diffusion in bulk monoclinic HfO2 . The activation enthalpy of oxygen tracer diffusion in bulk HfO2 was found to be ΔHD∗≈0.5 eV .

Evidence for monoclinic distortion in the ground state phase of underdoped La 1.95Sr 0.05CuO 4: A single crystal neutron diffraction study

DOE PAGES

Singh, Anar; Schefer, Jurg; Sura, Ravi; ...

2016-03-24

The existing controversy about the symmetry of the crystal structure of the ground state of the critical doped La 1.95Sr 0.05CuO 4 has been resolved by analyzing the single crystal neutron diffraction data collected between 5 and 730 K. We observed small but significant intensities for "forbidden" reflections given by extinction rules of the orthorhombic Bmab space group at low temperatures. A careful investigation of neutron diffraction data reveals that the crystal structure of La 1.95Sr 0.05CuO 4 at 5 K is monoclinic with B2/m (2/m 1 1) space group. The monoclinic structure emerges from the orthorhombic structure in amore » continuous way; however, the structure is stable below similar to 120K which agrees with other observed phenomena. Lastly, our results on symmetry changes are crucial for the interpretation of physical properties also in other high temperature superconductors with similar structures.« less

Remarkable features in lattice-parameter ratios of crystals. II. Monoclinic and triclinic crystals.

PubMed

de Gelder, R; Janner, A

2005-06-01

The frequency distributions of monoclinic crystals as a function of the lattice-parameter ratios resemble the corresponding ones of orthorhombic crystals: an exponential component, with more or less pronounced sharp peaks, with in general the most important peak at the ratio value 1. In addition, the distribution as a function of the monoclinic angle beta has a sharp peak at 90 degrees and decreases sensibly at larger angles. Similar behavior is observed for the three triclinic angular parameters alpha, beta and gamma, with characteristic differences between the organic and metal-organic, bio-macromolecular and inorganic crystals, respectively. The general behavior observed for the hexagonal, tetragonal, orthorhombic, monoclinic and triclinic crystals {in the first part of this series [de Gelder & Janner (2005). Acta Cryst. B61, 287-295] and in the present case} is summarized and commented. The data involved represent 366 800 crystals, with lattice parameters taken from the Cambridge Structural Database, CSD (294 400 entries), the Protein Data Bank, PDB (18 800 entries), and the Inorganic Crystal Structure Database, ICSD (53 600 entries). A new general structural principle is suggested.

Structural phase transition of BeTe: an ab initio molecular dynamics study.

PubMed

Alptekin, Sebahaddin

2017-08-11

Beryllium telluride (BeTe) with cubic zinc-blende (ZB) structure was studied using ab initio constant pressure method under high pressure. The ab initio molecular dynamics (MD) approach for constant pressure was studied and it was found that the first order phase transition occurs from the ZB structure to the nickel arsenide (NiAs) structure. It has been shown that the MD simulation predicts the transition pressure P T more than the value obtained by the static enthalpy and experimental data. The structural pathway reveals MD simulation such as cubic → tetragonal → orthorhombic → monoclinic → orthorhombic → hexagonal, leading the ZB to NiAs phase. The phase transformation is accompanied by a 10% volume drop and at 80 GPa is likely to be around 35 GPa in the experiment. In the present study, our obtained values can be compared with the experimental and theoretical results. Graphical abstract The energy-volume relation and ZB phase for the BeTe.

High Pressure Phase Transformations in Heavy Rare Earth Metals and Connections to Actinide Crystal Structures

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

Vohra, Yogesh K.; Sangala, Bagvanth Reddy; Stemshorn, Andrew K.

2008-07-01

High-pressure studies have been performed on heavy rare earth metals Terbium (Tb) to 155 GPa and Holmium (Ho) to 134 GPa in a diamond anvil cell at room temperature. The following crystal structure sequence was observed in both metals hcp {yields} Sm-type {yields} dhcp {yields} distorted fcc (hR-24) {yields} monoclinic (C2/m) with increasing pressure. The last transformation to a low symmetry monoclinic phase is accompanied by a volume collapse of 5 % for Tb at 51 GPa and a volume collapse of 3 % for Ho at 103 GPa. This volume collapse under high pressure is reminiscent of f-shell delocalizationmore » in light rare earth metal Cerium (Ce), Praseodymium (Pr), and heavy actinide metals Americium (Am) and Curium (Cm). The orthorhombic Pnma phase that has been reported in Am and Cm after f-shell delocalization is not observed in heavy rare earth metals under high pressures. (authors)« less

Pressure-induced effects and phase relations in Mg2NiH4

NASA Astrophysics Data System (ADS)

Gavra, Z.; Kimmel, G.; Gefen, Y.; Mintz, Moshe H.

1985-05-01

The low-temperature (<210 °C) crystallographic structure, electrical conductivity, and thermal stability of Mg2NiH4 powders compacted under isostatic pressures of up to 10 kbar were studied. A comparison is made with the corresponding properties of the noncompressed material. It has been concluded that under stress-free hydriding conditions performed below 210 °C, a two-phase hydride mixture is formed. Each of the hydride particles consists of an inner core composed of an hydrogen-deficient monoclinic phase coated by a layer of a stoichiometric orthorhombic phase. The monoclinic phase has a metalliclike electrical conductivity while the orthorhombic phase is insulating. High compaction pressures cause the transformation of the orthorhombic structure into the monoclinic one, thereby resulting in a pressure-induced insulator-to-conductor transition. Reduced decomposition temperatures are obtained for the compressed hydrides. This reduction is attributed to kinetic factors rather than to a reduced thermodynamic stability.

Non-Congruence of Thermally Induced Structural and Electronic Transitions in VO2

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

Nag, Joyeeta; HaglundJr., Richard F; Payzant, E Andrew

2012-01-01

The multifunctional properties of vanadium dioxide (VO2) arise from coupled first-order phase transitions: an insulator-to-metal transition (IMT) and a structural phase transition (SPT) from monoclinic to tetragonal. The characteristic signatures of the IMT and SPT are the hysteresis loops that track the phase transition from nucleation to stabilization of a new phase and back. A long-standing question about the mechanism of the VO2 phase transition is whether and how the almost-simultaneous electronic and structural transitions are related. Here we report independent measurements of the IMT and SPT hystereses in epitaxial VO2 films with differing morphologies. We show that, in bothmore » cases, the hystereses are not congruent, that the structural change requires more energy to reach completion. This result is independent of nanoscale morphology, so that the non- congruence is an intrinsic property of the VO2 phase transition. Our conclusion is supported by effective-medium calculations of the dielectric function incorporating the measured volume fractions of the monoclinic and tetragonal states. The results are consistent with the existence of an monoclinic correlated metallic state in which the electron- electron correlations characteristic of the monoclinic state begin to disappear before the transition to the tetragonal structural state.« less

Deviatoric stress-induced phase transitions in diamantane

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

Yang, Fan; Lin, Yu; Dahl, Jeremy E. P.

2014-10-21

The high-pressure behavior of diamantane was investigated using angle-dispersive synchrotron x-ray diffraction (XRD) and Raman spectroscopy in diamond anvil cells. Our experiments revealed that the structural transitions in diamantane were extremely sensitive to deviatoric stress. Under non-hydrostatic conditions, diamantane underwent a cubic (space group Pa3) to a monoclinic phase transition at below 0.15 GPa, the lowest pressure we were able to measure. Upon further compression to 3.5 GPa, this monoclinic phase transformed into another high-pressure monoclinic phase which persisted to 32 GPa, the highest pressure studied in our experiments. However, under more hydrostatic conditions using silicone oil as a pressuremore » medium, the transition pressure to the first high-pressure monoclinic phase was elevated to 7–10 GPa, which coincided with the hydrostatic limit of silicone oil. In another experiment using helium as a pressure medium, no phase transitions were observed to the highest pressure we reached (13 GPa). In addition, large hysteresis and sluggish transition kinetics were observed upon decompression. Over the pressure range where phase transitions were confirmed by XRD, only continuous changes in the Raman spectra were observed. This suggests that these phase transitions are associated with unit cell distortions and modifications in molecular packing rather than the formation of new carbon-carbon bonds under pressure.« less

Electronic structure of ferromagnetic semiconductor material on the monoclinic and rhombohedral ordered double perovskites La{sub 2}FeCoO{sub 6}

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

Fuh, Huei-Ru; Chang, Ching-Ray; Graduate Institute of Applied Physics, National Taiwan University, Taipei 106, Taiwan

2015-05-07

Double perovskite La{sub 2}FeCoO{sub 6} with monoclinic structure and rhombohedra structure show as ferromagnetic semiconductor based on density functional theory calculation. The ferromagnetic semiconductor state can be well explained by the superexchange interaction. Moreover, the ferromagnetic semiconductor state remains under the generalized gradient approximation (GGA) and GGA plus onsite Coulomb interaction calculation.

Post-patterning of an electronic homojunction in atomically thin monoclinic MoTe2

NASA Astrophysics Data System (ADS)

Kim, Sera; Kim, Jung Ho; Kim, Dohyun; Hwang, Geunwoo; Baik, Jaeyoon; Yang, Heejun; Cho, Suyeon

2017-06-01

Monoclinic group 6 transition metal dichalcogenides (TMDs) have been extensively studied for their intriguing 2D physics (e.g. spin Hall insulator) as well as for ohmic homojunction contacts in 2D device applications. A critical prerequisite for those applications is thickness control of the monoclinic 2D materials, which allows subtle engineering of the topological states or electronic bandgaps. Local thickness control enables the realization of clean homojunctions between different electronic states, and novel device operation in a single material. However, conventional fabrication processes, including chemical methods, typically produce non-homogeneous and relatively thick monoclinic TMDs, due to their distorted octahedral structures. Here, we report on a post-patterning technique using laser-irradiation to fabricate homojunctions between two different thickness areas in monoclinic MoTe2. A thickness-dependent electronic change from a metallic to semiconducting state, resulting in an electronic homojunction, was realized by the optical patterning of pristine MoTe2 flakes, and a pre-patterned device channel of monoclinic MoTe2 with a thickness-resolution of 5 nm. Our work provides insight on an optical post-process method for controlling thickness, as a promising approach for fabricating impurity-free 2D TMDs homojunction devices.

Synthesis, structures, and phase transitions of barium bismuth iridium oxide perovskites Ba{sub 2}BiIrO{sub 6} and Ba{sub 3}BiIr{sub 2}O{sub 9}

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

Ling, Chris D., E-mail: c.ling@chem.usyd.edu.a; Bragg Institute, ANSTO, PMB 1, Menai 2234; Kennedy, Brendan J.

The Ba-Bi-Ir-O system is found to contain two distinct perovskite-type phases: a rock-salt ordered double perovskite Ba{sub 2}BiIrO{sub 6}; and a 6H-type hexagonal perovskite Ba{sub 3}BiIr{sub 2}O{sub 9}. Ba{sub 2}BiIrO{sub 6} undergoes a series of symmetry-lowering phase transitions on cooling Fm3-barm->R3-barc->12/m(C2/m)->I1-bar(P1-bar), all of which are second order except the rhombohedral->monoclinic one, which is first order. The monoclinic phase is only observed in a 2-phase rhombohedral+monoclinic regime. The transition and 2-phase region lie very close to 300 K, making the room-temperature X-ray diffraction patterns extremely complex and potentially explaining why Ba{sub 2}BiIrO{sub 6} had not previously been identified and reported. Amore » solid solution Ba{sub 2}Bi{sub 1+x}Ir{sub 1-x}O{sub 6}, analogous to Ba{sub 2}Bi{sub 1+x}Ru{sub 1-x}O{sub 6}, 0<=x<=2/3, was not observed. The 6H-type phase Ba{sub 3}BiIr{sub 2}O{sub 9} undergoes a clean second-order phase transition P6{sub 3}/mmc->C2/c at 750 K, unlike 6H-type Ba{sub 3}LaIr{sub 2}O{sub 9}, the P6{sub 3}/mmc structure of which is highly strained below {approx}750 K but fails to distort coherently to the monoclinic phase. - Graphical abstract: Structure of Ba{sub 3}BiIr{sub 2}O{sub 9} at 300 K. BiO{sub 6} octahedra are purple, IrO{sub 6} octahedra are gold, and Ba atoms are green. Thermal ellipsoids at 90% probability.« less

Presence of a monoclinic (Pm) phase in the morphotropic phase boundary region of multiferroic (1 − x)Bi(Ni{sub 1/2}Ti{sub 1/2})O{sub 3}-xPbTiO{sub 3} solid solution: A Rietveld study

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

Pandey, Rishikesh, E-mail: akhilesh-bhu@yahoo.com, E-mail: aksingh.mst@itbhu.ac.in; Singh, Akhilesh Kumar, E-mail: akhilesh-bhu@yahoo.com, E-mail: aksingh.mst@itbhu.ac.in

2014-07-28

We present here the results of structural studies on multiferroic (1 − x)Bi(Ni{sub 1/2}Ti{sub 1/2})O{sub 3}-xPbTiO{sub 3} solid solution using Rietveld analysis on powder x-ray diffraction data in the composition range 0.35 ≤ x ≤ 0.55. The stability region of various crystallographic phases at room temperature for (1 − x)Bi(Ni{sub 1/2}Ti{sub 1/2})O{sub 3}-xPbTiO{sub 3} is determined precisely. Structural transformation from pseudo-cubic (x ≤ 0.40) to tetragonal (x ≥ 0.50) phase is observed via phase coexistence region demarcating the morphotropic phase boundary. The morphotropic phase boundary region consists of coexisting tetragonal and monoclinic structures with space group P4mm and Pm, respectively, stable in composition range 0.41 ≤ x ≤ 0.49 as confirmed by Rietveld analysis. The resultsmore » of Rietveld analysis completely rule out the coexistence of rhombohedral and tetragonal phases in the morphotropic phase boundary region reported by earlier workers. A comparison between the bond lengths for “B-site cations-oxygen anions” obtained after Rietveld refinement, with the bond length calculated using Shannon-Prewitt ionic radii, reveals the ionic nature of B-O (Ni/Ti-O) bonds for the cubic phase and partial covalent character for the other crystallographic phases.« less

Polarized Raman scattering of epitaxial vanadium dioxide films with low-temperature monoclinic phase

NASA Astrophysics Data System (ADS)

Shibuya, Keisuke; Sawa, Akihito

2017-07-01

A polarized Raman scattering study was carried out on epitaxial VO2 thin films on MgF2(001) and (110) substrates to investigate the Raman symmetry and tensor elements of the phonon modes of the films in a low-temperature monoclinic phase. From the polarization angular dependence of the Raman intensity, we assigned the phonon modes at 137, 194, 310, 340, 499, 612, and 663 cm-1 to Ag symmetry and the phonon modes at 143, 262, 442, 480, 582, and 820 cm-1 to Bg symmetry. The angular-dependence measurements also revealed that two phonon modes with Ag and Bg symmetries are present at about 224 and 393 cm-1, although only a single peak was observed in the Raman spectra at around these wavenumbers. On the basis of the experimental results, we evaluated the Raman tensors of the identified phonon modes. From the Raman tensors, we found that the atomic displacements of the 194 and 340 cm-1 phonon modes are approximately perpendicular and parallel, respectively, to the V-V dimer direction. This is consistent with a previous theoretical prediction, i.e., these modes are attributable to the tilting motion and the stretching vibration of the V-V dimers, respectively.

Structural phase transitions in Bi2Se3 under high pressure

PubMed Central

Yu, Zhenhai; Wang, Lin; Hu, Qingyang; Zhao, Jinggeng; Yan, Shuai; Yang, Ke; Sinogeikin, Stanislav; Gu, Genda; Mao, Ho-kwang

2015-01-01

Raman spectroscopy and angle dispersive X-ray diffraction (XRD) experiments of bismuth selenide (Bi2Se3) have been carried out to pressures of 35.6 and 81.2 GPa, respectively, to explore its pressure-induced phase transformation. The experiments indicate that a progressive structural evolution occurs from an ambient rhombohedra phase (Space group (SG): R-3m) to monoclinic phase (SG: C2/m) and eventually to a high pressure body-centered tetragonal phase (SG: I4/mmm). Evidenced by our XRD data up to 81.2 GPa, the Bi2Se3 crystallizes into body-centered tetragonal structures rather than the recently reported disordered body-centered cubic (BCC) phase. Furthermore, first principles theoretical calculations favor the viewpoint that the I4/mmm phase Bi2Se3 can be stabilized under high pressure (>30 GPa). Remarkably, the Raman spectra of Bi2Se3 from this work (two independent runs) are still Raman active up to ~35 GPa. It is worthy to note that the disordered BCC phase at 27.8 GPa is not observed here. The remarkable difference in atomic radii of Bi and Se in Bi2Se3 may explain why Bi2Se3 shows different structural behavior than isocompounds Bi2Te3 and Sb2Te3. PMID:26522818

Structural phase transitions in Bi 2Se 3 under high pressure

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

Yu, Zhenhai; Gu, Genda; Wang, Lin

2015-11-02

Raman spectroscopy and angle dispersive X-ray diffraction (XRD) experiments of bismuth selenide (Bi 2Se 3) have been carried out to pressures of 35.6 and 81.2 GPa, respectively, to explore its pressure-induced phase transformation. The experiments indicate that a progressive structural evolution occurs from an ambient rhombohedra phase (Space group (SG): R-3m) to monoclinic phase (SG: C2/m) and eventually to a high pressure body-centered tetragonal phase (SG: I4/mmm). Evidenced by our XRD data up to 81.2 GPa, the Bi 2Se 3 crystallizes into body-centered tetragonal structures rather than the recently reported disordered body-centered cubic (BCC) phase. Furthermore, first principles theoretical calculationsmore » favor the viewpoint that the I4/mmm phase Bi 2Se 3 can be stabilized under high pressure (>30 GPa). Remarkably, the Raman spectra of Bi 2Se 3 from this work (two independent runs) are still Raman active up to ~35 GPa. Furthermore, it is worthy to note that the disordered BCC phase at 27.8 GPa is not observed here. The remarkable difference in atomic radii of Bi and Se in Bi 2Se 3 may explain why Bi 2Se 3 shows different structural behavior than isocompounds Bi 2Te 3 and Sb 2Te 3.« less

Phonon anharmonicity of monoclinic zirconia and yttrium-stabilized zirconia

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

Li, Chen W.; Smith, Hillary L.; Lan, Tian

2015-04-13

Inelastic neutron scattering measurements on monoclinic zirconia (ZrO 2) and 8 mol% yttrium-stabilized zirconia were performed at temperatures from 300 to 1373 ωK. We reported temperature-dependent phonon densities of states (DOS) and Raman spectra obtained at elevated temperatures. First-principles lattice dynamics calculations with density functional theory gave total and partial phonon DOS curves and mode Grüneisen parameters. These mode Grüneisen parameters were used to predict the experimental temperature dependence of the phonon DOS with partial success. However, substantial anharmonicity was found at elevated temperatures, especially for phonon modes dominated by the motions of oxygen atoms. Yttrium-stabilized zirconia (YSZ) was somewhatmore » more anharmonic and had a broader phonon spectrum at low temperatures, owing in part to defects in its structure. YSZ also has a larger vibrational entropy than monoclinic zirconia.« less

Anisotropic crystal structure distortion of the monoclinic polymorph of acetaminophen at high hydrostatic pressures.

PubMed

Boldyreva, E V; Shakhtshneider, T P; Vasilchenko, M A; Ahsbahs, H; Uchtmann, H

2000-04-01

The anisotropy of structural distortion of the monoclinic polymorph of acetaminophen induced by hydrostatic pressure up to 4.0 GPa was studied by single-crystal X-ray diffraction in a Merrill-Bassett diamond anvil cell (DAC). The space group (P2(1)/n) and the general structural pattern remained unchanged with pressure. Despite the overall decrease in the molar volume with pressure, the structure expanded in particular crystallographic directions. One of the linear cell parameters (c) passed through a minimum as the pressure increased. The intramolecular bond lengths changed only slightly with pressure, but the changes in the dihedral and torsion angles were very large. The compressibility of the intermolecular hydrogen bonds NH...O and OH...O was measured. NH...O bonds were shown to be slightly more compressible than OH...O bonds. The anisotropy of structural distortion was analysed in detail in relation to the pressure-induced changes in the molecular conformations, to the compression of the hydrogen-bond network, and to the changes in the orientation of molecules with respect to each other in the pleated sheets in the structure. Dirichlet domains were calculated in order to analyse the relative shifts of the centroids of the hydrogen-bonded cycles and of the centroids of the benzene rings with pressure.

Strain-driven phase transitions and associated dielectric/piezoelectric anomalies in BiFeO3 thin films

NASA Astrophysics Data System (ADS)

Huang, C. W.; Chu, Y. H.; Chen, Z. H.; Wang, Junling; Sritharan, T.; He, Q.; Ramesh, R.; Chen, Lang

2010-10-01

Strain-driven phase transitions and related intrinsic polarization, dielectric, and piezoelectric properties for single-domain films were studied for BiFeO3 using phenomenological Landau-Devonshire theory. A stable and mixed structure between tetragonal and rhombohedral-like (monoclinic) phases is predicted at a compressive misfit strain of um=-0.0382 without an energy barrier. For a tensile misfit strain of um=0.0272, another phase transition between the monoclinic and orthorhombic phases was predicted with sharply high dielectric and piezoelectric responses.

Lifting the geometric frustration through a monoclinic distortion in “114” YBaFe{sub 4}O{sub 7.0}: Magnetism and transport

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

Duffort, V.; Sarkar, T.; Caignaert, V., E-mail: vincent.caignaert@ensicaen.fr

2013-09-15

The possibility to lift the geometric frustration in the “114” stoichiomeric tetragonal oxide YBaFe{sub 4}O{sub 7.0} by decreasing the temperature has been investigated using neutron and synchrotron powder diffraction techniques. Besides the structural transition from tetragonal to monoclinic symmetry that appears at T{sub S}=180 K, a magnetic transition is observed below T{sub N}=95 K. The latter corresponds to a lifting of the 3D geometric frustration toward an antiferromagnetic long range ordering, never observed to date in a cubic based “114’” oxide. The magnetic structure, characterized by the propagation vector k{sub 1}=(0,0,½), shows that one iron Fe2 exhibits a larger magneticmore » moment than the three others, suggesting a possible charge ordering according to the formula YBaFe{sup 3+}Fe{sub 3}{sup 2+}O{sub 7.0}. The magnetic M(T) and χ′(T) curves, in agreement with neutron data, confirm the structural and magnetic transitions and evidence the coexistence of residual magnetic frustration. Moreover, the transport measurements show a resistive transition from a thermally activated conduction mechanism to a variable range hopping mechanism at T{sub S}=180 K, with a significant increase of the dependence of the resistivity vs. temperature. Mössbauer spectroscopy clearly evidences a change in the electronic configuration of the iron framework at the structural transition as well as coexistence of several oxidation states. The role of barium underbonding in these transitions is discussed. - Graphical abstract: Atomic displacements at the tetragonal-monoclinic transition in YBaFe{sub 4}O{sub 7}. Display Omitted - Highlights: • The structural and magnetic phase transitions of YBaFe{sub 4}O{sub 7} were studied below room temperature. • The tetragonal to monoclinic transition, characterized by NPD and SXRD, was studied using mode crystallography approach. • Monoclinic distortion allows the lifting of the geometrical frustration on the iron

Pressure dependence of the monoclinic phase in (1–x)Pb(Mg 1/3Nb 2/3)O 3-xPbTiO₃ solid solutions

DOE PAGES

Ahart, Muhtar; Sinogeikin, Stanislav; Shebanova, Olga; ...

2012-12-26

We combine high-pressure x-ray diffraction, high-pressure Raman scattering, and optical microscopy to investigate a series of (1–x)Pb(Mg 1/3Nb 2/3)O 3-xPbTiO₃ (PMN-xPT) solid solutions (x=0.2, 0.3, 0.33, 0.35, 0.37, 0.4) in diamond anvil cells up to 20 GPa at 300 K. The Raman spectra show a peak centered at 380 cm⁻¹ starting above 6 GPa for all samples, in agreement with previous observations. X-ray diffraction measurements are consistent with this spectral change indicating a structural phase transition; we find that the triplet at the pseudocubic (220) Bragg peak merges into a doublet above 6 GPa. Our results indicate that the morphotropicmore » phase boundary region (x=0.33–0.37) with the presence of monoclinic symmetry persists up to 7 GPa. The pressure dependence of ferroelectric domains in PMN-0.32PT single crystals was observed using a polarizing optical microscope. The domain wall density decreases with pressure and the domains disappear at a modest pressure of 3 GPa. We propose a pressure-composition phase diagram for PMN-xPT solid solutions.« less

Controllable synthesis of Ln3+ (Ln = Tb, Eu) doped zinc phosphate nano-/micro-structured materials: phase, morphology and luminescence properties

NASA Astrophysics Data System (ADS)

Yue, Dan; Lu, Wei; Li, Chunyang; Zhang, Xinlei; Liu, Chunxia; Wang, Zhenling

2014-01-01

Ln3+ (Ln = Tb, Eu) doped zinc phosphate tetrahydrate (ZPT:Ln3+) and ammonium zinc phosphate (AZP:Ln3+) nano-/micro-structured materials were synthesized in aqueous solution without the addition of any structure-directing agent. The phase structures, morphologies and luminescence properties of the as-synthesized samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), photoluminescence (PL) spectroscopy and lifetime. These investigations indicate that different phosphate sources MnH(3-n)PO4 (M = NH4+ or Na+, n = 1, 2, 3) can lead to the altering of morphology from nanosheet to microflower, but have no significant effect on the phase structure of the samples. The microlump, nanosheet, and microflower (constructed by the primary microlumps or nanosheets) of orthorhombic ZPT:Ln3+ could be selectively prepared by adjusting the pH value from 3.5 to 7.0. A mixture of orthorhombic ZPT:Ln3+ and monoclinic AZP:Ln3+ with a microflower morphology was obtained when the pH value was adjusted to 8.0. Monoclinic AZP:Ln3+ microplate, microcube and nanoparticle morphologies were obtained at pH values of 8.5, 9.0 and 11.0 respectively. The phase transformation and growth mechanism of the diverse morphologies were proposed, and ZPT:Ln3+ (Ln3+ = Eu or Tb) samples exhibit red or green emission under the excitation of UV light.Ln3+ (Ln = Tb, Eu) doped zinc phosphate tetrahydrate (ZPT:Ln3+) and ammonium zinc phosphate (AZP:Ln3+) nano-/micro-structured materials were synthesized in aqueous solution without the addition of any structure-directing agent. The phase structures, morphologies and luminescence properties of the as-synthesized samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), photoluminescence (PL) spectroscopy and lifetime. These

On the structure of the disordered Bi 2Te 4O 11 phase

NASA Astrophysics Data System (ADS)

Masson, O.; Thomas, P.; Durand, O.; Hansen, T.; Champarnaud, J. C.; Mercurio, D.

2004-06-01

The structure of the disordered metastable Bi 2Te 4O 11 phase has been investigated using both neutron powder diffraction and reverse Monte Carlo (RMC) modelling. The average structure, of fluorite-type (space group Fm 3¯m ), is characterized by very high Debye-Waller parameters, especially for oxygen. Whereas the cations form a fairly well-defined FCC lattice, the oxygen sublattice is very disordered. It is shown that the local order is similar to that present in the stable monoclinic Bi 2Te 4O 11 phase. Clear differences are observed for the intermediate range order. The present phase is analogous to the "anti-glass" phases reported by Trömel in other tellurium-based mixed oxides. However, whereas Trömel defines anti-glass as having long range order but no short range order, it is shown here that this phase is best described as an intermediate state between the amorphous and crystalline states, i.e. having short and medium range order similar to that of tellurite glasses and a premise of long range order with the cations only.

Twin and habit plane microstructures due to the tetragonal to monoclinic transformation of zirconia

NASA Astrophysics Data System (ADS)

Simha, N. K.

1997-02-01

We first construct Bain strains for the tetragonal to monoclinic ( t → m) transformation of zirconia (ZrO 2), and then examine the resulting twin and habit plane microstructures. The ( t → m) transformation in zirconia occurs via two paths; transformation along path I has two Bain strains that involve shearing of a rectangular face of the tetragonal unit cell, and shearing of the square base corresponds to path II. The monoclinic variants resulting from each of the three Bain strains can form 12 twins, and four of the twins corresponding to path II are neither of type I nor of type II. Habit planes do not exist for the transformation along path I, whereas transformation along path II has: (± 0.8139, ± 0.3898, - 0.4309) t, (± 0.6489, ± 0.6271, - 0.4309) t, (± 0.7804, ± 0.4530, - 0.4309) t. We predict the exact twin planes observed by Bailey [(1964) Phase transformation at high temperatures in hafnia and zirconia. Proc. Roy. Soc.279A, 395-412], Bansal and Heuer [(1972) On a martensitic phase transformation in Zirconia ZrO 2—I. Metallographic evidence. Acta Metall.20, 1281-1289] and Buljan et al. [(1976) Optical and X-ray single crystal studies of the monoclinic ↔ tetragonal transition in ZrO 2. J. Am. Ceram. Soc.59, 351-354]; additional twins and habit planes that we predict have not yet been observed.

Evidence for monoclinic distortion in the ground state phase of underdoped La{sub 1.95}Sr{sub 0.05}CuO{sub 4}: A single crystal neutron diffraction study

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

Singh, Anar, E-mail: singhanar@gmail.com; Schefer, Jürg; Frontzek, Matthias

2016-03-28

The existing controversy about the symmetry of the crystal structure of the ground state of the critical doped La{sub 1.95}Sr{sub 0.05}CuO{sub 4} has been resolved by analyzing the single crystal neutron diffraction data collected between 5 and 730 K. We observed small but significant intensities for “forbidden” reflections given by extinction rules of the orthorhombic Bmab space group at low temperatures. A careful investigation of neutron diffraction data reveals that the crystal structure of La{sub 1.95}Sr{sub 0.05}CuO{sub 4} at 5 K is monoclinic with B2/m (2/m 1 1) space group. The monoclinic structure emerges from the orthorhombic structure in a continuous way;more » however, the structure is stable below ∼120 K which agrees with other observed phenomena. Our results on symmetry changes are crucial for the interpretation of physical properties also in other high temperature superconductors with similar structures.« less

Origin of phase transition in VO2

NASA Astrophysics Data System (ADS)

Basu, Raktima; Sardar, Manas; Dhara, Sandip

2018-04-01

Vanadium dioxide (VO2) exhibits a reversible first-order metal to insulator transition (MIT) along with a structural phase transition (SPT) from monoclinic M1 to rutile tetragonal R via another two intermediate phases of monoclinic M2 and triclinic T at a technologically important temperature of 340K. In the present work, besides synthesizing M1 phase of VO2, we also stabilized M2 and T phases at room temperature by introducing native defects in the system and observed an increase in transition temperature with increase in native defects. Raman spectroscopic measurements were carried out to confirm the pure VO2 phases. Since the MIT is accompanied by SPT in these systems, the origin of the phase transition is still under debate. The controversy between MIT and SPT, whether electron-phonon coupling or strong electron-electron correlation triggers the phase transition in VO2 is also resolved by examining the presence of intermediate phase M2 during phase transition.

Controllable synthesis of Ln3+ (Ln = Tb, Eu) doped zinc phosphate nano-/micro-structured materials: phase, morphology and luminescence properties.

PubMed

Yue, Dan; Lu, Wei; Li, Chunyang; Zhang, Xinlei; Liu, Chunxia; Wang, Zhenling

2014-02-21

Ln(3+) (Ln = Tb, Eu) doped zinc phosphate tetrahydrate (ZPT:Ln(3+)) and ammonium zinc phosphate (AZP:Ln(3+)) nano-/micro-structured materials were synthesized in aqueous solution without the addition of any structure-directing agent. The phase structures, morphologies and luminescence properties of the as-synthesized samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), photoluminescence (PL) spectroscopy and lifetime. These investigations indicate that different phosphate sources MnH(3-n)PO4 (M = NH4(+) or Na(+), n = 1, 2, 3) can lead to the altering of morphology from nanosheet to microflower, but have no significant effect on the phase structure of the samples. The microlump, nanosheet, and microflower (constructed by the primary microlumps or nanosheets) of orthorhombic ZPT:Ln(3+) could be selectively prepared by adjusting the pH value from 3.5 to 7.0. A mixture of orthorhombic ZPT:Ln(3+) and monoclinic AZP:Ln(3+) with a microflower morphology was obtained when the pH value was adjusted to 8.0. Monoclinic AZP:Ln(3+) microplate, microcube and nanoparticle morphologies were obtained at pH values of 8.5, 9.0 and 11.0 respectively. The phase transformation and growth mechanism of the diverse morphologies were proposed, and ZPT:Ln(3+) (Ln(3+) = Eu or Tb) samples exhibit red or green emission under the excitation of UV light.

What causes the Besnus transition in monoclinic pyrrhotite?

NASA Astrophysics Data System (ADS)

Gehring, A. U.; Koulialias, D.; Löffler, J. F.; Charilaou, M.

2016-12-01

Monoclinic 4C pyrrhotite (ideal formula Fe7S8) is a major magnetic remanence carrier in the Earth's crust and in extraterrestrial materials. Because of its low-temperature magnetic transition around 30 K also known as Besnus transition, this mineral phase is easily detectable in rock samples. An intrinsic origin of the Besnus transition due to a crystallographic change similar to that in the Verwey transition has generally been postulated (1). Although the physical properties of pyrrhotite have intensively been studied, the physics behind the pronounced change in magnetization at the low-temperature transition is still unresolved. To address this question we performed structural and magnetic analyses on a natural pyrrhotite single crystal (Fe6.6S8) from Auerbach, Germany (2,3). Chemical analysis, X-ray diffractometry and transmission electron microscopy show that this pyrrhotite consists of an intergrowth of 4C and an incommensurate 5C* superstructure that are polymorphs with different vacancy distributions. The occurrence of two superstructures is magnetically confirmed by symmetric inflection points in the hysteresis measurements above the transition at about 30 K. The disappearance of the inflection points and the associated change of the hysteresis parameters indicate that the two superstructures become embedded to form a unitary magnetic anisotropy system at the transition. This embedding of the 5C* into the 4C pyrrhotite at about 30 K is directly visible by the occurrence of additional 4-fold and 12-fold symmetry terms in magnetic anisotropy and anisotropic magnetic resistivity mesarurements, respectively. From this it follows that the Besnus transition in monoclinic pyrrhotite is an extrinsic magnetic phenomenon with respect to the 4C superstructure, i.e., a coupling effect, and therefore the physics behind it is in fact different from that of the well-known Verwey transition. (1) Rochette et al., The IRM Quarterly, 21, 1 (2011); (2) Charilaou et al., J

Pt-Bi Antibonding Interaction: The Key Factor for Superconductivity in Monoclinic BaPt2Bi2.

PubMed

Gui, Xin; Xing, Lingyi; Wang, Xiaoxiong; Bian, Guang; Jin, Rongying; Xie, Weiwei

2018-02-19

In the search for superconductivity in a BaAu 2 Sb 2 -type monoclinic structure, we have successfully synthesized the new compound BaPt 2 Bi 2 , which crystallizes in the space group P2 1 /m (No. 11; Pearson symbol mP10) according to a combination of powder and single-crystal X-ray diffraction and scanning electron microscopy. A sharp electrical resistivity drop and large diamagnetic magnetization below 2.0 K indicates it owns superconducting ground state. This makes BaPt 2 Bi 2 the first reported superconductor in a monoclinic BaAu 2 Sb 2 -type structure, a previously unappreciated structure for superconductivity. First-principles calculations considering spin-orbit coupling indicate that Pt-Bi antibonding interaction plays a critical role in inducing superconductivity.

Low temperature structural and transport studies of La{sub 0.175}Pr{sub 0.45}Ca{sub 0.375}MnO{sub 3-δ}

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

Sharma, Shivani; Shahee, Aga; Singh, Kiran

2016-05-23

The temperature (T) dependent x-ray diffraction (XRD) and resistivity measurements of La{sub 0.175}Pr{sub 0.45}Ca{sub 0.375}MnO{sub 3-δ} (LPCMO) have been performed down to 2 K to understand the structural and transport properties. From room temperature down to 220 K, LPCMO exists in orthorhombic phase with Pnma structure and at T~220 K, it transforms to charge ordered (CO) monoclinic phase with P2{sub 1}/m structure and remains as it is down to 2 K. The CO phase is evident from the occurrence of weak but well defined superlattice peaks in the XRD pattern. This structural transformation is of first order in nature asmore » evident from the phase coexistence across the transition region. These results thus clearly illustrate that LPCMO undergoes a first order structural phase transition from charge disordered orthorhombic phase to CO monoclinic phase at ~220 K, consistent with temperature dependent resistivity results. Our structural analysis of T dependent XRD data using Rietveld refinement infers that below 220 K, LPCMO forms commensurate CO monoclinic P2{sub 1}/m structure with four times structural modulation.« less

Direct observation of the M2 phase with its Mott transition in a VO2 film

NASA Astrophysics Data System (ADS)

Kim, Hoon; Slusar, Tetiana V.; Wulferding, Dirk; Yang, Ilkyu; Cho, Jin-Cheol; Lee, Minkyung; Choi, Hee Cheul; Jeong, Yoon Hee; Kim, Hyun-Tak; Kim, Jeehoon

2016-12-01

In VO2, the explicit origin of the insulator-to-metal transition is still disputable between Peierls and Mott insulators. Along with the controversy, its second monoclinic (M2) phase has received considerable attention due to the presence of electron correlation in undimerized vanadium ions. However, the origin of the M2 phase is still obscure. Here, we study a granular VO2 film using conductive atomic force microscopy and Raman scattering. Upon the structural transition from monoclinic to rutile, we observe directly an intermediate state showing the coexistence of monoclinic M1 and M2 phases. The conductivity near the grain boundary in this regime is six times larger than that of the grain core, producing a donut-like landscape. Our results reveal an intra-grain percolation process, indicating that VO2 with the M2 phase is a Mott insulator.

Quasielastic and inelastic neutron scattering study of the hydration of monoclinic and triclinic tricalcium silicate

NASA Astrophysics Data System (ADS)

Peterson, Vanessa K.; Brown, Craig M.; Livingston, Richard A.

2006-08-01

The hydration of Mg-stabilized triclinic and monoclinic tricalcium silicate samples were studied using quasielastic neutron scattering to follow the fixation of hydrogen into the reaction products and by applying hydration models to the data. The quantity of Ca(OH) 2 produced during hydration was also determined using inelastic neutron scattering. The monoclinic form was found to be intrinsically less reactive that the triclinic form. The monoclinic form was also confirmed to produce more product than the triclinic form after 50 h, a process found to occur through a longer, rather than earlier, nucleation and growth regime. Results indicated an increase in the permeability of the hydration layer product relative to the triclinic form and the increase in the length of the nucleation and growth regime was thus attributed to an alteration in morphology or structure of the hydration layer product, extending the time for diffusion limited mechanics to be reached.

Structural studies of degradation process of zirconium dioxide tetragonal phase induced by grinding with dental bur

NASA Astrophysics Data System (ADS)

Piosik, A.; Żurowski, K.; Pietralik, Z.; Hędzelek, W.; Kozak, M.

2017-11-01

Zirconium dioxide has been widely used in dental prosthetics. However, the improper mechanical treatment can induce changes in the microstructure of zirconium dioxide. From the viewpoint of mechanical properties and performance, the phase transitions of ZrO2 from the tetragonal to the monoclinic phase induced by mechanical processing, are particularly undesirable. In this study, the phase transitions of yttrium stabilized zirconium dioxide (Y-TZP) induced by mechanical treatment are investigated by the scanning electron microscopy (SEM), atomic force microscopy (AFM) and powder diffraction (XRD). Mechanical stress was induced by different types of drills used presently in dentistry. At the same time the surface temperature was monitored during milling using a thermal imaging camera. Diffraction analysis allowed determination of the effect of temperature and mechanical processing on the scale of induced changes. The observed phase transition to the monoclinic phase was correlated with the methods of mechanical processing.

Crystal structure and chemical bonding of the high-temperature phase of AgN3.

PubMed

Schmidt, Carsten L; Dinnebier, Robert; Wedig, Ulrich; Jansen, Martin

2007-02-05

The crystal structure of silver azide (AgN3) in its high-temperature (HT) modification was determined from X-ray powder diffraction data, recorded at T = 170 degrees C and was further refined by the Rietveld method. The structure is monoclinic (P21/c (No. 14), a = 6.0756(2) A, b = 6.1663(2) A, c = 6.5729(2) A, beta = 114.19(0) degrees, V = 224.62(14) A3, Z = 4) and consists of two-dimensional Ag and N containing layers in which the silver atoms are coordinated by four nitrogen atoms exhibiting a distorted square coordination environment. These sheets are linked together by weaker perpendicular Ag-N contacts, thus forming a 4 + 2 coordination geometry around the silver atoms. The phase transition has been characterized by DTA, DSC, and measurement of the density, as well as of the ionic conductivity. Both, the room-temperature and the HT phase are electrically insulating. This fact is getting support by DFT band structure calculations within the generalized gradient approximation, using the PBE functional. On the basis of the DFT band structure, the bonding characteristics of both phases are essentially the same. Finally, the implication of the existence of a low-symmetry HT-phase in a crystalline explosive concerning decomposition mechanisms is discussed.

The nature of photoinduced phase transition and metastable states in vanadium dioxide

PubMed Central

Tao, Zhensheng; Zhou, Faran; Han, Tzong-Ru T.; Torres, David; Wang, Tongyu; Sepulveda, Nelson; Chang, Kiseok; Young, Margaret; Lunt, Richard R.; Ruan, Chong-Yu

2016-01-01

Photoinduced threshold switching processes that lead to bistability and the formation of metastable phases in photoinduced phase transition of VO2 are elucidated through ultrafast electron diffraction and diffusive scattering techniques with varying excitation wavelengths. We uncover two distinct regimes of the dynamical phase change: a nearly instantaneous crossover into an intermediate state and its decay led by lattice instabilities over 10 ps timescales. The structure of this intermediate state is identified to be monoclinic, but more akin to M2 rather than M1 based on structure refinements. The extinction of all major monoclinic features within just a few picoseconds at the above-threshold-level (~20%) photoexcitations and the distinct dynamics in diffusive scattering that represents medium-range atomic fluctuations at two photon wavelengths strongly suggest a density-driven and nonthermal pathway for the initial process of the photoinduced phase transition. These results highlight the critical roles of electron correlations and lattice instabilities in driving and controlling phase transformations far from equilibrium. PMID:27982066

The nature of photoinduced phase transition and metastable states in vanadium dioxide

NASA Astrophysics Data System (ADS)

Tao, Zhensheng; Zhou, Faran; Han, Tzong-Ru T.; Torres, David; Wang, Tongyu; Sepulveda, Nelson; Chang, Kiseok; Young, Margaret; Lunt, Richard R.; Ruan, Chong-Yu

2016-12-01

Photoinduced threshold switching processes that lead to bistability and the formation of metastable phases in photoinduced phase transition of VO2 are elucidated through ultrafast electron diffraction and diffusive scattering techniques with varying excitation wavelengths. We uncover two distinct regimes of the dynamical phase change: a nearly instantaneous crossover into an intermediate state and its decay led by lattice instabilities over 10 ps timescales. The structure of this intermediate state is identified to be monoclinic, but more akin to M2 rather than M1 based on structure refinements. The extinction of all major monoclinic features within just a few picoseconds at the above-threshold-level (~20%) photoexcitations and the distinct dynamics in diffusive scattering that represents medium-range atomic fluctuations at two photon wavelengths strongly suggest a density-driven and nonthermal pathway for the initial process of the photoinduced phase transition. These results highlight the critical roles of electron correlations and lattice instabilities in driving and controlling phase transformations far from equilibrium.

Temperature controlled evolution of monoclinic to super-tetragonal phase of epitaxial BiFeO3 thin films on La0.67Sr0.33MnO3 buffered SrTiO3 substrate

NASA Astrophysics Data System (ADS)

Singh, Anar; Kaifeng, Dong; Chen, Jing-Sheng

2018-03-01

Epitaxial BiFeO3 thin films of 130nm were deposited by pulsed laser deposition (PLD) technique on La0.67Sr0.33MnO3 buffered SrTiO3 (001) substrate at various temperatures under different ambient oxygen pressures. Reciprocal space mapping reveals that, with decreasing temperature and oxygen pressure, the broadly reported monoclinic phase (MA) of BiFeO3 thin film initially transforms to a tetragonal phase (T1) with c/a =1.05 (1) in a narrow girth of deposition condition and then to a super-tetragonal phase (T2) with giant c/a = 1.24 (1), as confirmed by reciprocal space mapping using high resolution x-ray diffraction. The surface morphology of the films reveals the island growth of the BiFeO3 films deposited at low temperatures. We propose that the transformation from monoclinic to the super-tetragonal phase is essentially due to the manifestation of excess local strain as a result of the island growth. This study offers a recipe to grow the super-tetragonal phase of BiFeO3, with giant c/a =1.24 (1) which exhibits exceptionally large ferroelectric polarization, on ferromagnetic layer La0.67Sr0.33MnO3. This phase of BiFeO3 can be utilized for the ferroelectric control of magnetism at the interface of BiFeO3 and La0.67Sr0.33MnO3.

Structural and electronic properties of Ga2O3-Al2O3 alloys

NASA Astrophysics Data System (ADS)

Peelaers, Hartwin; Varley, Joel B.; Speck, James S.; Van de Walle, Chris G.

2018-06-01

Ga2O3 is emerging as an important electronic material. Alloying with Al2O3 is a viable method to achieve carrier confinement, to increase the bandgap, or to modify the lattice parameters. However, the two materials have very different ground-state crystal structures (monoclinic β-gallia for Ga2O3 and corundum for Al2O3). Here, we use hybrid density functional theory calculations to assess the alloy stabilities and electronic properties of the alloys. We find that the monoclinic phase is the preferred structure for up to 71% Al incorporation, in close agreement with experimental phase diagrams, and that the ordered monoclinic AlGaO3 alloy is exceptionally stable. We also discuss bandgap bowing, lattice constants, and band offsets that can guide future synthesis and device design efforts.

About structural phase state of coating based on zirconium oxide formed by microplasma oxidation method

NASA Astrophysics Data System (ADS)

Gubaidulina, Tatiana A.; Sergeev, Viktor P.; Kuzmin, Oleg S.; Fedorischeva, Marina V.; Kalashnikov, Mark P.

2017-12-01

The oxide-ceramic coating based of zirconium oxide is formed by the method of microplasma oxidation. The producing modes of the oxide layers on E110 zirconium alloy are under testing. It was found that using microplasma treatment of E110 zirconium in aluminosilicate electrolyte makes possible the formation of porous oxide-ceramic coatings based on zirconium alloyed by aluminum and niobium. The study is focused on the modes how to form heat-shielding coatings with controlled porosity and minimal amount of microcracks. The structural-phase state of the coating is studied by X-ray diffraction analysis and scanning electron microscopy (SEM). It was found that the ratio of the monoclinic and tetragonal phases changes with the change occurring in the coating formation modes.

Low temperature magnetic properties of monoclinic pyrrhotite with particular relevance to the Besnus transition

NASA Astrophysics Data System (ADS)

Volk, M.; Gilder, S.; Feinberg, J. M.

2016-12-01

Monoclinic pyrrhotite (Fe7S8) is an important mineral on earth as well as in some meteorites. It owes its ferrimagnetism to an ordered array of Fe vacancies. Its magnetic properties change markedly around 30 K, in what is known as the Besnus transition. Plausible explanations for the Besnus transition are either due to changes in crystalline anisotropy from a transformation in crystal symmetry or from the establishment of a two-phase system with magnetic interaction between the two phases. To help resolve this discrepancy, we measured hysteresis loops every 5° and back field curves every 10° in the basal plane of an oriented single crystal of monoclinic pyrrhotite at 300 K and at 21 temperature steps from 50 K through the Besnus transition until 20 K. Between 50 and 30 K, hysteresis loops possess double inflections between crystallographic a-axes and only a single inflection parallel to the a-axes. The second inflection phenomenon and relative differences of the loops show a six-fold symmetry in this temperature range. The Besnus transition is best characterized by changes in magnetic remanence and coercivity over a 6° temperature span with a maximum rate of change at 30 K. A surprising yet puzzling finding is that the coercivity ratio becomes less than unity below the transition when four-fold symmetry arises. The saturation magnetization of natural pyrrhotite cycled from room temperature to successively lower temperatures through the Besnus transition decreases 2-4 times less than equivalent grain sizes of magnetite, with less than a 10% loss in remanence between 300 K and 150 K in pseudo-single domain pyrrhotite. As pseudo-single domain monoclinic pyrrhotite carries the magnetic remanence in some meteorites, it is likely that low temperature cycling in space to the Earth's surface will have only a minor influence on paleointensity values derived from those meteorites.

Mesoscopic structural phase progression in photo-excited VO 2 revealed by time-resolved x-ray diffraction microscopy

DOE PAGES

Zhu, Yi; Cai, Zhonghou; Chen, Pice; ...

2016-02-26

Dynamical phase separation during a solid-solid phase transition poses a challenge for understanding the fundamental processes in correlated materials. Critical information underlying a phase transition, such as localized phase competition, is difficult to reveal by measurements that are spatially averaged over many phase seperated regions. The ability to simultanousely track the spatial and temporal evolution of such systems is essential to understanding mesoscopic processes during a phase transition. Using state-of- the-art time-resolved hard x-ray diffraction microscopy, we directly visualize the structural phase progression in a VO 2 film upon photoexcitation. Following a homogenous in-plane optical excitation, the phase transformation ismore » initiated at discrete sites and completed by the growth of one lattice structure into the other, instead of a simultaneous isotropic lattice symmetry change. The time-dependent x-ray diffraction spatial maps show that the in-plane phase progression in laser-superheated VO 2 is via a displacive lattice transformation as a result of relaxation from an excited monoclinic phase into a rutile phase. The speed of the phase front progression is quantitatively measured, which is faster than the process driven by in-plane thermal diffusion but slower than the sound speed in VO 2. Lastly, the direct visualization of localized structural changes in the time domain opens a new avenue to study mesoscopic processes in driven systems.« less

Mesoscopic structural phase progression in photo-excited VO2 revealed by time-resolved x-ray diffraction microscopy

NASA Astrophysics Data System (ADS)

Zhu, Yi; Cai, Zhonghou; Chen, Pice; Zhang, Qingteng; Highland, Matthew J.; Jung, Il Woong; Walko, Donald A.; Dufresne, Eric M.; Jeong, Jaewoo; Samant, Mahesh G.; Parkin, Stuart S. P.; Freeland, John W.; Evans, Paul G.; Wen, Haidan

2016-02-01

Dynamical phase separation during a solid-solid phase transition poses a challenge for understanding the fundamental processes in correlated materials. Critical information underlying a phase transition, such as localized phase competition, is difficult to reveal by measurements that are spatially averaged over many phase separated regions. The ability to simultaneously track the spatial and temporal evolution of such systems is essential to understanding mesoscopic processes during a phase transition. Using state-of-the-art time-resolved hard x-ray diffraction microscopy, we directly visualize the structural phase progression in a VO2 film upon photoexcitation. Following a homogenous in-plane optical excitation, the phase transformation is initiated at discrete sites and completed by the growth of one lattice structure into the other, instead of a simultaneous isotropic lattice symmetry change. The time-dependent x-ray diffraction spatial maps show that the in-plane phase progression in laser-superheated VO2 is via a displacive lattice transformation as a result of relaxation from an excited monoclinic phase into a rutile phase. The speed of the phase front progression is quantitatively measured, and is faster than the process driven by in-plane thermal diffusion but slower than the sound speed in VO2. The direct visualization of localized structural changes in the time domain opens a new avenue to study mesoscopic processes in driven systems.

Mesoscopic structural phase progression in photo-excited VO2 revealed by time-resolved x-ray diffraction microscopy.

PubMed

Zhu, Yi; Cai, Zhonghou; Chen, Pice; Zhang, Qingteng; Highland, Matthew J; Jung, Il Woong; Walko, Donald A; Dufresne, Eric M; Jeong, Jaewoo; Samant, Mahesh G; Parkin, Stuart S P; Freeland, John W; Evans, Paul G; Wen, Haidan

2016-02-26

Dynamical phase separation during a solid-solid phase transition poses a challenge for understanding the fundamental processes in correlated materials. Critical information underlying a phase transition, such as localized phase competition, is difficult to reveal by measurements that are spatially averaged over many phase separated regions. The ability to simultaneously track the spatial and temporal evolution of such systems is essential to understanding mesoscopic processes during a phase transition. Using state-of-the-art time-resolved hard x-ray diffraction microscopy, we directly visualize the structural phase progression in a VO2 film upon photoexcitation. Following a homogenous in-plane optical excitation, the phase transformation is initiated at discrete sites and completed by the growth of one lattice structure into the other, instead of a simultaneous isotropic lattice symmetry change. The time-dependent x-ray diffraction spatial maps show that the in-plane phase progression in laser-superheated VO2 is via a displacive lattice transformation as a result of relaxation from an excited monoclinic phase into a rutile phase. The speed of the phase front progression is quantitatively measured, and is faster than the process driven by in-plane thermal diffusion but slower than the sound speed in VO2. The direct visualization of localized structural changes in the time domain opens a new avenue to study mesoscopic processes in driven systems.

Local twin domains and tip-voltage-induced domain switching of monoclinic M C phase in Pb ( M g 1 / 3 N b 2 / 3 ) O 3 - 0.34 PbTi O 3 single crystal revealed by piezoresponse force microscopy

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

Wang, Ruixue; Yang, Bin; Luo, Zhenlin

2016-08-29

Here, the monoclinic (M) phases in high-performance relaxor-based ferroelectric single crystals have been recognized to be a vital structural factor for the outstanding piezoelectric property. However, due to the complexity of the structure in M phases, the understanding about it is still limited. In this paper, the local twin domains and tip-voltage-induced domain switching of the M C phase in Pb(Mg 1/3Nb 2/3)O 3 - 0.34PbTiO 3 (PMN-0.34PT) single crystal have been intensively investigated by piezoresponse force microscopy (PFM). By theoretically analyzing the experimental patterns of domain walls on the (001) C face, the specific M C twin domains inmore » the initial annealed state of a selected area have been clarified, and the polarization orientation of the M C phase in this sample is determined to be at an angle of 29 degrees to the < 001 > C directions. In addition, based on the evolution of domains and the motion of domain walls under the step-increased PFM tip dc voltage (V dc), the switching process and features of different types of M C domain variants are visually revealed« less

The nature of photoinduced phase transition and metastable states in vanadium dioxide

DOE PAGES

Tao, Zhensheng; Zhou, Faran; Han, Tzong-Ru T.; ...

2016-12-16

Photoinduced threshold switching processes that lead to bistability and the formation of metastable phases in photoinduced phase transition of VO 2 are elucidated through ultrafast electron diffraction and diffusive scattering techniques with varying excitation wavelengths. We uncover two distinct regimes of the dynamical phase change: a nearly instantaneous crossover into an intermediate state and its decay led by lattice instabilities over 10 ps timescales. The structure of this intermediate state is identified to be monoclinic, but more akin to M 2 rather than M1 based on structure refinements. The extinction of all major monoclinic features within just a few picosecondsmore » at the above-threshold-level (~20%) photoexcitations and the distinct dynamics in diffusive scattering that represents medium-range atomic fluctuations at two photon wavelengths strongly suggest a density-driven and nonthermal pathway for the initial process of the photoinduced phase transition. These results highlight the critical roles of electron correlations and lattice instabilities in driving and controlling phase transformations far from equilibrium.« less

Mechanics of monoclinal systems in the Colorado Plateau during the Laramide orogeny

NASA Astrophysics Data System (ADS)

Yin, An

1994-11-01

Monoclines developed in the Colorado Plateau region during the Laramide orogeny are divided into western and eastern groups by a broad NNW trending antiform through the central part of the plateau. In the western group the major monoclines verge to the east, whereas in the eastern group the major monoclines verge to the west. Paleogeographic reconstruction based on paleocurrent indicators and sedimentary facies distribution suggests that the broad antiform was developed during the Laramide orogeny and was coeval with the formation of the monoclines in the plateau. This relationship implies that the monoclines were drag folds verging towards the center of the plateau as a response to the antiformal warping of the plateau. To simulate the warping of the plateau region and the stress distribution that produced the variable trends of the monoclines, an elastic thin plate model considering in-plane stress was developed. This model assumes that (1) sedimentation in the Laramide basins provided vertical loading along the edge of the plateau region, (2) frictional sliding was operating along the Laramide faults on the northern and eastern boundaries, and (3) the greatest regional compressive stress was oriented in the N 60 deg E direction and was applied uniformly along the western and southwestern sides of the plateau. Buoyancy due to instantaneous isostatic adjustment of crustal thickening or magmatic addition was also considered. The result of the model suggests that the frictional strength of the Uinta thrust system on the northern side of the plateau is at least 2 times greater than that along the Park Range and Sangre de Cristo thrust systems on the eastern side of the plateau in order to explain the observed monoclinal trends and the warping pattern within the plateau during the Laramide orogeny.

Crystal structures and high-temperature phase-transitions in SrNdMRuO{sub 6} (M=Zn,Co,Mg,Ni) new double perovskites studied by symmetry-mode analysis

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

Iturbe-Zabalo, E., E-mail: iturbe@ill.fr; Fisika Aplikatua II Saila, Zientzia eta Teknologia Fakultatea, UPV/EHU, P.O. Box 644, 48080 Bilbao; Igartua, J.M.

2013-02-15

Crystal structures of SrNdZnRuO{sub 6}, SrNdCoRuO{sub 6}, SrNdMgRuO{sub 6} and SrNdNiRuO{sub 6} double perovskites have been studied by X-ray, synchrotron radiation and neutron powder diffraction method, at different temperatures, and using the symmetry-mode analysis. All compounds adopt the monoclinic space group P2{sub 1}/n at room-temperature, and contain a completely ordered array of the tilted MO{sub 6} and RuO{sub 6} octahedra, whereas Sr/Nd cations are completely disordered. The analysis of the structures in terms of symmetry-adapted modes of the parent phase allows the identification of the modes responsible for the phase-transition. The high-temperature study (300-1250 K) has shown that the compoundsmore » present a temperature induced structural phase-transition: P2{sub 1}/n{yields}P4{sub 2}/n{yields}Fm3{sup Macron }m. - Graphical abstract: Representation of the dominant distortion modes of the symmetry mode decomposition of the room-temperature (P2{sub 1}/n), intermediate (P4{sub 2}/n) and cubic (Fm-3m) phase SrNdMRuO{sub 6} (M=Zn,Co,Mg,Ni), with respect to the parent phase Fm-3m. The dominant distortion modes are: in the monoclinic phase-GM{sub 4}{sup +} (blue arrow), X{sub 3}{sup +} (green arrow) and X{sub 5}{sup +} acting on A-site cations (red arrow); in the tetragonal phase-GM{sub 4}{sup +} (pink arrow), X{sub 3}{sup +} (light blue arrow) and X{sub 5}{sup +} acting on A-site cations (brown arrow). Highlights: Black-Right-Pointing-Pointer Structural study of four ruthenate double perovskites. Black-Right-Pointing-Pointer Room-temperature structural determination using symmetry-mode procedure. Black-Right-Pointing-Pointer Determination of temperature induced structural phase-transitions. Black-Right-Pointing-Pointer Symmetry adapted-mode analysis.« less

Phase transformation of molecular beam epitaxy-grown nanometer-thick Gd₂O₃ and Y₂O₃ on GaN.

PubMed

Chang, Wen-Hsin; Wu, Shao-Yun; Lee, Chih-Hsun; Lai, Te-Yang; Lee, Yi-Jun; Chang, Pen; Hsu, Chia-Hung; Huang, Tsung-Shiew; Kwo, J Raynien; Hong, Minghwei

2013-02-01

High quality nanometer-thick Gd₂O₃ and Y₂O₃ (rare-earth oxide, R₂O₃) films have been epitaxially grown on GaN (0001) substrate by molecular beam epitaxy (MBE). The R₂O₃ epi-layers exhibit remarkable thermal stability at 1100 °C, uniformity, and highly structural perfection. Structural investigation was carried out by in situ reflection high energy electron diffraction (RHEED) and ex-situ X-ray diffraction (XRD) with synchrotron radiation. In the initial stage of epitaxial growth, the R₂O₃ layers have a hexagonal phase with the epitaxial relationship of R₂O₃ (0001)(H)<1120>(H)//GaN(0001)(H)<1120>(H). With the increase in R₂O₃ film thickness, the structure of the R₂O₃ films changes from single domain hexagonal phase to monoclinic phase with six different rotational domains, following the R₂O₃ (201)(M)[020](M)//GaN(0001)(H)<1120>(H) orientational relationship. The structural details and fingerprints of hexagonal and monoclinic phase Gd₂O₃ films have also been examined by using electron energy loss spectroscopy (EELS). Approximate 3-4 nm is the critical thickness for the structural phase transition depending on the composing rare earth element.

Crustal-scale thrusting and origin of the Montreal River monocline-A 35-km-thick cross section of the midcontinent rift in northern Michigan and Wisconsin

USGS Publications Warehouse

Cannon, W.F.; Peterman, Z.E.; Sims, P.K.

1993-01-01

A structurally simple, 35-km-thick, north facing stratigraphic succession of Late Archean to Middle Proterozoic rocks is exposed near the Montreal River, which forms the border between northern Wisconsin and Michigan. This structure, the Montreal River monocline, is composed of steeply dipping to vertical sedimentary rocks and flood basalts of the Keweenawan Supergroup (Middle Proterozoic) along the south limb of the Midcontinent rift, and disconformably underlying sedimentary rocks of the Marquette Range Supergroup (Early Proterozoic). These rocks lie on an Archean granite-greenstone complex, about 10 km of which is included in the monocline. This remarkable thickness of rocks appears to be essentially structurally intact and lacks evidence of tectonic thickening or repetition.Tilting to form the monocline resulted from southward thrusting on listric faults of crustal dimension. The faults responsible for the monocline are newly recognized components of a well-known regional fault system that partly closed and inverted the Midcontinent rift system. Resetting of biotite ages on the upper plate of the faults indicates that faulting and uplift occurred at about 1060 +/−20 Ma and followed very shortly after extension that formed the Midcontinent rift system.

Low symmetry phase in Pb(Zr0.52Ti0.48)O3 epitaxial thin films with enhanced ferroelectric properties

NASA Astrophysics Data System (ADS)

Yan, Li; Li, Jiefang; Cao, Hu; Viehland, D.

2006-12-01

The authors report the structural and ferroelectric properties of Pb(Zr0.52Ti0.48)O3 (PZT) epitaxial thin films grown on (001), (110), and (111) SrRuO3/SrTiO3 substrates by pulsed laser deposition. A monoclinic C (Mc) phase has been found for (101) films, whereas (001) and (111) ones were tetragonal (T ) and rhombohedral (R), respectively. The authors find that the ferroelectric polarization of the Mc phase is higher than that in either the T or R ones. These results are consistent with predictions (i) of epitaxial phase diagrams and (ii) that the enhanced ferroelectric properties of morphotropic phase boundary PZT are related to a low symmetry monoclinic phase.

Observation of reduced phase transition temperature in N-doped thermochromic film of monoclinic VO2

NASA Astrophysics Data System (ADS)

Wan, Meinan; Xiong, Mo; Li, Neng; Liu, Baoshun; Wang, Shuo; Ching, Wai-Yim; Zhao, Xiujian

2017-07-01

Research on monoclinic (M1) phase of VO2 has attracted a great of interest for smart coating applications due to its exceptional thermochromic property. Herein, we report the results using a novel approach to synthesize N-doped VO2(M1) thin films with high purity by heat treatment in NH3 atmosphere. The N dopant in the film can be regulated by varying NH3 concentration during the annealing process. We find that the N atoms are located at the interstitial sites or substitute oxygen atoms, and the V-N bonds in the VO2 thin films increase with NH3 concentration. The metal to insulator transition (MIT) temperature (τc,h) of the VO2 thin film is effectively reduced from 80.0 to 62.9 °C, while the solar modulation efficiency (ΔTsol) and the modulation efficiency at 2000 nm (ΔT2000nm) are 7.36% and 55.6% respectively. The band gap of N-doped VO2 thin films related to MIT (Eg1) is estimated to be as low as 0.18-0.25 eV whereas the band gap associated with the visible transparency (Eg2) is about 1.50-1.58 eV. Based on the highly accurate first-principles calculations, the Eg1 of VO2 (M1) is reduced after substituted or interstitial N-doping, while the Eg2 alters with the mode of N-doping, which is excellent agreement with experimental measurement.

Spontaneous ferroelectricity in strained low-temperature monoclinic Fe3O4: A first-principles study

NASA Astrophysics Data System (ADS)

Liu, Xiang; Mi, Wen-Bo

2018-04-01

As a single-phase multiferroic material, Fe3O4 exhibits spontaneous ferroelectric polarization below 38 K. However, the nature of the ferroelectricity in Fe3O4 and effect of external disturbances such as strain on it remains ambiguous. Here, the spontaneous ferroelectric polarization of low-temperature monoclinic Fe3O4 was investigated by first-principles calculations. The pseudo-centrosymmetric Fe B42-Fe B43 pair has a different valence state. The noncentrosymmetric charge distribution results in ferroelectric polarization. The initial ferroelectric polarization direction is in the - x and - z directions. The ferroelectricity along the y axis is limited owing to the symmetry of the Cc space group. Both the ionic displacement and charge separation at the Fe B42-Fe B43 pair are affected by strain, which further influences the spontaneous ferroelectric polarization of monoclinic Fe3O4. The ferroelectric polarization along the z axis exhibits an increase of 45.3% as the strain changes from 6% to -6%.

Relation between Kitaev magnetism and structure in α -RuCl3

NASA Astrophysics Data System (ADS)

Glamazda, A.; Lemmens, P.; Do, S.-H.; Kwon, Y. S.; Choi, K.-Y.

2017-05-01

Raman scattering has been employed to investigate lattice and magnetic excitations of the honeycomb Kitaev material α -RuCl3 and its Heisenberg counterpart CrCl3. Our phonon Raman spectra give evidence for a first-order structural transition from a monoclinic to a rhombohedral structure for both compounds. Significantly, only α -RuCl3 features a large thermal hysteresis, consistent with the formation of a wide phase of coexistence. In the related temperature interval of 70 -170 K, we observe a hysteretic behavior of magnetic excitations as well. The stronger magnetic response in the rhombohedral compared to the monoclinic phase evidences a coupling between the crystallographic structure and low-energy magnetic response. Our results demonstrate that the Kitaev magnetism concomitant with fractionalized excitations is susceptible to small variations of bonding geometry.

Phase transitions in heated Sr{sub 2}MgTeO{sub 6} double perovskite oxide probed by X-ray diffraction and Raman spectroscopy

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

Manoun, Bouchaib, E-mail: manounb@gmail.com; Tamraoui, Y.; Lazor, P.

2013-12-23

Double-perovskite oxide Sr{sub 2}MgTeO{sub 6} has been synthetized, and its crystal structure was probed by the technique of X-ray diffraction at room temperature. The structure is monoclinic, space group I2/m. Temperature-induced phase transitions in this compound were investigated by Raman spectroscopy up to 550 °C. Two low-wavenumber modes corresponding to external lattice vibrations merge at temperature of around 100 °C, indicating a phase transition from the monoclinic (I2/m) to the tetragonal (I4/m) structure. At 300 °C, changes in the slopes of temperature dependencies of external and O–Te–O bending modes are detected and interpreted as a second phase transition from the tetragonal (I4/m) tomore » the cubic (Fm-3m) structure.« less

The low-temperature structural behavior of sodium 1-carba-closo-decaborate: NaCB{sub 9}H{sub 10}

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

Wu, Hui, E-mail: hui.wu@nist.gov; Tang, Wan Si; Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742-2115

2016-11-15

Two ordered phases of the novel solid superionic conductor sodium 1-carba-closo-decaborate (NaCB{sub 9}H{sub 10}) were identified via synchrotron x-ray powder diffraction in combination with first-principles calculations and neutron vibrational spectroscopy. A monoclinic packing of the large ellipsoidal CB{sub 9}H{sub 10}{sup −} anions prevails at the lowest temperatures, but a first-order transformation to a slightly modified orthorhombic packing is largely complete by 240 K. The CB{sub 9}H{sub 10}{sup −} anion orientational alignments and Na{sup +} cation interstitial sitings in both phases are arranged so as to minimize the cation proximities to the uniquely more positive C-bonded H atoms of the anions.more » These results provide valuable structural information pertinent to understanding the relatively low-temperature, entropy-driven, order-disorder phase transition for this compound. - Graphical abstract: Ordered monoclinic and orthorhombic NaCB{sub 9}H{sub 10} phases were determined by XRD and DFT computations and corroborated by neutron vibrational spectroscopy. - Highlights: • Two T-dependent ordered structures of Na(1-CB{sub 9}H{sub 10}) were determined by XRD. • The lower-T monoclinic to higher-T orthorhombic transition occurs from 210 to 240 K. • The main structural differences involve changes in the canting of the CB{sub 9}H{sub 10}{sup −} anions. • DFT and neutron vibrational spectroscopy corroborate the lower-T monoclinic structure. • The results are important for understanding the nature of this superionic conductor.« less

Engineering the Structural and Electronic Phases of MoTe2 through W Substitution

NASA Astrophysics Data System (ADS)

Rhodes, D.; Chenet, D. A.; Janicek, B. E.; Nyby, C.; Lin, Y.; Jin, W.; Edelberg, D.; Mannebach, E.; Finney, N.; Antony, A.; Schiros, T.; Klarr, T.; Mazzoni, A.; Chin, M.; Chiu, Y.-c.; Zheng, W.; Zhang, Q. R.; Ernst, F.; Dadap, J. I.; Tong, X.; Ma, J.; Lou, R.; Wang, S.; Qian, T.; Ding, H.; Osgood, R. M., Jr.; Paley, D. W.; Lindenberg, A. M.; Huang, P. Y.; Pasupathy, A. N.; Dubey, M.; Hone, J.; Balicas, L.

2017-03-01

MoTe$_2$ is an exfoliable transition metal dichalcogenide (TMD) which crystallizes in three symmetries, the semiconducting trigonal-prismatic $2H-$phase, the semimetallic $1T^{\\prime}$ monoclinic phase, and the semimetallic orthorhombic $T_d$ structure. The $2H-$phase displays a band gap of $\\sim 1$ eV making it appealing for flexible and transparent optoelectronics. The $T_d-$phase is predicted to possess unique topological properties which might lead to topologically protected non-dissipative transport channels. Recently, it was argued that it is possible to locally induce phase-transformations in TMDs, through chemical doping, local heating, or electric-field to achieve ohmic contacts or to induce useful functionalities such as electronic phase-change memory elements. The combination of semiconducting and topological elements based upon the same compound, might produce a new generation of high performance, low dissipation optoelectronic elements. Here, we show that it is possible to engineer the phases of MoTe$_2$ through W substitution by unveiling the phase-diagram of the Mo$_{1-x}$W$_x$Te$_2$ solid solution which displays a semiconducting to semimetallic transition as a function of $x$. We find that only $\\sim 8$ \\% of W stabilizes the $T_d-$phase at room temperature. Photoemission spectroscopy, indicates that this phase possesses a Fermi surface akin to that of WTe$_2$.

Monoclinic β-Li2TiO3 nanocrystalline particles employing novel urea assisted solid state route: Synthesis, characterization and sintering behavior

NASA Astrophysics Data System (ADS)

Tripathi, Biranchi M.; Mohanty, Trupti; Prakash, Deep; Tyagi, A. K.; Sinha, P. K.

2017-07-01

Pure phase monoclinic nano-crystalline Li2TiO3 powder was synthesized by a novel urea assisted solid state synthesis method using readily available and economical precursors. A single phase and well crystalline Li2TiO3 powder has been obtained at slightly lower temperature (600-700 °C) and shorter duration (2 h) as compared to the conventional solid state method. The proposed method has significant advantages in comparison to other viable methods mainly in terms of phase purity, powder properties and sinterability. Analysis of chemical composition using inductively coupled plasma atomic emission spectroscopy (ICP-AES) shows no loss of lithium from Li2TiO3 in the proposed method. The emergence of monoclinic Li2TiO3 phase was confirmed by X-ray diffraction (XRD) pattern of as-synthesized powder. The crystallite size of Li2TiO3 powder was calculated to be in the range of 15-80 nm, which varied as a function of urea composition and temperature. The morphology of as-prepared Li2TiO3 powders was examined by scanning electron microscope (SEM). The effect of urea composition on phase and morphology was investigated so as to delineate the role of urea. Upon sintering at < 1000 °C temperature, the Li2TiO3 powder compact attained about 98% of the theoretical density with fine grained (grain size: 2-3 μm) microstructure. It indicates excellent sinter-ability of Li2TiO3 powder synthesized by the proposed method. The fine grained structure is desirable for better tritium breeding performance of Li2TiO3. Electrochemical impedance spectroscopy at variable temperature showed good electrical properties of Li2TiO3. The proposed method is simple, anticipated to be cost effective and convenient to realise for large scale production of phase pure nanocrystalline and having significantly enhanced sinter-ability Li2TiO3 powder.

Encapsulated monoclinic sulfur for stable cycling of li-s rechargeable batteries.

PubMed

Moon, San; Jung, Young Hwa; Jung, Wook Ki; Jung, Dae Soo; Choi, Jang Wook; Kim, Do Kyung

2013-12-03

Monoclinic S8 , an uncommon allotrope of sulfur at room temperature, can be formed when common orthorhombic S8 is heat-treated under enclosed environments in nanometer dimensions. Monoclinic S8 prevents the formation of soluble polysulfides during battery operation, resulting in unprecedented cycling performance over 1000 cycles under the highest sulfur content to date. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

A New Superhard Phase and Physical Properties of ZrB₃ from First-Principles Calculations.

PubMed

Zhang, Gangtai; Bai, Tingting; Zhao, Yaru; Hu, Yanfei

2016-08-22

Using the first-principles particle swarm optimization algorithm for crystal structural prediction, we have predicted a novel monoclinic C 2/ m structure for ZrB₃, which is more energetically favorable than the previously proposed FeB₃-, TcP₃-, MoB₃-, WB₃-, and OsB₃-type structures in the considered pressure range. The new phase is mechanically and dynamically stable, as confirmed by the calculations of its elastic constants and phonon dispersion curve. The calculated large shear modulus (227 GPa) and high hardness (42.2 GPa) show that ZrB₃ within the monoclinic phase is a potentially superhard material. The analyses of the electronic density of states and chemical bonding reveal that the strong B-B and B-Zr covalent bonds are attributed to its high hardness. By the quasi-harmonic Debye model, the heat capacity, thermal expansion coefficient and Grüneisen parameter of ZrB₃ are also systemically investigated.

Tuning the pure monoclinic phase of WO3 and WO3-Ag nanostructures for non-enzymatic glucose sensing application with theoretical insight from electronic structure simulations

NASA Astrophysics Data System (ADS)

Ponnusamy, Rajeswari; Gangan, Abhijeet; Chakraborty, Brahmananda; Sekhar Rout, Chandra

2018-01-01

Here, we report the controlled hydrothermal synthesis and tuning of the pure monoclinic phase of WO3 and WO3-Ag nanostructures. Comparative electrochemical nonenzymatic glucose sensing properties of WO3 and WO3-Ag were investigated by cyclic voltammetry and chronoamperometric tests. We observed enhanced glucose sensing performance of WO3-Ag porous spheres as compared to bare WO3 nanoslabs. The sensitivity of the pure WO3 nanoslabs is 11.1 μA μM-1 cm-2 whereas WO3-Ag porous spheres exhibit sensitivity of 23.3 μA μM-1 cm-2. The WO3-Ag porous spheres exhibited a good linear range (5-375 μM) with excellent anti-interference property. Our experimental observations are qualitatively supported by density functional theory simulations through investigation of bonding and charge transfer mechanism of glucose on WO3 and Ag doped WO3. As the binding energy of glucose is more on the Ag doped WO3 (100) surface compared to the bare WO3 (100) surface and the Ag doped WO3 (100) surface becomes more conducting due to enhancement of density of states near the Fermi level, we can infer that Ag doped WO3 exhibits a better charge transfer media compared to bare WO3 resulting in enhanced glucose sensitivity in consistency with our experimental data.

Possible bicollinear nematic state with monoclinic lattice distortions in iron telluride compounds

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

Bishop, Christopher B.; Herbrych, Jacek W.; Dagotto, Elbio R.

2017-07-15

Here, iron telluride (FeTe) is known to display bicollinear magnetic order at low temperatures together with a monoclinic lattice distortion. Because the bicollinear order can involve two different wave vectors (π/2,π/2) and (π/2,–π/2), symmetry considerations allow for the possible stabilization of a nematic state with short-range bicollinear order coupled to monoclinic lattice distortions at a T S higher than the temperature T N where long-range bicollinear order fully develops. As a concrete example, the three-orbital spin-fermion model for iron telluride is studied with an additional coupling ˜λ 12 between the monoclinic lattice strain and an orbital-nematic order parameter with Bmore » 2g symmetry. Monte Carlo simulations show that with increasing ˜λ 12 the first-order transition characteristic of FeTe splits and bicollinear nematicity is stabilized in a (narrow) temperature range. In this new regime, the lattice is monoclinically distorted and short-range spin and orbital order breaks rotational invariance. A discussion of possible realizations of this exotic state is provided.« less

Diffusion paths formation for Cu + ions in superionic Cu 6PS 5I single crystals studied in terms of structural phase transition

NASA Astrophysics Data System (ADS)

Gągor, A.; Pietraszko, A.; Kaynts, D.

2005-11-01

In order to understand the structural transformations leading to high ionic conductivity of Cu + ions in Cu 6PS 5I argyrodite compound, the detailed structure analysis based on single-crystal X-ray diffraction has been performed. Below the phase transition at T=(144-169) K Cu 6PS 5I belongs to monoclinic, ferroelastic phase (space group Cc) with ordered copper sublattice. Above Tc delocalization of copper ions begins and crystal changes the symmetry to cubic superstructure with space group F-43 c ( a'=19.528 Å, z=32). Finally, above T1=274 K increasing disordering of the Cu + ions heightens the symmetry to F-43 m ( a=9.794 Å, z=4). In this work, the final structural model of two cubic phases is presented including the detailed temperature evolution of positions and site occupation factors of copper ions ( R1=0.0397 for F-43 c phase, and 0.0245 for F-43 m phase). Possible diffusion paths for the copper ions are represented by means of the atomic displacement factors and split model. The structural results coincide well with the previously reported non-Arrhenius behavior of conductivity and indicate significant change in conduction mechanism.

Structural phase transitions of (Bi 1$-$xSb x ) 2(Te 1$-$y Se y) 3 compounds under high pressure and the influence of the atomic radius on the compression processes of tetradymites

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

Zhao, Jinggeng; Yu, Zhenhai; Hu, Qingyang

Recently, A 2B 3-type tetradymites have developed into a hot topic in physical and material research fields, where the A and B atoms represent V and VI group elements, respectively. In this study, in situ angle-dispersive X-ray diffraction measurements were performed on Bi 2Te 2Se, BiSbTeSe 2, and Sb 2Te 2Se tetradymites under high pressure. Bi 2Te 2Se transforms from a layered rhombohedral structure (phase I) into 7-fold monoclinic (phase II) and body-centered tetragonal (phase IV) structures at about 8.0 and 14.3 GPa, respectively, without an 8-fold monoclinic structure (phase III) similar to that in Bi 2Te 3. Thus, themore » compression behavior of Bi 2Te 2Se is the same as that of Bi 2Se 3, which could also be obtained from first-principles calculations and in situ high-pressure electrical resistance measurements. Under high pressure, BiSbTeSe 2 and Sb 2Te 2Se undergo similar structural phase transitions to Bi 2Te 2Se, which indicates that the compression process of tellurides can be modulated by doping Se in Te sites. According to these high-pressure investigations of A 2B 3-type tetradymites, the decrease of the B-site atomic radius shrinks the stable pressure range of phase III and expands that of phase II, whereas the decrease of the A-site atomic radius induces a different effect, i.e. expanding the stable pressure range of phase III and shrinking that of phase II. Lastly, the influence of the atomic radius on the compression process of tetradymites is closely related to the chemical composition and the atom arrangement in the quintuple layer.« less

Structural phase transitions of (Bi 1$-$xSb x ) 2(Te 1$-$y Se y) 3 compounds under high pressure and the influence of the atomic radius on the compression processes of tetradymites

DOE PAGES

Zhao, Jinggeng; Yu, Zhenhai; Hu, Qingyang; ...

2016-12-14

Recently, A 2B 3-type tetradymites have developed into a hot topic in physical and material research fields, where the A and B atoms represent V and VI group elements, respectively. In this study, in situ angle-dispersive X-ray diffraction measurements were performed on Bi 2Te 2Se, BiSbTeSe 2, and Sb 2Te 2Se tetradymites under high pressure. Bi 2Te 2Se transforms from a layered rhombohedral structure (phase I) into 7-fold monoclinic (phase II) and body-centered tetragonal (phase IV) structures at about 8.0 and 14.3 GPa, respectively, without an 8-fold monoclinic structure (phase III) similar to that in Bi 2Te 3. Thus, themore » compression behavior of Bi 2Te 2Se is the same as that of Bi 2Se 3, which could also be obtained from first-principles calculations and in situ high-pressure electrical resistance measurements. Under high pressure, BiSbTeSe 2 and Sb 2Te 2Se undergo similar structural phase transitions to Bi 2Te 2Se, which indicates that the compression process of tellurides can be modulated by doping Se in Te sites. According to these high-pressure investigations of A 2B 3-type tetradymites, the decrease of the B-site atomic radius shrinks the stable pressure range of phase III and expands that of phase II, whereas the decrease of the A-site atomic radius induces a different effect, i.e. expanding the stable pressure range of phase III and shrinking that of phase II. Lastly, the influence of the atomic radius on the compression process of tetradymites is closely related to the chemical composition and the atom arrangement in the quintuple layer.« less

Static high-pressure structural studies on Dy to 119 GPa

NASA Astrophysics Data System (ADS)

Patterson, Reed; Saw, Cheng K.; Akella, Jagannadham

2004-05-01

Structural phase transitions in the rare-earth metal dysprosium have been studied in a diamond anvil cell to 119 GPa by x-ray diffraction. Four transformations following the sequence hcp→Sm-type→dhcp→hR24 (hexagonal)→bcm (monoclinic) are observed at 6, 15, 43, and 73 GPa, respectively. The hexagonal to monoclinic transformation is accompanied by a 6% reduction in volume, which is attributed to delocalization of the 4f electrons, similar to that seen in Ce, Pr, and Gd.

Bicollinear antiferromagnetic order, monoclinic distortion, and reversed resistivity anisotropy in FeTe as a result of spin-lattice coupling

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

Bishop, Christopher B.; Moreo, Adriana; Dagotto, Elbio

2016-09-08

The bicollinear antiferromagnetic order experimentally observed in FeTe is shown to be stabilized by the coupling g ~ 12 between monoclinic lattice distortions and the spin-nematic order parameter with B 2g symmetry, within a three-orbital spin-fermion model studied with Monte Carlo techniques. A finite but small value of g ~ 12 is required, with a concomitant lattice distortion compatible with experiments, and a tetragonal-monoclinic transition strongly first order. Remarkably, the bicollinear state found here displays a planar resistivity with the reversed puzzling anisotropy discovered in transport experiments. Orthorhombic distortions are also incorporated, and phase diagrams interpolating between pnictides and chalcogenidesmore » are presented. Here, we conclude that the spin-lattice coupling we introduce is sufficient to explain the challenging properties of FeTe.« less

Structure analysis of BaCe{sub 0.8}Y{sub 0.2}O{sub 3−δ} in dry and wet atmospheres by high-temperature X-ray diffraction measurement

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

Han, Donglin, E-mail: han.donglin.8n@kyoto-u.ac.jp; Majima, Masatoshi; Uda, Tetsuya, E-mail: materials_process@aqua.mtl.kyoto-u.ac.jp

2013-09-15

High temperature X-ray diffraction measurements were performed under dry and wet atmospheres to investigate phase behavior of BaCe{sub 0.8}Y{sub 0.2}O{sub 3−δ} (BCY20). In the temperature range of 30–400 °C, BCY20 was identified to be rhombohedral and monoclinic structures in dry and wet atmospheres, respectively. Larger lattice volumes were obtained in a wet atmosphere due to a chemical expansion induced by water incorporation. A gradual change in diffraction peak shape due to a phase transformation from rhombohedral to monoclinic was observed at 300 °C when moisture was introduced into the atmosphere. These results indicated clearly the dependence of phase behavior ofmore » BCY20 on partial pressure of water vapor in atmosphere. - Graphical abstract: A BaCe{sub 0.8}Y{sub 0.2}O{sub 3−δ} rhombohedral phase transited to a monoclinic phase at 300 °C when moisture was introduced into the atmosphere. Display Omitted - Highlights: • Different structures for hydrated and dehydrated BaCe{sub 0.8}Y{sub 0.2}O{sub 3−δ} (BCY20). • Slow phase transition from rhombohedral to monoclinic at 300 °C in wet atmosphere. • Chemical expansion of BCY20 in wet atmosphere. • Importance of considering moisture when discussing phase behavior of BCY20.« less

Crystallographic phases in heavy rare earth metals under megabar pressures

NASA Astrophysics Data System (ADS)

Samudrala, G. K.; Vohra, Y. K.

2012-07-01

Experiments aimed at understanding the crystallographic phases of heavy rare earth metals were carried out in a diamond anvil cell at the Advanced Photon Source, Argonne National Laboratory. Heavy rare earth metals dysprosium (Dy), holmium (Ho), erbium (Er) and thulium (Tm) were compressed to multi-megabar pressures. The rare earth crystal sequence hcp→Sm-type→dhcp→distorted-fcc (dfcc) is observed in all four elements. Upon further compression, a structural transformation to a monoclinic C2/m phase has been observed. We summarize the results from these experiments and present Rietveld structural refinements on high pressure phases for the specific case of dysprosium.

Temperature-dependent index of refraction of monoclinic Ga2O3 single crystal.

PubMed

Bhaumik, Indranil; Bhatt, R; Ganesamoorthy, S; Saxena, A; Karnal, A K; Gupta, P K; Sinha, A K; Deb, S K

2011-11-01

We present temperature-dependent refractive index along crystallographic b[010] and a direction perpendicular to (100)-plane for monoclinic phase (β) Ga(2)O(3) single crystal grown by the optical floating zone technique. The experimental results are consistent with the theoretical result of Litimein et al.1. Also, the Sellmeier equation for wavelengths in the range of 0.4-1.55 μm is formulated at different temperatures in the range of 30-175 °C. The thermal coefficient of refractive index in the above specified range is ~10(-5)/°C. © 2011 Optical Society of America

Static High Pressure Structural studies on Dy to 119 GPa

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

Patterson, J R; Saw, C K; Akella, J

2003-11-12

Structural phase transitions in the rare-earth metal Dysprosium have been studied in a Diamond Anvil Cell (DAC) to 119 GPa by x-ray diffraction. Four transformations following the sequence hcp {yields} Sm-type {yields} dhcp {yields} hR24 (hexagonal) {yields} bcm (monoclinic) are observed at 6, 15, 43, and 73 GPa respectively. The hexagonal to monoclinic transformation is accompanied by a 6% reduction in volume, which is attributed to delocalization of the 4f electrons, similar to that seen in Ce, Pr, and Gd.

Temperature-driven Topological Phase Transition in MoTe2

NASA Astrophysics Data System (ADS)

Notis Berger, Ayelet; Andrade, Erick; Kerelsky, Alex; Cheong, Sang-Wook; Li, Jian; Bernevig, B. Andrei; Pasupathy, Abhay

The discovery of several candidates predicted to be weyl semimetals has made it possible to experimentally study weyl fermions and their exotic properties. One example is MoTe2, a transition metal dichalcogenide. At temperatures below 240 K it is predicted to be a type II Weyl semimetal with four Weyl points close to the fermi level. As with most weyl semimetals, the complicated band structure causes difficulty in distinguishing features related to bulk states and those related to topological fermi arc surface states characteristic of weyl semimetals. MoTe2 is unique because of its temperature-driven phase change. At high temperatures, MoTe2 is monoclinic, with trivial surface states. When cooled below 240K, it undergoes a first order phase transition to become an orthorhombic weyl semimetal with topologically protected fermi arc surface states. We present STM and STS measurements on MoTe2 crystals in both states. In the orthorhombic phase, we observe scattering that is consistent with the presence of the Fermi-arc surface states. Upon warming into the monoclinic phase, these features disappear in the observed interference patterns, providing direct evidence of the topological nature of the fermi arcs in the Weyl phase

Paramagnetic centers in two phases of manganese-doped lanthanum gallate

NASA Astrophysics Data System (ADS)

Vazhenin, V. A.; Potapov, A. P.; Guseva, V. B.; Artyomov, M. Yu.

2009-05-01

An EPR study of two phases of manganese-doped lanthanum gallate (with a first-order structural transition occurring at 430 K) has revealed Gd3+, Fe3+, and Mn4+ centers at room temperature and 438 K. The parameters of spin Hamiltonians are determined for the Gd3+, Fe3+, and Mn4+ rhombohedral centers in the high-temperature phase (with no other centers found here) and for the monoclinic center Gd3+ in the low-temperature phase. Both in the orthorhombic and in the rhombohedral phase, crystallographic twins (or ferroelastic domains) are observed.

Mechanical and Transport Properties of Poly(propylene-co-1-heptene) Copolymers and Their Dependence on Monoclinic and/or Mesomorphic Polymorphs.

PubMed

García-Peñas, Alberto; Gómez-Elvira, José M; Lorenzo, Vicente; Pérez, Ernesto; Cerrada, María L

2016-02-25

Poly(propylene-co-1-heptene) copolymers have been evaluated in a wide 1-heptene content range. Two different polymorphs can be observed in this interval under the processing conditions imposed: monoclinic crystallites and mesomorphic entities. The unique presence of one or the other lattice at specific content or the coexistence of both of them at the intermediate composition interval is a key factor affecting some of the exhibited properties. The influence is observed in characteristics mainly dependent on either crystalline structure, like rigidity, or the amorphous regions, as location and intensity of glass transition. To get a deeper understanding, the transition from mesophase to monoclinic crystallites has also been studied by DSC and by WAXS and SAXS experiments using synchrotron radiation. It is observed that this transformation is only partial and just involves one-third of the initial ordered mesomorphic entities. Permeability to different gases is mainly dependent on amorphous content and hindrance that monoclinic or mesomorphic entities impose on those disordered regions.

Computing the melting point and thermodynamic stability of the orthorhombic and monoclinic crystalline polymorphs of the ionic liquid 1-n-butyl-3-methylimidazolium chloride

NASA Astrophysics Data System (ADS)

Jayaraman, Saivenkataraman; Maginn, Edward J.

2007-12-01

The melting point, enthalpy of fusion, and thermodynamic stability of two crystal polymorphs of the ionic liquid 1-n-butyl-3-methylimidazolium chloride are calculated using a thermodynamic integration-based atomistic simulation method. The computed melting point of the orthorhombic phase ranges from 365 to 369 K, depending on the classical force field used. This compares reasonably well with the experimental values, which range from 337 to 339 K. The computed enthalpy of fusion ranges from 19 to 29 kJ/mol, compared to the experimental values of 18.5-21.5 kJ/mol. Only one of the two force fields evaluated in this work yielded a stable monoclinic phase, despite the fact that both give accurate liquid state densities. The computed melting point of the monoclinic polymorph was found to be 373 K, which is somewhat higher than the experimental range of 318-340 K. The computed enthalpy of fusion was 23 kJ/mol, which is also higher than the experimental value of 9.3-14.5 kJ/mol. The simulations predict that the monoclinic form is more stable than the orthorhombic form at low temperature, in agreement with one set of experiments but in conflict with another. The difference in free energy between the two polymorphs is very small, due to the fact that a single trans-gauche conformational difference in an alkyl sidechain distinguishes the two structures. As a result, it is very difficult to construct simple classical force fields that are accurate enough to definitively predict which polymorph is most stable. A liquid phase analysis of the probability distribution of the dihedral angles in the alkyl chain indicates that less than half of the dihedral angles are in the gauche-trans configuration that is adopted in the orthorhombic crystal. The low melting point and glass forming tendency of this ionic liquid is likely due to the energy barrier for conversion of the remaining dihedral angles into the gauche-trans state. The simulation procedure used to perform the melting point

Pressure induced structural phase transition in solid oxidizer KClO3: A first-principles study

NASA Astrophysics Data System (ADS)

Yedukondalu, N.; Ghule, Vikas D.; Vaitheeswaran, G.

2013-05-01

High pressure behavior of potassium chlorate (KClO3) has been investigated from 0 to 10 GPa by means of first principles density functional theory calculations. The calculated ground state parameters, transition pressure, and phonon frequencies using semiempirical dispersion correction scheme are in excellent agreement with experiment. It is found that KClO3 undergoes a pressure induced first order phase transition with an associated volume collapse of 6.4% from monoclinic (P21/m) → rhombohedral (R3m) structure at 2.26 GPa, which is in good accord with experimental observation. However, the transition pressure was found to underestimate (0.11 GPa) and overestimate (3.57 GPa) using local density approximation and generalized gradient approximation functionals, respectively. Mechanical stability of both the phases is explained from the calculated single crystal elastic constants. In addition, the zone center phonon frequencies have been calculated using density functional perturbation theory at ambient as well as at high pressure and the lattice modes are found to soften under pressure between 0.6 and 1.2 GPa. The present study reveals that the observed structural phase transition leads to changes in the decomposition mechanism of KClO3 which corroborates with the experimental results.

Pressure induced structural phase transition in solid oxidizer KClO3: a first-principles study.

PubMed

Yedukondalu, N; Ghule, Vikas D; Vaitheeswaran, G

2013-05-07

High pressure behavior of potassium chlorate (KClO3) has been investigated from 0 to 10 GPa by means of first principles density functional theory calculations. The calculated ground state parameters, transition pressure, and phonon frequencies using semiempirical dispersion correction scheme are in excellent agreement with experiment. It is found that KClO3 undergoes a pressure induced first order phase transition with an associated volume collapse of 6.4% from monoclinic (P2(1)/m) → rhombohedral (R3m) structure at 2.26 GPa, which is in good accord with experimental observation. However, the transition pressure was found to underestimate (0.11 GPa) and overestimate (3.57 GPa) using local density approximation and generalized gradient approximation functionals, respectively. Mechanical stability of both the phases is explained from the calculated single crystal elastic constants. In addition, the zone center phonon frequencies have been calculated using density functional perturbation theory at ambient as well as at high pressure and the lattice modes are found to soften under pressure between 0.6 and 1.2 GPa. The present study reveals that the observed structural phase transition leads to changes in the decomposition mechanism of KClO3 which corroborates with the experimental results.

The structural and magnetic phase transitions in Ca 0.73La 0.27FeAs 2 with electron overdoped FeAs layers.

DOE PAGES

Jiang, Shan; Liu, Chang; Cao, H.; ...

2016-02-26

Here we report a study of the Ca 0.73La 0.27FeAs 2 single crystals. We unravel a monoclinic to triclinic phase transition at 58 K, and a paramagnetic to stripe antiferromagnetic (AFM) phase transition at 54 K, below which spins order 45° away from the stripe direction. Furthermore, we demonstrate this material is substantially structurally untwinned at ambient pressure with the formation of spin rotation walls (S-walls). Finally, in addition to the central-hole and corner-electron Fermi pockets usually appearing in FPS, angle-resolved photoemission (ARPES) measurements resolve a Fermiology where an extra electron pocket of mainly As chain character exists at themore » Brillouin zone edge.« less

Engineering the Structural and Electronic Phases of MoTe 2 through W Substitution

DOE PAGES

Rhodes, D.; Chenet, D. A.; Janicek, B. E.; ...

2017-02-01

MoTe 2 is an exfoliable transition metal dichalcogenide (TMD) that crystallizes in three symmetries: the semiconducting trigonal-prismatic 2H- or α-phase, the semimetallic and monoclinic 1T'- or β-phase, and the semimetallic orthorhombic γ-structure. The 2H-phase displays a band gap of ~1 eV making it appealing for flexible and transparent optoelectronics. The γ-phase is predicted to possess unique topological properties that might lead to topologically protected nondissipative transport channels. Recently, it was argued that it is possible to locally induce phase-transformations in TMDs, through chemical doping, local heating, or electric-field to achieve ohmic contacts or to induce useful functionalities such as electronicmore » phase-change memory elements. The combination of semiconducting and topological elements based upon the same compound might produce a new generation of high performance, low dissipation optoelectronic elements. Here, we show that it is possible to engineer the phases of MoTe2 through W substitution by unveiling the phase-diagram of the Mo 1–xW xTe 2 solid solution, which displays a semiconducting to semimetallic transition as a function of x. We find that a small critical W concentration xc ~ 8% stabilizes the γ-phase at room temperature. Lastly, this suggests that crystals with x close to xc might be particularly susceptible to phase transformations induced by an external perturbation, for example, an electric field. Photoemission spectroscopy, indicates that the γ-phase possesses a Fermi surface akin to that of WTe 2.« less

Phase transformations in SrAl2Si2O8 glass

NASA Technical Reports Server (NTRS)

Drummond, Charles H., III; Bansal, Narottam P.

1992-01-01

Bulk glass of SrAl2Si2O8 composition crystallized at temperatures below 1000 C into hexacelsian, a hexagonal phase which undergoes a reversible, rapid transformation to an orthorhombic phase at 758 C, and at higher temperatures crystallized as celsian, a monoclinic phase. The glass transition temperature and crystallization onset temperature were determined to be 883 C and 1086 C, respectively, from DSC at a heating rate of 20 C/min. Thermal expansion of the various phases and density and bend strengths of cold isostatically pressed glass powder bars, sintered at various temperatures, were measured. The kinetics of the hexacelsian-to-celsian transformation for SrAl2Si2O8 were studied. Hexacelsian flakes were isothermally heat treated at temperatures from 1025-1200 C for various times. Avrami plots were determined by quantitatively measuring the amount of monoclinic celsian formed at various times using x ray diffraction. The Avrami constant was determined to be 1.1, suggesting a diffusionless, one dimensional transformation mechanism. The activation energy was determined from an Arrhenius plot of 1n k vs. 1/T to be 125 kilocal/mole. This value is consistent with a mechanism which transforms the layered hexacelsian structure to a three dimensional framework celsian structure and involves the breaking of Si-O bonds.

Equation of state and phase transformations study of Nd at ultra-high pressures

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

Akella, J.; Smith, G.S.; Weir, S.

1991-10-01

Neodymium was investigated to 96.0 GPa pressure in a diamond-anvil ell at room temperature. The observed structural sequence as a function of pressure is dhcp-fcc- six layered'' structure. In the diffraction pattern hexagonal doublets; notably 102, 006 and 100, 108; appear as single reflection when the c/a ratio is 4.899. However, when cc/a approaches 4.7, the splitting is clear. So far in this study, no monoclinic phase or tetragonal phase were observed. 1 fig., 18 refs.

Investigation of a Structural Phase Transition and Magnetic Structure of Na 2BaFe(VO 4) 2: A Triangular Magnetic Lattice with a Ferromagnetic Ground State

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

Sanjeewa, Liurukara D.; Garlea, Vasile O.; McGuire, Michael A.

The structural and magnetic properties of a glaserite-type Na 2BaFe(VO 4) 2 compound, featuring a triangular magnetic lattice of Fe 2+ (S = 2), are reported. Temperature dependent X-ray single crystal studies indicate that at room temperature the system adopts a trigonal Pmore » $$\\bar{3}$$ m1 structure and undergoes a structural phase transition to a C2/c monoclinic phase slightly below room temperature (T s = 288 K). This structural transition involves a tilting of Fe–O–V bond angles and strongly influences the magnetic correlation within the Fe triangular lattice. The magnetic susceptibility measurements reveal a ferromagnetic transition near 7 K. Single crystal neutron diffraction confirms the structural distortion and the ferromagnetic spin ordering in Na 2BaFe(VO 4) 2. The magnetic structure of the ordered state is modeled in the magnetic space group C2'/c' that implies a ferromagnetic order of the a and c moment components and antiferromagnetic arrangement for the b components. Altogether, the Fe magnetic moments form ferromagnetic layers that are stacked along the c-axis, where the spins point along one of the (111) facets of the FeO 6 octahedron.« less

Investigation of a Structural Phase Transition and Magnetic Structure of Na 2BaFe(VO 4) 2: A Triangular Magnetic Lattice with a Ferromagnetic Ground State

DOE PAGES

Sanjeewa, Liurukara D.; Garlea, Vasile O.; McGuire, Michael A.; ...

2017-12-07

The structural and magnetic properties of a glaserite-type Na 2BaFe(VO 4) 2 compound, featuring a triangular magnetic lattice of Fe 2+ (S = 2), are reported. Temperature dependent X-ray single crystal studies indicate that at room temperature the system adopts a trigonal Pmore » $$\\bar{3}$$ m1 structure and undergoes a structural phase transition to a C2/c monoclinic phase slightly below room temperature (T s = 288 K). This structural transition involves a tilting of Fe–O–V bond angles and strongly influences the magnetic correlation within the Fe triangular lattice. The magnetic susceptibility measurements reveal a ferromagnetic transition near 7 K. Single crystal neutron diffraction confirms the structural distortion and the ferromagnetic spin ordering in Na 2BaFe(VO 4) 2. The magnetic structure of the ordered state is modeled in the magnetic space group C2'/c' that implies a ferromagnetic order of the a and c moment components and antiferromagnetic arrangement for the b components. Altogether, the Fe magnetic moments form ferromagnetic layers that are stacked along the c-axis, where the spins point along one of the (111) facets of the FeO 6 octahedron.« less

Phase analysis of plasma-sprayed zirconia-yttria coatings

NASA Technical Reports Server (NTRS)

Shankar, N. R.; Berndt, C. C.; Herman, H.

1983-01-01

Phase analysis of plasma-sprayed 8 wt pct-yttria-stabilized zirconia (YSZ) thermal barrier coatings and powders was carried out by X-ray diffraction. Step scanning was used for increased peak resolution. Plasma spraying of the YSZ powder into water or onto a steel substrate to form a coating reduced the cubic and monoclinic phases with a simultaneous increase in the tetragonal phase. Heat treatment of the coating at 1150 C for 10 h in an Ar atmosphere increased the amount of cubic and monoclinic phases. The implications of these transformations on coating performance and integrity are discussed.

Measurement of elastic constants of monoclinic nickel-titanium and validation of first principles calculations

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

Stebner, A. P.; Brown, D. W.; Brinson, L. C.

2013-05-27

Polycrystalline, monoclinic nickel-titanium specimens were subjected to tensile and compressive deformations while neutron diffraction spectra were recorded in situ. Using these data, orientation-specific and macroscopic Young's moduli are determined from analysis of linear-elastic deformation exhibited by 13 unique orientations of monoclinic lattices and their relationships to each macroscopic stress and strain. Five of 13 elastic compliance constants are also identified: s{sub 11} = 1.15, s{sub 15} = -1.10, s{sub 22} = 1.34, s{sub 33} = 1.06, s{sub 35} = -1.54, all Multiplication-Sign 10{sup -2} GPa{sup -1}. Through these results, recent atomistic calculations of monoclinic nickel-titanium elastic constants are validated.

Structural stability, electronic structure and mechanical properties of alkali gallium hydrides AGaH{sub 4} (A = Li, Na)

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

Santhosh, M.; Rajeswarapalanichamy, R., E-mail: rrpalanichamy@gmail.com; Manikandan, M.

2016-05-06

Ab initio calculations are performed to investigate the structural stability, electronic structure and mechanical properties of alkali gallium hydrides AGaH{sub 4} (A = Li, Na) for three different crystal structures, namely tetragonal (P42{sub 1}c), tetragonal (P4{sub 2}/nmc) and monoclinic (P2{sub 1}/c). Among the considered structures, tetragonal (P42{sub 1}c) phase is found to be the most stable phase for these hydrides at normal pressure. A pressure induced structural phase transition from tetragonal (P42{sub 1}c) to tetragonal (P4{sub 2}/nmc) is observed. The electronic structure reveals that these hydrides are insulators. The calculated elastic constants indicate that these ternary imides are mechanically stablemore » at normal pressure.« less

Crystal structure of new AsS{sub 2} compound

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

Bolotina, N. B., E-mail: bolotina@ns.crys.ras.ru; Brazhkin, V. V.; Dyuzheva, T. I.

2013-01-15

AsS{sub 2} single crystals have been obtained for the first time from an As{sub 2}S{sub 3} melt at pressures above 6 GPa and temperatures above 800 K in the As{sub 2}S{sub 3} {yields} AsS + AsS{sub 2} reaction. The monoclinic structure of the new high-pressure phase is solved by X-ray diffraction analysis and compared to the structure of high-pressure AsS phase, which was studied previously.

Structural and magnetic phase transitions in CeCu 6-xT x (T = Ag,Pd)

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

Poudel, Lekhanath N.; De la cruz, Clarina; Payzant, E. Andrew

The structural and the magnetic properties of CeCu 6-xAg x (0 ≤ x ≤ 0.85) and CeCu 6-xPd x (0 ≤ x ≤ 0.4) have been studied using neutron diffraction, resonant ultrasound spectroscopy (RUS), x-ray diffraction measurements, and first principles calculations. The structural and magnetic phase diagrams of CeCu 6-xAg x and CeCu 6-xPd x as a function of Ag/Pd composition are reported. The end member, CeCu 6, undergoes a structural phase transition from an orthorhombic ( Pnma) to a monoclinic (P2 1/c) phase at 240 K. In CeCu 6-xAg x, the structural phase transition temperature (T s) decreases linearlymore » with Ag concentration and extrapolates to zero at x S ≈ 0.1. The structural transition in CeCu 6-xPd x remains unperturbed with Pd substitution within the range of our study. The lattice constant b slightly decreases with Ag/Pd doping, whereas a and c increase with an overall increase in the unit cell volume. Both systems, CeCu 6-xAg x and CeCu 6-xPd x, exhibit a magnetic quantum critical point (QCP), at x ≈ 0.2 and x ≈ 0.05, respectively. Near the QCP, long range antiferromagnetic ordering takes place at an incommensurate wave vector (δ 1 0 δ 2), where δ 1 ~ 0.62, δ 2 ~ 0.25, x = 0.125 for CeCu 6-xPd x and δ 1 ~ 0.64, δ 2 ~ 0.3, x = 0.3 for CeCu 6-xAg x. As a result, the magnetic structure consists of an amplitude modulation of the Ce moments which are aligned along the c axis of the orthorhombic unit cell.« less

Structural and magnetic phase transitions in CeCu 6-xT x (T = Ag,Pd)

DOE PAGES

Poudel, Lekhanath N.; De la cruz, Clarina; Payzant, E. Andrew; ...

2015-12-15

The structural and the magnetic properties of CeCu 6-xAg x (0 ≤ x ≤ 0.85) and CeCu 6-xPd x (0 ≤ x ≤ 0.4) have been studied using neutron diffraction, resonant ultrasound spectroscopy (RUS), x-ray diffraction measurements, and first principles calculations. The structural and magnetic phase diagrams of CeCu 6-xAg x and CeCu 6-xPd x as a function of Ag/Pd composition are reported. The end member, CeCu 6, undergoes a structural phase transition from an orthorhombic ( Pnma) to a monoclinic (P2 1/c) phase at 240 K. In CeCu 6-xAg x, the structural phase transition temperature (T s) decreases linearlymore » with Ag concentration and extrapolates to zero at x S ≈ 0.1. The structural transition in CeCu 6-xPd x remains unperturbed with Pd substitution within the range of our study. The lattice constant b slightly decreases with Ag/Pd doping, whereas a and c increase with an overall increase in the unit cell volume. Both systems, CeCu 6-xAg x and CeCu 6-xPd x, exhibit a magnetic quantum critical point (QCP), at x ≈ 0.2 and x ≈ 0.05, respectively. Near the QCP, long range antiferromagnetic ordering takes place at an incommensurate wave vector (δ 1 0 δ 2), where δ 1 ~ 0.62, δ 2 ~ 0.25, x = 0.125 for CeCu 6-xPd x and δ 1 ~ 0.64, δ 2 ~ 0.3, x = 0.3 for CeCu 6-xAg x. As a result, the magnetic structure consists of an amplitude modulation of the Ce moments which are aligned along the c axis of the orthorhombic unit cell.« less

Impedance spectroscopy of reduced monoclinic zirconia.

PubMed

Eder, Dominik; Kramer, Reinhard

2006-10-14

Zirconia doped with low-valent cations (e.g. Y3+ or Ca2+) exhibits an exceptionally high ionic conductivity, making them ideal candidates for various electrochemical applications including solid oxide fuel cells (SOFC) and oxygen sensors. It is nevertheless important to study the undoped, monoclinic ZrO2 as a model system to construct a comprehensive picture of the electrical behaviour. In pure zirconia a residual number of anion vacancies remains because of contaminants in the material as well as the thermodynamic disorder equilibrium, but electronic conduction may also contribute to the observed conductivity. Reduction of zirconia in hydrogen leads to the adsorption of hydrogen and to the formation of oxygen vacancies, with their concentration affected by various parameters (e.g. reduction temperature and time, surface area, and water vapour pressure). However, there is still little known about the reactivities of defect species and their effect on the ionic and electronic conduction. Thus, we applied electrochemical impedance spectroscopy to investigate the electric performance of pure monoclinic zirconia with different surface areas in both oxidizing and reducing atmospheres. A novel equivalent circuit model including parallel ionic and electronic conduction has previously been developed for titania and is used herein to decouple the conduction processes. The concentration of defects and their formation energies were measured using volumetric oxygen titration and temperature programmed oxidation/desorption.

Method of Preparing Monoclinic BaO.Al2O3.2SiO2

DTIC Science & Technology

Monoclinic celsian (BaO.Al2O3.2SiO2) is produced by heating a stoichiometric, powder mixture of BaCO3 (or BaC2O4), Al2O3, and SiO2 (preferably SiO2 gel) with monoclinic celsian seeds at from 1250 deg C to 1500 deg C.

Correlation of Structure, Tunable Colors, and Lifetimes of (Sr, Ca, Ba)Al₂O₄:Eu2+, Dy3+ Phosphors.

PubMed

Xie, Qidi; Li, Bowen; He, Xin; Zhang, Mei; Chen, Yan; Zeng, Qingguang

2017-10-18

(Sr, Ca, Ba)Al₂O₄:Eu 2+ , Dy 3+ phosphors were prepared via a high temperature solid-state reaction method. The correlation of phase structure, optical properties and lifetimes of the phosphors are investigated in this work. For the (Sr, Ca)Al₂O₄:Eu 2+ ,Dy 3+ phosphors, the different phase formation from monoclinic SrAl₂O₄ phase to hexagonal SrAl₂O₄ phase to monoclinic CaAl₂O₄ phase was observed when the Ca content increased. The emission color of SrAl₂O₄:Eu 2+ , Dy 3+ phosphors varied from green to blue. For the (Sr, Ba)Al₂O₄:Eu 2+ , Dy 3+ phosphors, different phase formation from the monoclinic SrAl₂O₄ phase to the hexagonal BaAl₂O₄ phase was observed, along with a shift of emission wavelength from 520 nm to 500 nm. More interestingly, the decay time of SrAl₂O₄:Eu 2+ , Dy 3+ changed due to the different phase formations. Lifetime can be dramatically shortened by the substitution of Sr 2+ with Ba 2+ cations, resulting in improving the performance of the alternating current light emitting diode (AC-LED). Finally, intense LEDs are successfully obtained by combining these phosphors with Ga(In)N near UV chips.

Effect of phase transformation on optical and dielectric properties of zirconium oxide nanoparticles

NASA Astrophysics Data System (ADS)

Chintaparty, Rajababu; Palagiri, Bhavani; Reddy Nagireddy, Ramamanohar; subbha Reddy Imma Reddy, Venkata

2015-09-01

Zirconium oxide nanoparticle (ZrO2) is synthesized by the hydrothermal method at different calcination temperatures. The structural analysis is carried out by X-ray diffraction and Raman spectra. The sample prepared at 400 °C and 1100 °C showed the cubic and monoclinic phase, respectively, and the sample calcined at 600 °C and 800 °C showed the mixed phase with co-existence of cubic and monoclinic phases. Furthermore, the morphology and particle size of these samples were investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM) analysis. The band gap estimated from UV-Vis spectra of ZrO2 (zirconia) nanocrystalline materials calcined at different temperatures from 400 °C to 1100 °C was in the range of 2.6-4.2 eV. The frequency dependence of dielectric constant and dielectric loss was investigated at room temperature. The low frequency region of dielectric constant is attributed to space charge effects.

Paleomagnetic and structural evidence for oblique slip in a fault-related fold, Grayback monocline, Colorado

USGS Publications Warehouse

Tetreault, J.; Jones, C.H.; Erslev, E.; Larson, S.; Hudson, M.; Holdaway, S.

2008-01-01

Significant fold-axis-parallel slip is accommodated in the folded strata of the Grayback monocline, northeastern Front Range, Colorado, without visible large strike-slip displacement on the fold surface. In many cases, oblique-slip deformation is partitioned; fold-axis-normal slip is accommodated within folds, and fold-axis-parallel slip is resolved onto adjacent strike-slip faults. Unlike partitioning strike-parallel slip onto adjacent strike-slip faults, fold-axis-parallel slip has deformed the forelimb of the Grayback monocline. Mean compressive paleostress orientations in the forelimb are deflected 15??-37?? clockwise from the regional paleostress orientation of the northeastern Front Range. Paleomagnetic directions from the Permian Ingleside Formation in the forelimb are rotated 16??-42?? clockwise about a bedding-normal axis relative to the North American Permian reference direction. The paleostress and paleomagnetic rotations increase with the bedding dip angle and decrease along strike toward the fold tip. These measurements allow for 50-120 m of fold-axis-parallel slip within the forelimb, depending on the kinematics of strike-slip shear. This resolved horizontal slip is nearly equal in magnitude to the ???180 m vertical throw across the fold. For 200 m of oblique-slip displacement (120 m of strike slip and 180 m of reverse slip), the true shortening direction across the fold is N90??E, indistinguishable from the regionally inferred direction of N90??E and quite different from the S53??E fold-normal direction. Recognition of this deformational style means that significant amounts of strike slip can be accommodated within folds without axis-parallel surficial faulting. ?? 2008 Geological Society of America.

Photo-induced Low Temperature Structural Transition in the "114" YbaFe 4O 7 oxide

DOE PAGES

Duffort, V.; Caignaert, Vincent; Pralong, V.; ...

2013-11-11

Synchrotron irradiation of the oxide YBaFe 4O 7.0 below 190 K converts the low temperature monoclinic structure to a higher symmetry tetragonal form analogous to the room temperature structure. This photo-induced metastable tetragonal form is stable even in the absence of irradiation over the range 4-60 K, however, above 60 K the photo-transition is reversible. These structural phenomena are correlated to the magnetic behaviour of this system, suggesting possible spin-lattice coupling. Lastly, a scenario explaining the low temperature photo-induced transition is proposed, based on the different distributions of the valence electrons in the iron sub-lattice of the monoclinic and tetragonalmore » phases.« less

In situ disordering of monoclinic titanium monoxide Ti5O5 studied by transmission electron microscope TEM.

PubMed

Rempel, А А; Van Renterghem, W; Valeeva, А А; Verwerft, M; Van den Berghe, S

2017-09-07

The superlattice and domain structures exhibited by ordered titanium monoxide Ti 5 O 5 are disrupted by low energy electron beam irradiation. The effect is attributed to the disordering of the oxygen and titanium sublattices. This disordering is caused by the displacement of both oxygen and titanium atoms by the incident electrons and results in a phase transformation of the monoclinic phase Ti 5 O 5 into cubic B1 titanium monoxide. In order to determine the energies required for the displacement of titanium or oxygen atoms, i.e. threshold displacement energies, a systematic study of the disappearance of superstructure reflections with increasing electron energy and electron bombardment dose has been performed in situ in a transmission electron microscope (TEM). An incident electron energy threshold between 120 and 140 keV has been observed. This threshold can be ascribed to the displacements of titanium atoms with 4 as well as with 5 oxygen atoms as nearest neighbors. The displacement threshold energy of titanium atoms in Ti 5 O 5 corresponding with the observed incident electron threshold energy lies between 6.0 and 7.5 eV. This surprisingly low value can be explained by the presence of either one or two vacant oxygen lattice sites in the nearest neighbors of all titanium atoms.

Phase Transformation of VO2 Nanoparticles Assisted by Microwave Heating

PubMed Central

Sikong, Lek.

2014-01-01

The microwave assisted synthesis nowadays attracts a great deal of attention. Monoclinic phase VO2 (M) was prepared from NH4VO3 and H2C2O4 · 2H2O by a rapid microwave assisted technique. The synthesis parameters, microwave irradiation time, microwave power, and calcinations temperature were systematically varied and their influences on the structure and morphology were evaluated. The microwave power level has been carried out in range 180–600 W. TEM analysis demonstrated nanosized samples. The structural and morphological properties were measured using XRD, TEM, and thermal analyses. The variations of vanadium phase led to thermochromic properties. PMID:24688438

Impact of the oxygen defects and the hydrogen concentration on the surface of tetragonal and monoclinic ZrO2 on the reduction rates of stearic acid on Ni/ZrO2

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

Foraita, Sebastian D.; Fulton, John L.; Chase, Zizwe A.

2015-02-02

The effect of the physicochemical properties of ZrO2 phases on the activity of Ni/ZrO2 catalysts for hydrodeoxygenation of stearic acid are described. A synergistic interaction between Ni and ZrO2 support was found. The effect is greatest for the monoclinic phase of ZrO2.

Structure of bayerite-based lithium-aluminum layered double hydroxides (LDHs): observation of monoclinic symmetry.

PubMed

Britto, Sylvia; Kamath, P Vishnu

2009-12-21

The double hydroxides of Li with Al, obtained by the imbibition of Li salts into bayerite and gibbsite-Al(OH)(3), are not different polytypes of the same symmetry but actually crystallize in two different symmetries. The bayerite-derived double hydroxides crystallize with monoclinic symmetry, while the gibbsite-derived hydroxides crystallize with hexagonal symmetry. Successive metal hydroxide layers in the bayerite-derived LDHs are translated by the vector ( approximately -1/3, 0, 1) with respect to each other. The exigency of hydrogen bonding drives the intercalated Cl(-) ion to a site with 2-fold coordination, whereas the intercalated water occupies a site with 6-fold coordination having a pseudotrigonal prismatic symmetry. The nonideal nature of the interlayer sites has implications for the observed selectivity of Li-Al LDHs toward anions of different symmetries.

Progressive evolution of deformation band populations during Laramide fault-propagation folding: Navajo Sandstone, San Rafael monocline, Utah, U.S.A.

NASA Astrophysics Data System (ADS)

Zuluaga, Luisa F.; Fossen, Haakon; Rotevatn, Atle

2014-11-01

Monoclinal fault propagation folds are a common type of structure in orogenic foreland settings, particularly on the Colorado Plateau. We have studied a portion of the San Rafael monocline, Utah, assumed to have formed through pure thrust- or reverse-slip (blind) fault movement, and mapped a particular sequence of subseismic cataclastic deformation structures (deformation bands) that can be related in terms of geometry, density and orientation to the dip of the forelimb or fold interlimb angle. In simple terms, deformation bands parallel to bedding are the first structures to form, increasing exponentially in number as the forelimb gets steeper. At about 30° rotation of the forelimb, bands forming ladder structures start to cross-cut bedding, consolidating themselves into a well-defined and regularly spaced network of deformation band zones that rotate with the layering during further deformation. In summary, we demonstrate a close relationship between limb dip and deformation band density that can be used to predict the distribution and orientation of such subseismic structures in subsurface reservoirs of similar type. Furthermore, given the fact that these cataclastic deformation bands compartmentalize fluid flow, this relationship can be used to predict or model fluid flow across and along comparable fault-propagation folds.

The New Superconductor tP-SrPd2Bi2: Structural Polymorphism and Superconductivity in Intermetallics.

PubMed

Xie, Weiwei; Seibel, Elizabeth M; Cava, Robert J

2016-04-04

We consider a system where structural polymorphism suggests the possible existence of superconductivity through the implied structural instability. SrPd2Bi2 has two polymorphs, which can be controlled by the synthesis temperature: a tetragonal form (CaBe2Ge2-type) and a monoclinic form (BaAu2Sb2-type). Although the crystallographic difference between the two forms may, at first, seem trivial, we show that tetragonal SrPd2Bi2 is superconducting at 2.0 K, whereas monoclinic SrPd2Bi2 is not. We rationalize this finding and place it in context with other 1-2-2 phases.

High-pressure phase transitions, amorphization, and crystallization behaviors in Bi2Se3.

PubMed

Zhao, Jinggeng; Liu, Haozhe; Ehm, Lars; Dong, Dawei; Chen, Zhiqiang; Gu, Genda

2013-03-27

The phase transition, amorphization, and crystallization behaviors of the topological insulator bismuth selenide (Bi2Se3) were discovered by performing in situ high-pressure angle-dispersive x-ray diffraction experiments during an increasing, decreasing, and recycling pressure process. In the compression process, Bi2Se3 transforms from the original rhombohedral structure (phase I(A)) to a monoclinic structure (phase II) at about 10.4 GPa, and further to a body-centered tetragonal structure (phase III) at about 24.5 GPa. When releasing pressure to ambient conditions after the complete transformation from phase II to III, Bi2Se3 becomes an amorphous solid (AM). In the relaxation process from this amorphous state, Bi2Se3 starts crystallizing into an orthorhombic structure (phase I(B)) about five hours after releasing the pressure to ambient. A review of the pressure-induced phase transition behaviors of A2B3-type materials composed from the V and VI group elements is presented.

Extended x-ray absorption fine structure study of phase transitions in the piezoelectric perovskite K0.5Na0.5NbO3

NASA Astrophysics Data System (ADS)

Kodre, A.; Tellier, J.; Arčon, I.; Malič, B.; Kosec, M.

2009-06-01

Following an x-ray diffraction study of phase transitions of the piezoelectric perovskite K0.5Na0.5NbO3 the structural changes of the material are studied using extended x-ray absorption fine structure analysis, whereby the neighborhood of Nb atom is determined in the temperature range of monoclinic, tetragonal, and cubic phases. Within the entire range Nb atom is displaced from the center of the octahedron of its immediate oxygen neighbors, as witnessed by the splitting of Nb-O distance. The model shows high prevalence of the displacement in the (111) crystallographic direction of the simple perovskite cell. The corresponding splitting of the Nb-Nb distance is negligible. There is no observable disalignment of the linear Nb-O-Nb bonds from the ideal cubic arrangement, judging from the intensity of the focusing of the photoelectron wave on the Nb-Nb scattering path by the interposed oxygen atom. As a general result, the phase transitions are found as an effect of the long-range order, while the placement of the atoms in the immediate vicinity remains largely unaffected.

X-Ray diffraction and mu-Raman investigation of the monoclinic-orthorhombic phase transition in Th(1-x)U(x)(C(2)O(4))(2).2H(2)O solid solutions.

PubMed

Clavier, Nicolas; Hingant, Nina; Rivenet, Murielle; Obbade, Saïd; Dacheux, Nicolas; Barré, Nicole; Abraham, Francis

2010-02-15

A complete Th(1-x)U(x)(C(2)O(4))(2).2H(2)O solid solution was prepared by mild hydrothermal synthesis from a mixture of hydrochloric solutions containing cations and oxalic acid. The crystal structure has been solved from twinned single crystals for x = 0, 0.5, and 1 with monoclinic symmetry, space group C2/c, leading to unit cell parameters of a approximately 10.5 A, b approximately 8.5 A, and c approximately 9.6 A. The crystal structure consists of a two-dimensional arrangement of actinide centers connected through bis-bidentate oxalate ions forming squares. The actinide metal is coordinated by eight oxygen atoms from four oxalate entities and two water oxygen atoms forming a bicapped square antiprism. The connection between the layers is assumed by hydrogen bonds between the water molecules and the oxygen of oxalate of an adjacent layer. Under these conditions, the unit cell contains two independent oxalate ions. From high-temperature mu-Raman and X-ray diffraction studies, the compounds were found to undergo a transition to an orthorhombic form (space group Ccca). The major differences in the structural arrangement concern the symmetry of uranium, which decreases from C2 to D2, leading to a unique oxalate group. Consequently, the nu(s)(C-O) double band observed in the Raman spectra recorded at room temperature turned into a singlet. This transformation was then used to make the phase transition temperature more precise as a function of the uranium content of the sample.

Raman spectra and phase transitions in Rb{sub 2}KInF{sub 6} elpasolite

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

Krylov, A. S.; Krylova, S. N., E-mail: slanky@iph.krasn.ru; Vtyurin, A. N.

2011-01-15

The Raman spectra of Rb{sub 2}KInF{sub 6} elpasolite crystal have been studied in a wide temperature range, including two phase transitions: from the cubic phase to the tetragonal phase and then to the monoclinic phase. Several anomalies of internal modes of InF{sub 6} octahedra and low-frequency lattice vibrations, which are related to the structural changes at the transition points, have been found and quantitatively analyzed. The results of a quantitative analysis of the temperature dependences of the parameters of spectral lines are in good agreement with the thermodynamic data on the phase transitions.

Theoretical calculations of high-pressure phases of NiF2: An ab initio constant-pressure study

NASA Astrophysics Data System (ADS)

Kürkçü, Cihan; Merdan, Ziya; Öztürk, Hülya

2016-12-01

We have studied the structural properties of the antiferromagnetic NiF2 tetragonal structure with P42/ mnm symmetry using density functional theory (DFT) under rapid hydrostatic pressure up to 400 GPa. For the exchange correlation energy we used the local density approximation (LDA) of Ceperley and Alder (CA). Two phase transformations are successfully observed through the simulations. The structures of XF2-type compounds crystallize in rutile-type structure. NiF2 undergoes phase transformations from the tetragonal rutile-type structure with space group P42/ mnm to orthorhombic CaCl2-type structure with space group Pnnm and from this orthorhombic phase to monoclinic structure with space group C2/ m at 152 GPa and 360 GPa, respectively. These phase changes are also studied by total energy and enthalpy calculations. According to these calculations, we perdict these phase transformations at about 1.85 and 30 GPa.

1-Nitro-4-(4-nitro-phen-oxy)benzene: a second monoclinic polymorph.

PubMed

Naz, Mehwish; Akhter, Zareen; McKee, Vickie; Nadeem, Arif

2013-11-06

In the title compound, C12H8N2O5, the aromatic rings are inclined to one another by 56.14 (7)°. The nitro groups are inclined by to the benzene rings to which they are attached by 3.86 (17) and 9.65 (15)°. In the crystal, mol-ecules are linked by C-H⋯O hydrogen bonds, forming a three-dimensional structure. The title compound is a new monoclinic polymorph, crystallizing in space group P21/c. The first polymorph crystallized in space group C2/c and the mol-ecule possesses twofold rotation symmetry. Two low-temperature structures of this polymorph (150 K and 100 K, respectively) have been reported [Meciarova et al. (2004). Private Communication (refcode IXOGAD). CCDC, Cambridge, England, and Dey & Desiraju (2005). Chem. Commun. pp. 2486-2488].

Correlation of Structure, Tunable Colors, and Lifetimes of (Sr, Ca, Ba)Al2O4:Eu2+, Dy3+ Phosphors

PubMed Central

Xie, Qidi; Li, Bowen; He, Xin; Zhang, Mei; Chen, Yan; Zeng, Qingguang

2017-01-01

(Sr, Ca, Ba)Al2O4:Eu2+, Dy3+ phosphors were prepared via a high temperature solid-state reaction method. The correlation of phase structure, optical properties and lifetimes of the phosphors are investigated in this work. For the (Sr, Ca)Al2O4:Eu2+, Dy3+ phosphors, the different phase formation from monoclinic SrAl2O4 phase to hexagonal SrAl2O4 phase to monoclinic CaAl2O4 phase was observed when the Ca content increased. The emission color of SrAl2O4:Eu2+, Dy3+ phosphors varied from green to blue. For the (Sr, Ba)Al2O4:Eu2+, Dy3+ phosphors, different phase formation from the monoclinic SrAl2O4 phase to the hexagonal BaAl2O4 phase was observed, along with a shift of emission wavelength from 520 nm to 500 nm. More interestingly, the decay time of SrAl2O4:Eu2+, Dy3+ changed due to the different phase formations. Lifetime can be dramatically shortened by the substitution of Sr2+ with Ba2+ cations, resulting in improving the performance of the alternating current light emitting diode (AC-LED). Finally, intense LEDs are successfully obtained by combining these phosphors with Ga(In)N near UV chips. PMID:29057839

Influence of frequency on the structure of zirconium oxide coatings deposited from aqueous electrolytes under microplasma oxidation

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

Gubaidulina, Tatiana A., E-mail: goub2002@mail.ru; Sergeev, Viktor P., E-mail: vserg@mail.tomsknet.ru; Fedorischeva, Marina V., E-mail: fmw@ispms.tsc.ru

2015-10-27

The work describes the microplasma oxidation (MPO) of zirconium surface resulting in the formation of zirconium oxide Zr-Al-Nb-O. We have used novel power supply to deposit oxide ceramic coatings by MPO and studied the effect of current density on the phase structure of oxide ceramic coatings. The size of microcracks in the coatings was determined at different frequencies. We have also used EVO50c scanning election microscope with an attachment for elemental analysis to study the morphology and elemental composition of oxide ceramic coating. In addition, we have established the influence of the frequency on the phase composition of the coating:more » at the frequency of 2500 Hz, the fraction of monoclinic phase was 18%, while the fraction of tetragonal phase amounted to 72%. The oxide ceramic coating produced at 250 Hz contained 38% of monoclinic phase and 62% of tetragonal phase; in addition, it had no buildups and craters.« less

Equation of state, phase stability, and phase transformations of uranium-6 wt. % niobium under high pressure and temperature

NASA Astrophysics Data System (ADS)

Zhang, Jianzhong; Vogel, Sven; Brown, Donald; Clausen, Bjorn; Hackenberg, Robert

2018-05-01

In-situ time-of-flight neutron diffraction experiments were conducted on the uranium-niobium alloy with 6 wt. % Nb (U-6Nb) at pressures up to 4.7 GPa and temperatures up to 1073 K. Upon static compression at room temperature, the monoclinic structure of U-6Nb (α″ U-6Nb) remains stable up to the highest experimental pressure. Based on the pressure-volume measurements at room temperature, the least-squares fit using the finite-strain equation of state (EOS) yields an isothermal bulk modulus of B0 = 127 ± 2 GPa for the α″-phase of U-6Nb. The calculated zero-pressure bulk sound speed from this EOS is 2.706 ± 0.022 km/s, which is in good agreement with the linear extrapolation of the previous Hugoniot data above 12 GPa for α″ U-6Nb, indicating that the dynamic response under those shock-loading conditions is consistent with the stabilization of the initial monoclinic phase of U-6Nb. Upon heating at ambient and high pressures, the metastable α″ U-6Nb exhibits complex transformation paths leading to the diffusional phase decomposition, which are sensitive to applied pressure, stress state, and temperature-time path. These findings provide new insight into the behavior of atypical systems such as U-Nb and suggest that the different U-Nb phases are separated by rather small energies and hence highly sensitive to compositional, thermal, and mechanical perturbations.

Structural transition in lanthanum gallate and transformation of the fine structure of the EPR spectrum of a Gd3+ impurity center

NASA Astrophysics Data System (ADS)

Vazhenin, V. A.; Guseva, V. B.; Fokin, A. V.; Potapov, A. P.; Artyomov, M. Yu.

2011-04-01

Abrupt changes in resonance positions, hysteretic temperature behavior, and coexistence of phases, which indicate a first-order phase transition, have been revealed from measurements of temperature dependences of the EPR spectra of Gd3+ and Mn4+ centers in the vicinity of the structural transition of lanthanum gallate. The transformation of monoclinic Gd3+ centers into trigonal Gd3+ centers upon the phase transition has been used to estimate the adequacy of two approximations of the superposition model for parameters of the zero-field splitting of the ground state.

Diffusion paths formation for Cu{sup +} ions in superionic Cu{sub 6}PS{sub 5}I single crystals studied in terms of structural phase transition

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

Gagor, A.; Pietraszko, A.; Kaynts, D.

2005-11-15

In order to understand the structural transformations leading to high ionic conductivity of Cu{sup +} ions in Cu{sub 6}PS{sub 5}I argyrodite compound, the detailed structure analysis based on single-crystal X-ray diffraction has been performed. Below the phase transition at T{sub c}=(144-169)K Cu{sub 6}PS{sub 5}I belongs to monoclinic, ferroelastic phase (space group Cc) with ordered copper sublattice. Above T{sub c} delocalization of copper ions begins and crystal changes the symmetry to cubic superstructure with space group F-43c (a{sup '}=19.528A, z=32). Finally, above T{sub 1}=274K increasing disordering of the Cu{sup +} ions heightens the symmetry to F-43m (a=9.794A, z=4). In this work,more » the final structural model of two cubic phases is presented including the detailed temperature evolution of positions and site occupation factors of copper ions (R{sub 1}=0.0397 for F-43c phase, and 0.0245 for F-43m phase). Possible diffusion paths for the copper ions are represented by means of the atomic displacement factors and split model. The structural results coincide well with the previously reported non-Arrhenius behavior of conductivity and indicate significant change in conduction mechanism.« less

Thermal effects on the structural properties of tungsten oxide nanoparticles

NASA Astrophysics Data System (ADS)

Yang, Tsung-Yeh; Wu, Chung-Yi; Tsai, Meng-Hung; Lin, Hong-Ming; Tsai, Wen-Li; Hwu, Yeukuang

2004-06-01

Tungsten oxide nanoparticles are prepared by evaporating and oxidizing the tungsten boat in helium and oxygen atmosphere and then quenched to the liquid nitrogen temperature. The as-prepared tungsten oxide nanoparticles are porous-free with uniform size. The morphology and particle size distribution of the as-prepared and after sinter treatments tungsten oxide nanoparticles are revealed by TEM and AFM. The long-range order of these nanoparticles can be examined by X-ray diffraction technique. The as-prepared nanoparticles exhibit a mixture structure of monoclinic and hexagonal crystals. Preliminary X-ray diffraction results indicate that the hexagonal structure is transformed to monoclinic structure after annealing to above 600°C. In order to better distinguish the structural properties of the tungsten oxide (WO3- x) nanoparticles before and after annealing, the X-ray absorption spectrum technique is utilized; thus, the detailed local atomic arrangement of oxygen and/or tungsten can be determined. According to the XAS result, the shape of the W L3-edge undergoes no considerable changes. This infers that structural transformation of tungsten oxide nanoparticle may be caused by the migration of oxygen after sintering. From the O K-edge of absorption spectrum, it suggests that a mixture phase structure is obtained when sintered below 300°C. And this result indicates that heat treatment to approximately 600°C produces a stable structure of a monoclinic crystal of WO3.

Lattice dynamics of A Sb2O6 (A =Cu , Co) with trirutile structure

NASA Astrophysics Data System (ADS)

Maimone, D. T.; Christian, A. B.; Neumeier, J. J.; Granado, E.

2018-03-01

Raman spectroscopy experiments on single crystals of CuSb2O6 and CoSb2O6 quasi-one-dimensional antiferromagnets with trirutile crystal structure were performed, with a focus on the first material. The observed Raman-active phonon modes and previously reported infrared-active modes were identified with the aid of ab initio lattice dynamics calculations. The structural transition between monoclinic β -CuSb2O6 and tetragonal α -CuSb2O6 phases at Ts=400 K is manifested in our spectra by a "repulsion" of two accidentally quasidegenerate symmetric modes below Ts, caused by a phonon mixing effect that is only operative in the monoclinic β -CuSb2O6 phase due to symmetry restrictions. Also, two specific phonons, associated with CuO6 octahedra rotation and with a Jahn-Teller elongation mode, soften and broaden appreciably as T →Ts . A crossover from a displacive to an order-disorder transition at Ts is inferred.

Cubic phase stability, optical and magnetic properties of Cu-stabilized zirconia nanocrystals

NASA Astrophysics Data System (ADS)

Pramanik, Prativa; Singh, Sobhit; Joshi, Deep Chandra; Mallick, Ayan; Pisane, Kelly; Romero, Aldo H.; Thota, Subhash; Seehra, M. S.

2018-06-01

By means of experimental and ab initio investigations, we report on the cubic phase stability of Cu doped zirconia (ZrO2) at room temperature, and further characterize its structural, optical and magnetic properties. Various compositions of Zr1‑x Cu x O2 (0.01  ⩽  x  ⩽  0.25) nanocrystallites of average size  ∼16 nm were synthesized using co-precipitation technique. Thermal analysis and kinetics of crystallization revealed that the cubic phase at ambient temperature can be stabilized by using a critical calcination temperature of 500 °C for 8 h in air and a critical composition of . For x  <  x c , some undigested monoclinic phase of ZrO2 exists together with the cubic structure. However, for x  >  x c , the monoclinic CuO emerges as a secondary phase with shrinkage of unit-cell volume with increasing the Cu content. At x  =  0.05 and 500 °C calcination temperature, we observe a high degree of cubic crystallinity which breaks down into monoclinic phase with increasing calcination temperature beyond 550 °C. Electron magnetic resonance studies provide evidence for the substitution of Cu2+ (2D5/9,3d9) ions at Zr4+ sites with g, g and average g a   =  (  +  2)/3  ∼  2.1. The temperature dependence of magnetic susceptibility measurements from 2 K to 300 K exhibits Curie–Weiss behaviour whose analysis using g a   =  2.1 and spin S  =  1/2 yields x  =  0.028 and x  =  0.068 for the nominal x  =  0.05 and x  =  0.20 samples, respectively. This magnetic analysis confirms the findings from x-ray diffraction that only a part of Cu is successfully doped into cubic phase of Cu-doped ZrO2. The optical bandgap decreases with increasing x, which is due to the emergence of Cu-d states at Fermi-level near the valence bands, thus making Cu-doped zirconia a hole doped (p-type) semiconductor.

Phase Stability and Transformations in Vanadium Oxide Nanocrystals

NASA Astrophysics Data System (ADS)

Bergerud, Amy Jo

Vanadium oxides are both fascinating and complex, due in part to the many compounds and phases that can be stabilized as well as the phase transformations which occur between them. The metal to insulator transitions (MITs) that take place in vanadium oxides are particularly interesting for both fundamental and applied study as they can be induced by a variety of stimuli ( i.e., temperature, pressure, doping) and utilized in many applications (i.e., smart windows, sensors, phase change memory). Nanocrystals also tend to demonstrate interesting phase behavior, due in part to the enhanced influence of surface energy on material thermodynamics. Vanadium oxide nanocrystals are thus expected to demonstrate very interesting properties in regard to phase stability and phase transformations, although synthesizing vanadium oxides in nanocrystal form remains a challenge. Vanadium sesquioxide (V2O3) is an example of a material that undergoes a MIT. For decades, the low temperature monoclinic phase and high temperature corundum phase were the only known crystal structures of V2O3. However, in 2011, a new metastable polymorph of V2O3 was reported with a cubic, bixbyite crystal structure. In Chapter 2, a colloidal route to bixbyite V2O 3 nanocrystals is presented. In addition to being one of the first reported observations of the bixbyite phase in V2O3, it is also one of the first successful colloidal syntheses of any of the vanadium oxides. The nanocrystals possess a flower-like morphology, the size and shape of which are dependent on synthesis time and temperature, respectively. An aminolysis reaction mechanism is determined from Fourier transform infrared spectroscopy data and the bixbyite crystal structure is confirmed by Rietveld refinement of X-ray diffraction (XRD) data. Phase stability is assessed in both air and inert environments, confirming the metastable nature of the material. Upon heating in an inert atmosphere above 700°C, the nanocrystals irreversibly transform

Phase transformations and equation of state of praseodymium metal to 103 GPa

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

Chesnut, Gary N.; Vohra, Yogesh K.

2000-08-01

Pressure-induced structural phase transformations in a trivalent rare-earth metal praseodymium (Pr) were studied at room temperature in a diamond anvil cell to 103 GPa by energy dispersive x-ray diffraction using a synchrotron source. Our x-ray diffraction studies document the following crystal structure sequence: dhcp{yields}fcc{yields}distorted fcc(hR24 type){yields}monoclinic(C2/m){yields}{alpha}-uranium with increasing pressure. We measure a 16.7% volume collapse at the transition to the {alpha}-uranium phase at 20 GPa. The high-pressure {alpha}-uranium phase in Pr was found to be stable to the highest pressure of 103 GPa, which corresponds to a volume compression V/V{sub 0}=0.407. (c) 2000 The American Physical Society.

In situ investigations of the phase change behaviour of tungsten oxide nanostructures

NASA Astrophysics Data System (ADS)

Thummavichai, Kunyapat; Wang, Nannan; Xu, Fang; Rance, Graham; Xia, Yongda; Zhu, Yanqiu

2018-04-01

This study uses two in situ techniques to investigate the geometry and phase change behaviour of bundled ultrathin W18O49 nanowires and WO3 nanoparticles. The in situ X-ray diffraction (XRD) results have shown that the phase transition of WO3 nanoparticles occurs in sequence from monoclinic (room temperature) → orthorhombic (350°C) → tetragonal (800°C), akin to bulk WO3; however, W18O49 nanowires remain stable as the monoclinic phase up to 500°C, after which a complete oxidation to WO3 and transformation to the orthorhombic β-phase at 550°C is observed. The in situ Raman spectroscopy investigations have revealed the Raman peak downshifts as the temperature increases, and have identified the 187.6 cm-1 as the fingerprint band for the phase transition from γ- to β-phase of the WO3 nanoparticle. Furthermore, WO3 nanoparticles exhibit the γ- to β-phase conversion at 275°C, which is about 75°C lower than the relaxation temperature of 350°C for the monoclinic γ-W18O49 nanowires. These new fundamental understandings on the phase transition behaviour offer important guidance for the design and development of tungsten oxide-based nanodevices by defining their allowed operating conditions.

Crystal Structure and Superconductivity of PH 3 at High Pressures

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

Liu, Hanyu; Li, Yinwei; Gao, Guoying

2016-02-04

We have performed a systematic structure search on solid PH3 at high pressures using the particle swarm optimization method. At 100–200 GPa, the search led to two structures which along with others have P–P bonds. These structures are structurally and chemically distinct from those predicted for the high-pressure superconducting H2S phase, which has a different topology (i.e., does not contain S–S bonds). Phonon and electron–phonon coupling calculations indicate that both structures are dynamically stable and superconducting. The pressure dependence and critical temperature for the monoclinic (C2/m) phase of 83 K at 200 GPa are in excellent agreement with a recentmore » experimental report.« less

Phase Stabilization of Zirconia.

DTIC Science & Technology

1997-01-30

preparing stabilized zirconia pursuant to this disclosure, an insoluble alumina powder is mixed with zirconia powder using a liquid dispersant, such...in a drying oven or a furnace. When mixing the alumina and zirconia powders , it is not necessary to have zirconia in any particular phase to achieve...phase stabilization, as disclosed herein. When mixed with alumina powder, zirconia powder can be in cubic, tetragonal or 20 monoclinic phases

Multi-phase structures of boron-doped copper tin sulfide nanoparticles synthesized by chemical bath deposition for optoelectronic devices

NASA Astrophysics Data System (ADS)

Rakspun, Jariya; Kantip, Nathakan; Vailikhit, Veeramol; Choopun, Supab; Tubtimtae, Auttasit

2018-04-01

We investigated the influence of boron doping on the structural, optical, and electrical properties of copper tin sulfide (CTS) nanoparticles coated on a WO3 surface and synthesized using chemical bath deposition. Boron doping at concentrations of 0.5, 1.0, 1.5, and 2.0 wt% was investigated. The X-ray diffraction pattern of CTS showed the presence of monoclinic Cu2Sn3S7, cubic Cu2SnS3, and orthorhombic Cu4SnS4. Boron doping influenced the preferred orientation of the nanoparticles for all phase structures and produced a lattice strain effect and changes in the dislocation density. Increasing the concentration of boron in CTS from 0.5 wt% to 2.0 wt% reduced the band gap for all phases of CTS from 1.46 to 1.29 eV and reduced the optical transmittance. Optical constants, such as the refractive index, extinction coefficient, and dissipation factor, were also obtained for B-doped CTS. The dispersion behavior of the refractive index was investigated in terms of a single oscillator model and the physical parameters were determined. Fourier transform infrared spectroscopy confirmed the successful synthesis of CTS nanoparticles. Cyclic voltammetry indicated that optimum boron doping (<1.5 wt% for all phases) resulted in desirable p-n junction behavior for optoelectronic applications.

Vapor-crystal phase transition in synthesis of paracetamol films by vacuum evaporation and condensation

NASA Astrophysics Data System (ADS)

Belyaev, A. P.; Rubets, V. P.; Antipov, V. V.; Bordei, N. S.; Zarembo, V. I.

2014-03-01

We report on the structural and technological investigations of the vapor-crystal phase transition during synthesis of paracetamol films of the monoclinic system by vacuum evaporation and condensation in the temperature range 220-320 K. The complex nature of the transformation accompanied by the formation of a gel-like phase is revealed. The results are interpreted using a model according to which the vapor-crystal phase transition is not a simple first-order phase transition, but is a nonlinear superposition of two phase transitions: a first-order transition with a change in density and a second-order phase transition with a change in ordering. Micrographs of the surface of the films are obtained at different phases of formation.

Phase transitions in the (Ni,Zn)TiF 6 · 6H 2O system

NASA Astrophysics Data System (ADS)

Lichti, R. L.; Jan, I.-Yuan; Casey, K. G.

1989-02-01

Measurements of the transformation rates and the characteristic temperatures of the trigonal ≡ monoclinic structural change in (Ni 1- xZn x)TiF 6 · 6H 2O show a double transition up to x = 0.5. The relationships between the phase changes generally observed in the ABF 6 · 6H 2O system and the internal motions of the octahedral ionic complexes are discussed, and a phase diagram for the mixed nickel/zinc fluorotitanate is established.

Initial stages of ion beam-induced phase transformations in Gd2O3 and Lu2O3

NASA Astrophysics Data System (ADS)

Chen, Chien-Hung; Tracy, Cameron L.; Wang, Chenxu; Lang, Maik; Ewing, Rodney C.

2018-02-01

The atomic-scale evolution of lanthanide sesquioxides Gd2O3 and Lu2O3 irradiated with 1 MeV Kr ions at room temperature and 120 K, up to fluences of 1 × 1016 ions/cm2 (˜20 dpa), has been characterized by in situ transmission electron microscopy. At room temperature, both oxides exhibited high radiation tolerance. Irradiation did not cause any observable structural change in either material, likely due to the mobility of irradiation-induced point defects, causing efficient defect annihilation. For Gd2O3, having the larger cation ionic radius of the two materials, an irradiation-induced stacking fault structure appeared at low fluences in the low temperature irradiation. As compared with the cubic-to-monoclinic phase transformations known to result from higher energy (˜GeV) ion irradiation, Kr ions of lower energies (˜MeV) yield much lower rates of damage accumulation and thus less extensive structural modification. At a fluence of 2.5 × 1015 ions/cm2, only the initial stages of the cubic-to-monoclinic (C to B) phase transformation process, consisting of the formation and aggregation of defects, have been observed.

On the P 21/m and Pmmn pathways of the B1 B2 phase transition in NaCl: a quantum-mechanical study

NASA Astrophysics Data System (ADS)

Catti, Michele

2004-06-01

The monoclinic P 21/m and orthorhombic Pmmn (Watanabe et al' s-type) mechanisms of the high-pressure phase transition of NaCl between the B1 (rocksalt, Fm\\overline 3 m ) and B2 (CsCl-like, Pm\\overline 3 m ) cubic phases were investigated by ab initio DFT techniques with all-electron localized basis sets. Enthalpy profiles versus the order parameter were computed at constant pressures of 15, 26.3 (equilibrium) and 35 GPa for each pathway. The monoclinic path shows a lower activation enthalpy at the equilibrium pressure, but at different p values (hysteresis effects) the other mechanism becomes competitive. In the P 21/m case, sharp jumps of structural parameters are observed along the transformation coordinate, which can be explained by a mechanism based on discontinuous sliding of alternating pairs of (100) atomic layers. This accounts also for the predicted formation of a metastable intermediate Cmcm phase with TlI-like structure, similar to that observed experimentally at high pressure in AgCl, and the relations with the KOH structure are discussed, too. On the other hand, along the Pmmn pathway the structural parameters vary quite smoothly, indicating a continuous motion of neighbouring atomic planes within the constraint of the additional mirror symmetry.

Crystal Structure and Superconductivity of PH 3 at High Pressures

DOE PAGES

Liu, Hanyu; Li, Yinwei; Gao, Guoying; ...

2016-01-20

Here, we performed systematic structure search on solid PH 3 at high pressures using particle swarm optimization method. Furthermore, at 100-200 GPa, the search led to two structures consisting of P-P bonds that different from these predicted for H 2S. Phonon and electron-phonon calculations indicate both structures are dynamically stable and superconductive. Particularly, the estimated critical temperature for the monoclinic (C2/m) phase of 83 K at 200 GPa is in excellent agreement with a recent experimental report.

Structural, electronic and optical properties of monoclinic Na{sub 2}Ti{sub 3}O{sub 7} from density functional theory calculations: A comparison with XRD and optical absorption measurements

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

Araújo-Filho, Adailton A.; Silva, Fábio L.R.; Righi, Ariete

Powder samples of bulk monoclinic sodium trititanate Na{sub 2}Ti{sub 3}O{sub 7} were prepared carefully by solid state reaction, and its monoclinic P2{sub 1}/m crystal structure and morphology were characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM), respectively. Moreover, the sodium trititanate main energy band gap was estimated as E{sub g}=3.51±0.01 eV employing UV–Vis spectroscopy, which is smaller than the measured 3.70 eV energy gap published previously by other authors. Aiming to achieve a better understanding of the experimental data, density functional theory (DFT) computations were performed within the local density and generalized gradient approximations (LDA and GGA,more » respectively) taking into account dispersion effects through the scheme of Tkatchenko and Scheffler (GGA+TS). Optimal lattice parameters, with deviations relative to measurements Δa=−0.06 Å, Δb=0.02 Å, and Δc=−0.09 Å, were obtained at the GGA level, which was then used to simulate the sodium trititanate electronic and optical properties. Indirect band transitions have led to a theoretical gap energy value of about 3.25 eV. Our results, however, differ from pioneer DFT results with respect to the specific Brillouin zone vectors for which the indirect transition with smallest energy value occurs. Effective masses for electrons and holes were also estimated along a set of directions in reciprocal space. Lastly, our calculations revealed a relatively large degree of optical isotropy for the Na{sub 2}Ti{sub 3}O{sub 7} optical absorption and complex dielectric function. - Graphical abstract: Monoclinic sodium trititanate Na2Ti3O7 was characterized by experiment and dispersion-corrected DFT calculations. An indirect gap of 3.5 eV is predicted, with heavy electrons and anisotropic holes ruling its conductivity. - Highlights: • Monoclinic Na2Ti3O7 was characterized by experiment (XRD, SEM, UV–Vis spectroscopy). • DFT GGA+TS optimized

(Zr,Ti)O2 interface structure in ZrO2-TiO2 nanolaminates with ultrathin periodicity

NASA Astrophysics Data System (ADS)

Aita, C. R.; DeLoach, J. D.; Yakovlev, V. V.

2002-07-01

A mixed cation interfacial structure in ZrO2-TiO2 nanolaminate films with ultrathin bilayer periodicity grown by sputter deposition at 297 K was identified by x-ray diffraction and nonresonant Raman spectroscopy. This structure consists of an amorphous phase at a ZrO2-on-TiO2 bilayer interface, followed by an extensive crystalline monoclinic (Zr,Ti)O2 solid solution predicted by Vegard's law. Monoclinic (Zr,Ti)O2 has previously been reported only once, in bulk powder of a single composition (ZrTiO4) at high pressure. Its stabilization in the nanolaminates is explained by the Gibbs-Thomson effect. This complex interfacial structure is shown to be a means of accommodating chemical mixing in the absence of a driving force for heteroepitaxy.

Hydrogen-induced morphotropic phase transformation of single-crystalline vanadium dioxide nanobeams.

PubMed

Hong, Woong-Ki; Park, Jong Bae; Yoon, Jongwon; Kim, Bong-Joong; Sohn, Jung Inn; Lee, Young Boo; Bae, Tae-Sung; Chang, Sung-Jin; Huh, Yun Suk; Son, Byoungchul; Stach, Eric A; Lee, Takhee; Welland, Mark E

2013-04-10

We report a morphotropic phase transformation in vanadium dioxide (VO2) nanobeams annealed in a high-pressure hydrogen gas, which leads to the stabilization of metallic phases. Structural analyses show that the annealed VO2 nanobeams are hexagonal-close-packed structures with roughened surfaces at room temperature, unlike as-grown VO2 nanobeams with the monoclinic structure and with clean surfaces. Quantitative chemical examination reveals that the hydrogen significantly reduces oxygen in the nanobeams with characteristic nonlinear reduction kinetics which depend on the annealing time. Surprisingly, the work function and the electrical resistance of the reduced nanobeams follow a similar trend to the compositional variation due mainly to the oxygen-deficiency-related defects formed at the roughened surfaces. The electronic transport characteristics indicate that the reduced nanobeams are metallic over a large range of temperatures (room temperature to 383 K). Our results demonstrate the interplay between oxygen deficiency and structural/electronic phase transitions, with implications for engineering electronic properties in vanadium oxide systems.

Fragile morphotropic phase boundary and phase stability in the near-surface region of the relaxor ferroelectric (1 -x ) Pb (Z n1 /3N b2 /3) O3-x PbTi O3 : [001] field-cooled phase diagrams

NASA Astrophysics Data System (ADS)

Wang, Yaojin; Wang, Ding; Yuan, Guoliang; Ma, He; Xu, Feng; Li, Jiefang; Viehland, D.; Gehring, Peter M.

2016-11-01

We have examined the effects of field cooling on the phase diagram of the relaxor system (1 -x ) Pb (Z n1 /3N b2 /3) O3-x PbTi O3 (PZN-x PT ) for compositions near the morphotropic phase boundary (MPB). High-resolution diffraction measurements using Cu Kα x rays, which probe ≈3 μ m below the crystal surface, were made on field-cooled (FC) single-crystal specimens of PZN-4.5 %PT and PZN-6.5 %PT under electric fields of 1 and 2 kV/cm applied along [001] and combined with previous neutron diffraction data, which probe the entire crystal volume, for FC PZN-8 %PT [Ohwada et al., Phys. Rev. B 67, 094111 (2003), 10.1103/PhysRevB.67.094111]. A comparison to the zero-field-cooled (ZFC) PZN-x PT phase diagram reveals several interesting features: (1) The short-range monoclinic phase observed in the ZFC state on the low-PT side of the MPB is replaced by a monoclinic MA phase; (2) field cooling extends the tetragonal phase to higher temperatures and lower-PT concentrations; (3) the orthorhombic phase near the MPB is replaced by a monoclinic MC phase; (4) the vertical MPB in the ZFC phase diagram bends significantly towards the low-PT side in the FC state. These results demonstrate that both the phase stability and the nature of the MPB in PZN-PT within the near-surface regions are fragile in the presence of electric fields.

In situ investigations of the phase change behaviour of tungsten oxide nanostructures.

PubMed

Thummavichai, Kunyapat; Wang, Nannan; Xu, Fang; Rance, Graham; Xia, Yongda; Zhu, Yanqiu

2018-04-01

This study uses two in situ techniques to investigate the geometry and phase change behaviour of bundled ultrathin W 18 O 49 nanowires and WO 3 nanoparticles. The in situ X-ray diffraction (XRD) results have shown that the phase transition of WO 3 nanoparticles occurs in sequence from monoclinic (room temperature) → orthorhombic (350°C) → tetragonal (800°C), akin to bulk WO 3 ; however, W 18 O 49 nanowires remain stable as the monoclinic phase up to 500°C, after which a complete oxidation to WO 3 and transformation to the orthorhombic β-phase at 550°C is observed. The in situ Raman spectroscopy investigations have revealed the Raman peak downshifts as the temperature increases, and have identified the 187.6 cm -1 as the fingerprint band for the phase transition from γ- to β-phase of the WO 3 nanoparticle. Furthermore, WO 3 nanoparticles exhibit the γ- to β-phase conversion at 275°C, which is about 75°C lower than the relaxation temperature of 350°C for the monoclinic γ-W 18 O 49 nanowires. These new fundamental understandings on the phase transition behaviour offer important guidance for the design and development of tungsten oxide-based nanodevices by defining their allowed operating conditions.

Determination of Stress Coefficient Terms in Cracked Solids for Monoclinic Materials with Plane Symmetry at x3 = 0

NASA Technical Reports Server (NTRS)

Yuan, F. G.

1998-01-01

Determination of all the coefficients in the crack tip field expansion for monoclinic materials under two-dimensional deformation is presented in this report. For monoclinic materials with a plane of material symmetry at x(sub 3) = 0, the in-plane deformation is decoupled from the anti-plane deformation. In the case of in-plane deformation, utilizing conservation laws of elasticity and Betti's reciprocal theorem, together with selected auxiliary fields, T-stress and third-order stress coefficients near the crack tip are evaluated first from path-independent line integrals. To determine the T-stress terms using the J-integral and Betti's reciprocal work theorem, auxiliary fields under a concentrated force and moment acting at the crack tip are used respectively. Through the use of Stroh formalism in anisotropic elasticity, analytical expressions for all the coefficients including the stress intensity factors are derived in a compact form that has surprisingly simple structure in terms of the Barnett-Lothe tensors, L. The solution forms for degenerated materials, orthotropic, and isotropic materials are presented.

Crystal grain growth at the α -uranium phase transformation in praseodymium

NASA Astrophysics Data System (ADS)

Cunningham, Nicholas C.; Velisavljevic, Nenad; Vohra, Yogesh K.

2005-01-01

Structural phase transformations under pressure are examined in praseodymium metal for the range 0-40GPa at ambient temperature. Pressure was generated with a diamond-anvil cell, and data were collected using high-resolution synchrotron x-ray diffraction and the image plate technique. The structural sequence double hexagonal close packed (dhcp)→face centered cubic (fcc)→distorted-fcc (d-fcc)→ α -uranium (α-U) is observed with increasing pressure. Rietveld refinement of all crystallographic phases provided confirmation of the hR24 structure for the d-fcc phase while the previously reported monoclinic phase between the d-fcc and the α-U phase was not confirmed. We observe dramatic crystal grain growth during the volume collapse concurrent with the symmetry-lowering transition to the α-U structure. No preferred orientation axis is observed, and the formation process for these large grains is expected to be via a nucleation and growth mechanism. An analogous effect in rare earth metal cerium suggests that the grain growth during transformation to the α-U structure is a common occurrence in f -electron metals at high pressures.

Method of forming high density oxide pellets by hot pressing at 50$sup 0$ to 100$sup 0$C above the cubic to monoclinic phase transformation temperature

DOEpatents

Pasto, A.E.

1975-08-01

A process for low temperature sintering of rare earth and actinide oxides which have a cubic to monoclinic transformation is described. The process involves hot pressing a powder compact at a temperature just above the transformation temperature. (auth)

Structures, phase transitions and microwave dielectric properties of the 6H perovskites Ba 3BSb 2O 9, B=Mg, Ca, Sr, Ba

NASA Astrophysics Data System (ADS)

Ling, Chris D.; Rowda, Budwy; Avdeev, Maxim; Pullar, Robert

2009-03-01

We present a complete temperature-composition phase diagram for Ba 3BSb 2O 9, B=Mg, Ca, Sr, Ba, along with their electrical behavior as a function of B. These compounds have long been recognized as 6H-type perovskites, but (with the exception of B=Mg) their exact structures and properties were unknown due to their low symmetries, temperature-dependent phase transitions, and difficulties in synthesizing pure samples. The full range of possible space group symmetries is observed, from ideal hexagonal P6 3/ mmc to monoclinic C2/ c to triclinic P1¯. Direct second-order transitions between these phases are plausible according to group theory, and no evidence was seen for any further intermediate phases. The phase diagram with respect to temperature and the effective ionic radius of B is remarkably symmetrical for B=Mg, Ca, and Sr. For B=Ba, a first-order phase transition to a locally distorted phase allows a metastable hexagonal phase to persist to lower temperatures than expected before decomposing around 600 K. Electrical measurements revealed that dielectric permittivity corrected for porosity does not change significantly as a function of B and is in a good agreement with the values predicted by the Clausius-Mossotti equation.

Electronic structure, charge transfer, and intrinsic luminescence of gadolinium oxide nanoparticles: Experiment and theory

NASA Astrophysics Data System (ADS)

Zatsepin, D. A.; Boukhvalov, D. W.; Zatsepin, A. F.; Kuznetsova, Yu. A.; Mashkovtsev, M. A.; Rychkov, V. N.; Shur, V. Ya.; Esin, A. A.; Kurmaev, E. Z.

2018-04-01

The cubic (c) and monoclinic (m) polymorphs of Gd2O3 were studied using the combined analysis of several materials science techniques - X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) spectroscopy. Density functional theory (DFT) based calculations for the samples under study were performed as well. The cubic phase of gadolinium oxide (c-Gd2O3) synthesized using a precipitation method exhibits spheroidal-like nanoclusters with well-defined edges assembled from primary nanoparticles with an average size of 50 nm, whereas the monoclinic phase of gadolinium oxide (m-Gd2O3) deposited using explosive pyrolysis has a denser structure compared with natural gadolinia. This phase also has a structure composed of three-dimensional complex agglomerates without clear-edged boundaries that are ∼21 nm in size plus a cubic phase admixture of only 2 at.% composed of primary edge-boundary nanoparticles ∼15 nm in size. These atomic features appear in the electronic structure as different defects ([Gd…Osbnd OH] and [Gd…Osbnd O]) and have dissimilar contributions to the charge-transfer processes among the appropriate electronic states with ambiguous contributions in the Gd 5р - O 2s core-like levels in the valence band structures. The origin of [Gd…Osbnd OH] defects found by XPS was well-supported by PL analysis. The electronic and atomic structures of the synthesized gadolinias calculated using DFT were compared and discussed on the basis of the well-known joint OKT-van der Laan model, and good agreement was established.

Temperature dependence of the dielectric tensor of monoclinic Ga2O3 single crystals in the spectral range 1.0-8.5 eV

NASA Astrophysics Data System (ADS)

Sturm, C.; Schmidt-Grund, R.; Zviagin, V.; Grundmann, M.

2017-08-01

The full dielectric tensor of monoclinic Ga2O3 (β-phase) was determined by generalized spectroscopic ellipsometry in the spectral range from 1.0 eV up to 8.5 eV and temperatures in the range from 10 K up to 300 K. By using the oriented dipole approach, the energies and broadenings of the excitonic transitions are determined as a function of the temperature, and the exciton-phonon coupling properties are deduced.

Phase stability and B-site ordering in La{sub 2}NiMnO{sub 6} thin films

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

Jin, Xiao-Wei; Lu, Lu; Liu, Ming

2016-07-18

Thin films of multiferroic double-perovskite La{sub 2}NiMnO{sub 6} are prepared on (001)-oriented SrTiO{sub 3}, (La{sub 0.289}Sr{sub 0.712})(Al{sub 0.633}Ta{sub 0.356})O{sub 3}, and LaSrAlO{sub 4} substrates by pulsed laser deposition. Microstructure investigation by advanced electron microscopy shows that the La{sub 2}NiMnO{sub 6} films have a monoclinic structure on the SrTiO{sub 3} substrates and a rhombohedral structure on the (La{sub 0.289}Sr{sub 0.712})(Al{sub 0.633}Ta{sub 0.356})O{sub 3} and LaSrAlO{sub 4} substrates. Atomic-scale elemental maps of the monoclinic and rhombohedral phases reveal a short-range and/or partial ordering of the B-sites. In addition, domains and columnar grains are found in the films. Our results demonstrate that themore » phase and microstructure of the La{sub 2}NiMnO{sub 6} films can be tuned by epitaxial strains induced by different substrates.« less

Structural properties of zirconia - in-situ high temperature XRD characterization

NASA Astrophysics Data System (ADS)

Kurpaska, Lukasz

2018-07-01

In this work, the effect of high temperature on structural properties of pure zirconium have been investigated. In-situ X-ray diffraction analysis of the oxide layer formed at temperature window 25-600 °C on pure zirconium were performed. Conducted experiment aimed at investigation of the zirconia phases developed on surface of the metallic substrate. Based on the conducted studies, possible stress state (during heating, continuous oxidation and cooling), cell parameters and HWHM factor were analyzed. A tetragonal and monoclinic phases peak shifts and intensities change were observed, suggesting that different phases react in different way upon temperature effect.

Crystal structure of cobalt hydroxide carbonate Co2CO3(OH)2: density functional theory and X-ray diffraction investigation.

PubMed

González-López, Jorge; Cockcroft, Jeremy K; Fernández-González, Ángeles; Jimenez, Amalia; Grau-Crespo, Ricardo

2017-10-01

The cobalt carbonate hydroxide Co 2 CO 3 (OH) 2 is a technologically important solid which is used as a precursor for the synthesis of cobalt oxides in a wide range of applications. It also has relevance as a potential immobilizer of the toxic element cobalt in the natural environment, but its detailed crystal structure is so far unknown. The structure of Co 2 CO 3 (OH) 2 has now been investigated using density functional theory (DFT) simulations and powder X-ray diffraction (PXRD) measurements on samples synthesized via deposition from aqueous solution. Two possible monoclinic phases are considered, with closely related but symmetrically different crystal structures, based on those of the minerals malachite [Cu 2 CO 3 (OH) 2 ] and rosasite [Cu 1.5 Zn 0.5 CO 3 (OH) 2 ], as well as an orthorhombic phase that can be seen as a common parent structure for the two monoclinic phases, and a triclinic phase with the structure of the mineral kolwezite [Cu 1.34 Co 0.66 CO 3 (OH) 2 ]. The DFT simulations predict that the rosasite-like and malachite-like phases are two different local minima of the potential energy landscape for Co 2 CO 3 (OH) 2 and are practically degenerate in energy, while the orthorhombic and triclinic structures are unstable and experience barrierless transformations to the malachite phase upon relaxation. The best fit to the PXRD data is obtained using a rosasite model [monoclinic with space group P112 1 /n and cell parameters a = 3.1408 (4) Å, b = 12.2914 (17) Å, c = 9.3311 (16) Å and γ = 82.299 (16)°]. However, some features of the PXRD pattern are still not well accounted for by this refinement and the residual parameters are relatively poor. The relationship between the rosasite and malachite phases of Co 2 CO 3 (OH) 2 is discussed and it is shown that they can be seen as polytypes. Based on the similar calculated stabilities of these two polytypes, it is speculated that some level of stacking disorder could account for the poor

Properties of the correlated metal phase induced by electrolyte gating of insulating vanadium dioxide nanobeams

NASA Astrophysics Data System (ADS)

Singh, Sujay; Horrocks, Gregory; Marley, Peter; Banerjee, Sarbajit; Sambandamurthy, G.

2014-03-01

Vanadium oxide (VO2) undergoes a first order metal to insulator transition (MIT) and a structural phase transition (monoclinic insulator to rutile metal) near 340 K. Over the past few years, several attempts are made to trigger the MIT in VO2 using ionic liquids (IL). Parkin's group has recently showed that IL gating leads to the creation of oxygen vacancies in VO2 and stabilizes the metallic phase. Our goal is to study the electronic properties, changes in the stoichiometry and structure of this metallic phase created by oxygen vacancies. Electrical transport measurements on single crystal nanobeams show that the metallic phase has a higher resistance while IL gating is applied and results from Raman spectroscopy studies on any structural change during IL gating will be presented. The role of substitutional dopants (such as W, Mo) on the creation of oxygen vacancies and subsequent stabilization of metallic phase in IL gated experiments will also be discussed. The work is supported by NSF DMR 0847324 and 0847169.

High pressure phase transitions in lawsonite at simultaneous high pressure and temperature: A single crystal study

NASA Astrophysics Data System (ADS)

O'Bannon, E. F., III; Vennari, C.; Beavers, C. C. G.; Williams, Q. C.

2015-12-01

Lawsonite (CaAl2Si2O7(OH)2.H2O) is a hydrous mineral with a high overall water content of ~11.5 wt.%. It is a significant carrier of water in subduction zones to depths greater than ~150 km. The structure of lawsonite has been extensively studied under room temperature, high-pressure conditions. However, simultaneous high-pressure and high-temperature experiments are scarce. We have conducted synchrotron-based simultaneous high-pressure and temperature single crystal experiments on lawsonite up to a maximum pressure of 8.4 GPa at ambient and high temperatures. We used a natural sample of lawsonite from Valley Ford, California (Sonoma County). At room pressure and temperature lawsonite crystallizes in the orthorhombic system with Cmcm symmetry. Room temperature compression indicates that lawsonite remains in the orthorhombic Cmcm space group up to ~9.0 GPa. Our 5.0 GPa crystal structure is similar to the room pressure structure, and shows almost isotropic compression of the crystallographic axes. Unit cell parameters at 5.0 GPa are a- 5.7835(10), b- 8.694(2), and c- 13.009(3). Single-crystal measurements at simultaneous high-pressure and temperature (e.g., >8.0 GPa and ~100 oC) can be indexed to a monoclinic P-centered unit cell. Interestingly, a modest temperature increase of ~100 oC appears to initiate the orthorhombic to monoclinic phase transition at ~0.6-2.4 GPa lower than room temperature compression studies have shown. There is no evidence of dehydration or H atom disorder under these conditions. This suggests that the orthorhombic to monoclinic transition could be kinetically impeded at 298 K, and that monoclinic lawsonite could be the dominant water carrier through much of the depth range of upper mantle subduction processes.

High-pressure phase transitions in rare earth metal thulium to 195 GPa.

PubMed

Montgomery, Jeffrey M; Samudrala, Gopi K; Tsoi, Georgiy M; Vohra, Yogesh K

2011-04-20

We have performed image plate x-ray diffraction studies on a heavy rare earth metal, thulium (Tm), in a diamond anvil cell to a pressure of 195 GPa and volume compression V/V₀ = 0.38 at room temperature. The rare earth crystal structure sequence, hcp →Sm-type→ dhcp →fcc → distorted fcc, is observed in Tm below 70 GPa with the exception of a pure fcc phase. The focus of our study is on the ultrahigh-pressure phase transition and Rietveld refinement of crystal structures in the pressure range between 70 and 195 GPa. The hexagonal hR-24 phase is seen to describe the distorted fcc phase between 70 and 124 GPa. Above 124 ± 4 GPa, a structural transformation from hR 24 phase to a monoclinic C 2/m phase is observed with a volume change of -1.5%. The equation of state data shows rapid stiffening above the phase transition at 124 GPa and is indicative of participation of f-electrons in bonding. We compare the behavior of Tm to other heavy rare-earths and heavy actinide metals under extreme conditions of pressure.

High-pressure phase transitions in rare earth metal thulium to 195 GPa

NASA Astrophysics Data System (ADS)

Montgomery, Jeffrey M.; Samudrala, Gopi K.; Tsoi, Georgiy M.; Vohra, Yogesh K.

2011-04-01

We have performed image plate x-ray diffraction studies on a heavy rare earth metal, thulium (Tm), in a diamond anvil cell to a pressure of 195 GPa and volume compression V/Vo = 0.38 at room temperature. The rare earth crystal structure sequence, {hcp}\\to {Sm {-}type} \\to {dhcp} \\to {fcc} \\to distorted fcc, is observed in Tm below 70 GPa with the exception of a pure fcc phase. The focus of our study is on the ultrahigh-pressure phase transition and Rietveld refinement of crystal structures in the pressure range between 70 and 195 GPa. The hexagonal hR- 24 phase is seen to describe the distorted fcc phase between 70 and 124 GPa. Above 124 ± 4 GPa, a structural transformation from hR 24 phase to a monoclinic C 2/m phase is observed with a volume change of - 1.5%. The equation of state data shows rapid stiffening above the phase transition at 124 GPa and is indicative of participation of f-electrons in bonding. We compare the behavior of Tm to other heavy rare-earths and heavy actinide metals under extreme conditions of pressure.

High-pressure crystal structures of an insensitive energetic crystal: 1,1-diamino-2,2-dinitroethene

DOE PAGES

Dreger, Zbigniew A.; Stash, Adam I.; Yu, Zhi -Gang; ...

2015-12-03

Understanding the insensitivity/stability of insensitive high explosive crystals requires detailed structural information at high pressures and high temperatures of interest. Synchrotron single crystal x-ray diffraction experiments were used to determine the high-pressure structures of 1,1-diamino-2,2-dinitroethene (FOX-7), a prototypical insensitive high explosive. The phase transition around 4.5 GPa was investigated and the structures were determined at 4.27 GPa (α’-phase) and 5.9 GPa (ε-phase). The α’-phase (monoclinic, P2 1/ n), structurally indistinguishable from the ambient α-phase, transforms to the new ε-phase (triclinic, P1). The most notable features of the ε-phase, compared to the α’-phase, are: formation of planar layers and flattening ofmore » molecules. Density functional theory (DFT-D2) calculations complemented the experimental results. Furthermore, the results presented here are important for understanding the molecular and crystalline attributes governing the high-pressure insensitivity/stability of insensitive high explosive crystals.« less

High-pressure crystal structures of an insensitive energetic crystal: 1,1-diamino-2,2-dinitroethene

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

Dreger, Zbigniew A.; Stash, Adam I.; Yu, Zhi -Gang

Understanding the insensitivity/stability of insensitive high explosive crystals requires detailed structural information at high pressures and high temperatures of interest. Synchrotron single crystal x-ray diffraction experiments were used to determine the high-pressure structures of 1,1-diamino-2,2-dinitroethene (FOX-7), a prototypical insensitive high explosive. The phase transition around 4.5 GPa was investigated and the structures were determined at 4.27 GPa (α’-phase) and 5.9 GPa (ε-phase). The α’-phase (monoclinic, P2 1/ n), structurally indistinguishable from the ambient α-phase, transforms to the new ε-phase (triclinic, P1). The most notable features of the ε-phase, compared to the α’-phase, are: formation of planar layers and flattening ofmore » molecules. Density functional theory (DFT-D2) calculations complemented the experimental results. Furthermore, the results presented here are important for understanding the molecular and crystalline attributes governing the high-pressure insensitivity/stability of insensitive high explosive crystals.« less

Magnetic specific heat and structural phase transitions in (CH3)4NMnCl3 (TMMC) and TMMC:Cu2+ studied by crystal optics

NASA Astrophysics Data System (ADS)

Levola, T.; Kleemann, W.

1985-10-01

High-resolution refractive index (RI) and linear birefringence (LB) measurements are performed on the one-dimensional antiferromagnet tetramethyl ammonium manganese trichloride (TMMC) in order to reveal the temperature dependence of the magnetic short-range order. In agreement with values obtained by other methods an exchange constant J/kB=-7.3 K is reliably extracted. Anomalies of the in-plane LB and of the ordinary RI at the hexagonal-to-monoclinic structural phase transition (Tc=126 K) are successfully described with the use of linear elasto-optic response theory and the Landau approximation, which accounts for symmetry-adapted coupling between the components of the order parameter and of the spontaneous strain. Cu2+ ions, substituting Mn2+ ions of TMMC at a rate exceeding x=1.5%, are shown to stabilize an intermediate, possibly incommensurate phase. Its stability range is marked by very drastic decreases &=145 K and &=55 K for x=4.5%, respectively.

Band-to-band transitions, selection rules, effective mass, and excitonic contributions in monoclinic β -Ga2O3

NASA Astrophysics Data System (ADS)

Mock, Alyssa; Korlacki, Rafał; Briley, Chad; Darakchieva, Vanya; Monemar, Bo; Kumagai, Yoshinao; Goto, Ken; Higashiwaki, Masataka; Schubert, Mathias

2017-12-01

We employ an eigenpolarization model including the description of direction dependent excitonic effects for rendering critical point structures within the dielectric function tensor of monoclinic β -Ga2O3 yielding a comprehensive analysis of generalized ellipsometry data obtained from 0.75-9 eV. The eigenpolarization model permits complete description of the dielectric response. We obtain, for single-electron and excitonic band-to-band transitions, anisotropic critical point model parameters including their polarization vectors within the monoclinic lattice. We compare our experimental analysis with results from density functional theory calculations performed using the Gaussian-attenuation-Perdew-Burke-Ernzerhof hybrid density functional. We present and discuss the order of the fundamental direct band-to-band transitions and their polarization selection rules, the electron and hole effective mass parameters for the three lowest band-to-band transitions, and their excitonic contributions. We find that the effective masses for holes are highly anisotropic and correlate with the selection rules for the fundamental band-to-band transitions. The observed transitions are polarized close to the direction of the lowest hole effective mass for the valence band participating in the transition.

Surface crystalline phases and nanoindentation hardness of explanted zirconia femoral heads.

PubMed

Catledge, Shane A; Cook, Monique; Vohra, Yogesh K; Santos, Erick M; McClenny, Michelle D; David Moore, K

2003-10-01

One new and nine explanted zirconia femoral heads were studied using glancing angle X-ray diffraction, scanning electron microscopy, and nanoindentation hardness techniques. All starting zirconia implants consisted only of tetragonal zirconia polycrystals (TZP). For comparison, one explanted alumina femoral head was also studied. Evidence for a surface tetragonal-to-monoclinic zirconia phase transformation was observed in some implants, the extent of which was varied for different in-service conditions. A strong correlation was found between increasing transformation to the monoclinic phase and decreasing surface hardness. Microscopic investigations of some of the explanted femoral heads revealed ultra high molecular weight polyethylene and metallic transfer wear debris.

Phase transition in crystalline benzil : an infrared study of vibrational excitons.

NASA Astrophysics Data System (ADS)

Le Roy, A.; Et-Tabti, O.; Guérin, R.

1993-03-01

The molecular crystal of benzil, [C 6 H 5 CO] 2, is known to undergo a phase transition at T c = 84 K. The phase transition is from a high temperature trigonal phase with space group D 43 (P3 121) to a low temperature monoclinic phase with space group C 32 (C 2). This paper reports a study of the exciton structure of the infrared bands of benzil as a function of temperature in the vicinity of T c = 84 K. The benzil molecule belongs to the C 2 molecular point group. Group theoretical analysis of the exciton structure of infrared bands predicts two components for molecular B modes and one component for molecular A modes in the high temperature phase. Below T c all the internal modes of benzil are expected to split into two components. Our experimental results show that the A molecular modes are resolved in a doublet structure in the low temperature phase whereas only one component is observed above T c. The doublet structure of infrared bands is studied as a function of temperature in the vicinity of T c. These splittings of crystal states in the low temperature phase are found to be described by a ¦T c - T¦ β law. The temperature dependence of the doublet structure of internal B modes is also studied below and above T c.

[Effect of pigmentation on the strength of dental Y-TZP/porcelain bilayered structure].

PubMed

Ma, Ting-ting; Yi, Yuan-fu; Shao, Long-quan; Liu, Hong-chen; Tian, Jie-mo; Hou, Kang-lin; Zhang, Wei-wei; Wen, Ning; Deng, Bin

2010-05-01

To investigate the effect of two methods of pigmentation on the flexural strength of dental Y-TZP/porcelain layered structure. KaVo zirconia substructures were pigmented by dipping presintered blocks in the coloring solution VITA LL1 and LL5, and colored TZ-3YS zirconia substructures were fabricated by adding pigments before isostatic pressing. The colors No.1 and No.5 were used for the test. The specimens were made in monolithic or bilayered forms, and the flexural strength was tested. XRD and SEM with EDX were used to analyze the characteristics of the surface structure. In KaVo group, no significant differences were found in the flexural strength between white and LL1 and LL5 colored monoclinic materials, nor in bilayered structures. While in TZ-3YS group, significant differences were noted in the flexural strength between color No.5 white and color No.1 monoclinic materials, but not between the latter two subgroups. The flexural strength was significantly lowered by veneering with porcelain in both zirconia groups, and similar findings were observed with the monoclinic materials. Only the tetragonal phase was detected in both of the zirconia groups. Pigmentation has no apparent effects on the bonding strength between the veneering porcelain and zirconia. Both coloring methods are appropriate when the concentration of the pigments is under deliberate control.

Crystal structure stability and electronic properties of the layered nickelate La4Ni3O10

NASA Astrophysics Data System (ADS)

Puggioni, Danilo; Rondinelli, James M.

2018-03-01

We investigate the crystal structure and the electronic properties of the trilayer nickelate La4Ni3O10 by means of quantum-mechanical calculations in the framework of the density-functional theory. We find that, at low temperature, La4Ni3O10 undergoes a hitherto unreported structural phase transition and transforms to a new monoclinic P 21/a phase. This phase exhibits electronic properties in agreement with recent angle-resolved photoemission spectroscopy data reported in H. Li et al., [Nat. Commun. 8, 704 (2017), 10.1038/s41467-017-00777-0] and should be considered in models focused on explaining the observed ˜140 K metal-to-metal phase transition.

New insights into the structure, chemistry, and properties of Cu 4SnS 4

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

Choudhury, Amitava; Mohapatra, Sudip; Yaghoobnejad Asl, Hooman

The ambient temperature structure of Cu 4SnS 4 has been revisited and the recently reported low temperature structure has been confirmed from single-crystal X-ray diffraction data. A structural phase transition from a large monoclinic unit cell at low temperature to a smaller orthorhombic unit cell at high temperature has been observed. The room temperature phase exhibited disorder in the two copper sites, which is a different finding from earlier reports. The low temperature monoclinic form crystallizes in P2 1/c space group, which is isostructural with Cu 4GeS 4. The phase transition has also been studied with variable temperature powder X-raymore » diffraction and 119Sn Mössbauer spectroscopy. The Seebeck coefficients and electrical resistivity of polycrystalline Cu 4SnS 4 are reported from 16 to 400 K on hot pressed pellets. Thermal conductivity measurements at high temperatures, 350 – 750 K exhibited very low thermal conductivities in the range 0.28 – 0.35 W K –1 m –1. In all the transport measurements the phase transition has been observed at around 232 K. Resistivity decreases, while Seebeck coefficient increases after the phase transition during warming up from low to high temperatures. This change in resistivity has been correlated with the results of first-principles electronic band structure calculations using highly-accurate screened-exchange local density approximation. It was found that both the low hole effective mass of 0.63 me for the Γ→Y crystallographic direction and small band gap, 0.49 eV, are likely to contribute to the observed higher conductivity of the orthorhombic phase. Cu 4SnS 4 is also electrochemically active and shows reversible reaction with lithium between 1.7 and 3.5 volts.« less

New insights into the structure, chemistry, and properties of Cu 4SnS 4

DOE PAGES

Choudhury, Amitava; Mohapatra, Sudip; Yaghoobnejad Asl, Hooman; ...

2017-05-25

The ambient temperature structure of Cu 4SnS 4 has been revisited and the recently reported low temperature structure has been confirmed from single-crystal X-ray diffraction data. A structural phase transition from a large monoclinic unit cell at low temperature to a smaller orthorhombic unit cell at high temperature has been observed. The room temperature phase exhibited disorder in the two copper sites, which is a different finding from earlier reports. The low temperature monoclinic form crystallizes in P2 1/c space group, which is isostructural with Cu 4GeS 4. The phase transition has also been studied with variable temperature powder X-raymore » diffraction and 119Sn Mössbauer spectroscopy. The Seebeck coefficients and electrical resistivity of polycrystalline Cu 4SnS 4 are reported from 16 to 400 K on hot pressed pellets. Thermal conductivity measurements at high temperatures, 350 – 750 K exhibited very low thermal conductivities in the range 0.28 – 0.35 W K –1 m –1. In all the transport measurements the phase transition has been observed at around 232 K. Resistivity decreases, while Seebeck coefficient increases after the phase transition during warming up from low to high temperatures. This change in resistivity has been correlated with the results of first-principles electronic band structure calculations using highly-accurate screened-exchange local density approximation. It was found that both the low hole effective mass of 0.63 me for the Γ→Y crystallographic direction and small band gap, 0.49 eV, are likely to contribute to the observed higher conductivity of the orthorhombic phase. Cu 4SnS 4 is also electrochemically active and shows reversible reaction with lithium between 1.7 and 3.5 volts.« less

The optical gap in VO2 insulating phases is dominated by Coulomb repulsion

NASA Astrophysics Data System (ADS)

Hendriks, Christopher; Walter, Eric; Krakauer, Henry; Huffman, Tyler; Qazilbash, Mumtaz

Under doping, tensile strain or heating, vanadium dioxide (VO2) transforms from an insulating monoclinic (M1) to a metallic rutile (R) phase, progressing through intermediate insulating triclinic (T) and magnetic (M2) phases. Broadband optical spectroscopy data have been obtained on the T and M2 phases in the same sample. While only half the V atoms are dimerized in M2 compared to M1 and T, the measured optical gap is essentially unaltered by the first-order structural phase transition between them. Moreover, the optical interband features in the T and M2 phases are remarkably similar to those previously observed in the well-studied M1 phase. This shows that the electronic structure is insensitive to the lattice structure. Our ab-initio HSE optical conductivity calculations on the insulating phases of VO2 are in excellent agreement with the experimental measurements. We will discuss the choice of α, the fraction of exact exchange. As the energy gap is insensitive to the different lattice structures of the three insulating phases, we rule out Peierls effects as the dominant contributor to the opening of the gap. Rather, the energy gap arises from intra-atomic Coulomb correlations. Supported by ONR.

Bis[2-(hy-droxy-imino-meth-yl)phenolato]nickel(II): a second monoclinic polymorph.

PubMed

Rusanova, Julia A; Buvaylo, Elena A; Rusanov, Eduard B

2011-01-15

The title compound, [Ni(C(7)H(6)NO(2))(2)], (I), is a second monoclinic polymorph of the compound, (II), reported by Srivastava et al. [Acta Cryst. (1967), 22, 922] and Mereiter [Private communication (2002) CCDC refcode NISALO01]. The bond lengths and angles are similar in both structures. The mol-ecule in both structures lies on a crystallographic inversion center and both have an inter-nal hydrogen bond. The title compound crystallizes in the space group P2(1)/c (Z = 2), whereas compound (II) is in the space group P2(1)/n (Z = 2) with a similar cell volume but different cell parameters. In both polymorphs, mol-ecules are arranged in the layers but in contrast to the previously published compound (II) where the dihedral angle between the layers is 86.3°, in the title polymorph the same dihedral angle is 29.4°. The structure of (I) is stabilized by strong intra-molecular O-H⋯O hydrogen bonding between the O-H group and the phenolate O atom.

Phase stability and mechanical properties of Mo1-xNx with 0 ≤ x ≤ 1

NASA Astrophysics Data System (ADS)

Balasubramanian, Karthik; Huang, Liping; Gall, Daniel

2017-11-01

First-principle density-functional calculations coupled with the USPEX evolutionary phase-search algorithm are employed to calculate the convex hull of the Mo-N binary system. Eight molybdenum nitride compound phases are found to be thermodynamically stable: tetragonal β-Mo3N, hexagonal δ-Mo3N2, cubic γ-Mo11N8, orthorhombic ɛ-Mo4N3, cubic γ-Mo14N11, monoclinic σ-MoN and σ-Mo2N3, and hexagonal δ-MoN2. The convex hull is a straight line for 0 ≤ x ≤ 0.44 such that bcc Mo and the five listed compound phases with x ≤ 0.44 are predicted to co-exist in thermodynamic equilibrium. Comparing the convex hulls of cubic and hexagonal Mo1-xNx indicates that cubic structures are preferred for molybdenum rich (x < 0.3) compounds, and hexagonal phases are favored for nitrogen rich (x > 0.5) compositions, while similar formation enthalpies for cubic and hexagonal phases at intermediate x = 0.3-0.5 imply that kinetic factors play a crucial role in the phase formation. The volume per atom Vo of the thermodynamically stable Mo1-xNx phases decreases from 13.17 to 9.56 Å3 as x increases from 0.25 to 0.67, with plateaus at Vo = 11.59 Å3 for hexagonal and cubic phases and Vo = 10.95 Å3 for orthorhombic and monoclinic phases. The plateaus are attributed to the changes in the average coordination numbers of molybdenum and nitrogen atoms, which increase from 2 to 6 and decrease from 6 to 4, respectively, indicating an increasing covalent bonding character with increasing x. The change in bonding character and the associated phase change from hexagonal to cubic/orthorhombic to monoclinic cause steep increases in the isotropic elastic modulus E = 387-487 GPa, the shear modulus G = 150-196 GPa, and the hardness H = 14-24 GPa in the relatively narrow composition range x = 0.4-0.5. This also causes a drop in Poisson's ratio from 0.29 to 0.24 and an increase in Pugh's ratio from 0.49 to 0.64, indicating a ductile-to-brittle transition between x = 0.44 and 0.5.

Monoclinic distortion and magnetic coupling in the double perovskite Sr{sub 2−x}Ca{sub x}YRuO{sub 6}

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

Bernardo, P.L.; Ghivelder, L.; Eslava, G.G.

2014-12-15

Abstracts: This work investigates in the insulating double perovskites Sr{sub 2−x}Ca{sub x}YRuO{sub 6}. We address the angular dependence of the strength of the magnetic coupling due to the deviation from planarity of the basal layers of the monoclinic structure, characterized by the in-plane angle α<180°, in order to probe the impact of the structural distortions in the magnetic properties of the compounds. High resolution x-ray powder diffraction, susceptibility, and specific heat measurements were performed. The deviation from planarity significantly increases (α=144° for x=1) while the bond distances vary in a complex way as a consequence of the strong monoclinic distortion.more » We found that the magnetic transition temperature, T{sub M}, shows a linear dependence on cos [(π−α)/2]. This result is discussed in terms of t{sub 2g}(π)–e{sub g}(σ) mixing of the magnetic orbitals of the Ru{sup 5+} ions and unbalanced competitive super-exchange interactions. The deleterious effect of Ca doping for the magnetic coupling is confirmed by the reduction in the short-range antiferromagnetic correlations characteristic of the parent compound at T>>T{sub M} and the enhancement of magnetic frustration for T« less

Ultrafast diffraction conoscopy of the structural phase transition in VO2: Evidence of two lattice distortions

NASA Astrophysics Data System (ADS)

Kumar, Nardeep; Rúa, Armando; Fernández, Félix E.; Lysenko, Sergiy

2017-06-01

Photoinduced phase transitions in complex correlated systems occur very rapidly and involve the interplay between various electronic and lattice degrees of freedom. For these materials to be considered for practical applications, it is important to discover how their phase transitions take place. Here we use a novel ultrafast diffraction conoscopy technique to study the evolution of vanadium dioxide (VO2) from biaxial to uniaxial symmetry. A key finding in this study is an additional relaxation process through which the phase transition takes place. Our results show that the biaxial monoclinic crystal initially, within the first 100-300 fs, transforms to a transient biaxial crystal, and within the next 300-400 fs converts into a uniaxial rutile crystal. The characteristic times for these transitions depend on film morphology and are presumably altered by misfit strain. We take advantage of Landau phenomenology to describe the complex dynamics of VO2 phase transition in the femtosecond regime.

Layered-structural monoclinic–orthorhombic perovskite La{sub 2}Ti{sub 2}O{sub 7} to orthorhombic LaTiO{sub 3} phase transition and their microstructure characterization

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

Herrera, G., E-mail: manuel.herrera@enp.unam.mx; Departamento de Química Inorgánica, Universidad de Valencia, 46100 Burjasot, Valencia; Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, 04510 México D. F.

2014-03-01

The layered-structural ceramics, such as lanthanum titanate (La{sub 2}Ti{sub 2}O{sub 7}), have been known for their good temperature and low dielectric loss at microwave frequencies that make them good candidate materials for high frequency applications. However, few studies have been conducted on the synthesis optimization by sol gel reaction, in particular by acrylamide polymerization route. The interest in La{sub 2}Ti{sub 2}O{sub 7} ceramic has been greatly increased recently due to the effect of oriented grains. This anisotropy of the microstructure leads to anisotropy in dielectric, electrical and mechanical properties. In this study, grain oriented lanthanum titanate was produced by themore » sol–gel acrylamide polymerization route. The characterizations of the samples were achieved by thermal analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). X-ray diffraction indicates that the formation of monoclinic perovskite La{sub 2}Ti{sub 2}O{sub 7} nanocrystals is a necessary first step to obtain orthorhombic LaTiO{sub 3} nanocomposites (with space group Pbnm). In this work we identified that the monoclinic perovskite La{sub 2}Ti{sub 2}O{sub 7} with space group P2{sub 1} transforms its structure into one with the orthorhombic space group Cmc2{sub 1} at approximately 1073 K. The microstructure associated consisted of flaky monoclinic La{sub 2}Ti{sub 2}O{sub 7} nanocomposites in comparison with round-shaped LaTiO{sub 3} nanocomposites. - Highlights: • The flaky-like La{sub 2}Ti{sub 2}O{sub 7} compound was synthesized by sol–gel acrylamide route. • Simultaneous monitoring of the DTA and XRD with temperature was performed. • Phase transformation characterization of La{sub 2}Ti{sub 2}O{sub 7} has been carried out. • The variation of the La{sub 2}Ti{sub 2}O{sub 7} and LaTiO{sub 3} grain morphology has been compared.« less

Structures, thermal expansion properties and phase transitions of Er{sub x}Fe{sub 2-x}(MoO{sub 4}){sub 3} (0.0 {le} x {le} 2.0).

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

Cheng, Y. Z.; Wu, M. M.; Peng, J.

2007-05-03

Structures, thermal expansion properties and phase transitions of Er{sub x}Fe{sub 2-x}(MoO{sub 4}){sub 3} (0.0 {le} x {le} 2.0) have been investigated by X-ray diffraction and differential thermal analysis. The partial substitution of Er{sup 3+} for Fe{sup 3+} induces pronounced decreases in the phase transition temperature from monoclinic to orthorhombic structure. Rietveld analysis of the XRD data shows that both the monoclinic and orthorhombic Fe{sub 2}(MoO{sub 4}){sub 3}, as well as the orthorhombic Er{sub x}Fe{sub 2-x}(MoO{sub 4}){sub 3} (x {le} 0.8) have positive thermal expansion coefficients. However, the linear thermal expansion coefficients of Er{sub x}Fe{sub 2-x}(MoO{sub 4}){sub 3} (x = 0.6-2.0)more » decrease with increasing content of Er{sup 3+} and for x {ge} 1.0, compounds Er{sub x}Fe{sub 2-x}(MoO{sub 4}){sub 3} show negative thermal expansion properties. Attempts for making zero thermal expansion coefficient materials result in that very low negative thermal expansion coefficient of -0.60 x 10{sup -6} C in Er{sub 1.0}Fe{sub 1.0}(MoO{sub 4}){sub 3} is observed in the temperature range of 180-400 C, and zero thermal expansion is observed in Er{sub 0.8}Fe{sub 1.2}(MoO{sub 4}){sub 3} in the temperature range of 350-450 C. In addition, anisotropic thermal expansions are found for all the orthorhombic Er{sub x}Fe{sub 2-x}(MoO{sub 4}){sub 3} compounds, with negative thermal expansion coefficients along the a axes.« less

Imaging the Dynamics of the Ferroelectric Stripe Phase Near a Field-Driven Phase Transition in Bismuth Ferrite

NASA Astrophysics Data System (ADS)

Laanait, Nouamane; Li, Qian; Zhang, Zhan; Kalinin, Sergei

Electric field-driven phase transitions in multiferroic systems such as Bismuth Ferrite could potentially host interesting domain dynamics due to the coexistence of multiple order parameters. Structural imaging of these dynamics under a host of elastic and electric boundary conditions is therefore of interest. Here, we present X-ray diffraction microscopy (XDM) studies of the domain wall dynamics in a bismuth ferrite thin-film near the field-driven transition from rhombohedral to monoclinic (R to M). XDM is a novel full-field imaging technique that uses Bragg diffraction contrast to image structural configurations with sub-100nm lateral resolutions and fast acquisition times (milliseconds to seconds per image). We find that under electric fields 100 kV/cm, a bismuth ferrite thin-film (100 nm BiFeO3/DyScO3 (110)) undergoes a structural phase transition but that this new phase (M) is pinned by the preexisting ferroelectric/ferroelastic stripe phase (R). At higher fields ( 300 kV/cm), we observe unusually slow domain wall dynamics in the stripe phase, consisting of periodicity doubling, domain wall roughening and crowding. These observed ferroelastic domain wall spatial dynamics are weakly constrained by the crystal symmetry of the orthorhombic substrate but exhibit nonlinear dynamics more commonly associated with disordered nematic systems. This work was supported by the Eugene P. Wigner Fellowship program at Oak Ridge National Laboratory, a U.S. Department of Energy facility.

Pressure-induced phase transitions of β-type pyrochlore CsTaWO 6

DOE PAGES

Zhang, F. X.; Tracy, C. L.; Shamblin, J.; ...

2016-09-30

The β-type pyrochlore CsTaWO 6 was studied by synchrotron X-ray diffraction (XRD) and Raman scattering methods up to pressures of 43 GPa using a diamond anvil cell (DAC). With increasing pressure, the cubic pyrochlore in space group of Fd-3¯m with combining macron]m transforms to an orthorhombic structure (space group: Pnma) at 5.9 GPa and then to a monoclinic structure (space group: P2 1/c) at ~18 GPa. The structural evolution in CsTaWO 6 is a continuous process and experimental results suggest that the initial cubic phase has a tetragonal distortion at ambient conditions. Both XRD and Raman measurements indicate that themore » pressure-induced phase transitions in CsTaWO 6 are reversible. Lastly, these results may provide a structural explanation of previous experimental resistivity measurement results for the isostructural superconductor K(Cs)Os 2O 6 at high pressure conditions.« less

Pressure-induced phase transitions of β-type pyrochlore CsTaWO 6

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

Zhang, F. X.; Tracy, C. L.; Shamblin, J.

The β-type pyrochlore CsTaWO 6 was studied by synchrotron X-ray diffraction (XRD) and Raman scattering methods up to pressures of 43 GPa using a diamond anvil cell (DAC). With increasing pressure, the cubic pyrochlore in space group of Fd-3¯m with combining macron]m transforms to an orthorhombic structure (space group: Pnma) at 5.9 GPa and then to a monoclinic structure (space group: P2 1/c) at ~18 GPa. The structural evolution in CsTaWO 6 is a continuous process and experimental results suggest that the initial cubic phase has a tetragonal distortion at ambient conditions. Both XRD and Raman measurements indicate that themore » pressure-induced phase transitions in CsTaWO 6 are reversible. Lastly, these results may provide a structural explanation of previous experimental resistivity measurement results for the isostructural superconductor K(Cs)Os 2O 6 at high pressure conditions.« less

Structural Studies of dielectric HDPE+ZrO2 polymer nanocomposites: filler concentration dependences

NASA Astrophysics Data System (ADS)

Nabiyev, A. A.; Islamov, A. Kh; Maharramov, A. M.; Nuriyev, M. A.; Ismayilova, R. S.; Doroshkevic, A. S.; Pawlukojc, A.; Turchenko, V. A.; Olejniczak, A.; Rulev, M. İ.; Almasan, V.; Kuklin, A. I.

2018-03-01

Structural properties of HDPE+ZrO2 polymer nanocomposites thin films of 80-100μm thicknesses were investigated using SANS, XRD, Laser Raman and FTIR spectroscopy. The mass fraction of the filler was 1, 3, 10, and 20%. Results of XRD analysis showed that ZrO2 powder was crystallized both in monoclinic and in cubic phase under normal conditions. The percentages of monoclinic and cubic phase were found to be 99.8% and 0.2%, respectively. It was found that ZrO2 nanoparticles did not affect the main crystal and chemical structure of HDPE, but the degree of crystallinity of the polymer decreases with increasing concentration of zirconium oxide. SANS experiments showed that at ambient conditions ZrO2 nanoparticles mainly distributed like mono-particles in the polymer matrix at all concentrations of filler.The structure of HDPE+ZrO2 does not changes up to 132°C at 1-3% of filler, excepting changing of the polymer structure at temperatures upper 82°C. At high concentrations of filler 10-20% the aggregation of ZrO2 nanoparticles occurs, forming domains of 2.5μm. The results of Raman and FTIR spectroscopy did not show additional specific chemical bonds between the filler and the polymer matrix. New peaks formation was not observed. These results suggest that core-shell structure does not exist in the polymer nanocomposite system.

Interplay of phase sequence and electronic structure in the modulated martensites of Mn2NiGa from first-principles calculations

NASA Astrophysics Data System (ADS)

Kundu, Ashis; Gruner, Markus E.; Siewert, Mario; Hucht, Alfred; Entel, Peter; Ghosh, Subhradip

2017-08-01

We investigate the relative stability, structural properties, and electronic structure of various modulated martensites of the magnetic shape memory alloy Mn2NiGa by means of density functional theory. We observe that the instability in the high-temperature cubic structure first drives the system to a structure where modulation shuffles with a period of six atomic planes are taken into account. The driving mechanism for this instability is found to be the nesting of the minority band Fermi surface, in a similar way to that established for the prototype system Ni2MnGa . In agreement with experiments, we find 14M modulated structures with orthorhombic and monoclinic symmetries having energies lower than other modulated phases with the same symmetry. In addition, we also find energetically favorable 10M modulated structures which have not been observed experimentally for this system yet. The relative stability of various martensites is explained in terms of changes in the electronic structures near the Fermi level, affected mostly by the hybridization of Ni and Mn states. Our results indicate that the maximum achievable magnetic field-induced strain in Mn2NiGa would be larger than in Ni2MnGa . However, the energy costs for creating nanoscale adaptive twin boundaries are found to be one order of magnitude higher than that in Ni2MnGa .

Identification of monoclinic calcium pyrophosphate dihydrate and hydroxyapatite in human sclera using Raman microspectroscopy.

PubMed

Chen, Ko-Hua; Li, Mei-Jane; Cheng, Wen-Ting; Balic-Zunic, Tonci; Lin, Shan-Yang

2009-02-01

Raman microspectroscopy was first used to determine the composition of a calcified plaque located at the pterygium-excision site of a 51-year-old female patient's left nasal sclera after surgery. It was unexpectedly found that the Raman spectrum of the calcified sample at 1149, 1108, 1049, 756, 517, 376 and 352/cm was similar to the Raman spectrum of monoclinic form of calcium pyrophosphate dihydrate (CPPD) crystal, but differed from the Raman spectrum of triclinic form of CPPD. An additional peak at 958/cm was also observed in the Raman spectrum of the calcified plaque, which was identical to the characteristic peak at 958/cm of hydroxyapatite (HA). This is the first study to report the spectral biodiagnosis of both monoclinic CPPD and HA co-deposited in the calcified plaque of a patient with sclera dystrophic calcification using Raman microspectroscopy.

Identification of monoclinic calcium pyrophosphate dihydrate and hydroxyapatite in human sclera using Raman microspectroscopy

PubMed Central

Chen, Ko-Hua; Li, Mei-Jane; Cheng, Wen-Ting; Balic-Zunic, Tonci; Lin, Shan-Yang

2009-01-01

Raman microspectroscopy was first used to determine the composition of a calcified plaque located at the pterygium-excision site of a 51-year-old female patient's left nasal sclera after surgery. It was unexpectedly found that the Raman spectrum of the calcified sample at 1149, 1108, 1049, 756, 517, 376 and 352/cm was similar to the Raman spectrum of monoclinic form of calcium pyrophosphate dihydrate (CPPD) crystal, but differed from the Raman spectrum of triclinic form of CPPD. An additional peak at 958/cm was also observed in the Raman spectrum of the calcified plaque, which was identical to the characteristic peak at 958/cm of hydroxyapatite (HA). This is the first study to report the spectral biodiagnosis of both monoclinic CPPD and HA co-deposited in the calcified plaque of a patient with sclera dystrophic calcification using Raman microspectroscopy. PMID:19200254

High pressure phase transitions in the rare earth metal erbium to 151 GPa.

PubMed

Samudrala, Gopi K; Thomas, Sarah A; Montgomery, Jeffrey M; Vohra, Yogesh K

2011-08-10

High pressure x-ray diffraction studies have been performed on the heavy rare earth metal erbium (Er) in a diamond anvil cell at room temperature to a pressure of 151 GPa and Er has been compressed to 40% of its initial volume. The rare earth crystal structure sequence hcp → Sm type → dhcp → distorted fcc (hcp: hexagonal close packed; fcc: face centered cubic; dhcp: double hcp) is observed in Er below 58 GPa. We have carried out Rietveld refinement of crystal structures in the pressure range between 58 GPa and 151 GPa. We have examined various crystal structures that have been proposed for the distorted fcc (dfcc) phase and the post-dfcc phase in rare earth metals. We find that the hexagonal hR 24 structure is the best fit between 58 and 118 GPa. Above 118 GPa, a structural transformation from hR 24 phase to a monoclinic C 2/m phase is observed with a volume change of - 1.9%. We have also established a clear trend for the pressure at which a post-dfcc phase is formed in rare earth metals and show that there is a monotonic increase in this pressure with the filling of 4f shell.

High pressure phase transitions in the rare earth metal erbium to 151 GPa

NASA Astrophysics Data System (ADS)

Samudrala, Gopi K.; Thomas, Sarah A.; Montgomery, Jeffrey M.; Vohra, Yogesh K.

2011-08-01

High pressure x-ray diffraction studies have been performed on the heavy rare earth metal erbium (Er) in a diamond anvil cell at room temperature to a pressure of 151 GPa and Er has been compressed to 40% of its initial volume. The rare earth crystal structure sequence {hcp} \\to {Sm}~ {type} \\to {dhcp} \\to {distorted} fcc (hcp: hexagonal close packed; fcc: face centered cubic; dhcp: double hcp) is observed in Er below 58 GPa. We have carried out Rietveld refinement of crystal structures in the pressure range between 58 GPa and 151 GPa. We have examined various crystal structures that have been proposed for the distorted fcc (dfcc) phase and the post-dfcc phase in rare earth metals. We find that the hexagonal hR 24 structure is the best fit between 58 and 118 GPa. Above 118 GPa, a structural transformation from hR 24 phase to a monoclinic C 2/m phase is observed with a volume change of - 1.9%. We have also established a clear trend for the pressure at which a post-dfcc phase is formed in rare earth metals and show that there is a monotonic increase in this pressure with the filling of 4f shell.

Domain wall and interphase boundary motion in (1-x)Bi(Mg 0.5 Ti 0.5 )O 3 –xPbTiO 3 near the morphotropic phase boundary

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

Tutuncu, Goknur; Chen, Jun; Fan, Longlong

Electric field-induced changes in the domain wall motion of (1-x)Bi(Mg 0.5Ti 0.5)O 3–xPbTiO 3 (BMT-xPT) near the morphotropic phase boundary (MPB) where x = 0.37 (BMT-37PT) and x =0.38 (BMT-38PT), are studied by means of synchrotron x-ray diffraction. Through Rietveld analysis and profile fitting, a mixture of coexisting monoclinic (Cm) and tetragonal (P4mm) phases is identified at room temperature. Extrinsic contributions to the property coefficients are evident from electric-field-induced domain wall motion in both the tetragonal and monoclinic phases, as well as through the interphase boundary motion between the two phases. Domain wall motion in the tetragonal and monoclinic phasesmore » for BMT-37PT is larger than that of BMT-38PT, possibly due to this composition's closer proximity to the MPB. Increased interphase boundary motion was also observed in BMT-37PT. Lattice strain, which is a function of both intrinsic piezoelectric strain and elastic interactions of the grains (the latter originating from domain wall and interphase boundary motion), is similar for the respective tetragonal and monoclinic phases.« less

Pressure induced phase transitions in ceramic compounds containing tetragonal zirconia

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

Sparks, R.G.; Pfeiffer, G.; Paesler, M.A.

Stabilized tetragonal zirconia compounds exhibit a transformation toughening process in which stress applied to the material induces a crystallographic phase transition. The phase transition is accompanied by a volume expansion in the stressed region thereby dissipating stress and increasing the fracture strength of the material. The hydrostatic component of the stress required to induce the phase transition can be investigated by the use of a high pressure technique in combination with Micro-Raman spectroscopy. The intensity of Raman lines characteristic for the crystallographic phases can be used to calculate the amount of material that has undergone the transition as a functionmore » of pressure. It was found that pressures on the order of 2-5 kBar were sufficient to produce an almost complete transition from the original tetragonal to the less dense monoclinic phase; while a further increase in pressure caused a gradual reversal of the transition back to the original tetragonal structure.« less

Selective scanning tunneling microscope light emission from rutile phase of VO2.

PubMed

Sakai, Joe; Kuwahara, Masashi; Hotsuki, Masaki; Katano, Satoshi; Uehara, Yoichi

2016-09-28

We observed scanning tunneling microscope light emission (STM-LE) induced by a tunneling current at the gap between an Ag tip and a VO2 thin film, in parallel to scanning tunneling spectroscopy (STS) profiles. The 34 nm thick VO2 film grown on a rutile TiO2 (0 0 1) substrate consisted of both rutile (R)- and monoclinic (M)-structure phases of a few 10 nm-sized domains at room temperature. We found that STM-LE with a certain photon energy of 2.0 eV occurs selectively from R-phase domains of VO2, while no STM-LE was observed from M-phase. The mechanism of STM-LE from R-phase VO2 was determined to be an interband transition process rather than inverse photoemission or inelastic tunneling processes.

Why the Heyd-Scuseria-Ernzerhof hybrid functional description of VO2 phases is not correct

NASA Astrophysics Data System (ADS)

Grau-Crespo, Ricardo; Wang, Hao; Schwingenschlögl, Udo

2012-08-01

In contrast with recent claims that the Heyd-Scuseria-Ernzerhof (HSE) screened hybrid functional can provide a good description of the electronic and magnetic structures of VO2 phases [Eyert, Phys. Rev. Lett.PRLTAO0031-900710.1103/PhysRevLett.107.016401 107, 016401 (2011)], we show here that the HSE lowest-energy solutions for both the low-temperature monoclinic (M1) phase and the high-temperature rutile (R) phase, which are obtained upon inclusion of spin polarization, are at odds with experimental observations. For the M1 phase the ground state is (but should not be) magnetic, while the ground state of the R phase, which is also spin polarized, is not (but should be) metallic. The energy difference between the low-temperature and high-temperature phases has strong discrepancies with the experimental latent heat.

Instability induced by orthopyroxene phase transformation and implications for deep earthquakes below 300 km depth

NASA Astrophysics Data System (ADS)

Shi, F.; Wang, Y.; Zhang, J.; Yu, T.; Zhu, L.

2017-12-01

Global earthquake occurrence rate falls exponentially from the surface to 300 km depth, and then peaks again near 500 km depth. Unassisted frictional sliding will not function at depth below the brittle-ductile transition depth (10-15 km) because increasing pressure trends to inhibit frictional sliding and increasing temperature promotes ductile flow. Two main hypotheses have been proposed and demonstrated in the laboratory for the generation of earthquakes at depth, including dehydration embrittlement (e.g., Rayleigh and Paterson, 1965) for intermediate-depth (70-300 km) earthquakes, metastable olivine phase transformation induced anticrack faulting (e.g., Green and Burnley, 1989) for deep-focus (410-660 km) earthquakes. However, the possibility of earthquake generation by pyroxene phase transformation, another important constituent mineral in the upper mantle and transition zone has never been explored in the laboratory. Here we report axial deformation experiments on hypersthene [(Mg,Fe)SiO3], which has the same structure as enstatite, with the phase transformation to high-pressure monoclinic phase (same structure as the high-pressure clinoenstatite) occurring at lower pressures, in a deformation-DIA (D-DIA) apparatus interfaced with an acoustic emission (AE) monitoring system. Our results show that hypersthene deformed within its stability field (<2GPa and 1000 oC) behaves in a ductile manner without any AE activity. In contrast, numerous AE events were observed during the deformation of metastable hyposthene in its high pressure monoclinic phase field (>5GPa, 1000-1300 oC). This finding provides an additional viable mechanism for earthquakes at depths >300km and moonquakes at 700 - 1200 km depths. Reference: Barcheck, C. Grace, et al. EPSL,349 (2012): 153-160;van Keken, Peter E., et al.JGR,116.B1 (2011);Green II, H. W., and P. C. Burnley. Nature 341.6244 (1989): 733-737.

Phase Transformation and Lattice Parameter Changes of Non-trivalent Rare Earth-Doped YSZ as a Function of Temperature

NASA Astrophysics Data System (ADS)

Jiang, Shengli; Huang, Xiao; He, Zhang; Buyers, Andrew

2018-01-01

To examine the effect of doping/co-doping on high-temperature phase compositions of YSZ, stand-alone YSZ and CeO2 and Nb2O5 co-doped YSZ samples were prepared using mechanical alloy and high-temperature sintering. XRD analysis was performed on these samples from room temperature to 1100 °C. The results show that the structure for the co-doped samples tends to be thermally stable when the test temperature is higher than a critical value. Monoclinic phase was dominant in Nb2O5 co-doped YSZ at temperatures lower than 600 °C, while for the YSZ and CeO2 co-doped YSZ, cubic/tetragonal phase was dominant in the whole test temperature range. The lattice parameters for all the samples increase with increasing test temperature generally. The lattice parameters for the two non-trivalent rare earth oxides co-doped YSZ show that the lattice parameter a for the cubic phase of the Ce4+ co-doped YSZ is consistently greater than that of 7YSZ which is related to the presence of larger radius of Ce4+ in the matrix. The lattice parameters a, b, c for the monoclinic phase of Ce4+ co-doped YSZ are much closer to each other than that of the Nb5+ co-doped YSZ, indicating the former has better tendency to form cubic/tetragonal phase, which is desired for vast engineering applications.

Phase Transformation and Lattice Parameter Changes of Non-trivalent Rare Earth-Doped YSZ as a Function of Temperature

NASA Astrophysics Data System (ADS)

Jiang, Shengli; Huang, Xiao; He, Zhang; Buyers, Andrew

2018-05-01

To examine the effect of doping/co-doping on high-temperature phase compositions of YSZ, stand-alone YSZ and CeO2 and Nb2O5 co-doped YSZ samples were prepared using mechanical alloy and high-temperature sintering. XRD analysis was performed on these samples from room temperature to 1100 °C. The results show that the structure for the co-doped samples tends to be thermally stable when the test temperature is higher than a critical value. Monoclinic phase was dominant in Nb2O5 co-doped YSZ at temperatures lower than 600 °C, while for the YSZ and CeO2 co-doped YSZ, cubic/tetragonal phase was dominant in the whole test temperature range. The lattice parameters for all the samples increase with increasing test temperature generally. The lattice parameters for the two non-trivalent rare earth oxides co-doped YSZ show that the lattice parameter a for the cubic phase of the Ce4+ co-doped YSZ is consistently greater than that of 7YSZ which is related to the presence of larger radius of Ce4+ in the matrix. The lattice parameters a, b, c for the monoclinic phase of Ce4+ co-doped YSZ are much closer to each other than that of the Nb5+ co-doped YSZ, indicating the former has better tendency to form cubic/tetragonal phase, which is desired for vast engineering applications.

Magnetostructural phase transitions and magnetocaloric effect in (Gd 5-xSc x)Si 1.8Ge 2.2

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

Rudolph, Kirk; Pathak, Arjun K.; Mudryk, Yaroslav

Future advancements in magnetocaloric refrigeration/heat pumping technologies depend on the discovery of new materials that demonstrate large, tunable magnetocaloric effects (MCEs) in the vicinity of coupled magnetic and structural phase transitions that occur reversibly with minimum hysteresis. Here, with this in mind, we investigate phase transitions, microstructure, magnetic, thermal, magnetocaloric, and transport properties of (Gd 5-xSc x)Si 1.8Ge 2.2 compounds. Replacement of magnetic Gd with non-magnetic Sc in Gd 5-xSc xSi 1.8Ge 2.2 increases the ferromagnetic to paramagnetic first order phase transition temperature, T C, with only a minor reduction in MCE when x ≤ 0.2. We also demonstrate thatmore » hydrostatic pressure further increases T C and reduces the hysteresis of the first order phase transition in Gd 4.8Sc 0.2Si 1.8Ge 2.2 from 7 to 4 K. Temperature-dependent x-ray powder diffraction study of Gd 4.8Sc 0.2Si 1.8Ge 2.2 confirms the monoclinic ↔ orthorhombic structural transformation at T C, in agreement with magnetic, calorimetric, and electrical transport measurements. In addition to the substantial magnetocaloric effect, a large magnetoresistance of ~20% is also observed in Gd 4.8Sc 0.2Si 1.8Ge 2.2 for ΔH = 50 kOe in the vicinity of the magnetostructural transition. Finally, in a drastic reversal of the initial doping behavior further additions of Sc (x > 0.2) suppress formation of the monoclinic phase, change the nature of the transition from first-to second-order, and reduce both the transition temperature and magnetocaloric effect.« less

Magnetostructural phase transitions and magnetocaloric effect in (Gd 5-xSc x)Si 1.8Ge 2.2

DOE PAGES

Rudolph, Kirk; Pathak, Arjun K.; Mudryk, Yaroslav; ...

2017-12-21

Future advancements in magnetocaloric refrigeration/heat pumping technologies depend on the discovery of new materials that demonstrate large, tunable magnetocaloric effects (MCEs) in the vicinity of coupled magnetic and structural phase transitions that occur reversibly with minimum hysteresis. Here, with this in mind, we investigate phase transitions, microstructure, magnetic, thermal, magnetocaloric, and transport properties of (Gd 5-xSc x)Si 1.8Ge 2.2 compounds. Replacement of magnetic Gd with non-magnetic Sc in Gd 5-xSc xSi 1.8Ge 2.2 increases the ferromagnetic to paramagnetic first order phase transition temperature, T C, with only a minor reduction in MCE when x ≤ 0.2. We also demonstrate thatmore » hydrostatic pressure further increases T C and reduces the hysteresis of the first order phase transition in Gd 4.8Sc 0.2Si 1.8Ge 2.2 from 7 to 4 K. Temperature-dependent x-ray powder diffraction study of Gd 4.8Sc 0.2Si 1.8Ge 2.2 confirms the monoclinic ↔ orthorhombic structural transformation at T C, in agreement with magnetic, calorimetric, and electrical transport measurements. In addition to the substantial magnetocaloric effect, a large magnetoresistance of ~20% is also observed in Gd 4.8Sc 0.2Si 1.8Ge 2.2 for ΔH = 50 kOe in the vicinity of the magnetostructural transition. Finally, in a drastic reversal of the initial doping behavior further additions of Sc (x > 0.2) suppress formation of the monoclinic phase, change the nature of the transition from first-to second-order, and reduce both the transition temperature and magnetocaloric effect.« less

Phase-field model of insulator-to-metal transition in VO2 under an electric field

NASA Astrophysics Data System (ADS)

Shi, Yin; Chen, Long-Qing

2018-05-01

The roles of an electric field and electronic doping in insulator-to-metal transitions are still not well understood. Here we formulated a phase-field model of insulator-to-metal transitions by taking into account both structural and electronic instabilities as well as free electrons and holes in VO2, a strongly correlated transition-metal oxide. Our phase-field simulations demonstrate that in a VO2 slab under a uniform electric field, an abrupt universal resistive transition occurs inside the supercooling region, in sharp contrast to the conventional Landau-Zener smooth electric breakdown. We also show that hole doping may decouple the structural and electronic phase transitions in VO2, leading to a metastable metallic monoclinic phase which could be stabilized through a geometrical confinement and the size effect. This work provides a general mesoscale thermodynamic framework for understanding the influences of electric field, electronic doping, and stress and strain on insulator-to-metal transitions and the corresponding mesoscale domain structure evolution in VO2 and related strongly correlated systems.

Ab initio velocity-field curves in monoclinic β-Ga2O3

NASA Astrophysics Data System (ADS)

Ghosh, Krishnendu; Singisetti, Uttam

2017-07-01

We investigate the high-field transport in monoclinic β-Ga2O3 using a combination of ab initio calculations and full band Monte Carlo (FBMC) simulation. Scattering rate calculation and the final state selection in the FBMC simulation use complete wave-vector (both electron and phonon) and crystal direction dependent electron phonon interaction (EPI) elements. We propose and implement a semi-coarse version of the Wannier-Fourier interpolation method [Giustino et al., Phys. Rev. B 76, 165108 (2007)] for short-range non-polar optical phonon (EPI) elements in order to ease the computational requirement in FBMC simulation. During the interpolation of the EPI, the inverse Fourier sum over the real-space electronic grids is done on a coarse mesh while the unitary rotations are done on a fine mesh. This paper reports the high field transport in monoclinic β-Ga2O3 with deep insight into the contribution of electron-phonon interactions and velocity-field characteristics for electric fields ranging up to 450 kV/cm in different crystal directions. A peak velocity of 2 × 107 cm/s is estimated at an electric field of 200 kV/cm.

Structural, thermal, dielectric and phonon properties of perovskite-like imidazolium magnesium formate.

PubMed

Mączka, Mirosław; Marinho Costa, Nathalia Leal; Gągor, Anna; Paraguassu, Waldeci; Sieradzki, Adam; Hanuza, Jerzy

2016-05-18

We report the synthesis and characterisation of a magnesium formate framework templated by protonated imidazole. Single-crystal X-ray diffraction data showed that this compound crystallizes in the monoclinic structure in the P21/n space group with lattice parameters a = 12.1246(4) Å, b = 12.2087(5) Å, c = 12.4991(4) Å and β = 91.39(1)°. The antiparallel arrangement of the dipole moments associated with imidazolium cations suggests the antiferroelectric character of the room-temperature phase. The studied compound undergoes a structural phase transition at 451 K associated with a halving of the c lattice parameter and the disappearance of the antiferroelectric order. The monoclinic symmetry is preserved and the new metrics are a = 12.261(7) Å, b = 12.290(4) Å, c = 6.280(4) Å, and β = 90.62(5)°. Raman and IR data are consistent with the X-ray diffraction data. They also indicate that the disorder of imidazolium cations plays a significant role in the mechanism of the phase transition. Dielectric data show that the phase transition is associated with a relaxor nature of electric ordering. We also report high-pressure Raman scattering studies of this compound that revealed the presence of two pressure-induced phase transitions near 3 and 7 GPa. The first transition is most likely associated with a rearrangement of the imidazolium cations without any significant distortion of these cations and the magnesium formate framework, whereas the second transition leads to strong distortion of both the framework and imidazolium cations. High-pressure data also show that imidazolium magnesium formate does not show any signs of amorphization up to 11.4 GPa.

Chinese Script vs Plate-Like Precipitation of Beta-Al9Fe2Si2 Phase in an Al-6.5Si-1Fe Alloy

NASA Astrophysics Data System (ADS)

Ferdian, Deni; Josse, Claudie; Nguyen, Patrick; Gey, Nathalie; Ratel-Ramond, Nicolas; de Parseval, Philippe; Thebault, Yannick; Malard, Benoit; Lacaze, Jacques; Salvo, Luc

2015-07-01

The microstructure of a high-purity Al-6.5Si-1Fe (wt pct) alloy after solidification at various cooling rates was investigated. In most of the cases, the monoclinic beta-Al9Fe2Si2 phase was observed as long and thin lamellae. However, at a very slow cooling rate, Fe-bearing precipitates with Chinese script morphology appeared together with lamellae. Further analysis showed all these Chinese script precipitates correspond also to the monoclinic beta phase. This finding stresses that differentiating second phases according to their shape may be misleading.

Highly infrared sensitive VO2 nanowires for a nano-optical device.

PubMed

Bhuyan, Prabal Dev; Gupta, Sanjeev K; Kumar, Ashok; Sonvane, Yogesh; Gajjar, P N

2018-04-25

Recent studies on the electronic, magnetic and optical properties of VO2 (vanadium dioxide) materials have motivated the exploration of one dimensional VO2 nanowires. First principles calculations were performed to investigate the structural, electronic, magnetic and optical properties of the monoclinic (M) and rutile (R) phases of VO2 nanowires. The monoclinic phase shows semiconducting behaviour with a band gap of 1.17 eV, whereas the rutile phase of VO2 nanowires behaves as a spin gapless semiconducting material, as band lines cross the Fermi level due only to up spin contribution. The monoclinic structure of VO2 nanowires is found to be paramagnetic and the rutile structure shows ferromagnetic half metal behavior. The conductivity calculation for VO2 nanowires shows the metal-insulator transition (MIT) temperature to be 250 K. The possible mechanism of VO2 nanowires to be used as smart windows has been discussed, as the nanowires are highly sensitive in the infrared (IR) region. Interestingly, at low temperature, the VO2 monoclinic structure allows infrared light to be transmitted, while VO2 with the rutile phase blocks light in the IR region. Furthermore, we adsorbed CO2, N2 and SO2 gas molecules on 1D VO2 monoclinic nanowire to investigate their interaction behaviour. It was observed that the absorption and transmission properties of VO2 dramatically change upon the adsorption of CO2 and SO2 gas molecules, which is likely to open up its application as an optical gas sensor.

[Structure and properties of colored dental tetragonal zirconia stabilized by yttrium ceramics].

PubMed

Yi, Yuan-fu; Wang, Chen; Wen, Ning; Lin, Yong-zhao; Tian, Jie-mo

2009-10-01

To investigate the structure, mechanical and low temperature aging properties of colored dental zirconia ceramics. 5 graded colored dental zirconia ceramics were made by adding colorants and their combinations into a 3Y-TZP (tetragonal zirconia stabilized by 3mol% yttrium) powder, the green body were compacted at 200 MPa, pre-sinter at 1,050 degrees C and maintained for 2 h, then densely sintered at 1,500 degrees C for 2 h. Specimens were cut from each of the 5 graded colored blocks. Physical, mechanical properties as well as chemical stability were tested, microstructure were observed, crystalline phase were identified by X-ray diffraction (XRD), aging properties were assessed by measurement of the relative content of monoclinic phase and bending strength testing. The overall density of colored zirconia ceramics was over 99.7%, linear shrinkage was about 20%, while thermal expansion coefficient was about 11 x 10(-6) x degrees C(-1), the crystalline phase was tetragonal, bending strength was over 900 MPa which was slightly lowered than that of the uncolored zirconia, fracture toughness was slightly higher. Good chemical stability in acetic acid was observed. After aging treatment, tetragonal-to-monoclinic phase transformation was detected up to 40%, while bending strength was not significantly degraded. The results showed that colored 3Y-TZP ceramics presented good mechanical properties even after aging treatments, and was suitable for dental clinical use.

Phase Composition of Samarium Niobate and Tantalate Thin Films Prepared by Sol-Gel Method

NASA Astrophysics Data System (ADS)

Bruncková, H.; Medvecký, Ľ.; Múdra, E.; Kovalčiková, A.; Ďurišin, J.; Šebek, M.; Girman, V.

2017-12-01

Samarium niobate SmNbO4 (SNO) and tantalate SmTaO4 (STO) thin films ( 100 nm) were prepared by sol-gel/spin-coating process on alumina substrates with PZT interlayer and annealing at 1000°C. The precursors of films were synthesized using Nb or Ta tartrate complexes. The improvement of the crystallinity of monoclinic M'-SmTaO4 phase via heating was observed through the coexistence of small amounts of tetragonal T-SmTa7O19 phase in STO precursor at 1000°C. The XRD results of SNO and STO films confirmed monoclinic M-SmNbO4 and M'-SmTaO4 phases, respectively, with traces of orthorhombic O-SmNbO4 (in SNO). In STO film, the single monoclinic M'-SmTaO4 phase was revealed. The surface morphology and topography of thin films were investigated by SEM and AFM analysis. STO film was smoother with roughness 3.2 nm in comparison with SNO (6.3 nm). In the microstructure of SNO film, small spherical ( 50 nm) and larger cuboidal particles ( 100 nm) of the SmNbO4 phase were observed. In STO, compact clusters composed of fine spherical SmTaO4 particles ( 20-50 nm) were found. Effect of samarium can contribute to the formation different polymorphs of these films for the application to environmental electrolytic thin film devices.

Characterization of monoclinic crystals in tablets by pattern-fitting procedure using X-ray powder diffraction data.

PubMed

Yamamura, Shigeo; Momose, Yasunori

2003-06-18

The purpose of this study is to characterize the monoclinic crystals in tablets by using X-ray powder diffraction data and to evaluate the deformation feature of crystals during compression. The monoclinic crystals of acetaminophen and benzoic acid were used as the samples. The observed X-ray diffraction intensities were fitted to the analytic expression, and the fitting parameters, such as the lattice parameters, the peak-width parameters, the preferred orientation parameter and peak asymmetric parameter were optimized by a non-linear least-squares procedure. The Gauss and March distribution functions were used to correct the preferred orientation of crystallites in the tablet. The March function performed better in correcting the modification of diffraction intensity by preferred orientation of crystallites, suggesting that the crystallites in the tablets had fiber texture with axial orientation. Although a broadening of diffraction peaks was observed in acetaminophen tablets with an increase of compression pressure, little broadening was observed in the benzoic tablets. These results suggest that "acetaminophen is a material consolidating by fragmentation of crystalline particles and benzoic acid is a material consolidating by plastic deformation then occurred rearrangement of molecules during compression". A pattern-fitting procedure is the superior method for characterizing the crystalline drugs of monoclinic crystals in the tablets, as well as orthorhombic isoniazid and mannitol crystals reported in the previous paper.

Pressure-induced Td to 1T' structural phase transition in WTe 2

DOE PAGES

Zhou, Yonghui; Chen, Xuliang; Li, Nana; ...

2016-07-01

WTe 2 is provoking immense interest owing to its extraordinary properties, such as large positive magnetoresistance, pressure-driven superconductivity and possible type-II Weyl semimetal state. Here we report results of high-pressure synchrotron X-ray diffraction (XRD), Raman and electrical transport measurements on WTe 2. Both the XRD and Raman results reveal a structural transition upon compression, starting at 6.0 GPa and completing above 15.5 GPa. We have determined that the high-pressure lattice symmetry is monoclinic 1T' with space group of P21/m. This transition is related to a lateral sliding of adjacent Te-W-Te layers and results in a collapse of the unit cellmore » volume by ~20.5%. The structural transition also casts a pressure range with the broadened superconducting transition, where the zero resistance disappears.« less

Domain wall and interphase boundary motion in (1−x)Bi(Mg{sub 0.5}Ti{sub 0.5})O{sub 3}–xPbTiO{sub 3} near the morphotropic phase boundary

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

Tutuncu, Goknur; Chen, Jun; Fan, Longlong

Electric field-induced changes in the domain wall motion of (1−x)Bi(Mg{sub 0.5}Ti{sub 0.5})O{sub 3}–xPbTiO{sub 3} (BMT-xPT) near the morphotropic phase boundary (MPB) where x = 0.37 (BMT-37PT) and x = 0.38 (BMT-38PT), are studied by means of synchrotron x-ray diffraction. Through Rietveld analysis and profile fitting, a mixture of coexisting monoclinic (Cm) and tetragonal (P4mm) phases is identified at room temperature. Extrinsic contributions to the property coefficients are evident from electric-field-induced domain wall motion in both the tetragonal and monoclinic phases, as well as through the interphase boundary motion between the two phases. Domain wall motion in the tetragonal and monoclinic phases for BMT-37PT ismore » larger than that of BMT-38PT, possibly due to this composition's closer proximity to the MPB. Increased interphase boundary motion was also observed in BMT-37PT. Lattice strain, which is a function of both intrinsic piezoelectric strain and elastic interactions of the grains (the latter originating from domain wall and interphase boundary motion), is similar for the respective tetragonal and monoclinic phases.« less

Structural, vibrational, and electrical properties of 1 T -TiT e2 under hydrostatic pressure: Experiments and theory

NASA Astrophysics Data System (ADS)

Rajaji, V.; Dutta, Utpal; Sreeparvathy, P. C.; Sarma, Saurav Ch.; Sorb, Y. A.; Joseph, B.; Sahoo, Subodha; Peter, Sebastian C.; Kanchana, V.; Narayana, Chandrabhas

2018-02-01

We report the structural, vibrational, and electrical transport properties up to ˜16 GPa of 1 T -TiT e2 , a prominent layered 2D system. We clearly show signatures of two isostructural transitions at ˜2 GPa and ˜4 GPa obtained from the minima in c /a ratio concomitant with the phonon linewidth anomalies of Eg and A1 g modes around the same pressures, providing a strong indication of unusual electron-phonon coupling associated with these transitions. Resistance measurements present nonlinear behavior over similar pressure ranges shedding light on the electronic origin of these pressure-driven isostructural transitions. These multiple indirect signatures of an electronic transition at ˜2 GPa and ˜4 GPa are discussed in connection with the recent theoretical proposal for 1 T -TiT e2 and also the possibility of an electronic topological transition from our electronic Fermi surface calculations. Between 4 GPa and ˜8 GPa , the c /a ratio shows a plateau suggesting a transformation from an anisotropic 2D layer to a quasi-3D crystal network. First-principles calculations suggest that the 2D to quasi-3D evolution without any structural phase transitions is mainly due to the increased interlayer Te-Te interactions (bridging) via the charge density overlap. In addition, we observed a first-order structural phase transition from the trigonal (P 3 ¯m 1 ) to monoclinic (C 2 /m ) phase at higher pressure regions. We estimate the start of this structural phase transition to be ˜8 GPa and also the coexistence of two phases [trigonal (P 3 ¯m 1 ) and monoclinic (C 2 /m )] was observed from ˜8 GPa to ˜16 GPa .

Phase stabilization in transparent Lu2O3:Eu ceramics by lattice expansion

NASA Astrophysics Data System (ADS)

Seeley, Z. M.; Dai, Z. R.; Kuntz, J. D.; Cherepy, N. J.; Payne, S. A.

2012-11-01

Gadolinium lutetium oxide transparent ceramics doped with europium (Gd,Lu)2O3:Eu were fabricated via vacuum sintering and hot isostatic pressing (HIP). Nano-scale starting powder with the composition GdxLu1.9-xEu0.1O3 (x = 0, 0.3, 0.6, 0.9, 1.0, and 1.1) were uniaxially pressed and sintered under high vacuum at 1625 °C to obtain ˜97% dense structures with closed porosity. Sintered compacts were then subjected to 200 MPa argon gas at temperatures between 1750 and 1900 °C to reach full density. It was observed that a small portion of the Eu3+ ions were exsolved from the Lu2O3 cubic crystal lattice and concentrated at the grain boundaries, where they precipitated into a secondary monoclinic phase creating optical scattering defects. Addition of Gd3+ ions into the Lu2O3 cubic lattice formed the solid solution (Gd,Lu)2O3:Eu and stretched the lattice parameter allowing the larger Eu3+ ions to stay in solid solution, reducing the secondary phase and improving the transparency of the ceramics. Excess gadolinium, however, resulted in a complete phase transformation to monoclinic at pressures and temperatures sufficient for densification. Light yield performance was measured and all samples show equal amounts of the characteristic Eu3+ luminescence, indicating gadolinium addition had no adverse effect. This material has potential to improve the performance of high energy radiography devices.

The crystalline structure of copper phthalocyanine films on ZnO(1100).

PubMed

Cruickshank, Amy C; Dotzler, Christian J; Din, Salahud; Heutz, Sandrine; Toney, Michael F; Ryan, Mary P

2012-09-05

The structure of copper phthalocyanine (CuPc) thin films (5-100 nm) deposited on single-crystal ZnO(1100) substrates by organic molecular beam deposition was determined from grazing-incidence X-ray diffraction reciprocal space maps. The crystal structure was identified as the metastable polymorph α-CuPc, but the molecular stacking was found to vary depending on the film thickness: for thin films, a herringbone arrangement was observed, whereas for films thicker than 10 nm, coexistence of both the herringbone and brickstone arrangements was found. We propose a modified structure for the herringbone phase with a larger monoclinic β angle, which leads to intrastack Cu-Cu distances closer to those in the brickstone phase. This structural basis enables an understanding of the functional properties (e.g., light absorption and charge transport) of (opto)electronic devices fabricated from CuPc/ZnO hybrid systems.

Electronic structure and insulating gap in epitaxial VO 2 polymorphs

DOE PAGES

Lee, Shinbuhm; Meyer, Tricia L.; Sohn, Changhee; ...

2015-12-24

Here, determining the origin of the insulating gap in the monoclinic VO 2(M1) is a long-standing issue. The difficulty of this study arises from the simultaneous occurrence of structural and electronic transitions upon thermal cycling. Here, we compare the electronic structure of the M1 phase with that of single crystalline insulating VO 2(A) and VO 2(B) thin films to better understand the insulating phase of VO 2. As these A and B phases do not undergo a structural transition upon thermal cycling, we comparatively study the origin of the gap opening in the insulating VO 2 phases. By x-ray absorptionmore » and optical spectroscopy, we find that the shift of unoccupied t 2g orbitals away from the Fermi level is a common feature, which plays an important role for the insulating behavior in VO 2 polymorphs. The distinct splitting of the half-filled t 2g orbital is observed only in the M1 phase, widening the bandgap up to ~0.6 eV. Our approach of comparing all three insulating VO 2 phases provides insight into a better understanding of the electronic structure and the origin of the insulating gap in VO 2.« less

Lithium ion conduction in sol-gel synthesized LiZr2(PO4)3 polymorphs

NASA Astrophysics Data System (ADS)

Kumar, Milind; Yadav, Arun Kumar; Anita, Sen, Somaditya; Kumar, Sunil

2018-04-01

Safety issue associated with the high flammability and volatility of organic electrolytes used in commercial rechargeable lithium ion batteries has led to significant attention to ceramic-based solid electrolytes. In the present study, lithium ion conduction in two polymorphs of LiZr2(PO4)3 synthesized via the sol-gel route has been investigated. Rietveld refinement of room temperature X-ray diffraction data of LiZr2(PO4)3 powders calcined at 900 °C and 1300 °C confirmed these to be the monoclinic phase with P21/n structure and rhombohedral phase with R3¯c structure, respectively. Increase in calcination temperature and resultant phase transformation improved the room temperature conductivity from 2.27×10-6 ohm-1m-1 for the monoclinic phase to 1.41×10-4 ohm-1m-1 for rhombohedral phase. Temperature dependence of conductivity was modeled using Arrhenius law and activation energy of ˜ 0.59 eV (for monoclinic phase) and ˜0.50 eV (for rhombohedral phase) were obtained.

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.

Absence of pressure-induced amorphization in LiKSO4.

PubMed

Machon, D; Pinheiro, C B; Bouvier, P; Dmitriev, V P; Crichton, W A

2010-08-11

Angle-resolved synchrotron radiation diffraction was used to investigate lithium potassium sulfate (LiKSO(4)) crystals under high pressure. We confirm that the title compound undergoes three phase transitions, α →β, β → γ and γ →δ, observed at around 0.8 GPa, 4.0 GPa and 7.0 GPa, respectively. Two competitive structures are proposed for the β-phase after powder diffraction data Rietveld refinements: an orthorhombic (space group Cmc 2(1)) or a monoclinic (space group Cc) structure. These structures correspond to the models of the low temperature phases. The γ-phase is indexed by a monoclinic structure. Finally, the δ-phase is found to be highly disordered. No evidence of any pressure-induced amorphous phase was observed up to 24 GPa, even under imposed highly non-hydrostatic conditions, contrary to previous propositions.

Phase transitions and proton ordering in hemimorphite: new insights from single-crystal EPR experiments and DFT calculations

NASA Astrophysics Data System (ADS)

Mao, Mao; Li, Zucheng; Pan, Yuanming

2013-02-01

Single-crystal electron paramagnetic resonance spectra of gamma-ray-irradiated hemimorphite (Mapimi, Durango, Mexico) after storage at room temperature for 3 months, measured from 4 to 275 K, reveal a hydroperoxy radical HO2 derived from the water molecule in the channel. The EPR spectra of the HO2 radical confirm that hemimorphite undergoes two reversible phase transitions at ~98 and ~21 K and allow determinations of its spin Hamiltonian parameters, including superhyperfine coupling constants of two more-distant protons from the neighboring hydroxyl groups, at 110, 85, 40 and 7 K. These EPR results show that the HO2 radical changes in site symmetry from monoclinic to triclinic related to the ordering and rotation of its precursor water molecule in the channel at <98 K. The monoclinic structure of hemimorphite with completely ordered O-H systems at low temperature has been evaluated by both the EPR spectra of the HO2 radical at <21 K and periodic density functional theory calculations.

In Operando XRD and TXM Study on the Metastable Structure Change of NaNi 1/3Fe 1/3Mn 1/3O 2 under Electrochemical Sodium-Ion Intercalation

DOE PAGES

Xie, Yingying; Wang, Hong; Xu, Guiliang; ...

2016-09-02

In operando XRD and TXM-XANES approaches demonstrate that structure evolution in NaNi 1/3Fe 1/3Mn 1/3O 2 during cycling follows a continuous change, and the formation of a nonequilibrium solid solution phase in the existence of two phases. Here, an O3' and P3' monoclinic phase occur, and redox couples of Ni 3+/Ni 4+ and Fe 3+/Fe 4+ are mainly responsible in the charge voltage range from 4.0 to 4.3 V.

Ultrafast dynamics during the photoinduced phase transition in VO2

NASA Astrophysics Data System (ADS)

Wegkamp, Daniel; Stähler, Julia

2015-12-01

The phase transition of VO2 from a monoclinic insulator to a rutile metal, which occurs thermally at TC = 340 K, can also be driven by strong photoexcitation. The ultrafast dynamics during this photoinduced phase transition (PIPT) have attracted great scientific attention for decades, as this approach promises to answer the question of whether the insulator-to-metal (IMT) transition is caused by electronic or crystallographic processes through disentanglement of the different contributions in the time domain. We review our recent results achieved by femtosecond time-resolved photoelectron, optical, and coherent phonon spectroscopy and discuss them within the framework of a selection of latest, complementary studies of the ultrafast PIPT in VO2. We show that the population change of electrons and holes caused by photoexcitation launches a highly non-equilibrium plasma phase characterized by enhanced screening due to quasi-free carriers and followed by two branches of non-equilibrium dynamics: (i) an instantaneous (within the time resolution) collapse of the insulating gap that precedes charge carrier relaxation and significant ionic motion and (ii) an instantaneous lattice potential symmetry change that represents the onset of the crystallographic phase transition through ionic motion on longer timescales. We discuss the interconnection between these two non-thermal pathways with particular focus on the meaning of the critical fluence of the PIPT in different types of experiments. Based on this, we conclude that the PIPT threshold identified in optical experiments is most probably determined by the excitation density required to drive the lattice potential change rather than the IMT. These considerations suggest that the IMT can be driven by weaker excitation, predicting a transiently metallic, monoclinic state of VO2 that is not stabilized by the non-thermal structural transition and, thus, decays on ultrafast timescales.

Doping-Based Stabilization of the M2 Phase in Free-Standing VO2 Nanostructures at Room Temperature

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

Strelcov, Evgheni; Tselev, Alexander; Ivanov, Ilia N

2012-01-01

A new high-yield method of doping VO2 nanostructures with aluminum is proposed, which renders possible stabilization of the monoclinic M2 phase in free-standing nanoplatelets in ambient conditions and opens an opportunity for realization of a purely electronic Mott Transition Field-Effect Transistor without an accompanying structural transition. The synthesized free-standing M2-phase nanostructures are shown to have very high crystallinity and an extremely sharp temperature-driven metal-insulator transition. A combination of x-ray microdiffraction, micro-Raman spectroscopy, Energy-Dispersive X-ray spectroscopy, and four-probe electrical measurements allowed thorough characterization of the doped nanostructures. Light is shed onto some aspects of the nanostructure growth, and the temperature-doping levelmore » phase diagram is established.« less

In situ TEM observation of heterogeneous phase transition of a constrained single-crystalline Ag2Te nanowire.

PubMed

In, Juneho; Yoo, Youngdong; Kim, Jin-Gyu; Seo, Kwanyong; Kim, Hyunju; Ihee, Hyotchel; Oh, Sang Ho; Kim, Bongsoo

2010-11-10

Laterally epitaxial single crystalline Ag2Te nanowires (NWs) are synthesized on sapphire substrates by the vapor transport method. We observed the phase transitions of these Ag2Te NWs via in situ transmission electron microscopy (TEM) after covering them with Pt layers. The constrained NW shows phase transition from monoclinic to a body-centered cubic (bcc) structure near the interfaces, which is ascribed to the thermal stress caused by differences in the thermal expansion coefficients. Furthermore, we observed the nucleation and growth of bcc phase penetrating into the face-centered cubic matrix at 200 °C by high-resolution TEM in real time. Our results would provide valuable insight into how compressive stresses imposed by overlayers affect behaviors of nanodevices.

Morphotropic phase boundary of heterovalent perovskite solid solutions: Experimental and theoretical investigation of PbSc{sub 1/2}Nb{sub 1/2}O{sub 3}-PbTiO{sub 3}

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

Haumont, R.; Al-Barakaty, A.; Dkhil, B.

2005-03-01

X-ray and neutron diffraction techniques are combined with first-principles-based simulations to derive and understand the structural properties of Pb(Sc,Nb,Ti)O{sub 3} (PSN-PT) near its morphotropic phase boundary (MPB). An analysis of our measurements yields, at room and low temperatures, an overall tetragonal T--monoclinic M{sub C}--monoclinic M{sub B}--rhombohedral R path (when adopting the notations of Vanderbilt and Cohen, Phys. Rev. B 63, 94108 (2001) for the monoclinic phases) as the Ti composition decreases across the MPB. A composition- and temperature-dependent significant mixing between some of these phases is also measured and reported here. The overall T-M{sub C}-M{sub B}-R path, which has alsomore » been proposed for Pb(Mg,Nb,Ti)O{sub 3} [A. K. Singh and D. Pandey, Phys. Rev. B 67, 64102 (2003)] is rather complex since it involves a change in the polarization path: this polarization first rotates in a (100) plane for the T-M{sub C} part of the path and then in a (1-10) plane for the M{sub B}-R part of the path. Moreover, a comparison between these measurements and first-principles-based calculations raises the possibility that this complex path, and the associated M{sub C} and M{sub B} phases, can only occur if the samples exhibit a deviation from a perfectly homogeneous and disordered situation, e.g. possess nanoscale chemically-ordered regions. If not, homogeneously disordered PSN-PT is predicted to exhibit at low temperature the same polarization path as Pb(Zr,Ti)O{sub 3}, that is T-monoclinic M{sub A}-R which involves a 'single' polarization rotation in a (1-10) plane. Nanoscale inhomogeneity may thus play a key role on the macroscopic properties of PSN-PT, in particular, and of other heterovalent complex solid solutions, in general, near their MPB.« less

Postaragonite phases of CaCO3 at lower mantle pressures

NASA Astrophysics Data System (ADS)

Smith, Dean; Lawler, Keith V.; Martinez-Canales, Miguel; Daykin, Austin W.; Fussell, Zachary; Smith, G. Alexander; Childs, Christian; Smith, Jesse S.; Pickard, Chris J.; Salamat, Ashkan

2018-01-01

The stability, structure, and properties of carbonate minerals at lower mantle conditions have significant impact on our understanding of the global carbon cycle and the composition of the interior of the Earth. In recent years there has been significant interest in the behavior of carbonates at lower mantle conditions, specifically in their carbon hybridization, which has relevance for the storage of carbon within the deep mantle. Using high-pressure synchrotron x-ray diffraction in a diamond anvil cell coupled with direct laser heating of CaCO3 using a CO2 laser, we identify a crystalline phase of the material above 40 GPa—corresponding to a lower mantle depth of around 1000 km—which has first been predicted by ab initio structure predictions. The observed s p2 carbon hybridized species at 40 GPa is monoclinic with P 21/c symmetry and is stable up to 50 GPa, above which it transforms into a structure which cannot be indexed by existing known phases. A combination of ab initio random structure search (AIRSS) and quasiharmonic approximation (QHA) calculations are used to re-explore the relative phase stabilities of the rich phase diagram of CaCO3. Nudged elastic band (NEB) calculations are used to investigate the reaction mechanisms between relevant crystal phases of CaCO3 and we postulate that the mineral is capable of undergoing s p2-s p3 hybridization change purely in the P 21/c structure—forgoing the accepted postaragonite P m m n structure.

Spiral chain structure of high pressure selenium-II{sup '} and sulfur-II from powder x-ray diffraction

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

Fujihisa, Hiroshi; Yamawaki, Hiroshi; Sakashita, Mami

2004-10-01

The structure of high pressure phases, selenium-II{sup '} (Se-II{sup '}) and sulfur-II (S-II), for {alpha}-Se{sub 8} (monoclinic Se-I) and {alpha}-S{sub 8} (orthorhombic S-I) was studied by powder x-ray diffraction experiments. Se-II{sup '} and S-II were found to be isostructural and to belong to the tetragonal space group I4{sub 1}/acd, which is made up of 16 atoms in the unit cell. The structure consisted of unique spiral chains with both 4{sub 1} and 4{sub 3} screws. The results confirmed that the structure sequence of the pressure-induced phase transitions for the group VIb elements depended on the initial molecular form. The chemicalmore » bonds of the phases are also discussed from the interatomic distances that were obtained.« less

Probing the electronic and local structural changes across the pressure-induced insulator-to-metal transition in VO2

NASA Astrophysics Data System (ADS)

Marini, C.; Bendele, M.; Joseph, B.; Kantor, I.; Mitrano, M.; Mathon, O.; Baldini, M.; Malavasi, L.; Pascarelli, S.; Postorino, P.

2014-11-01

Local and electronic structures of vanadium in \\text{VO}2 are studied across the high-pressure insulator-to-metal (IMT) transition using V K-edge x-ray absorption spectroscopy. Unlike the temperature-induced IMT, pressure-induced metallization leads to only subtle changes in the V K-edge prepeak structure, indicating a different mechanism involving smaller electronic spectral weight transfer close to the chemical potential. Intriguingly, upon application of the hydrostatic pressure, the electronic structure begins to show substantial changes well before the occurrence of the IMT and the associated structural transition to an anisotropic compression of the monoclinic metallic phase.

Tungsten Incorporation into Gallium Oxide: Crystal Structure, Surface and Interface Chemistry, Thermal Stability and Interdiffusion

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

Rubio, E. J.; Mates, T. E.; Manandhar, S.

Tungsten (W) incorporated gallium oxide (Ga2O3) (GWO) thin films were deposited by radio-frequency magnetron co-sputtering of W-metal and Ga2O3-ceramic targets. Films were produced by varying sputtering power applied to the W-target in order to achieve variable W-content (0-12 at%) into Ga2O3 while substrate temperature was kept constant at 500 °C. Chemical composition, chemical valence states, microstructure and crystal structure of as-deposited and annealed GWO films were evaluated as a function of W-content. The structural and chemical analyses indicate that the samples deposited without any W-incorporation are stoichiometric, nanocrystalline Ga2O3 films, which crystallize in β-phase monoclinic structure. While GWO films alsomore » crystallize in monoclinic β-Ga2O3 phase, W-incorporation induces surface amorphization as revealed by structural studies. The chemical valence state of Ga ions probed by X-ray photoelectron spectroscopic (XPS) analyses is characterized by the highest oxidation state i.e., Ga3+. No changes in Ga chemical state are noted for variable W-incorporation in the range of 0-12 at%. Rutherford backscattering spectrometry (RBS) analyses indicate the uniform distribution of W-content in the GWO films. However, XPS analyses indicate the formation of mixed valence states for W ions, which may be responsible for surface amorphization in GWO films. GWO films were stable up to 900 oC, at which point thermally induced secondary phase (W-oxide) formation was observed. A transition to mesoporous structure coupled with W interdiffusion occurs due to thermal annealing as derived from the chemical analyses at the GWO films’ surface as well as depth-profiling towards the GWO-Si interface. A model has been formulated to account for the mechanism of W-incorporation, thermal stability and interdiffusion via pore formation in GWO films.« less

Prediction and synthesis of a family of atomic laminate phases with Kagomé-like and in-plane chemical ordering

PubMed Central

Dahlqvist, Martin; Lu, Jun; Meshkian, Rahele; Tao, Quanzheng; Hultman, Lars; Rosen, Johanna

2017-01-01

The enigma of MAX phases and their hybrids prevails. We probe transition metal (M) alloying in MAX phases for metal size, electronegativity, and electron configuration, and discover ordering in these MAX hybrids, namely, (V2/3Zr1/3)2AlC and (Mo2/3Y1/3)2AlC. Predictive theory and verifying materials synthesis, including a judicious choice of alloying M from groups III to VI and periods 4 and 5, indicate a potentially large family of thermodynamically stable phases, with Kagomé-like and in-plane chemical ordering, and with incorporation of elements previously not known for MAX phases, including the common Y. We propose the structure to be monoclinic C2/c. As an extension of the work, we suggest a matching set of novel MXenes, from selective etching of the A-element. The demonstrated structural design on simultaneous two-dimensional (2D) and 3D atomic levels expands the property tuning potential of functional materials. PMID:28776034

Electrochemical characterization of monoclinic and orthorhombic Li3CrF6 as positive electrodes in lithium-ion batteries synthesized by a sol-gel process with environmentally benign chemicals

NASA Astrophysics Data System (ADS)

Lieser, Georg; Winkler, Volker; Geßwein, Holger; de Biasi, Lea; Glatthaar, Sven; Hoffmann, M. J.; Ehrenberg, Helmut; Binder, Joachim R.

2015-10-01

Lithium transition metal fluorides (Li3MF6; M = Fe, V) with cryolite structure are investigated as positive electrode materials for lithium-ion batteries. A novel sol-gel process with trifluoroacetic acid as fluorine source was used to synthesize monoclinic and orthorhombic Li3CrF6. A ball milling process with Li3CrF6, binder, and conductive agent was applied to form a Li3CrF6 composite, which was electrochemically characterized against lithium metal for the first time. The electrochemical properties of two different modifications are quite similar, with a reversible specific capacity of 111 mAhg-1 for monoclinic Li3CrF6 and 106 mAhg-1 for orthorhombic Li3CrF6 (1 eq. Li ≙ 143 mAhg-1). The electrochemically active redox couple CrIII/CrII was confirmed by X-ray photoelectron spectroscopy.

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.

Crystal structure and superconducting properties of KSr2Nb3O10

NASA Astrophysics Data System (ADS)

Kawaguchi, T.; Horigane, K.; Itoh, Y.; Kobayashi, K.; Horie, R.; Kambe, T.; Akimitsu, J.

2018-05-01

We performed X-ray diffraction (XRD) and DC magnetic susceptibility measurements to elucidate the crystal structure and superconducting properties of KSr2Nb3O10. From the diffraction pattern indexing, it was found that KSr2Nb3O10 crystallizes with monoclinic symmetry, space group P21/m(11). We succeeded in preparing high temperature (HT) and low temperature (LT) phases of KSr2Nb3O10 powder samples synthesized by a conventional solid state reaction and an ion-exchange reaction, respectively. Superconductivity was observed at 4 K by Li intercalation and it was found that the superconducting volume fraction of the LT phase ( 1.4%) is clearly larger than that of the HT phase (0.07%).

B1-B2 phase transition mechanism and pathway of PbS under pressure

NASA Astrophysics Data System (ADS)

Adeleke, Adebayo A.; Yao, Yansun

2018-03-01

Experimental studies at finite Pressure-Temperature (P-T) conditions and a theoretical study at 0 K of the phase transition in lead sulphide (PbS) have been inconclusive. Many studies that have been done to understand structural transformation in PbS can broadly be classified into two main ideological streams—one with Pnma and another with Cmcm orthorhombic intermediate phase. To foster better understanding of this phenomenon, we present the result of the first-principles study of phase transition in PbS at finite temperature. We employed the particle swarm-intelligence optimization algorithm for the 0 K structure search and first-principles metadynamics simulations to study the phase transition pathway of PbS from the ambient pressure, 0 K Fm-3m structure to the high-pressure Pm-3m phase under experimentally achievable P-T conditions. Significantly, our calculation shows that both streams are achievable under specific P-T conditions. We further uncover new tetragonal and monoclinic structures of PbS with space group P21/c and I41/amd, respectively. We propose the P21/c and I41/amd as a precursor phase to the Pnma and Cmcm phases, respectively. We investigated the stability of the new structures and found them to be dynamically stable at their stability pressure range. Electronic structure calculations reveal that both P21/c and I41/amd phases are semiconducting with direct and indirect bandgap energies of 0.69(5) eV and 0.97(3) eV, respectively. In general, both P21/c and I41/amd phases were found to be energetically competitive with their respective orthorhombic successors.

Phase diagram and polarization of stable phases of (Ga1- x In x )2O3

NASA Astrophysics Data System (ADS)

Maccioni, Maria Barbara; Fiorentini, Vincenzo

2016-04-01

The full phase diagram of (Ga1- x In x )2O3 is obtained theoretically. The phases competing for the ground state are monoclinic β (low x), hexagonal (x ˜ 0.5), and bixbyite (large x). Three disconnected mixing regions interlace with two distinct phase-separation regions, and at x ˜ 0.5, the coexistence of hexagonal and β alloys with phase-separated binary components is expected. We also explore the permanent polarization of the phases, but none of them are polar. On the other hand, we find that ɛ-Ga2O3, which was stabilized in recent experiments, is pyroelectric with a large polarization and piezoelectric coupling, and could be used to produce high-density electron gases at interfaces.

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

Brgoch, Jakoah; Klob, Simon D.; Denault, Kristin A.

The preparation of Eu 2+-substituted barium aluminum silicates is achieved using a rapid microwave-assisted preparation. The phase evolution of two BaAl 2Si 2O 8:Eu 2+ polymorphs, the higher temperature hexagonal phase (hexacelsian), and the lower temperature monoclinic phase (celsian), is explored by varying the ramp time and soak time. This preparation method significantly reduces the reaction time needed to form these phases compared to conventional solid state routes. The luminescent properties of the two phases are identified under UV excitation with the hexagonal phase emitting in the UV region (λ em = 372 nm) and the monoclinic phase emitting inmore » the blue region (λ em = 438 nm). The differences in optical properties of the two polymorphs are correlated to the coordination number and arrangement of the alkali earth site. The optical properties of the monoclinic phase can be further tuned through the substitution of Sr 2+, forming the solid solution (Ba 1–xSr x)Al 2Si 2O 8:Eu 2+. Changes in the crystal structure due to Sr 2+ substitution produce a surprising blue-shift in the emission spectrum, which is explained by a greater dispersion of bond lengths in the (Ba/Sr)–O polyhedra. The entire monoclinic solid solution exhibits excellent quantum yields of nearly 90 %, owing to the structural rigidity provided by the highly connected tetrahedral network.« less

Anisotropy and phonon modes from analysis of the dielectric function tensor and the inverse dielectric function tensor of monoclinic yttrium orthosilicate

NASA Astrophysics Data System (ADS)

Mock, A.; Korlacki, R.; Knight, S.; Schubert, M.

2018-04-01

We determine the frequency dependence of the four independent Cartesian tensor elements of the dielectric function for monoclinic symmetry Y2SiO5 using generalized spectroscopic ellipsometry from 40-1200 cm-1. Three different crystal cuts, each perpendicular to a principle axis, are investigated. We apply our recently described augmentation of lattice anharmonicity onto the eigendielectric displacement vector summation approach [A. Mock et al., Phys. Rev. B 95, 165202 (2017), 10.1103/PhysRevB.95.165202], and we present and demonstrate the application of an eigendielectric displacement loss vector summation approach with anharmonic broadening. We obtain an excellent match between all measured and model-calculated dielectric function tensor elements and all dielectric loss function tensor elements. We obtain 23 Au and 22 Bu symmetry long-wavelength active transverse and longitudinal optical mode parameters including their eigenvector orientation within the monoclinic lattice. We perform density functional theory calculations and obtain 23 Au symmetry and 22 Bu transverse and longitudinal optical mode parameters and their orientation within the monoclinic lattice. We compare our results from ellipsometry and density functional theory and find excellent agreement. We also determine the static and above reststrahlen spectral range dielectric tensor values and find a recently derived generalization of the Lyddane-Sachs-Teller relation for polar phonons in monoclinic symmetry materials satisfied [M. Schubert, Phys Rev. Lett. 117, 215502 (2016), 10.1103/PhysRevLett.117.215502].

Crystal structures of the ionic conductors Bi{sub 46}M{sub 8}O{sub 89} (M=P, V) related to the fluorite-type structure

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

Darriet, J.; Launay, J.C.; Zuniga, F.J.

2005-06-15

The crystal structures of the two oxides Bi{sub 46}M{sub 8}O{sub 89} (M=P, V) have been solved from single crystals X-ray data at room temperature. Bi{sub 46}P{sub 8}O{sub 89} crystallizes in the monoclinic symmetry (space group C2/m) with the cell parameters a=19.6073(4)A, b=11.4181(4)A, c=21.1119(4)A and {beta}=112.14(3){sup o}. The symmetry of Bi{sub 46}V{sub 8}O{sub 89} is also monoclinic but the space group is P2{sub 1}/c with the unit-cell parameters: a=20.0100(4)A, b=11.6445(4)A, c=20.4136(4)A and {beta}=107.27(3){sup o}. Both structures derive from an oxygen deficient fluorite-type structure where the Bi and M cations (M=P, V) are ordered in the framework. The structures are characterised bymore » isolated MO{sub 4} tetrahedra (M=P, V) which contradicts the previous results. The difference between the two structures is only due to a different order of the M atoms (M=P, V) in the fluorite-type superstructure. It will be shown that some oxygen sites are partially occupied in both structures which can explain the ion conduction properties of these phases. A structural building principle will be proposed that can explain the large domain of solid solution related to the fluorite-type observed in both systems.« less

Effect of different annealing condition on the structural and magnetic properties of Mn2NiGa Heusler alloys

NASA Astrophysics Data System (ADS)

Vagadia, Megha; Hester, James; Nigam, A. K.

2018-04-01

We studied the effect of different annealing conditions on structural and magnetic properties of Mn2NiGa Heusler alloys. Reitveld refinement of neutron diffraction pattern at RT confirms the tetragonal structure with cubic phase for I-W quenched alloy whereas Le Bail fitting trials performed on neutron diffraction pattern collected for other three alloys confirm 7M monoclinic structure with cubic phase. It is found that starting and finish temperatures associated with martensite and austenite phase transformation depends strongly on the cooling rate corresponding to different cooling techniques. Slow furnace cooled sample possesses the highest martensite start temperature above room temperature ˜ 326K which decreases to ˜ 198K for ice -water quenched sample. Variation in the drop in the magnetization around MS obtained upon warming from martensite to austenite phase under ZFC cycle suggests that change in the cooling condition strongly affects the magnetization in the low temperature martensite phase. Present results suggest that by varying the cooling rate, martensite transformation as well as the martensite structure can be tuned.

Electrochemically Induced Insulator-Metal-Insulator Transformations of Vanadium Dioxide Nanocrystal Films

NASA Astrophysics Data System (ADS)

Milliron, Delia; Dahlman, Clayton; Leblanc, Gabriel; Bergerud, Amy

Vanadium dioxide (VO2) undergoes significant optical, electronic, and structural changes as it transforms between the low-temperature monoclinic and high-temperature rutile phases. The low-temperature state is insulating and transparent, while the high-temperature state is metallic and IR blocking. Alternative stimuli have been utilized to trigger insulator-to-metal transformations in VO2, including electrochemical gating. Here, VO2 nanocrystal films have been prepared by solution deposition of V2O3 nanocrystals followed by oxidative annealing. Nanocrystalline VO2 films are electrochemically reduced, inducing changes in their electronic and optical properties. We observe a reversible transition between infrared transparent insulating phases and a darkened metallic phase by in situ visible-near-infrared spectroelectrochemistry and correlate these observations with structural and electronic changes monitored by X-ray absorption spectroscopy, X-ray diffraction, Raman spectroscopy, and conductivity measurements. Reduction causes an initial transformation to a metallic, IR-colored distorted monoclinic phase. However, an unexpected reversible transition from conductive, reduced monoclinic VO2 to an infrared-transparent insulating phase is observed upon further reduction.

In situ probing of doping- and stress-mediated phase transitions in a single-crystalline VO2 nanobeam by spatially resolved Raman spectroscopy

NASA Astrophysics Data System (ADS)

Chang, Sung-Jin; Park, Jong Bae; Lee, Gaehang; Kim, Hae Jin; Lee, Jin-Bae; Bae, Tae-Sung; Han, Young-Kyu; Park, Tae Jung; Huh, Yun Suk; Hong, Woong-Ki

2014-06-01

We demonstrate an experimental in situ observation of the temperature-dependent evolution of doping- and stress-mediated structural phase transitions in an individual single-crystalline VO2 nanobeam on a Au-coated substrate under exposure to hydrogen gas using spatially resolved Raman spectroscopy. The nucleation temperature of the rutile R structural phase in the VO2 nanobeam upon heating under hydrogen gas was lower than that under air. The spatial structural phase evolution behavior along the length of the VO2 nanobeam under hydrogen gas upon heating was much more inhomogeneous than that along the length of the same nanobeam under air. The triclinic T phase of the VO2 nanobeam upon heating under hydrogen gas transformed to the R phase and this R phase was stabilized even at room temperature in air after sample cooling. In particular, after the VO2 nanobeam with the R phase was annealed at approximately 250 °C in air, it exhibited the monoclinic M1 phase (not the T phase) at room temperature during heating and cooling cycles. These results were attributed to the interplay between hydrogen doping and stress associated with nanobeam-substrate interactions. Our study has important implications for engineering metal-insulator transition properties and developing functional devices based on VO2 nanostructures through doping and stress.We demonstrate an experimental in situ observation of the temperature-dependent evolution of doping- and stress-mediated structural phase transitions in an individual single-crystalline VO2 nanobeam on a Au-coated substrate under exposure to hydrogen gas using spatially resolved Raman spectroscopy. The nucleation temperature of the rutile R structural phase in the VO2 nanobeam upon heating under hydrogen gas was lower than that under air. The spatial structural phase evolution behavior along the length of the VO2 nanobeam under hydrogen gas upon heating was much more inhomogeneous than that along the length of the same nanobeam under

Introduction of Structural Variations into Oxide Polymers and their Effect on Sintering and Crystallization.

DTIC Science & Technology

1985-02-11

any information, apparatus, product or process disclosed or represents that its use would not S infringe privately-owned rights. S S.i : -- :’.- S’. S...hydrolytic 57 - condensation product of Zr(OC3H7)n at 100*C indi- cates cubic phase structure (top); the same material develops traces of monoclinic...interactions. *. 13 Oak" Alcohol Effect Hydroysis edium Etao - etanol I-Proponel IV Fiur 4. Moeua-egtdstiuincre f oyraoioa etanl an 2-rp o afte 24hsa 0C

Donor-acceptor pair recombination luminescence from monoclinic Cu{sub 2}SnS{sub 3} thin film

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

Aihara, Naoya; Tanaka, Kunihiko, E-mail: tanaka@vos.nagaokaut.ac.jp; Uchiki, Hisao

2015-07-20

The defect levels in Cu{sub 2}SnS{sub 3} (CTS) were investigated using photoluminescence (PL) spectroscopy. A CTS thin film was prepared on a soda-lime glass/molybdenum substrate by thermal co-evaporation and sulfurization. The crystal structure was determined to be monoclinic, and the compositional ratios of Cu/Sn and S/Metal were determined to be 1.8 and 1.2, respectively. The photon energy of the PL spectra observed from the CTS thin film was lower than that previously reported. All fitted PL peaks were associated with defect related luminescence. The PL peaks observed at 0.843 and 0.867 eV were assigned to donor-acceptor pair recombination luminescence, the thermalmore » activation energies of which were determined to be 22.9 and 24.8 meV, respectively.« less

Structural Rheology of the Smectic Phase

PubMed Central

Fujii, Shuji; Komura, Shigeyuki; Lu, Chun-Yi David

2014-01-01

In this review article, we discuss the rheological properties of the thermotropic smectic liquid crystal 8CB with focal conic domains (FCDs) from the viewpoint of structural rheology. It is known that the unbinding of the dislocation loops in the smectic phase drives the smectic-nematic transition. Here we discuss how the unbinding of the dislocation loops affects the evolution of the FCD size, linear and nonlinear rheological behaviors of the smectic phase. By studying the FCD formation from the perpendicularly oriented smectic layers, we also argue that dislocations play a key role in the structural development in layered systems. Furthermore, similarities in the rheological behavior between the FCDs in the smectic phase and the onion structures in the lyotropic lamellar phase suggest that these systems share a common physical origin for the elasticity. PMID:28788123

Raman and infrared spectroscopic investigations of a ferroelastic phase transition in B a2ZnTe O6 double perovskite

NASA Astrophysics Data System (ADS)

Moreira, Roberto L.; Lobo, Ricardo P. S. M.; Ramos, Sérgio L. L. M.; Sebastian, Mailadil T.; Matinaga, Franklin M.; Righi, Ariete; Dias, Anderson

2018-05-01

The low-temperature vibrational properties of B a2ZnTe O6 double-perovskite ceramics obtained by the solid-state route were investigated by Raman scattering and Fourier-transform infrared reflectivity. We found that this material undergoes a reversible ferroelastic phase transition at around 140 K, well compatible with a recently proposed rhombohedral-to-monoclinic structural change that would occur below 165 K. Complementary calorimetric measurements showed that the phase transition has a first-order character, with an entropy jump compatible with a displacive mechanism. The vibrational spectra show clearly the splitting of the doubly degenerate E modes into nondegenerate representations of the low-symmetry phase. In particular, the lowest-frequency Raman mode presents soft-mode behavior and splits below the critical temperature, confirming the in-plane ferroelastic deformation in the low-temperature phase.

Solution-processed phase-change VO(2) metamaterials from colloidal vanadium oxide (VO(x)) nanocrystals.

PubMed

Paik, Taejong; Hong, Sung-Hoon; Gaulding, E Ashley; Caglayan, Humeyra; Gordon, Thomas R; Engheta, Nader; Kagan, Cherie R; Murray, Christopher B

2014-01-28

We demonstrate thermally switchable VO2 metamaterials fabricated using solution-processable colloidal nanocrystals (NCs). Vanadium oxide (VOx) NCs are synthesized through a nonhydrolytic reaction and deposited from stable colloidal dispersions to form NC thin films. Rapid thermal annealing transforms the VOx NC thin films into monoclinic, nanocrystalline VO2 thin films that show a sharp, reversible metal-insulator phase transition. Introduction of precise concentrations of tungsten dopings into the colloidal VOx NCs enables the still sharp phase transition of the VO2 thin films to be tuned to lower temperatures as the doping level increases. We fabricate "smart", differentially doped, multilayered VO2 films to program the phase and therefore the metal-insulator behavior of constituent vertically structured layers with temperature. With increasing temperature, we tailored the optical response of multilayered films in the near-IR and IR regions from that of a strong light absorber, in a metal-insulator structure, to that of a Drude-like reflector, characteristic of a pure metallic structure. We demonstrate that nanocrystal-based nanoimprinting can be employed to pattern multilayered subwavelength nanostructures, such as three-dimensional VO2 nanopillar arrays, that exhibit plasmonic dipolar responses tunable with a temperature change.

Physical Properties of Phase Pure 4C Pyrrhotite (Fe7S8) during its Low Temperature Besnus Transition

NASA Astrophysics Data System (ADS)

Volk, M.; Feinberg, J. M.; McCalla, E.; Leighton, C.; Voigt, B.

2017-12-01

Of all magnetic minerals that play a role in recording terrestrial and extraterrestrial magnetic fields, the low temperature phase transition of monoclinic Fe7S8 is the least well understood. At room temperature an array of ordered vacancies gives rise to ferrimagnetism in pyrrhotite. The mineral's physical properties change dramatically at ≈30 K during what is known as the Besnus transition. The mechanism driving these changes, however, is not fully understood. Several explanations have been proposed, including changes in crystalline anisotropy, a transformation of the crystal symmetry, and magnetic interactions within in a two-phase (4C/5C*) system among them. To better understand the transition we studied magnetic, electric and structural properties as well as the heat capacity of a large, phase pure monoclinic crystal (Fe6.8±0.1S8). The single-phase sample shows a clear peak at 32 K in the heat capacity associated with a second order phase transition. Zero field cooling of 2.5 T saturating isothermal remanent magnetizations acquired at 300 and 20 K, as well electrical conductivity exhibit sudden changes between 30-33 K. Susceptibility shows a secondary peak within the same temperature interval. These phenomena can be related to the peak in heat capacity, indicating that the changes are related to the phase transition. In-field measurements show that the magnetic and electric transitions are mildly field dependent. Repeated measurements on different instruments show that the transition temperature for susceptibility is 1 K higher when measured parallel to the crystallographic c-axis as compared to within the c-plane. Similar trends could be found in magnetoresistivity, which is negative (≈ -2%) in the c-plane and larger and positive (≈ 5%) along the c-axis. While this comprehensive data set is not able to unambiguously explain the mechanism driving the transition, it indicates the coupling of structural and magnetocrystalline properties and suggests that

Ab Initio Study of the Structure and Stability of High-Pressure Iron-Bearing Dolomite

NASA Astrophysics Data System (ADS)

Solomatova, N. V.; Asimow, P. D.

2016-12-01

Carbon is subducted into the mantle primarily in the form of metasomatically calcium-enriched basaltic rock, calcified serpentinites and carbonaceous ooze, all of which often contain dolomite. End-member CaMg(CO3)2 dolomite typically breaks down upon compression into two carbonates at 5-6 GPa in the temperature range of 800-1200 K [1]. However, high-pressure X-ray diffraction experiments have recently shown that the presence of iron may be sufficient to stabilize high-pressure dolomite over single-cation carbonates above 35 GPa [2,3]. The structure and equation of state of high-pressure dolomite phases have been debated, creating a need for theoretical calculations. Using density functional theory interfaced with a genetic algorithm that predicts crystal structures (USPEX), we have found a monoclinic phase with space group C2/c. The C2/c structure has a lower energy than previously reported dolomite structures at relevant pressures. It is possible that this phase is not achieved experimentally due to a large energy barrier and a correspondingly large required volume drop, resulting in the transformation to metastable dolomite II. We calculate the equation of state of trigonal dolomite, dolomite III and monoclinic C2/c dolomite to 80 GPa with 0 and 50 mol% CaFe(CO3)2 and compare their enthalpies to single-carbonate assemblages. Although end-member C2/c CaMg(CO3)2 dolomite is not stable relative to single-cation carbonates, C2/c CaMg0.5Fe0.5(CO3)2 is preferred over single-cation carbonates at high pressures. Thus, iron-bearing C2/c dolomite may be an important host phase for carbon in slabs subducted into the lower mantle. [1] Shirasaka, M., et al. (2002) American Mineralogist, 87, 922-930. [2] Mao, Z. et al. (2011) Geophysical Research Letters, 38. [3] Merlini, M. et al. (2012) Proceedings of the National Academy of Sciences, 109, 13509-13514.

Two spin-peierls-like compounds exhibiting divergent structural features, lattice compression, and expansion in the low- temperature phase.

PubMed

Tian, Zhengfang; Duan, Haibao; Ren, Xiaoming; Lu, Changsheng; Li, Yizhi; Song, You; Zhu, Huizhen; Meng, Qingjin

2009-06-18

Two quasi-one-dimensional (quasi-1D) compounds, [4'-CH(3)Bz-4-RPy][Ni(mnt)(2)] (mnt(2-) = maleonitriledithiolate), where 4'-CH(3)Bz-4-RPy(+) = 1-(4'-methylbenzyl)pyridinium (denoted as compound 1) and 1-(4'-methylbenzyl)-4-aminopyridinium (denoted as compound 2), show a spin-Peierls-like transition with T(C) approximately 182 K for 1 and T(C) approximately 155 K for 2. The enthalpy changes for the transition are estimated to be DeltaH = 316.6 J.mol(-1) for 1 and 1082.1 J.mol(-1) for 2. From fits to the magnetic susceptibility, the magnetic exchange constants in the gapless state are calculated to be J = 166(2) K with g = 2.020(23) for 1 versus J = 42(0) K with g = 2.056(5) for 2. In the high-temperature (HT) phase, 1 and 2 are isostructural and crystallize in the monoclinic space group P2(1)/c. The nonmagnetic cations and paramagnetic anions form segregated columns with regular anionic and cationic stacks. In the low-temperature (LT) phase, the crystals of the two compounds undergo a transformation to the triclinic space group P-1, and both anionic and cationic stacks dimerize. In the transformation from the HT to LT phases, the two compounds exhibit divergent structural features, with lattice compression for 1 but lattice expansion for 2, due to intermolecular slippage. Combined with our previous studies, it is also noted that the transition temperature, T(C), is qualitatively related to the cell volume in the HT phase for the series of compounds [1-(4'-R-benzylpyridinium][Ni(mnt)(2)] (where R represents the substituent). When there is a single substituent in the para position of benzene, giving a larger cell volume, the transition temperature increases.

Structure, dielectric and electric properties of diisobutylammonium hydrogen sulfate crystal

NASA Astrophysics Data System (ADS)

Bednarchuk, Tamara J.; Kinzhybalo, Vasyl; Markiewicz, Ewa; Hilczer, Bożena; Pietraszko, Adam

2018-02-01

Diisobutylammonium hydrogen sulfate, a new organic-inorganic hybrid compound, was successfully synthesized and three structural phases in 298-433 K temperature range were revealed by differential scanning calorimetry and X-ray powder diffraction studies. Single crystal X-ray diffraction data were used to describe the crystal structures in each particular case. In phase III (below 336/319 K on heating/cooling) the crystal arrangement appears to be within the triclinic symmetry with P-1 space group. During heating in the 336-339 K region (and 319-337 K on cooling) the crystal exists in the phase II, characterized by monoclinic symmetry with P21/c space group. Consequently, above 339 K (during heating, and 337 K during cooling temperature sequences), i.e. in phase I the crystal exhibits orthorhombic symmetry (Cmce space group). Ferroelastic domain structure was observed in phase III. These phase boundaries (III→II and II→I) were accompanied by the presence of small anomalies, apparent in the dielectric permittivity and electric conductivity experimental data. Fast proton transport with activation energy of 0.23 eV was observed in the high temperature phase I and related to phonon assisted proton diffusion conditioned by disorder of diisobutylammonium (diba) cations, as well as by high thermal displacements of oxygen and sulfur atoms of hydrogen sulfate anion (hs).

Revealing the hidden structural phases of FeRh

NASA Astrophysics Data System (ADS)

Kim, Jinwoong; Ramesh, R.; Kioussis, Nicholas

2016-11-01

Ab initio electronic structure calculations reveal that tetragonal distortion has a dramatic effect on the relative stability of the various magnetic structures (C-, A-, G-, A'-AFM, and FM) of FeRh giving rise to a wide range of novel stable/metastable structures and magnetic phase transitions between these states. We predict that the cubic G-AFM structure, which was believed thus far to be the ground state, is metastable and that the tetragonally expanded G-AFM is the stable structure. The low energy barrier separating these states suggests phase coexistence at room temperature. We propose an A'-AFM phase to be the global ground state among all magnetic phases which arises from the strain-induced tuning of the exchange interactions. The results elucidate the underlying mechanism for the recent experimental findings of electric-field control of magnetic phase transition driven via tetragonal strain. The magnetic phase transitions open interesting prospects for exploiting strain engineering for the next-generation memory devices.

Atomic structure and pressure-induced phase transformations in a phase-change alloy

NASA Astrophysics Data System (ADS)

Xu, Ming

Phase-change materials exist in at least two phases under the ambient condition. One is the amorphous state and another is crystalline phase. These two phases have vastly different physical properties, such as electrical conductivity, optical reflectivity, mass density, thermal conductivity, etc. The distinct physical properties and the fast transformation between amorphous and crystalline phases render these materials the ability to store information. For example, the DVD and the Blue-ray discs take advantage of the optical reflectivity contrast, and the newly developed solid-state memories make use of the large conductivity difference. In addition, both the amorphous and crystalline phases in phase-change memories (PCMs) are very stable at room temperature, and they are easy to be scaled up in the production of devices with large storage density. All these features make phase-change materials the ideal candidates for the next-generation memories. Despite of the fast development of these new memory materials in industry, many fundamental physics problems underlying these interesting materials are still not fully resolved. This thesis is aiming at solving some of the key issues in phase-change materials. Most of phase-change materials are composed of Ge-Sb-Te constituents. Among all these Ge-Sb-Te based materials, Ge2Sb2Te5 (GST) has the best performance and has been frequently studied as a prototypical phase-change material. The first and foremost issue is the structure of the two functioning phases. In this thesis, we investigate the unique atomic structure and bonding nature of amorphous GST (a-GST) and crystalline GST ( c-GST), using ab initio tools and X-ray diffraction (XRD) methods. Their local structures and bonding scenarios are then analyzed using electronic structure calculations. In order to gain insight into the fast phase transformation mechanism, we also carried out a series of high-pressure experiments on GST. Several new polymorphs and their

Structural phase transitions in monolayer molybdenum dichalcogenides

NASA Astrophysics Data System (ADS)

Choe, Duk-Hyun; Sung, Ha June; Chang, Kee Joo

2015-03-01

The recent discovery of two-dimensional materials such as graphene and transition metal dichalcogenides (TMDs) has provided opportunities to develop ultimate thin channel devices. In contrast to graphene, the existence of moderate band gap and strong spin-orbit coupling gives rise to exotic electronic properties which vary with layer thickness, lattice structure, and symmetry. TMDs commonly appear in two structures with distinct symmetries, trigonal prismatic 2H and octahedral 1T phases which are semiconducting and metallic, respectively. In this work, we investigate the structural and electronic properties of monolayer molybdenum dichalcogenides (MoX2, where X = S, Se, Te) through first-principles density functional calculations. We find a tendency that the semiconducting 2H phase is more stable than the metallic 1T phase. We show that a spontaneous symmetry breaking of 1T phase leads to various distorted octahedral (1T') phases, thus inducing a metal-to-semiconductor transition. We discuss the effects of carrier doping on the structural stability and the modification of the electronic structure. This work was supported by the National Research Foundation of Korea (NRF) under Grant No. NRF-2005-0093845 and Samsung Science and Technology Foundation under Grant No. SSTFBA1401-08.

Intermediate phases in [111]- and [001]-oriented PbMg1/3Nb2/3O3-29PbTiO3 single crystals

NASA Astrophysics Data System (ADS)

Kamzina, L. S.

2017-09-01

Phase transformations in [111]- and [001]-oriented PbMg1/3Nb2/3O3-29PbTiO3 single crystals have been studied using dielectric and optical measurements before and after applying an electric field. It is shown that the subsequence of phase transitions rhombohedral ( R)—tetragonal ( T)—cubic ( C) phases is observed in nonpolarized samples of both orientations as temperature increases. In the [111]-oriented crystal, an additional intermediate monoclinic phase (it is possible, M a ) is induced after preliminary polarization at room temperature and the R- M a - T- C phase transitions are observed on heating. In the [001]-oriented crystal, after its polarization, the monoclinic phase forms instead of the rhombohedral phase even at room temperature and the M a - T- C transitions occur on heating. The results are discussed from the point of view of the existence polar nanoregions with different local symmetries in a glasslike matrix.

Study of thermal stability of Cu{sub 2}Se thermoelectric material

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

Bohra, Anil, E-mail: anilbohra786@gmail.com; Bhatt, Ranu; Bhattacharya, Shovit

2016-05-23

Sustainability of thermoelectric parameter in operating temperature range is a key consideration factor for fabricating thermoelectric generator or cooler. In present work, we have studied the stability of thermoelectric parameter of Cu{sub 2}Se within the temperature range of 50-800°C. Temperature dependent Seebeck coefficients and electrical resistivity measurement are performed under three continuous thermal cycles. X-ray diffraction pattern shows the presence of mixed cubic-monoclinic Cu{sub 2}Se phase in bare pellet which transforms to pure α-Cu{sub 2}Se phase with repeating thermal cycle. Significant enhancement in Seebeck coefficient and electrical resistivity is observed which may be attributed to (i) Se loss observed inmore » EDS and (ii) the phase transformation from mixed cubic-monoclinic structure to pure monoclinic α-Cu{sub 2}Se phase.« less

Electric field induced cubic to monoclinic phase transition in multiferroic 0.65Bi(Ni{sub 1/2}Ti{sub 1/2})O{sub 3}-0.35PbTiO{sub 3} solid solution

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

Pandey, Rishikesh; Singh, Akhilesh Kumar, E-mail: akhilesh-bhu@yahoo.com

2014-10-20

The results of x-ray diffraction studies on 0.65Bi(Ni{sub 1/2}Ti{sub 1/2})O{sub 3}-0.35PbTiO{sub 3} solid solution poled at various electric fields are presented. After poling, significant value of planar electromechanical coupling coefficient (k{sub P}) is observed for this composition having cubic structure in unpoled state. The cubic structure of 0.65Bi(Ni{sub 1/2}Ti{sub 1/2})O{sub 3}-0.35PbTiO{sub 3} transforms to monoclinic structure with space group Pm for the poling field ≥5 kV/cm. Large c-axis microscopic lattice strain (1.6%) is achieved at 30 kV/cm poling field. The variation of the c-axis strain and unit cell volume with poling field shows a drastic jump similar to that observed for strainmore » versus electric field curve in (1 − x)Pb(Mg{sub 1/3}Nb{sub 2/3}) O{sub 3}-xPbTiO{sub 3} and (1 − x)Pb(Zn{sub 1/3}Nb{sub 2/3})O{sub 3}-xPbTiO{sub 3}.« less

Difference in the luminescence properties of orthorhombic and monoclinic forms of Y{sub 2}GeO{sub 5}:Ln (Ln = Tb{sup 3+} and Dy{sup 3+})

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

Tyagi, Adish; Shah, Alpa; Sudarsan, V., E-mail: vsudar@barc.gov.in

2015-04-15

Highlights: • Improved emission colour purity with orthorhombic form of Y{sub 2}GeO{sub 5}. • Non-stationary quenching exists in orthorhombic and monoclinic forms of Y{sub 2}GeO{sub 5}:Tb. • Ion pair formation and cross relaxation quenching operating for Y{sub 2}GeO{sub 5}:Dy samples. - Abstract: The luminescence properties of Tb{sup 3+} and Dy{sup 3+} doped orthorhombic and monoclinic forms of Y{sub 2}GeO{sub 5} are significantly different. Orthorhombic Y{sub 2}GeO{sub 5} doped with Tb{sup 3+} and Dy{sup 3+} ions gives bright green and blue emission upon UV light excitation with CIE coordinates (0.25, 0.46) and (0.25, 0.24), respectively. The monoclinic Y{sub 2}GeO{sub 5} dopedmore » with these ions exhibits light green and yellowish white emissions, respectively. This has been attributed to the differences in crystallographic environments around Y{sup 3+} ions in orthorhombic and monoclinic forms of Y{sub 2}GeO{sub 5}. Quantum yield of emission for orthorhombic Y{sub 2}GeO{sub 5}:Tb (∼29%) is significantly higher than that of the monoclinic Y{sub 2}GeO{sub 5}:Tb (∼14%). Lifetime values corresponding to {sup 4}F{sub 9/2} level of Dy{sup 3+} ions in both monoclinic and orthorhombic forms of Y{sub 2}GeO{sub 5} follow an opposite trend with respect to {sup 5}D{sub 4} level of Tb{sup 3+} ions. This is attributed to difference in the concentration quenching mechanism operating for Tb{sup 3+} and Dy{sup 3+} ions.« less

Symmetry of Epitaxial BiFeO3 Films in the Ultrathin Regime

NASA Astrophysics Data System (ADS)

Yang, Yongsoo; Schlep&üTz, Christian; Adamo, Carolina; Schlom, Darrell; Clarke, Roy

2013-03-01

BiFeO3 (BFO) films grown on SrTiO3 (STO) with a SrRuO3 buffer layer exhibit a monoclinic structure at thicknesses greater than 40 nm, but higher structural symmetry can be observed for thinner films [Phys. Rev. B 81, 144115 (2010)]. We report a structural phase transition from monoclinic to tetragonal in ultra-thin BFO films grown directly on (100)-oriented STO. X-ray diffraction measurements of 3-dimensional reciprocal space maps reveal half-integer order peaks due to oxygen octahedral tilting. When the film thickness is decreased below 20 unit cells, the integer-order Bragg peak splitting associated with the presence of multiple domains of the monoclinic phase disappears. Instead, a single peak that is commensurate with the STO substrate lattice appears. The diffraction pattern has four-fold symmetry, ruling out the presence of a single monoclinic domain in favor of a tetragonal film structure. The evolution of the oxygen octahedra tilt pattern inferred from the intensities of half-order peaks suggests that this transition originates from the corner-connectivity of oxygen atoms at the interface between BFO and STO, and also strongly supports this monoclinic to tetragonal transition. Supported in part by the U.S. Department of Energy (DE-FG02-06ER46273). Measurements performed at Sectors 13-BMC, 33-IDD, 33-BMC of the Advanced Photon Source, Argonne National Laboratory, USA (DOE contract No. DE-AC02-06CH11357).

Structural and magnetic phase transitions in gadolinium under high pressures and low temperatures

DOE PAGES

Samudrala, Gopi K.; Tsoi, Georgiy M.; Weir, Samuel T.; ...

2014-11-07

High pressure structural transition studies have been carried out on rare earth metal gadolinium in a diamond anvil cell at room temperature to 169 GPa. Gadolinium has been compressed to 38% of its initial volume at this pressure. With increasing pressure, a crystal structure sequence of hcp → Smtype→ dhcp → fcc → dfcc → monoclinic has been observed in our studies on gadolinium. The measured equation of state of gadolinium is presented to 169 GPa at ambient temperature. Magnetic ordering temperature of gadolinium has been studied using designer diamond anvils to a pressure of 25 GP and a temperaturemore » of 10 K. The magnetic ordering temperature has been determined from the four-point electrical resistivity measurements carried out on gadolinium. Furthermore, our experiments show that the magnetic transition temperature decreases with increasing pressure to 19 GPa and then increases when gadolinium is subjected to higher pressures.« less

Structural and magnetic phase transitions in gadolinium under high pressures and low temperatures

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

Samudrala, Gopi K.; Tsoi, Georgiy M.; Weir, Samuel T.

High pressure structural transition studies have been carried out on rare earth metal gadolinium in a diamond anvil cell at room temperature to 169 GPa. Gadolinium has been compressed to 38% of its initial volume at this pressure. With increasing pressure, a crystal structure sequence of hcp → Smtype→ dhcp → fcc → dfcc → monoclinic has been observed in our studies on gadolinium. The measured equation of state of gadolinium is presented to 169 GPa at ambient temperature. Magnetic ordering temperature of gadolinium has been studied using designer diamond anvils to a pressure of 25 GP and a temperaturemore » of 10 K. The magnetic ordering temperature has been determined from the four-point electrical resistivity measurements carried out on gadolinium. Furthermore, our experiments show that the magnetic transition temperature decreases with increasing pressure to 19 GPa and then increases when gadolinium is subjected to higher pressures.« less

Structural and magnetic phase transitions in gadolinium under high pressures and low temperatures

NASA Astrophysics Data System (ADS)

Samudrala, Gopi K.; Tsoi, Georgiy M.; Weir, Samuel T.; Vohra, Yogesh K.

2014-10-01

High pressure structural transition studies have been carried out on rare earth metal gadolinium in a diamond anvil cell at room temperature to 169 GPa. Gadolinium has been compressed to 38% of its initial volume at this pressure. With increasing pressure, a crystal structure sequence of hcp → Sm-type → dhcp → fcc → dfcc → monoclinic has been observed in our studies on gadolinium. The measured equation of state of gadolinium is presented to 169 GPa at ambient temperature. Magnetic ordering temperature of gadolinium has been studied using designer diamond anvils to a pressure of 25 GPa and a temperature of 10 K. The magnetic ordering temperature has been determined from the four-point electrical resistivity measurements carried out on gadolinium. Our experiments show that the magnetic transition temperature decreases with increasing pressure to 19 GPa and then increases when gadolinium is subjected to higher pressures.

Polymorphism in Strontium Tungstate SrWO4 under Quasi-Hydrostatic Compression.

PubMed

Santamaria-Perez, David; Errandonea, Daniel; Rodriguez-Hernandez, Placida; Muñoz, Alfonso; Lacomba-Perales, Raul; Polian, Alain; Meng, Yue

2016-10-03

The structural and vibrational properties of SrWO 4 have been studied experimentally up to 27 and 46 GPa, respectively, by angle-dispersive synchrotron X-ray diffraction and Raman spectroscopy measurements as well as using ab initio calculations. The existence of four polymorphs upon quasi-hydrostatic compression is reported. The three phase transitions were found at 11.5, 19.0, and 39.5 GPa. The ambient-pressure SrWO 4 tetragonal scheelite-type structure (S.G. I4 1 /a) undergoes a transition to a monoclinic fergusonite-type structure (S.G. I2/a) at 11.5 GPa with a 1.5% volume decrease. Subsequently, at 19.0 GPa, another structural transformation takes place. Our calculations indicate two possible post-fergusonite phases, one monoclinic and the other orthorhombic. In the diffraction experiments, we observed the theoretically predicted monoclinic LaTaO 4 -type phase coexisting with the fergusonite-type phase up to 27 GPa. The coexistence of the two phases and the large volume collapse at the transition confirm a kinetic hindrance typical of first-order phase transitions. Significant changes in Raman spectra suggest a third pressure-induced transition at 39.5 GPa. The conclusions extracted from the experiments are complemented and supported by ab initio calculations. Our data provides insight into the structural mechanism of the first transition, with the formation of two additional W-O contacts. The fergusonite-type phase can be therefore considered as a structural bridge between the scheelite structure, composed of [WO 4 ] tetrahedra, and the new higher pressure phases, which contain [WO 6 ] octahedra. All the observed phases are compatible with the high-pressure structural systematics predicted for ABO 4 compounds using crystal-chemistry arguments such as the diagram proposed by Bastide.

Kinetics of hexacelsian to celsian phase transformation in SrAl2Si2O8

NASA Technical Reports Server (NTRS)

Bansal, Narottam P.; Drummond, Charles H., III

1992-01-01

The kinetics of hexacelsian to celsian phase transformation in SrAl2Si2O8 have been investigated. Phase pure hexacelsian was prepared by heat treatment of glass flakes at 990 C for 10 h. Bulk hexacelsian was isothermally heat treated at 1026, 1050, 1100, 1152, and 1200 C for various times. The amounts of monoclinic celsian formed were determined using quantitative X-ray diffraction. Values of reaction rate constant, k, at various temperatures were evaluated from the Avrami equation. The Avrami parameter was determined to be 1.1, suggesting a diffusionless, one-dimensional transformation mechanism. From the temperature dependence of k, the activation energy for this reaction was evaluated to be 527 plus or minus 50 kJ/mole (126 plus or minus 12 kcal/mole). This value is consistent with a mechanism involving the transformation of the layered hexacelsian structure to a three-dimensional network celsian structure which necessitates breaking of the strongest bonds, the Si-O bonds.

Kinetics of hexacelsian-to-celsian phase transformation in SrAl2Si2O8

NASA Technical Reports Server (NTRS)

Bansal, Narottam P.; Drummond, Charles H., III

1993-01-01

The kinetics of hexacelsian to celsian phase transformation in SrAl2Si2O8 have been investigated. Phase pure hexacelsian was prepared by heat treatment of glass flakes at 990 C for 10 h. Bulk hexacelsian was isothermally heat treated at 1026, 1050, 1100, 1152, and 1200 C for various times. The amounts of monoclinic celsian formed were determined using quantitative X-ray diffraction. Values of reaction rate constant, k, at various temperatures were evaluated from the Avrami equation. The Avrami parameter was determined to be 1.1, suggesting a diffusionless, one-dimensional transformation mechanism. From the temperature dependence of k, the activation energy for this reaction was evaluated to be 527 plus or minus 50 kJ/mole (126 plus or minus 12 kcal/mole). This value is consistent with a mechanism involving the transformation of the layered hexacelsian structure to a three-dimensional network celsian structure which necessitates breaking of the strongest bonds, the Si-O bonds.

On the symmetry and crystal structures of Ba{sub 2}LaIrO{sub 6}

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

Fu, W.T., E-mail: w.fu@chem.leidenuniv.n; Goetz, R.J.; IJdo, D.J.W.

2010-02-15

Accurate profile analysis of X-ray diffraction data was carried out to settle recent dispute on the symmetry and crystal structures of the double perovskite Ba{sub 2}LaIrO{sub 6}. Even through careful comparison of the full-width at half-maximum values, we found no evidence for Ba{sub 2}LaIrO{sub 6} adopting either monoclinic (I2/m) or mixed rhombohedral (R3-bar) and monoclinic (I2/m) structures at room temperature, becoming triclinic (I1-bar) at below about 200 K. The correct space group is just R3-bar at temperatures between 82 and 653 K. Furthermore, the R3-bar->Fm3-barm phase transition does occur in Ba{sub 2}LaIrO{sub 6}, but the transition temperature is found tomore » be much higher than the reported value. - Graphical abstract: Observed (crosses) and calculated (continuous line) profiles of Ba{sub 2}LaIrO{sub 6} at some selected temperature showing the region containing the basic (222), (321) and (400) reflections. Tick marks below indicate the positions of the allowed Bragg's reflections.« less

Structural metatransition of energetically tangled crystalline phases.

PubMed

Zhou, Dan; Li, Quan; Zheng, Weitao; Ma, Yanming; Chen, Changfeng

2017-02-08

We solve the longstanding puzzle of pressure induced structural evolution of SnSe using a swarm structure search method combined with first-principles phonon and kinetic barrier calculations. Our results identify a dynamic set of nearly degenerate crystalline SnSe phases that are separated by low kinetic barriers and undergo an unusual type of structural transitions characterized by a dynamically changing mix of the constituent phases. We introduce a new concept of structural metatransition to highlight the transitional nature of such phase transitions. Our theoretical prediction is corroborated by X-ray diffraction measurements, and this intriguing phenomenon offers insights into the enigmatic property variations of SnSe under pressure. This work raises prospects of considerably improving characterization and understanding of intrinsic multiphase crystals and their dynamic evolution.

THE STRUCTURE, MAGNETISM AND CONDUCTIVITY OF Li3V2(PO4)3: A THEORETICAL AND EXPERIMENTAL STUDY

NASA Astrophysics Data System (ADS)

Lin, Zhi-Ping; Zhao, Yu-Jun; Zhao, Yan-Ming

2013-10-01

In this paper, we present a combination of first-principles and experimental investigations on the structural, magnetic and electronic properties of monoclinic Li3V2(PO4)3. The change of dielectric constant indicates that the structural phase transition appear around the temperature 120°C. The first-principles calculation and magnetic measurement display that Li3V2(PO4)3 is a compound with weak ferromagnetism, with Curie constant of C = 0.004 and Curie temperature of 140 K. The experimental and theoretical results demonstrated that the Li3V2(PO4)3 is a typical semiconductor.

High-pressure polymorphism of As2S3 and new AsS2 modification with layered structure

NASA Astrophysics Data System (ADS)

Bolotina, N. B.; Brazhkin, V. V.; Dyuzheva, T. I.; Katayama, Y.; Kulikova, L. F.; Lityagina, L. V.; Nikolaev, N. A.

2014-01-01

At normal pressure, the As2S3 compound is the most stable equilibrium modification with unique layered structure. The possibility of high-pressure polymorphism of this substance remains questionable. Our research showed that the As2S3 substance was metastable under pressures P > 6 GPa decomposing into two high-pressure phases: As2S3 → AsS2 + AsS. New AsS2 phase can be conserved in the single crystalline form in metastable state at room pressure up to its melting temperature (470 K). This modification has the layered structure with P1211 monoclinic symmetry group; the unit-cell values are a = 7.916(2) Å, b = 9.937(2) Å, c = 7.118(1) Å, β = 106.41° ( Z = 8, density 3.44 g/cm3). Along with the recently studied AsS high-pressure modification, the new AsS2 phase suggests that high pressure polymorphism is a very powerful tool to create new layered-structure phases with "wrong" stoichiometry.

Ordered vs. disordered perovskites; structural studies of Fe-doped SrIrO{sub 3} and SrRuO{sub 3}

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

Qasim, Ilyas; Blanchard, Peter E.R.; Liu, Samuel

2013-10-15

The structures of the two Fe containing perovskites Sr{sub 2}IrFeO{sub 6} and SrRu{sub 0.5}Fe{sub 0.5}O{sub 3} have been established using a combination of synchrotron and neutron diffraction methods. Sr{sub 2}IrFeO{sub 6} and SrRu{sub 0.5}Fe{sub 0.5}O{sub 3} are shown to be monoclinic I2/m and tetragonal I4/mcm respectively The former exhibits a rock-salt like ordering of the Fe and Ir cations and displays a sequence of phase transitions associated with the loss of the octahedral tilts upon heating; 12/m→I4/m→Fm3{sup ¯}m. The Fe and Ru cations are disordered in SrRu{sub 0.5}Fe{sub 0.5}O{sub 3} and this shows a single structural phase transition upon heatingmore » due to the loss of the in-phase tilts, viz. I4/mcm→Pm3{sup ¯}m. In both cases XANES measurements show partial oxidation of the Fe{sup 3+} to Fe{sup 4+}. The difference in the structures between the two is remarkable given the similar size of Ir{sup 5+} and Ru{sup 5+}, and this is reflected in their magnetic properties. - Graphical abstract: Sr{sub 2}IrFeO{sub 6} and SrRu{sub 0.5}Fe{sub 0.5}O{sub 3} are shown, using a combination of synchrotron and neutron diffraction, to be monoclinic I2/m with cation ordering and tetragonal I4/mcm with disordered Fe and Ir, respectively. Both undergo phase transitions upon heating due to the loss of the octahedral tilts. Display Omitted - Highlights: • Sr{sub 2}IrFeO{sub 6} shown to be monoclinic and shows the transitions upon heating I2/m→I4/m→ Fm3{sup ¯}m. • SrRu{sub 0.5}Fe{sub 0.5}O{sub 3} is tetragonal and shows a single I4/mcm→Pm3m transition upon heating. • The Fe and Ru cations are disordered in SrRu{sub 0.5}Fe{sub 0.5}O{sub 3} but ordered in Sr{sub 2}FeIrO{sub 6}. • XANES measurements show partial oxidation of the Fe{sup 3+} to Fe{sup 4+}.« less

Photocatalytic degradation and removal mechanism of ibuprofen via monoclinic BiVO4 under simulated solar light.

PubMed

Li, Fuhua; Kang, Yapu; Chen, Min; Liu, Guoguang; Lv, Wenying; Yao, Kun; Chen, Ping; Huang, Haoping

2016-05-01

Characterized as by X-ray diffraction, scanning electron microscopy and UV-vis diffuse reflectance spectra techniques, BiVO4 photocatalyst was hydrothermally synthesized. The photocatalytic degradation mechanisms of ibuprofen (IBP) were evaluated in aqueous media via BiVO4. Results demonstrated that the prepared photocatalyst corresponded to phase-pure monoclinic scheelite BiVO4. The synthesized BiVO4 showed superior photocatalytic properties under the irradiation of visible-light. The photocatalytic degradation rate of IBP decreased with an increase in the initial IBP concentration. The degradation process followed first-order kinetics model. At an IBP concentration of 10 mg L(-1), while a BiVO4 concentration of 5.0 g L(-1) with pH value of 4.5, the rate of IBP degradation was obtained as 90% after 25 min. The photocatalytic degradation of IBP was primarily accomplished via the generation of superoxide radical (O2(•-)) and hydroxyl radicals ((•)OH). During the process of degradation, part of the (•)OH was converted from the O2(•-). The direct oxidation of holes (h(+)) made a minimal contribution to the degradation of IBP. Copyright © 2016 Elsevier Ltd. All rights reserved.

Pressure-induced phase transition and fracture in α-MoO3 nanoribbons

NASA Astrophysics Data System (ADS)

Silveira, Jose V.; Vieira, Luciana L.; Aguiar, Acrisio L.; Freire, Paulo T. C.; Mendes Filho, Josue; Alves, Oswaldo L.; Souza Filho, Antonio G.

2018-03-01

MoO3 nanoribbons were studied under different pressure conditions ranging from 0 to 21 GPa at room temperature. The effect of the applied pressure on the spectroscopic and morphologic properties of the MoO3 nanoribbons was investigated by means of Raman spectroscopy and scanning electron microscopy techniques. The pressure dependent Raman spectra of the MoO3 nanoribbons indicate that a structural phase transition occurs at 5 GPa from the orthorhombic α-MoO3 phase (Pbnm) to the monoclinic MoO3-II phase (P21/m), which remains stable up to 21 GPa. Such phase transformation occurs at considerably lower pressure than the critical pressure for α-MoO3 microcrystals (12 GPa). We suggested that the applanate morphology combined with the presence of crystalline defects in the sample play an important role in the phase transition of the MoO3 nanoribbons. Frequencies and linewidths of the Raman bands as a function of pressure also suggest a pressure-induced morphological change and the decreasing of the nanocrystal size. The observed spectroscopic changes are supported by electron microscopy images, which clearly show a pressure-induced morphologic change in MoO3 nanoribbons.

Process-structure-property relationships of micron thick gadolinium oxide films deposited by reactive electron beam-physical vapor deposition (EB-PVD)

NASA Astrophysics Data System (ADS)

Grave, Daniel A.

Gadolinium oxide (Gd2O3) is an attractive material for solid state neutron detection due to gadolinium's high thermal neutron capture cross section. Development of neutron detectors based on Gd2 O3 requires sufficiently thick films to ensure neutron absorption. In this dissertation work, the process-structure-property relationships of micron thick Gd2O3 films deposited by reactive electron-beam physical vapor deposition (EB-PVD) were studied. Through a systematic design of experiments, fundamental studies were conducted to determine the effects of processing conditions such as deposition temperature, oxygen flow rate, deposition rate, and substrate material on Gd2O3 film crystallographic phase, texture, morphology, grain size, density, and surface roughness. Films deposited at high rates (> 5 A/s) were examined via x-ray diffraction (XRD) and Raman spectroscopy. Quantitative phase volume calculations were performed via a Rietveld refinement technique. All films deposited at high rates were found to be fully monoclinic or mixed cubic/monoclinic phase. Generally, increased deposition temperature and increased oxygen flow resulted in increased cubic phase volume. As film thickness increased, monoclinic phase volume increased. Grazing incidence x-ray diffraction (GIXRD) depth profiling analysis showed that cubic phase was only present under large incidence angle (large penetration depth) measurements, and after a certain point, only monoclinic phase was grown. This was confirmed by transmission electron microscopy (TEM) analysis with selected area diffraction (SAD). Based on this information, a large compressive stress was hypothesized to cause the formation of the monoclinic phase and this hypothesis was confirmed by demonstrating the existence of a stress induced phase transition. An experiment was designed to introduce compressive stress into the Gd2O 3 films via ion beam assisted deposition (IBAD). This allowed for systematic increase in compressive stress while

Reconstructive structural phase transitions in dense Mg

NASA Astrophysics Data System (ADS)

Yao, Yansun; Klug, Dennis D.

2012-07-01

The question raised recently about whether the high-pressure phase transitions of Mg follow a hexagonal close-packed (hcp) → body centered cubic (bcc) or hcp → double hexagonal close-packed (dhcp) → bcc sequence at room temperature is examined by the use of first principles density functional methods. Enthalpy calculations show that the bcc structure replaces the hcp structure to become the most stable structure near 48 GPa, whereas the dhcp structure is never the most stable structure in the pressure range of interest. The characterized phase-transition mechanisms indicate that the hcp → dhcp transition is also associated with a higher enthalpy barrier. At room temperature, the structural sequence hcp → bcc is therefore more energetically favorable for Mg. The same conclusion is also reached from the simulations of the phase transitions using metadynamics methods. At room temperature, the metadynamics simulations predict the onset of a hcp → bcc transition at 40 GPa and the transition becomes more prominent upon further compression. At high temperatures, the metadynamics simulations reveal a structural fluctuation among the hcp, dhcp, and bcc structures at 15 GPa. With increasing pressure, the structural evolution at high temperatures becomes more unambiguous and eventually settles to a bcc structure once sufficient pressure is applied.

Reconstructive structural phase transitions in dense Mg.

PubMed

Yao, Yansun; Klug, Dennis D

2012-07-04

The question raised recently about whether the high-pressure phase transitions of Mg follow a hexagonal close-packed (hcp) → body centered cubic (bcc) or hcp → double hexagonal close-packed (dhcp) → bcc sequence at room temperature is examined by the use of first principles density functional methods. Enthalpy calculations show that the bcc structure replaces the hcp structure to become the most stable structure near 48 GPa, whereas the dhcp structure is never the most stable structure in the pressure range of interest. The characterized phase-transition mechanisms indicate that the hcp → dhcp transition is also associated with a higher enthalpy barrier. At room temperature, the structural sequence hcp → bcc is therefore more energetically favorable for Mg. The same conclusion is also reached from the simulations of the phase transitions using metadynamics methods. At room temperature, the metadynamics simulations predict the onset of a hcp → bcc transition at 40 GPa and the transition becomes more prominent upon further compression. At high temperatures, the metadynamics simulations reveal a structural fluctuation among the hcp, dhcp, and bcc structures at 15 GPa. With increasing pressure, the structural evolution at high temperatures becomes more unambiguous and eventually settles to a bcc structure once sufficient pressure is applied.

Structural evolution of plasma-sprayed nanoscale 3 mol% and 5 mol% yttria-stabilized zirconia coatings during sintering

NASA Astrophysics Data System (ADS)

Zhao, Yan; Gao, Yang

2017-12-01

The microstructure of plasma-sprayed nanostructured yttria-stabilized zirconia (YSZ) coatings may change during high-temperature exposure, which would influence the coating performance and service lifetime. In this study, the phase structure and the microstructural evolution of 3YSZ (zirconia-3 mol% yttria) and 5YSZ (zirconia-5 mol% yttria) nanostructured coatings were investigated by means of sintering at 1400 °C for 50-100 h. The microhardness, elastic moduli, and thermal shock cycles of the 3YSZ and 5YSZ nanostructured coatings were also investigated. The results showed that the redistribution of yttrium ions at 1400 °C caused the continuous increase of monoclinic-phase zirconia, but no obvious inter-splat cracking formed at the cross-sections, even after 100 h. Large voids appeared around the nanoporous zone because of the sintering of nanoscale granules upon high-temperature exposure. The microhardness and elastic moduli of the nanostructured coatings first increased and then decreased with increasing sintering times. The growth rate of the nanograins in the 3YSZ coating was lower than that in 5YSZ, which slowed the changes in 3YSZ coating porosity during sintering. Although the 3YSZ coating was prone to monoclinic phase transition, the experimental results showed that the thermal shock resistance of the 3YSZ coating was better than that of the 5YSZ coating.

A very promising piezoelectric property of Ta{sub 2}O{sub 5} thin films. II: Birefringence and piezoelectricity

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

Audier, M., E-mail: Marc.Audier@grenoble-inp.fr; Chenevier, B.; Roussel, H.

2011-08-15

Birefringent and piezoelectric properties of Ta{sub 2}O{sub 5} ceramic thin films of monoclinic and trigonal structures were analyzed. The birefringence, observed by reflected polarized light microscopy, yields information on thin film microstructures, crystal shapes and sizes and on crystallographic orientations of grains of trigonal structure. Such an information was considered for investigating piezoelectric properties by laser Doppler vibrometry and by piezoresponse force microscopy. The vibration velocity was measured by applying an oscillating electric field between electrodes on both sides of a Ta{sub 2}O{sub 5} film deposited on a Si substrate which was pasted on an isolating mica sheet. In thismore » case, it is shown that the vibration velocity results were not only from a converse piezoelectric effect, proportional to the voltage, but also from the Coulomb force, proportional to the square of the voltage. A huge piezoelectric strain effect, up to 7.6%, is found in the case of Ta{sub 2}O{sub 5} of trigonal structure. From an estimation of the electrical field through the Ta{sub 2}O{sub 5} thin film, this strain likely corresponds to a very high longitudinal coefficient d{sub 33} of several thousand picometers. Results obtained by piezoresponse force microscopy show that trigonal grains exhibit a polarization at zero field, which is probably due to stress caused expansion in the transition monoclinic-trigonal, presented in a previous article (part I). - Graphical abstract: Image of cross-polarized optical microscopy showing grains of trigonal structure embedded in the monoclinic phase (on the left); (a) mounting of the sample for Laser Doppler Vibrometry, sample constituted of several layers and its equivalent electrical circuit; (b) longitudinal displacements due to converse piezoelectric and Coulomb effects and corresponding piezoelectric strain-U{sub app.}. hystereses. Highlights: > A new Ta{sub 2}O{sub 5} trigonal phase is shown to be birefringent

Using a plenoptic sensor to reconstruct vortex phase structures.

PubMed

Wu, Chensheng; Ko, Jonathan; Davis, Christopher C

2016-07-15

A branch point problem and its solution commonly involve recognizing and reconstructing a vortex phase structure around a singular point. In laser beam propagation through random media, the destructive phase contributions from various parts of a vortex phase structure will cause a dark area in the center of the beam's intensity profile. This null of intensity can, in turn, prevent the vortex phase structure from being recognized. In this Letter, we show how to use a plenoptic sensor to transform the light field of a vortex beam so that a simple and direct reconstruction algorithm can be applied to reveal the vortex phase structure. As a result, we show that the plenoptic sensor is effective in detecting branch points and can be used to reconstruct phase distortion in a beam in a wide sense.

Heterophase-structured nanocrystals as superior supports for Ru-based catalysts in selective hydrogenation of benzene

PubMed Central

Peng, Zhikun; Liu, Xu; Li, Shuaihui; Li, Zhongjun; Li, Baojun; Liu, Zhongyi; Liu, Shouchang

2017-01-01

ZrO2 heterophase structure nanocrystals (HSNCs) were synthesized with tunable ratios of monoclinic ZrO2 (m-ZrO2) to tetragonal ZrO2 (t-ZrO2). The phase mole ratio of m-ZrO2 versus t-ZrO2 in ZrO2 HSNCs was tuned from 40% to 100%. The concentration of the surface hydroxyl groups on m-ZrO2 is higher than that on t-ZrO2. ZrO2 HSNCs have different surface hydroxyl groups on two crystalline phases. This creates more intimate synergistic effects than their single-phase counterparts. The ZrO2 HSNCs were used as effective supports to fabricate heterophase-structured Ru/ZrO2 catalysts for benzene-selective hydrogenation. The excellent catalytic performance including high activity and selectivity is attributed to the heterogeneous strong/weak hydrophilic interface and water layer formed at the m-ZrO2/t-ZrO2 catalyst junction. PMID:28057914

Grain growth and phase transformations induced by shock waves on alpha-GeO2 powder

NASA Astrophysics Data System (ADS)

Rosales, Ivonne; Thions-Renero, Claude; Martinez, Erendira; Bucio, Lauro; Orozco, Eligio

2011-09-01

An impact experiment on a mixture of water and microcrystalline alpha-GeO2 powder was performed with a single-stage gas gun. The recovered sample contained micrometer-scale crystals of different sizes and morphologies that correspond to 88% of alpha-GeO2, 6.0% of monoclinic phase (P21/c, space group No. 14), 4.9% of orthorhombic phase (Pnnm, space group No. 58) and 1.1% of rutile-type phase.

Unraveling the nature of electric field- and stress- induced structural transformations in soft PZT by a new powder poling technique.

PubMed

Kalyani, Ajay Kumar; V, Lalitha K; James, Ajit R; Fitch, Andy; Ranjan, Rajeev

2015-02-25

A 'powder-poling' technique was developed to study electric field induced structural transformations in ferroelectrics exhibiting a morphotropic phase boundary (MPB). The technique was employed on soft PZT exhibiting a large longitudinal piezoelectric response (d(33) ∼ 650 pC N(-1)). It was found that electric poling brings about a considerable degree of irreversible tetragonal to monoclinic transformation. The same transformation was achieved after subjecting the specimen to mechanical stress, which suggests an equivalence of stress and electric field with regard to the structural mechanism in MPB compositions. The electric field induced structural transformation was also found to be accompanied by a decrease in the spatial coherence of polarization.

Investigation of ZrO x /ZrC-ZrN/Zr thin-film structural evolution and their degradation using X-ray diffraction and Raman spectrometry

NASA Astrophysics Data System (ADS)

Usmani, B.; Vijay, V.; Chhibber, R.; Dixit, A.

2016-11-01

The thin-film structures of DC/FR magnetron-sputtered ZrO x /ZrC-ZrN/Zr tandem solar-selective coatings are investigated using X-ray diffraction and room-temperature Raman spectroscopic measurements. These studies suggest that the major contribution is coming from h-ZrN0.28, c-ZrC, h-Zr3C2 crystallographic phases in ZrN-ZrC absorber layer, in conjunction with mixed ZrO x crystallographic phases. The change in structure for thermally annealed samples has been examined and observed that cubic and hexagonal ZrO x phase converted partially into tetragonal and monoclinic ZrO x phases, whereas hexagonal and cubic ZrN phases, from absorber layer, have not been observed for these thermally treated samples in air. These studies suggest that thermal treatment may lead to the loss of ZrN phase in absorber, degrading the thermal response for the desired wavelength range in open ambient conditions in contrast to vacuum conditions.

Modelling the structure of Zr-rich Pb(Zr1-xTix)O3, x = 0.4 by a multiphase approach.

PubMed

Bogdanov, Alexander; Mysovsky, Andrey; Pickard, Chris J; Kimmel, Anna V

2016-10-12

Solid solution perovskite Pb(Zr 1-x Ti x )O 3 (PZT) is an industrially important material. Despite the long history of experimental and theoretical studies, the structure of this material is still under intensive discussion. In this work, we have applied structure searching coupled with density functional theory methods to provide a multiphase description of this material at x = 0.4. We demonstrate that the permutational freedom of B-site cations leads to the stabilisation of a variety of local phases reflecting a relatively flat energy landscape of PZT. Using a set of predicted local phases we reproduce the experimental pair distribution function (PDF) profile with high accuracy. We introduce a complex multiphase picture of the structure of PZT and show that additional monoclinic and rhombohedral phases account for a better description of the experimental PDF profile. We propose that such a multiphase picture reflects the entropy reached in the sample during the preparation process.

Structural comparison of nickel electrodes and precursor phases

NASA Technical Reports Server (NTRS)

Cornilsen, Bahne C.; Shan, Xiaoyin; Loyselle, Patricia

1989-01-01

Researchers summarize previous Raman spectroscopic results and discuss important structural differences in the various phases of active mass and active mass precursors. Raman spectra provide unique signatures for these phases, and allow one to distinguish each phase, even when the compound is amorphous to x rays (i.e., does not scatter x rays because of a lack of order and/or small particle size). The structural changes incurred during formation, charge and discharge, cobalt addition, and aging are discussed. The oxidation states and dopant contents are explained in terms of the nonstoichiometric structures.

Structure and temperature-dependent phase transitions of lead-free Bi 1/2Na 1/2TiO 3-Bi 1/2K 1/2TiO 3-K 0.5Na 0.5NbO 3 piezoceramics

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

Anton, Eva-Maria; Schmitt, Ljubomira Ana; Hinterstein, Manuel

2014-05-28

Structure and phase transitions of (1-y)((1-x)Bi 1/2Na 1/2TiO 3-xBi 1/2K 1/2TiO 3)-yK 0.5Na 0.5NbO 3 (x; y) piezoceramics (0.1 ≤ x ≤ 0.4; 0 ≤ y ≤ 0.05) were investigated by transmission electron microscopy, neutron diffraction, temperature-dependent x-ray diffraction, and Raman spectroscopy. The local crystallographic structure at room temperature (RT) does not change by adding K 0.5Na 0.5NbO 3 to Bi 1/2Na 1/2TiO 3-xBi 1/2K 1/2TiO 3 for x = 0.2 and 0.4. The average crystal structure and microstructure on the other hand develop from mainly long-range polar order with ferroelectric domains to short-range order with polar nanoregions displaying amore » more pronounced relaxor character. The (0.1; 0) and (0.1; 0.02) compositions exhibit monoclinic Cc space group symmetry, which transform into Cc + P4bm at 185 and 130 °C, respectively. This high temperature phase is stable at RT for the morphotropic phase boundary compositions of (0.1; 0.05) and all compositions with x = 0.2. For the compositions of (0.1; 0) and (0.1; 0.02), local structural changes on heating are evidenced by Raman; for all other compositions, changes in the long-range average crystal structure were observed.« less

Effect of crystal structure and cationic order on phonon modes across ferroelectric phase transformation in Pb(Fe{sub 0.5-x}Sc{sub x}Nb{sub 0.5})O{sub 3} bulk ceramics

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

Mallesham, B.; Ranjith, R., E-mail: ranjith@iith.ac.in; Viswanath, B.

Pb(Fe{sub 0.5-x}Sc{sub x}Nb{sub 0.5})O{sub 3} [(PFSN) (0 ≤ x ≤ 0.5)] multiferroic relaxors were synthesized and the temperature dependence of phonon modes across ferroelectric to paraelectric transition was studied. With varying Sc content from x = 0 to 0.25 the structure remains monoclinic and with further addition (x = 0.3 - 0.5) the structure transforms into rhombohedral symmetry. Structural refinement studies showed that the change in crystal structure from monoclinic to rhombohedral symmetry involves a volume increment of 34-36%. Associated changes in the tolerance factor (1.024 ≤ t ≤ 0.976) and bond angles were observed. Structure assisted B′-B″ cation orderingmore » was confirmed through the superlattice reflections in selected area electron diffraction (SAED) pattern of Pb(Sc{sub 0.5}Nb{sub 0.5})O{sub 3} (x = 0.5). Cation ordering is also evident from the evolution of Pb-O phonon mode in Raman spectra of compositions with rhombohedral symmetry (x ≥ 0.3). The high temperature Raman scattering studies show that the B-localized mode [F{sub 1u}, ∼250 cm{sup −1}] and BO{sub 6} octahedral rotational mode [F{sub 1g}, ∼200 cm{sup −1}], both originating from polar nano regions (PNRs) behave like coupled phonon modes in rhombohedral symmetry. However, in monoclinic symmetry they behave independently across the transition. Softening of B localized mode across the transition followed by the hardening for all compositions confirms the diffusive nature of the ferroelectric transformation. The presence of correlation between the B localized and BO{sub 6} rotational modes introduces a weak relaxor feature for systems with rhombohedral symmetry in PFSN ceramics, which was confirmed from the macroscopic dielectric studies.« less

Electronic structure and electron energy-loss spectroscopy of ZrO2 zirconia

NASA Astrophysics Data System (ADS)

Dash, L. K.; Vast, Nathalie; Baranek, Philippe; Cheynet, Marie-Claude; Reining, Lucia

2004-12-01

The atomic and electronic structures of zirconia are calculated within density functional theory, and their evolution is analyzed as the crystal-field symmetry changes from tetrahedral [cubic (c-ZrO2) and tetragonal (t-ZrO2) phases] to octahedral (hypothetical rutile ZrO2 ), to a mixing of these symmetries (monoclinic phase, m-ZrO2 ). We find that the theoretical bulk modulus in c-ZrO2 is 30% larger than the experimental value, showing that the introduction of yttria in zirconia has a significant effect. Electronic structure fingerprints which characterize each phase from their electronic spectra are identified. We have carried out electron energy-loss spectroscopy experiments at low momentum transfer and compared these results to the theoretical spectra calculated within the random phase approximation. We show a dependence of the valence and 4p ( N2,3 edge) plasmons on the crystal structure, the dependence of the latter being brought into the spectra by local-field effects. Last, we attribute low energy excitations observed in EELS of m-ZrO2 to defect states 2eV above the top of the intrinsic valence band, and the EELS fundamental band gap value is reconciled with the 5.2 or 5.8eV gaps determined by vacuum ultraviolet spectroscopy.

Electrolysis-induced protonation of VO2 thin film transistor for the metal-insulator phase modulation

NASA Astrophysics Data System (ADS)

Katase, Takayoshi; Endo, Kenji; Ohta, Hiromichi

2016-02-01

Compared to state-of-the-art modulation techniques, protonation is the most ideal to control the electrical and optical properties of transition metal oxides (TMOs) due to its intrinsic non-volatile operation. However, the protonation of TMOs is not typically utilized for solid-state devices because of imperative high-temperature annealing treatment in hydrogen source. Although one solution for room temperature (RT) protonation of TMOs is liquid-phase electrochemistry, it is unsuited for practical purposes due to liquid-leakage problem. Herein we demonstrate solid-state RT-protonation of vanadium dioxide (VO2), which is a well-known thermochromic TMO. We fabricated the three terminal thin-film-transistor structure on an insulating VO2 film using a water-infiltrated nanoporous glass, which serves as a solid electrolyte. For gate voltage application, water electrolysis and protonation/deprotonation of VO2 film surface occurred, leading to reversible metal-insulator phase conversion of ~11-nm-thick VO2 layer. The protonation was clearly accompanied by the structural change from an insulating monoclinic to a metallic tetragonal phase. Present results offer a new route for the development of electro-optically active solid-state devices with TMO materials by engineering RT protonation.

Structural contribution to the ferroelectric fatigue in lead zirconate titanate ceramics

NASA Astrophysics Data System (ADS)

Hinterstein, M.; Rouquette, J.; Haines, J.; Papet, Ph.; Glaum, J.; Knapp, M.; Eckert, J.; Hoffman, M.

2014-09-01

Many ferroelectric devices are based on doped lead zirconate titanate (PZT) ceramics with compositions near the morphotropic phase boundary (MPB), at which the relevant material's properties approach their maximum. Based on a synchrotron x-ray diffraction study of MPB PZT, bulk fatigue is unambiguously found to arise from a less effective field induced tetragonal-to-monoclinic transformation, at which the degradation of the polarization flipping is detected by a less intense and more diffuse anomaly in the atomic displacement parameter of lead. The time dependence of the ferroelectric response on a structural level down to 250 μs confirms this interpretation in the time scale of the piezolectric strain response.

Effect of Zn-doping on structural and magnetic properties of copper ferrite nanoparticles

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

Gautam, Nisha; Thirupathi, Gadipelly; Singh, Rajender

2016-05-23

The nanoparticles of CuFe{sub 2}O{sub 4} (CF) and Cu{sub 0.8}Zn{sub 0.2}Fe{sub 2}O{sub 4} (CZF) were synthesized using co-precipitation method to study the effect of Zn doping in Cu-ferrite. The X-ray diffraction (XRD) patterns were well fitted with two-phase structure using Rietveld analysis as Fd-3 m space group (spinel system) and C12/c1 space group (monoclinic system CuO-phase). The average crystallite size of the CF and CZF nanoparticles for spinel structure are 6 and 7 nm respectively. The spinel phase fraction is increased from 56% to 71% with Zn-doping of 20% in CF. The transmission electron micrograph analysis showed the narrow size distribution formore » CZF nanoparticles. The magnetization plots as a function of magnetic field (M (H)) of CF and CZF nanoparticles indicate superparamagnetic behavior. The magnetization is increased with Zn-doping in CF. The stable spinel Cu-ferrite can be obtained with Zn-doping in CF.« less

Structural comparison of nickel electrodes and precursor phases

NASA Technical Reports Server (NTRS)

Cornilsen, Bahne C.; Shan, Xiaoyin; Loyselle, Patricia

1989-01-01

A summary of previous Raman spectroscopic results and a discussion of important structural differences in the various phases of active mass and active mass precurors are presented. Raman spectra provide unique signatures for these phases, and allow one to distinguish each phase, even when the compound is amorphous to X-rays (i.e., does not scatter X-rays because of a lack of order and/or small particle size). The structural changes incurred during formation, charge and discharge, cobalt addition, and aging will be discussed and related to electrode properties. Important structural differences include NiO2 layer stacking, nonstoichiometry (especially cation-deficit nonstoichiometry), disorder, dopant content, and water content. The results indicate that optimal nickel active mass is non-close packed and nonstoichiometric. The formation process transforms precursor phases into this structure. Therefore, the precursor disorder, or lack thereof, influences this final active mass structure and the rate of formation. Aging processes induce structural change which is believed to be detrimental. The role of cobalt addition can be appreciated in terms of structures favored or stabilized by the dopant. In recent work, the in situ Raman technique to characterize the critical structural parameters was developed. An in situ method relates structure, electrochemistry, and preparation. In situ Raman spectra of cells during charge and discharge, either during cyclic voltammetry or under constant current conditions were collected. With the structure-preparation knowledge and the in situ Raman tool, it will be possible to define the structure-property-preparation relations in more detail. This instrumentation has application to a variety of electrode systems.

Phase-selective vanadium dioxide (VO2) nanostructured thin films by pulsed laser deposition

NASA Astrophysics Data System (ADS)

Masina, B. N.; Lafane, S.; Wu, L.; Akande, A. A.; Mwakikunga, B.; Abdelli-Messaci, S.; Kerdja, T.; Forbes, A.

2015-10-01

Thin films of monoclinic nanostructured vanadium dioxide are notoriously difficult to produce in a selective manner. To date, post-annealing, after pulsed laser deposition (PLD), has been used to revert the crystal phase or to remove impurities, and non-glass substrates have been employed, thus reducing the efficacy of the transparency switching. Here, we overcome these limitations in PLD by optimizing a laser-ablation and deposition process through optical imaging of the laser-induced plasma. We report high quality monoclinic rutile-type vanadium dioxide (VO2) (M1) nanoparticles without post-annealing, and on a glass substrate. Our samples demonstrate a reversible metal-to-insulator transition at ˜43 °C, without any doping, paving the way to switchable transparency in optical materials at room temperature.

LaCu6-xAgx : A promising host of an elastic quantum critical point

NASA Astrophysics Data System (ADS)

Poudel, L.; Cruz, C. de la; Koehler, M. R.; McGuire, M. A.; Keppens, V.; Mandrus, D.; Christianson, A. D.

2018-05-01

Structural properties of LaCu6-xAgx have been investigated using neutron and x-ray diffraction, and resonant ultrasound spectroscopy (RUS) measurements. Diffraction measurements indicate a continuous structural transition from orthorhombic (Pnma) to monoclinic (P21 / c) structure. RUS measurements show softening of natural frequencies at the structural transition, consistent with the elastic nature of the structural ground state. The structural transition temperatures in LaCu6-xAgx decrease with Ag composition until the monoclinic phase is completely suppressed at xc = 0.225 . All of the evidence is consistent with the presence of an elastic quantum critical point in LaCu6-xAgx .

LaCu 6-xAg x: A promising host of an elastic quantum critical point

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

Poudel, Lekh; Dela Cruz, Clarina R.; Koehler, Michael R.

Structural properties of LaCu 6-xAg x have been investigated using neutron and x-ray diffraction, and resonant ultrasound spectroscopy (RUS) measurements. Diffraction measurements indicate a continuous structural transition from orthorhombic (Pnma) to monoclinic (P2₁/C) structure. RUS measurements show softening of natural frequencies at the structural transition, consistent with the elastic nature of the structural ground state. The structural transition temperatures in LaCu 6-xAg x decrease with Ag composition until the monoclinic phase is completely suppressed at x c=0.225. All of the evidence is consistent with the presence of an elastic quantum critical point in LaCu 6-xAg x.

Neutral line chaos and phase space structure

NASA Technical Reports Server (NTRS)

Burkhart, Grant R.; Speiser, Theodore W.; Martin, Richard F., Jr.; Dusenbery, Paul B.

1991-01-01

Phase space structure and chaos near a neutral line are studied with numerical surface-of-section (SOS) techniques and analytic methods. Results are presented for a linear neutral line model with zero crosstail electric field. It was found that particle motion can be divided into three regimes dependening on the value of the conserved canonical momentum, Py, and the conserved Hamiltonian, h. The phase space structure, using Poincare SOS plots, is highly sensitive to bn = Bn/B0 variations, but not to h variations. It is verified that the slow motion preserves the action, Jz, as evaluated by Sonnerup (1971), when the period of the fast motion is smaller than the time scale of the slow motion. Results show that the phase space structure and particle chaos depend sensitively upon Py and bn, but are independent of h.

Effect of annealing on structure, morphology and optoelectronic properties of nanocrystalline CuO thin films

NASA Astrophysics Data System (ADS)

Jundale, D. M.; Pawar, S. G.; Patil, S. L.; Chougule, M. A.; Godse, P. R.; Patil, V. B.

2011-10-01

The nanocrystalline CuO thin films were prepared on glass substrates by the sol-gel method. The structural, morphological, electrical and optical properties of CuO thin films, submitted to an annealing treatment in the 400-700 °C ranges are studied by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Four Probe Technique and UV-visible spectroscopic. XRD measurements show that all the films are crystallized in the monoclinic phase and present a random orientation. Four prominent peaks, corresponding to the (110) phase (2θ≈32.70°), (002) phase (2θ≈35.70°), (111) phase (2θ≈38.76°) and (202) phase (2θ≈49.06°) appear on the diffractograms. The crystallite size increases with increasing annealing temperature. These modifications influence the microstructure, electrical and optical properties. The optical band gap energy decreases with increasing annealing temperature. These mean that the optical quality of CuO films is improved by annealing.

Effect of O 2 gas partial pressure on structures and dielectric characteristics of rf sputtered ZrO 2 thin films

NASA Astrophysics Data System (ADS)

Ma, C. Y.; Lapostolle, F.; Briois, P.; Zhang, Q. Y.

2007-08-01

Amorphous and polycrystalline zirconium oxide thin films have been deposited by reactive rf magnetron sputtering in a mixed argon/oxygen or pure oxygen atmosphere with no intentional heating of the substrate. The films were characterized by high-resolution transmission electron microscopy (HR-TEM), atomic force microscopy (AFM), spectroscopic ellipsometry (SE), and capacitance versus voltage ( C- V) measurements to investigate the variation of structure, surface morphology, thickness of SiO 2-like interfacial layer as well as dielectric characteristics with different oxygen partial pressures. The films deposited at low oxygen partial pressures (less than 15%) are amorphous and dense with a smooth surface. In contrast, the films prepared at an oxygen partial pressure higher than 73% are crystallized with the microstructure changing from the mixture of monoclinic and tetragonal phases to a single monoclinic structure. The film structural transition is believed to be consequences of decrease in the oxygen vacancy concentration in the film and of increase of the energetically neutral particles in the plasma due to an increased oxygen partial pressure. SE measurements showed that significant interfacial SiO 2 growth has taken place above approximately 51%. The best C- V results in terms of relative dielectric constant values are obtained for thin films prepared at an oxygen partial pressure of 15%.

EDTA-assisted phase conversion synthesis of (Gd0.95RE0.05)PO4 nanowires (RE = Eu, Tb) and investigation of photoluminescence

PubMed Central

Wang, Zhihao; Li, Ji-Guang; Zhu, Qi; Ai, Zhengrong; Li, Xiaodong; Sun, Xudong; Kim, Byung-Nam; Sakka, Yoshio

2017-01-01

Abstract Hexagonal (Gd0.95RE0.05)PO4·nH2O nanowires ~300 nm in length and ~10 nm in diameter have been converted from (Gd0.95RE0.05)2(OH)5NO3·nH2O nanosheets (RE = Eu, Tb) in the presence of monoammonium phosphate (NH4H2PO4) and ethylene diamine tetraacetic acid (EDTA). They were characterized by X-ray diffraction, thermogravimetry, electron microscopy, and Fourier transform infrared and photoluminescence spectroscopies. It is shown that EDTA played an essential role in the morphology development of the nanowires. The hydrothermal products obtained up to 180 °C are of a pure hexagonal phase, while monoclinic phosphate evolved as an impurity at 200 °C. The nanowires undergo hexagonal→monoclinic phase transformation upon calcination at ≥600 °C to yield a pure monoclinic phase at ~900 °C. The effects of calcination on morphology, excitation/emission, and fluorescence decay kinetics were investigated in detail with (Gd0.95Eu0.05)PO4 as example. The abnormally strong 5D0→7F4 electric dipole Eu3+ emission in the hexagonal phosphates was ascribed to site distortion. The process of energy migration was also discussed for the optically active Gd3+ and Eu3+/Tb3+ ions. PMID:28740561

Structural phase transition, electronic structure and optical properties of half Heusler alloys LiBeZ (Z = As, Sb)

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

Amudhavalli, A.; Rajeswarapalanichamy, R., E-mail: rajeswarapalanichamy@gmail.com

2016-05-23

Ab initio calculations are performed to investigate the structural stability, electronic structure, mechanical properties and optical properties of half Heusler alloys (LiBeAs and LiBeSb) for three different phases of zinc blende crystal structure. Among the considered phases, α- phase is found to be the most stable phase for these alloys at normal pressure. A pressure induced structural phase transition from α-phase to β- phase is observed for LiBeAs. The electronic structure reveals that these alloys are semiconductors. The optical properties confirm that these alloys are semiconductor in nature.

Structural-Phase Transformations of CuZn Alloy Under Thermal-Impact Cycling

NASA Astrophysics Data System (ADS)

Potekaev, A. I.; Chaplygina, A. A.; Kulagina, V. V.; Chaplygin, P. A.; Starostenkov, M. D.; Grinkevich, L. S.

2017-02-01

Using the Monte Carlo method, special features of structural - phase transformations in β-brass are investigated during thermal impact using thermal cycling as an example (a number of successive order - disorder and disorder - order phase transitions in the course of several heating - cooling cycles). It is shown that a unique hysteresis is observed after every heating and cooling cycle, whose presence indicates irreversibility of the processes, which suggests a difference in the structural - phase states both in the heating and cooling stages. A conclusion is drawn that the structural - phase transformations in the heating and cooling stages occur within different temperature intervals, where the thermodynamic stimuli of one or the other structural - phase state are low. This is also demonstrated both in the plots of configurational energy, long- and short-range order parameter, atomic structure variations, and structural - phase state distributions. Simultaneously, there coexist ordered and disordered phases and a certain collection of superstructure domains. This implies the presence of low - stability states in the vicinity of the order - disorder phase transition. The results of investigations demonstrate that the structural - phase transitions within two successive heating and cooling cycles at the same temperature are different in both stages. These changes, though not revolutionary, occur in every cycle and decrease with the increasing cycle number. In fact, the system undergoes training with a tendency towards a certain sequence of structural - phase states.

A reinvestigation of the crystal structure of α-Pb 2BiVO 6

NASA Astrophysics Data System (ADS)

Labidi, O.; Wignacourt, J. P.; Roussel, P.; Drache, M.; Conflant, P.; Steinfink, H.

2004-08-01

A previously reported β phase for Pb 2BiVO 6 has been characterized as a stable phase, and the phase transitions α→ β and β→ δ were identified in the mother phase as well as in Pb 2BiV 1- xM xO 6- y solid solutions (M=Cr, Mn); the high temperature form δ-Pb 2BiVO 6 eventually decomposes at 480 °C to a mixture of PbBiVO 5 and Pb 4BiVO 8 before showing recombination at 650 °C. The related substituted compositions behave the same way. The crystal structure of α-Pb 2BiVO 6 (I) is monoclinic, P2 1/ n, a=7.717(3) Å, b=5.845(3) Å, c=29.081(8) Å, β=94.27(1)°, Z=8. Oxygen atoms are in tetrahedral interstices formed by four Bi and Pb atoms. These tetrahedra articulate by BiPb edge sharing in two dimensions parallel to the b axis to form infinite chains. Mixed O(V Bi Pb) 4 tetrahedra bridge the O(Bi Pb) 4 ribbons by edge sharing to complete the three-dimensional articulation of the structure. α-Pb 2BiV 1- xMn xO 6- y ( x=0.06) (II) is monoclinic, P2 1/ m, a=7.684(3) Å, b=5.822(3) Å, c=14.708(6) Å, β=100.92(1)°, Z=4. Tetrahedral units of O(2Bi 2Pb) are also present in (II). They form dimers O 2Bi 4Pb 4 by BiBi edge sharing. The dimers form a chain along the b axis by sharing BiPb edges. Two independent MO 4 tetrahedra (M=V, Mn) are present in which one has V/Mn mixed occupancy. Both tetrahedra show statistical disorder by rotation around a VO apex. The disordered tetrahedral oxygen atoms are part of the coordination sphere of Bi and Pb. The matrix {-1 0 0, 0 -1 0, 1 0 2} relates the structures and unit cells of [I], and [II].

The new Zintl phases Eu{sub 21}Cd{sub 4}Sb{sub 18} and Eu{sub 21}Mn{sub 4}Sb{sub 18}

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

Wang, Yi; Darone, Gregory M.; Bobev, Svilen, E-mail: bobev@udel.edu

Crystals of two new Zintl compounds, Eu{sub 21}Mn{sub 4}Sb{sub 18} and Eu{sub 21}Cd{sub 4}Sb{sub 18} have been synthesized using the molten metal flux method, and their structures have been established by single-crystal X-ray diffraction. Both compounds are isotypic and crystallize in the monoclinic space group C2/m (No. 12, Z=4). The structures are based on edge- and corner-shared MnSb{sub 4} or CdSb{sub 4} tetrahedra, which make octameric [Mn{sub 8}Sb{sub 22}] or [Cd{sub 8}Sb{sub 22}] polyanions. Homoatomic Sb–Sb bonds are present in both structures. The Eu atoms take the role of Eu{sup 2+}cations with seven unpaired 4f electrons, as suggested by themore » temperature-dependent magnetization measurements. The magnetic susceptibilities of Eu{sub 21}Mn{sub 4}Sb{sub 18} and Eu{sub 21}Cd{sub 4}Sb{sub 18} indicate that both phases order anti-ferromagnetically with Néel temperatures of ca. 7 K and ca. 10 K, respectively. The unpaired 3d electrons of the Mn atoms in Eu{sub 21}Mn{sub 4}Sb{sub 18} do contribute to the magnetic response, however, the bulk magnetization measurements do not provide evidence for long-range ordering of the Mn spins down to 5 K. Electrical resistivity measurements suggest that both compounds are narrow band gap semiconductors. - Graphical abstract: Eu{sub 21}Mn{sub 4}Sb{sub 18} and Eu{sub 21}Cd{sub 4}Sb{sub 18} have complex monoclinic structures, based on MnSb{sub 4} and CdSb{sub 4} tetrahedra, both edge- and corner-shared. A perspective of the crystal structure is shown, as viewed along the b axis. Display Omitted - Highlights: • Eu{sub 21}Mn{sub 4}Sb{sub 18} and Eu{sub 21}Cd{sub 4}Sb{sub 18} are novel compounds in the respective ternary phase diagrams. • For both structures, the Zintl-Klemm rules are followed, and both are small gap semiconductors. • Eu{sub 21}Mn{sub 4}Sb{sub 18} and Eu{sub 21}Cd{sub 4}Sb{sub 18} are air-stable Zintl phases and could be new thermoelectric materials.« less

Role of Y2O3, CaO, MgO additives on structural and microstructural behavior of zirconia/mullite aggregates

NASA Astrophysics Data System (ADS)

Mishra, D. K.; Prusty, Sasmita; Mohapatra, B. K.; Singh, S. K.; Behera, S. N.

2012-07-01

Zirconia mullite (MUZ), Y2O3-MUZ, CaO-MUZ and MgO-MUZ composites, synthesized through plasma fusion technique, are becoming important due to their commercial scale of production within five minutes of plasma treatment from sillimanite, zircon and alumina mixture. The X-ray diffraction studies reveal the monoclinic zirconia phase in MUZ composite whereas mixed monoclinic, tetragonal and cubic phases of zirconia have been observed in Y2O3, CaO, MgO added MUZ composites. The Y2O3, CaO and MgO additives act as sintering aids to favour the transformation and stabilisation of tetragonal and cubic zirconia phases at room temperature. These additives also play a key role in the development of various forms of microstructure to achieve dense MUZ composites.

Electron-electron correlations in Raman spectra of VO2

NASA Astrophysics Data System (ADS)

Goncharuk, I. N.; Ilinskiy, A. V.; Kvashenkina, O. E.; Shadrin, E. B.

2013-01-01

It has been shown that, in single crystals and films of a strongly correlated material, namely, vanadium dioxide, upon a thermally stimulated phase transition from the low-temperature monoclinic phase to the high-temperature tetragonal phase, the narrow-line Raman spectrum of the insulating (monoclinic) phase transforms into the broad-band Raman spectrum, which contains two peaks at 500 and 5000 cm-1 with widths of 400 and 3500 cm-1, respectively. It has been found that, as the temperature of the monoclinic phase approaches the structural phase transition temperature (340 K), the line profile of soft-mode phonons at a frequency of 149 cm-1 with A g symmetry and the line profile of phonons at a frequency of 201 cm-1 with A g symmetry acquire an asymmetric shape with a Fano antiresonance that is characteristic of the interaction of a single phonon vibration with a continuum of strongly correlated electrons. It has been demonstrated that the thermal transformation of peaks in the Raman spectra of the VO2 metallic phase is in quantitative agreement with the theory of Raman scattering in strongly correlated materials.

High-pressure phases of Mg2Si from first principles

NASA Astrophysics Data System (ADS)

Huan, Tran Doan; Tuoc, Vu Ngoc; Le, Nam Ba; Minh, Nguyen Viet; Woods, Lilia M.

2016-03-01

First-principles calculations are presented to resolve the possible pressure-dependent phases of Mg2Si . Although previous reports show that Mg2Si is characterized by the cubic antifluorite F m 3 ¯m structure at low pressures, the situation at higher pressures is less clear with many contradicting results. Here we utilize several methods to examine the stability, electron, phonon, and transport properties of this material as a function of pressure and temperature. We find that Mg2Si is thermodynamically stable at low and high pressures. Between 6 and 24 GPa, Mg2Si can transform into Mg9Si5 , a defected compound, and vice versa, without energy cost. Perhaps this result is related to the aforementioned inconsistency in the structures reported for Mg2Si within this pressure range. Focusing solely on Mg2Si , we find a new monoclinic C 2 /m structure of Mg2Si , which is stable at high pressures within thermodynamical considerations. The calculated electrical conductivity and Seebeck coefficient taking into account results from the electronic structure calculations help us understand better how transport can be affected in this material by modulating pressure and temperature.

Investigation of the crystallization features, atomic structure, and microstructure of chromium-doped monticellite

NASA Astrophysics Data System (ADS)

Subbotin, K. A.; Iskhakova, L. D.; Zharikov, E. V.; Lavrishchev, S. V.

2008-12-01

A series of Cr4+:CaMgSiO4 single crystals is grown using floating zone melting, and their microstructure, composition, and crystal structure are investigated. It is shown that regions with inclusions of second phases, such as forsterite, akermanite, MgO, and Ca4Mg2Si3O12, can form over the length of the sample. The composition of the single-phase regions of the single crystals varies from the stoichiometric monticellite CaMgSiO4 to the solid solution Ca(1 - x)Mg(1 + x)SiO4( x = 0.22). The Cr:(Ca0.88Mg0.12)MgSiO4 crystal is studied using X-ray diffraction. It is revealed that, in this case, the olivine-like orthorhombic crystal lattice is distorted to the monoclinic lattice with the parameters a = 6.3574(5) Å, b = 4.8164(4) Å, c = 11.0387(8) Å, β = 90.30(1)o, Z = 4, V = 337.98 Å3, and space group P21/ c. In the monoclinic lattice, the M(1) position of the initial olivine structure is split into two nonequivalent positions with the center of symmetry, which are occupied only by Mg2+ cations with the average length of the Mg-O bond R av = 2.128 Å. The overstoichiometric Mg2+ cations partially replace Ca2+ cations (in the M(2) position of the orthorhombic prastructure) with the average bond length of 2.347 Å in the [(Ca,Mg)-O6] octahedron. The average distance in SiO4 distorted tetrahedra is 1.541 Å.

Non-convergent ordering and displacive phase transition in pigeonite: in situ HT XRD study

NASA Astrophysics Data System (ADS)

Cámara, F.; Carpenter, M. A.; Domeneghetti, M. C.; Tazzoli, V.

A natural Ca-rich pigeonite (En47Fs43Wo10), free of augite exsolution products, was studied by in situ high-temperature single-crystal X-ray diffraction. The sample, monoclinic P21/c (a=9.719(7) Å, b=8.947(9) Å, c=5.251(3) Å, β=108.49(5), V=433.0(6) Å3), was annealed up to 1000 °C to induce a phase transition from P21/c to C2/c symmetry. Complete single-crystal X-ray diffraction data collections were carried out in situ at 650, 750, 850 and 950 °C after the crystal had reached equilibrium for the Fe-Mg intracrystalline exchange reaction at each temperature. The variation, with increasing temperature, of lattice parameters, of intensity of hkl reflections with h + k=2n + 1 (which vanish at high temperature) and of some geometrical parameters from structure refinement, showed that the displacive phase transition P21/cC2/c was continuous in character. This contrasts with the first-order character for the HT phase transition in pigeonite containing significantly less calcium.

La{sup 3+} doping of the Sr{sub 2}CoWO{sub 6} double perovskite: A structural and magnetic study

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

Lopez, C.A.; Viola, M.C.; Pedregosa, J.C.

2008-11-15

La-doped Sr{sub 2}CoWO{sub 6} double perovskites have been prepared in air in polycrystalline form by solid-state reaction. These materials have been studied by X-ray powder diffraction (XRPD), neutron powder diffraction (NPD) and magnetic susceptibility. The structural refinement was performed from combined XRPD and NPD data (D2B instrument, {lambda}=1.594 A). At room temperature, the replacement of Sr{sup 2+} by La{sup 3+} induces a change of the tetragonal structure, space group I4/m of the undoped Sr{sub 2}CoWO{sub 6} into the distorted monoclinic crystal structure, space group P2{sub 1}/n, Z=2. The structure of La-doped phases contains alternating CoO{sub 6} and (Co/W)O{sub 6} octahedra,more » almost fully ordered. On the other hand, the replacement of Sr{sup 2+} by La{sup 3+} induces a partial replacement of W{sup 6+} by Co{sup 2+} into the B sites, i.e. Sr{sub 2-x}La{sub x}CoW{sub 1-y}Co{sub y}O{sub 6} (y=x/4) with segregation of SrWO{sub 4}. Magnetic and neutron diffraction measurements indicate an antiferromagnetic ordering below T{sub N}=24 K independently of the La-substitution. - Graphical abstract: La-doped Sr{sub 2}CoWO{sub 6} double perovskites have been prepared in polycrystalline form by solid-state reaction. The general formula of these compounds is Sr{sub 2-x}La{sub x}CoW{sub 1-y}Co{sub y}O{sub 6} (y=x/4). XRPD, NPD and magnetic susceptibility studies were performed. The structure of monoclinic La-doped phases contains alternating CoO{sub 6} and (Co/W)O{sub 6} octahedra, almost fully ordered. NPD and magnetic measurements indicate an antiferromagnetic ordering at low temperature.« less

Pump-probe STM light emission spectroscopy for detection of photo-induced semiconductor-metal phase transition of VO2

NASA Astrophysics Data System (ADS)

Sakai, Joe; Katano, Satoshi; Kuwahara, Masashi; Uehara, Yoichi

2017-10-01

We attempted to observe pump-probe scanning tunneling microscopy (STM)-light emission (LE) from a VO2 thin film grown on a rutile TiO2(0 0 1) substrate, with an Ag tip fixed over a semiconducting domain. Laser pulses from a Ti:sapphire laser (wavelength 920 nm pulse width less than 1.5 ps) irradiated the tip-sample gap as pump and probe light sources. With a photon energy of 2.7 eV, suggesting phase transition from semiconducting monoclinic (M) to metallic rutile (R) phases in relation to the electronic band structure, faint LE was observed roughly 30 ps after the irradiation of the pump pulse, followed by retention for roughly 20 ps. The incident energy fluence of the pump pulse at the gap was five orders of magnitude lower than the threshold value for reported photo-induced M-R phase transition. The mechanism that makes it possible to reduce the threshold fluence is discussed.

Pump-probe STM light emission spectroscopy for detection of photo-induced semiconductor-metal phase transition of VO2.

PubMed

Sakai, Joe; Katano, Satoshi; Kuwahara, Masashi; Uehara, Yoichi

2017-10-11

We attempted to observe pump-probe scanning tunneling microscopy (STM)-light emission (LE) from a VO 2 thin film grown on a rutile TiO 2 (0 0 1) substrate, with an Ag tip fixed over a semiconducting domain. Laser pulses from a Ti:sapphire laser (wavelength 920 nm; pulse width less than 1.5 ps) irradiated the tip-sample gap as pump and probe light sources. With a photon energy of 2.7 eV, suggesting phase transition from semiconducting monoclinic (M) to metallic rutile (R) phases in relation to the electronic band structure, faint LE was observed roughly 30 ps after the irradiation of the pump pulse, followed by retention for roughly 20 ps. The incident energy fluence of the pump pulse at the gap was five orders of magnitude lower than the threshold value for reported photo-induced M-R phase transition. The mechanism that makes it possible to reduce the threshold fluence is discussed.

Ultrafast Dynamics in Vanadium Dioxide: Separating Spatially Segregated Mixed Phase Dynamics in the Time-domain

NASA Astrophysics Data System (ADS)

Hilton, David

2011-10-01

In correlated electronic systems, observed electronic and structural behavior results from the complex interplay between multiple, sometimes competing degrees-of- freedom. One such material used to study insulator-to-metal transitions is vanadium dioxide, which undergoes a phase transition from a monoclinic-insulating phase to a rutile-metallic phase when the sample is heated to 340 K. The major open question with this material is the relative influence of this structural phase transition (Peirels transition) and the effects of electronic correlations (Mott transition) on the observed insulator-to-metal transition. Answers to these major questions are complicated by vanadium dioxide's sensitivity to perturbations in the chemical structure in VO2. For example, related VxOy oxides with nearly a 2:1 ratio do not demonstrate the insulator-to- metal transition, while recent work has demonstrated that W:VO2 has demonstrated a tunable transition temperature controllable with tungsten doping. All of these preexisting results suggest that the observed electronic properties are exquisitely sensitive to the sample disorder. Using ultrafast spectroscopic techniques, it is now possible to impulsively excite this transition and investigate the photoinduced counterpart to this thermal phase transition in a strongly nonequilibrium regime. I will discuss our recent results studying the terahertz-frequency conductivity dynamics of this photoinduced phase transition in the poorly understood near threshold temperature range. We find a dramatic softening of the transition near the critical temperature, which results primarily from the mixed phase coexistence near the transition temperature. To directly study this mixed phase behavior, we directly study the nucleation and growth rates of the metallic phase in the parent insulator using non-degenerate optical pump-probe spectroscopy. These experiments measure, in the time- domain, the coexistent phase separation in VO2 (spatially

A new set of K3Fe3(PO4)4·yH2O (0 ≤ y ≤ 1) layered phases obtained by topotactic reactions

NASA Astrophysics Data System (ADS)

Trad, Khiem; Wattiaux, Alain; Ben Amara, Mongi; Delmas, Claude; Carlier, Dany

2018-06-01

K3Fe3(PO4)4·H2O powder was synthesized by Na+/K+ exchange reaction from Na3Fe3(PO4)4 in aqueous medium. The replacement of the sodium cations by the potassium larger ones and water molecules causes a structural distortion leading to P2/n monoclinic K3Fe3(PO4)4·H2O. This new layered phase was characterized by XRD, Mössbauer spectroscopy and magnetic measurements. The study of its thermal stability reveals that other new layered K3Fe3(PO4)4·yH2O with (0 ≤ y ≤ 1) phases can be stabilized up to 600 °C and finally at higher temperature a new K3Fe3(PO4)4 polymorph with a different structural type is irreversibility formed.

Local Crystalline Structure in an Amorphous Protein Dense Phase

PubMed Central

Greene, Daniel G.; Modla, Shannon; Wagner, Norman J.; Sandler, Stanley I.; Lenhoff, Abraham M.

2015-01-01

Proteins exhibit a variety of dense phases ranging from gels, aggregates, and precipitates to crystalline phases and dense liquids. Although the structure of the crystalline phase is known in atomistic detail, little attention has been paid to noncrystalline protein dense phases, and in many cases the structures of these phases are assumed to be fully amorphous. In this work, we used small-angle neutron scattering, electron microscopy, and electron tomography to measure the structure of ovalbumin precipitate particles salted out with ammonium sulfate. We found that the ovalbumin phase-separates into core-shell particles with a core radius of ∼2 μm and shell thickness of ∼0.5 μm. Within this shell region, nanostructures comprised of crystallites of ovalbumin self-assemble into a well-defined bicontinuous network with branches ∼12 nm thick. These results demonstrate that the protein gel is comprised in part of nanocrystalline protein. PMID:26488663

Unraveling the Origin of Structural Disorder in High Temperature Transition Al2O3: Structure of θ-Al2O3

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

Kovarik, Libor; Bowden, Mark E.; Shi, Dachuan

The crystallography of transition Al2O3 has been extensively studied in the past due to the advantageous properties of the oxide in catalytic and a range of other technological applications. However, existing crystallographic models are insufficient to describe the structure of many important Al2O3 polymorphs due to their highly disordered nature. In this work, we investigate structure and disorder in high-temperature treated transition Al2O3, and provide a structural description for θ-Al2O3 by using a suite of complementary imaging, spectroscopy and quantum calculation techniques. Contrary to current understanding, our high-resolution imaging shows that θ-Al2O3 is a disordered composite phase of at leastmore » two different end members. By correlating imaging and spectroscopy results with DFT calculations, we propose a model that describes θ-Al2O3 as a disordered intergrowth of two crystallographic variants at the unit cell level. One variant is based on β-Ga2O3, and the other on a monoclinic phase that is closely-related to δ-Al2O3. The overall findings and interpretations afford new insight into the origin of poor crystallinity in transition Al2O3, and also provide new perspectives on structural complexity that can emerge from intergrowth of closely related structural polymorphs.« less

Epitaxial structure and electronic property of β-Ga2O3 films grown on MgO (100) substrates by pulsed-laser deposition

NASA Astrophysics Data System (ADS)

Wakabayashi, Ryo; Yoshimatsu, Kohei; Hattori, Mai; Ohtomo, Akira

2017-10-01

We investigated heteroepitaxial growth of Si-doped Ga2O3 films on MgO (100) substrates by pulsed-laser deposition as a function of growth temperature (Tg) to find a strong correlation between the structural and electronic properties. The films were found to contain cubic γ-phase and monoclinic β-phase, the latter of which indicated rotational twin domains when grown at higher Tg. The formation of the metastable γ-phase and twin-domain structure in the stable β-phase are discussed in terms of the in-plane epitaxial relationships with a square MgO lattice, while crystallinity of the β-phase degraded monotonically with decreasing Tg. The room-temperature conductivity indicated a maximum at the middle of Tg, where the β-Ga2O3 layer was relatively highly crystalline and free from the twin-domain structure. Moreover, both crystallinity and conductivity of β-Ga2O3 films on the MgO substrates were found superior to those on α-Al2O3 (0001) substrates. A ratio of the conductivity, attained to the highest quantity on each substrate, was almost three orders of magnitude.

Hydrogen bonds in PC{sub 61}BM solids

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

Sheng, Chun-Qi; Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Normal University, Hangzhou 311121; Li, Wen-Jie

2015-09-15

We have studied the hydrogen bonds in PC{sub 61}BM solids. Inter-molecular interaction is analyzed theoretically for the well-defined monoclinic (P2{sub 1}/n) structure. The results indicate that PC{sub 61}BM combines into C–H⋯O{sub d} bonded molecular chains, where O{sub d} denotes the doubly-bonded O atom of PC{sub 61}BM. The molecular chains are linked together by C–H⋯O{sub s} bonds, where O{sub s} denotes the singly-bonded O atom of PC{sub 61}BM. To reveal the consequences of hydrogen bond formation on the structural properties of PC{sub 61}BM solids (not limited to the monoclinic structure), we design and perform some experiments for annealed samples with themore » monoclinic (P2{sub 1}/n) PC{sub 61}BM as starting material. The experiments include differential scanning calorimetry, X-ray diffraction and infrared absorption measurements. Structural phase transitions are observed below the melting point. The C–H⋯O{sub d} bonds seem persisting in the altered structures. The inter-molecular hydrogen bonds can help to understand the phase separation in polymer/PC{sub 61}BM blends and may be responsible for the existence of liquid PC{sub 61}BM.« less

Lattice-Rotation Vortex at the Charged Monoclinic Domain Boundary in a Relaxor Ferroelectric Crystal

NASA Astrophysics Data System (ADS)

Shao, Yu-Tsun; Zuo, Jian-Min

2017-04-01

We present evidence of lattice-rotation vortices having an average radius of ˜7 nm at the ferroelectric domain boundary of (1 -x )Pb (Zn1 /3Nb2 /3)O3-xPbTiO3 (x =0.08 ). Maps of crystal orientations and domain symmetry breaking are obtained using scanning convergent beam electron diffraction, which show fractional rotation vortices near the 50° monoclinic domain walls. The merging of 2D and 1D topological defects is consistent with inhomogeneous boundary charge and expected to have a large impact on the domain-switching mechanisms in relaxor ferroelectric crystals and ferroelectric devices.

Polymorphism of Alprazolam (Xanax): a review of its crystalline phases and identification, crystallographic characterization, and crystal structure of a new polymorph (form III).

PubMed

de Armas, Héctor Novoa; Peeters, Oswald M; Van den Mooter, Guy; Blaton, Norbert

2007-05-01

A new polymorphic form of Alprazolam (Xanax), 8-chloro-1-methyl-6-phenyl-4H-[1,2,4]triazolo-[4,3-alpha][1,4]benzodiazepine, C(17)H(13)ClN(4), has been investigated by means of X-ray powder diffraction (XRPD), single crystal X-ray diffraction, and differential scanning calorimetry (DSC). This polymorphic form (form III) was obtained during DSC experiments after the exothermic recrystallization of the melt of form I. The crystal unit cell dimensions for form III were determined from diffractometer methods. The monoclinic unit cell found for this polymorph using XRPD after indexing the powder diffractogram was confirmed by the cell parameters obtained from single crystal X-ray diffractometry on a crystal isolated from the DSC pans. The single crystal unit cell parameters are: a = 28.929(9), b = 13.844(8), c = 7.361(3) angstroms, beta = 92.82(3) degrees , V = 2944(2) angstroms(3), Z = 8, space group P2(1) (No.4), Dx = 1.393 Mg/m(3). The structure obtained from single crystal X-ray diffraction was used as initial model for Rietveld refinement on the powder diffraction data of form III. The temperature phase transformations of alprazolam were also studied using high temperature XRPD. A review of the different phases available in the Powder Diffraction File (PDF) database for this drug is described bringing some clarification and corrections. (c) 2007 Wiley-Liss, Inc. and the American Pharmacists Association.

Misfit strain phase diagrams of epitaxial PMN–PT films

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

Khakpash, N.; Khassaf, H.; Rossetti, G. A.

Misfit strain–temperature phase diagrams of three compositions of (001) pseudocubic (1 − x)·Pb (Mg{sub l/3}Nb{sub 2/3})O{sub 3} − x·PbTiO{sub 3} (PMN–PT) thin films are computed using a phenomenological model. Two (x = 0.30, 0.42) are located near the morphotropic phase boundary (MPB) of bulk PMN–PT at room temperature (RT) and one (x = 0.70) is located far from the MPB. The results show that it is possible to stabilize an adaptive monoclinic phase over a wide range of misfit strains. At RT, the stability region of this phase is much larger for PMN–PT compared to barium strontium titanate and lead zirconate titanate films.

Effect of NiO substitution on the structural and dielectric behaviour of NaNbO3

NASA Astrophysics Data System (ADS)

George, R. T.; Joshi, D. C.; Nayak, S.; Tiwari, N.; Chauhan, R. N.; Pramanik, P.; Dar, T. A.; Ghosh, S.; Thota, S.

2018-02-01

The structural and dielectric properties of NiO substituted NaNbO3 ceramics are reported. The orthorhombic (Pmna) crystal structure of NaNbO3 transforms to a lower symmetry monoclinic phase (Pbma) after the dilute dispersion of NiO. X-ray photoelectron spectroscopy reveals pentavalent "Nb," monovalent "Na," and divalent "Ni" states along with the signatures of non-local screening effects. The antiferroelectric to paraelectric transition (TAFE) accompanied by a structural change from the orthorhombic to the tetragonal phase shifts by 55 °C toward the low-temperature side, whereas the morphotropic phase boundary (TO-M) moves toward a higher temperature by 28 °C for nominal substitutions ( x ≤0.10 ). The generalized Lyddane-Sachs-Teller expression (ε0/-S'ε∞)= (ωl/ωt ) 2 and thermodynamic free energy models are employed to explain the anomalous behaviour of the temperature dependence of relative dielectric permittivity ( εr (T)) across TAFE and TO-M. The frequency dependence of ac-conductivity σac(ω) follows the Jonscher power law (σac = σ(0) + Aωs), suggesting the dominance of the phonon-assisted hopping mechanism, whereas the frequency independent term (σ(0)) was explained by Funke's Jump-Relaxation Model.

Modelling Assisted Design and Synthesis of Highly Porous Materials for Chemical Adsorbents

DTIC Science & Technology

2010-10-01

two phases of crystal, a monoclinic phase within the solution, and after removal from solution a trigonal phase is obtained. The single crystal...days. Single crystal X-ray data showed there existed a monoclinic phase within the solution that, upon removal from solution, rapidly converted to a... monoclinic to trigonal upon desolvation, as the new peak which has emerged matches the simulated PXRD of the trigonal phase. Also, as the sample is

Charge disproportionation of mixed-valent Cr triggered by Bi lone-pair effect in the A -site-ordered perovskite BiC u3C r4O12

NASA Astrophysics Data System (ADS)

Etter, Martin; Isobe, Masahiko; Sakurai, Hiroya; Yaresko, Alexander; Dinnebier, Robert E.; Takagi, Hidenori

2018-05-01

A new A -site-ordered perovskite BiC u3C r4O12 is synthesized under a high pressure of 7.7 GPa. A phase transition from a paramagnetic metal to a ferrimagnetic metal is observed at Tc=190 K accompanied with a structural change from cubic to monoclinic. Structural analysis of the low-temperature monoclinic phase reveals that this transition represents a charge disproportionation of C r3.75 + into C r4 + and C r3.5 + . We argue that the asymmetric displacement of Bi caused by a lone-pair effect triggers the formation of a dimeric Cr4+2O5 unit and leads to an ordering of C r4 + and C r3.5 + below the transition.

Phase-selective vanadium dioxide (VO{sub 2}) nanostructured thin films by pulsed laser deposition

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

Masina, B. N., E-mail: BMasina@csir.co.za, E-mail: slafane@cdta.dz; School of Physics, University of KwaZulu-Natal, Private Bag X54001, Durban 4000; Lafane, S., E-mail: BMasina@csir.co.za, E-mail: slafane@cdta.dz

2015-10-28

Thin films of monoclinic nanostructured vanadium dioxide are notoriously difficult to produce in a selective manner. To date, post-annealing, after pulsed laser deposition (PLD), has been used to revert the crystal phase or to remove impurities, and non-glass substrates have been employed, thus reducing the efficacy of the transparency switching. Here, we overcome these limitations in PLD by optimizing a laser-ablation and deposition process through optical imaging of the laser-induced plasma. We report high quality monoclinic rutile-type vanadium dioxide (VO{sub 2}) (M1) nanoparticles without post-annealing, and on a glass substrate. Our samples demonstrate a reversible metal-to-insulator transition at ∼43 °C, withoutmore » any doping, paving the way to switchable transparency in optical materials at room temperature.« less

Three-phase inductive-coupled structures for contactless PHEV charging system

NASA Astrophysics Data System (ADS)

Lee, Jia-You; Shen, Hung-Yu; Li, Cheng-Bin

2016-07-01

In this article, a new-type three-phase inductive-coupled structure is proposed for the contactless plug-in hybrid electric vehicle (PHEV) charging system regarding with SAE J-1773. Four possible three-phase core structures are presented and subsequently investigated by the finite element analysis. To study the correlation between the core geometric parameter and the coupling coefficient, the magnetic equivalent circuit model of each structure is also established. In accordance with the simulation results, the low reluctance and the sharing of flux path in the core material are achieved by the proposed inductive-coupled structure with an arc-shape and three-phase symmetrical core material. It results in a compensation of the magnetic flux between each phase and a continuous flow of the output power in the inductive-coupled structure. Higher coupling coefficient between inductive-coupled structures is achieved. A comparison of coupling coefficient, mutual inductance, and self-inductance between theoretical and measured results is also performed to verify the proposed model. A 1 kW laboratory scale prototype of the contactless PHEV charging system with the proposed arc-shape three-phase inductive-coupled structure is implemented and tested. An overall system efficiency of 88% is measured when two series lithium iron phosphate battery packs of 25.6 V/8.4 Ah are charged.

Pressure-induced structural phase transformation and superconducting properties of titanium mononitride

NASA Astrophysics Data System (ADS)

Li, Qian; Guo, Yanan; Zhang, Miao; Ge, Xinlei

2018-03-01

In this work, we have systematically performed the first-principles structure search on titanium mononitride (TiN) within Crystal Structure AnaLYsis by Particle Swarm Optimization (CALYPSO) methodology at high pressures. Here, we have confirmed a phase transition from cubic rock-salt (fcc) phase to CsCl (bcc) phase of TiN at ∼348 GPa. Further simulations reveal that the bcc phase is dynamically stable, and could be synthesized experimentally in principle. The calculated elastic anisotropy decreases with the phase transformation from fcc to bcc structure under high pressures, and the material changes from ductile to brittle simultaneously. Moreover, we found that both structures are superconductive with the superconducting critical temperature of 2-12 K.

Cr incorporated phase transformation in Y 2O 3 under ion irradiation

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

Li, Nan; Yadav, Satyesh Kumar; Xu, Yun

Under irradiation, chemical species can redistribute in ways not expected from equilibrium behavior. In oxide-dispersed ferritic alloys, the phenomenon of irradiation-induced Cr redistribution at the metal/oxide interfaces has drawn recent attention. Here, the thermal and irradiation stability of the FeCr/Y 2O 3 interface has been systematically studied. Trilayer thin films of 90 nm Fe - 20 at.% Cr (1 st layer)/100 nm Y 2O 3 (2 nd layer)/135 nm Fe - 20 at.% Cr (3 rd layer) were deposited on MgO substrates at 500 °C. After irradiation, Cr diffuses towards and enriches the FeCr/Y 2O 3 interface. Further, correlated withmore » Cr redistributed into the oxide, an amorphous layer is generated at the interface. In the Y 2O 3 layer, the original cubic phase is observed to transform to the monoclinic phase after irradiation. Meanwhile, nanosized voids, with relatively larger size at interfaces, are also observed in the oxide layer. First-principles calculations reveal that Cr substitution of Y interstitials in Y 2O 3 containing excess Y interstitials is favored and the irradiation-induced monoclinic phase enhances this process. Lastly, our findings provide new insights that may aid in the development of irradiation resistant oxide-dispersed ferritic alloys.« less

Cr incorporated phase transformation in Y2O3 under ion irradiation

PubMed Central

Li, N.; Yadav, S. K.; Xu, Y.; Aguiar, J. A.; Baldwin, J. K.; Wang, Y. Q.; Luo, H. M.; Misra, A.; Uberuaga, B. P.

2017-01-01

Under irradiation, chemical species can redistribute in ways not expected from equilibrium behavior. In oxide-dispersed ferritic alloys, the phenomenon of irradiation-induced Cr redistribution at the metal/oxide interfaces has drawn recent attention. Here, the thermal and irradiation stability of the FeCr/Y2O3 interface has been systematically studied. Trilayer thin films of 90 nm Fe - 20 at.% Cr (1st layer)/100 nm Y2O3 (2nd layer)/135 nm Fe - 20 at.% Cr (3rd layer) were deposited on MgO substrates at 500 °C. After irradiation, Cr diffuses towards and enriches the FeCr/Y2O3 interface. Further, correlated with Cr redistributed into the oxide, an amorphous layer is generated at the interface. In the Y2O3 layer, the original cubic phase is observed to transform to the monoclinic phase after irradiation. Meanwhile, nanosized voids, with relatively larger size at interfaces, are also observed in the oxide layer. First-principles calculations reveal that Cr substitution of Y interstitials in Y2O3 containing excess Y interstitials is favored and the irradiation-induced monoclinic phase enhances this process. Our findings provide new insights that may aid in the development of irradiation resistant oxide-dispersed ferritic alloys. PMID:28091522

Cr incorporated phase transformation in Y 2O 3 under ion irradiation

DOE PAGES

Li, Nan; Yadav, Satyesh Kumar; Xu, Yun; ...

2017-01-16

Under irradiation, chemical species can redistribute in ways not expected from equilibrium behavior. In oxide-dispersed ferritic alloys, the phenomenon of irradiation-induced Cr redistribution at the metal/oxide interfaces has drawn recent attention. Here, the thermal and irradiation stability of the FeCr/Y 2O 3 interface has been systematically studied. Trilayer thin films of 90 nm Fe - 20 at.% Cr (1 st layer)/100 nm Y 2O 3 (2 nd layer)/135 nm Fe - 20 at.% Cr (3 rd layer) were deposited on MgO substrates at 500 °C. After irradiation, Cr diffuses towards and enriches the FeCr/Y 2O 3 interface. Further, correlated withmore » Cr redistributed into the oxide, an amorphous layer is generated at the interface. In the Y 2O 3 layer, the original cubic phase is observed to transform to the monoclinic phase after irradiation. Meanwhile, nanosized voids, with relatively larger size at interfaces, are also observed in the oxide layer. First-principles calculations reveal that Cr substitution of Y interstitials in Y 2O 3 containing excess Y interstitials is favored and the irradiation-induced monoclinic phase enhances this process. Lastly, our findings provide new insights that may aid in the development of irradiation resistant oxide-dispersed ferritic alloys.« less

Oxygen self-diffusion mechanisms in monoclinic Zr O2 revealed and quantified by density functional theory, random walk analysis, and kinetic Monte Carlo calculations

NASA Astrophysics Data System (ADS)

Yang, Jing; Youssef, Mostafa; Yildiz, Bilge

2018-01-01

In this work, we quantify oxygen self-diffusion in monoclinic-phase zirconium oxide as a function of temperature and oxygen partial pressure. A migration barrier of each type of oxygen defect was obtained by first-principles calculations. Random walk theory was used to quantify the diffusivities of oxygen interstitials by using the calculated migration barriers. Kinetic Monte Carlo simulations were used to calculate diffusivities of oxygen vacancies by distinguishing the threefold- and fourfold-coordinated lattice oxygen. By combining the equilibrium defect concentrations obtained in our previous work together with the herein calculated diffusivity of each defect species, we present the resulting oxygen self-diffusion coefficients and the corresponding atomistically resolved transport mechanisms. The predicted effective migration barriers and diffusion prefactors are in reasonable agreement with the experimentally reported values. This work provides insights into oxygen diffusion engineering in Zr O2 -related devices and parametrization for continuum transport modeling.

Microstructure characterization and phase transformation kinetic study of ball-milled m-ZrO 2-30 mol% a-TiO 2 mixture by Rietveld method

NASA Astrophysics Data System (ADS)

Pradhan, S. K.; Dutta, H.

2005-05-01

High-energy ball milling of monoclinic ZrO 2-30 mol% anatase TiO 2 mixture at different durations results in the formation of m-ZrO 2-a-TiO 2 solid solution from which the nucleation of nanocrystalline cubic (c) ZrO 2 polymorphic phase sets in. Post-annealing of 12 h ball-milled sample at different elevated temperatures for 1 h results in almost complete formation of c-ZrO 2 phase. Microstructure of the unmilled, all the ball milled and annealed samples has been characterized by Rietveld's X-ray powder structure refinement method. Particle size, rms lattice strain, change in lattice parameters and phase content of individual phases have been estimated from Rietveld analysis, and are utilized to interpret the results. In course of milling, (1 1 1) of cubic lattice became parallel to ( 1bar 1 1) plane of monoclinic lattice due to the orientation effect and cubic phase may have been formed on the (0 0 1) of the m-ZrO 2-a-TiO 2 solid solution lattice. A comparative study of microstructure and phase transformation kinetics of ZrO 2-10, 20 and 30 mol% a-TiO 2 ball-milled and post-annealed samples reveals that rate of phase transformation m→c-ZrO 2 increases with increasing a-TiO 2 concentration and ∼30 mol% of nanocrystalline c-ZrO 2 phase can be obtained within 4 h of milling time in the presence of 30 mol% of a-TiO 2. The post-annealing treatment at 773, 873 and 973 K for 1 h duration each reveals that rate of c-ZrO 2 formation with increasing temperature is retarded with increasing a-TiO 2 concentration but the amount of c-ZrO 2 becomes almost equal (∼95 mol%) at 973 K. It suggests that almost fully stabilized nanocrystalline c-ZrO 2 can be formed by adding a tetravalent solute to m-ZrO 2.

Emergent Low-Symmetry Phases and Large Property Enhancements in Ferroelectric KNbO 3 Bulk Crystals [Emergent Low-Symmetry Phases with Large Property Enhancement in Ferroelectric KNbO 3 Bulk Crystals

DOE PAGES

Lummen, Tom T. A.; Leung, J.; Kumar, Amit; ...

2017-06-19

The design of new or enhanced functionality in materials is traditionally viewed as requiring the discovery of new chemical compositions through synthesis. Large property enhancements may however also be hidden within already well-known materials, when their structural symmetry is deviated from equilibrium through a small local strain or field. Here, the discovery of enhanced material properties associated with a new metastable phase of monoclinic symmetry within bulk KNbO 3 is reported. This phase is found to coexist with the nominal orthorhombic phase at room temperature, and is both induced by and stabilized with local strains generated by a network ofmore » ferroelectric domain walls. While the local microstructural shear strain involved is only ≈0.017%, the concurrent symmetry reduction results in an optical second harmonic generation response that is over 550% higher at room temperature. Moreover, the meandering walls of the low-symmetry domains also exhibit enhanced electrical conductivity on the order of 1 S m -1. In conclusion, this discovery reveals a potential new route to local engineering of significant property enhancements and conductivity through symmetry lowering in ferroelectric crystals.« less

High-throughput determination of structural phase diagram and constituent phases using GRENDEL

NASA Astrophysics Data System (ADS)

Kusne, A. G.; Keller, D.; Anderson, A.; Zaban, A.; Takeuchi, I.

2015-11-01

Advances in high-throughput materials fabrication and characterization techniques have resulted in faster rates of data collection and rapidly growing volumes of experimental data. To convert this mass of information into actionable knowledge of material process-structure-property relationships requires high-throughput data analysis techniques. This work explores the use of the Graph-based endmember extraction and labeling (GRENDEL) algorithm as a high-throughput method for analyzing structural data from combinatorial libraries, specifically, to determine phase diagrams and constituent phases from both x-ray diffraction and Raman spectral data. The GRENDEL algorithm utilizes a set of physical constraints to optimize results and provides a framework by which additional physics-based constraints can be easily incorporated. GRENDEL also permits the integration of database data as shown by the use of critically evaluated data from the Inorganic Crystal Structure Database in the x-ray diffraction data analysis. Also the Sunburst radial tree map is demonstrated as a tool to visualize material structure-property relationships found through graph based analysis.

The Effect of Poling on the Properties of 0.65Pb(Mg1/3Nb2/3)O3-0.35PbTiO3 Ceramics

NASA Astrophysics Data System (ADS)

Uršič, Hana; Tellier, Jenny; Hrovat, Marko; Holc, Janez; Drnovšek, Silvo; Bobnar, Vid; Alguero, Miguel; Kosec, Marija

2011-03-01

The effects of the poling field on the structural and electrical properties of 0.65Pb(Mg1/3Nb2/3)O3-0.35PbTiO3 (0.65PMN-0.35PT) ceramics were investigated. The highest piezoelectric coefficient d33, coupling coefficients kp, kt, and mechanical quality factor Qm were achieved for ceramics poled at electric fields between 2 and 3.5 kV/mm, whereas the d33, kp, kt, and Qm of ceramics poled at higher electric fields, i.e., 4 and 4.5 kV/mm, were lower. The non-poled ceramics contained 86% of the monoclinic phase with the space group Pm and 14% of the tetragonal phase with the space group P4mm. However, the ceramics poled at 2.5 kV/mm contained 99% of the monoclinic phase and the rest is the tetragonal phase. The results show that the ratio of the monoclinic to the tetragonal phases can be changed by the application of a poling electric field and that the extent of this change is dependent on the field strength.

Structures of chloralide, ?-lactic acid chloralide, malic acid chloralide and citric acid chloralide

NASA Astrophysics Data System (ADS)

Koh, L. L.; Huang, H. H.; Chia, L. H. L.; Liang, E. P.

1995-06-01

The crystal and molecular structures of chloralide ( 1), D-lactic acid chloralide ( 2), malic acid chloralide ( 3) and citric acid chloralide ( 4) have been determined by X-ray diffraction methods. Compound 1 crystallizes in the monoclinic space group, {P2 1}/{c}, a = 6.201(2), b = 17.11(2), c = 10.357(6) Å, β = 95.21(4)°, Z = 4; compound 2 in the monoclinic space group P2 1, a = 7.600(4), b = 5.902(4), c = 9.743(6) Å, β = 99.20(5), Z = 2; compound 3 in the monoclinic space group {P2 1}/{c}, a = 16.500(6), b = 5.819(3), c = 10.120(4) Å, β = 91.41(3), Z = 4; compound 4 in the monoclinic space group {P2 1}/{c}, a = 12.041(3), b = 6.1190(10), c = 17.259(4) Å, β = 101.85(2), Z = 4. The five-membered ring systems of all the compounds are slightly twisted out-of-plane, that of compound 4 being the most puckered. The CCl 3 group is trans to the second CCl 3 group in 1, to the CH 3 group in 2 and to the CH 2COOH group in 3. The two CH 2COOH groups in 4 are disposed axially with respect to the ring. Dipole moment and Kerr constant data for D-lactic acid chloralide suggest a structure in solution which is consistent with the X-ray results. The IR spectra of 2, 3 and 4 are discussed in relation to the structures of these compounds.

Phase transitions, mechanical properties and electronic structures of novel boron phases under high-pressure: A first-principles study

PubMed Central

Fan, Changzeng; Li, Jian; Wang, Limin

2014-01-01

We have explored the mechanical properties, electronic structures and phase transition behaviors of three designed new phases for element boron from ambient condition to high-pressure of 120 GPa including (1) a C2/c symmetric structure (m-B16); (2) a symmetric structure (c-B56) and (3) a Pmna symmetric structure (o-B24). The calculation of the elastic constants and phonon dispersions shows that the phases are of mechanical and dynamic stability. The m-B16 phase is found to transform into another new phase (the o-B16 phase) when pressure exceeds 68 GPa. This might offer a new synthesis strategy for o-B16 from the metastable m-B16 at low temperature under high pressure, bypassing the thermodynamically stable γ-B28. The enthalpies of the c-B56 and o-B24 phases are observed to increase with pressure. The hardness of m-B16 and o-B16 is calculated to be about 56 GPa and 61 GPa, approaching to the highest value of 61 GPa recorded for α-Ga-B among all available Boron phases. The electronic structures and bonding characters are analyzed according to the difference charge-density and crystal orbital Hamilton population (COHP), revealing the metallic nature of the three phases. PMID:25345910

In pursuit of the rhabdophane crystal structure: from the hydrated monoclinic LnPO4.0.667H2O to the hexagonal LnPO4 (Ln = Nd, Sm, Gd, Eu and Dy)

NASA Astrophysics Data System (ADS)

Mesbah, Adel; Clavier, Nicolas; Elkaim, Erik; Szenknect, Stéphanie; Dacheux, Nicolas

2017-05-01

The dehydration process of the hydrated rhabdophane LnPO4.0.667H2O (Ln = La to Dy) was thoroughly studied over the combination of in situ high resolution synchrotron powder diffraction and TGA experiments. In the case of SmPO4.0.667H2O (monoclinic, C2), a first dehydration step was identified around 80 °C leading to the formation of SmPO4.0.5H2O (Monoclinic, C2) with Z =12 and a =17.6264(1) Å, b =6.9704(1) Å, c =12.1141(1) Å, β=133.74(1) °, V =1075.33(1) Å3. In agreement with the TGA and dilatometry experiments, all the water molecules were evacuated above 220 °C yielding to the anhydrous form, which crystallizes in the hexagonal P3121 space group with a =7.0389(1) Å, c =6.3702(1) Å and V =273.34(1) Å3. This study was extended to selected LnPO4.0.667H2O samples (Ln= Nd, Gd, Eu, Dy) and the obtained results confirmed the existence of two dehydration steps before the stabilization of the anhydrous form, with the transitory formation of LnPO4.0.5H2O.

Structural, optical and magnetic behaviour of nanocrystalline Volborthite

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

Arvind, Hemant K., E-mail: hemantarvind@gmail.com; Kumar, Sudhish, E-mail: skmlsu@gmail.com; Kalal, Sangeeta

2016-05-06

Nanocrystalline sample of Volborthite (Copper Pyrovanadate: Cu{sub 3}V{sub 2} (OH){sub 2}O{sub 7}.2H{sub 2}O) has been synthesized using wet chemical route and characterized by XRD, SEM, FTIR, UV-Vis-NIR spectroscopic and magnetization measurements. Room temperature X-ray diffraction analysis confirms the single phase monoclinic structure and nanocrystalline nature of Volborthite. The UV-Visible optical absorption spectrum displays two broad absorption peaks in the range of 200-350 nm and 400-1000 nm. The direct band gap is found to be E{sub g}= ∼2.74 eV. Bulk Volborthite was reported to be a natural frustrated antiferromagnet, however our nanocrystalline Volborthite display week ferromagnetic hysteresis loop with very small coercivity andmore » retentivity at room temperature.« less

New cubic structure compounds as actinide host phases

NASA Astrophysics Data System (ADS)

Stefanovsky, S. V.; Yudintsev, S. V.; Livshits, T. S.

2010-03-01

Various compounds with fluorite (cubic zirconia) and fluorite-derived (pyrochlore, zirconolite) structures are considered as promising actinide host phases at immobilization of actinide-bearing nuclear wastes. Recently some new cubic compounds — stannate and stannate-zirconate pyrochlores, murataite and related phases, and actinide-bearing garnet structure compounds were proposed as perspective matrices for complex actinide wastes. Zirconate pyrochlore (ideally Gd2Zr2O7) has excellent radiation resistance and high chemical durability but requires high temperatures (at least 1500 °C) to be produced by hot-pressing from sol-gel derived precursor. Partial Sn4+ substitution for Zr4+ reduces production temperature and the compounds REE2ZrSnO7 may be hot-pressed or cold pressed and sintered at ~1400 °C. Pyrochlore, A2B2O7-x (two-fold elementary fluorite unit cell), and murataite, A3B6C2O20-y (three-fold fluorite unit cell), are end-members of the polysomatic series consisting of the phases whose structures are built from alternating pyrochlore and murataite blocks (nano-sized modules) with seven- (2C/3C/2C), five- (2C/3C), eight- (3C/2C/3C) and three-fold (3C — murataite) fluorite unit cells. Actinide content in this series reduces in the row: 2C (pyrochlore) > 7C > 5C > 8C > 3C (murataite). Due to congruent melting murataite-based ceramics may be produced by melting and the firstly segregated phase at melt crystallization is that with the highest fraction of the pyrochlore modules in its structure. The melts containing up to 10 wt. % AnO2 (An = Th, U, Np, Pu) or REE/An fraction of HLW form at crystallization zoned grains composed sequentially of the 5C → 8C → 3C phases with the highest actinide concentration in the core and the lowest — in the rim of the grains. Radiation resistance of the "murataite" is comparable to titanate pyrochlores. One more promising actinide hosts are ferrites with garnet structure. The matrices containing sometime complex fluorite