Science.gov

Sample records for fluorite baf2 structure

  1. Dynamical properties and instability of local fluorite BaF(2) structure around doped Mn(2+) ions-EPR and electron spin echo studies.

    PubMed

    Lijewski, S; Hoffmann, S K; Goslar, J; Wencka, M; Ulanov, V A

    2008-09-24

    The electron paramagnetic resonance (EPR) and electron spin echo (ESE) were measured at the X-band for Mn(2+) in a BaF(2) crystal in the temperature range 4.2-300 K. In addition to the cubic symmetry centre, two other lower concentration tetragonal centres were identified. Temperature variations and computer simulation of the EPR spectrum confirm that the cubic symmetry of the MnF(8) centre is deformed to two T(d) tetrahedra of different dimensions at around 45 K. Electron spin relaxation was measured in the temperature range 4.2-35 K, where the ESE signal was detectable. For higher temperature the Mn(2+) dynamics produces homogeneously broadened EPR lines. At the lowest temperatures the spin-lattice relaxation is governed by ordinary phonon processes with 1/T(1)∼T(5). The efficiency of these processes rapidly decreases and at about 11 K a local mode of energy 17 cm(-1) becomes the relaxation mechanism. Phase relaxation observed as ESE signal dephasing indicates that after the local deformation jumps (tunnelling with frequency 4 × 10(8) s(-1)) between the two tetrahedral configurations appear, with the energy barrier being the local mode energy. This motion is directly visible as a resonance-type enhancement of the ESE dephasing rate 1/T(M) around 11 K. Only the cubic centre displays the above dynamics.

  2. Platinum nitride with fluorite structure

    SciTech Connect

    Yu, Rong; Zhang, Xiao-Feng

    2005-01-31

    The mechanical stability of platinum nitride has been studied using first-principles calculations. By calculating the single-crystal elastic constants, we show that platinum nitride can be stabilized in the fluorite structure, in which the nitrogen atoms occupy all the tetrahedral interstitial sites of the metal lattice. The stability is attributed to the pseudogap effect from analysis of the electronic structure.

  3. Studies of EPR spectra and defect structure for Er3+ ions in BaF2 and SrF2 crystals

    NASA Astrophysics Data System (ADS)

    Li, Hui; Kuang, Xiao-Yu; Mao, Ai-Jie; Li, Cheng-Gang

    2013-02-01

    The local lattice structure and electron paramagnetic resonance (EPR) spectra have been studied systematically by diagonalizing 364 × 364 complete energy matrices for a f11 ion in a trigonal ligand-field. By simulating the calculated Stark levels and EPR parameters to the experimental results, the shift parameters are determined for Er3+ ions in BaF2 and SrF2. The results show that the trigonal center is attributed to an interstitial F- ion located at the [1 1 1] axis of the cube, and the nearest ligand close to the charge compensator has a displacement towards central ion by 0.042 Å for L center in BaF2:Er3+ and 0.026 Å for J center in SrF2:Er3+, respectively. Moreover, the relationships between g-factors and shift parameter ΔZ as well as orbit reduction factor k' are discussed.

  4. A Study of Mechanochemical Doping of Fluoride Crystals with a Fluorite Structure by Er3+ Ions via Electron Paramagnetic Resonance Spectra

    NASA Astrophysics Data System (ADS)

    Irisova, I. A.; Rodionov, A. A.; Tayurskii, D. A.; Yusupov, R. V.

    2014-05-01

    Using electron paramagnetic resonance (EPR) spectroscopy, we have shown that, upon mecha- noactivated doping of powders of compounds CaF2, SrF2, and BaF2 with Er3+ ions, impurity centers of single erbium ions with cubic symmetry are formed. Investigations of dependences of EPR spectra intensities on the particle size show that the process of mechanochemical doping with Er3+ ions proceeds differently for CaF2, SrF2, and BaF2 host matrices. In the case of CaF2, impurity centers are localized in a very thin near-surface layer of CaF2 particles, in SrF2, the impurity is distributed over the volume of particles, while, in BaF2, there is a layer of a finite thickness for which the probability of doping in the course of mechanosynthesis is very small and the impurity of the rare-earth element is localized in the core of large particles. These data can be explained assuming that the result of mechanosynthesis of particles of fluorides with a fluorite structure doped with Er3+ ions at room temperature is governed by two processes—mechanoactivated diffusion of rare-earth ions into particles and segregation of impurity ions at grain boundaries. In this case, the typical scales for compounds CaF2, SrF2, and BaF2 considerably differ from each other.

  5. Structural properties of Bi2Te3 topological insulator thin films grown by molecular beam epitaxy on (111) BaF2 substrates

    NASA Astrophysics Data System (ADS)

    Fornari, Celso I.; Rappl, Paulo H. O.; Morelhão, Sérgio L.; Abramof, Eduardo

    2016-04-01

    Structural properties of topological insulator bismuth telluride films grown epitaxially on (111) BaF2 with a fixed Bi2Te3 beam flux were systematically investigated as a function of substrate temperature and additional Te flux. A layer-by-layer growth mode is observed since the early stages of epitaxy and remains throughout the whole deposition. Composition of the epitaxial films produced here stays between Bi2Te3 and Bi4Te5, as determined from the comparison of the measured x-ray diffraction curves with calculations. The substrate temperature region, where the growth rate remains constant, is found to be the most appropriate to obtain ordered Bi2Te3 films. Line width of the L = 18 Bi2Te3 diffraction peaks as low as 140 arcsec was obtained, indicating high crystalline quality. Twinning domains density rises with increasing growth temperature and reducing Te extra flux. X-ray reflectivity curves of pure Bi2Te3 films with thickness from 165 to 8 nm exhibited well defined interference fringes, evidencing homogeneous layers with smooth surface. Our results demonstrate that Bi2Te3 films with very well controlled structural parameters can be obtained. High structural quality Bi2Te3 films as thin as only eight quintuple layers grown here are promising candidates for intrinsic topological insulator.

  6. Radiation Resistance of Fluorite-Structured Nuclear Oxides

    SciTech Connect

    Garrido, Frederico; Moll, Sandra; Thome, Lionel; Vincent, Laetitia; Nowicki, Lech; Sattonnay, Gaeel

    2009-03-10

    Fluorite-structure oxides are radiation-resistant materials making them ideal candidates for uses as nuclear fuels or as inert matrices for actinide transmutation. The radiation tolerance of urania and cubic zirconia single crystals was investigated by external ion irradiation in predominating domains of electronic and nuclear stopping of bombarding particles. Damage kinetics show that the behavior of the two investigated fluorite-type oxides is almost the same: (i) at low-energy a two-stage disordering process is exhibited--first a ballistic step due to the formation of radiation-induced defects and second a crystal fragmentation induced by the formation of gas bubbles at large concentration-; (ii) at high energy a one-stage damage kinetics associated with the formation of ion tracks whose overlapping at high fluence results in the formation of nanometer-sized domains with a small disorientation.

  7. Glass structure and NIR emission of Er3+ at 1.5 μm in oxyfluoride BaF2-Al2O3-B2O3 glasses

    NASA Astrophysics Data System (ADS)

    Shinozaki, Kenji; Pisarski, Wojciech; Affatigato, Mario; Honma, Tsuyoshi; Komatsu, Takayuki

    2015-12-01

    The glass structure, photoluminescence properties of Eu3+, Judd-Ofelt analysis, and near infrared emissions of Er3+ at 1.5 μm in the oxyfluoride glasses and glass-ceramics of 1Eu2O3- or 1Er2O3-doped 50BaF2-xAl2O3-(50 - x)B2O3 (x = 0-25 mol%) were investigated. It was clarified on the ground of Raman scattering spectroscopy and F1s and O1s XPS measurements that the glass with no Al2O3 (1Er2O3-50BaF2-50B2O3) is composed of BO3, BO2F and BO3F units with F-Ba bonds. The glasses with 25Al2O3 (1Er2O3-50BaF2-25Al2O3-25B2O3) is mainly composed of BO3- and Al(O,F)x units. Existence of non-bridging oxygen was not detected by O1s-XPS spectra. It was proposed that these structures are largely affected on crystallization behavior, e.g., the glass with no Al2O3 forms BaF2 and β-BaB2O4 due to Ba-F bonds and the glass with 25Al2O3 forms BaAlBO3F2 because the glass structure composed of BO3 and Al(O,F) units is similar to the BaAlBO3F2 crystal structure. Judd-Ofelt parameters of Er3+ and Eu3+ in the glasses showed almost the same values in Ω4 and Ω6 for each glass, on the other hand Ω2 decreased with addition of Al2O3. The emission spectra of Er3+ at 1.5 μm in the glasses and glass-ceramics with BaAlBO3F2 crystals showed broad peaks. It is proposed that oxyfluoride glasses and glass-ceramics based on the BaF2-Al2O3-B2O3 system have a high potential for optical device applications such as broadband optical amplifiers.

  8. Giant Mechanocaloric Effects in Fluorite-Structured Superionic Materials

    NASA Astrophysics Data System (ADS)

    Cazorla, Claudio; Errandonea, Daniel

    2016-05-01

    Mechanocaloric materials experience a change in temperature when a mechanical stress is adiabatically applied on them. Thus far, only ferroelectrics and superelastic metallic alloys have been considered as potential mechanocaloric compounds to be exploited in solid-state cooling applications. Here we show that giant mechanocaloric effects occur in hitherto overlooked fast ion conductors (FIC), a class of multicomponent materials in which above a critical temperature, Ts, a constituent ionic species undergoes a sudden increase in mobility. Using first-principles and molecular dynamics simulations, we found that the superionic transition in fluorite-structured FIC, which is characterised by a large entropy increase of the order of 100 J/K*Kg, can be externally tuned with hydrostatic, biaxial or uniaxial stresses. In particular, Ts can be reduced several hundreds of degrees through the application of moderate tensile stresses due to the concomitant drop in the formation energy of Frenkel pair defects. We predict that the adiabatic temperature change in CaF2 and PbF2, two archetypal fluorite-structured FIC, close to their critical points are of the order of 100 and 10 K, respectively. This work advocates that FIC constitute a new family of mechanocaloric materials showing great promise for prospective solid-state refrigeration applications.

  9. Giant Mechanocaloric Effects in Fluorite-Structured Superionic Materials.

    PubMed

    Cazorla, Claudio; Errandonea, Daniel

    2016-05-11

    Mechanocaloric materials experience a change in temperature when a mechanical stress is applied on them adiabatically. Thus, far, only ferroelectrics and superelastic metallic alloys have been considered as potential mechanocaloric compounds to be exploited in solid-state cooling applications. Here we show that giant mechanocaloric effects occur in hitherto overlooked fast ion conductors (FIC), a class of multicomponent materials in which above a critical temperature, Ts, a constituent ionic species undergoes a sudden increase in mobility. Using first-principles and molecular dynamics simulations, we found that the superionic transition in fluorite-structured FIC, which is characterized by a large entropy increase of the order of 10(2) JK(-1) kg(-1), can be externally tuned with hydrostatic, biaxial, or uniaxial stresses. In particular, Ts can be reduced several hundreds of degrees through the application of moderate tensile stresses due to the concomitant drop in the formation energy of Frenkel pair defects. We predict that the adiabatic temperature change in CaF2 and PbF2, two archetypal fluorite-structured FIC, close to their critical points are of the order of 10(2) and 10(1) K, respectively. This work advocates that FIC constitute a new family of mechanocaloric materials showing great promise for prospective solid-state refrigeration applications.

  10. Vibrational spectra of (BaF2)n (n=1-6) clusters

    NASA Astrophysics Data System (ADS)

    Pandey, Ratnesh K.; Waters, Kevin; Nigam, Sandeep; Pandey, Ravindra; Pandey, Avinash C.

    2016-05-01

    The vibrational properties of alkaline-earth metal fluoride clusters (BaF2)n (n=1-6) are investigated in the framework of density functional theory. The calculated Raman and Infrared (IR) spectra reveals shift in Raman and IR peak position towards lower frequency region with the increase in the cluster size. Further the calculated spectra have been compared with the experimental vibrational spectra of bulk BaF2 crystal. Even though the smaller size cluster lacks translational symmetry, the structural and vibrational characteristic of (BaF2)5-6 are nearer to bulk counterpart.

  11. Low Dimensional String-like Relaxation Underpins Superionic Conduction in Fluorites and Related Structures.

    PubMed

    Annamareddy, Ajay; Eapen, Jacob

    2017-03-27

    Among the superionic conductors that show a Faraday transition - the continuous increase in the ionic conductivity over a range of temperatures - the fluorite structures have enjoyed incisive examinations over the past four decades; yet the fundamental nature of superionicity has remained largely inconclusive. Departing from the traditional quasi-static defect framework, we provide weighty evidence for string-like dynamical structures that govern the fast ion conduction process in fluorites. We show that lower temperatures encourage the growth of longer but slowly relaxing strings and vice-versa - a direct manifestation of heterogeneous dynamics. Remarkably, the ionic conductivity is inversely correlated to the lifetime of the ions that participate in the strings and not explicitly to the ion population. Our analysis methodology, which resolves a long-standing disagreement on defect structures and the mechanism of ionic transport in fcc fluorite structures, is well-positioned to describe the dynamics of low dimensional conduction in a larger class of superionic conductors.

  12. Thermodynamics of solid electrolytes and related oxide ceramics based on the fluorite structure

    SciTech Connect

    Navrotsky, Alexandra

    2010-01-01

    Oxides based on the fluorite structure are important as electrolytes in solid oxide fuel cells, thermal barrier coatings, gate dielectrics, catalysts, and nuclear materials. Though the parent fluorite structure is simple, the substitution of trivalent for tetravalent cations, coupled with the presence of charge-balancing oxygen vacancies, leads to a wealth of short-range and long-range ordered structures and complex thermodynamic properties. The location of vacancies and the nature of clusters affect the energetics of mixing in rare earth doped zirconia, hafnia, ceria, urania, and thoria, with systematic trends in energetics as a function of cation radius. High temperature oxide melt solution calorimetry has provided direct measurement of formation enthalpies of these refractory materials. Surface and interfacial energies have also been measured in yttria stabilized zirconia (YSZ) nanomaterials. Other ionic conductors having perovskite, apatite, and mellilite structures are discussed briefly.

  13. Activation volumes of oxygen self-diffusion in fluorite structured oxides

    SciTech Connect

    Christopoulos, S-R G.; Kordatos, A.; Cooper, Michael William D.; Fitzpatrick, M. E.; Chroneos, A.

    2016-10-27

    In this study, fluorite structured oxides are used in numerous applications and as such it is necessary to determine their materials properties over a range of conditions. In the present study we employ molecular dynamics calculations to calculate the elastic and expansivity data, which are then used in a thermodynamic model (the cBΩ model) to calculate the activation volumes of oxygen self-diffusion coefficient in ThO2, UO2 and PuO2 fluorite structured oxides over a wide temperature range. We present relations to calculate the activation volumes of oxygen self-diffusion coefficient in ThO2, UO2 and PuO2 for a wide range of temperature (300–1700 K) and pressure (–7.5 to 7.5 GPa).

  14. Activation volumes of oxygen self-diffusion in fluorite structured oxides

    DOE PAGES

    Christopoulos, S-R G.; Kordatos, A.; Cooper, Michael William D.; ...

    2016-10-27

    In this study, fluorite structured oxides are used in numerous applications and as such it is necessary to determine their materials properties over a range of conditions. In the present study we employ molecular dynamics calculations to calculate the elastic and expansivity data, which are then used in a thermodynamic model (the cBΩ model) to calculate the activation volumes of oxygen self-diffusion coefficient in ThO2, UO2 and PuO2 fluorite structured oxides over a wide temperature range. We present relations to calculate the activation volumes of oxygen self-diffusion coefficient in ThO2, UO2 and PuO2 for a wide range of temperature (300–1700more » K) and pressure (–7.5 to 7.5 GPa).« less

  15. Activation volumes of oxygen self-diffusion in fluorite structured oxides

    NASA Astrophysics Data System (ADS)

    Christopoulos, S.-R. G.; Kordatos, A.; Cooper, M. W. D.; Fitzpatrick, M. E.; Chroneos, A.

    2016-10-01

    Fluorite structured oxides are used in numerous applications and as such it is necessary to determine their materials properties over a range of conditions. In the present study we employ molecular dynamics calculations to calculate the elastic and expansivity data, which are then used in a thermodynamic model (the cBΩ model) to calculate the activation volumes of oxygen self-diffusion coefficient in ThO2, UO2 and PuO2 fluorite structured oxides over a wide temperature range. We present relations to calculate the activation volumes of oxygen self-diffusion coefficient in ThO2, UO2 and PuO2 for a wide range of temperature (300-1700 K) and pressure (-7.5 to 7.5 GPa).

  16. Low Dimensional String-like Relaxation Underpins Superionic Conduction in Fluorites and Related Structures

    PubMed Central

    Annamareddy, Ajay; Eapen, Jacob

    2017-01-01

    Among the superionic conductors that show a Faraday transition – the continuous increase in the ionic conductivity over a range of temperatures – the fluorite structures have enjoyed incisive examinations over the past four decades; yet the fundamental nature of superionicity has remained largely inconclusive. Departing from the traditional quasi-static defect framework, we provide weighty evidence for string-like dynamical structures that govern the fast ion conduction process in fluorites. We show that lower temperatures encourage the growth of longer but slowly relaxing strings and vice-versa – a direct manifestation of heterogeneous dynamics. Remarkably, the ionic conductivity is inversely correlated to the lifetime of the ions that participate in the strings and not explicitly to the ion population. Our analysis methodology, which resolves a long-standing disagreement on defect structures and the mechanism of ionic transport in fcc fluorite structures, is well-positioned to describe the dynamics of low dimensional conduction in a larger class of superionic conductors. PMID:28344314

  17. Fast ion conductivity in strained defect-fluorite structure created by ion tracks in Gd2Ti2O7

    PubMed Central

    Aidhy, Dilpuneet S.; Sachan, Ritesh; Zarkadoula, Eva; Pakarinen, Olli; Chisholm, Matthew F.; Zhang, Yanwen; Weber, William J.

    2015-01-01

    The structure and ion-conducting properties of the defect-fluorite ring structure formed around amorphous ion-tracks by swift heavy ion irradiation of Gd2Ti2O7 pyrochlore are investigated. High angle annular dark field imaging complemented with ion-track molecular dynamics simulations show that the atoms in the ring structure are disordered, and have relatively larger cation-cation interspacing than in the bulk pyrochlore, illustrating the presence of tensile strain in the ring region. Density functional theory calculations show that the non-equilibrium defect-fluorite structure can be stabilized by tensile strain. The pyrochlore to defect-fluorite structure transformation in the ring region is predicted to be induced by recrystallization during a melt-quench process and stabilized by tensile strain. Static pair-potential calculations show that planar tensile strain lowers oxygen vacancy migration barriers in pyrochlores, in agreement with recent studies on fluorite and perovskite materials. In view of these results, it is suggested that strain engineering could be simultaneously used to stabilize the defect-fluorite structure and gain control over its high ion-conducting properties. PMID:26555848

  18. Fast ion conductivity in strained defect-fluorite structure created by ion tracks in Gd2Ti2O7

    DOE PAGES

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

    2015-11-10

    The structure and ion-conducting properties of the defect-fluorite ring structure formed around amorphous ion-tracks by swift heavy ion irradiation of Gd2Ti2O7 pyrochlore are investigated. High angle annular dark field imaging complemented with ion-track molecular dynamics simulations show that the atoms in the ring structure are disordered, and have relatively larger cation-cation interspacing than in the bulk pyrochlore, illustrating the presence of tensile strain in the ring region. Density functional theory calculations show that the non-equilibrium defect-fluorite structure can be stabilized by tensile strain. The pyrochlore to defect-fluorite structure transformation in the ring region is predicted to be induced by recrystallizationmore » during a melt-quench process and stabilized by tensile strain. Static pair-potential calculations show that planar tensile strain lowers oxygen vacancy migration barriers in pyrochlores, in agreement with recent studies on fluorite and perovskite materials. Lastly, in view of these results, it is suggested that strain engineering could be simultaneously used to stabilize the defect-fluorite structure and gain control over its high ion-conducting properties.« less

  19. Fast ion conductivity in strained defect-fluorite structure created by ion tracks in Gd2Ti2O7.

    PubMed

    Aidhy, Dilpuneet S; Sachan, Ritesh; Zarkadoula, Eva; Pakarinen, Olli; Chisholm, Matthew F; Zhang, Yanwen; Weber, William J

    2015-11-10

    The structure and ion-conducting properties of the defect-fluorite ring structure formed around amorphous ion-tracks by swift heavy ion irradiation of Gd2Ti2O7 pyrochlore are investigated. High angle annular dark field imaging complemented with ion-track molecular dynamics simulations show that the atoms in the ring structure are disordered, and have relatively larger cation-cation interspacing than in the bulk pyrochlore, illustrating the presence of tensile strain in the ring region. Density functional theory calculations show that the non-equilibrium defect-fluorite structure can be stabilized by tensile strain. The pyrochlore to defect-fluorite structure transformation in the ring region is predicted to be induced by recrystallization during a melt-quench process and stabilized by tensile strain. Static pair-potential calculations show that planar tensile strain lowers oxygen vacancy migration barriers in pyrochlores, in agreement with recent studies on fluorite and perovskite materials. In view of these results, it is suggested that strain engineering could be simultaneously used to stabilize the defect-fluorite structure and gain control over its high ion-conducting properties.

  20. Novel and easy access to highly luminescent Eu and Tb doped ultra-small CaF2, SrF2 and BaF2 nanoparticles - structure and luminescence.

    PubMed

    Ritter, Benjamin; Haida, Philipp; Fink, Friedrich; Krahl, Thoralf; Gawlitza, Kornelia; Rurack, Knut; Scholz, Gudrun; Kemnitz, Erhard

    2017-02-28

    A universal fast and easy access at room temperature to transparent sols of nanoscopic Eu(3+) and Tb(3+) doped CaF2, SrF2 and BaF2 particles via the fluorolytic sol-gel synthesis route is presented. Monodisperse quasi-spherical nanoparticles with sizes of 3-20 nm are obtained with up to 40% rare earth doping showing red or green luminescence. In the beginning luminescence quenching effects are only observed for the highest content, which demonstrates the unique and outstanding properties of these materials. From CaF2:Eu10 via SrF2:Eu10 to BaF2:Eu10 a steady increase of the luminescence intensity and lifetime occurs by a factor of ≈2; the photoluminescence quantum yield increases by 29 to 35% due to the lower phonon energy of the matrix. The fast formation process of the particles within fractions of seconds is clearly visualized by exploiting appropriate luminescence processes during the synthesis. Multiply doped particles are also available by this method. Fine tuning of the luminescence properties is achieved by variation of the Ca-to-Sr ratio. Co-doping with Ce(3+) and Tb(3+) results in a huge increase (>50 times) of the green luminescence intensity due to energy transfer Ce(3+) → Tb(3+). In this case, the luminescence intensity is higher for CaF2 than for SrF2, due to a lower spatial distance of the rare earth ions.

  1. Special quasirandom structure modeling of fluorite-structured oxide solid solutions with aliovalent cation substitutions

    NASA Astrophysics Data System (ADS)

    Wolff-Goodrich, Silas; Hanken, Benjamin E.; Solomon, Jonathan M.; Asta, Mark

    2015-07-01

    The accuracy of the special quasirandom structure (SQS) approach for modeling the structure and energetics of fluorite-structured oxide solid solutions with aliovalent cation substitutions is assessed in an ionic-pair potential study of urania and ceria based systems mixed with trivalent rare-earth ions. Mixing enthalpies for SQS supercells containing 96 and 324 lattice sites were calculated using ionic pair potentials for U0.5La0.5O1.75, U0.5Y0.5O1.75, Ce0.5La0.5O1.75, Ce0.5Y0.5O1.75, and Ce0.5Gd0.5O1.75, which all have stoichiometries of pyrochlores. The SQS results were compared to benchmark values for random substitutional disorder obtained using large supercell models. The calculations show significant improvement of the mixing enthalpy for the larger 324 site SQS, which is attributed to a better description of the structural distortions, as characterized by the radial distribution functions in relaxed systems.

  2. Interconversion of perovskite and fluorite structures in Ce-Sc-O system.

    PubMed

    Shukla, Rakesh; Arya, Ashok; Tyagi, Avesh K

    2010-02-01

    CeScO(3) was synthesized by a two-step synthesis route involving a combustion method followed by vacuum heating at 1100 degrees C in the presence of Zr sponge which acts as an oxygen getter. The compound was characterized by various techniques such as X-ray diffraction (XRD), high temperature XRD, thermogravimetry, diffuse reflectance (DR)-UV visible spectrophotometry, and Raman spectroscopy. Fluorite-type (F-type) solid solution with composition Ce(0.5)Sc(0.5)O(1.75) was observed as an intermediate during the synthesis of CeScO(3). Only by mere redox reaction was a reversible transformation between fluorite-type structure and perovskites structure observed. CeScO(3) was found as semiconducting oxide with band gap of 3.2 eV arising mainly between O p states in the valence band and Sc d and Ce d states in the conduction band with small contributions coming from Ce f and Sc p states. First-principles potential plane-wave-based calculations were performed for the band gap and its origin in CeScO(3). Photoluminescence measurement showed that CeScO(3) is a potential host material giving broad blue emission. This was further confirmed by demonstrating CeScO(3) doped with 2 mol % Tb(3+) compound as an efficient green light emitter.

  3. Reflectors for canning a BaF 2 crystal

    NASA Astrophysics Data System (ADS)

    Sinha, B. K.; Bhattacharya, R.

    1989-03-01

    The performance of BaF 2 crystals with various reflectors like teflon tape, ZrO 2, Al 2O 3 and CaF 2 powders is tested by measuring the energy and time resolution at different energies. It is shown that CaF 2 powder is a somewhat better reflector than the Al 2O 3 powder so far used by different workers. It is also seen that the reflectivity of CaF 2 powder for the BaF 2 emission spectrum is not much inferior to that of teflon tape which is, however, the best reported so far. Based on these observations a method of canning a BaF 2 crystal is proposed.

  4. Determination of solubility limit of Sn(4+) in fluorite structured terbia with simultaneous evaluation of photocatalytic function.

    PubMed

    Tripathi, Vikash Kumar; Nagarajan, Rajamani

    2016-07-05

    Although the fluorite structure is highly common among stoichiometric and non-stoichiometric terbia compositions, high pressures are necessary to stabilize SnO2 in the fluorite structure. With this objective, the extent of solubility of Sn(4+) in terbia possessing the fluorite structure has been determined by conducting its synthesis via an epoxide mediated sol-gel method. Up to 40% of Sn(4+) can be incorporated in terbia, which retains its fluorite structure, as concluded from PXRD, FTIR, Raman spectroscopy, FESEM, HR-TEM and SAED measurements. The cubic lattice constant decreases systematically, as inferred from successful Rietveld refinements of PXRD patterns. The stretching vibration of the Tb-O bond is manifested as broad band at 734 cm(-1) for the terbia, and moves to lower wavenumber for the tin substituted samples. The broad band at 611 cm(-1) in the Raman spectrum of terbia became even broader with the maxima shifting towards higher values, which indicated the strain of the lattice and generation of oxygen vacancies with progressive tin substitution. The band gap value increased from 1.78 eV for terbia to 2.05 eV for the 40% tin substituted sample. Emission in the blue region became intense upon tin substitution, which was indicative of increased oxygen vacancies and this has been constructively utilized for environmental remediation as a catalyst to degrade aqueous Rhodamine-6G and Methylene Blue dye solutions.

  5. Density functional theory study of the electronic structure of fluorite Cu2Se

    NASA Astrophysics Data System (ADS)

    Råsander, Mikael; Bergqvist, Lars; Delin, Anna

    2013-03-01

    We have investigated the electronic structure of fluorite Cu2Se using density functional theory calculations within the LDA, PBE and AM05 approximations as well as the non-local hybrid PBE0 and HSE approximations. We find that Cu2Se is a zero gap semiconductor when using either a local or semi-local density functional approximation while the PBE0 functional opens up a gap. For the HSE approximation, we find that the presence of a gap depends on the range separation for the non-local exchange. For the occupied part in the density of states we find that LDA, PBE, AM05, PBE0 and HSE agree with regard to the overall electronic structure. However, the hybrid functionals result in peaks shifted towards lower energy compared to LDA, PBE and AM05. The valence bands obtained using the hybrid functionals are in good agreement with experimental valence band spectra. We also find that the PBE, PBE0 and HSE approximations give similar results regarding bulk properties, such as lattice constants and bulk modulus. In addition, we have investigated the localization of the Cu d-states and its effect on the band gap in the material using the LDA + U approach. We find that a sufficiently high U indeed opens up a gap; however, this U leads to valence bands that disagree with experimental observations.

  6. Density functional theory study of the electronic structure of fluorite Cu2Se.

    PubMed

    Råsander, Mikael; Bergqvist, Lars; Delin, Anna

    2013-03-27

    We have investigated the electronic structure of fluorite Cu2Se using density functional theory calculations within the LDA, PBE and AM05 approximations as well as the non-local hybrid PBE0 and HSE approximations. We find that Cu2Se is a zero gap semiconductor when using either a local or semi-local density functional approximation while the PBE0 functional opens up a gap. For the HSE approximation, we find that the presence of a gap depends on the range separation for the non-local exchange. For the occupied part in the density of states we find that LDA, PBE, AM05, PBE0 and HSE agree with regard to the overall electronic structure. However, the hybrid functionals result in peaks shifted towards lower energy compared to LDA, PBE and AM05. The valence bands obtained using the hybrid functionals are in good agreement with experimental valence band spectra. We also find that the PBE, PBE0 and HSE approximations give similar results regarding bulk properties, such as lattice constants and bulk modulus. In addition, we have investigated the localization of the Cu d-states and its effect on the band gap in the material using the LDA + U approach. We find that a sufficiently high U indeed opens up a gap; however, this U leads to valence bands that disagree with experimental observations.

  7. La2Hf2O7 crystal and local structure changes on the fluorite - pyrochlore phase transition

    NASA Astrophysics Data System (ADS)

    Popov, V. V.; Menushenkov, A. P.; Yastrebtsev, A. A.; Zubavichus, Ya V.

    2016-09-01

    The process of La2Hf2O7 (rLa3+/rHf4+ = 1.63) nanocrystals formation and evolution upon calcinations up to 1400 °C has been investigated by means of synchrotron radiation X-ray diffraction (XRD) and Raman spectroscopy. It has been shown that isothermal calcination at 800 °C/3h of the X-ray amorphous precursor firstly leads to the formation of oxide nanocrystalline powders with a defect fluorite structure. In the temperature range 900 - 1000 °C we observed the nucleation and growth of pyrochlore nanodomains inside a well crystalline fluorite matrix. The pyrochlore-type superstructural ordering of cations and anions appears at calcinations temperature higher than 1000 °C.

  8. Surface reconstruction of fluorites in vacuum and aqueous environment

    NASA Astrophysics Data System (ADS)

    Fisicaro, Giuseppe; Sicher, Michael; Amsler, Maximilian; Saha, Santanu; Genovese, Luigi; Goedecker, Stefan

    2017-08-01

    Surfaces and interfaces of bulk materials with liquids are of importance for a wide range of chemical processes. In this work, we systematically explore reconstructions on the (100) surface of calcium fluoride (CaF2) and other fluorites (M F2 ), M ={Sr,Cd,Ba} by sampling the configurational space with the minima hopping structure prediction method in conjunction with density functional theory calculations. We find a large variety of structures that are energetically very close to each other and are connected by very low barriers, resulting in a high mobility of the topmost surface anions. This high density of configurational states makes the CaF2 (100) surface a very dynamic system. The majority of the surface reconstructions found in CaF2 are also present in SrF2,CdF2, and BaF2. Furthermore, we investigate in detail the influence of these reconstructions on the crystal growth of CaF2 in solvents by modeling the fluorite-water interface and its wetting properties. We perform a global structural search both by explicitly including water molecules and by employing a recently developed soft-sphere solvation model to simulate an implicit aqueous environment. The implicit approach correctly reproduces both our findings with the explicit-water model and the experimentally reported contact angles for the partial-hydrophobic (111) and hydrophilic (100) surfaces. Our simulations show that the high anion mobility and the low coordination of the (100) surface atoms strongly favors the adsorption of water molecules over the (111) surface. The aqueous environment makes terminations with low-coordination surface atoms more stable, promoting (100) growth instead of the (111).

  9. The role of the coordination defect (CD) in the structures of anion-deficient, fluorite-related compounds.

    PubMed

    Bevan, D J M; Martin, Lisandra L; Martin, Raymond L

    2013-02-01

    The various superstructure phases that occur with the anion-deficient compositions of binary oxides MO(2-x) with the fluorite structure as parent are explored here in terms of the original 'coordination defect' (or CD) concept in which each vacant oxygen site, □, is 'coordinated' by six O atoms thereby creating the octahedral 'structure-determining' entity [M(3.5)□O(6)]. It emerges that the structure and composition of each anion-deficient (polymorph) phase can be described in terms of crystallographic `motifs' which comprise sets of parallel coplanar polygons based on ½<210>(F) and ½<111>(F) CD linkages.

  10. Investigating the electronic structure of fluorite-structured oxide compounds: comparison of experimental EELS with first principles calculations

    SciTech Connect

    Aguiar, Jeff; Ramasse, Q. M.; Asta, Mark D.; Browning, Nigel D.

    2012-06-27

    Energy loss spectra from fluorite-structured ZrO2, CeO2, and UO2 compounds are compared with theoretical calculations based on density functional theory (DFT) and its extensions, including the use of Hubbard-U corrections (DFT + U) and hybrid functionals. Electron energy loss spectra (EELS) were obtained from each oxide using a scanning transmission electron microscope (STEM). The same spectra were computed within the framework of the full-potential linear augmented plane-wave (FLAPW) method. The theoretical and experimental EEL spectra are compared quantitatively using non-linear least squares peak fitting and a cross-correlation approach, with the best level of agreement between experiment and theory being obtained using the DFT + U and hybrid computational approaches.

  11. Investigating the electronic structure of fluorite-structured oxide compounds: comparison of experimental EELS with first principles calculations.

    PubMed

    Aguiar, J A; Ramasse, Q M; Asta, M; Browning, N D

    2012-07-25

    Energy loss spectra from fluorite-structured ZrO(2), CeO(2), and UO(2) compounds are compared with theoretical calculations based on density functional theory (DFT) and its extensions, including the use of Hubbard-U corrections (DFT + U) and hybrid functionals. Electron energy loss spectra (EELS) were obtained from each oxide using a scanning transmission electron microscope (STEM). The same spectra were computed within the framework of the full-potential linear augmented plane-wave (FLAPW) method. The theoretical and experimental EEL spectra are compared quantitatively using non-linear least squares peak fitting and a cross-correlation approach, with the best level of agreement between experiment and theory being obtained using the DFT + U and hybrid computational approaches.

  12. Novel fluorite structured superparamagnetic RbGdF₄ nanocrystals as versatile upconversion host.

    PubMed

    Ahmad, Shahzad; Nagarajan, Rajamani; Raj, Packiya; Prakash, G Vijaya

    2014-10-06

    Fluorite structured nanocrystals of RbGdF4 in cubic symmetry have successfully been synthesized by employing a simple, one-step, and template-free wet chemical method at room temperature. Considering the structural model of cubic KLaF4 in the Fm̅3m space group, the observed powder X-ray diffraction (PXRD) pattern was fitted by the Le Bail procedure with the cubic lattice constant of a = 5.8244 (1) Å. Both high-resolution transmission electron microscopic (HR-TEM) and dynamic light scattering (DLS) measurements revealed the monodispersity of the nanocrystals with their size in the range of 2-18 nm. Upon excitation at 980 nm, Yb(3+), (Er(3+)/Ho(3+)/Tm(3+)) codoped RbGdF4 nanocrystals showed multicolor upconversion including red, yellow, blue, and the combination of basic color (near-white) emissions. Also, near-white upconversion emission from Yb(3+), Ho(3+), Tm(3+) triply doped cubic RbGdF4 nanocrystals was observed at varying laser power densities. RbGdF4 nanocrystals exhibited superparamagnetic behavior with a molar magnetic susceptibility of 2.61 × 10(-2) emu·Oe(-1)·mol(-1) at room temperature, while at low temperature (5 K) a saturation magnetization value of 90.41 emu·g(-1) at an applied field of at 10 kOe was observed. Non-interaction of the localized magnetic moment of Gd(3+) ions in the host matrix has been reasoned out for the observed superparamagnetic behavior. From the Langevin fit of the magnetic data, the average particle diameter obtained was approximately 2.2 nm, matching well with the values from other measurements. RbGdF4 nanocrystals exhibited a large ionic longitudinal relaxivity (r1 = 2.30 s(-1)·mM(-1)), suggesting their potential applicability as a promising agent for T1 contrast magnetic resonance imaging (MRI) in addition to the applications arising from the coupling of optical and magnetic functions such as multiplexing biodetection, bioimaging (optical and MRI), and other optical technologies.

  13. Redetermination of the structure of ALa2WO7 (A=Ba, Sr) with fluorite-like metal ordering

    NASA Astrophysics Data System (ADS)

    Fu, W. T.; IJdo, D. J. W.; Bontenbal, A.

    2013-05-01

    The crystal structures of ALa2WO7 (A=Ba, and Sr) at room temperature were re-determined by the Rietveld method using the combined X-ray and neutron powder diffraction data. The compounds are confirmed to be isomorphic, crystallizing in the space group P1121/b. In ALa2WO7 the ordering of metal atoms is fluorite-like, but it differs from that of the fluorite-defect compounds of the formula Ln3MO7 (Ln=lanthanide or Y, M=pentavalent metal). The structure of ALa2WO7 consists of isolated WO6 octahedra, whereas in the normal Ln3MO7 the MO6 octahedra share corners forming one-dimensional chains. Although ALa2WO7 has a centric space group, La ions are not situate at the centre of symmetry, which explains the 5D0→7F2 transition being dominant in emission spectrum of Eu-doped materials.

  14. Fragmentation of the fluorite type in Fe8Al(17.4)Si(7.6): structural complexity in intermetallics dictated by the 18 electron rule.

    PubMed

    Fredrickson, Rie T; Fredrickson, Daniel C

    2012-10-01

    This Article presents the synthesis, structure determination, and bonding analysis of Fe(8)Al(17.4)Si(7.6). Fe(8)Al(17.4)Si(7.6) crystallizes in a new monoclinic structure type based on columns of the fluorite (CaF(2)) structure type. As such, the compound can be seen as part of a structural series in which the fluorite structure-adopted by several transition metal disilicides (TMSi(2))-is fragmented by the incorporation of Al. Electronic structure analysis using density functional theory (DFT) and DFT-calibrated Hückel calculations indicates that the fluorite-type TMSi(2) phases (TM = Co, Ni) exhibit density of states (DOS) pseudogaps near their Fermi energies. An analogous pseudogap occurs for Fe(8)Al(17.4)Si(7.6), revealing that its complex structure serves to preserve this stabilizing feature of the electronic structure. Pursuing the origins of these pseudogaps leads to a simple picture: the DOS minimum in the TMSi(2) structures arises via a bonding scheme analogous to those of 18 electron transition metal complexes. Replacement of Si with Al leads to the necessity of increasing the (Si/Al):TM ratio to maintain this valence electron concentration. The excess Si/Al atoms are accommodated through the fragmentation of the fluorite type. The resulting picture highlights how the elucidating power of bonding concepts from transition metal complexes can extend into the intermetallic realm.

  15. Solid State Structure-Reactivity Studies on Bixbyites, Fluorites and Perovskites Belonging to the Vanadate, Titanate and Cerate Families

    NASA Astrophysics Data System (ADS)

    Shafi, Shahid P.

    This thesis primarily focuses on the systematic understanding of structure-reactivity relationships in two representative systems: bixbyite and related structures as well as indium doped CeO2. Topotactic reaction routes have gained significant attention over the past two decades due to their potential to access kinetically controlled metastable materials. This has contributed substantially to the understanding of solid state reaction pathways and provided first insights into mechanisms. Contrary to the widely used ex-situ methods, in-situ techniques including powder x-ray diffraction and thermogravimetric-differential thermal analysis have been employed extensively throughout this work in order to follow the reaction pathways in real time. Detailed analysis of the AVO3 (A = In, Sc) bixbyite reactivity under oxidative conditions has been carried out and a variety of novel metastable oxygen defect phases have been identified and characterized. The novel metastable materials have oxygen deficient fluorite structures and consequently are potential ion conductors. Structural aspects of the topotactic vs. reconstructive transformations are illustrated with this model system. The structure-reactivity study of AVO3 phases was extended to AVO3 perovskite family. Based on the research methodologies and results from AVO3 bixbyite reactivity studies a generalized mechanistic oxidation pathway has been established with a non-vanadium phase, ScTiO3 bixbyite. However, there is stark contrast in terms of structural stability and features beyond this stability limit during AVO3 and ScTiO3 bixbyite reaction pathways. A series of complex reaction sequences including phase separation and phase transitions were identified during the investigation of ScTiO3 reactivity. The two-step formation pathway for the fluorite-type oxide ion conductor Ce1-xInxO2-delta (0 ≤ x ≤ 0.3) is being reported. The formation of the BaCe1-xInxO 3-delta perovskites and the subsequent CO2-capture reaction

  16. Structure of fluorite-like compound based on Nd₅Mo₃O₁₆ with lead partly substituting for neodymium.

    PubMed

    Antipin, Alexander M; Sorokina, Natalia I; Alekseeva, Olga A; Kuskova, Alexandra N; Kharitonova, Elena P; Orlova, Ekaterina I; Voronkova, Valentina I

    2015-04-01

    A single crystal of Nd5Mo3O16 with lead partly substituting for neodymium, which has a fluorite-like structure, was studied by precision X-ray diffraction, high-resolution transmission microscopy and EDX microanalysis. The crystal structure is determined in the space group Pn3¯n. It was found that the Pb atoms substitute in part for Nd atoms in the structure and are located in the vicinity of Nd2 positions. Partial substitutions of Mo cations for Nd positions and of Nd for Mo positions in crystals of the Ln5Mo3O16 oxide family are corroborated by X-ray diffraction for the first time. The first experimental verification of the location of an additional oxygen ion in the voids abutting MoO4 tetrahedra was obtained.

  17. Redetermination of the structure of ALa₂WO₇ (A=Ba, Sr) with fluorite-like metal ordering

    SciTech Connect

    Fu, W.T.; IJdo, D.J.W.; Bontenbal, A.

    2013-05-01

    The crystal structures of ALa₂WO₇ (A=Ba, and Sr) at room temperature were re-determined by the Rietveld method using the combined X-ray and neutron powder diffraction data. The compounds are confirmed to be isomorphic, crystallizing in the space group P112₁/b. In ALa₂WO₇ the ordering of metal atoms is fluorite-like, but it differs from that of the fluorite-defect compounds of the formula Ln₃MO₇ (Ln=lanthanide or Y, M=pentavalent metal). The structure of ALa₂WO₇ consists of isolated WO₆ octahedra, whereas in the normal Ln₃MO₇ the MO₆ octahedra share corners forming one-dimensional chains. Although ALa₂WO₇ has a centric space group, La ions are not situate at the centre of symmetry, which explains the ⁵D₀→⁷F₂ transition being dominant in emission spectrum of Eu-doped materials. - Graphical abstract: Schematic drawing of the crystal structures of ALa₂WO₇ (A=Ba, Sr) (left) and Ln₃MO₇ (right) showing the different arrangement of metal octahedra. Note that the example of Ln₃MO₇ is the structure of La₃TaO₇ with the space group Cmcm. Highlights: • The structural ambiguity of BaLa₂WO₇ has been resolved. • The details of the crystal structure of SrLa₂WO₇ are reported for the first time. • ALa₂WO₇ (A=Ba, Sr) closely resemble β-La₃RuO₇ and one of the La₃IrO₇ polymorph.

  18. Divalent europium doped CaF2 and BaF2 nanocrystals from ionic liquids

    DOE PAGES

    Anghel, Sergiu; Golbert, Sebastian; Meijerink, Andries; ...

    2016-10-11

    A new, facile and quick synthesis method for Eu2+ doped the alkaline earth fluorides was developed using ionic liquids as solvent, precursor and capping agent. Reductive atmosphere and very high temperatures were avoided, while still attaining the desired structure, small particle sizes and divalent oxidation state of the lanthanide. Here, this opens the door for the development of new Ln2+ doped nanomaterials. Here, the successful Eu2+ incorporation was proven by optical spectroscopic measurements which showed the spin and parity allowed f-d transitions of Eu2+ in CaF2:Eu2+/BaF2:Eu2+. 4f7-4f7 transitions could be observed at low temperatures (7 K).

  19. Core-hole effect on XANES and electronic structure of minor actinide dioxides with fluorite structure

    NASA Astrophysics Data System (ADS)

    Suzuki, Chikashi; Nishi, Tsuyoshi; Nakada, Masami; Akabori, Mitsuo; Hirata, Masaru; Kaji, Yoshiyuki

    2012-02-01

    The authors investigated theoretically core-hole effects on X-ray absorption near-edge structures (XANES) of Np and Am LIII in neptunium dioxide (NpO2) and americium dioxide (AmO2) with CaF2-type crystal lattices using the all-electron full-potential linearized augmented plane-wave (FP-LAPW) method. The peak creation mechanism of XANES was shown by examining the electronic structures of these oxides, which indicated that core-hole screening was more marked for AmO2 than for NpO2 because of the difference in the charge transfer between these oxides. Furthermore, the results of charge density analysis suggested that the white line was assigned to the quasi-bound state composed of the localized Np d or Am d components and O components, and that the tail structure was created as a result of delocalized standing waves between the Np or Am atoms.

  20. Luminescence and scintillation properties of BaF2sbnd Ce transparent ceramic

    NASA Astrophysics Data System (ADS)

    Luo, Junming; Sahi, Sunil; Groza, Michael; Wang, Zhiqiang; Ma, Lun; Chen, Wei; Burger, Arnold; Kenarangui, Rasool; Sham, Tsun-Kong; Selim, Farida A.

    2016-08-01

    Cerium doped Barium Fluoride (BaF2sbnd Ce) transparent ceramic was fabricated and its luminescence and scintillation properties were studied. The photoluminescence shows the emission peaks at 310 nm and 323 nm and is related to the 5d-4f transitions in Ce3+ ion. Photo peak at 511 keV and 1274 keV were obtained with BaF2sbnd Ce transparent ceramic for Na-22 radioisotopes. Energy resolution of 13.5% at 662 keV is calculated for the BaF2sbnd Ce transparent ceramic. Light yield of 5100 photons/MeV was recorded for BaF2sbnd Ce(0.2%) ceramic and is comparable to its single crystal counterpart. Scintillation decay time measurements shows fast component of 58 ns and a relatively slow component of 434 ns under 662 keV gamma excitation. The slower component in BaF2sbnd Ce(0.2%) ceramic is about 200 ns faster than the STE emission in BaF2 host and is associated with the dipole-dipole energy transfer from the host matrix to Ce3+ luminescence center.

  1. The fluorite related modulated structures of the Gd2(Zr2-xCex)O7 solid solution: An analogue for Pu disposition

    NASA Astrophysics Data System (ADS)

    Reid, D. P.; Stennett, M. C.; Hyatt, N. C.

    2012-07-01

    We present an overview of the Gd2(Zr2-xCex)O7 phase diagram, of interest as a model system for ceramic disposition of Pu (with Ce as a Pu surrogate). The fluorite related structures of this solid solution were determined using a modulated structure approach, to identify the underlying cation and vacancy ordering mechanisms from analysis of key satellite reflections in selected zone axis electron diffraction patterns. This revealed the formation of four structure types: pyrochlore for x<0.25, defect fluorite for 0.5structure for x=1.00, and a C-type structure for x>1.50. X-ray absorption (XAS) and electron energy loss (EELS) spectra confirmed the presence of Ce4+ as the dominant species in compositions across this system, remaining analogous to Pu4+.

  2. Characterization and Defect Structure of Anion-Excess Fluorite-Related Phases in LnOF-LnF

    SciTech Connect

    Laval, J. P.; Taoudi, A.; Abaouz, A.

    2001-02-15

    The phases LnO{sub 1-x}F{sub 1+2x}, derived from the fluorite-type structure by anionic excess, form, for medium size Ln cations (Ln=Nd, Sm, Eu, Gd), a homogeneous series of orthorhombic symmetry (space group Pmmn, a{approx}b{approx}a{sub F}{radical}2/2; c=a{sub F}). Structural comparison of these phases by X-ray powder diffraction allows us to make a better determination of their defect structure: O/F order in sheets perpendicular to the [001] axis and formation of 1:0:2 clusters which can be associated in dimer units. For Nd, an orthorhombic-tetragonal transition is explained by a change from 1:0:2 to denser 1:0:3 clusters. Comparison of the structural features of these phases with ''vernier'' modulated phases, isolated for Sm-Lu cations, allows a general classification of LnO{sub 1-x}F{sub 1+2x} phases at low temperature to be proposed. All are characterized by the preservation of an anionic O/F order in sheets perpendicular to the [001]{sub F} axis and by a progressive loss of freedom in the accommodation of the anionic excess (nature and orientation of the clusters, tendency to long-range ordering) from La to Lu, caused by the decrease in Ln size and therefore in Ln coordination.

  3. Formation, structure and magnetism of the metastable defect fluorite phases AVO{sub 3.5+x} (A=In, Sc)

    SciTech Connect

    Shafi, Shahid P.; Lundgren, Rylan J.; Cranswick, Lachlan M.D.; Bieringer, Mario

    2007-12-15

    We report the preparation and stability of ScVO{sub 3.5+x} and the novel phase InVO{sub 3.5+x}. AVO{sub 3.5+x} (A=Sc, In) defect fluorite structures are formed as metastable intermediates during the topotactic oxidation of AVO{sub 3} bixbyites. The oxidation pathway has been studied in detail by means of thermogravimetric/differential thermal analysis and in-situ powder X-ray diffraction. The oxidation of the bixbyite phase follows a topotactic pathway at temperatures between 300 and 400 deg. C in air/carbon dioxide. The range of accessible oxygen stoichiometries for the AVO{sub 3.5+x} structures following this pathway are 0.00{<=}x{<=}0.22. Rietveld refinements against powder X-ray and neutron data revealed that InVO{sub 3.54} and ScVO{sub 3.70} crystallize in the defect fluorite structure in space group Fm-3 m (227) with a=4.9863(5) and 4.9697(3)A, respectively with A{sup 3+}/V{sup 4+} disorder on the (4a) cation site. Powder neutron diffraction experiments indicate clustering of oxide defects in all samples. Bulk magnetic measurements showed the presence of V{sup 4+} and the absence of magnetic ordering at low temperatures. Powder neutron diffraction experiments confirmed the absence of a long range ordered magnetic ground state. - Graphical abstract: Topotactic oxidation of AVO{sub 3} bixbyite to AVO{sub 3.5} defect fluorite structure followed by in-situ powder X-ray diffraction. The upper structural diagram shows a six coordinated (A/V)-O{sub 6} fragment in bixbyite, the lower structure illustrates the same seven-fold coordinated (A/V)-O{sub 7} cubic environment in the defect fluorite structure.

  4. Fluorite and mixed-metal Kagome-related topologies in metal-organic framework compounds: synthesis, structure, and properties.

    PubMed

    Mahata, Partha; Raghunathan, Rajamani; Banerjee, Debamalya; Sen, Diptiman; Ramasesha, S; Bhat, S V; Natarajan, S

    2009-06-02

    Two new three-dimensional metal-organic frameworks (MOFs) [Mn(2)(mu(3)-OH)(H(2)O)(2)(BTC)] x 2 H(2)O, I, and [NaMn(BTC)], II (BTC = 1,2,4-benzenetricarboxylate = trimellitate) were synthesized and their structures determined by single-crystal X-ray diffraction (XRD). In I, the Mn(4) cluster, [Mn(4)(mu(3)-OH)(2)(H(2)O)(4)O(12)], is connected with eight trimellitate anions and each trimellitate anion connects to four different Mn(4) clusters, resulting in a fluorite-like structure. In II, the Mn(2)O(8) dimer is connected with two Na(+) ions through carboxylate oxygen to form mixed-metal distorted Kagome-related two-dimensional -M-O-M- layers, which are pillared by the trimellitate anions forming the three-dimensional structure. The extra-framework water molecules in I are reversibly adsorbed and are also corroborated by powder XRD studies. The formation of octameric water clusters involving free and coordinated water molecules appears to be new. Interesting magnetic behavior has been observed for both compounds. Electron spin resonance (ESR) studies indicate a broadening of the signal below the ordering temperature and appear to support the findings of the magnetic studies.

  5. An oxygen deficient fluorite with a tunnel structure: Bi sub 8 La sub 10 O sub 27

    SciTech Connect

    Michel, C.; Caignaert, V.; Raveau, B. )

    1991-02-01

    A new lanthanum bismuth oxide, Bi{sub 8}La{sub 10}O{sub 27}, has been synthesized. It crystallizes in the Immm space group with the following parameters: a = 12.079 (2) {angstrom}, b = 16.348 (4) {angstrom}, c = 4.0988 (5) {angstrom}. Its structure was determined from powder X-ray and neutron diffraction data. It can be described as an oxygen deficient fluorite superstructure (a {approx} 3a{sub F}/{radical}2, b {approx} 3a{sub F}, c {approx} a{sub F}/{radical}2) in which bismuth and lanthanum, as well as oxygen vacancies, are ordered. The structure consists of fully occupied (110) or ({bar 1}10) lanthanum planes (La) which alternate with mixed {l brace}{sub La}{sup Ba}{r brace} planes and fully occupied oxygen planes (A) which alternate with two sorts of oxygen deficient (110) or ({bar 1}10) planes (B and C) according to the sequence -{l brace} La A B {l brace}{sub La}{sup Bi}{r brace} C C {l brace}{sub La}{sup Bi}{r brace} B A {l brace}-. The anionic distribution determines tunnels where the bismuth ions are located, forming diamond-shaped based tunnels. The coordination of bismuth and lanthanum is discussed. The high thermal factor of some oxygen atoms suggests that this oxide exhibits ionic conductivity.

  6. Response of MEDEA BaF 2 detectors to 20-280 MeV photons

    NASA Astrophysics Data System (ADS)

    Bellia, G.; Alba, R.; Coniglione, R.; Del Zoppo, A.; Finocchiaro, P.; Maiolino, C.; Migneco, E.; Piattelli, P.; Sapienza, P.; Frascaria, N.; Lhenry, I.; Roynette, J. C.; Suomijärvi, T.; Alamanos, N.; Auger, F.; Gillibert, A.; Pierroutsakou, D.; Sida, J. L.; Silveira Gomes, P. R.

    1993-05-01

    The response function of MEDEA BaF 2 crystals to high energy photons, up to 280 MeV, has been studied using monochromatic γ-rays from the in flight annihilation of positron beams. The experimental response functions are compared to the results of Monte Carlo simulations based on the EGS3 code and parametrized over the whole investigated energy range.

  7. Clock distribution for BaF2 readout electronics at CSNS-WNS

    NASA Astrophysics Data System (ADS)

    He, Bing; Cao, Ping; Zhang, De-Liang; Wang, Qi; Zhang, Ya-Xi; Qi, Xin-Cheng; An, Qi

    2017-01-01

    A BaF2 (Barium Fluoride) detector array is designed to precisely measure the (n, γ) cross section at the CSNS-WNS (white neutron source at China Spallation Neutron Source). It is a 4π solid angle-shaped detector array consisting of 92 BaF2 crystal elements. To discriminate signals from the BaF2 detector, a pulse shape discrimination method is used, supported by a waveform digitization technique. There are 92 channels for digitizing. The precision and synchronization of clock distribution restricts the performance of waveform digitizing. In this paper, a clock prototype for the BaF2 readout electronics at CSNS-WNS is introduced. It is based on the PXIe platform and has a twin-stage tree topology. In the first stage, clock is synchronously distributed from the tree root to each PXIe crate through a coaxial cable over a long distance, while in the second stage, the clock is further distributed to each electronic module through a PXIe dedicated differential star bus. With the help of this topology, each tree node can fan out up to 20 clocks with 3U size. Test results show the clock jitter is less than 20 ps, which meets the requirements of the BaF2 readout electronics. Besides, this clock system has the advantages of high density, simplicity, scalability and cost saving, so it can be useful for other clock distribution applications. Supported by National Research and Development plan (2016 YFA0401602) NSAF (U1530111) and National Natural Science Foundation of China (11005107)

  8. Atomized BaF2-CaF7 for Better-Flowing Plasma-Spray Feedstock

    NASA Technical Reports Server (NTRS)

    DellaCorte, Christopher; Stanford, Malcolm K.

    2008-01-01

    Atomization of a molten mixture of BaF2 and CaF2 has been found to be superior to crushing of bulk solid BaF2- CaF2 as a means of producing eutectic BaF2-CaF2 powder for use as an ingredient of the powder feedstock of a high-temperature solid lubricant material known as PS304. Developed to reduce friction and wear in turbomachines that incorporate foil air bearings, PS304 is applied to metal substrates by plasma spraying. The constituents of PS304 are: a) An alloy of 80 weight percent Ni and 20 weight percent Cr, b) Cr2O3, c) Ag, and d) The BaF2-CaF2 eutectic, specifically, 62 weight percent BaF2 and 38 weight percent CaF2. The superiority of atomization as a means of producing the eutectic BaF2-CaF2 powder lies in (1) the shapes of the BaF2-CaF2 particles produced and (2) the resulting flow properties of the PS304 feedstock powder: The particles produced through crushing are angular, whereas those produced through atomization are more rounded. PS304 feedstock powder containing the more rounded BaF2-CaF2 particles flows more freely and more predictably, as is preferable for plasma spraying.

  9. Biologic precipitation of fluorite.

    PubMed

    Lowenstam, H A; McConnell, D

    1968-12-27

    X-ray diffraction patterns show that the statoliths of marine mysid crustaceans are composed of fluorite, and that this mineral is also a principal phase of the gizzard plates of some tectibranch gastropods. A phosphatic phase is also indicated by chemical analyses in the gizzard plates, but its crystallochemical characterization has not been feasible by x-ray diffraction. The occurrence of fluorite in mysid statoliths confirms the earlier interpretations based on insufficient documentation. Fixation of fluorine in hard tissues of marine invertebrates is extensive in the shelf seawaters and minor in the bathyal zone of the oceans.

  10. Water orientation and hydrogen-bond structure at the fluorite/water interface

    PubMed Central

    Khatib, Rémi; Backus, Ellen H. G.; Bonn, Mischa; Perez-Haro, María-José; Gaigeot, Marie-Pierre; Sulpizi, Marialore

    2016-01-01

    Water in contact with mineral interfaces is important for a variety of different processes. Here, we present a combined theoretical/experimental study which provides a quantitative, molecular-level understanding of the ubiquitous and important CaF2/water interface. Our results show that, at low pH, the surface is positively charged, causing a substantial degree of water ordering. The surface charge originates primarily from the dissolution of fluoride ions, rather than from adsorption of protons to the surface. At high pH we observe the presence of Ca-OH species pointing into the water. These OH groups interact remarkably weakly with the surrounding water, and are responsible for the “free OH” signature in the VSFG spectrum, which can be explained from local electronic structure effects. The quantification of the surface termination, near-surface ion distribution and water arrangement is enabled by a combination of advanced phase-resolved Vibrational Sum Frequency Generation spectra of CaF2/water interfaces and state-of-the-art ab initio molecular dynamics simulations which include electronic structure effects. PMID:27068326

  11. Water orientation and hydrogen-bond structure at the fluorite/water interface.

    PubMed

    Khatib, Rémi; Backus, Ellen H G; Bonn, Mischa; Perez-Haro, María-José; Gaigeot, Marie-Pierre; Sulpizi, Marialore

    2016-04-12

    Water in contact with mineral interfaces is important for a variety of different processes. Here, we present a combined theoretical/experimental study which provides a quantitative, molecular-level understanding of the ubiquitous and important CaF2/water interface. Our results show that, at low pH, the surface is positively charged, causing a substantial degree of water ordering. The surface charge originates primarily from the dissolution of fluoride ions, rather than from adsorption of protons to the surface. At high pH we observe the presence of Ca-OH species pointing into the water. These OH groups interact remarkably weakly with the surrounding water, and are responsible for the "free OH" signature in the VSFG spectrum, which can be explained from local electronic structure effects. The quantification of the surface termination, near-surface ion distribution and water arrangement is enabled by a combination of advanced phase-resolved Vibrational Sum Frequency Generation spectra of CaF2/water interfaces and state-of-the-art ab initio molecular dynamics simulations which include electronic structure effects.

  12. Fast ion conductivity in strained defect-fluorite structure created by ion tracks in Gd2Ti2O7

    SciTech Connect

    Aidhy, Dilpuneet S.; Sachan, Ritesh; Zarkadoula, Eva; Pakarinen, Olli; Chisholm, Matthew F.; Zhang, Yanwen; Weber, William J.

    2015-11-10

    In this research, the structure and ion-conducting properties of the defect-fluorite ring structure formed around amorphous ion-tracks by swift heavy ion irradiation of Gd2Ti2O7 pyrochlore are investigated. High angle annular dark field imaging complemented with ion-track molecular dynamics simulations show that the atoms in the ring structure are disordered, and have relatively larger cation-cation interspacing than in the bulk pyrochlore, illustrating the presence of tensile strain in the ring region. Density functional theory calculations show that the non-equilibrium defect-fluorite structure can be stabilized by tensile strain. The pyrochlore to defect-fluorite structure transformation in the ring region is predicted to be induced by recrystallization during a melt-quench process and stabilized by tensile strain. Static pair-potential calculations show that planar tensile strain lowers oxygen vacancy migration barriers in pyrochlores, in agreement with recent studies on fluorite and perovskite materials. From these results, it is suggested that strain engineering could be simultaneously used to stabilize the defect-fluorite structure and gain control over its high ion-conducting properties.

  13. Investigating the Electronic Structure of Fluorite Oxides: Comparsion of EELS and First Principles Calculations

    SciTech Connect

    Aguiar, J; Asta, M; Gronbech-Jensen, N; Perlov, A; Milman, V; Gao, S; Pickard, C; Browning, N

    2009-06-05

    Energy loss spectra from a variety of cubic oxides are compared with ab-initio calculations based on the density functional plane wave method (CASTEP). In order to obtain agreement between experimental and theoretical spectra, unique material specific considerations were taken into account. The spectra were calculated using various approximations to describe core-hole effects and electronic correlations. All the calculations are based on the local spin density approximation to show qualitative agreement with the sensitive oxygen K-edge spectra in ceria, zirconia, and urania. Comparison of experimental and theoretical results let us characterize the main electronic interactions responsible for both the electronic structure and the resulting EEL spectra of the compounds in question.

  14. Analysis of anharmonic elastic properties of anti-fluorite structure A 2MX 6 single crystals

    NASA Astrophysics Data System (ADS)

    Chaudhry, Pradeepti; Singh, S. P.; Gupta, B. R. K.

    1995-06-01

    In the present paper and analysis of vibrational anharmonic properties of a number of antifluorite structure crystals [K 2SnCl 6, K 2ReCl 6, K 2PtBr 6, (NH 4) 2SnCl 6,(NH 4) 2PtBr 6 and (NH 4) 2TeCl 6] has been performed in terms of higher order elastic constants and their linear combinations, pressure and temperature derivatives of their second order elastic constants (SOEC) and Gruneisen parameters. These physical quantities have been calculated by making use of a phenomonological potential model. It has been found that the three-body interactions considered in the present potential model plays an important and significant role in explaining the characteristic behaviour of the crystals under study. Good agreement between the experimental and theoretical values has been obtained.

  15. Relaxation and interaction of electronic excitations induced by intense ultra short light pulses in BaF2 scintillator

    NASA Astrophysics Data System (ADS)

    Kirm, M.; Nagirnyi, V.; Vielhauer, S.; Feldbach, E.

    2011-06-01

    Excitation density effects have a pronounced influence on relaxation processes in solids. They come into play in scintillating and dosimetric materials exposed to ionizing radiation or in laser materials operating in intense ultraviolet light fields. The scientific understanding of the underlying process is poor, mainly because most of the studies of light emitting materials under short wavelength excitation have been performed at weak and moderate excitation intensities due to limited availability of powerful light sources. Disembodied data on excitation density effects have been reported for wide-gap dielectrics studied by luminescence spectroscopy, by using such excitation sources as powerful ion beams,1,2pulsed electron beams,2,3 and wide-band hard X-ray synchrotron radiation.4 It is obvious that such non-selective excitation is a good tool for revealing density-related phenomena in these materials in general, but for investigating specific features of relaxation processes in insulators, light sources with well defined parameters are necessary. Since the shortwavelength free electron laser (FEL) technology has been devised by an international consortium at HASYLAB of DESY, resulting in the development of TESLA Test facility (TTF)5 and later in the construction of a dedicated FEL source FLASH in Hamburg,6 more advanced studies became possible. The range of interests towards this light source covers the fields from material science and various other branches of physics to structural biology. The pioneering luminescence study revealed excitation density effects in the decay of Ce3+ 5d-4f luminescence in Y3Al5O12 crystals and luminescence of BaF2 crystals in UV-visible range.7 These results motivated systematic investigations of excitation density effects in wide gap crystals using FEL8,9 and high-harmonic-generated VUV radiation,10 and, at lower energies, femtosecond laser pulses in the UV.11,12 The main goal of the present work is to analyze the same phenomenon

  16. First-principles identification of charge-transition levels of native defects in BaF2

    NASA Astrophysics Data System (ADS)

    Ibraheem, A. M.; Eisa, M. H.; Adlan, W.; Amolo, George O.; Khalafalla, M. A. H.

    2017-03-01

    This paper reports on semilocal and hybrid density functional analysis of charge-transition levels of native defects in BaF2 structure. The transition level is defined as the Fermi level where two defect charge states have the same formation energy. The errors arising from the small supercell size effects have been relieved through extrapolating the formation energies to the limit of infinite supercell size. The level placement in the corrected band gap is achieved using a correction factor obtained from the difference between the valence band maxima in semilocal and hybrid calculations. The band gap size from hybrid calculation is validated using the full-potential, linearized augmented planewave method with the modified Becke-Johnson exchange potential. Our results are sufficiently accurate and, thus, significant for direct comparison with experiments.

  17. Craters and nanostructures on BaF2 sample induced by a focused 46.9nm laser

    NASA Astrophysics Data System (ADS)

    Cui, Huaiyu; Zhang, Shuqing; Li, Jingjun; Lu, Haiqiang; Zhao, Yongpeng

    2017-08-01

    We successfully damaged BaF2 samples by a 46.9nm capillary discharge laser of 100μJ focused by a toroidal mirror at a grazing incidence. Ablation craters with clear boundaries were detected by optical microscope and atomic force microscope (AFM). Laser-induced nanostructures with a period of ˜1μm were observed in the ablation area under single pulse irradiation and multiple pulses irradiation. The surface behavior was compared and analyzed with that induced by the laser of 50μJ. The nanostructures were supposed to be attributed to the thermoelastic effect and the period of the structures was effected by the energy of the laser.

  18. Evaluation Software for BaF2 Detector Array Electronics at CSNS-WNS

    NASA Astrophysics Data System (ADS)

    Zhang, Yaxi; Cao, Ping; Wang, Qi; Zhang, Deliang; He, Bing; Qi, Xincheng; An, Qi

    2017-06-01

    The “in programming barium fluoride (BaF2) detector array” is one of the experiment facilities at China Spallation Neutron Source-White Neutron Sources, which is designed for the measurement of neutron capture cross section with high accuracy. It consists of 92 crystal elements with completely 4π solid-angle coverage, which needs 92 analog channels and 92 digitization channels for data acquisition. Accordingly, the readout electronics is comprised with four distributed readout PXIe crates, containing 46 field digitization modules (FDMs). Each FDM supports two valid channels for signal digitizing. In this paper, evaluation software is designed for evaluating the performance of BaF2 readout electronics. It focuses on evaluating the performance of data transmission, waveform digitizing, and working status monitoring. Test results show that the evaluation software can correctly acquire and assemble data from the BaF2 readout system and evaluate the analog digital converter (ADC) performance under ADC test mode. FDMs in the readout system can also be monitored and controlled by this software in real time.

  19. Prototype of Field Waveform Digitizer for BaF2 Detector Array at CSNS-WNS

    NASA Astrophysics Data System (ADS)

    Wang, Qi; Cao, Ping; Zhang, Deliang; Qi, Xincheng; Yu, Tao; Jiang, Di; He, Bing; Zhang, Yaxi; An, Qi

    2017-07-01

    At the China spallation neutron source-white neutron sources (CSNS-WNS), the BaF2 (barium fluoride) detector array in planning is designed for neutron capture cross section measurements with high accuracy and efficiency. Once proton beam collides with the spallation target, the neutrons will fly from the target to specimen surrounded by BaF2 array and produce cascaded γ rays eventually. The time of flight (TOF) corresponds to the neutron energy. To identify γ signals from the high α-particle background, pulse shape discrimination (PSD) technique is usually used according to the ratio of fast to slow component in the signal. Waveform digitization is a valid supporting technology for PSD. In order to precisely obtain the wave and time information carried by detector signal, and to maximally cover the signal dynamic range, a universal digitizer with 1 GSps sampling rate and 12-b resolution has been designed based on a 3U PXIe platform in this paper. Besides waveform digitization, this customized digitizer also measures TOF precisely based on the digitized waveform data and technique of time-to-digital converting on field-programming gate array. Test results show that this digitizer can achieve good static and dynamic performance. The specification of effective number of bits is better than 9.43 b within 198 MHz. Digitizer proposed in this paper can meet the requirements for BaF2 spectrum at CSNS-WNS.

  20. Formation of defect-fluorite structured NdNiOxHy epitaxial thin films via a soft chemical route from NdNiO3 precursors.

    PubMed

    Onozuka, T; Chikamatsu, A; Katayama, T; Fukumura, T; Hasegawa, T

    2016-07-26

    A new phase of oxyhydride NdNiOxHy with a defect-fluorite structure was obtained by a soft chemical reaction of NdNiO3 epitaxial thin films on a substrate of SrTiO3 (100) with CaH2. The epitaxial relationship of this phase relative to SrTiO3 could be controlled by changing the reaction temperature. At 240 °C, NdNiOxHy grew with a [001] orientation, forming a thin layer of infinite-layer NdNiO2 at the interface between the NdNiOxHy and the substrate. Meanwhile, a high-temperature reaction at 400 °C formed [110]-oriented NdNiOxHy without NdNiO2.

  1. Commissioning the DANTE array of BaF2 detectors at TRIUMF-ISAC using a fast-timing lifetime measurement

    NASA Astrophysics Data System (ADS)

    Cross, D. S.; Ball, G. C.; Garrett, P. E.; Triambak, S.; Williams, S. J.; Andreoiu, C.; Churchman, R.; Garnsworthy, A. B.; Hackman, G.; Leslie, J. R.; Orce, J. N.; Sumithrarachchi, C. S.; Svensson, C. E.

    2011-08-01

    The Di-pentagonal Array for Nuclear Timing Experiments (DANTE) is an array of ten BaF2 detectors used in conjunction with the 8π gamma-ray spectrometer at the TRIUMF-ISAC radioactive-ion beam facility. DANTE is used to conduct direct lifetime measurements of nuclear excited states in the picosecond - nanosecond range. This, in turn, will aid in probing the collective structures of deformed nuclei. The capability of DANTE to measure nanosecond-scale lifetimes is demonstrated by using a 152Eu source. The half-life of the Iπ = 21+ state of 152Sm is measured to be 1.426 ± 0.018 ns.

  2. From fluorite to pyrochlore: Characterisation of local and average structure of neodymium zirconate, Nd{sub 2}Zr{sub 2}O{sub 7}

    SciTech Connect

    Payne, Julia L.; Tucker, Matthew G.; Evans, Ivana Radosavljević

    2013-09-15

    The structural characterisation of Nd{sub 2}Zr{sub 2}O{sub 7} prepared via a precursor route was performed using a combination of local and average structure probes (neutron total scattering, X-ray and neutron diffraction). We present the first total scattering and reverse Monte Carlo (RMC) modelling study of Nd{sub 2}Zr{sub 2}O{sub 7}, which provides compelling evidence for the adoption of a disordered fluorite-type structure by Nd{sub 2}Zr{sub 2}O{sub 7} prepared by a low-temperature precursor route. Annealing the material at high temperatures leads to a transformation to a pyrochlore-type structure; however, Rietveld refinement using powder neutron diffraction data shows that the oxygen sublattice retains a degree of disorder. - Graphical abstract: Display Omitted - Highlights: • The first total scattering and RMC modelling study of Nd{sub 2}Zr{sub 2}O{sub 7}. • Demonstration that the synthetic route influences the crystal structure adopted. • Insight into the importance of total scattering in studies of complex superstructures, especially for nano-sized materials.

  3. Hardness of CaF2 and BaF2 solid lubricants at 25 to 670 deg C

    NASA Technical Reports Server (NTRS)

    Deadmore, Daniel L.; Sliney, Harold E.

    1987-01-01

    Plastic deformation is a prominent factor in determining the lubricating value of solid lubricants. Little information is available and its direct measurement is difficult so hardness, which is an indirect measure of this property was determined for fluoride solid lubricant compositions. The Vickers hardness of BaF2 and CaF2 single crystals was measured up to 670 C in a vacuum. The orientation of the BaF2 was near the (013) plane and the CaF2 was about 16 degrees from the degrees from the (1'11) plane. The BaF2 has a hardness of 83 kg/sq mm at the 25 C and 9 at the 600 C. The CaF2 is 170 at 25 C and 13 at 670 C. The decrease in hardness in the temperature range of 25 to 100 C is very rapid and amounts to 40% for both materials. Melts of BaF2 and CaF2 were made in a platinum crucible in ambient air with compositions of 50 to 100 wt% BaF2. The Vickers hardness of these polycrystalline binary compositions at 25 C increased with increasing CaF2 reaching a maximum of 150 kn/sq mm near the eutectic. The polycrystalline CaF2 was 14% softer than that of the single crystal surface and BsF2 was 30% harder than the single crystal surface. It is estimated that the brittle to ductile transition temperature for CaF2 and BaF2 is less than 100 C for the conditions present in the hardness tester.

  4. Dislocations in YbF3 doped BaF2 crystals

    NASA Astrophysics Data System (ADS)

    Cirlan, Florina; Buse, Gabriel; Nicoara, Irina

    2014-11-01

    Three concentrations of YbF3 -doped BaF2 crystals were grown using the conventional Bridgman method. Transparent colorless crystals were obtained in graphite crucible in vacuum (˜10-1 Pa) using a shaped graphite furnace. The crystals have been cooled to room temperature using an established procedure. In order to study the etch pits morphology and the dislocations density the etching method have been used. This method consists in immersing the cleaved sample in 2N HCl at 60°C for 2 minutes. Small etch pits are developed at the emergence points of the dislocations. The etch pits have triangular or hexagonal shapes, depending on the dopant concentration. The dislocations density depends on the dopant concentration.

  5. Pr2M2O7 (M=Zr, Hf) local and crystal structure changes in a fluorite-pyrochlore phase transition.

    NASA Astrophysics Data System (ADS)

    Menushenkov, A. P.; Popov, V. V.; Gaynanov, B. R.; Zubavichus, Ya V.

    2016-09-01

    The formation of nanocrystal powders of Pr2M2O7 (M=Zr, Hf) compounds upon calcinations up to 1400°C has been investigated by using of X-ray absorption fine structure (XAFS) spectroscopy combined with X-ray powder diffraction (XRD). The structure ordering upon calcination was estimated independently from XRD and EXAFS data. It was found that the phase transition fluorite-pyrochlore occurs at calcination temperature in the range of 900-1000°C. The appearance of the ordered pyrochlore structure at calcination temperatures above 1000°C was manifested as splitting of the first oxygen coordination shell of the local praseodymium environment and supported by a sharp reduction of the Debye-Waller factor values of interatomic bonds. Besides we observed the L3 -Hf X-ray absorption near edge line shape splitting with increasing of annealing temperature of amorphous precursor due to crystal field influence in high ordered pyrochlore-type structure.

  6. Studies on phase transition temperature of rare earth niobates Ln3NbO7 (Ln = Pr, Sm, Eu) with orthorhombic fluorite-related structure

    NASA Astrophysics Data System (ADS)

    Hinatsu, Yukio; Doi, Yoshihiro

    2017-06-01

    The phase transition of ternary rare earth niobates Ln3NbO7 (Ln = Pr, Sm, Eu) was investigated by the measurements of high-temperature and low-temperature X-ray diffraction, differential scanning calorimetry (DSC) and differential thermal analysis (DTA). These compounds crystallize in an orthorhombic superstructure derived from the structure of cubic fluorite (space group Pnma for Ln = Pr; C2221 for Ln = Sm, Eu). Sm3NbO7 undergoes the phase transition when the temperature is increased through ca. 1080 K and above the transition temperature, its structure is well described with space group Pnma. For Eu3NbO7, the phase transition was not observed up to 1273 K Pr3NbO7 indicates the phase transition when the temperature is increased through ca. 370 K. The change of the phase transition temperature against the Ln ionic radius for Ln3NbO7 is quite different from those for Ln3MO7 (M = Mo, Ru, Re, Os, or Ir), i.e., no systematic relationship between the phase transition temperature and the Ln ionic radius has been observed for Ln3NbO7 compounds.

  7. A novel nickel metal-organic framework with fluorite-like structure: gas adsorption properties and catalytic activity in Knoevenagel condensation.

    PubMed

    Almáši, Miroslav; Zeleňák, Vladimír; Opanasenko, Maksym; Cejka, Jíří

    2014-03-07

    A new non-interpenetrating 3D metal-organic framework {[Ni4(μ6-MTB)2(μ2-H2O)4(H2O)4]·10DMF·11H2O}n (DMF = N,N'-dimethylformamide) built from nickel(ii) ions as connectors and methanetetrabenzoate ligands (MTB(4-)) as linkers has been synthesized and characterized. The single crystal X-ray diffraction showed that complex exhibits CaF2-like fluorite structure topology and four types of 3D channels with sizes about 12.6 × 9.4 Å(2), 9.4 × 8.0 Å(2), 12.6 × 11.7 Å(2) and 14.9 × 14.9 Å(2), which are filled with guest molecules. Conditions of the activation of the compound have been studied and optimized by powder X-ray diffraction during in situ heating, thermogravimetric analysis and infrared spectroscopy. Nitrogen and carbon dioxide adsorption showed that the activated sample exhibits a BET specific surface area of 700 m(2) g(-1) and a carbon dioxide uptake of 12.36 wt% at 0 °C, which are the highest values reported for the compounds of the MTB(4-) series. The complex was tested in Knoevenagel condensation of aldehydes and active methylene compounds. Straightforward dependence of the substrate conversion on the size of used aldehyde was established. A possible mechanism of Knoevenagel condensation over a MTB(4-) containing a metal-organic framework was proposed.

  8. CaF2, BaF2 and SrF2 crystals’ optical anisotropy parameters

    NASA Astrophysics Data System (ADS)

    Snetkov, I. L.; Yakovlev, A. I.; Palashov, O. V.

    2015-09-01

    Using the original method, based on measurements of thermally induced depolarization, the optical anisotropy parameters of CaF2, BaF2 and SrF2 cubic crystals were measured and compared with what is known from the literature. Euler angles of crystallographic axis orientation [C], in which the thermally induced depolarization is minimal, were determined using experimental results for studied fluorides.

  9. Density functional analysis of fluorite-structured (Ce, Zr)O2/CeO2 interfaces [Density functional analysis of fluorite-structured (Ce, Zr)O2/CeO2 interfaces: Implications for catalysis and energy applications

    DOE PAGES

    Weck, Philippe F.; Juan, Pierre -Alexandre; Dingreville, Remi; ...

    2017-06-21

    The structures and properties of Ce1–xZrxO2 (x = 0–1) solid solutions, selected Ce1–xZrxO2 surfaces, and Ce1–xZrxO2/CeO2 interfaces were computed within the framework of density functional theory corrected for strong electron correlation (DFT+U). The calculated Debye temperature increases steadily with Zr content in (Ce, Zr)O2 phases, indicating a significant rise in microhardness from CeO2 to ZrO2, without appreciable loss in ductility as the interfacial stoichiometry changes. Surface energy calculations for the low-index CeO2(111) and (110) surfaces show limited sensitivity to strong 4f-electron correlation. The fracture energy of Ce1–xZrxO2(111)/CeO2(111) increases markedly with Zr content, with a significant decrease in energy for thickermore » Ce1–xZrxO2 films. These findings suggest the crucial role of Zr acting as a binder at the Ce1–xZrxO2/CeO2 interfaces, due to the more covalent character of Zr–O bonds compared to Ce–O. Finally, the impact of surface relaxation upon interface cracking was assessed and found to reach a maximum for Ce0.25Zr0.75O2/CeO2 interfaces.« less

  10. Synthesis, crystal structures and magnetic properties of fluorite-related compounds Ce3MO7 (M = Nb, Ta)

    NASA Astrophysics Data System (ADS)

    Inabayashi, Masaki; Doi, Yoshihiro; Wakeshima, Makoto; Hinatsu, Yukio

    2017-10-01

    Ternary oxides Ce3NbO7 and Ce3TaO7 were successfully synthesized by the solid state reaction under flowing hydrogen atmosphere. The structures were determined by the powder X-ray diffraction. Both the compounds were crystallized in the orthorhombic space groups Pnma (for Ce3NbO7) and Cmcm (for Ce3TaO7). Both the structures have similar features: two kinds of infinite chains formed by corner-sharing NbO6 (TaO6) octahedra and edge-sharing Ce(1)O8 cubes, the slabs consisting of alternate chains, and 7-coordinated Ce(2) ions existing between the slabs. In the structure of Ce3NbO7, the NbO6 octahedra running along the a-axis are titled towards the 0 0 1 direction, while in the Ce3TaO7 structure, the TaO6 octahedra running along the c-axis are titled towards the 0 1 0 direction. Magnetic susceptibility measurements for Ce3NbO7 and Ce3TaO7 show that both compounds are paramagnetic down to 1.8 K, and confirm that the Ce ion is in the trivalent state. From specific heat (Cp) measurements, a rapid increase of Cp/T has been observed below 3 K for both the compounds, indicating the onset of magnetic ordering between Ce3+ ions at further lower temperatures.

  11. Magnetic properties and structural transitions of orthorhombic fluorite-related compounds Ln{sub 3}MO{sub 7} (Ln=rare earths, M=transition metals)

    SciTech Connect

    Wakeshima, Makoto; Hinatsu, Yukio

    2010-11-15

    Magnetic properties and structural transitions of ternary rare-earth transition-metal oxides Ln{sub 3}MO{sub 7} (Ln=rare earths, M=transition metals) were investigated. In this study, we prepared a series of molybdates Ln{sub 3}MoO{sub 7} (Ln=La-Gd). They crystallize in an orthorhombic superstructure of cubic fluorite with space group P2{sub 1}2{sub 1}2{sub 1}, in which Ln{sup 3+} ions occupy two different crystallographic sites (the 8-coordinated and 7-coordinated sites). All of these compounds show a phase transition from the space group P2{sub 1}2{sub 1}2{sub 1} to Pnma in the temperature range between 370 and 710 K. Their magnetic properties were characterized by magnetic susceptibility measurements from 1.8 to 400 K and specific heat measurements from 0.4 to 400 K. Gd{sub 3}MoO{sub 7} shows an antiferromagnetic transition at 1.9 K. Measurements of the specific heat for Sm{sub 3}MoO{sub 7} and the analysis of the magnetic specific heat indicate a 'two-step' antiferromagnetic transition due to the ordering of Sm magnetic moments in different crystallographic sites, i.e., with decreasing temperature, the antiferromagnetic ordering of the 7-coordinated Sm ions occur at 2.5 K, and then the 8-coordinated Sm ions order at 0.8 K. The results of Ln{sub 3}MoO{sub 7} were compared with the magnetic properties and structural transitions of Ln{sub 3}MO{sub 7} (M=Nb, Ru, Sb, Ta, Re, Os, or Ir). -- Graphical Abstract: Magnetic properties and structural transitions of ternary rare-earth transition-metal oxides Ln{sub 3}MO{sub 7} (Ln=rare earths, M=transition metals) were investigated. In this study, we prepared a series of molybdates Ln{sub 3}MoO{sub 7} (Ln=La{approx}Gd). These compounds show a phase transition from the space group P2{sub 1}2{sub 1}2{sub 1} to Pnma in the temperature range between 370 and 710 K. Their magnetic properties were characterized by magnetic susceptibility and specific heat measurements from 0.4 to 400 K. The results of Ln{sub 3}MoO{sub 7} were

  12. First-principles calculations of Mg2X (X = Si, Ge, Sn) semiconductors with the calcium fluorite structure

    NASA Astrophysics Data System (ADS)

    Sandong, Guo

    2015-05-01

    The electronic structures of Mg2X (X = Si, Ge, Sn) have been calculated by using generalized gradient approximation, various screened hybrid functionals, as well as Tran and Blaha's modified Becke and Johnson exchange potential. It was found that the Tran and Blaha's modified Becke and Johnson exchange potential provides a more realistic description of the electronic structures and the optical properties of Mg2X (X = Si, Ge, Sn) than else exchange-correlation potential, and the theoretical gaps and dielectric functions of Mg2X (X = Si, Ge, Sn) are quite compatible with the experimental data. The elastic properties of Mg2X (X = Si, Ge, Sn) have also been studied in detail with the generalized gradient approximation, including bulk modulus, shear modulus, Young's modulus, Poisson's ratio, sound velocities, and Debye temperature. The phonon dispersions of Mg2X (X = Si, Ge, Sn) have been calculated within the generalized gradient approximation, suggesting no structural instability, and the measurable phonon heat capacity as a function of the temperature has been also calculated. Project supported by the Fundamental Research Funds for the Central Universities (No. 2013QNA32) and the National Natural Science Foundation of China (No. 11404391).

  13. Characterization of electron-beam induced damage structures in natural fluorite, CaF2, by analytical electron microscopy

    NASA Technical Reports Server (NTRS)

    Blake, D. F.; Freund, Friedemann; Allard, L. F.; Echer, C. J.

    1988-01-01

    This paper describes the damage structure induced in natural CaF2 by the electron beam when using TEM. The observed 10-20 nm periodic features with coherent fringe patterns and the pronounced loss of fluorine found after the TEM exposure of 100-line-oriented and 111-oriented sections of CaF2 provides support for the mechanism of damage by decomposition of CaF2 into 2F and Ca, with the Ca precipitates maintaining a close topotaxial relationship with the parent CaF2.

  14. Atomistic simulation of a superionic transition in fluorite type structures UO2, UN2, TiH2

    NASA Astrophysics Data System (ADS)

    Korneva, M. A.; Starikov, S. V.

    2016-11-01

    The results of the atomistic simulation of a superionic transition and melting of stoichiometric UN2, UO2 and TiH2 have been presented. Simulation shows that superionic transitions of UO2 and TiH2 take place at temperature below melting temperature, while UN2 did not show such feature. This difference in properties of studied structures is caused by various gap between formation energies of Frenkel pair defects for sublattices. The possibility of describing the superionic transition within the theory of second-order phase transitions has been discussed, the conditions of the existence of superionic transition have been discussed.

  15. Characterization of electron-beam induced damage structures in natural fluorite, CaF2, by analytical electron microscopy

    NASA Technical Reports Server (NTRS)

    Blake, D. F.; Freund, Friedemann; Allard, L. F.; Echer, C. J.

    1988-01-01

    This paper describes the damage structure induced in natural CaF2 by the electron beam when using TEM. The observed 10-20 nm periodic features with coherent fringe patterns and the pronounced loss of fluorine found after the TEM exposure of 100-line-oriented and 111-oriented sections of CaF2 provides support for the mechanism of damage by decomposition of CaF2 into 2F and Ca, with the Ca precipitates maintaining a close topotaxial relationship with the parent CaF2.

  16. High-pressure experimental and theoretical investigations on the fluorite structured compound AuAl{sub 2}

    SciTech Connect

    Garg, Alka B.; Verma, Ashok K.; Vijayakumar, V.; Rao, R.S.; Godwal, B.K.

    2005-07-01

    Diamond anvil cell (DAC) based resistance measurements are carried out in AuAl{sub 2} to the pressures of about 25 GPa for the first time. The data indicate possibility of phase transition around 12 GPa. High pressure data obtained using Elettra synchrotron source confirm the transformation of AuAl{sub 2} from ambient CaF{sub 2} to orthorhombic phase. The high precision P-V data when transformed to universal equation of state does not show any deviation from linearity. The first principles electronic band structure calculations up to these pressures in the cubic phase do not reveal any change corroborating the experimental findings of nonobservation of electronic topological transition in this compound.

  17. Stability of fluorite-type La2Ce2O7 under extreme conditions

    DOE PAGES

    Zhang, F. X.; Tracy, C. L.; Lang, M.; ...

    2016-03-03

    Here, the structural stability of fluorite-type La2Ce2O7 was studied at pressure up to ~40 GPa and under hydrothermal conditions (~1 GPa, 350 °C), respectively, using synchrotron x-ray diffraction (XRD) and Raman scattering measurements. XRD measurements indicated that fluorite-type La2Ce2O7 is not stable at pressures greater than 22.6 GPa and slowly transforms to a high-pressure phase. The high-pressure phase is not stable and changes back to the fluorite-type structure when pressure is released. The La2Ce2O7 fluorite is also not stable under hydrothermal conditions and begins to react with water at 200~250 °C. Both Raman and XRD results suggest that lanthanum hydroxidemore » La(OH)3 and La3+-doped CeO2 fluorite are the dominant products after hydrothermal treatment.« less

  18. Nanostructured crystals of fluorite phases Sr1- x R x F2 + x and their ordering: 11. Influence of structural cluster ordering on fluorine ionic conductivity of the Sr1- x Lu x F2 + x Phase

    NASA Astrophysics Data System (ADS)

    Sorokin, N. I.; Sobolev, B. P.

    2015-11-01

    Ionic conductivity σ of the ordered phase (binary compound) Sr4Lu3F17 (sp. gr. Roverline 3 , Z = 6) with a fluorite-derivative structure obtained from a melt has been studied for the first time by impedance spectroscopy. The octahedral-cubic clusters {Sr8[Lu6F37]F32} formed in the structure of the disordered (fluorite) phase Sr1- x Lu x F2 + x (sp. gr. Fmoverline 3 m, Z = 4) become structural blocks with long-range order in the Sr4Lu3F17 compound. The effect of the clusters on the conductivity of nonstoichiometric phases Sr1- x Lu x F2 + x (2-25 mol % LuF3) and Sr4Lu3F17 (42.86 mol % LuF3) has been studied for the first time. Cluster ordering in Sr4Lu3F17 is accompanied by a decrease in the conductivity (σ = 6.3 × 10-7 S/cm at 673 K and Δ H σ = 1.12 eV) by a factor of 25 with respect to a disordered Sr0.75Lu0.25F2.75 crystal (experiment) and by a factor of 125 with respect to the saturated fluorite phase Sr0.63Lu0.37F2.37 (extrapolation). The unit-cell volume changes per F- ion during ordering are small and should not significantly affect the fluorine ion conductivity.

  19. Structure and crystal chemistry of fluorite-related Bi{sub 38}Mo{sub 7}O{sub 78} from single crystal X-ray diffraction and ab initio calculations

    SciTech Connect

    Sharma, Neeraj; Macquart, Rene B.; Christensen, Mogens; Avdeev, Maxim; Chen Yusheng; Ling, Chris D.

    2009-06-15

    The floating-zone furnace method was used to synthesize single crystals of the fluorite-related delta-Bi{sub 2}O{sub 3}-type phase Bi{sub 38}Mo{sub 7}O{sub 78} for the first time. Single crystal synchrotron X-ray diffraction data, in conjunction with ab initio (density functional theory) calculations, were used to solve, optimize, and refine the 5x3x3 commensurate superstructure of fluorite-type delta-Bi{sub 2}O{sub 3} in Pbcn (a=28.7058(11) A, b=16.8493(7) A and c=16.9376(6) A, Z=4, R{sub F}=11.26%, wR{sub I}=21.67%). The structure contains stepped channels of Mo{sup 6+} in tetrahedral environments along the b axis and chains of Mo{sup 6+} in octahedral environments along the ac plane. The role of the stepped channels in oxide ion conduction is discussed. The simultaneous presence of both tetrahedral and octahedral coordination environments for Mo{sup 6+}, something not previously observed in Mo{sup 6+}-doped delta-Bi{sub 2}O{sub 3}-type phases, is supported by charge balance considerations in addition to the results of crystallographic and ab initio analysis. - Graphical abstract: The crystal structure of Bi{sub 38}Mo{sub 7}O{sub 78} with MoO{sub 4} and MoO{sub 6} represented as polyhedra, Bi atoms as gray spheres, and fluorite-type O atoms as white vertices of Bi-O-Bi bonds.

  20. Preparation and topotactical oxidation of ScVO3 with bixbyte structure: a low-temperature route to stabilize the new defect fluorite ScVO(3.5) metastable phase.

    PubMed

    Alonso, José Antonio; Casais, María Teresa; Martínez-Lope, María Jesús

    2004-05-07

    ScVO3 has been prepared by controlled reduction of a ScVO4 precursor under an H2/N2 flow at 1250 degrees C. The crystal structure of this material has been studied at room temperature by Rietveld refinement of high-resolution neutron powder diffraction (NPD) data. Sc3+ and V3+ are distributed at random over the metal sites of a C-M2O3 bixbyite-type structure, space group Ia3, a = 9.6182(1) Angstroms. The thermal analysis of ScVO3 in an air flow shows two subsequent oxidation processes, with a final reversal to ScVO4 above 600 degrees C. An intermediate phase of composition ScVO(3.5), containing V4+ cations, can be isolated by isothermal annealing at 350 degrees C in air. This metastable phase has been identified by X-ray diffraction (XRD) as a fluorite-type oxide (space group Fm3m, a = 4.947(2) Angstroms), also showing a random distribution of Sc and V cations over the metal positions. The Rietveld refinement of the ScVO(3.5) structure from powder XRD data in a fluorite structural model yields abnormally high thermal factors for the oxygen atoms, suggesting oxygen mobility in this metastable material.

  1. Response of BaF 2 detectors to photons of 3-50 MeV energy

    NASA Astrophysics Data System (ADS)

    Matulewicz, T.; Grosse, E.; Emling, H.; Freifelder, R.; Grein, H.; Henning, W.; Herrmann, N.; Holzmann, R.; Kulessa, R.; Simon, R. S.; Wollersheim, H. J.; Schoch, B.; Vogt, J.; Wilhelm, M.; Kratz, J. V.; Schmidt, R.; Janssens, R. V. F.

    1990-04-01

    BaF 2 detectors of 20 cm length (10 radiation lengths) and hexagonal cross section (diameter 5.2 cm) were tested using monochromatic photons from the tagged-photon facility at the electron accelerator MAMIA at Mainz. The experimental spectra the deposited energy for a single detector and for an array of seven modules compare very well with the predictions of Monte Carlo shower simulations using the code GEANT3. At high photon energies a significant improvement (more than a factor 2) of the energy resolution is observed for the summed energy spectra as compared to the resolution of one single module.

  2. Formation of nanostructures induced by capillary-discharge soft X-ray laser on BaF2 surfaces

    NASA Astrophysics Data System (ADS)

    Zhao, Yongpeng; Cui, Huaiyu; Zhang, Shuqing; Zhang, Wenhong; Li, Wei

    2017-02-01

    BaF2 was ablated by a capillary-discharge pumped soft X-ray laser at 46.9 nm focused by a toroidal mirror at a grazing incidence of 83°. The damaged area, induced by both single and multiple laser pulses, was determined to be covered with fringe-like nanostructures with spacings of approximately 400 nm and mastoid nanostructures with diameters of approximately 600 nm. In this study, we analyze the morphology of the detected damage patterns and discuss the damage mechanism. Results indicate that the depth of the nanostructures varies with different pulse numbers and laser power densities.

  3. System design for precise digitization and readout of the CSNS-WNS BaF2 spectrometer

    NASA Astrophysics Data System (ADS)

    Zhang, De-Liang; Cao, Ping; Wang, Qi; He, Bing; Zhang, Ya-Xi; Qi, Xin-Cheng; Yu, Tao; An, Qi

    2017-02-01

    The BaF2 (barium fluoride) spectrometer is one of the experiment facilities at the CSNS-WNS (White Neutron Source at China Spallation Neutron Source), currently under construction. It is designed to precisely measure the (n, γ) cross section, with 92 crystal elements and complete 4π steradian coverage. In order to improve the precision of measurement, in this paper, a new precise digitization and readout method is proposed. Waveform digitizing with 1 GSps sampling rate and 12-bit resolution is used to precisely capture the detector signal. To solve the problem of massive data readout and processing, the readout electronics is designed as a distributed architecture with 4 PXIe crates. The digitized signal is concentrated to the PXIe crate controller through a PCIe bus on the backplane and transmitted to the data acquisition system over gigabit Ethernet in parallel. Besides, the clock and trigger can be fanned out synchronously to every electronic channel over a high-precision distribution network. Test results show that the prototype of the readout electronics can achieve good performance and meet the requirements of the CSNS-WNS BaF2 spectrometer. Supported by National Research and Development plan (2016YFA0401602) and NSAF (U1530111)

  4. Light scattering and computer simulation studies of superionic pure and La-doped BaF2

    NASA Astrophysics Data System (ADS)

    Rammutla, K. E.; Comins, J. D.; Erasmus, R. M.; Netshisaulu, T. T.; Ngoepe, P. E.; Chadwick, A. V.

    2016-03-01

    A combination of both Raman and Brillouin scattering experiments as well as Molecular Dynamics (MD) was used to study the superionic behaviour of BaF2 doped with a wide range of LaF3 concentrations (0 ⩽ x ⩽ 50 mol%). Raman spectroscopy reveals that for undoped BaF2 and those doped with 5% and 10% LaF3, the room temperature spectra show the usual T2g symmetry mode at 241 cm-1 whereas for those doped with 20%, 30% and 50% LaF3, the dominant Raman mode is of the Eg symmetry situated at ∼263, 275 and 286 cm-1, respectively. The Raman linewidths show near linear increases with temperature followed by rapid increases above the characteristic transition temperatures (Tc), being at 1200, 850, 800, 975, 950 and 920 K for LaF3 concentrations of 0, 5, 10, 20, 30 and 50; respectively. The temperature dependence of the squares of the Brillouin frequencies (ΔωB)2 of the LA and TA acoustic modes respectively related to elastic constants C11 and C44 showed linear decreases followed by significant deviations around the same temperatures (Tc), at which the Raman linewidths start to show substantial increases. The complementary studies using MD simulations show that the diffusion coefficients increase markedly above the same temperatures observed experimentally. The extrinsic fluorine ion trajectories were also determined from the MD simulations to better understand the mechanisms of diffusion.

  5. Temperature Dependence of the Surface Energy of the Low-index Planes of CaF2, BaF2 and SrF2

    NASA Astrophysics Data System (ADS)

    Bebelis, S.; Nikolopoulos, P.

    2017-02-01

    The temperature dependence of the surface energy γ of the low-index (111), (110) and (100) planes of CaF2, BaF2 and SrF2 was evaluated for the first time, using a simple method which is based on physical and thermodynamic quantities and considerations. The extrapolated to 0 K surface energy values agree well with the available theoretical data reported in the literature for the (111) plane and are close to those for the (110) plane, whereas they are significantly lower than those for the (100) plane. The calculated γ111 values for 298 K agree well with the corresponding experimentally determined literature values. The calculated γ100/γ111 ratio revealed formation of crystals with truncated octahedron structure in thermodynamic equilibrium. This structure remains practically constant with temperature. The surface energy values of fluoride melts formed by melting of octahedron structures, calculated via extrapolation of the surface energy of the solids to the melting points, were compared with experimentally determined literature values for melts resulting from single or polycrystalline materials.

  6. Temperature Dependence of the Surface Energy of the Low-index Planes of CaF2, BaF2 and SrF2

    NASA Astrophysics Data System (ADS)

    Bebelis, S.; Nikolopoulos, P.

    2017-03-01

    The temperature dependence of the surface energy γ of the low-index (111), (110) and (100) planes of CaF2, BaF2 and SrF2 was evaluated for the first time, using a simple method which is based on physical and thermodynamic quantities and considerations. The extrapolated to 0 K surface energy values agree well with the available theoretical data reported in the literature for the (111) plane and are close to those for the (110) plane, whereas they are significantly lower than those for the (100) plane. The calculated γ111 values for 298 K agree well with the corresponding experimentally determined literature values. The calculated γ100/γ111 ratio revealed formation of crystals with truncated octahedron structure in thermodynamic equilibrium. This structure remains practically constant with temperature. The surface energy values of fluoride melts formed by melting of octahedron structures, calculated via extrapolation of the surface energy of the solids to the melting points, were compared with experimentally determined literature values for melts resulting from single or polycrystalline materials.

  7. Structural elucidation of the Bi(2(n + 2))Mo(n)O(6(n + 1)) (n = 3, 4, 5 and 6) family of fluorite superstructures by transmission electron microscopy.

    PubMed

    Landa-Cánovas, Angel R; Vila, Eladio; Hernández-Velasco, Jorge; Galy, Jean; Castro, Alicia

    2009-08-01

    The cationic framework structure of a whole new family of compounds with the general formula Bi(2(n + 2))Mo(n)O(6(n + 1)) (n = 3, 4, 5 and 6) has been elucidated by transmission electron microscopy (TEM) methods. High-resolution transmission electron microscopy (HRTEM) has been used to postulate heavy-atom models based on the known structure of the n = 3 phase, Bi(10)Mo(3)O(24). These models were tested by HRTEM image simulation, electron diffraction and powder X-ray diffraction simulation methods which agreed with the experimental results. The four known phases of this family correspond to n = 3, 4, 5 and 6 members and all show fluorite superstructures. They consist of a common delta-Bi(2)O(3) fluorite-type framework, inside of which are distributed ribbons of {MoO(4)} tetrahedra which are infinite along b, one tetrahedron thick along c, and of variable widths of 3, 4, 5 or 6 {MoO(4)} tetrahedra along a depending on the family member (n value). These {MoO(4)} tetrahedra are isolated, i.e. without sharing any corner as in the [Bi(12)O(14)] columnar structural-type phase Bi[Bi(12)O(14)][MoO(4)](4)[VO(4)]. The structure of all these family members can be described as crystallographic shear derivatives from Aurivillius-type phases such as Bi(2)MoO(6), the n = infinity end member. All these compounds are good oxygen-ion conductors.

  8. Intervalence charge transfer luminescence: interplay between anomalous and 5d - 4f emissions in Yb-doped fluorite-type crystals.

    PubMed

    Barandiarán, Zoila; Seijo, Luis

    2014-12-21

    In this paper, we report the existence of intervalence charge transfer (IVCT) luminescence in Yb-doped fluorite-type crystals associated with Yb(2+)-Yb(3+) mixed valence pairs. By means of embedded cluster, wave function theory ab initio calculations, we show that the widely studied, very broad band, anomalous emission of Yb(2+)-doped CaF2 and SrF2, usually associated with impurity-trapped excitons, is, rather, an IVCT luminescence associated with Yb(2+)-Yb(3+) mixed valence pairs. The IVCT luminescence is very efficiently excited by a two-photon upconversion mechanism where each photon provokes the same strong 4f(14)-1A1g→ 4f(13)((2)F7/2)5deg-1T1u absorption in the Yb(2+) part of the pair: the first one, from the pair ground state; the second one, from an excited state of the pair whose Yb(3+) moiety is in the higher 4f(13)((2)F5/2) multiplet. The Yb(2+)-Yb(3+) → Yb(3+)-Yb(2+) IVCT emission consists of an Yb(2+) 5deg → Yb(3+) 4f7/2 charge transfer accompanied by a 4f7/2 → 4f5/2 deexcitation within the Yb(2+) 4f(13) subshell: [(2)F5/25deg,(2)F7/2] → [(2)F7/2,4f(14)]. The IVCT vertical transition leaves the oxidized and reduced moieties of the pair after electron transfer very far from their equilibrium structures; this explains the unexpectedly large band width of the emission band and its low peak energy, because the large reorganization energies are subtracted from the normal emission. The IVCT energy diagrams resulting from the quantum mechanical calculations explain the different luminescent properties of Yb-doped CaF2, SrF2, BaF2, and SrCl2: the presence of IVCT luminescence in Yb-doped CaF2 and SrF2; its coexistence with regular 5d-4f emission in SrF2; its absence in BaF2 and SrCl2; the quenching of all emissions in BaF2; and the presence of additional 5d-4f emissions in SrCl2 which are absent in SrF2. They also allow to interpret and reproduce recent experiments on transient photoluminescence enhancement in Yb(2+)-doped CaF2 and SrF2, the

  9. Intervalence charge transfer luminescence: Interplay between anomalous and 5d - 4f emissions in Yb-doped fluorite-type crystals

    NASA Astrophysics Data System (ADS)

    Barandiarán, Zoila; Seijo, Luis

    2014-12-01

    In this paper, we report the existence of intervalence charge transfer (IVCT) luminescence in Yb-doped fluorite-type crystals associated with Yb2+-Yb3+ mixed valence pairs. By means of embedded cluster, wave function theory ab initio calculations, we show that the widely studied, very broad band, anomalous emission of Yb2+-doped CaF2 and SrF2, usually associated with impurity-trapped excitons, is, rather, an IVCT luminescence associated with Yb2+-Yb3+ mixed valence pairs. The IVCT luminescence is very efficiently excited by a two-photon upconversion mechanism where each photon provokes the same strong 4f14-1A1g→ 4f13(2F7/2)5deg-1T1u absorption in the Yb2+ part of the pair: the first one, from the pair ground state; the second one, from an excited state of the pair whose Yb3+ moiety is in the higher 4f13(2F5/2) multiplet. The Yb2+-Yb3+ → Yb3+-Yb2+ IVCT emission consists of an Yb2+ 5deg → Yb3+ 4f7/2 charge transfer accompanied by a 4f7/2 → 4f5/2 deexcitation within the Yb2+ 4f13 subshell: [2F5/25deg,2F7/2] → [2F7/2,4f14]. The IVCT vertical transition leaves the oxidized and reduced moieties of the pair after electron transfer very far from their equilibrium structures; this explains the unexpectedly large band width of the emission band and its low peak energy, because the large reorganization energies are subtracted from the normal emission. The IVCT energy diagrams resulting from the quantum mechanical calculations explain the different luminescent properties of Yb-doped CaF2, SrF2, BaF2, and SrCl2: the presence of IVCT luminescence in Yb-doped CaF2 and SrF2; its coexistence with regular 5d-4f emission in SrF2; its absence in BaF2 and SrCl2; the quenching of all emissions in BaF2; and the presence of additional 5d-4f emissions in SrCl2 which are absent in SrF2. They also allow to interpret and reproduce recent experiments on transient photoluminescence enhancement in Yb2+-doped CaF2 and SrF2, the appearance of Yb2+ 4f-5d absorption bands in the excitation

  10. Defect-induced wetting on BaF 2(111) and CaF 2(111) at ambient conditions

    NASA Astrophysics Data System (ADS)

    Cardellach, M.; Verdaguer, A.; Fraxedas, J.

    2011-12-01

    The interaction of water with freshly cleaved (111) surfaces of isostructural BaF2 and CaF2 single crystals at ambient conditions (room temperature and under controlled humidity) has been studied using scanning force microscopy in different operation modes and optical microscopy. Such surfaces exhibit contrasting behaviors for both materials: while on BaF2(111) two-dimensional water layers are formed after accumulation at step edges, CaF2(111) does not promote the formation of such layers. We attribute such opposed behavior to lattice match (mismatch) between hexagonal water ice and the hexagonal (111) surfaces of BaF2(CaF2). Optical microscope images reveal that this behavior also determines the way the surfaces become wetted at a macroscopic level.

  11. Test of the statistical model in Mo96 with the BaF2γ calorimeter DANCE array

    NASA Astrophysics Data System (ADS)

    Sheets, S. A.; Agvaanluvsan, U.; Becker, J. A.; Bečvář, F.; Bredeweg, T. A.; Haight, R. C.; Jandel, M.; Krtička, M.; Mitchell, G. E.; O'Donnell, J. M.; Parker, W.; Reifarth, R.; Rundberg, R. S.; Sharapov, E. I.; Ullmann, J. L.; Vieira, D. J.; Wilhelmy, J. B.; Wouters, J. M.; Wu, C. Y.

    2009-02-01

    The γ-ray cascades following the Mo95(n,γ)Mo96 reaction were studied with the γ calorimeter DANCE (Detector for Advanced Neutron Capture Experiments) consisting of 160 BaF2 scintillation detectors at the Los Alamos Neutron Science Center. The γ-ray energy spectra for different multiplicities were measured for s- and p-wave resonances below 2 keV. The shapes of these spectra were found to be in very good agreement with simulations using the DICEBOX statistical model code. The relevant model parameters used for the level density and photon strength functions were identical with those that provided the best fit of the data from a recent measurement of the thermal Mo95(n,γ)Mo96 reaction with the two-step-cascade method. The reported results strongly suggest that the extreme statistical model works very well in the mass region near A=100.

  12. Ion mobility and conductivity in the M0.5-xPbxBi0.5F2+x (M=K, Rb) solid solutions with fluorite structure

    NASA Astrophysics Data System (ADS)

    Kavun, V. Ya.; Uvarov, N. F.; Slobodyuk, A. B.; Polyantsev, M. M.; Merkulov, E. B.; Ulihin, A. S.; Goncharuk, V. K.

    2017-05-01

    Ionic mobility and conductivity in the K0.5-xPbxBi0.5F2+x and Rb0.5-xPbxBi0.5F2+x (x=0.05, 0.09) solid solutions with the fluorite structure have been investigated using the methods of 19F NMR, X-ray diffraction and impedance spectroscopy. Types of ionic motions in the fluoride sublattice of solid solutions have been established and temperature ranges of their realization have been determined (150-450 K). Diffusion of fluoride ions is a dominating type of ionic motions in the fluoride sublattice of solid solutions under study above 350 K. Due to high ionic conductivity, above 10-3 S/cm at 450 K, these solid solutions can be used as solid electrolytes in various electrochemical devices and systems.

  13. Structures and magnetic properties of new fluorite-related quaternary rare earth oxides LnY{sub 2}TaO{sub 7} and LaLn{sub 2}RuO{sub 7} (Ln=rare earths)

    SciTech Connect

    Hinatsu, Yukio Doi, Yoshihiro

    2016-01-15

    New fluorite-related quaternary rare earth oxides LnY{sub 2}TaO{sub 7} (Ln=La–Dy) and LaLn{sub 2}RuO{sub 7} (Ln=Eu–Tb) have been prepared. They crystallize in an orthorhombic superstructure of cubic fluorite with space groups C222{sub 1} and Cmcm, respectively. The results of the Rietveld analysis for LnY{sub 2}TaO{sub 7} (Ln=La–Dy) indicate that there exists no structural ordering of the Ln and Y ions at the eight-coordinate 4b site and the seven-coordinate 8c site, but the larger Ln ions occupy the eight-coordinate 4b site rather than the seven-coordinate 8c site, with increasing the Ln ionic radius. On the other hand, the results of the Rietveld analysis for the X-ray diffraction profiles of LaTb{sub 2}RuO{sub 7} showed that La and Tb atoms are almost situated at the eight-coordinated site (4a site) and seven-coordinated site (8g site), respectively, i.e., cation ordering occurs on the 4a and 8g sites. All compounds LnY{sub 2}TaO{sub 7} (Ln=La–Dy) are paramagnetic down to 1.8 K. On the other hand, LaTb{sub 2}RuO{sub 7} shows an antiferromagnetic transition at 17 K. In addition, another magnetic anomaly has been found at 10 K. Analysis of the magnetic specific heat for LaTb{sub 2}RuO{sub 7} indicates that the magnetic transitions at 10 and 17 K are due to the magnetic ordering of Tb{sup 3+} and Ru{sup 5+} ions, respectively. - Graphical abstract: Two magnetic anomalies have been observed in the temperature dependence of ∂(χT)/∂T for LaTb{sub 2}RuO{sub 7}. It is discussed that the anomalies at 10 and 17 K are due to the magnetic ordering of Tb{sup 3+} and Ru{sup 5+} ions, respectively. - Highlights: • New fluorite-related quaternary rare earth oxides Ln′Ln″{sub 2}MO{sub 7} have been prepared. • All compounds LnY{sub 2}TaO{sub 7} (Ln=La–Dy) are paramagnetic down to 1.8 K. • LaTb{sub 2}RuO{sub 7} shows two magnetic anomalies at 10 and 17 K.

  14. Effect of high-energy electron irradiation in an electron microscope column on fluorides of alkaline earth elements (CaF2, SrF2, and BaF2)

    NASA Astrophysics Data System (ADS)

    Nikolaichik, V. I.; Sobolev, B. P.; Zaporozhets, M. A.; Avilov, A. S.

    2012-03-01

    The effect of high-energy (150 eV) electron irradiation in an electron microscope column on crystals of fluorides of alkaline earth elements CaF2, SrF2, and BaF2 is studied. During structural investigations by electron diffraction and electron microscopy, the electron irradiation causes chemical changes in MF2 crystals such as the desorption of fluorine and the accumulation of oxygen in the irradiated area with the formation of oxide MO. The fluorine desorption rate increases significantly when the electron-beam density exceeds the threshold value of ˜2 × 103 pA/cm2). In BaF2 samples, the transformation of BaO into Ba(OH)2 was observed when irradiation stopped. The renewal of irradiation is accompanied by the inverse transformation of Ba(OH)2 into BaO. In the initial stage of irradiation of all MF2 compounds, the oxide phase is in the single-crystal state with a lattice highly matched with the MF2 matrix. When the irradiation dose is increased, the oxide phase passes to the polycrystalline phase. Gaseous products of MF2 destruction (in the form of bubbles several nanometers in diameter) form a rectangular array with a period of ˜20 nm in the sample.

  15. Structures and magnetic properties of new fluorite-related quaternary rare earth oxides LnY2TaO7 and LaLn2RuO7 (Ln=rare earths)

    NASA Astrophysics Data System (ADS)

    Hinatsu, Yukio; Doi, Yoshihiro

    2016-01-01

    New fluorite-related quaternary rare earth oxides LnY2TaO7 (Ln=La-Dy) and LaLn2RuO7 (Ln=Eu-Tb) have been prepared. They crystallize in an orthorhombic superstructure of cubic fluorite with space groups C2221 and Cmcm, respectively. The results of the Rietveld analysis for LnY2TaO7 (Ln=La-Dy) indicate that there exists no structural ordering of the Ln and Y ions at the eight-coordinate 4b site and the seven-coordinate 8c site, but the larger Ln ions occupy the eight-coordinate 4b site rather than the seven-coordinate 8c site, with increasing the Ln ionic radius. On the other hand, the results of the Rietveld analysis for the X-ray diffraction profiles of LaTb2RuO7 showed that La and Tb atoms are almost situated at the eight-coordinated site (4a site) and seven-coordinated site (8g site), respectively, i.e., cation ordering occurs on the 4a and 8g sites. All compounds LnY2TaO7 (Ln=La-Dy) are paramagnetic down to 1.8 K. On the other hand, LaTb2RuO7 shows an antiferromagnetic transition at 17 K. In addition, another magnetic anomaly has been found at 10 K. Analysis of the magnetic specific heat for LaTb2RuO7 indicates that the magnetic transitions at 10 and 17 K are due to the magnetic ordering of Tb3+ and Ru5+ ions, respectively.

  16. Impurity-controlled recrystallization in natural fluorite

    NASA Astrophysics Data System (ADS)

    Duschl, Florian; Wischhöfer, Philipp; Vollbrecht, Axel

    2017-04-01

    Microfabrics in natural fluorite from stratiform fluorite occurrences in Zechstein carbonate rocks (Ca2) near Eschwege (Germany) exhibit complex recrystallization features with different stages of accretive crystallization. Thin sections were studied using standard petrographic microscopy and hot-cathodoluminescence microscopy (CL); to identify fluid inclusion composition microthermometric analysis was applied. Though fluorite occurs in various forms at the locality, the focus of this study lies on early-diagenetic fluorite that replaced aragonitic ooids in the Zechstein carbonate. It can be subdivided into three groups: (I) brown or violet, impurity-rich replacement fluorite, (II) aggregates of parallel, bar-shaped or fibrous crystals with brown to violet grain boundaries, and (III) white fluorite grains with rectangular to mostly polygonal grain boundaries. Type (III) is the product of merged type (II) crystal aggregates. Artificial decoration of fluorite grain surfaces due to CL-induced electron irradiation (acceleration voltage: 14 kV) helped to visualize otherwise invisible crystallographic features. This technique revealed a strong crystallographic control on bar-shaped fluorite (type II) at an early stage of recrystallization. Parallel bundles of type (II) crystals show a crystallographic preferred orientation after {100}, that is no longer apparent after consumption by type (III) fluorite and formation of polygonal grains. Impurities such as fluid and solid inclusions in type (I) fluorite were segregated during progressive recrystallization; subsequently, solid and fluid inclusions accumulated along newly formed grain boundaries. Increase in grain size due to recrystallization is locally hindered by the concentration of impurities along grain boundaries. Therefore, we assume that impurity-controlled recrystallization not only influenced the formation of bar-shaped crystals prior to the development of a polygonal fabric, but locally also strongly affected

  17. Phase Transitions and Equations of State of Alkaline Earth Fluorides CaF2 SrF2 and BaF2 to Mbar Pressures

    SciTech Connect

    S Dorfman; F Jiang; Z Mao; A Kubo; Y Meng; V Prakapenda; T Duffy

    2011-12-31

    Phase transitions and equations of state of the alkaline earth fluorides CaF{sub 2}, SrF{sub 2}, and BaF{sub 2} were examined by static compression to pressures as high as 146 GPa. Angle-dispersive x-ray diffraction experiments were performed on polycrystalline samples in the laser-heated diamond-anvil cell. We confirmed that at pressures less than 10 GPa all three materials undergo a phase transition from the cubic (Fm3{sup -}m) fluorite structure to the orthorhombic (Pnam) cotunnite-type structure. This work has characterized an additional phase transition in CaF{sub 2} and SrF{sub 2}: these materials were observed to transform to a hexagonal (P6{sub 3}/mmc) Ni{sub 2}In-type structure between 63-79 GPa and 28-29 GPa, respectively, upon laser heating. For SrF{sub 2}, the Ni{sub 2}In-type phase was confirmed by Rietveld refinement. Volumes were determined as a function of pressure for all high-pressure phases and fit to the third-order Birch-Murnaghan equation of state. For CaF{sub 2} and SrF{sub 2}, the fluorite-cotunnite transition results in a volume decrease of 8-10%, while the bulk modulus of the cotunnite-type phase is the same or less than that of the fluorite phase within uncertainty. For all three fluorides, the volume reduction associated with the further transition to the Ni{sub 2}In-type phase is {approx}5%. The percentage increase in the bulk modulus ({Delta}K) across the transition is greater when the cation is smaller. While for BaF{sub 2}, {Delta}K is 10-30%, {Delta}K values for SrF{sub 2} and CaF{sub 2} are 45-65% and 20-40%. Although shock data for CaF{sub 2} have been interpreted to show a transition to a highly incompressible phase above 100 GPa, this is not consistent with our static equation of state data.

  18. Variable helium diffusion characteristics in fluorite

    NASA Astrophysics Data System (ADS)

    Wolff, R.; Dunkl, I.; Kempe, U.; Stockli, D.; Wiedenbeck, M.; von Eynatten, H.

    2016-09-01

    Precise analysis of the diffusion characteristics of helium in fluorite is crucial for establishing the new fluorite (U-Th-Sm)/He thermochronometer (FHe), which potentially provides a powerful tool for dating ore deposits unsuitable for the application of conventional geochronometers. Incremental helium outgassing experiments performed on fluorites derived from a spectrum of geological environments suggest a thermally activated volume diffusion mechanism. The diffusion behaviour is highly variable and the parameters range between log D0/a2 = 0.30 ± 0.27-7.27 ± 0.46 s-1 and Ea = 96 ± 3.5-182 ± 3.8 kJ/mol. Despite the fact that the CaF2 content of natural fluorites in most cases exceeds 99 weight percent, the closure temperature (Tc) of the fluorite (U-Th-Sm)/He thermochronometer as calculated from these diffusion parameters varies between 46 ± 14 °C and 169 ± 9 °C, considering a 125 μm fragment size. Here we establish that minor substitutions of calcium by rare earth elements and yttrium (REE + Y) and related charge compensation by sodium, fluorine, oxygen and/or vacancies in the fluorite crystal lattice have a significant impact on the diffusivity of helium in the mineral. With increasing REE + Y concentrations F vacancies are reduced and key diffusion pathways are narrowed. Consequently, a higher closure temperature is to be expected. An empirical case study confirms this variability: two fluorite samples from the same deposit (Horni Krupka, Czech Republic) with ca. 170 °C and ca. 43 °C Tc yield highly different (U-Th-Sm)/He ages of 290 ± 10 Ma and 79 ± 10 Ma, respectively. Accordingly, the fluorite sample with the high Tc could have quantitatively retained helium since the formation of the fluorite-bearing ores in the Permian, despite subsequent Mesozoic burial and associated regional hydrothermal heating. In contrast, the fluorite with the low Tc yields a Late Cretaceous age close to the apatite fission track (AFT) and apatite (U-Th)/He ages (AHe

  19. The pathogenicity of dust from a fluorite mine.

    PubMed

    Xu, Y H; Zhang, S J; Wu, C Q; Zhou, J; Chang, H F; Yu, S Q

    1987-08-01

    The pathogenicity of mixed dust from a fluorite mine was studied by animal experiments and in vitro tests. Animal experiments showed that calcium fluorite can induce only a foreign body reaction in the lungs; the fibrous nodular lesions induced by the fluorite mine dust are due mainly to its silica component. It was demonstrated that either silica or the mixed dust of a fluorite mine can stimulate pulmonary alveolar macrophages (PAMs) to release fibrogenetic factors in vitro, but calcium fluorite cannot. It was also demonstrated that having engulfed calcium fluorite, silica, or fluorite mine mixed dust, PAMs release an elastase-active substance. The authors suggest that the emphysematous lesion seen in autopsy material of pneumoconiosis of fluorite mine workers may be caused by calcium fluorite and silica.

  20. Crystal growth, observation, and characterization of the low-temperature structure of the fluorite-related ruthenates: Sm(3)RuO(7) and Eu(3)RuO(7).

    PubMed

    Gemmill, William R; Smith, Mark D; Zur Loye, Hans-Conrad

    2004-07-12

    The compounds Sm(3)RuO(7) and Eu(3)RuO(7) were grown as single crystals from molten hydroxide fluxes. They crystallize in the orthorhombic space group Cmcm and are part of a well-known family of fluorite-related oxides of stoichiometry Ln(3)MO(7). This structure contains rare earth cations in two different coordination environments, 8-fold pseudocubic and 7-fold pentagonal bipyramidal, and contains Ru(V) cations that are octahedrally coordinated. The RuO(6) octahedra are trans vertex-sharing to yield chains oriented along the c-axis. Upon cooling, single crystals of Sm(3)RuO(7) and Eu(3)RuO(7) undergo a structural transition at 190 and 280 K, respectively, from space group Cmcm to P2(1)nb. The structure transition results in a loss of lattice centering, a doubling of the b-axis, a distortion of the vertex-shared Ru-O chains, and a reduction in the coordination of one of the rare earth cations from 8-fold to 7-fold. Accompanying this structural transition are anomalies in the magnetic susceptibility at about 190 and 280 K for Sm(3)RuO(7) and Eu(3)RuO(7), respectively. The structures of these low-temperature phases of Ln(3)RuO(7) have been determined for the first time and are described.

  1. Remarkable transition from rocksalt/perovskite layered structure to fluorite/rocksalt layered structure in rapidly cooled Ln₂CuO₄.

    PubMed

    Patabendige, Chami N K; Azad, Abul K; Connor, Paul A; Rolle, Aurélie; Irvine, John T S

    2013-01-01

    Lanthanide cuprates of formula Ln₂CuO₄ exist in two principal forms, T and T' which are renowned for their exhibition at low temperatures of hole and electronic types of superconductivity, respectively. These structures differ primarily in the arrangement of oxygen between the perovskite layers and also in nature of the copper oxygen planes. The Cu-O distance in the T structure (~1.90 Å) is much shorter than the T' (1.97Å), reflecting a transition between partial Cu⁺and partial Cu³⁺ character. In seeking to find compositions that bridge these two structure/electron carrier types, we observed the transition from a T structure to a T' type structure, resulting in the metastable form T″ with slightly larger volume but similar character to T'. This transition from T to T″ is associated with 5% increase in a and a 5% decrease in c parameters of the tetragonal unit cells, which results in disintegration of ceramic bodies.

  2. Ion conductivity in cubically-stabilized fluorite-like structured Er5CeMoO12.5 and Yb5MMoO12.5 (M = Ce, Zr) solid solutions

    NASA Astrophysics Data System (ADS)

    Schildhammer, Daniel; Fuhrmann, Gerda; Petschnig, Lucas; Kogler, Michaela; Penner, Simon; Weinberger, Nikolaus; Schottenberger, Herwig; Huppertz, Hubert

    2016-12-01

    For the first time, the usually rhombohedral R 3 bar (no. 148) defect fluorite structured rare earth molybdenum oxides RE6MoO12 (RE = Er, Yb) could be stabilized in the cubic defect fluorite structure Fm 3 bar m (no. 225) through partial substitution of Er3+ and Yb3+ by M4+ cations with M4+ = Ce for Er6MoO12 and M4+ = Ce, Zr for Yb6MoO12. The solution combustion (SC) method and the classical solid state reaction are the synthetic approaches that were used (ambient atmosphere and temperatures of 1250 °C). The obtained oxide powders were characterized by energy dispersive X-ray (EDX) analysis, powder X-ray diffraction (PXRD), IR, and UV-Vis spectroscopy. The PXRD-data were used for Rietveld refinements. Electrochemical impedance spectroscopy (EIS) was conducted to identify oxygen vacancies in the cubic structure type, revealing oxygen ion conductivity starting at 500 °C. Additionally, the influence on ion conductivity by the cation substitutions are discussed. Calculation of the contributing activation energies for the bulk (best value for Yb5CeMoO12.5 is 119.8 kJmol-1), and the grain boundary (e.g. Er5CeMoO12.5: 152.1 kJmol-1) analyzed by means of the Arrhenius plot, shows similarities to the conventional stabilization of zirconia with yttrium (8-YSZ) (110 kJ mol-1 and 110-163 kJmol-1 respectively). The best-calculated conductivity values σ = 1.03 × 10-4 Scm-1 obtained for Er5CeMoO12.5 at 1000 °C are comparable to the values published for 8-YSZ with σ = 3.94 × 10-5 Scm-1 at similar temperatures. These promising preliminary results underline the potential of the title compounds for application in solid oxide fuel cells (SOFCs).

  3. NUCLEATION OF YBA(2)CU(3)O(7-D) IN THE BAF(2) PROCESS

    SciTech Connect

    WU,L.; SOLOVYOV,V.F.; WIESMANN,H.J.; ZHU,Y.; SUENAGA,M.

    2001-06-24

    The nucleation of the c-axis aligned and non-c-axis YBa{sub 2}Cu{sub 3}O{sub {approximately}6.1} (YBCO) from precursor films on [001]-cut SrTiO{sub 3} was investigated for the so-called BaF{sub 2} process. Specimens with different thickness were quenched from 735 C, then studied by transmission electron microscopy and x-ray diffraction techniques. Preceding the formation of YBCO nuclei, three intermediate phases of (Y,Ba) oxy-fluoride and a transition phase without F were found in the precursor films. These were structurally and chemically related to the nuclei of YBCO which was found to be deficient in Cu relative to its stoichiometric composition.

  4. Some physical properties of naturally irradiated fluorite

    USGS Publications Warehouse

    Berman, Robert

    1955-01-01

    Five samples of purple fluorite found in association with radioactive, materials, and a synthetic colorless control sample were studied and compared.  Before and after heating, observations were made on specific gravity, index of refraction, unit-cell size, breadth of X-ray diffraction lines, and fluorescence.  The purple samples became colorless on heating above 175° C.  During the process, observations were made on color, thermoluminescence, and differential thermal analysis curves.  There were strong correlations between the various physical properties, and it was found possible to arrange the samples in order of increasing difference in their physical properties from the control sample. This order apparently represents increasing structural damage by radiation; if so, it correlates with decreasing specific gravity, increasing index of refraction, broadening of X-ray lines, and increasingly strong exothermic reactions on annealing. The differences between the samples in index of refraction and X-ray pattern are largely eliminated on annealing.  Annealing begins at 1750 C; thermoluminescence at lower temperatures is due to electrons escaping from the metastable potential traps, not the destruction of those traps which takes place on annealing.

  5. High-temperature tribological properties of NiCoCrAlY-WSe2-BaF2·CaF2 solid lubricant coatings prepared by plasma spraying

    NASA Astrophysics Data System (ADS)

    Chen, X. H.; Yuan, X. J.; Xia, J.; Yu, Z. H.

    2015-12-01

    In this paper, NiCoCrAlY-WSe2-BaF2·CaF2 solid lubricant coatings were produced on a substrate by plasma spray and investigated at the high temperature, such as 500 °C and 800 °C. The structure of the coatings was characterized using XRD pattern and scanning electron microscopy. The TC1 (83wt% NiCoCrAlY) coating has a low friction coefficient at 500C, where the WSe2 is a good solid lubricant. The TC2 (65wt% NiCoCrAlY) coating has the low friction coefficient (0.279) at 800°C, due to the formation of BaCrO4 on the surfaces. As a result, the TC2 coating has the optimal tribological property in the wide temperature.

  6. Complex refractive index measurements for BaF2 and CaF2 via single-angle infrared reflectance spectroscopy

    NASA Astrophysics Data System (ADS)

    Kelly-Gorham, Molly Rose K.; DeVetter, Brent M.; Brauer, Carolyn S.; Cannon, Bret D.; Burton, Sarah D.; Bliss, Mary; Johnson, Timothy J.; Myers, Tanya L.

    2017-10-01

    We have re-investigated the optical constants n and k for the homologous series of inorganic salts barium fluoride (BaF2) and calcium fluoride (CaF2) using a single-angle near-normal incidence reflectance device in combination with a calibrated Fourier transform infrared (FTIR) spectrometer. Our results are in good qualitative agreement with most previous works. However, certain features of the previously published data near the reststrahlen band exhibit distinct differences in spectral characteristics. Notably, our measurements of BaF2 do not include a spectral feature in the ∼250 cm-1 reststrahlen band that was previously published. Additionally, CaF2 exhibits a distinct wavelength shift relative to the model derived from previously published data. We confirmed our results with recently published works that use significantly more modern instrumentation and data reduction techniques.

  7. Test of AN Electromagnetic Calorimeter Using BaF2 Scintillators and Photosensitive Wire Chambers Between 1 and 9 GeV

    NASA Astrophysics Data System (ADS)

    Bouclier, R.; Charpak, G.; Gao, W.; Million, G.; MinÉ, P.; Paul, S.; Santiard, J. C.; Scigocki, D.; Solomey, N.; Suffert, M.

    We describe an electromagnetic calorimeter constructed from layers of BaF2 crystals, coupled to low pressure MWPCs with hot TMAE gas as the photosensitive constituent. By making use of the fast component from the BaF2 scintillation, this detector is well suited for a high rale, intense radiation environment. We present the results of a test performed with our prototype in a 1-9 GeV/c beam, which gives an energy resolution better than 4%/√{E}. a position resolution of 1 mm. and a time resolution better than 1 ns. The detector is highly segmented, with tracking capabilities and good e/π rejection. We discuss the possible application to experiments with intense colliders.

  8. Simultaneous measurement of (n, γ) and (n, fission) cross sections with the DANCE 4π BaF 2 array

    NASA Astrophysics Data System (ADS)

    Bredeweg, T. A.; Fowler, M. M.; Becker, J. A.; Bond, E. M.; Chadwick, M. B.; Clement, R. R. C.; Esch, E.-I.; Ethvignot, T.; Granier, T.; Jandel, M.; Macri, R. A.; O'Donnell, J. M.; Reifarth, R.; Rundberg, R. S.; Ullmann, J. L.; Vieira, D. J.; Wilhelmy, J. B.; Wouters, J. M.; Wu, C. Y.

    2007-08-01

    We have recently begun a program of high precision measurements of the key production and destruction reactions of important radiochemical diagnostic isotopes, including several isotopes of uranium, plutonium and americium. The detector for advanced neutron capture experiments (DANCE), a 4π BaF2 array located at the Los Alamos Neutron Science Center, will be used to measure the neutron capture cross sections for most of the isotopes of interest. However, neutron capture measurements on many of the actinides are complicated by the presence of prompt γ-rays arising from low energy neutron-induced fission, which competes with neutron capture to varying degrees. Previous measurements of 235U using the DANCE array have shown that we can partially resolve capture from fission events based on total γ-ray calorimetry (i.e. total γ-ray energy versus γ-ray multiplicity). The addition of a dedicated fission-tagging detector to the DANCE array has greatly improved our ability to separate these two competing processes. In addition to higher quality neutron capture data, the addition of a fission-tagging detector offers a means to determine the capture-to-fission ratio (σγ/σf) in a single measurement, which should reduce the effect of systematic uncertainties. We are currently using a dual parallel-plate avalanche counter (PPAC) with the target material electro-deposited directly on the center cathode foil. This design provides a high efficiency for detecting fission fragments and allows loading of pre-assembled target/detector assemblies into the neutron beam line at DANCE. Results from tests of the fission-tag detector, as well as preliminary results from measurements on 235U and 252Cf that utilized the fission-tag detector will be presented.

  9. A DFT study of the structural, electronic and optical properties of transition metal doped fluorite oxides: Ce0.75M0.25O2 (M=Fe, Co, Ni)

    NASA Astrophysics Data System (ADS)

    Tian, Dong; Zeng, Chunhua; Fu, Yunchang; Wang, Hua; Luo, Hongchun; Xiang, Chao; Wei, Yonggang; Li, Kongzhai; Zhu, Xing

    2016-04-01

    The structural, electronic and optical properties of Ce1-xMxO2 (M=Fe, Co, Ni; x=0, 0.25) fluorite type oxides are studied by using the method of density functional theory (DFT) + U method. The calculated equilibrium lattice parameter, cell volume, bulk modulus and optical properties for CeO2 are in good agreement with the available experimental data and other theoretical results. The lattice parameter, cell volume, bulk modulus and bond length of Ce-O decreased after substituting Ce atom with Fe (or Co, Ni). Meanwhile, the band-gap (Eg) reduction is observed. It is interesting to find that doping of Fe (or Co, Ni) in CeO2 obviously decreases the O 2p-Ce 4f transition intensity and the covalent character of the Ce-O bond. On the contrary, the static dielectric constant ε0 and refractive index n0 for the doped system increased. Compared with the undoped CeO2, the doped system has steep absorption peaks ranging from 1.0 eV to 2.0 eV at lower energy, and it can be used for visible light absorption applications. The Ce0.75Co0.25O2 has a high refractive index and reflectivity, therefore it is suitable as a high-refractive index film material in single and multilayered optical coatings.

  10. FUNDAMENTAL AREAS OF PHENOMENOLOGY (INCLUDING APPLICATIONS): Cooperative Quantum Cutting of Nano-Crystalline BaF2:Tb3+, Yb3+ in Oxyfluoride Glass Ceramics

    NASA Astrophysics Data System (ADS)

    Chen, Jing-Xin; Ye, Song; Wang, Xi; Qiu, Jian-Rong

    2008-06-01

    We report on cooperative quantum cutting in Tb3+-Yb3+ codoped glass ceramics. Precipitation of BaF2 nano-crystals is confirmed by XRD and HRTEM analysis. Near-infrared emission due to transition of Yb3+ ions under 485 nm excitation indicates cooperative energy transfer from Tb3+ to Yb3+. The quantum efficiency of this process reaches 145%. The realization of quantum cutting in glass ceramics may have promising applications in solar cells.

  11. Optical characterization of Tm3+ doped Bi2O3-GeO2-Ga2O3 glasses in absence and presence of BaF2

    PubMed Central

    Han, Kexuan; Zhang, Peng; Wang, Shunbin; Guo, Yanyan; Zhou, Dechun; Yu, Fengxia

    2016-01-01

    In this paper, Two new Bi2O3-GeO2-Ga2O3 glasses (one presence of BaF2) doped with 1mol% Tm2O3 were prepared by melt-quenching technique. Differential thermal analysis (DTA), the absorption, Raman, IR spectra and fluorescence spectra were measured. The Judd–Ofelt intensity parameters, emission cross section, absorption cross section, and gain coefficient of Tm3+ ions were comparatively investigated. After the BaF2 introduced, the glass showed a better thermal stability, lower phonon energy and weaker OH− absorption coefficient, meanwhile, a larger ~1.8 μm emission cross section σem (7.56 × 10−21 cm2) and a longer fluorescence lifetime τmea (2.25 ms) corresponding to the Tm3+: 4F3 → 3H6 transition were obtained, which is due to the addition of fluoride in glass could reduce the quenching rate of hydroxyls and raise the cross-relaxation (3H6 + 3H4 → 3F4 + 3F4) rate. Our results suggest that the Tm3+ doped Bi2O3-GeO2-Ga2O3 glass with BaF2 might be potential to the application in efficient ~1.8 μm lasers system. PMID:27506152

  12. Spectroscopic properties and mechanism of Tm3+/Er3+/Yb3+co-doped oxyfluorogermanate glass ceramics containing BaF2 nanocrystals

    NASA Astrophysics Data System (ADS)

    Hu, Yue-Bo; Qiu, Jian-Bei; Zhou, Da-Cheng; Song, Zhi-Guo; Yang, Zheng-Wen; Wang, Rong-Fei; Jiao, Qing; Zhou, Da-Li

    2014-02-01

    Transparent Tm3+/Er3+/Yb3+ co-doped oxyfluorogermanate glass ceramics containing BaF2 nanocrystals are prepared. Under excitation of a 980-nm laser diode (LD), compared with the glass before heat treatment, the Tm3+/Er3+/Yb3+co-doped oxyfluorogermanate glass ceramics can emit intense blue, green and red up-conversion luminescence and Stark-split peaks; X-ray diffraction (XRD) and transmission electron microscope (TEM) results show that BaF2 nanocrystals with an average diameter of 20 nm are precipitated from the glass matrix. Stark splitting of the up-conversion luminescence peaks in the glass ceramics indicates that Tm3+, Er3+ and (or) Yb3+ ions are incorporated into the BaF2 nanocrystals. The up-conversion luminescence intensities of Tm3+, Er3+ and the splitting degree of luminescence peaks in the glass ceramics increase significantly with the increase of heat treat temperature and heat treat time extension. In addition, the possible energy transfer process between rare earth ions and the up-conversion luminescence mechanism are also proposed.

  13. Optical characterization of Tm(3+) doped Bi2O3-GeO2-Ga2O3 glasses in absence and presence of BaF2.

    PubMed

    Han, Kexuan; Zhang, Peng; Wang, Shunbin; Guo, Yanyan; Zhou, Dechun; Yu, Fengxia

    2016-08-10

    In this paper, Two new Bi2O3-GeO2-Ga2O3 glasses (one presence of BaF2) doped with 1mol% Tm2O3 were prepared by melt-quenching technique. Differential thermal analysis (DTA), the absorption, Raman, IR spectra and fluorescence spectra were measured. The Judd-Ofelt intensity parameters, emission cross section, absorption cross section, and gain coefficient of Tm(3+) ions were comparatively investigated. After the BaF2 introduced, the glass showed a better thermal stability, lower phonon energy and weaker OH(-) absorption coefficient, meanwhile, a larger ~1.8 μm emission cross section σem (7.56 × 10(-21) cm(2)) and a longer fluorescence lifetime τmea (2.25 ms) corresponding to the Tm(3+): (4)F3 → (3)H6 transition were obtained, which is due to the addition of fluoride in glass could reduce the quenching rate of hydroxyls and raise the cross-relaxation ((3)H6 + (3)H4 → (3)F4 + (3)F4) rate. Our results suggest that the Tm(3+) doped Bi2O3-GeO2-Ga2O3 glass with BaF2 might be potential to the application in efficient ~1.8 μm lasers system.

  14. Optical characterization of Tm3+ doped Bi2O3-GeO2-Ga2O3 glasses in absence and presence of BaF2

    NASA Astrophysics Data System (ADS)

    Han, Kexuan; Zhang, Peng; Wang, Shunbin; Guo, Yanyan; Zhou, Dechun; Yu, Fengxia

    2016-08-01

    In this paper, Two new Bi2O3-GeO2-Ga2O3 glasses (one presence of BaF2) doped with 1mol% Tm2O3 were prepared by melt-quenching technique. Differential thermal analysis (DTA), the absorption, Raman, IR spectra and fluorescence spectra were measured. The Judd–Ofelt intensity parameters, emission cross section, absorption cross section, and gain coefficient of Tm3+ ions were comparatively investigated. After the BaF2 introduced, the glass showed a better thermal stability, lower phonon energy and weaker OH‑ absorption coefficient, meanwhile, a larger ~1.8 μm emission cross section σem (7.56 × 10‑21 cm2) and a longer fluorescence lifetime τmea (2.25 ms) corresponding to the Tm3+: 4F3 → 3H6 transition were obtained, which is due to the addition of fluoride in glass could reduce the quenching rate of hydroxyls and raise the cross-relaxation (3H6 + 3H4 → 3F4 + 3F4) rate. Our results suggest that the Tm3+ doped Bi2O3-GeO2-Ga2O3 glass with BaF2 might be potential to the application in efficient ~1.8 μm lasers system.

  15. Luminescence properties of dual valence Eu doped nano-crystalline BaF2 embedded glass-ceramics and observation of Eu2+ → Eu3+ energy transfer.

    PubMed

    Biswas, Kaushik; Sontakke, Atul D; Sen, R; Annapurna, K

    2012-03-01

    Europium doped glass-ceramics containing BaF(2) nano-crystals have been prepared by using the controlled crystallization of melt-quenched glasses. X-ray diffraction and transmission electron microscopy have confirmed the presence of cubic BaF(2) nano-crystalline phase in glass matrix in the ceramized samples. Incorporation of rare earth ions into the formed crystalline phase having low phonon energy of 346 cm(-1) has been demonstrated from the emission spectra of Eu(3+) ions showing the transitions from upper excitation states (5)D(J) (J = 1, 2, and 3) to ground states for the glass-ceramics samples. The presence of divalent europium ions in glass and glass-ceramics samples is confirmed from the dominant blue emission corresponding to its 5d-4f transition under an excitation of 300 nm. Increase in the reduction of trivalent europium (Eu(3+)) ions to divalent (Eu(2+)) with the extent of ceramization is explained by charge compensation model based on substitution defect mechanisms. Further, the phenomenon of energy transfer from Eu(2+) to Eu(3+) ion by radiative trapping or re-absorption is evidenced which increases with the degree of ceramization. For the first time, the reduction of Eu(3+) to Eu(2+) under normal air atmospheric condition has been observed in a BaF(2) containing oxyfluoride glass-ceramics system.

  16. Crystal structure of fluorite-related Ln{sub 3}SbO{sub 7} (Ln=La–Dy) ceramics studied by synchrotron X-ray diffraction and Raman scattering

    SciTech Connect

    Siqueira, K.P.F.; Borges, R.M.; Granado, E.; Malard, L.M.; Paula, A.M. de; Moreira, R.L.; Bittar, E.M.; Dias, A.

    2013-07-15

    Ln{sub 3}SbO{sub 7} (Ln=La, Pr, Nd, Sm, Eu, Gd, Tb and Dy) ceramics were synthesized by solid-state reaction in optimized conditions of temperature and time to yield single-phase ceramics. The crystal structures of the obtained ceramics were investigated by synchrotron X-ray diffraction, second harmonic generation (SHG) and Raman scattering. All samples exhibited fluorite-type orthorhombic structures with different oxygen arrangements as a function of the ionic radius of the lanthanide metal. For ceramics with the largest ionic radii (La–Nd), the ceramics crystallized into the Cmcm space group, while the ceramics with intermediate and smallest ionic radii (Sm–Dy) exhibited a different crystal structure belonging to the same space group, described under the Ccmm setting. The results from SHG and Raman scattering confirmed these settings and ruled out any possibility for the non-centrosymmetric C222{sub 1} space group describing the structure of the small ionic radii ceramics, solving a recent controversy in the literature. Besides, the Raman modes for all samples are reported for the first time, showing characteristic features for each group of samples. - Graphical abstract: Raman spectrum for La{sub 3}SbO{sub 7} ceramics showing their 22 phonon modes adjusted through Lorentzian lines. According to synchrotron X-ray diffraction and Raman scattering, this material belongs to the space group Cmcm. - Highlights: • Ln{sub 3}SbO{sub 7} ceramics belonging to the space groups Cmcm and Ccmm are synthesized. • SXRD, SHG and Raman scattering confirmed the orthorhombic structures. • Ccmm instead of C222{sub 1} is the correct one based on SHG and Raman data.

  17. Fluorite solubility equilibria in selected geothermal waters

    USGS Publications Warehouse

    Nordstrom, D.K.; Jenne, E.A.

    1977-01-01

    Calculation of chemical equilibria in 351 hot springs and surface waters from selected geothermal areas in the western United States indicate that the solubility of the mineral fluorite, CaF2, provides an equilibrium control on dissolved fluoride activity. Waters that are undersaturated have undergone dilution by non-thermal waters as shown by decreased conductivity and temperature values, and only 2% of the samples are supersaturated by more than the expected error. Calculations also demonstrate that simultaneous chemical equilibria between the thermal waters and calcite as well as fluorite minerals exist under a variety of conditions. Testing for fluorite solubility required a critical review of the thermodynamic data for fluorite. By applying multiple regression of a mathematical model to selected published data we have obtained revised estimates of the pK (10,96), ??Gof (-280.08 kcal/mole), ??Hof (-292.59 kcal/mole), S?? (16.39 cal/deg/mole) and CoP (16.16 cal/deg/mole) for CaF2 at 25??C and 1 atm. Association constants and reaction enthalpies for fluoride complexes with boron, calcium and iron are included in this review. The excellent agreement between the computer-based activity products and the revised pK suggests that the chemistry of geothermal waters may also be a guide to evaluating mineral solubility data where major discrepancies are evident. ?? 1977.

  18. Ho3+/Yb3+ co-doped TeO2-BaF2-Y2O3 glasses for ∼1.2 μm laser applications

    NASA Astrophysics Data System (ADS)

    Wang, Shunbin; Li, Chengzhi; Yao, Chuanfei; Jia, Shijie; Jia, Zhixu; Qin, Guanshi; Qin, Weiping

    2017-02-01

    Intense ∼1.2 μm fluorescence is observed in Ho3+/Yb3+ co-doped TeO2-BaF2-Y2O3 glasses under 915 nm laser diode excitation. The 1.2 μm emission can be ascribed to the transition 5I6→5I8 of Ho3+. With the introducing of BaF2, the content of OH in the glasses drops markedly, and the 1.2 μm emission intensity increases gradually as increasing the concentration percentage of BaF2. Furthermore, microstructured fibers based on the TeO2-BaF2-Y2O3 glasses are fabricated by using a rod-in-tube method, and a relative positive gain of ∼9.42 dB at 1175.3 nm is obtained in a 5 cm long fiber.

  19. Facile conversion of layered Ruddlesden-Popper-related structure Y2O3-doped Sr2CeO4 into fast oxide ion-conducting fluorite-type Y2O3-doped CeO2.

    PubMed

    Gerlach, Ryan Georg; Bhella, Surinderjit Singh; Thangadurai, Venkataraman

    2009-01-05

    The present work shows a new solid- and gas-phase reaction technique for the preparation of a fast oxide-ion-conducting Y(2)O(3)-doped Ce(1-x)Y(x)O(2-delta) (x = 0.1, 0.2) (YCO), which involves the reaction of layered (Ruddlesden-Popper K(2)NiF(4)-type) structure Y(2)O(3)-doped Sr(2)CeO(4) (YSCO) with CO(2) at an elevated temperature and subsequent acid-washing. A powder X-ray diffraction study revealed the formation of a single-phase cubic fluorite-type YCO for the CO(2)-reacted and subsequent acid-washed product. Energy dispersive X-ray analysis showed the absence of Sr in the CO(2)-treated and subsequent acid-washed product, confirming the transformation of layered YSCO into YCO. The cubic lattice constant was found to decrease with increasing Y content in YCO, which is consistent with the other YCO samples reported in the literature. The scanning electron microscopy study showed smaller-sized particles for the product obtained after CO(2)- and acid-washed YCO samples, while the high-temperature sintered YCO and the precursor YSCO exhibit larger-sized particles. The bulk ionic conductivity of the present CO(2)-capture-method-prepared YCO exhibits about one and half orders of magnitude higher electrical conductivity than that of the undoped CeO(2) and was found to be comparable to those of ceramic- and wet-chemical-method synthesized rare-earth-doped CeO(2).

  20. Fluorite-related one-dimensional units in natural bismuth oxysulfates: the crystal structures of Bi14O16(SO4)5 and Bi30O33(SO4)9(AsO4)2.

    PubMed

    Pinto, Daniela; Garavelli, Anna; Bindi, Luca

    2015-10-01

    The crystal structures of two new natural Bi oxysulfates with the formula Bi14O16(SO4)5 [labelled new phase I; monoclinic, space group C2, a = 21.658 (4), b = 5.6648 (9), c = 15.092 (3) Å, β = 119.433 (11)° and Z = 2] and Bi30O33(SO4)9(AsO4)2 [labelled new phase II; triclinic, space group P1, a = 5.670 (3), b = 13.9408 (9), c = 22.7908 (18) Å, α = 80.903 (5), β = 82.854 (14), γ = 78.27 (2)° and Z = 1] from the high-temperature fumarole deposit of the La Fossa crater at Vulcano (Aeolian Islands, Italy) are reported. The structures are built up by a combination of fluorite-related Bi-O units and isolated (SO4)(2-) tetrahedra (new phase I) or both (SO4)(2-) and (AsO4)(3-) tetrahedra (new phase II). Owing to the effect of stereoactive lone pairs of Bi(3+), Bi-O units in both the structures can be suitably described in terms of oxo-centered OBi4 tetrahedra. The structure of Bi14O16(SO4)5 is based upon one-dimensional [O16Bi14](10+) ribbons formed by six chains of edge-sharing OBi4 tetrahedra extending along [010]. In the structure of Bi30O33(SO4)9(AsO4)2 the same ribbon type coexists with another one-dimensional ribbon formed by seven chains of edge-sharing OBi4 tetrahedra and with the composition [O17Bi16](14+). Ribbons of the same type are joined by (SO4)(2-) and (AsO4)(3-) tetrahedra along [010] – if a reduced triclinic unit-cell setting is considered – so forming two different (001) slabs which alternate to each other along [001] and are joined by additional (SO4)(2-) tetrahedra. New phase I represents the natural analogues of synthetic Bi14O16(SO4)5, but with an ordered structure model.

  1. The Detector for Advanced Neutron Capture Experiments: A 4{pi} BaF2 Detector for Neutron Capture Measurements at LANSCE

    SciTech Connect

    Ullmann, J.L.; Esch, E.-I.; Haight, R.C.; Hunt, L.; O'Donnell, J.M.; Reifarth, R.; Agvaanluvsan, U.; Alpizar, A.; Hatarik, R.; Bond, E.M.; Bredeweg, T.A.; Kronenberg, A.; Rundberg, R.S.; Vieira, D.J.; Wilhelmy, J.B.; Folden, C.M.; Hoffman, D.C.; Greife, U.; Schwantes, J.M.; Strottman, D.D.

    2005-05-24

    The Detector for Advanced Neutron Capture Experiments (DANCE) is a 162-element 4{pi} BaF2 array designed to make neutron capture cross-section measurements on rare or radioactive targets with masses as little as one milligram. Accurate capture cross sections are needed in many research areas, including stellar nucleosynthesis, advanced nuclear fuel cycles, waste transmutation, and other applied programs. These cross sections are difficult to calculate accurately and must be measured. The design and initial performance results of DANCE is discussed.

  2. Temperature dependence of optical anisotropy parameter of CaF2, BaF2 and SrF2 materials

    NASA Astrophysics Data System (ADS)

    Snetkov, Ilya; Yakovlev, Alexey; Palashov, Oleg

    2017-07-01

    Thermally induced depolarization in undoped CaF2, BaF2 and SrF2 crystals was measured. The obtained results were used to determine temperature dependence of the optical anisotropy parameter ξ of these materials in the 80-300 K range. Temperature dependence of the orientation of crystallographic axes corresponding to the ;zero depolarization; orientation was found. It was revealed that thermally induced depolarization in the studied materials cooled to 80 K reduces by more than 100 times, with changes in the parameter ξ being 5.8%, 6.4% and 28%, respectively.

  3. Hyperstoichiometric Oxygen in Fluorite-type U3O8 Formed at Extreme Conditions

    NASA Astrophysics Data System (ADS)

    Zhang, Fuxiang; Lang, Maik; Ewing, Rod; Department of Earth and Environmental Sciences Team

    2014-03-01

    U3O8 was obtained by annealing UO3 in a reducing atmosphere at 200 °C. Powder sample of β-U3O8 was pressurized at room temperature up to 37.5 GPa and XRD patterns clearly indicated that a phase transition occurred between 3-11 GPa. The high-pressure phase is a fluorite-like structure. The high-pressure phase was then laser heated to over 1700 K in the diamond anvil cell at high pressure conditions. No phase transition was found at high pressure/ temperature conditions, and the fluorite-like structure of U3O8 is even fully quenchable. The lattice parameter of the fluorite-like high-pressure phase is 5.425 Å at ambient conditions, which is smaller than that of the stoichiometric UO2. Previous experiments have shown that the stoichiometric uranium dioxide (UO2) is not stable at high pressure conditions and starts to transform to a cotunnite structure at ~ 30 GPa. When heating the sample at high pressure, the critical transition pressure is greatly reduced. However, the fluorite-like high-pressure phase of U3O8 is very stable at high pressure/high temperature conditions. The enhanced phase stability is believed to be related to the presence of extra oxygen (or U vacancies) in the structure.

  4. Optical properties of a new Bi38Mo7O78 semiconductor with fluorite-type δ-Bi2O3 structure

    NASA Astrophysics Data System (ADS)

    Wang, Zuoshan; Bi, Shala; Wan, Yingpeng; Huang, Pengjie; Zheng, Min

    2017-03-01

    Bi3+-containing inorganic materials usually show rich optical and electronic properties due to the hybridization between 6s and 6p electronic components together with the lone pair in Bi3+ ions. In this work, a new semiconductor of bismuth molybdate Bi38Mo7O78 (19Bi2O3·7MoO3) was synthesized by the sol-gel film coating and the following heat process. The samples developed into nanoparticles with average size of 40 nm. The phase formation was verified via the XRD Rietveld structural refinement. Orthorhombic Bi38Mo7O78 can be regarded to be derived from the cubic δ-phase Bi2O3 structure. The microstructure was investigated by SEM, EDX, TEM, BET and XPS measurements. The UV-vis absorption spectra showed that the band gap of Bi38Mo7O78 (2.38 eV) was greatly narrowed in comparison with Bi2O3 (2.6 eV). This enhances the efficient absorption of visible light. Meanwhile, the conduction band of is wider and shows more dispersion, which greatly benefits the mobility of the light-induced charges taking part in the photocatalytic reactions. Bi38Mo7O78 nanoparticles possess efficient activities on the photodegradation of methylene blue (MB) solutions under the excitation of visible-light. The photocatalysis activities and mechanisms were discussed on the crystal structure characteristics and the measurements such as photoluminescence, exciton lifetime and XPS results.

  5. Formation of metal nanoparticles in MgF2, CaF2 and BaF2 crystals under the electron beam irradiation

    NASA Astrophysics Data System (ADS)

    Bochkareva, Elizaveta S.; Sidorov, Alexander I.; Yurina, Uliana V.; Podsvirov, Oleg A.

    2017-07-01

    It is shown experimentally that electron beam action with electrons energies of 50 and 70 keV on MgF2, CaF2 and BaF2 crystals results in local formation in the crystal near-surface layer of Mg, Ca or Ba nanoparticles which possess plasmon resonance. In the case of MgF2 spheroidal nanoparticles are formed, in the cases of CaF2 and BaF2 - spherical. The formation of metal nanoparticles is confirmed by computer simulation in dipole quasistatic approximation. The dependence of absorption via electron irradiation dose is non-linear. It is caused by the increase of nanoparticles concentration and by the increase of nanoparticles sizes during irradiation. In the irradiated zones of MgF2 crystals, for irradiation doses less than 80 mC/cm2, the intense luminescence in a visible range appears. The practical application of fabricated composite materials for multilevel optical information recording is discussed.

  6. Nanostructured crystals of fluorite phases Sr1 - x R x F2 + x and their ordering: 7. A procedure for cluster modeling of Sr1 - x R x F2 + x Based on the Structure of an Ordered Phase ( R = Lu)

    NASA Astrophysics Data System (ADS)

    Sul'yanova, E. A.; Verin, I. A.; Sobolev, B. P.

    2012-01-01

    Single crystals of the Sr1 - x Lu x F2 + x nonstoichiometric phase ( x = 0.095 and 0.189), which crystallizes in the CaF2 structure type (space group Fm bar 3 m), are studied by X-ray diffraction. A procedure for the cluster modeling of Sr1 - x Lu x F2 + x based on the fine structural features of the Sr4Lu3F17 stoichiometric compound with a fluorite derivative structure (one of the ordered phases in the SrF2-LuF3 system) is proposed. The description of the structure of the Sr1- x Lu x F2+ x solid solutions is based on the octacubic configuration of the { M 14 - n R n F64 + n } defect cluster. Structural data on the displacements of cations and anions in the ordered Sr4Lu3F17 phase (in relation to the undistorted fluorite phase) allow one to identify the distortions of the crystal lattice of the disordered (nonstoichiometric) Sr1- x Lu x F2 + x phase.

  7. Fluorine, fluorite, and fluorspar in central Colorado

    USGS Publications Warehouse

    Wallace, Alan R.

    2010-01-01

    Fluorine (F) is a widespread element that was deposited in a variety of rocks, minerals, and geologic environments in central Colorado. It occurs as a trace element, as a major component of the mineral fluorite (CaFs), and as a major economic source of fluorine in fluorspar deposits, which are massive concentrations of fluorite. This study has compiled available geochemical analyses of rocks, both unmineralized and mineralized, to determine the distribution of fluorine in specific age-lithologic categories, ranging from 1.8-giga-annum (Ga) metamorphic rocks to modern soils, throughout central Colorado. It also draws upon field studies of fluorine-rich mineral deposits, including fluorspar deposits, to decipher the nearly two-billion-year-long geologic history of fluorine in the study area, with implications for mineral-resource evaluations and exploration. The resulting compilation provides an important inventory of the naturally occurring levels and sources of fluorine that ultimately weather, erode, and become part of surface waters that are used for domestic water supplies in densely populated areas along the Colorado Front Range. Most commonly, fluorine is a trace element in virtually all rocks in the region. In the 3,798 unmineralized rocks that were analyzed for fluorine in the study area, the average fluorine content was 1,550 parts per million (ppm). The median was 640 ppm, nearly identical to the average crustal abundance of 650 ppm, and some high-fluorine rocks in the Pikes Peak area skewed the average to a value much greater than the median. Most unmineralized age-lithologic rock suites, including Proterozoic metamorphic rocks, 1.7- and 1.4-Ga granitic batholiths, Cambrian igneous rocks, Phanerozoic sedimentary rocks, and Laramide and Tertiary igneous rocks, had median fluorine values of 400 to 740 ppm fluorine. In all suites, however, a small number of analyzed samples contained more than 1 percent (10,000 ppm) fluorine. The 1.1-Ga plutonic rocks

  8. Ordered lattice defects in colored fluorite: direct observations.

    PubMed

    Murr, L E

    1974-01-18

    Ordered arrays of defect aggregates in the (111) planes of natural fluorite have been observed by transmission electron microscopy. The intense blue coloration observed in corresponding sample areas after 200-kilovolt electron microscopy confirms the conclusion that these are color-center aggregates and, conversely, that color centers are primarily responsible for fluorite coloration.

  9. Absolute refractive indices and thermal coefficients of CaF2, SrF2, BaF2, and LiF near 157 nm.

    PubMed

    Burnett, John H; Gupta, Rajeev; Griesmann, Ulf

    2002-05-01

    We present high-accuracy measurements for wavelengths near 157 nm of the absolute index of refraction, the index dispersion, and the temperature dependence of the index for the ultraviolet optical materials with cubic symmetry: CaF2, SrF2, BaF2, and LiF. Accurate values of these quantities for these materials are needed for designs of the lens systems for F2 excimer-laser-based exposure tools for 157-nm photolithography. These tools are expected to use CaF2 as the primary optical material and possibly one of the others to correct for chromatic aberrations. These optical properties were measured by the minimum deviation method. Absolute refractive indices were obtained with an absolute accuracy of 5 x 10(-6) to 6 x 10(-6).

  10. Stabilization of the fluorite phase in the ZrO2-Y2O3 system

    NASA Astrophysics Data System (ADS)

    Popov, V. V.; Yastrebtsev, A. A.; Smirnov, I. S.; Monakhov, I. S.; Novoselova, E. G.; Tsarenko, N. A.; Udovskii, A. L.

    2016-09-01

    The calculated and experimental vertical ZrO2-Y2O3 sections of the Zr-Y-O system are compared to find the region of a stable fluorite structure of yttrium-stabilized zirconia (YSZ). X-ray diffraction (XRD) and Raman scattering are used to study the crystal and local structures of mixed oxide 0.82ZrO2 · 0.18Y2O3 (18YSZ) powders prepared by isothermal annealing of a precursor precipitated from a salt solution. The formation of a fluorite-type fcc structure (space group Fmoverline 3 m) in the powders is detected by XRD. Raman scattering study of the local structure of the cubic 18YSZ powders revealed traces of the tetragonal phase in them.

  11. Sm-Nd dating of fluorite from the worldclass Montroc fluorite deposit, southern Massif Central, France

    USGS Publications Warehouse

    Munoz, M.; Premo, W.R.; Courjault-Rade, P.

    2005-01-01

    A three-point Sm-Nd isotope isochron on fluorite from the very large Montroc fluorite vein deposit (southern Massif Central, France) defines an age of 111??13 Ma. Initial ??Nd of -8.6 and initial 87Sr/86Sr of ???0.71245 suggest an upper crustal source of the hydrothermal system, in agreement with earlier work on fluid inclusions which indicated a basinal brine origin. The mid-Cretaceous age of ???111 Ma suggests the Albian/Aptian transition as the most likely period for large-scale fluid circulation during a regional extensional tectonic event, related to the opening of the North Atlantic ocean. ?? Springer-Verlag 2004.

  12. Geochemistry and microfabrics of syndiagenetic strata-bound fluorite from Eschwege, Germany - Implications for fluorite formation and remobilization in Zechstein carbonates from the Lower Saxony Basin

    NASA Astrophysics Data System (ADS)

    Duschl, Florian; Wischhöfer, Philipp; Vollbrecht, Axel

    2017-04-01

    Core samples of Stassfurt carbonate rocks (Zechstein, Ca2) from various locations in the Lower Saxony Basin (LSB) often contain fluorite which occurs as pore filling cement, replacement fluorite, or as fault-related fracture mineralizations. Recent studies on fluorite geochemistry and fluid migration in the LSB suggest a sedimentary rather than a hydrothermal fluorite source for some of these deep-seated (> 2500 mbs) accumulations. Outcrop samples from lens-shaped and stratiform fluorite occurrences within oolithic limestone (Ca2) near Eschwege, Germany, give insight into syndiagenetic fluorite formation in Zechstein carbonates. They serve as a shallow-burial analogue for remobilized fluorite within deeply buried carbonate rocks of the LSB. Samples were studied using petrographic microscopy, hot-cathodoluminescence microscopy, and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Five different fluorite types were identified: a first generation (I) is represented by a dark brownish to violet fluorite that replaced ooids during early diagenesis. As pressure increased during burial the replacement fluorite recrystallized forming white aggregates of parallel bar-shaped fluorite crystals (type II), type (III) consists of white fluorite grains with rectangular and mostly polygonal grain boundaries; type (IV) is a colorless to blueish pore-filling fluorite, and type (V) is a colorless fracture-hosted fluorite. In-situ LA-ICP-MS analyses of respective fluorite types revealed relatively low REE concentrations in general with Tb/Ca vs. Tb/La signatures that are typical for sediment-hosted fluorite. The REE distribution patterns reflect the processes of recrystallization and remobilization. Though stylolitization affected both host rock and replacement fluorite, only little fluorite remobilization did occur due to pressure solution. Geochemical analyses prove that fluorite formation was controlled by precipitation from a sedimentary parental fluid

  13. Colloid formation and laser-induced bleaching in fluorite

    SciTech Connect

    LeBret, Joel B.; Cramer, Loren P.; Norton, M. Grant; Dickinson, J. T.

    2004-11-08

    Colloid formation and subsequent laser-induced bleaching in fluorite has been studied by transmission electron microscopy and electron diffraction. At high incident electron-beam (e-beam) energies, Ca colloids with diameter {approx}10 nm form a simple cubic superlattice with lattice parameter a{approx}18 nm. The colloids themselves are topotactic with the fluorite matrix forming low-energy interfaces close to a {sigma}=21 special grain boundary in cubic materials. Laser irradiation using {lambda}=532 nm has been shown to effectively bleach the e-beam-irradiated samples returning the fluorite to its monocrystalline state. The bleached samples appear more resistant to further colloid formation.

  14. 10BaF2:NaF, Na3AlF6/TiO2 composite as a novel visible-light-driven photocatalyst based on upconversion emission

    NASA Astrophysics Data System (ADS)

    Liu, En-Zhou; Fan, Jun; Hu, Xiao-Yun; Hou, Wen-Qian; Dai, Hong-Zhe

    2012-04-01

    A rare-earth free upconversion luminescent material, 10BaF2:NaF, Na3AlF6, is synthesized by a hydrothermal method. The study of fluorescent spectrum indicates that it can convert visible light (550 nm—610 nm) into ultraviolet light (290 nm—350 nm), and two emission peaks at 304 nm and 324 nm are observed under the excitation of 583 nm at room temperature. Subsequently, 10BaF2:NaF, Na3AlF6/TiO2 composite photocatalyst is prepared and its catalytic activity is evaluated by the photocatalytic reduction of CO2 under visible light irradiation (λ > 515 nm). The results show that 10BaF2:NaF, Na3AlF6/TiO2 is a more effective photocatalyst for CO2 reduction than pure TiO2, their corresponding methanol yields are 179 and 0 μmol/g-cat under the same conditions. Additionally, the mechanism of photocatalytic reduction of CO2 on 10BaF2:NaF, Na3AlF6/TiO2 is proposed.

  15. Turbidimetric study of fluorite nucleation in solution.

    PubMed

    Lundager Madsen, Hans E

    2007-03-15

    Precipitation of fluorite (calcium fluoride) from solution at 30 and 37 degrees C has been studied by measurement of turbidity at different wavelengths, analyzing the data with the aid of the Mie theory of light scattering from small particles. In this way both number density and average particle size were obtained as functions of time. The values were confirmed by sedimentation analysis. In the later part of the process the results were strongly influenced by agglomeration and sedimentation, but the initial part of the curve of number density vs time was useful in determination of nucleation kinetics. The trend of nucleation frequency vs supersaturation agreed well with the classical theory of Becker, Döring, Volmer, Zel'dovich, and Frenkel, and surface energies equal to 102 mJ/m(2) at 30 degrees C and 89 mJ/m(2) at 37 degrees C were found. On the other hand, the absolute rate of nucleation was many orders of magnitude lower than that predicted by the theory, which is ascribed to an overestimate of the rate of growth of postcritical nuclei.

  16. Ionic, electronic and ion-diffusion controlled relaxation processes in CaF2, BaF2 and LiBaF3 crystals

    NASA Astrophysics Data System (ADS)

    Ziraps, V.; Kulis, P.; Tale, I.; Veispals, A.

    The ionic, electronic and anion-diffusion controlled thermally stimulated relaxation (TSR) processes at 80-700 K in CaF2 BaF2 and LiBaF3 crystals (X-ray irradiated or non-irradiated) have been investigated by means of ionic conductivity, ionic thermally stimulated (TS) depolarization current (TSDC); as well as current (TSC), luminescence (TSL) and bleaching (TSB) techniques. Above 250-290 K broad and overlapping anion TSDC peaks and correlated TSB stages are detected. The TSB kinetics is initiated and controlled by anion detrapping and interaction with the localized charges, i.e., the anion-diffusion controlled TSR processes take place in fluorides. The TSL and TSC data for LiBaF3 indicate that the lifetime and drift of electrons at 80-250 K is very small because of deep retrapping. The main TSL peaks at 132K, 170K and 220 K are caused by Vk center detrapping and hole-diffusion controlled tunnel recombination within pairs like .

  17. White up-conversion emission in Ho3+/Tm3+/Yb3+ tri-doped glass ceramics embedding BaF2 nanocrystals

    NASA Astrophysics Data System (ADS)

    Li, Chenxia; Xu, Shiqing; Ye, Rengguang; Deng, Degang; Hua, Youjie; Zhao, Shilong; Zhuang, Songlin

    2011-04-01

    Ho3+/Tm3+/Yb3+ tri-doped glass ceramics with white light emitting have been developed and demonstrated. Pumped by 980 nm laser diode (LD), intensive red, green and blue up-conversions (UC) were obtained. The green emission is assigned to Ho3+ ion and the blue emission is assigned to Tm3+ ion, whereas the red emission is the combination contribution of the Ho3+ and Tm3+ ions. The RGB intensities could be adjusted by tuning the rare-earth ion concentration and pump power intensity. Thus, multicolor of the luminescence, including perfect white light with CIE-X=0.329 and CIE-Y=0.342 in the 1931 CIE chromaticity diagram can be obtained in 0.15 Ho3+/0.2Tm3+/3Yb3+ tri-doped glass ceramics embedding BaF2 nanocrystals pumped by a single infrared laser diode source of 980 nm at 500 mW. The up-conversion luminescence mechanism of Yb3+ sensitize Ho3+ and Tm3+ ions and the energy transfer from Ho3+ to Tm3+ in oxy-fluoride silicate glass ceramics were analyzed.

  18. The reasons for the color green fluorite Mehmandooye cover using UV spectroscopy and XRF results

    NASA Astrophysics Data System (ADS)

    Pirzadeh, Sara; Zahiri, Reza

    2016-04-01

    Fluorite mineral or fluorine with chemical formula CaF2 is most important mineralfluor in nature. This mineral crystallization to colors yellow, green, pink, blue, purple, colorless and sometimes black andin cubic system crystallized.assemi transparent and glass with polished.fluoritethe purity include 48/9% fluoreand 51/9% calcium. How the creation colors in minerals different greatly indebted to Kurt Nassau research from Bell Labs, Murray Hill, New Jersey.almostall the mechanisms that cause color in minerals, are the result of the interaction of light waves with the electrons The main factors affecting the color generation include the following: 1)the presence of a constructive element inherent (essential ingredient mineral composition) 2)The presence of a minor impurities (such a element as involved in latticesolid solution) 3) appearancedefects in the crystal structure 4) There are some physical boundaries with distances very small and delicate, like blades out of the solution (which may be the play of colors or Chatvyansy) 5) Mixing mechanical impurities dispersed in a host mineral Based on the results of the analysis, XRF and UV spectrum and also based on the results of ICP, because the color green fluorite examined, the focus color (F_center) and also the presence of some elementsintermediate (such as Y (yttrium). [1] Bill, H., Calas, G. Color centres associated rare earth ions and the origin of coloration in natural fluorites// PhysChem Min, (1978), v 3, pp. 117-131.

  19. A Fluorite Isotype of SnO2 and a New Modification of TiO2: Implications for the Earth's Lower Mantle.

    PubMed

    Liu, L G

    1978-01-27

    The existence of a cubic fluorite-type SnO(2) and a hexagonal TiO(2) (which may be related to the fluorite structure) have been demonstrated by an in situ x-ray diffraction study in which a diamond-anvil pressure cell was used after the samples had been heated by a continuous yttrium-aluminum-garnet laser. At room temperature, the lattice parameter for SnO(2) (fluorite) is a = 4.925 +/- 0.005 angstroms and those for TiO(2) (fluorite-related) are a = 9.22 +/- 0.01 angstroms and c = 5.685 +/- 0.006 angstroms at about 250 kilobars. The volume change associated with the transition from rutile to fluorite (or related structure) is about -8 percent for SnO(2) and -10.5 percent for TiO(2) at transition. Upon release of pressure, both the fluorite-type SnO(2) and the TiO(2) reverted to the alpha-PbO(2) structure at room temperature. The hypothesis that the earth's lower mantle is composed of oxide phases might be feasible if it were possible for SiO(2) to possess the fluorite structure or its related forms at high pressure, as shown for SnO(2) and TiO(2) in this study. The oxide hypothesis proposed here differs from that postulated by Birch in that the primary coordination of silicon is 6 for Birch's hypothesis and 8 for the hypothesis presented here.

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

    SciTech Connect

    Masui, Toshiyuki; Nagai, Ryosuke; Imanaka, Nobuhito

    2014-12-15

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

  1. Samarium-neodymium direct dating of fluorite mineralization.

    PubMed

    Chesley, J T; Halliday, A N; Scrivener, R C

    1991-05-17

    The direct dating of many styles of hydrothermal mineralization has proved difficult, limiting understanding of the geological processes that lead to crustal fluid flow and the formation of major ore deposits. The hydrothermal mineral fluorite (CaF(2)) displays large variations in rare earth element (REE) abundance and samarium/neodymium ratios within a single vein. Samarium-neodymium dating of fluorite from the classic granite-hosted tin deposits of southwest England demonstrates its use as a precise chronometer of mineralization. The concentrations of light rare earth elements (LREEs) in the fluorites are highly variable and suggest the coeval precipitation of an LREE-rich phase as the most likely cause of the extreme variation in samarium/neodymium ratios.

  2. Fluid inclusion study of some Sarrabus fluorite deposits, Sardinia, Italy.

    USGS Publications Warehouse

    Belkin, H.E.; de Vivo, B.; Valera, R.

    1984-01-01

    Fluid inclusions in six deposits of fluorite fracture fillings associated with Hercynian (Carboniferous) cycle magmatism were studied by microthermometric techniques. All the inclusions were liquid dominated, aqueous, and homogenized in the liquid phase. One-phase (liquid), two-phase (liquid + vapour) and three-phase (liquid, vapour, and solid NaCl daughter mineral) fluid inclusions were noted. This study indicates that five of the fluorite deposits formed from 95o-125oC fluids with approx 15 wt.% NaCl. One other deposit appears to have been formed by very dilute solutions at approx 125oC. It is suggested that the local fluorite-forming process was the formation of fracture-localized hydrothermal systems in which magmatic water interaction with some other fluid-connate, meteoric, or marine.-G.J.N.

  3. Ba(OH)2 Equilibria in the System Ba-O-H-F, With Application to the Formation of Ba2YCu3O6.5 + x From BaF2-Precursors

    PubMed Central

    Cook, L. P.; Wong-Ng, W.; Feenstra, R.

    2005-01-01

    The ex situ process for fabricating Ba2YCu3O6.5 + x superconducting tapes from BaF2- based precursors involves a hydration/oxidation reaction at ≈730 °C to 750 °C generally written as: (2BaF2+Y+3Cu)(amorphous)+(2H2O+2.25O2)(g)→Ba2YCu3O6.5+x(s)+4HF(g). However, microscopic observations of partially processed films suggest the presence of a transient liquid phase during conversion. Alternatively, the conversion reaction can be rewritten as the sum of several intermediate steps, including the formation of a barium hydroxide liquid: (BaF2)(amorphous)+2H2O(g)→Ba(OH)2(liq)+2HF(g). To evaluate the possibility of a hydroxide liquid conversion step, thermodynamic calculations on the stability of Ba(OH)2(liq) have been completed from 500 °C to 900 °C at 0.1 MPa ptotal. Based on currently available data, the calculated phase diagrams suggest that a viable hydroxide reaction path exists in the higher part of this temperature range. The calculations indicate that Ba(OH)2(liq) may be stable at log pH2O (Pa) values from ≈4 to 5, provided log pHF (Pa) values can be maintained below 0 to −1. Limited experimental confirmation is provided by results of an experiment on BaF2(s) at 815 °C, 0.1 MPa pH2O, in which essentially all F at the surface was replaced by O. It is therefore possible that processing routes exist for producing Ba2YCu3O6.5 + x based on the presence of a Ba(OH)2 liquid, which might have an effect on conversion rates and texturing in the superconducting film. PMID:27308108

  4. The quasi-binary phase diagram BaF2-BaBr2 and its relation to the x-ray storage phosphor BaFBr : Eu2+

    NASA Astrophysics Data System (ADS)

    Kolb, R.; Schlapp, M.; Hesse, S.; Schmechel, R.; von Seggern, H.; Fasel, C.; Riedel, R.; Ehrenberg, H.; Fuess, H.

    2002-08-01

    In order to understand the formation and stoichiometry of the x-ray storage phosphor BaFBr : Eu2+, the phase diagram of the quasi-binary BaF2-BaBr2 system has been investigated. The phase diagram was obtained by means of differential thermal analysis and temperature controlled x-ray diffraction experiments. The resulting phase diagram indicates that BaFBr forms a compound with no detectable solid solubility for neither BaF2 nor BaBr2. Experiments to obtain non-stoichiometric BaFBr via the synthesis route using BaF2 and NH4Br as proposed in the literature could not be verified. It will be shown that the type of colour centre created during x-ray irradiation is related to the non-stoichiometry of the starting compositions before sintering. A surplus of either barium fluoride or barium bromide during sintering allows the controlled formation of F(Br-)- and F(F-)-centres, respectively.

  5. Stability of fluorite-type La2Ce2O7 under extreme conditions

    SciTech Connect

    Zhang, F. X.; Tracy, C. L.; Lang, M.; Ewing, R. C.

    2016-03-03

    Here, the structural stability of fluorite-type La2Ce2O7 was studied at pressure up to ~40 GPa and under hydrothermal conditions (~1 GPa, 350 °C), respectively, using synchrotron x-ray diffraction (XRD) and Raman scattering measurements. XRD measurements indicated that fluorite-type La2Ce2O7 is not stable at pressures greater than 22.6 GPa and slowly transforms to a high-pressure phase. The high-pressure phase is not stable and changes back to the fluorite-type structure when pressure is released. The La2Ce2O7 fluorite is also not stable under hydrothermal conditions and begins to react with water at 200~250 °C. Both Raman and XRD results suggest that lanthanum hydroxide La(OH)3 and La3+-doped CeO2 fluorite are the dominant products after hydrothermal treatment.

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

    SciTech Connect

    Karthik, Chinnathambi; Anderson, Thomas J.; Gout, Delphine; Ubic, Rick

    2012-10-15

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

  7. Adsorption and self-assembly of octyl hydroxamic acid at a fluorite surface as revealed by sum-frequency vibrational spectroscopy.

    PubMed

    Wang, Xuming; Liu, Jin; Miller, Jan D

    2008-09-15

    In the study described here, the surface structure of a self-assembly octyl hydroxamic acid at a calcium fluoride (CaF(2)) surface is evaluated using sum-frequency vibrational spectroscopy (SFVS). Of particular significance are the results that show octyl hydroxamic acid adsorbs at the fluorite surface from octanol solution and has more ordering and molecular conformation than the octyl hydroxamic acid adsorbed from solution. At the fluorite/0.1 M octyl hydroxamic acid octanol solution interface a bilayer-like structure consisting of an octyl hydroxamic acid layer in contact with fluorite and a tilted alcohol layer was observed by SFVS. The alcohol molecules are oriented with respect to the hydroxamic acid monolayer with the OH groups directed towards the bulk alcohol phase and the terminal CH(3) group oriented to face the alkyl chains of the hydroxamic acid monolayer.

  8. Fluorite transition metal hydride induced destabilization of the MgH2 system in MgH2/TMH2 multilayers ( TM=Sc , Ti, V, Cr, Y, Zr, Nb, La, Hf)

    NASA Astrophysics Data System (ADS)

    Tao, S. X.; Notten, P. H. L.; van Santen, R. A.; Jansen, A. P. J.

    2010-09-01

    The structural changes in MgH2 induced by contact with fluorite transition metal hydrides ( TMH2 , TM=Sc , Ti, V, Cr, Y, Zr, Nb, La, Hf) have been studied using density-functional theory calculations. Models of MgH2(rutile)/TiH2(fluorite) and MgH2(fluorite)/TiH2(fluorite) multilayers with different Mg:TM ratios have been designed. With a fixed thickness of the TMH2 layer, structure transformation of MgH2 from rutile to fluorite occurs with a decrease in thickness of the MgH2 layer. The hydrogen desorption energy from the fluorite MgH2 layer in the multilayers is significantly lower than that of the bulk rutile MgH2 . The structural deformation of the MgH2 layer due to the strain induced by TMH2 is found to be responsible for the destabilization of the Mg-H bond: the more structural deformation, the more destabilization of the Mg-H. Our results provide an important insight for the development of new hydrogen-storage materials with desirable thermodynamic properties.

  9. Influence of ions on two-dimensional and three-dimensional atomic force microscopy at fluorite-water interfaces

    NASA Astrophysics Data System (ADS)

    Miyazawa, K.; Watkins, M.; Shluger, A. L.; Fukuma, T.

    2017-06-01

    Recent advancement in liquid-environment atomic force microscopy (AFM) has enabled us to visualize three-dimensional (3D) hydration structures as well as two-dimensional (2D) surface structures with subnanometer-scale resolution at solid-water interfaces. However, the influence of ions present in solution on the 2D- and 3D-AFM measurements has not been well understood. In this study, we perform atomic-scale 2D- and 3D-AFM measurements at fluorite-water interfaces in pure water and a supersaturated solution of fluorite. The images obtained in these two environments are compared to understand the influence of the ions in solution on these measurements. In the 2D images, we found clear difference in the nanoscale structures but no significant difference in the atomic-scale contrasts. However, the 3D force images show clear difference in the subnanometer-scale contrasts. The force contrasts measured in pure water largely agree with those expected from the molecular dynamics simulation and the solvent tip approximation model. In the supersaturated solution, an additional force peak is observed over the negatively charged fluorine ion site. This location suggests that the observed force peak may originate from cations adsorbed on the fluorite surface. These results demonstrate that the ions can significantly alter the subnanometer-scale force contrasts in the 3D-AFM images.

  10. [Assessment of Soil Fluorine Pollution in Jinhua Fluorite Ore Areas].

    PubMed

    Ye, Qun-feng; Zhou, Xiao-ling

    2015-07-01

    The contents of. soil total fluorine (TF) and water-soluble fluorine (WF) were measured in fluorite ore areas located in Jinhua City. The single factor index, geoaccumulation index and health risk assessment were used to evaluate fluorine pollution in soil in four fluorite ore areas and one non-ore area, respectively. The results showed that the TF contents in soils were 28. 36-56 052. 39 mg.kg-1 with an arithmetic mean value of 8 325.90 mg.kg-1, a geometric mean of 1 555. 94 mg.kg-1, and a median of 812. 98 mg.kg-1. The variation coefficient of TF was 172. 07% . The soil WF contents ranged from 0. 83 to 74. 63 mg.kg-1 with an arithmetic mean value of 16. 94 mg.kg-1, a geometric mean of 10. 59 mg.kg-1, and a median of 10. 17 mg.kg-1. The variation coefficient of WF was 100. 10%. The soil TF and WF contents were far higher than the national average level of the local fluorine epidemic occurrence area. The fluoride pollution in soil was significantly affected by human factors. Soil fluorine pollution in Yangjia, Lengshuikeng and Huajie fluorite ore areas was the most serious, followed by Daren fluorite ore area, and in non-ore area there was almost no fluorine pollution. Oral ingestion of soils was the main exposure route. Sensitivity analysis of model parameters showed that children's weight exerted the largest influence over hazard quotient. Furthermore, a significant positive correlation was found among the three kinds of evaluation methods.

  11. Thermal stability and thermal expansion studies of cubic fluorite-type MgF{sub 2} up to 135 GPa

    SciTech Connect

    Sun, X.W.; Song, T.; Wei, X.P.; Quan, W.L.; Liu, X.B.; Su, W.F.

    2014-04-01

    Highlights: • The thermal expansion of MgF{sub 2} with a fluorite-type structure has been investigated. • The quasi-harmonic Debye model is applied to take into account the thermal effect. • Particular attention is paid to the prediction of thermal expansion for the first time. - Abstract: The thermal expansion of MgF{sub 2} with a fluorite structure has been investigated at high pressures using plane-wave pseudopotential scheme within the local density approximation correction in the frame of density functional theory based on the analysis of thermal stability using classical molecular dynamics simulations up to 6500 K. To investigate the thermodynamic properties like as the P–V–T equation of state and volumetric thermal expansion coefficient α{sub V} of cubic fluorite-type MgF{sub 2} at extended pressure and temperature ranges, we apply the quasi-harmonic Debye model in which the phononic effects are considered. The P–V relationship and α{sub V} dependence of the pressure up to 135 GPa at different temperatures, and the V–T relationship and α{sub V} dependence of the temperature up to the melting temperature 1500 K at different pressures have been obtained.

  12. Methods of capturing and immobilizing radioactive nuclei with metal fluorite-based inorganic materials

    SciTech Connect

    Wang, Yifeng; Miller, Andy; Bryan, Charles R; Kruichar, Jessica Nicole

    2015-04-07

    Methods of capturing and immobilizing radioactive nuclei with metal fluorite-based inorganic materials are described. For example, a method of capturing and immobilizing radioactive nuclei includes flowing a gas stream through an exhaust apparatus. The exhaust apparatus includes a metal fluorite-based inorganic material. The gas stream includes a radioactive species. The radioactive species is removed from the gas stream by adsorbing the radioactive species to the metal fluorite-based inorganic material of the exhaust apparatus.

  13. Methods of capturing and immobilizing radioactive nuclei with metal fluorite-based inorganic materials

    SciTech Connect

    Wang, Yifeng; Miller, Andy; Bryan, Charles R.; Kruichak, Jessica Nicole

    2015-11-17

    Methods of capturing and immobilizing radioactive nuclei with metal fluorite-based inorganic materials are described. For example, a method of capturing and immobilizing radioactive nuclei includes flowing a gas stream through an exhaust apparatus. The exhaust apparatus includes a metal fluorite-based inorganic material. The gas stream includes a radioactive species. The radioactive species is removed from the gas stream by adsorbing the radioactive species to the metal fluorite-based inorganic material of the exhaust apparatus.

  14. Characterization of epitaxial EuS(111) thin films on BaF2(111) and SrF2(111) substrates grown by molecular beam epitaxy

    NASA Astrophysics Data System (ADS)

    Senba, Shinya; Matsumoto, Naoki; Jomura, Mitsuhiro; Asada, Hironori; Koyanagi, Tsuyoshi; Kishimoto, Kengo; Fukuma, Yasuhiro

    2013-06-01

    We have successfully grown EuS (111) epitaxial films on BaF2 (111) and SrF2 (111) substrates by using molecular beam epitaxy at substrate temperatures ( T S 's) between 100 and 500 °C. Pole figures of X-ray diffraction indicate a high degree of in-plane orientation, and all of the samplesshow very high resistivity. The surface roughness for 10-nm-thick EuS films on BaF2 (111) and SrF2 (111) substrates measured by using atomic force microscopy (AFM) are 0.122 and 0.092 nm, respectively. The Curie temperature of the EuS films increases up to ˜16 K with increasing T S . We also try to manipulate the coercive force, which is an important magnetic property, by Te-doping to achieve an anti-parallel magnetization state between two ferromagnetic layers in spin devices. The obtained coercive force for the Te-doped film (110 Oe) is large compared with that for the undoped one (20 Oe).

  15. Origin of fluorite mineralizations in the Nuba Mountains, Sudan and their rare earth element geochemistry

    NASA Astrophysics Data System (ADS)

    Ismail, Ibrahim; Baioumy, Hassan; Ouyang, Hegen; Mossa, Hesham; Aly, Hisham Fouad

    2015-12-01

    Among other mineralizations in the basement complex of the Nuba Mountains, fluorite occurs as lenses and veins in a number of localities. The rare earth elements (REE) geochemistry in these fluorites along with their petrography and fluid inclusion was investigated in this study to discuss the origin the fluorites and shed the light on the economic importance of the REE. Fluorites in the Nuba Mountains are classified into four categories based on their petrography. Category I (F1) is characterized by pink color and free of inclusions. Category II (F2) is zoned of alternating pink and colorless zones with euhedral outline or anhedral patchy pink and colorless fluorite enclosing category I fluorite and is usually sieved with submicroscopic silicate minerals. Category III (F3) is colorless, euhedral to anhedral fluorite and associated with quartz and/or orthoclase. Category IV (F4) is colorless, either massive or dispersed, corroded grains associated with calcite and pertain to the late introduced carbonatites in Dumbeir area. Gangue minerals in the studied fluorites include quartz, calcite, orthoclase and muscovite. The ΣREE ranges between 541 and 10,430 ppm with an average of 3234 ppm. Chondrite-normalized REE patterns for fluorite from different localities exhibit LREE enrichment relative to HREE as shown by (La/Yb)N ratios that vary from 16 to 194 and significant positive Eu anomalies that are pronounced with Eu/Eu* from 1.1 to 2.5. The Tb/La and Tb/Ca ratios of fluorites in the present study indicate that they plot mainly in the pegmatitic or high-hydrothermal field with the characteristics of primary crystallization and remobilization trend. The clear heterogeneity of fluorite, abundance of growth zones, irregular shapes of grains, presence of fluorite inclusions in other minerals as well as the relatively high concentration of REE in the studied fluorites are supportive for this interpretation. The relatively high Tb/La (0.002-0.013) and low Tb/Ca (0

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

    NASA Astrophysics Data System (ADS)

    Masui, Toshiyuki; Nagai, Ryosuke; Imanaka, Nobuhito

    2014-12-01

    Cubic fluorite-type solid solutions based on Pr6O11 and CeO2 were synthesized and oxide anion vacancies were intentionally introduced into the cubic fluorite-type lattice through the charge compensating mechanism by Mg2+ and/or Ca2+ doping into their lattices. The oxide anion vacancies bring about positive effect on NO decomposition catalysis. The reason for the increase in the catalytic activity was attributed to defect fluorite-type structures close to the C-type cubic one, because C-type cubic rare earth oxides, in which one-quarter of the oxygen atoms in the fluorite-type structure are removed, show high NO decomposition activity. In particular, the positive effect of the formation of oxide anion vacancies was significant for Pr6O11 and its solid solutions, because the molar volume of Pr6O11 is larger than that of CeO2, and Pr6O11 contains Pr3+ as well as Pr4+ and thereby a small amount of oxide anion vacancies exist inherently in the lattice.

  17. Electrolytic coloration and spectral properties of natural fluorite crystals containing oxygen impurities.

    PubMed

    Gu, Hongen; Ma, Dongliang; Chen, Weiwei; Zhu, Rui; Li, Yutong; Li, Yang

    2011-11-01

    Natural fluorite crystals containing oxygen impurities are colored electrolytically by using a pointed cathode and a flat anode at various temperatures and voltages. F and F(2) color centers are produced in colored fluorite crystals. O(2-)-V(a)(+), O(2-)-V(a)(+) aggregate, Yb(2+), Ce(3+) and Sm(2+) absorption bands are observed in absorption spectra of uncolored fluorite crystals. O(2-)-V(a)(+), O(2-)-V(a)(+) aggregate, Yb(2+), Ce(3+), Sm(2+), F, M (F(2)) absorption bands and group of four absorption bands are observed simultaneously in absorption spectra of colored fluorite crystals. Current-time curve for electrolytic coloration of natural fluorite crystal and its relationship with electrolytic coloration process are given. Production and conversion of color centers are explained.

  18. A new fluorite type compound Pb 5Bi 17X 5O 43: synchrotron and neutron structure determination (X=P) and conduction properties (X=P, V and As)

    NASA Astrophysics Data System (ADS)

    Roussel, Pascal; Giraud, Sophie; Suard, Emmanuelle; Wignacourt, Jean-Pierre; Steinfink, Hugo

    2002-09-01

    Recently, many new compounds have been discovered in the ternary system PbO-Bi 2O 3-P 2O 5: PbBi 6P 4O 20, nPbO-BiPO 4 (n=1, 2, 4), and Pb 5Bi 18P 4O 42. The structure of Pb 5Bi 18P 4O 42 was solved in a monoclinic I-centered cell, space group I2/ m. The structure of a new compound Pb 5Bi 17P 5O 43, monoclinic I-centered cell, is closely related and has been solved in the non centro symmetric Im space group from a joint refinement of X ray synchrotron single-crystal data and a powder neutron diffraction data; the cell parameters are a=11.341(2) Å, b=16.604(3) Å, c=11.432(2) Å, β=93.73(3)°. The atoms of lead, bismuth and phosphorus take the same positions as the bismuth atoms in the structure of δ-Bi 2O 3 with a distorted 3×3×3 superstructure of a tetragonal variety, as in Pb 5Bi 18P 4O 42. The homologous vanadate and arsenate phases are isostructural. Conductivity measurements were performed on the compounds of the series from 300°C to 800°C. The vanadate phase has the highest conductivity. An increase of the volume of the unit cell from the phosphate to the vanadate eases the ion migration through the structure.

  19. Manganese Vanadate Chemistry in Hydrothermal BaF 2 Brines: Ba 3 Mn 2 (V 2 O 7 ) 2 F 2 and Ba 7 Mn 8 O 2 (VO 4 ) 2 F 23

    DOE PAGES

    Sanjeewa, Liurukara D.; McMillen, Colin D.; McGuire, Michael A.; ...

    2016-12-05

    We synthesized manganese vanadate fluorides using high-temperature hydrothermal techniques with BaF2 as a mineralizer. Ba3Mn2(V2O7)2F2 crystallizes in space group C2/c and consists of dimers built from edge-sharing MnO4F2 trigonal prisms with linking V2O7 groups. Ba7Mn8O2(VO4)2F23 crystallizes in space group Cmmm, with a manganese oxyfluoride network built from edge- and corner-sharing Mn2+/3+(O,F)6 octahedra. The resulting octahedra form alternating Mn2+ and Mn2+/3+ layers separated by VO4 tetrahedra. This latter compound exhibits a canted antiferromagnetic order below TN = 25 K.

  20. Symmetry-guided synthesis of highly porous metal-organic frameworks with fluorite topology.

    PubMed

    Zhang, Muwei; Chen, Ying-Pin; Bosch, Mathieu; Gentle, Thomas; Wang, Kecheng; Feng, Dawei; Wang, Zhiyong U; Zhou, Hong-Cai

    2014-01-13

    Two stable, non-interpenetrated MOFs, PCN-521 and PCN-523, were synthesized by a symmetry-guided strategy. Augmentation of the 4-connected nodes in the fluorite structure with a rigid tetrahedral ligand and substitution of the 8-connected nodes by the Zr/Hf clusters yielded MOFs with large octahedral interstitial cavities. They are the first examples of Zr/Hf MOFs with tetrahedral linkers. PCN-521 has the largest BET surface area (3411 m(2) g(-1)), pore size (20.5×20.5×37.4 Å) and void volume (78.5%) of MOFs formed from tetrahedral ligands. This work not only demonstrates a successful implementation of rational design of MOFs with desired topology, but also provides a systematic way of constructing non-interpenetrated MOFs with high porosity.

  1. Symmetry-Guided Synthesis of Highly Porous Metal-Organic Frameworks with Fluorite Topology

    SciTech Connect

    Zhang, MW; Chen, YP; Bosch, M; Gentle, T; Wang, KC; Feng, DW; Wang, ZYU; Zhou, HC

    2013-11-11

    Two stable, non-interpenetrated MOFs, PCN-521 and PCN-523, were synthesized by a symmetry-guided strategy. Augmentation of the 4-connected nodes in the fluorite structure with a rigid tetrahedral ligand and substitution of the 8-connected nodes by the Zr/Hf clusters yielded MOFs with large octahedral interstitial cavities. They are the first examples of Zr/Hf MOFs with tetrahedral linkers. PCN-521 has the largest BET surface area (3411 m(2) g(-1)), pore size (20.5 x 20.5 x 37.4 angstrom) and void volume (78.5%) of MOFs formed from tetrahedral ligands. This work not only demonstrates a successful implementation of rational design of MOFs with desired topology, but also provides a systematic way of constructing non-interpenetrated MOFs with high porosity.

  2. Crystal growth, structural transitions, and magnetic properties of the fluorite-related osmates: Sm3OsO7, Eu3OsO7, and Gd3OsO7.

    PubMed

    Gemmill, William R; Smith, Mark D; Mozharivsky, Yurij A; Miller, Gordon J; zur Loye, Hans-Conrad

    2005-10-03

    The Ln3OsO7 (Ln = Sm, Eu, Gd) compounds were grown as single crystals from molten hydroxide fluxes. At temperatures above 235, 330, and 430 K, respectively, the Ln3OsO7 (Ln = Sm, Eu, Gd) compounds exist in the orthorhombic space group Cmcm. When they are cooled below these temperatures, the compounds undergo a structural phase transition from space group Cmcm to P2(1)nb. The structure transition results in a loss of lattice centering, a doubling of the b axis, a distortion of the vertex-shared Os-O chains, and a reduction in the coordination of one of the rare earth cations from 8-fold to 7-fold. Sm3OsO7 and Eu3OsO7 exhibit complex magnetic behavior below about 50 K, and Gd3OsO7 shows a ferromagnetic-like order at 34 K in applied fields of less than 10 kG.

  3. Damage evolution of ion irradiated defected-fluorite La 2 Zr 2 O 7 epitaxial thin films

    DOE PAGES

    Kaspar, Tiffany C.; Gigax, Jonathan G.; Shao, Lin; ...

    2017-05-01

    Pyrochlore-structure oxides, A2B2O7, may exhibit remarkable radiation tolerance due to the ease with which they can accommodate disorder by transitioning to a defected fluorite structure. In this paper, the mechanism of defect formation was explored by evaluating the radiation damage behavior of high quality epitaxial La2Zr2O7 thin films with the defected fluorite structure, irradiated with 1 MeV Zr+ at doses up to 10 displacements per atom (dpa). The level of film damage was evaluated as a function of dose by Rutherford backscattering spectrometry in the channeling geometry (RBS/c) and scanning transmission electron microscopy (STEM). At lower doses, the surface ofmore » the La2Zr2O7 film amorphized, and the amorphous fraction as a function of dose fit well to a stimulated amorphization model. As the dose increased, the surface amorphization slowed, and amorphization appeared at the interface. Even at a dose of 10 dpa, the core of the film remained crystalline, despite the prediction of amorphization from the model. To inform future ab initio simulations of La2Zr2O7, the bandgap of a thick La2Zr2O7 film was measured to be indirect at 4.96 eV, with a direct transition at 5.60 eV.« less

  4. Damage evolution of ion irradiated defected-fluorite La2Zr2O7 epitaxial thin films

    DOE PAGES

    Kaspar, Tiffany C.; Gigax, Jonathan G.; Shao, Lin; ...

    2017-01-13

    Pyrochlore-structure oxides, A2B2O7, may exhibit remarkable radiation tolerance due to the ease with which they can accommodate disorder by transitioning to a defected fluorite structure. In this paper, the mechanism of defect formation was explored by evaluating the radiation damage behavior of high quality epitaxial La2Zr2O7 thin films with the defected fluorite structure, irradiated with 1 MeV Zr+ at doses up to 10 displacements per atom (dpa). The level of film damage was evaluated as a function of dose by Rutherford backscattering spectrometry in the channeling geometry (RBS/c) and scanning transmission electron microscopy (STEM). At lower doses, the surface ofmore » the La2Zr2O7 film amorphized, and the amorphous fraction as a function of dose fit well to a stimulated amorphization model. As the dose increased, the surface amorphization slowed, and amorphization appeared at the interface. Even at a dose of 10 dpa, the core of the film remained crystalline, despite the prediction of amorphization from the model. To inform future ab initio simulations of La2Zr2O7, the bandgap of a thick La2Zr2O7 film was measured to be indirect at 4.96 eV, with a direct transition at 5.60 eV.« less

  5. Accommodation of multivalent cations in fluorite-type solid solutions: Case of Am-bearing UO2

    NASA Astrophysics Data System (ADS)

    Prieur, Damien; Martin, Philippe; Lebreton, Florent; Delahaye, Thibaud; Banerjee, Dipanjan; Scheinost, Andreas C.; Jankowiak, Aurélien

    2013-03-01

    The radiotoxicity of spent nuclear fuel is mainly due to the content of minor actinides, which could be substantially reduced by Partitioning and Transmutation. A possible transmutation method would be to employ americium-bearing uranium oxide materials as blanket fuels in fast neutron reactors. In order to maintain fuel performance and reactor safety, it is mandatory to control the structural homogeneity and oxygen stoichiometry during the sintering process. In this work, U0.85Am0.15O2-x materials, fabricated by a solid state chemistry process, were sintered at 2023 K under three oxygen potentials, i.e. -375, -350 and -325 kJ mol-1, thereby significantly extending the range of a previous study. By coupling X-ray diffraction and X-ray absorption spectroscopy measurements, it was shown that fluorite solid solutions are obtained whatever the sintering conditions. The presence of U(+V), pointed out in a previous work for oxygen potentials equal to -520 and -450 kJ mol-1, was confirmed. This result constitutes an experimental proof of the existence of U(+V) in An-doped UO2 fluorite-type structure materials. These experimental data are major results in view of the developing thermodynamical model of the U-Am-O system. Considering the now available extended range, the effect of the oxygen potential is discussed in terms of charge distribution and local structure.

  6. Elastic deformation in ceria nanorods via a fluorite-to-rutile phase transition.

    PubMed

    Sayle, Thi X T; Sayle, Dean C

    2010-02-23

    Atomistic simulations reveal that ceria nanorods, under uniaxial tension, can accommodate over 6% elastic deformation. Moreover, a reversible fluorite-to-rutile phase change occurs above 6% strain for a ceria nanorod that extends along [110]. We also observe that during unloading the stress increases with decreasing strain as the rutile reverts back to fluorite. Ceria nanorods may find possible application as vehicles for elastic energy storage.

  7. A new anion-deficient fluorite-related superstructure of Bi{sub 28}V{sub 8}O{sub 62}

    SciTech Connect

    Đorđević, T.; Karanović, Lj.

    2014-12-15

    New hydrothermally synthesized Bi{sub 28}V{sub 8}O{sub 62} was structurally characterized using single-crystal X-ray diffraction data. Bi{sub 28}V{sub 8}O{sub 62} crystallizes in the novel type of defect fluorite structure related to the face-centered cubic δ-Bi{sub 2}O{sub 3}. It is monoclinic, s. g. P2{sub 1}/c, and the relation to the fluorite subcell is given as a∼(3/2)a{sub F}+(3/2)c{sub F}; b∼ −b{sub F}; c∼2a{sub F} −4c{sub F} (F in subscript indicate the unit cell parameter of fluorite). Its structure is characterized by slabs of edge sharing OBi{sub 4} tetrahedra surrounded by the OBi{sub 3} triangles. As a part of these OBi{sub 3} triangles, two positionally disordered Bi{sup 3+} cations were observed in the marginal part of the slabs. The slabs are extending along b axis and are linked by inter-slab portion of the structure composed of VO{sub 4} tetrahedra and BiO{sub 6−x} coordination polyhedra, where x is a number of vacant oxygen sites. Raman spectra verified the coordination environment of vanadium atoms in the structure. - Graphical abstract: The [4{sup ¯}01] projection of two slabs and inter-slab part of the structure in one layer parallel to the (3{sup ¯}08)=(002{sup ¯}){sub F} plane (F in subscript indicate a fluorite type structure). The large green circles are Bi atoms. Small blue circles represent partly and fully occupied O sites, respectively. Pink (hatched black) are V1O{sub 4} and blue (hatched white) are V2O{sub 4} coordination tetrahedra. - Highlights: • Single crystals of Bi{sub 28}V{sub 8}O{sub 62} were grown using hydrothermal technique. • The crystal structure of Bi{sub 28}V{sub 8}O{sub 62} was solved using single-crystal XRD method. • Bi{sub 28}V{sub 8}O{sub 62} has an anion-deficient fluorite-related superstructure. • Raman spectrum confirmed the coordination environment of vanadium atoms. • Relation to the structurally related compound was discussed.

  8. Isoelectric point of fluorite by direct force measurements using atomic force microscopy.

    PubMed

    Assemi, Shoeleh; Nalaskowski, Jakub; Miller, Jan D; Johnson, William P

    2006-02-14

    Interaction forces between a fluorite (CaF2) surface and colloidal silica were measured by atomic force microscopy (AFM) in 1 x 10(-3) M NaNO3 at different pH values. Forces between the silica colloid and fluorite flat were measured at a range of pH values above the isoelectric point (IEP) of silica so that the forces were mainly controlled by the fluorite surface charge. In this way, the IEP of the fluorite surface was deduced from AFM force curves at pH approximately 9.2. Experimental force versus separation distance curves were in good agreement with theoretical predictions based on long-range electrostatic interactions, allowing the potential of the fluorite surface to be estimated from the experimental force curves. AFM-deduced surface potentials were generally lower than the published zeta potentials obtained from electrokinetic methods for powdered samples. Differences in methodology, orientation of the fluorite, surface carbonation, and equilibration time all could have contributed to this difference.

  9. Enhanced ultraviolet upconversion luminescence of Tm and Yb codoped ZrF4-BaF2-LaF3-AlF3-NaF glass.

    PubMed

    He, Chunfeng; Zhao, Dan; Qin, Guanshi; Zheng, Kezhi; Qin, Weiping

    2011-11-01

    Ultraviolet (UV) upconversion (UC) luminescence properties of Tm3+ ions sensitized by Yb3+ ions in ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) glass were studied in detail. Under the excitation from a 980 nm continuous wave (CW) diode laser, red, blue, and even UV emissions were observed in the fluorozirconate glass. Several fluorescence bands appeared in the UC emission spectrum from 292.8 nm to 805.8 nm. The UC emission peaks at 291 nm, 347 nm, 363 nm, 454 nm, 475 nm, 647 nm, 687 nm, and 804 nm correspond to the transitions of 1I6 --> 3H6, 1I6 --> 3F4, 1D2 --> 3H6, 1D2 --> 3F4, 1G4 --> 3H6, 1G4 --> 3F4, 3F3 --> 3H6, and 3H4 --> 3H6, respectively. Experimental results of intensity dependence of the up-converted fluorescence on the pump power indicate a five-photon excitation scheme of 1I6 energy level.

  10. DANCE (Detector for Advanced Neutron Capture Experiments) is a 4π array of BaF2 crystals installed at LANSCE, Lujan Center. Neutron capture measurements on ^157Gd and ^89Y nuclei were conducted using this facility.

    NASA Astrophysics Data System (ADS)

    Chyzh, A.; Mitchell, G.; Vieira, D.; Bredeweg, T.; Ullmann, J.; Jandel, M.; Couture, A.; Keksis, A.; Rundberg, R.; Wilhelmy, J.; O'Donnell, J.; Baramsai, B.; Haight, R.; Wouters, J.; Krticka, M.; Parker, W.; Becker, J.; Agvaanlusan, U.

    2009-10-01

    DANCE (Detector for Advanced Neutron Capture Experiments) is a 4π array of BaF2 crystals installed at LANSCE, Lujan Center. Neutron capture measurements on ^157Gd and ^89Y nuclei were conducted using this facility. The absolute cross sections of the ^89Y(n,γ) reaction was measured for the first time ever in the neutron energy range of 10 eV -- 10 keV and improvements were made in the 10 -- 300 keV range. The error bars were significantly reduced and number of cross section points was increased since the past ^89Y(n,γ) experiments. The ^157Gd(n,γ) cross section was determined at En = 20 eV -- 300 keV by normalizing the experimental DANCE data to a well known resonance taken from the ENDF/B-VII library. Computer simulations of the ^157Gd(n,γ) cascades and DANCE pulse height function were made using DICEBOX and GEANT4 codes and simulated Esum and Eγ spectra are compared to the experimental DANCE data. Values of spin and photon strength function (PSF) of the ^157Gd(n,γ) resonances are provided in the range of En = 2 -- 300 eV using spin dependence upon a γ-ray multiplicity.

  11. Vacuum ultraviolet thin films. I - Optical constants of BaF2, CaF2, LaF3, MgF2, Al2O3, HfO2, and SiO2 thin films. II - Vacuum ultraviolet all-dielectric narrowband filters

    NASA Technical Reports Server (NTRS)

    Zukic, Muamer; Torr, Douglas G.; Spann, James F.; Torr, Marsha R.

    1990-01-01

    An iteration process matching calculated and measured reflectance and transmittance values in the 120-230 nm VUV region is presently used to ascertain the optical constants of bulk MgF2, as well as films of BaF2, CaF2, LaF3, MgF2, Al2O3, HfO2, and SiO2 deposited on MgF2 substrates. In the second part of this work, a design concept is demonstrated for two filters, employing rapidly changing extinction coefficients, centered at 135 nm for BaF2 and 141 nm for SiO2. These filters are shown to yield excellent narrowband spectral performance in combination with narrowband reflection filters.

  12. Metastable NaYF 4 fluorite at high pressures and high temperatures

    NASA Astrophysics Data System (ADS)

    Grzechnik, Andrzej; Bouvier, Pierre; Crichton, Wilson A.; Farina, Luca; Köhler, Jürgen

    2002-06-01

    High-pressure high-temperature behavior of metastable NaYF 4 fluorite (Fm 3¯m, Z=4), with the Na and Y atoms randomly distributed in the cationic sublattice, is studied with synchrotron angle-dispersive powder X-ray diffraction in diamond anvil (DAC) and large-volume Paris-Edinburgh cells and synthesis in a multi-anvil apparatus. The onset of a pressure-induced phase transition at room temperature takes place above 10 GPa as observed in DACs loaded with different hydrostatic and non-hydrostatic pressure media (nitrogen, paraffin oil, or ethanol:methanol media). In situ powder X-ray diffraction measurements in the Paris-Edinburgh cell and syntheses using the multi-anvil apparatus at high pressures and high temperatures show that the new polymorph is of the gagarinite-type (P6 3/m, Z=1) with partially ordered cations, the formula being Na 1.5Y 1.5F 6. This phase is structurally related to the Na 1.5Y 1.5F 6 modification (P 6¯, Z=1) stable at ambient conditions. At higher temperatures, the new pressure-induced hexagonal variant of NaYF 4 eventually decomposes into a non-stoichiometric gagarinite-like phase and yttrium fluoride YF 3 (Pnma, Z=4).

  13. Fluid Origin of the Stratabound Fluorite and Celestite Deposits in Coahuila, Mexico.

    NASA Astrophysics Data System (ADS)

    Tritlla, J.; Gonzalez-Partida, E.; Banks, D.; Levresse, G.; Baca-Gasca, S.; Rodriguez-Santos, A.

    2004-12-01

    The Mesozoic units that outcrop north of Coahuila State in northern Mexico host numerous celestite, celestite-fluorite and fluorite deposits, mainly enclosed in the Cretaceous limestone units. Celestite and celestite-fluorite deposits are lens-shaped bodies, up to 2 m thick and a total length exceeding 500 meters, intercalated within the carbonates of the Cupido Formation (Aptian) or equivalents. Celestite-free fluorite bodies, mainly enclosed in the Aurora Formation (Albian-Cenomanian), appear as sub-concordant lenses with abundant evidences of hydraulic fracturing, usually near low-angle fractures. The celestite brine halogen composition on a Cl/Br vs Na/Br molar ratio plot on the trend defined by the evaporation of seawater, while in a ppm Cl vs Cl/Br plot away from the seawater evaporation line but parallel to it, indicating that the solution was mainly modified by dilution. The fluids involved in the genesis of the La Encantada fluorite deposit probably are evolved seawater that undergone some degree of evaporation. Mixing of fluids is clearly delineated by both microthemometric and halogen data, where salinities calculated are too low for the halogen ratios found, indicating a mixing with a low-salinity end-member. Hydrocarbon-bearing fluid inclusions repeatedly show the presence of solid bitumen trapped along with heavy oils, indicative of thermal degradation. Both, mixing and thermal degradation of hydrocarbon-rich fluids along with hydraulic fracture of the host rock points to an "in situ" organic matter maturation, due to the mixing of a saline, oxidant, sulphate and CaCl2-rich "bittern", which probably transported fluorite, with an organic-matter rich fluid present in the cretaceous carbonates. It is noteworthy that these previous results suggest that both celestite and fluorite-dominated deposits formed from brines from similar origin, showing the same halogen systematics. Strontium enrichment can be explained by leaching of Sr from the carbonate series

  14. Influence of glass composition on the properties of glass polyalkenoate cements. Part III: influence of fluorite content.

    PubMed

    De Barra, E; Hill, R G

    2000-03-01

    The influence of fluorite content of the glass on the formation and properties of glass polyalkenoate cements was investigated. A series of glass powders based on 1.5SiO2 x 0.5P2O5 x Al2O3 x CaO x XCaF2 were synthesised. The glass transition temperature of the glass fell with increasing fluorite content. Setting and working times of the cement pastes decreased with increasing fluorite content of the glass. Compressive strength and un-notched fracture strength increased with increasing fluorite content of the glass. Fracture toughness and toughness of the cements were relatively insensitive to fluorite content.

  15. Synthesis, crystal structures and ionic conductivities of Bi{sub 14}P{sub 4}O{sub 31} and Bi{sub 50}V{sub 4}O{sub 85}. Two members of the series Bi{sub 18-4m}M{sub 4m}O{sub 27+4m} (M=P, V) related to the fluorite-type structure

    SciTech Connect

    Mauvy, F.; Launay, J.C.; Darriet, J. . E-mail: darriet@icmcb-bordeaux.cnrs.fr

    2005-06-15

    The two hitherto unknown compounds Bi{sub 14}P{sub 4}O{sub 31} and Bi{sub 50}V{sub 4}O{sub 85} were prepared by the direct solid-state reaction of Bi{sub 2}O{sub 3} and (NH{sub 4})H{sub 2}PO{sub 4} or V{sub 2}O{sub 5}, respectively. Bi{sub 14}P{sub 4}O{sub 31} crystallizes in a C-centred monoclinic symmetry (C2/c space group) with the unit-cell parameters: a=19.2745(2)A, b=11.3698(1)A, c=52.4082(2)A and {beta}=93.63(1){sup o} (Z=16). The symmetry of Bi{sub 50}V{sub 4}O{sub 85} is also monoclinic (I2/m space group) with lattice parameters of a=11.8123(3)A, b=11.7425(2)A, c=16.5396(2)A and {beta}=90.14(1){sup o} (Z=2). Both structures correspond to a fluorite-type superstructure where the Bi and P or V atoms are ordered in the framework. An idealized structural model is proposed where the structures result of the stacking of mixed atomic layers of composition [Bi{sub 14}M{sub 4}O{sub 31}] and [Bi{sub 18}O{sub 27}] respectively. This new family can be formulated Bi{sub 18-4m}M{sub 4m}O{sub 27+4m} with M=P, V and where the parameter m (0=structural sequence is simply one [Bi{sub 14}V{sub 4}O{sub 31}] layer to two [Bi{sub 18}O{sub 27}] layers. As predicted by the proposed structural building principle, Bi{sub 14}P{sub 4}O{sub 31} is not a good ionic conductor. The conductivity at 650 deg. C is 4 orders of magnitude lower from those found in Bi{sub 46}M{sub 8}O{sub 89} (M=P, V) (m=2/3) and Bi{sub 50}V{sub 4}O{sub 85} (m=1/3)

  16. La10W2O21: an anion-deficient fluorite-related superstructure with oxide ion conduction.

    PubMed

    Chambrier, Marie-Hélène; Le Bail, Armel; Giovannelli, Fabien; Redjaïmia, Abdelkrim; Florian, Pierre; Massiot, Dominique; Suard, Emmanuelle; Goutenoire, François

    2014-01-06

    The crystal structure of La10W2O21, which has to be reformulated (La5.667W0.333)LaWO14□2, is best described, on average, by a 2 × 2 × 2 anion-deficient fluorite-related superstructure cubic cell, with space group F4 3m, Z = 4, and a = 11.17932(6) Å, similar to Y7ReO14--δ. The 32 cations are distributed with lanthanum on the 4a-site, tungsten on the 4b-site, and a partial occupancy of the 24g-site by La (94%) and W. The 56 oxygen atoms occupy four 16e-sites, three of them fully and with an occupancy of 1/2 for the fourth one. Others M10W2O21 (M = Er, Y) adopt a 3 × 2 × 2 fluorite superstructure with W in octahedral sites, whereas W is mainly in tetrahedral sites in La10W2O21. Several powerful techniques such as crystal image furnace synthesis, (139)La nuclear magnetic resonance (NMR) and convergent beam electron diffraction (CBED) were used to achieve our results. Transmission electron microscopy (microdiffraction, CBED, and Tanaka patterns) brought us the real symmetry, showing that indeed classical cubic twinning along the 3-fold axis does take place. The surprising La/W mixed site is nicely confirmed by (139)La NMR. This compound exhibits interesting fast oxide ion conducting properties, comparable with LAMOX (Lacorre et al. Nature 2000, 404, 856-858) at low temperature. As opposed to many ionic conductors, no temperature structural transition is observed. Its conductivity is about 6.4 × 10(-4) S·cm(-1) at 700 °C.

  17. SnK, PbL 3, and BaL 3 EXAFS, X-ray diffraction and 119Sn Mössbauer spectroscopic studies of ordered MSnF 4 ( M = Pb and Ba) fluoride ionic conductors with the α-PbSnF 4 structure

    NASA Astrophysics Data System (ADS)

    Denes, G.; Yu, Y. H.; Tyliszczak, T.; Hitchcock, A. P.

    1991-03-01

    The high performance fluoride ion conductors α-PbSnF 4 and BaSnF 4 have been studied using X-ray diffraction, 119Sn Mössbauer spectroscopy, and EXAFS. X-ray diffraction shows the unit-cell is a tetragonally distorted fluorite-type, with ordering of the metals which results in a superstructure along the c axis. The SnK, Pb L3, and Ba L3 X-ray absorption spectra of α-PbSnF 4 and BaSnF 4 as well as model compounds (SnO, β-PbF 2, and BaF 2) have been recorded at 300 and 77 K. Analysis of the extended fine structure (EXAFS) of α-PbSnF 4 and BaSnF 4 indicates they have a similar local structure around the corresponding metal atoms with average nearest neighbor distances of RSnF = 2.08(3) Å, RPbF = 2.50(3) Å and RBaF = 2.67(2) Å. The good agreement between the PbF and BaF distances derived from EXAFS with diffraction results and ion pair estimates indicates that the Pb and Ba ions determine the close-packing arrangement of the crystal structure. EXAFS shows that the local structure is much better defined around Pb and Ba than around Sn. This and the weak temperature dependence of the SnK EXAFS indicates a lower rigidity of next-neighbor fluorine shells around Sn than around Pb or Ba. In the α-PbSnF 4 structural type, the metal M (Pb or Ba) is in an eightfold coordination site similar to the cubic coordination in the fluorite-type, whereas Sn is in a unique SnF 5E pseudooctahedral coordination, with the lone pair E being stereoactive and making the materials strongly anisotropic. This is confirmed by the large quadrupole splitting observed in Mössbauer spectroscopy, Δ = 1.52(2) mm/s for α-PbSnF 4 and 1.52(1) mm/s for BaSnF 4. The mobile fluoride ions are probably disordered and widely spread over conduction paths, and they remain disordered even at low temperature when their long range motion is frozen. The structural results are discussed in relationship to proposed mechanisms for ionic conduction in these materials.

  18. Experimental observation of charge-shift bond in fluorite CaF2.

    PubMed

    Stachowicz, Marcin; Malinska, Maura; Parafiniuk, Jan; Woźniak, Krzysztof

    2017-08-01

    On the basis of a multipole refinement of single-crystal X-ray diffraction data collected using an Ag source at 90 K to a resolution of 1.63 Å(-1), a quantitative experimental charge density distribution has been obtained for fluorite (CaF2). The atoms-in-molecules integrated experimental charges for Ca(2+) and F(-) ions are +1.40 e and -0.70 e, respectively. The derived electron-density distribution, maximum electron-density paths, interaction lines and bond critical points along Ca(2+)...F(-) and F(-)...F(-) contacts revealed the character of these interactions. The Ca(2+)...F(-) interaction is clearly a closed shell and ionic in character. However, the F(-)...F(-) interaction has properties associated with the recently recognized type of interaction referred to as `charge-shift' bonding. This conclusion is supported by the topology of the electron localization function and analysis of the quantum theory of atoms in molecules and crystals topological parameters. The Ca(2+)...F(-) bonded radii - measured as distances from the centre of the ion to the critical point - are 1.21 Å for the Ca(2+) cation and 1.15 Å for the F(-) anion. These values are in a good agreement with the corresponding Shannon ionic radii. The F(-)...F(-) bond path and bond critical point is also found in the CaF2 crystal structure. According to the quantum theory of atoms in molecules and crystals, this interaction is attractive in character. This is additionally supported by the topology of non-covalent interactions based on the reduced density gradient.

  19. Active Laser and Raman Materials for 1.3-5 Micron Spectral Range

    DTIC Science & Technology

    2006-03-01

    transitions of rare- earth ions in fluorite type crystals”, in the Proceedings volume of the Advanced Solid-State Photonics 2004, author: Gregory Quarles...comparable with those for low phonon fluoride crystals like LaF3 and fluorite - type crystals like SrF2, CdF2, BaF2, and PbF2. Interestingly, the measured...impurities present in the samples. Oxygen - free CaF2: LaF3(0.25%): NdF3(0.25%) and SrF2: LaF3(1%): NdF3(0.2%) crystals with fluorite structure were

  20. Sedimentary fluorite in a lacustrine zeolitic tuff of the Gila Conglomerate near Buckhorn, Grant County, New Mexico.

    USGS Publications Warehouse

    Sheppard, R.A.; Mumpton, F.A.

    1984-01-01

    Fluorite makes up 20-30% of a zeolitic tuff in a Pliocene or Pleistocene lacustrine facies of the Gila Conglomerate that has not been subjected to hydrothermal activity. The fluorite occurs as prolate pellets and, rarely, as ooids that are mostly 0.1-0.3 mm in size are probably the result of primary precipitation of fluorite and magadiite where dilute, calcium-bearing water from springs or streams mixed with the saline, alkaline lake water that had a high fluorine content.-from Authors

  1. Integrated VLF and AMT survey for the exploration of a fluorite deposit at eastern Inner Mongolia, China

    NASA Astrophysics Data System (ADS)

    Zhang, L.; Wei, W.; Jin, S.; Ye, G.; Jia, D.; Dong, H.; Xie, C.

    2010-12-01

    Fluorite (calcium fluoride) is a common mineral in deposits of hydrothermal origin. It is a kind of important raw material for industrial production. China is the world's largest fluorite producer, taking almost 54% of the total production all over the world. The U.S. has also been relied heavily on Chinese fluorite imports. In China, VLF method is widely used for the exploration of fluorite deposits. But the distribution of fluorite in depth can not be imaged through this method because of its limited detecting depth. In this study, we adopted both VLF and AMT method for the exploration of a fluorite deposit at eastern Inner Mongolia of China. Fluorite deposit is mainly surrounded by granites in the target area. The electrical properties of host-rock and fluorite are all comparatively resistive. But the fluorite deposit still present as conductive anomaly in this region because the vein deposit of fluorite is distributed in fracture zones with water and metallic minerals. This survey consists of 18 VLF profiles with average length of 700m and 107 AMT sites distributed on 4 profiles overlapping with part of the VLF profiles. The VLF result shows two north-south stretching conductive anomaly zones, which are considered as two vein deposits of fluorite. The electrical model from 2D inversion of AMT data agree with the VLF result very well at shallow part, and give us more information about the distribution of fluorite deposit in deep part. Furthermore, the drilling results are also consistent with our 2D electrical model. While processing AMT data, we found that static shift could cause deeply extended false conductive anomalies in inversion results, especially in 1D TM mode. VLF models are very indicative for the removal of static shift caused by conductive objects near surface. We have also carried out a comprehensive study of the electrical model by using different algorithms for both 1D and 2D inversion of AMT data. It is concluded that 1D Occam inversion result

  2. Synthesis, properties and phase transitions of pyrochlore- and fluorite-like Ln{sub 2}RMO{sub 7} (Ln = Sm, Ho; R = Lu, Sc; M = Nb, Ta)

    SciTech Connect

    Shlyakhtina, A.V.; Belov, D.A.; Pigalskiy, K.S.; Shchegolikhin, A.N.; Kolbanev, I.V.; Karyagina, O.K.

    2014-01-01

    Graphical abstract: Temperature dependences of bulk conductivity for Sm{sub 2}ScTaO{sub 7} pyrochlore prepared at (1) 1400 °C, 20 h; and (2) 1200 °C, 40 h. - Highlights: • The phase formation of Ln{sub 2}RMO{sub 7} (Ln = Sm, Ho; R = Lu, Sc; M = Nb, Ta) at 1200–1600 °C. • The bulk conductivity and magnetic susceptibility were measured. • The bulk conductivity of Sm{sub 2}ScTaO{sub 7} has oxygen ion type at T ≥ 750 °C. • The first-order structural phase transition was observed in Sm{sub 2}ScTaO{sub 7} at ∼650–700 °C. • This phase transformation is not typical for defect fluorites. - Abstract: We have studied the new compounds with fluorite-like (Ho{sub 2}RNbO{sub 7} (R = Lu, Sc)) and pyrochlore-like (Sm{sub 2}ScTaO{sub 7}) structure as potential oxide ion conductors. The phase formation process (from 1200 to 1600 °C) and physical properties (electrical, thermo mechanical, and magnetic) for these compounds were investigated. Among the niobate materials the highest bulk conductivity is offered by the fluorite-like Ho{sub 2}ScNbO{sub 7} synthesized at 1600 °C: 3.8 × 10{sup −5} S/cm at 750 °C, whereas in Sm system the highest bulk conductivity, 7.3 × 10{sup −6} S/cm at 750 °C, is offered by the pyrochlore Sm{sub 2}ScTaO{sub 7} synthesized at 1400 °C. In Sm{sub 2}ScTaO{sub 7} pyrochlore we have observed the first-order phase transformation at ∼650–700 °C is related to rearrangement process in the oxygen sublattice of the pyrochlore structure containing B-site cations in different valence state and actually is absent in the defect fluorites. The two holmium niobates show Curie–Weiss paramagnetic behavior, with the prevalence of antiferromagnetic coupling. The magnetic susceptibility of Sm{sub 2}ScTaO{sub 7} is a weak function of temperature, corresponding to Van Vleck paramagnetism.

  3. Mineralogical, textural, geochemical and thermometric characteristics of Central Anatolian fluorites (Turkey): Tracing the origin of post-magmatic fluids

    NASA Astrophysics Data System (ADS)

    Cosanay, Pelin; Mutlu, Halim; Koc, Sükrü; Cevik, Nihal; Oztürk, Ceyda; Varol, Ece

    2016-04-01

    In this study, we investigate the spatial distribution of fluorite veins in Central Anatolia with emphasis on mineralogical, textural, geochemical and thermometric variations. The studied fluorite mineralizations (Kaman, Akçakent, Pöhrenk and Şefaatli mineralizations from west to east) are located on northern part of Kırşehir Massif which is a part of Central Anatolian Crystalline Complex that is bordered by the İzmir-Ankara-Erzincan Suture Zone. The Kaman, Akçakent and Şefaatli fluorite deposits are formed in association with magmatic rocks such as syenite and monzonite / monzodiorite in composition which are of Upper Cretaceous age. Fluorite in these deposits occurs as purple- and green-colored stockwork veins and/or disseminations along fault/fracture systems and is accompanied by quartz and rare pyrite. The Pöhrenk ore, however, is precipitated as space filling-breccia type within karstic voids of Eocene limestones and marl levels. The silicification/carbonatization and barite occurrences are found as the main alteration and secondary products of mineralization. Thickness of fluorite veins is between 2 and 30 cm. ΣREE contents of host rocks and fluorite veins are in the range of 2-806 ppm and 20-390 ppm, respectively. In element variation diagrams constructed for both host rocks and fluorite mineralizations, LREE concentrations are found to be greater than HREEs. REE contents of green-colored fluorites are about 10-fold higher than those of purple-colored ones. Negative Ce and Eu anomalies indicate high oxygen fugacity for the mineralizing fluids. Fluid inclusion studies indicated three different types of inclusions: 1) two-phase (liquid-vapor) primary and secondary inclusions, 2) single-phase (liquid) primary and secondary inclusions and 3) two-phase (liquid-vapor) and single-phase (liquid) pseudo-secondary inclusions. Results of homogenization temperatures from a number of about 200 measurements chiefly on fluorite and less often quartz and barite

  4. Geochemical associations between fluorite mineralization and A-type shoshonitic magmatism in the Keban-Elazig area, East Anatolia, Turkey

    NASA Astrophysics Data System (ADS)

    Akgul, Bunyamin

    2015-11-01

    Keban fluorite mineralizations are closely related Coniacian-Campanian subvolcanics intruded into Permian-Triassic Keban metamorphites; this event caused pyrometasomatic, porphyry, and vein-type Pb-Zn-Ag, Cu, W, and Pb-Zn-Ag-Mo-F mineralizations. These rocks are syenitic and syenomonzonitic in composition and have high Al2O3, alkali (Na2O + K2O), FeO*/MgO, Zr, Nb, Ta, Ga, Rb, Y, and rare earth element (REE) contents. They are A-type, metaluminous, and all fall in the shoshonitic series field in K2O vs SiO2 and Th/Yb vs Ta/Yb diagrams. The trace element contents and discriminations indicate that the Keban syenitoids were derived from lithospheric mantle metasomatized by oceanic-crust/sediment fluids. The metal and halogen contents of the Keban mineralizations apparently originated from metasomatized mantle and were transported to the crust by syenitoid magmas. Clear resemblances in chondrite-normalized REE patterns, LREE-HREE partionation, and high LILE contents of the magmatics and fluorites indicate a close kinship between the syenitoids and fluorite mineralizations. The HFSE contents of the fluorites are lower than those of the magmatics, as HFSEs are not soluble in aqueous fluids. The fluorites are products of early-phase alkali magmatism (LREE > HREE). The high contents of Rb, Sr, and Ba of fluorites are inherited from the magma, which also has very high contents of these elements. In Sc-∑REE, (La/Yb)n-(Eu/Eu*)n and (Tb/La)n-(Tb/Ca)n diagrams, Keban fluorites fall into distinct areas from Akcakisla-Akdagmadeni and Celikhan-Adiyaman fluorites.

  5. Chemical ordering in substituted fluorite oxides: a computational investigation of Ho2Zr2O7 and RE2Th2O7 (RE=Ho, Y, Gd, Nd, La)

    DOE PAGES

    Solomon, Jonathan M.; Shamblin, Jacob; Lang, Maik; ...

    2016-12-12

    Fluorite-structured oxides find widespread use for applications spanning nuclear energy and waste containment, energy conversion, and sensing. In such applications the host tetravalent cation is often partially substituted by trivalent cations, with an associated formation of charge-compensating oxygen vacancies. The stability and properties of such materials are known to be influenced strongly by chemical ordering of the cations and vacancies, and the nature of such ordering and associated energetics are thus of considerable interest. Here we employ density-functional theory (DFT) calculations to study the structure and energetics of cation and oxygen-vacancy ordering in Ho2Zr2O7. In a recent neutron total scatteringmore » study, solid solutions in this system were reported to feature local chemical ordering based on the fluorite-derivative weberite structure. The calculations show a preferred chemical ordering qualitatively consistent with these findings, and yield values for the ordering energy of 9.5 kJ/mol-cation. Similar DFT calculations are applied to additional RE2Th2O7'' fluorite compounds, spanning a range of values for the ratio of the tetravalent and trivalent (RE) cation radii. Finally, the results demonstrate that weberite-type order becomes destabilized with increasing values of this size ratio, consistent with an increasing energetic preference for the tetravalent cations to have higher oxygen coordination.« less

  6. Chemical ordering in substituted fluorite oxides: a computational investigation of Ho2Zr2O7 and RE2Th2O7 (RE=Ho, Y, Gd, Nd, La).

    PubMed

    Solomon, Jonathan M; Shamblin, Jacob; Lang, Maik; Navrotsky, Alexandra; Asta, Mark

    2016-12-12

    Fluorite-structured oxides find widespread use for applications spanning nuclear energy and waste containment, energy conversion, and sensing. In such applications the host tetravalent cation is often partially substituted by trivalent cations, with an associated formation of charge-compensating oxygen vacancies. The stability and properties of such materials are known to be influenced strongly by chemical ordering of the cations and vacancies, and the nature of such ordering and associated energetics are thus of considerable interest. Here we employ density-functional theory (DFT) calculations to study the structure and energetics of cation and oxygen-vacancy ordering in Ho2Zr2O7. In a recent neutron total scattering study, solid solutions in this system were reported to feature local chemical ordering based on the fluorite-derivative weberite structure. The calculations show a preferred chemical ordering qualitatively consistent with these findings, and yield values for the ordering energy of 9.5 kJ/mol-cation. Similar DFT calculations are applied to additional RE2Th2O7 fluorite compounds, spanning a range of values for the ratio of the tetravalent and trivalent (RE) cation radii. The results demonstrate that weberite-type order becomes destabilized with increasing values of this size ratio, consistent with an increasing energetic preference for the tetravalent cations to have higher oxygen coordination.

  7. Chemical ordering in substituted fluorite oxides: a computational investigation of Ho2Zr2O7 and RE2Th2O7 (RE=Ho, Y, Gd, Nd, La)

    NASA Astrophysics Data System (ADS)

    Solomon, Jonathan M.; Shamblin, Jacob; Lang, Maik; Navrotsky, Alexandra; Asta, Mark

    2016-12-01

    Fluorite-structured oxides find widespread use for applications spanning nuclear energy and waste containment, energy conversion, and sensing. In such applications the host tetravalent cation is often partially substituted by trivalent cations, with an associated formation of charge-compensating oxygen vacancies. The stability and properties of such materials are known to be influenced strongly by chemical ordering of the cations and vacancies, and the nature of such ordering and associated energetics are thus of considerable interest. Here we employ density-functional theory (DFT) calculations to study the structure and energetics of cation and oxygen-vacancy ordering in Ho2Zr2O7. In a recent neutron total scattering study, solid solutions in this system were reported to feature local chemical ordering based on the fluorite-derivative weberite structure. The calculations show a preferred chemical ordering qualitatively consistent with these findings, and yield values for the ordering energy of 9.5 kJ/mol-cation. Similar DFT calculations are applied to additional RE2Th2O7 fluorite compounds, spanning a range of values for the ratio of the tetravalent and trivalent (RE) cation radii. The results demonstrate that weberite-type order becomes destabilized with increasing values of this size ratio, consistent with an increasing energetic preference for the tetravalent cations to have higher oxygen coordination.

  8. Chemical ordering in substituted fluorite oxides: a computational investigation of Ho2Zr2O7 and RE2Th2O7 (RE=Ho, Y, Gd, Nd, La)

    PubMed Central

    Solomon, Jonathan M.; Shamblin, Jacob; Lang, Maik; Navrotsky, Alexandra; Asta, Mark

    2016-01-01

    Fluorite-structured oxides find widespread use for applications spanning nuclear energy and waste containment, energy conversion, and sensing. In such applications the host tetravalent cation is often partially substituted by trivalent cations, with an associated formation of charge-compensating oxygen vacancies. The stability and properties of such materials are known to be influenced strongly by chemical ordering of the cations and vacancies, and the nature of such ordering and associated energetics are thus of considerable interest. Here we employ density-functional theory (DFT) calculations to study the structure and energetics of cation and oxygen-vacancy ordering in Ho2Zr2O7. In a recent neutron total scattering study, solid solutions in this system were reported to feature local chemical ordering based on the fluorite-derivative weberite structure. The calculations show a preferred chemical ordering qualitatively consistent with these findings, and yield values for the ordering energy of 9.5 kJ/mol-cation. Similar DFT calculations are applied to additional RE2Th2O7 fluorite compounds, spanning a range of values for the ratio of the tetravalent and trivalent (RE) cation radii. The results demonstrate that weberite-type order becomes destabilized with increasing values of this size ratio, consistent with an increasing energetic preference for the tetravalent cations to have higher oxygen coordination. PMID:27941870

  9. Hyper-Parametric Oscillations in a Whispering Gallery Mode Fluorite Resonator

    NASA Technical Reports Server (NTRS)

    Savchenkov, Anatoliy; Strekalov, Dmitry; Mohageg, Makan; Ilchenko, Vladimir; Matsko, Andrey; Maleki, Lute

    2004-01-01

    This viewgraph presentation summarizes the hyper-parametric oscillations observations of the fluorite resonator. The reporters have observed various nonlinear effects in ultra-high Q crystalline whispering gallery mode (WGM) resonators. In particular, it was demonstrated a low threshold optical hyper-parametric oscillations in a high-Q (Q=1010) CaF2 WGM resonator. The oscillations result from the resonantly enhanced four-wave-mixing occurring due to Kerr nonlinearity of the material.

  10. Growth, Structure, and Thermal Conductivity of Yttria-Stabilized Hafnia Thin Films (Postprint)

    DTIC Science & Technology

    2012-01-01

    heated to temperatures higher than 1700 °C.9,10 Further transformation into the cubic polymorphic form having the fluorite structure takes place at 2700...of fluorite -structured (CaF2) oxygen superionic conductors.11 Interest in yttria-stabilized zirconia and hafnia materials is generated because of a

  11. Layer-engineering of high-Tc superconductors: (Cu,Mo)Sr2(Ce,Y)4Cu2O13+delta with a quadruple-fluorite-layer block between CuO2 planes.

    PubMed

    Grigoraviciute, Inga; Yamauchi, Hisao; Karppinen, Maarit

    2007-03-07

    Among high-Tc superconductive copper oxides, there have been known phases that contain fluorite-structured layers as an additional "blocking block" between adjacent CuO2 planes. Here, we report that even a phase with the CuO2 planes separated by a 12-A thick quadruple-fluorite-layer block can be synthesized in a single phase and strongly oxygenated form to exhibit superconductivity with a Tc value as high as 55 K. The new phase is the fourth member of the (Cu,Mo)Sr2(Ce,Y)sCu2O(5+2s+delta) or (Cu,Mo)-12s2 homologous series. Comparison with the previously known s = 1, 2, and 3 members of the series reveals the amazing conclusion that Tc remains essentially unaffected upon inserting additional fluorite-structured layers between the two CuO2 planes as long as the hole-doping level of the planes is kept constant.

  12. A kinetic study of the replacement of calcite marble by fluorite

    NASA Astrophysics Data System (ADS)

    Trindade Pedrosa, Elisabete; Boeck, Lena; Putnis, Christine V.; Putnis, Andrew

    2016-04-01

    Replacement reactions are relevant in any situation that involves the reequilibration between a solid and an aqueous fluid phase and are commonly controlled by an interface-coupled dissolution-precipitation mechanism (Putnis and Putnis, 2007). These reactions control many large-scale Earth processes whenever aqueous fluids are available, such as during metamorphism, metasomatism, and weathering. An important consequence of coupled dissolution-precipitation is the generation of porosity in the product phase that then allows the infiltration of the fluid within the mineral being replaced. Understanding the mechanism and kinetics of the replacement of carbonates by fluorite has application in earth sciences and engineering. Fluorite (CaF2) occurs in all kinds of rocks (igneous, sedimentary, and metamorphic) and its origin is commonly associated with hydrothermal fluids. Moreover, calcium carbonate has been suggested as a successful seed material for the sequestration of fluoride from contaminated waters (Waghmare and Arfin, 2015). The aim of the present work is to investigate aspects of the replacement of calcium carbonate by fluorite to better understand the mechanism and kinetics of this reaction. Small cubes (˜ 3 × 3 × 3 mm) of Carrara marble (CaCO3 > 99 %) were cut and reacted with a 4 M ammonium fluoride (NH4F) solution for different times (1 to 48 hours) and temperatures (60, 80, 100, and 140 ° C). The microstructure of the product phases was analysed using SEM. The kinetics of replacement was monitored from the Rietveld analysis of X-ray powder diffraction patterns of the products as a function of temperature and reaction time. After reaction, all samples preserved their size and external morphology (a pseudomorphic replacement) and the product phase (fluorite) was highly porous. The activation energy Ea (kJ/mol) of the replacement reaction was empirically determined by both model-fitting and model-free methods. The isoconversional method yielded an

  13. Hydrothermal uranium deposits containing molybdenum and fluorite in the Marysvale volcanic field, west-central Utah

    USGS Publications Warehouse

    Cunningham, C.G.; Rasmussen, J.D.; Steven, T.A.; Rye, R.O.; Rowley, P.D.; Romberger, S.B.; Selverstone, J.

    1998-01-01

    Uranium deposits containing molybdenum and fluorite occur in the Central Mining Area, near Marysvale, Utah, and formed in an epithermal vein system that is part of a volcanic/hypabyssal complex. They represent a known, but uncommon, type of deposit; relative to other commonly described volcanic-related uranium deposits, they are young, well-exposed and well-documented. Hydrothermal uranium-bearing quartz and fluorite veins are exposed over a 300 m vertical range in the mines. Molybdenum, as jordisite (amorphous MoS2, together with fluorite and pyrite, increase with depth, and uranium decreases with depth. The veins cut 23-Ma quartz monzonite, 20-Ma granite, and 19-Ma rhyolite ash-flow tuff. The veins formed at 19-18 Ma in a 1 km2 area, above a cupola of a composite, recurrent, magma chamber at least 24 ?? 5 km across that fed a sequence of 21- to 14-Ma hypabyssal granitic stocks, rhyolite lava flows, ash-flow tuffs, and volcanic domes. Formation of the Central Mining Area began when the intrusion of a rhyolite stock, and related molybdenite-bearing, uranium-rich, glassy rhyolite dikes, lifted the fractured roof above the stock. A breccia pipe formed and relieved magmatic pressures, and as blocks of the fractured roof began to settle back in place, flat-lying, concave-downward, 'pull-apart' fractures were formed. Uranium-bearing, quartz and fluorite veins were deposited by a shallow hydrothermal system in the disarticulated carapace. The veins, which filled open spaces along the high-angle fault zones and flat-lying fractures, were deposited within 115 m of the ground surface above the concealed rhyolite stock. Hydrothermal fluids with temperatures near 200??C, ??18OH2O ~ -1.5, ?? -1.5, ??DH2O ~ -130, log fO2 about -47 to -50, and pH about 6 to 7, permeated the fractured rocks; these fluids were rich in fluorine, molybdenum, potassium, and hydrogen sulfide, and contained uranium as fluoride complexes. The hydrothermal fluids reacted with the wallrock resulting in

  14. C-type related order in the defective fluorites La2Ce2O7 and Nd2Ce2O7 studied by neutron scattering and ab initio MD simulations.

    PubMed

    Kalland, Liv-Elisif; Norberg, Stefan T; Kyrklund, Jakob; Hull, Stephen; Eriksson, Sten G; Norby, Truls; Mohn, Chris E; Knee, Christopher S

    2016-09-14

    This work presents a structural investigation of La2-xNdxCe2O7 (x = 0.0, 0.5, 1.0, 1.5, 2.0) using X-ray powder diffraction and total scattering neutron powder diffraction, analysed using Rietveld and the reverse Monte Carlo method (RMC). Ab initio molecular dynamics (MD) modelling is also performed for further investigations of the local order. The main intensities in the neutron diffraction data for the La2-xNdxCe2O7 series correspond to the fluorite structure. However, additional C-type superlattice peaks are visible for x > 0 and increase in intensity with increasing x. The Nd-containing compositions (x > 0) are best fitted with Rietveld analysis by using a combination of oxygen deficient fluorite and oxygen excess C-type structures. No indications of cation order are found in the RMC or Rietveld analysis, and the absence of cation order is supported by the MD modelling. We argue that the superlattice peaks originate from oxygen vacancy ordering and associated shift in the cation position away from the ideal fluorite site similar to that in the C-type structure, which is seen from the Rietveld refinements and the observed ordering in the MD modelling. The vacancies favour alignments in the 〈110〉, 〈111〉 and especially the 〈210〉 direction. Moreover, we find that such ordering might also be found to a small extent in La2Ce2O7, explaining the discernible modulated background between the fluorite peaks. The observed overlap of the main Bragg peaks between the fluorite and C-type phase supports the co-existence of vacancy ordered and more disordered domains. This is further supported by the observed similarity of the radial distribution functions as modelled with MD. The increase in long range oxygen vacancy order with increasing Nd-content in La2-xNdxCe2O7 corresponds well with the lower oxide ion conductivity in Nd2Ce2O7 compared to La2Ce2O7 reported earlier.

  15. Evaluation in situ of genotoxicity and stress in South American common toad Rhinella arenarum in environments related to fluorite mine.

    PubMed

    Pollo, Favio E; Grenat, Pablo R; Salinas, Zulma A; Otero, Manuel A; Salas, Nancy E; Martino, Adolfo L

    2017-06-20

    Little attention has been paid to the impact of wastewater generated by mining activities on fluoride. In this study, we evaluated the hematology responses of common South American toad Rhinella arenarum inhabiting natural and artificial environments associated with a fluorite mine from central Argentina. We analyzed three sampling stations associated with the fluorite mine: (I) Los Cerros Negros stream (CN), which runs on granitic rock with a high fluorite content; (II) Los Vallecitos stream (LV), which runs on metamorphic rock with low fluorite content; and (III) artificial decantation ponds (DP) containing sediments produced by fluorite flotation process. We calculated frequencies of micronuclei, erythrocyte nuclear abnormalities, mitosis, and immature erythrocytes. In addition, we performed a differential leukocyte count and determined neutrophils/lymphocyte ratio as a stress response estimator. We found high micronucleus (MN) and erythrocyte nuclear abnormality (ENA) frequencies in DP and CN but low frequencies in LV. The neutrophil/lymphocyte ratio was different among sites, with a significant increase in individuals from DP. Values registered in DP could be caused by exposure to mixture of compounds registered in dams that hold wastewater, while high values registered in CN stream might be due to natural concentrations of fluoride. Our results suggest that blood is an effective and non-destructive sensitive indicator for monitoring genotoxic agents in freshwater ecosystems.

  16. Ytterbium Clusters in Fluorite CaF{sub 2}

    SciTech Connect

    Nikiforov, Anatoliy E.; Chernyshev, Vladimir A.; Volodin, Vasiliy P.; Avram, Nicolae M.; Avram, Calin N.

    2009-05-22

    The crystal structure of the ytterbium hexameric cluster in CaF{sub 2} has been calculated in the framework of the embedded--cluster method within the shell model, in pair potential approximation. The crystal field parameters and scheme of the energy levels for Yb{sub 6}F{sub 36} hexameric cluster were calculated in the exchange charges model. Also, the optical spectra of Yb{sup 3+} single center in CaF{sub 2}(cubic and tetragonal symmetry) was calculated and discussed.

  17. Synthesis and thermoelectric properties of oxygen deficient fluorite derivative Ga3-xIn5+xSn2O16

    NASA Astrophysics Data System (ADS)

    Bhame, S. D.; Zhou, T.; Guilmeau, E.; Kinemuchi, Y.; Delorme, F.; Raveau, B.

    2010-11-01

    Exploring the possibility to obtain n-type oxide thermoelectrics, the transparent conducting oxide Ga3-xIn5+xSn2O16 (0.3structural relationships with In2O3 and its much lower indium content, more promising for applications. The absolute value of the Seebeck coefficient decreases almost linearly with x, whereas improvement in the power factor is obtained by increasing the indium content. The figure of merit ZT value reaches about 0.28 at 1000 K for x =0.6. The observed results are explained taking into account the peculiarities of the oxygen-deficient fluorite structure.

  18. Spectral properties and anti-Stokes luminescence of TeO2-BaF2:Ho3+, Ho3+/Yb3+ ceramics and glass excited by 1.9-μm radiation of a Tm:LiYF4 laser

    NASA Astrophysics Data System (ADS)

    Savikin, A. P.; Egorov, A. S.; Budruev, A. V.; Perunin, I. Yu.; Krasheninnikova, O. V.; Grishin, I. A.

    2017-07-01

    We demonstrate the up-conversion of Tm:LiYF4 infrared (IR) laser radiation with 1908-nm wavelength into visible light with a spectral maximum at 650 nm by ceramics with a composition of (100 - x)TeO2- xBaF2 - 1 wt % HoF3- yYbF3, where x = 20, 30, or 40 mol % and y = 0 or 0.5 wt %. The samples of 60TeO2-40BaF2 - 1 wt % HoF3 - 0.5 wt % YbF3 exhibited anti-Stokes luminescence at a threshold radiation power density of 1.0-1.5 W cm-2.

  19. Hexamer clusters in MeF2:Yb3+ (Me = Ca,Sr,Ba)

    NASA Astrophysics Data System (ADS)

    Chernyshev, V. A.; Nikiforov, A. E.; Nazemnikh, A. D.

    2011-10-01

    The electronic structure and g factors of simple impurity centers and hexamer ytterbium clusters in fluorites have been calculated in terms of the exchange charge model. Preliminarily, their local crystal structures have been calculated in the framework of the shell model with the inclusion of lattice distortions near the impurity. Crystal structure and g factors have been calculated of linear cluster Yb-F-Yb-F oriented along a trigonal axis in BaF2.

  20. Systematics of stretching of fluid inclusions I: fluorite and sphalerite at 1 atmosphere confining pressure.

    USGS Publications Warehouse

    Bodnar, R.J.; Bethke, P.M.

    1984-01-01

    Measured homogenization T of fluid inclusions in fluorite and sphalerite may be higher than the true homogenization T of samples that have been previously heated in the laboratory or naturally in post-entrapment events. As T and with it internal P is increased, the resulting volume increase may become inelastic. If the volume increase exceeds the precision of T measurement, the inclusion is said to have stretched. More than 1300 measurements on fluid inclusions in fluorite and sphalerite indicate that stretching is systematically related to P-V-T-X properties of the fluid, inclusion size and shape, physical properties of the host mineral, and the confining P. Experimental methods are detailed in an appendix. The mechanism of stretching is probably plastic deformation or - not observed - microfracturing. The systematic relationship between the internal P necessary to initiate stretching and the inclusion volume provides a means of recognizing previously stretched inclusions and estimating the magnitude of post-entrapment thermal events. -G.J.N.

  1. Mechanical loss of the reflective coating and fluorite at low temperature

    NASA Astrophysics Data System (ADS)

    Yamamoto, K.; Miyoki, S.; Uchiyama, T.; Ishitsuka, H.; Ohashi, M.; Kuroda, K.; Tomaru, T.; Sato, N.; Suzuki, T.; Haruyama, T.; Yamamoto, A.; Shintomi, T.; Numata, K.; Waseda, K.; Ito, K.; Watanabe, K.

    2004-03-01

    In some future interferometric gravitational wave detector projects, LCGT and EGO, the mirrors will be cooled so as to reduce the thermal noise. In order to evaluate the thermal noise of such cryogenic interferometers, the mechanical loss of the reflective coating of the mirrors was measured at low temperature. The measured loss angles were about 4 × 10-4 and constant between 4 K and 300 K. This result implies that the thermal noise of the coating is proportional to the square root of the temperature. The cooling mirror suppresses the thermal noise of the coating loss effectively, which satisfies the goal sensitivity of the future projects (h ~ 3 × 10-24 Hz-1/2 at 100 Hz). The Q-values of fluorite, which is a candidate for the mirror substrate of the cryogenic interferometer, were also measured at low temperature. The measurement showed that the Q-value below 12 K is at least 107. The fluorite mirrors must be cooled down to less than 5 K if they are to be used in future projects.

  2. Genesis of rare-metal pegmatites and alkaline apatite-fluorite rocks of Burpala massi, Northern Baikal folded zone

    NASA Astrophysics Data System (ADS)

    Sotnikova, Irina; Vladykin, Nikolai

    2015-04-01

    Burpalinsky rare metal alkaline massif in the Northern Baikal folded zone in southern margin of Siberian Platform, is a of intrusion central type, created 287 Ma covering area of about 250 km2. It is composed of nepheline syenites and pulaskites grading to quartz syenites in the contacts. Veines and dykes are represented by shonkinites, sodalite syenite, leucocratic granophyres, alkali granites and numerous rare metal alkaline syenite pegmatites and two dykes of carbonatites. All rocks except for granites are cut by a large apatite-fluorite dyke rocks with mica and magnetite, which in turn is cut by alaskite granites dyke. The massif has been studied by A.M. Portnov, A.A. Ganzeev et al. (1992) Burpalinsky massif is highly enriched with trace elements, which are concentrated in pegmatite dykes. About 70 rare-metal minerals we found in massif. Zr-silicates: zircon, eudialyte, lovenite, Ti-lovenite, velerite, burpalite, seidozerite, Ca- seidozerite, Rosenbuschite, vlasovite, katapleite, Ca-katapleite, elpidite. Ti- minerals:- sphene, astrophyllite, ramsaite, Mn-neptunite bafertisite, chevkinite, Mn-ilmenite, pirofanite, Sr-perrerit, landauite, rutile, anatase, brookite; TR- minerals - loparite, metaloparite, britolite, rinkolite, melanocerite, bastnesite, parisite, ankilite, monazite, fluocerite, TR-apatite; Nb- minerals - pyrochlore, loparite. Other rare minerals leucophanite, hambergite, pyrochlore, betafite, torite, thorianite, tayniolite, brewsterite, cryolite and others. We have proposed a new scheme massif: shonkinites - nepheline syenites - alkaline syenite - quartz syenites - veined rocks: mariupolites, rare-metal pegmatites, apatite, fluorite rock alyaskite and alkaline granites and carbonatites (Sotnikova, 2009). Apatite-fluorite rocks are found in the central part of massif. This is a large vein body of 2 km length and a 20 m width cutting prevailing pulaskites. Previously, these rocks were regarded as hydrothermal low-temperature phase. New geological and

  3. Regional stratigraphy and distribution of epigenetic stratabound celestine, fluorite, barite and Pb-Zn deposits in the MVT province of northeastern Mexico

    NASA Astrophysics Data System (ADS)

    González-Sánchez, Francisco; Camprubí, Antoni; González-Partida, Eduardo; Puente-Solís, Rafael; Canet, Carles; Centeno-García, Elena; Atudorei, Viorel

    2009-04-01

    Northeastern Mexico hosts numerous epigenetic stratabound carbonate-hosted low-temperature hydrothermal deposits of celestine, fluorite, barite and zinc-lead, which formed by replacement of Mesozoic evaporites or carbonate rocks. Such deposits can be permissively catalogued as Mississippi Valley-type (MVT) deposits. The deposits studied in the state of Coahuila are associated with granitic and metasedimentary basement highs (horsts) marginal or central to the Mesozoic Sabinas Basin. These horsts controlled the stratigraphy of the Mesozoic basins and subsequently influenced the Laramide structural pattern. The Sabinas Basin consists of ~6,000-m-thick Jurassic to Cretaceous siliciclastic, carbonate and evaporitic series. The MVT deposits are mostly in Barremian and in Aptian-Albian to Cenomanian formations and likely formed from basinal brines that were mobilized during the Laramide orogeny, although earlier diagenetic replacement of evaporite layers (barite and celestine deposits) and lining of paleokarstic cavities in reef carbonates (Zn-Pb deposits) is observed. Fluid inclusion microthermometry and isotopic studies suggest ore formation due to mixing of basinal brines and meteoric water. Homogenization temperatures of fluid inclusions range from 45°C to 210°C; salinities range from 0 to 26 wt.% NaCl equiv., and some inclusions contain hydrocarbons or bitumen. Sulfur isotope data suggest that most of the sulfur in barite and celestine is derived from Barremian to Cenomanian evaporites. Regional geology and a compilation of metallogenic features define the new MVT province of northeastern Mexico, which comprises most of the state of Coahuila and portions of the neighboring states of Nuevo León, Durango and, perhaps extends into Zacatecas and southern Texas. This province exhibits a regional metal zonation, with celestine deposits to the south, fluorite deposits to the north and barite and Zn-Pb deposits mostly in the central part.

  4. Thermal behaviour of ytterbium-doped fluorite crystals under high power pumping.

    PubMed

    Boudeile, J; Didierjean, J; Camy, P; Doualan, J L; Benayad, A; Ménard, V; Moncorgé, R; Druon, F; Balembois, F; Georges, P

    2008-07-07

    We report an in situ thermal study of Yb-doped fluorite crystals Yb:CaF(2) and Yb:SrF(2) under high power pumping, with or without laser operation. The experiment combines simultaneously thermography and measurement of the thermal aberrations. This setup allows us to measure temperature gradients, thermal lens, and absorption coefficients. From these measurements, we evaluate the thermal conductivity, fractional thermal load, and thermo-optic coefficient. Great differences are observed between the lasing and non lasing regimes. Our measured thermal lenses are greater than what are expected from the thermo-optic parameters found in previous work. Based on this thermal study, we design a laser cavity operating with large output power and TEM(00), leading to better performances for Yb:CaF(2) than Yb:SrF(2).

  5. Ion irradiation-induced diffusion in bixbyite-fluorite related oxides: Dislocations and phase transformation

    NASA Astrophysics Data System (ADS)

    Rolly, Gaboriaud; Fabien, Paumier; Bertrand, Lacroix

    2014-05-01

    Ion-irradiation induced diffusion and the phase transformation of a bixbyite-fluorite related rare earth oxide thin films are studied. This work is focused on yttrium sesquioxide, Y2O3, thin films deposited on Si (1 0 0) substrates using the ion beam sputtering technique (IBS). As-deposited samples were annealed ant then irradiated at cryogenic temperature (80 K) with 260 keV Xe2+ at different fluences. The irradiated thin oxide films are characterized by X-ray diffraction. A cubic to monoclinic phase transformation was observed. Analysis of this phenomenon is done in terms of residual stresses. Stress measurements as a function of irradiation fluences were realised using the XRD-sin2ψ method. Stress evolution and kinetic of the phase transformation are compared and leads to the role-played by the nucleation of point and extended defects.

  6. Design of a new family of inorganic compounds Ae2F2SnX3 (Ae = Sr, Ba; X = S, Se) using rock salt and fluorite 2D building blocks.

    PubMed

    Kabbour, Houria; Cario, Laurent; Danot, Michel; Meerschaut, Alain

    2006-01-23

    We could predict the structure of a new family of compounds Ae(2)F(2)SnX(3) (Ae = Sr, Ba; X = S, Se) from the stacking of known 2D building blocks of the rock salt and fluorite types. With a high-temperature ceramic method we have then succeeded to synthesize the four compounds Ba(2)F(2)SnS(3), Ba(2)F(2)SnSe(3), Sr(2)F(2)SnS(3), and Sr(2)F(2)SnSe(3). The structure refinements from X-ray powder diffraction patterns have confirmed the structure predictions and showed their good accuracy. The structure of the four compounds results from the alternated stacking of fluorite [Ae(2)F(2)] (Ae = Sr, Ba) and distorted rock salt [SnX(3)] (X = S, Se) 2D building blocks. As shown by band structure calculations, these blocks behave as a charge reservoir and a charge acceptor, respectively. Sr(2)F(2)SnS(3) and Ba(2)F(2)SnS(3) are transparent with optical gaps of 3.06 and 3.21 eV, respectively. However, an attempt to obtain a transparent conductor by substituting Ba per La in Ba(2)F(2)SnS(3) was unsuccessful.

  7. Thermal and chemical variations of the Nigerian Benue trough lead-zinc-barite-fluorite deposits

    NASA Astrophysics Data System (ADS)

    Ogundipe, Ibukun Emmanuel

    2017-08-01

    The Benue trough is an intra-continental rift initiated in the Cretaceous during the opening of the South Atlantic Ocean. Lead-zinc-barite-fluorite mineralization occurs along the 600 km axis of the trough in three discrete sub-basins which coincide with the lower, middle and upper mineral districts of the Benue Valley. Lithologically these sub-basins are dominated by black carbonaceous shale in the Lower Benue, platform carbonates in the Middle Benue and sandstones in the Upper Benue. Micro-thermometric analysis of fluid inclusions in sphalerite, fluorite, barite and quartz have shown that each mineral district has its own unique thermal and chemical imprint. For example, the temperature can be bracketed between 109 °C and 160 °C for lower Benue, 89 °C-144 °C for the Middle Benue and 176 °C-254 °C for the Upper Benue. Chemical differentiation also exists between each mineral district with the Lower Benue having 22 wt % equivalent NaCl while the Middle and Upper Benue have 18 and 16 wt % equivalent NaCl respectively. This study shows that inter-district thermal and chemical variations exist between the ore-stage sulfide and post-sulfide gangue minerals of the entire Benue Valley. Similarly, intra-district thermal and chemical variations have also been observed among all the paragenetic minerals of each district. The thermal variations may be as a result of variations in the geothermal gradient accompanying continental rifting from one district to the other. The variations in the chemistry between the Lower Benue and the Upper Benue paragenic minerals may be as a result of the distinct lithological differences across the Benue Trough.

  8. Compact watt-level passively Q-switched ZrF4-BaF2-LaF3-AIF3-NaF fiber laser at 2.8 μm using Fe2+:ZnSe saturable absorber mirror

    NASA Astrophysics Data System (ADS)

    Zhang, Tao; Feng, Guoying; Zhang, Hong; Ning, Shougui; Lan, Bin; Zhou, Shouhuan

    2016-08-01

    A midinfrared (mid-IR) saturable absorber mirror (SAM) was fabricated by coating aluminum film on Fe2+:ZnSe crystal, based on the vacuum evaporation method. By employing the prepared SAM, we demonstrated a high-power passively Q-switched Er3+-doped ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fiber laser at 2.8 μm. The highest output power in excess of 1.01 W was obtained with a pulse energy of 11.37 μJ and pulse duration of 0.73 μs, corresponding to a repetition rate of 88.84 kHz. To the best of our knowledge, these values represent the highest output power/pulse energy from a passively Q-switched ZBLAN fiber laser around 2.8 μm. Our results demonstrate that Fe2+:ZnSe SAM is a promising device for high-power/high-energy pulse generation in compact mid-IR fiber lasers.

  9. Highly-thermostable metal-organic frameworks (MOFs) of zinc and cadmium 4,4'-(hexafluoroisopropylidene)diphthalates with a unique fluorite topology.

    PubMed

    Zou, Ru-Qiang; Zhong, Rui-Qin; Du, Miao; Kiyobayashi, Tetsu; Xu, Qiang

    2007-06-28

    Two novel zinc and cadmium 4,4'-(hexafluoroisopropylidene)diphthalate metal-organic frameworks have been synthesized and characterized using single crystal X-ray diffraction analysis, and exhibit a unique fluorite topology and high thermal stabilities.

  10. Chemical ordering in substituted fluorite oxides: a computational investigation of Ho2Zr2O7 and RE2Th2O7 (RE=Ho, Y, Gd, Nd, La)

    SciTech Connect

    Solomon, Jonathan M.; Shamblin, Jacob; Lang, Maik; Navrotsky, Alexandra; Asta, Mark

    2016-12-12

    Fluorite-structured oxides find widespread use for applications spanning nuclear energy and waste containment, energy conversion, and sensing. In such applications the host tetravalent cation is often partially substituted by trivalent cations, with an associated formation of charge-compensating oxygen vacancies. The stability and properties of such materials are known to be influenced strongly by chemical ordering of the cations and vacancies, and the nature of such ordering and associated energetics are thus of considerable interest. Here we employ density-functional theory (DFT) calculations to study the structure and energetics of cation and oxygen-vacancy ordering in Ho2Zr2O7. In a recent neutron total scattering study, solid solutions in this system were reported to feature local chemical ordering based on the fluorite-derivative weberite structure. The calculations show a preferred chemical ordering qualitatively consistent with these findings, and yield values for the ordering energy of 9.5 kJ/mol-cation. Similar DFT calculations are applied to additional RE2Th2O7'' fluorite compounds, spanning a range of values for the ratio of the tetravalent and trivalent (RE) cation radii. Finally, the results demonstrate that weberite-type order becomes destabilized with increasing values of this size ratio, consistent with an increasing energetic preference for the tetravalent cations to have higher oxygen coordination.

  11. Experimental replacement of calcium carbonates by fluorite: high volume changes and porosity generation

    NASA Astrophysics Data System (ADS)

    Trindade Pedrosa, Elisabete; Putnis, Andrew

    2015-04-01

    Pseudomorphic mineral replacement reactions are a common phenomena in nature, and often described as interface-coupled dissolution-reprecipitation processes. The generation of porosity is a key factor for its progression since it creates the pathway for fluid infiltration towards an ongoing reaction front. The generation of porosity depends on two key factors: the molar volume differences between parent and product phase, and the relative solubilities of the parent and product in the fluid at the mineral-fluid interface (Pollok et al., 2011). Jamtveit et al., (2009) demonstrated that the permeability of the parent rock may also be enhanced by the development of fractures as a response to stresses generated by local volume changes at the reaction interface, which in turn increases the reaction rate. The replacement of calcite (CaCO3) by fluorite (CaF2) involves a molar volume decrease of 33.5 %. If indeed high volume changes generate high local stresses, a fragmentation process is expected to be driven by this replacement reaction. To test this hypothesis, a number of hydrothermal experiments were performed. Small cubes of calcite rock (Carrara marble), and single crystals of calcite were used as parent materials. Two fluoride solutions (ammonium fluoride and sodium fluoride) were used as reactants. Samples were reacted at temperatures up to 200°C for various times and quenched to room temperature. After drying, samples were mounted in epoxy holders, cross sections through the centre of the samples were cut and polished, and analysed using scanning electron microscopy (SEM), X-ray diffraction (XRD), and electron microprobe analysis (EMP). The replacement end product of all experiments was confirmed to be fluorite. In every case the external shape of the samples was perfectly maintained. No reaction induced fracturing was visible in any of the samples (rock or single crystals) although the texture of the replaced material was quite complex, often with a 'V' shaped

  12. Laser-excited fluorescence of rare earth elements in fluorite: Initial observations with a laser Raman microprobe

    USGS Publications Warehouse

    Burruss, R.C.; Ging, T.G.; Eppinger, R.G.; Samson, a.M.

    1992-01-01

    Fluorescence emission spectra of three samples of fluorite containing 226-867 ppm total rare earth elements (REE) were excited by visible and ultraviolet wavelength lines of an argon ion laser and recorded with a Raman microprobe spectrometer system. Narrow emission lines ( 0.9 for Eu2+ and 0.99 for Er3+. Detection limits for three micrometer spots are about 0.01 ppm Eu2+ and 0.07 ppm Er3+. These limits are less than chondrite abundance for Eu and Er, demonstrating the potential microprobe analytical applications of laser-excited fluorescence of REE in fluorite. However, application of this technique to common rock-forming minerals may be hampered by competition between fluorescence emission and radiationless energy transfer processes involving lattice phonons. ?? 1992.

  13. Stable isotope variations in the Quaternary epithermal calcite-fluorite deposit at Monte delle Fate near Cerveteri (Latium, central Italy)

    USGS Publications Warehouse

    Masi, U.; O'Neil, J.R.

    1980-01-01

    Carbon, oxygen and hydrogen isotope variations have been measured in samples from the epithermal fluorite vein deposit at Monte delle Fate, Latium. The ranges in ?? 13C and ??18O of calcite are -1.3 to 3.4 and 9.5 to 17.3, respectively. ??D values of water extracted from fluid inclusions are -49 to -39 for calcite and -41 to -34 for fluorite. Fluid inclusion filling temperatures (225??-240??C) and salinites (3.75) are nearly the same for both fluorite and sparry calcite. An elongated form of calcite, of minor abundance, precipitated at lower temperatures. The data indicate that (1) the CO2 involved in the mineralization was provided by the local marine limestones, (2) the waters were meteoric in origin and underwent an 18O shift of ??? 10 permil by exchange with marine country rocks, and (3) all geochemical features can be explained by the action of two hydrothermal fluids. Hot brines recently discovered in the Cesano geothermal area, 30 km to the east, have temperatures and some chemical characteristics similar to the hydrothermal fluids at Monte delle Fate. ?? 1980 Springer-Verlag.

  14. Fluoride removal by calcite: evidence for fluorite precipitation and surface adsorption.

    PubMed

    Turner, Brett D; Binning, Philip; Stipp, S L S

    2005-12-15

    Fluoride contamination of groundwater, both anthropogenic and natural, is a major problem worldwide. In this study, fluoride removal by crushed limestone (99% pure calcite) was investigated by batch studies and surface-sensitive techniques from solutions with fluoride concentrations from 150 micromol/L (3 mg/L) to 110 mM (approximately 2100 mg/L). Surface-sensitive techniques, including atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) as well as zeta potential measurements, confirm that, in addition to precipitation reactions, adsorption of fluoride also occurs. Results indicate that fluoride adsorption occurs immediately over the entire calcite surface with fluorite precipitating at step edges and kinks, where dissolved Ca2+ concentration is highest. The PHREEQ geochemical model was applied to the observed data and indicates that existing models, especially at low fluoride concentrations and high pH (>7.5) are not equipped to describe this complex system, largely because the PHREEQ model includes only precipitation reactions, whereas a combination of adsorption and precipitation parameters are required.

  15. Equilibrium and non-equilibrium kinetics of self-assembled surfactant monolayers: a vibrational sum-frequency study of dodecanoate at the fluorite-water interface.

    PubMed

    Schrödle, Simon; Richmond, Geraldine L

    2008-04-16

    The adsorption, desorption, and equilibrium monomer exchange processes of sodium dodecanoate at the fluorite(CaF 2)-water interface have been studied. For the first time, we use in situ vibrational sum-frequency spectroscopy (VSFS) to gain insights into the mechanism and kinetics of monolayer self-assembly at the mineral-water interface. By exploiting the nonlinear optical response of the adsorbate, the temporal correlation of headgroup adsorption and alignment of the surfactant's alkyl chain was monitored. Because of the unique surface-specificity of VSFS, changes in the interfacial water structure were also tracked experimentally. The spectra clearly reveal that the structure of interfacial water molecules is severely disturbed at the start of the adsorption process. With the formation of a well-ordered adsorbate layer, it is partially reestablished; however, the molecular orientation and state of coordination is significantly altered. Even at very low surfactant concentrations, overcharging of the mineral surface (i.e., the adsorption of adsorbates past the point of electrostatic equilibrium) was observed. This points out the importance of effects other than electrostatic interactions and it is proposed that cooperative effects of both water structure and surfactant hemimicelle formation at the interface are key factors. The present study also investigates desorption kinetics of partially and fully established monolayers and a statistical model for data analysis is proposed. Additional experiments were performed in the presence of electrolytes and showed that uni- and divalent anions affect the nonequilibrium kinetics of self-assembled monolayers in strikingly different ways.

  16. Ae2Sb2X4F2 (Ae = Sr, Ba): new members of the homologous series Ae2M(1+n)X(3+n)F2 designed from rock salt and fluorite 2D building blocks.

    PubMed

    Kabbour, Houria; Cario, Laurent

    2006-03-20

    We have designed new compounds within the homologous series Ae2F2M(1+n)X(3+n) (Ae = Sr, Ba; M = main group metal; n = integer) built up from the stacking of 2D building blocks of rock salt and fluorite types. By incrementally increasing the size of the rock salt 2D building blocks, we have obtained two new n = 1 members of this homologous series, namely, Sr2F2Sb2Se4 and Ba2F2Sb2Se4. We then succeeded in synthesizing these compounds using a high-temperature ceramic method. The structure refinements from the powder or single-crystal X-ray diffraction data confirmed presence of the expected alternating stacking of fluorite [Ae2F2] (Ae = Sr, Ba) and rock salt [Sb2Se4] 2D building blocks. However the Ba derivative shows a strong distortion of the [Sb2Se4] block and a concomitant change of the Sb atom coordination likely related to the lone-pair activity.

  17. The pyrochlore to 'defect fluorite' transition in the Y{sub 2}(Zr{sub y}Ti{sub 1-y}){sub 2}O{sub 7} system and its underlying crystal chemistry

    SciTech Connect

    Liu Yun; Withers, Ray Leslie . E-mail: withers@rsc.anu.edu.au; Noren, Lasse

    2004-12-01

    A careful investigation of the previously reported single phase, pyrochlore structure type Y{sub 2}(Zr{sub y}Ti{sub 1-y}){sub 2}O{sub 7}, 0=fluorite-related stabilized zirconia systems and the consequent difficulty in achieving equilibrium, careful attention was paid to synthesis procedures while Guinier XRD and electron diffraction were used to investigate the synthesized materials. As a consequence, a subtle but nonetheless clear two-phase region separating a pyrochlore-type solid solution field (from 0=fluorite' type solid solution field (from {approx}0.68=

  18. High-throughput continuous hydrothermal flow synthesis of Zn-Ce oxides: unprecedented solubility of Zn in the nanoparticle fluorite lattice.

    PubMed

    Kellici, Suela; Gong, Kenan; Lin, Tian; Brown, Sonal; Clark, Robin J H; Vickers, Martin; Cockcroft, Jeremy K; Middelkoop, Vesna; Barnes, Paul; Perkins, James M; Tighe, Christopher J; Darr, Jawwad A

    2010-09-28

    High-throughput continuous hydrothermal flow synthesis has been used as a rapid and efficient synthetic route to produce a range of crystalline nanopowders in the Ce-Zn oxide binary system. High-resolution powder X-ray diffraction data were obtained for both as-prepared and heat-treated (850 degrees C for 10 h in air) samples using the new robotic beamline I11, located at Diamond Light Source. The influence of the sample composition on the crystal structure and on the optical and physical properties was studied. All the nanomaterials were characterized using Raman spectroscopy, UV-visible spectrophotometry, Brunauer-Emmett-Teller surface area and elemental analysis (via energy-dispersive X-ray spectroscopy). Initially, for 'as-prepared' Ce(1-x)Zn(x)O(y), a phase-pure cerium oxide (fluorite) structure was obtained for nominal values of x=0.1 and 0.2. Biphasic mixtures were obtained for nominal values of x in the range of 0.3-0.9 (inclusive). High-resolution transmission electron microscopy images revealed that the phase-pure nano-CeO(2) (x=0) consisted of ca 3.7 nm well-defined nanoparticles. The nanomaterials produced herein generally had high surface areas (greater than 150 m(2) g(-1)) and possessed combinations of particle properties (e.g. bandgap, crystallinity, size, etc.) that were unobtainable or difficult to achieve by other more conventional synthetic methods.

  19. Energetics of mixing in ThO 2-CeO 2 fluorite solid solutions

    NASA Astrophysics Data System (ADS)

    Shvareva, Tatiana Y.; Alexandrov, Vitaly; Asta, Mark; Navrotsky, Alexandra

    2011-12-01

    Mixing enthalpies (Δ Hmix) of ThO 2-CeO 2 solid solutions with respect to cubic fluorite ThO 2 and CeO 2 have been measured by high temperature oxide melt solution calorimetry. The system shows a slightly positive mixing enthalpy, with a maximum value of Δ Hmix = 3.7 ± 2.5 kJ/mol at 50% Ce/(Ce + Th). Based on the regular-solution model, with an interaction parameter of 15.1 ± 2.2 kJ/mol fit to the measured data, the phase diagram is predicted to feature a miscibility gap with a calculated critical temperature of 908 ± 132 K. The results are complemented by density-functional-theory and Monte-Carlo calculations, which provide positive mixing enthalpies and a miscibility-gap phase diagram, in qualitative agreement with calorimetric results. The calculations suggest small effects of short-range order (clustering) on the mixing enthalpy above the miscibility gap. The calculated values of Δ Hmix are within the error bars of the measured values, but consistently smaller in magnitude. An analysis of the calculated results indicates that the dominant contribution to the mixing enthalpy arises from the elastic energy associated with cation size mismatch, allowing predictions of the behavior in ThO 2-UO 2 and ThO 2-PuO 2 systems. The analysis also suggests that the slightly smaller values of the computed Δ Hmix relative to experiment can be attributed to an underestimation of the magnitude of the elastic moduli in the calculations.

  20. Assessment in situ of genotoxicity in tadpoles and adults of frog Hypsiboas cordobae (Barrio 1965) inhabiting aquatic ecosystems associated to fluorite mine.

    PubMed

    Pollo, Favio E; Grenat, Pablo R; Otero, Manuel A; Salas, Nancy E; Martino, Adolfo L

    2016-11-01

    Non-lethal biological techniques such as blood biomarkers have gained attention due to their value as early signals of anthropic effects of contamination representing significant tools to evaluate ecosystems health. We evaluate and characterize in situ genotoxicity of water samples collected from aquatic ecosystems around a fluorite mine using amphibian frogs Hypsiboas cordobae as bioindicator species complemented with 16 physicochemical parameters. Four stations associated with fluorite mine sampling were sampled: a stream running on granitic rock with natural high fluorite content; two streams both running on metamorphic rock with low fluorite content; and an artificial decantation pond containing sediments produced by fluorite flotation process with high variation in physicochemical parameters. We analyses the blood of tadpoles and adults of H. Cordobae, calculated frequencies of micronuclei, erythrocyte nuclear abnormalities, mitosis, immature and enucleated erythrocytes. Individuals were measured and weighed and body condition was calculated. The results of this study indicate that individuals of decantation pond are exposed to compounds or mixtures which are causing cell damage when compared to those that were collected of stream. Larval stage was more vulnerable than the adult phase and it could be related mainly to the higher exposure time to xenobiotics, which can penetrate easily by skin, mouth and gills; additionally this site offers a reduced availability of food than other sites. Therefore, chronic exposure to pollutants could derive in degenerative and neoplastic diseases in target organs. Moreover these individuals may experience reproductive and behavioral disturbances which could lead to population decline in the long term.

  1. A look inside epitaxial cobalt-on-fluorite nanoparticles with three-dimensional reciprocal space mapping using GIXD, RHEED and GISAXS

    PubMed Central

    Suturin, S. M.; Fedorov, V. V.; Korovin, A. M.; Valkovskiy, G. A.; Konnikov, S. G.; Tabuchi, M.; Sokolov, N. S.

    2013-01-01

    In this work epitaxial growth of cobalt on CaF2(111), (110) and (001) surfaces has been extensively studied. It has been shown by atomic force microscopy that at selected growth conditions stand-alone faceted Co nanoparticles are formed on a fluorite surface. Grazing-incidence X-ray diffraction (GIXD) and reflection high-energy electron diffraction (RHEED) studies have revealed that the particles crystallize in the face-centered cubic lattice structure otherwise non-achievable in bulk cobalt under normal conditions. The particles were found to inherit lattice orientation from the underlying CaF2 layer. Three-dimensional reciprocal space mapping carried out using X-ray and electron diffraction has revealed that there exist long bright 〈111〉 streaks passing through the cobalt Bragg reflections. These streaks are attributed to stacking faults formed in the crystal lattice of larger islands upon coalescence of independently nucleated smaller islands. Distinguished from the stacking fault streaks, crystal truncation rods perpendicular to the {111} and {001} particle facets have been observed. Finally, grazing-incidence small-angle X-ray scattering (GISAXS) has been applied to decouple the shape-related scattering from that induced by the crystal lattice defects. Particle faceting has been verified by modeling the GISAXS patterns. The work demonstrates the importance of three-dimensional reciprocal space mapping in the study of epitaxial nanoparticles. PMID:24046491

  2. A look inside epitaxial cobalt-on-fluorite nanoparticles with three-dimensional reciprocal space mapping using GIXD, RHEED and GISAXS.

    PubMed

    Suturin, S M; Fedorov, V V; Korovin, A M; Valkovskiy, G A; Konnikov, S G; Tabuchi, M; Sokolov, N S

    2013-08-01

    In this work epitaxial growth of cobalt on CaF2(111), (110) and (001) surfaces has been extensively studied. It has been shown by atomic force microscopy that at selected growth conditions stand-alone faceted Co nanoparticles are formed on a fluorite surface. Grazing-incidence X-ray diffraction (GIXD) and reflection high-energy electron diffraction (RHEED) studies have revealed that the particles crystallize in the face-centered cubic lattice structure otherwise non-achievable in bulk cobalt under normal conditions. The particles were found to inherit lattice orientation from the underlying CaF2 layer. Three-dimensional reciprocal space mapping carried out using X-ray and electron diffraction has revealed that there exist long bright 〈111〉 streaks passing through the cobalt Bragg reflections. These streaks are attributed to stacking faults formed in the crystal lattice of larger islands upon coalescence of independently nucleated smaller islands. Distinguished from the stacking fault streaks, crystal truncation rods perpendicular to the {111} and {001} particle facets have been observed. Finally, grazing-incidence small-angle X-ray scattering (GISAXS) has been applied to decouple the shape-related scattering from that induced by the crystal lattice defects. Particle faceting has been verified by modeling the GISAXS patterns. The work demonstrates the importance of three-dimensional reciprocal space mapping in the study of epitaxial nanoparticles.

  3. Evidence for an Early Cretaceous mineralizing event above the basement/sediment unconformity in the intracratonic Paris Basin: paragenetic sequence and Sm-Nd dating of the world-class Pierre-Perthuis stratabound fluorite deposit

    NASA Astrophysics Data System (ADS)

    Gigoux, Morgane; Delpech, Guillaume; Guerrot, Catherine; Pagel, Maurice; Augé, Thierry; Négrel, Philippe; Brigaud, Benjamin

    2015-04-01

    World-class stratabound fluorite deposits are spatially associated with the basement/sediment unconformity of the intracratonic Paris Basin and the Morvan Massif in Burgundy (France). The reserves are estimated to be about 5.5 Mt of fluorite within six fluorite deposits. In this study, we aim to determine the age of the major fluorite mineralization event of the Pierre-Perthuis deposit (1.4 Mt fluorite) by a combined study of the paragenetic mineral sequence and Sm-Nd dating on fluorite crystals. Fluorite occurs as isolated cubes or filling geodes in a Triassic, silicified, dolomitic formation. Three fluorite stages associated with sphalerite, pyrite, galena, barite, and quartz have been distinguished using optical, cathodoluminescence, and scanning electron microscopes. Seven crystals of the geodic fluorite stage were analyzed for their rare earth element (REE) contents and their 147Sm/144Nd and 143Nd/144Nd isotopic compositions. The normalized REE distribution displays homogeneous bell-shaped patterns for all the geodic fluorite samples with a Mid-REE enrichment over the Light-REE and Heavy-REE. The 147Sm/144Nd varies from 0.3108 to 0.5504 and the 143Nd/144Nd from 0.512313 to 0.512518. A six-point Sm-Nd isochron defines an age of 130 ± 15 Ma (initial 143Nd/144Nd = 0.512054, MSWD = 0.21). This Sm-Nd isochron provides the first age for the stratabound fluorite sediment-hosted deposit, related to an unconformity in the Paris Basin, and highlights a major Early Cretaceous fluid circulation event mainly above the basement/sediment unconformity during a flexural deformation of the Paris Basin, which relates to the rifting of the Bay of Biscay and the formation of the Ligurian Sea in the Western Europe domain.

  4. A universal empirical expression for the isotope surface exchange coefficients (k*) of acceptor-doped perovskite and fluorite oxides.

    PubMed

    De Souza, R A

    2006-02-21

    The isotope surface exchange coefficient k* determined in an 18O/16O exchange experiment characterises the exchange flux of the dynamic equilibrium between oxygen in the gas phase and oxygen in a solid oxide. At present there is no atomistic expression that relates measured exchange coefficients to materials' parameters. In this study an empirical, atomistic expression is developed that describes the exchange kinetics of gaseous oxygen with diverse acceptor-doped perovskite and fluorite oxides at temperatures above T approximately 900 K. The expression is used to explain the observed correlations between surface exchange coefficients k* and oxygen tracer diffusion coefficients D* and to identify compounds that exhibit high surface exchange coefficients.

  5. Stable isotope study of fluid inclusions in fluorite from Idaho: implications for continental climates during the Eocene

    USGS Publications Warehouse

    Seal, R.R.; Rye, R.O.

    1993-01-01

    Isotopic studies of fluid inclusions from meteoric water-dominated epithermal ore deposits offer a unique opportunity to study paleoclimates because the fluids can provide direct samples of ancient waters. Fluorite-hosted fluid inclusions from the Eocene (51-50 Ma) epithermal deposits of the Bayhorse mining district, have low salinities and low to moderate homogenization temperatures indicating meteoric origins for the fluids. Oxygen and hydrogen isotope data on inclusion fluids are almost identical to those of modern meteoric waters in the area. The equivalence of the isotope composition of the Eocene inclusion fluids and modern meteoric waters indicates that the Eocene climatic conditions were similar to those today. -from Authors

  6. Atmospheric dust deposition on soils around an abandoned fluorite mine (Hammam Zriba, NE Tunisia).

    PubMed

    Djebbi, Chaima; Chaabani, Fredj; Font, Oriol; Queralt, Ignasi; Querol, Xavier

    2017-10-01

    The present study focuses on the eolian dispersion and dust deposition, of major and trace elements in soils in a semi-arid climate, around an old fluorite (CaF2) and barite (BaSO4) mine, located in Hammam Zriba in Northern Tunisia. Ore deposits from this site contain a high amount of metal sulphides constituting heavy metal pollution in the surrounding environment. Samples of waste from the surface of mine tailings and agricultural topsoil samples in the vicinity of the mine were collected. The soil samples and a control sample from unpolluted area, were taken in the direction of prevailing northwest and west winds. Chemical analysis of these solids was performed using both X-ray fluorescence and X-ray diffraction. To determine the transfer from mine wastes to the soils, soluble fraction was performed by inductively coupled plasma and ionic chromatography. The fine grained size fraction of the un-restored tailings, still contained significant levels of barium, strontium, sulphur, fluorine, zinc and lead with mean percentages (wt%) of 30 (calculated as BaO), 13 (as SrO), 10 (as SO3), 4 (F), 2 (Zn) and 1.2 (Pb). Also, high concentrations of cadmium (Cd), arsenic (As) and mercury (Hg) were found with an averages of 36, 24 and 1.2mgkg(-1), respectively. As a result of the eolian erosion of the tailings and their subsequent wind transport, the concentrations of Ba, Sr, S, F, Zn and Pb were extremely high in the soils near to the tailings dumps, with 5%, 4%, 7%, 1%, 0.8% and 0.2%, respectively. Concentration of major pollutants decreases with distance, but they were high even in the farthest samples. Same spatial distribution was observed for Cd, As and Hg. While, the other elements follow different spatial patterns. The leaching test revealed that most elements in the mining wastes, except for the anions, had a low solubility despite their high bulk concentrations. According the 2003/33/CE Decision Threshold, some of these tailings samples were considered as hazardous

  7. Optical and Electronic Analysis of the Two Fluorite-Related Phases in the Ga-In-Sn-O System

    NASA Astrophysics Data System (ADS)

    Dolgonos, Alex

    Optical transparency and electrical conductivity, traditionally mutually-exclusive materials properties, are both critical for the performance of electrode materials in applications such as flat-panel displays and organic photovoltaics (OPVs). Owing to their wide band gaps which are amenable to degenerate doping, transparent conducting oxides (TCOs) are widely utilized for organic optoelectronic applications. The two fluorite-related phases in the Ga-In-Sn-O (GITO) pseudo-ternary system---Ga,Sn co-substituted bixbyite 2O3 and the so-called "T-phase" Ga3--xIn5+xSn2O 16---have the potential to overcome the issues associated with Sn-doped 2O3 (ITO) by reducing the indium content, utilizing compositions that are more stable in acidic conditions, and offering work function levels that are in better alignment with adjacent layers in organic optoelectronic devices. In this work, the composition and processing effects on the optical and electronic properties of the bixbyite phase and the T-phase in the GITO system were determined. Electronic properties were analyzed by conductivity and Seebeck coefficient measurements, and the combination of these two properties revealed carrier characteristics for these two phases. The optical band gaps of the phases were determined from absorption data by methods developed in this work, which account for the band-altering effects of doping in TCOs. The optical characterization methods were experimentally confirmed through a combination of diffuse-reflectance and photoluminescence measurements of un-doped and Sn-doped In2O3. The bixbyite phase's conductivity, Seebeck coefficient, and optical band gap are dominated by the concentration of Sn-on-In substitution defects, as expected. Remarkably, Ga substition was found to have a negligible effect on the electronic properties. A conductivity of 3000 S/cm was achieved in bulk specimens containing a relatively small Sn content of 1 atomic percent and 4.5 atomic percent Ga. Ga behaves as an

  8. Fluid evolution and mineralogy of Mn-Fe-barite-fluorite mineralizations at the contact of the Thuringian Basin, Thüringer Wald and Thüringer Schiefergebirge in Germany

    NASA Astrophysics Data System (ADS)

    Majzlan, Juraj; Brey-Funke, Maria; Malz, Alexander; Donndorf, Stefan; Milovský, Rastislav

    2016-02-01

    Numerous small deposits and occurrences of Mn-Fe-fluorite-barite mineralization have developed at the contact of the Thuringian Basin, Thüringer Wald and Thüringer Schiefergebirge in central Germany. The studied mineralizations comprise the assemblages siderite+ankerite-calcite-fluorite-barite and hematite-Mn oxides-calcite-barite, with the precipitation sequence in that order within each assemblage. A structural geological analysis places the origin of the barite veins between the Middle Jurassic and Early Cretaceous. Primary fluid inclusions contain water vapour and an aqueous phase with NaCl and CaCl2 as the main solutes, with salinities mostly between 24-27 mass. % CaCl2 eq. Th measurements range between 85 °C and 160 °C in barite, between 139 °C and 163 °C in siderite, and between 80 °C and 130 °C in fluorite and calcite. Stable isotopes (S, O) point to the evaporitic source of sulphur in the observed mineralizations. The S,C,O isotopic compositions suggest that barite and calcite could not have precipitated from the same fluid. The isotopic composition of the fluid that precipitated barite is close to the sea water in the entire Permo-Mesozoic time span whereas calcite is isotopically distinctly heavier, as if the fluids were affected by evaporation. The fluid evolution in the siliciclastic/volcanic Rotliegend sediments (as determined by a number of earlier petrological and geochemical studies) can be correlated with the deposition sequence of the ore minerals. In particular, the bleaching of the sediments by reduced Rotliegend fluids (basinal brines) could be the event that mobilized Fe and Mn. These elements were deposited as siderite+ankerite within the Zechstein carbonate rocks and as hematite+Mn oxides within the oxidizing environment of the Permian volcanic and volcanoclastic rocks. A Middle-Jurassic illitization event delivered Ca, Na, Ba, and Pb from the feldspars into the basinal brines. Of these elements, Ba was deposited as massive barite

  9. Complex ceramic structures. I. Weberites.

    PubMed

    Cai, Lu; Nino, Juan C

    2009-06-01

    The weberite structure (A2B2X7) is an anion-deficient fluorite-related superstructure. Compared with fluorites, the reduction in the number of anions leads to a decrease in the coordination number of the B cations (VI coordination) with respect to the A cations (VIII coordination), thus allowing the accommodation of diverse cations. As a result, weberite compounds have a broad range of chemical and physical properties and great technological potential. This article summarizes the structural features of weberite and describes the structure in several different ways. This is the first time that the stacking vector and stacking angle are used to represent the weberite structure. This paper also discusses the crystallographic relationship between weberite, fluorite and pyrochlore (another fluorite-related structure). The cation sublattices of weberite and pyrochlore are correlated by an axial transformation. It has been shown that the different coordination environment of anions is due to the alternating layering of the AB3 and A3B close-packed cation layers. A stability field of weberite oxides is proposed in terms of the ratio of ionic radius of cations and relative bond ionicity. In addition, a selection of weberite compounds with interesting properties is discussed.

  10. EFFECT OF LASER LIGHT ON MATTER. LASER PLASMAS: Optical strength of mirrors on fluorite substrates subjected to cw radiation from a chemical CO laser

    NASA Astrophysics Data System (ADS)

    Dudkin, V. A.; Rukhin, V. B.

    1994-02-01

    Optimisation of the energy characteristics of a chemical CO laser revealed damage to the optical resonator mirrors. Estimates indicated that when the power density of the incident cw laser radiation was 2-2.5 kW cm-2 the induced thermoelastic stresses could reach the damage threshold of mirrors on fluorite substrates.

  11. Fluorite as an Sm-Nd geochronometer of hydrothermal processes: Dating of mineralization hosted in the Strel'tsovka uranium ore field, eastern Baikal region

    NASA Astrophysics Data System (ADS)

    Chernyshev, I. V.; Golubev, V. N.; Aleshin, A. P.; Larionova, Yu. O.; Gol'tsman, Yu. V.

    2016-11-01

    The possibility of using hydrothermal fluorite as an Sm-Nd geochronometer is based on the results of an REE pattern study of this mineral (Chernyshev et al., 1986). As a result of REE fractionation, in many cases, the Sm/Nd ratio achieves a multifold increase compared with its level in terrestrial rocks, and the radiogenic shift of the 143Nd/144Nd isotope ratio reaches 10-20 ɛNd units over a short time interval (as soon as tens of Ma). This is a necessary prerequisite for Sm-Nd isochron dating of fluorite. Zonal polychrome fluorite from a vein referred to the final stage of large-scale uranium mineralization at the Sterl'tsovka deposit in the ore field of the same name located in the eastern Transbaikal region has been dated using the 143Nd/144Nd method. To optimize isochron construction, local probes with high and contrasting Sm/Nd ratios have been sampled from the polished surfaces of two samples, taking into account the REE pattern of zonal fluorite. Sm-Nd isochron dating has been carried out separately for each sample. The 147Sm/144Nd i 143Nd/144Nd ratios vary within the intervals 0.5359-2.037 and 0.512799-0.514105, respectively. Two isochrons, each based on six fluorite probes, have been obtained with the following parameters, which coincide within 2σ uncertainty limits: (1) t = 134.8 ± 1.3 Ma, (143Nd/144Nd)0 = 0.512310 ± 13, MWSD = 0.43 and (2) t = 135.8 ± 1.6 Ma, (143Nd/144Nd)0 = 0.512318 ± 10, MWSD = 1.5. The mean age of fluorite based on two isochron datings is 135.3 ± 1 Ma. Comparison of this value with the most precise dating of pitchblende related to the ore stage in the Strel'tsovka ore field (135.5 ± 1 Ma) shows that four mineralization stages, distinguished by geological and mineralogical data, that were completed with the formation of polychrome fluorite veins 135.3 ± 1 Ma ago, represent a single and indivisible hydrothermal process whose duration does not exceed 1 Ma.

  12. Transformation of proton-conducting Perovskite-type into fluorite-type fast oxide ion electrolytes using a CO2 capture technique and their electrical properties.

    PubMed

    Trobec, Francesca; Thangadurai, Venkataraman

    2008-10-06

    Fast oxide ion conducting Ce 1- x M x O 2-delta (M = In, Sm; x = 0.1, 0.2) and Ce 0.8Sm 0.05Ca 0.15O 1.825 were prepared from the corresponding perovskite-like structured materials with nominal chemical composition of BaCe 1- x M x O 3-delta and BaCe 0.8Sm 0.05Ca 0.15O 2.825, respectively, by reacting with CO 2 at 800 degrees C for 12 h. Powder X-ray diffraction (PXRD) analysis showed the formation of fluorite-type CeO 2 and BaCO 3 just after reaction with CO 2. The amount of CO 2 gained per ceramic gram was found to be consistent with the Ba content. The CO 2 reacted samples were washed with dilute HCl and water, and the resultant solid product was characterized structurally and electrically employing various solid-state characterization methods, including PXRD, and alternating current (ac) impedance spectroscopy. The lattice constant of presently prepared Ce 1- x M x O 2-delta and Ce 0.8Sm 0.05Ca 0. 15O 1.825 by a CO 2 capture technique follows the expected ionic radii trend. For example, In-doped Ce 0.9In 0.1O 1.95 (In (3+) (VIII) = 0.92 A) sample showed a fluorite-type cell constant of 5.398(1) A, which is lower than the parent CeO 2 (5.411 A, Ce (4+) (VIII) = 0.97 A). Our attempt to prepare single-phase In-doped CeO 2 samples at 800, 1000, and 1500 degrees C using the ceramic method was unsuccessful. However, we were able to prepare single-phase Ce 0.9In 0.1O 1.95 and Ce 0.8In 0.2O 1.9 by the CO 2 capture method from the corresponding barium perovskites. The PXRD studies showed that the In-doped samples are thermodynamically unstable above 800 degrees C. The ac electrical conductivity studies using Pt electrodes showed the presence of bulk, grain-boundary, and electrode contributions over the investigated temperature range in the frequency range of 10 (-2)-10 (7) Hz. The bulk ionic conductivity and activation energy for the electrical conductivity of presently prepared Sm- and (Sm + Ca)-doped CeO 2 samples shows conductivities similar to those of materials

  13. Accelerator-based analytical technique in the evaluation of some Nigeria’s natural minerals: Fluorite, tourmaline and topaz

    NASA Astrophysics Data System (ADS)

    Olabanji, S. O.; Ige, O. A.; Mazzoli, C.; Ceccato, D.; Akintunde, J. A.; De Poli, M.; Moschini, G.

    2005-10-01

    For the first time, the complementary accelerator-based analytical technique of PIXE and electron microprobe analysis (EMPA) were employed for the characterization of some Nigeria's natural minerals namely fluorite, tourmaline and topaz. These minerals occur in different areas in Nigeria. The minerals are mainly used as gemstones and for other scientific and technological applications and therefore are very important. There is need to characterize them to know the quality of these gemstones and update the geochemical data on them geared towards useful applications. PIXE analysis was carried out using the 1.8 MeV collimated proton beam from the 2.5 MV AN 2000 Van de Graaff accelerator at INFN, LNL, Legnaro, Padova, Italy. The novel results which show many elements at different concentrations in these minerals are presented and discussed.

  14. Magnetic interactions in new fluorite-related rare earth oxides LnLn'2RuO7 (Ln, Ln'=rare earths)

    NASA Astrophysics Data System (ADS)

    Hinatsu, Yukio; Doi, Yoshihiro

    2016-07-01

    New fluorite-related quaternary rare earth oxides Pr2YRuO7 and La2TbRuO7 have been prepared. They crystallize in an orthorhombic superstructure of cubic fluorite with space group Cmcm. Through magnetic susceptibility and specific heat measurements, Pr2YRuO7 shows an antiferromagnetic transition at 27 K, which is considerably lowered compared with that for Pr3RuO7. Analysis of the magnetic specific heat indicates that the magnetic behavior observed at 27 K for Pr2YRuO7 is predominantly due to the magnetic interactions between Ru ions, and that the interactions between the Pr3+ and Ru5+ ions are also important. La2TbRuO7 shows magnetic ordering at 9.0 K, which is ascribed to the magnetic ordering between Ru5+ ions from the analysis of the magnetic specific heat data.

  15. Laser-excited fluorescence of rare earth elements in fluorite: Initial observations with a laser Raman microprobe

    NASA Astrophysics Data System (ADS)

    Burruss, Robert C.; Ging, Tom G.; Eppinger, Robert G.; Samson, ain M.

    1992-07-01

    Fluorescence emission spectra of three samples of fluorite containing 226-867 ppm total rare earth elements (REE) were excited by visible and ultraviolet wavelength lines of an argon ion laser and recorded with a Raman microprobe spectrometer system. Narrow emission lines (< 1 nm) due to 4f-4f electron transitions in individual trivalent REE (Pr, Nd, Sm, Eu?, Tb, Dy, Ho, Er, Tm) were observed in the wavelength range of 400-900 nm. Emission from individual REE occur in bands of overlapping lines in the wavelength intervals of 470-495 nm, 535-560 nm, 565-580 nm, 585-620 nm, 640-643 nm, 671.4 nm, and 758.2 nm. A broad band at 419 nm excited by the 363.8-nm ultraviolet line of the laser is due to a 4f-5 d transition in Eu 2+. Two bands of enigmatic origin are a narrow line at 682.8 nm present at all excitation wavelengths in only one sample and a broad band at 720 nm. We have tentatively assigned individual REE to specific lines in each emission band based on selection rules for strongly and weakly allowed 4f-4f transitions and the position of absorption and emission bands documented in the literature for REE in CaF 2 and LaF 3 host crystals. Working curves of integrated peak intensity of emission from Er 3+ and Eu 2+ vs. ppm measured by ICP-MS give linear log-log fits with R2 > 0.9 for Eu 2+ and 0.99 for Er 3+. Detection limits for three micrometer spots are about 0.01 ppm Eu 2+ and 0.07 ppm Er 3+. These limits are less than chondrite abundance for Eu and Er, demonstrating the potential microprobe analytical applications of laser-excited fluorescence of REE in fluorite. However, application of this technique to common rock-forming minerals may be hampered by competition between fluorescence emission and radiationless energy transfer processes involving lattice phonons.

  16. Magnetic interactions in new fluorite-related rare earth oxides LnLn’{sub 2}RuO{sub 7} (Ln, Ln’=rare earths)

    SciTech Connect

    Hinatsu, Yukio Doi, Yoshihiro

    2016-07-15

    New fluorite-related quaternary rare earth oxides Pr{sub 2}YRuO{sub 7} and La{sub 2}TbRuO{sub 7} have been prepared. They crystallize in an orthorhombic superstructure of cubic fluorite with space group Cmcm. Through magnetic susceptibility and specific heat measurements, Pr{sub 2}YRuO{sub 7} shows an antiferromagnetic transition at 27 K, which is considerably lowered compared with that for Pr{sub 3}RuO{sub 7}. Analysis of the magnetic specific heat indicates that the magnetic behavior observed at 27 K for Pr{sub 2}YRuO{sub 7} is predominantly due to the magnetic interactions between Ru ions, and that the interactions between the Pr{sup 3+} and Ru{sup 5+} ions are also important. La{sub 2}TbRuO{sub 7} shows magnetic ordering at 9.0 K, which is ascribed to the magnetic ordering between Ru{sup 5+} ions from the analysis of the magnetic specific heat data. - Graphical abstract: New fluorite-related quaternary rare earth oxides Pr{sub 2}YRuO{sub 7} and La{sub 2}TbRuO{sub 7} have been prepared. Through magnetic susceptibility and specific heat measurements, Pr{sub 2}YRuO{sub 7} and La{sub 2}TbRuO{sub 7} show an antiferromagnetic transition at 27 and 9.0 K, respectively. Display Omitted - Highlights: • New fluorite-related quaternary rare earth oxides LnLn’{sub 2}RuO{sub 7} have been prepared. • Pr{sub 2}YRuO{sub 7} shows an antiferromagnetic transition at 27 K. • La{sub 2}TbRuO{sub 7} shows magnetic ordering at 9.0 K. • Their magnetic exchange mechanism has been elucidated by the magnetic entropy change.

  17. Charge distribution and local structure of americium-bearing thorium oxide solid solutions.

    PubMed

    Carvajal-Nunez, U; Prieur, D; Vitova, T; Somers, J

    2012-11-05

    The electronical and structural properties of Th(0.80)Am(0.20)O(2-x) materials have been studied by the coupling of X-ray diffraction and X-ray absorption spectroscopy techniques. A substoichiometric fluorite Th(IV)(0.80)Am(III)(0.20)O(1.90) solid solution is found following sintering in moisturized Ar-H(2). In contrast, heating of this sample in air leads to a nondefective fluorite Th(IV)(0.80)Am(IV)(0.20)O(2.00) solid solution. The structures of these solid solution compounds were fully characterized by assessing the interatomic distances, the coordination numbers, and the structural disorder. The effect of the sintering atmosphere on these crystallographical parameters and on the cation valences has been determined and the capability of ThO(2) to accommodate tri- and tetravalent actinides in the fluorite structure assessed.

  18. Formation and properties of silicon/fluorite heterostructures. Final report, June 1, 1994--May 31, 1997

    SciTech Connect

    Olmstead, M.A.

    1997-10-01

    Our primary goal during the previous support periods was to determine the interrelationship among the structure of the CaF{sub 2}/Si interface, the growth kinetics, the structure and morphology of the overlying film, and the intrinsic dielectric properties of the interface itself. The experiments were designed (i) to determine the CaF{sub 2} overlayer structure and morphology as a function of kinetic conditions [Den93a, Den93b, Won93, Den95, Hes95], (ii) to develop and test models explaining the observed structures [Den95, Hes95], (iii) to investigate the stability of the interface structure [Les97] and (iv) to determine and interpret the photoelectron kinetic energy distributions as a function of the emitting atom location [Rot93, Rot94, Rot96]. As summarized below, we were successful in accomplishing these experiments and in largely fulfilling our original goal.

  19. Effect of Molar Ratio of (CaxBay)F2 Films on Electrical Characteristics of Metal/Fluoride/InP Structures

    NASA Astrophysics Data System (ADS)

    Hidaka, Atsushi; Ikoma, Hideaki

    1995-09-01

    (Ca xBa y)F2 (x:y=7:3, 1:1 and 3:7)/n-type InP (100) metal-insulator-semiconductor (MIS) structures were investigated by means of capacitance-voltage ( C-V) measurements and X-ray photoelectron spectroscopy (XPS) to study the effect of the molar ratio (x:y) of the fluoride films on the quality of the interface between the fluoride and InP. The fluoride films were deposited on the InP substrates by co-evaporating CaF2 and BaF2 powders in a vacuum of about 1-2×10-6 Torr. The best C-V characteristics were obtained for the lattice-matched (CaBa)F2 (Ca:Ba=1:1 in molar ratio)/InP and the next-best, for the (CaBa)F2 (Ca:Ba=7:3)/InP MIS structures. On the other hand, poor C-V characteristics were obtained for the (CaBa)F2 (Ca:Ba=3:7)/InP and the SrF2/InP structures, though their lattice matchings were better than that of the (CaBa)F2 (Ca:Ba=7:3)/InP structure. It is therefore concluded that the lattice matching between the fluoride film and the InP substrate is probably the most important factor for obtaining a good-quality interface; however, the presence of a fair amount of the “stable” Ca-P bonds is also effective to obtain a stable interface between fluoride and InP. The XPS data indicated that fluoride films with distinct and uniform chemical compositions were formed from the top surface to the interface with the InP substrate even by the conventional vacuum evaporation technique. The peak binding energy of the XPS signal from the Ba 3d core level shifts to the lower energy side, while that from the Ca 2p core level shifts to the higher energy side with increase in the ratio of Ba to Ca. These shifts are nearly linear with the change of the mixed molar ratio. These findings can be explained by a difference in ionic radii between Ba and Ca in the fluorite structure and indicate that the molar ratios in the fluoride films are the same as the mixed molar ratios of the source powders. Native oxides of InP such as In2O3, InPO3 and InPO4 were found to be removed or

  20. Preparation, XRD and Raman spectroscopic studies on new compounds RE {sub 2}Hf{sub 2}O{sub 7} (RE=Dy, Ho, Er, Tm, Lu, Y): Pyrochlores or defect-fluorite?

    SciTech Connect

    Mandal, B.P.; Garg, Nandini; Sharma, Surinder M.; Tyagi, A.K. . E-mail: aktyagi@barc.ernet.in

    2006-07-15

    A series of compositions with the general formula RE {sub 2}Hf{sub 2}O{sub 7} (RE=Dy, Ho, Er, Tm, Y and Lu) was prepared by a standard solid-state route and characterized by powder X-ray diffraction (XRD) and Raman spectroscopy. As per theoretical modeling reported in literature, some of these materials were predicted to exist in pyrochlore lattice. However, a careful X-ray diffraction, Raman spectroscopic and synchrotron radiation-XRD study revealed that under the experimental conditions used in the present investigation, out of all the RE {sub 2}Hf{sub 2}O{sub 7} samples only Dy{sub 2}Hf{sub 2}O{sub 7} has got a tendency to form a pyrochlore structure. All the other (Ho, Er, Tm, Lu, Y) hafnates crystallize in a defect-fluorite structure. In order to further ascertain these inferences, a few more RE {sub 2}Hf{sub 2}O{sub 7} samples (La, Nd, Sm) i.e., with larger RE{sup 3+} ions were also prepared and the results were compared. - Graphical abstract: Display Omitted.

  1. Structure of Ce(1-x)Sn(x)O(2) and its relation to oxygen storage property from first-principles analysis.

    PubMed

    Gupta, Asha; Kumar, Anil; Hegde, M S; Waghmare, U V

    2010-05-21

    CeO(2)-SnO(2) solid solution has been reported to possess high oxygen storage/release property which possibly originates from local structural distortion. We have performed first-principles based density functional calculations of Ce(1-x)Sn(x)O(2) structure (x=0, 0.25, 0.5, 1) to understand its structural stability in fluorite in comparison to rutile structure of the other end-member SnO(2), and studied the local structural distortion induced by the dopant Sn ion. Analysis of relative energies of fluorite and rutile phases of CeO(2), SnO(2), and Ce(1-x)Sn(x)O(2) indicates that fluorite structure is the most stable for Ce(1-x)Sn(x)O(2) solid solution. An analysis of local structural distortions reflected in phonon dispersion show that SnO(2) in fluorite structure is highly unstable while CeO(2) in rutile structure is only weakly unstable. Thus, Sn in Ce(1-x)Sn(x)O(2)-fluorite structure is associated with high local structural distortion whereas Ce in Ce(1-x)Sn(x)O(2)-rutile structure, if formed, will show only marginal local distortion. Determination of M-O (M=Ce or Sn) bond lengths and analysis of Born effective charges for the optimized structure of Ce(1-x)Sn(x)O(2) show that local coordination of these cations changes from ideal eightfold coordination expected of fluorite lattice to 4+4 coordination, leading to generation of long and short Ce-O and Sn-O bonds in the doped structure. Bond valence analyses for all ions show the presence of oxygen with bond valence approximately 1.84. These weakly bonded oxygen ions are relevant for enhanced oxygen storage/release properties observed in Ce(1-x)Sn(x)O(2) solid solution.

  2. Bubble Formation and Lattice Parameter Changes Resulting from He Irradiation of Defect-Fluorite Gd2Zr2O7

    SciTech Connect

    Taylor, Caitlin A.; Patel, Maulik K.; Aguiar, Jeffery A.; Zhang, Yanwen; Crespillo, Miguel L.; Wen, Juan; Xue, Haizhou; Wang, Yongqiang; Weber, William J.

    2016-08-15

    Pyrochlores have long been considered as potential candidates for advanced ceramic waste-forms for the immobilization of radioactive waste nuclides. This work provides evidence that Gd2Zr2O7, often considered the most radiation tolerant pyrochlore, could be susceptible to radiation damage in the form of bubble nucleation at the highest He doses expected over geological time. Ion irradiations were utilized to experimentally simulate the radiation damage and He accumulation produced by ..alpha..-decay. Samples were pre-damaged using 7 MeV Au3+ to induce the pyrochlore to defect-fluorite phase transformation, which would occur due to ..alpha..-recoil damage within several hundred years of storage in a Gd2Zr2O7 waste-form. These samples were then implanted to various He concentrations in order to study the long-term effects of He accumulation. Helium bubbles 1-3 nm in diameter were observed in TEM at a concentration of 4.6 at.% He. Some bubbles remained isolated, while others formed chains 10-30 nm in length parallel to the surface. GIXRD measurements showed lattice swelling after irradiating pristine Gd2Zr2O7 with 7 MeV Au3+ to a fluence of 2.2 x 1015 Au/cm2. An increase in lattice swelling was also measured after 2.2 x 1015 Au/cm2 + 2 x 1015 He/cm2 and 2.2 x 1015 Au/cm2 + 2 x 1016 He/cm2. A decrease in lattice swelling was measured after irradiation with 2.2 x 1015 Au/cm2 + 2 x 1017 He/cm2, the fluence where bubbles and bubble chains were observed in TEM. Bubble chains are thought to form in order to reduce lattice strain normal to the surface, which is produced by the Au and He irradiation damage.

  3. Application of the sublattice method to the investigation of phonon spectra and frequency density of fluorite-structure crystals

    NASA Astrophysics Data System (ADS)

    Kirienko, T. P.; Poplavnoy, A. S.

    2010-09-01

    Phonon spectra and state densities of MeF2 (Me = Ca, Sr, Cd, Ba, or Pb) crystals are calculated in the basis of sublattice state vectors using the Born-Mayer model. The phonon spectra and the sublattice state densities are calculated in the field of the second frozen sublattice. It is demonstrated that optical crystal branches are mainly due to oscillations of fluorine ions; moreover, the topology of optical branches in the spectrum and the crystal state densities are close to the topology of the spectra and state densities of the fluorine sublattice in the frozen metal sublattice. Exception is CaF2 whose ion and cation masses are close in values.

  4. Thermodynamics of fluorite type solid solutions containing plutonium, lanthanide elements or alkaline earth metals in uranium dioxide host lattices

    NASA Astrophysics Data System (ADS)

    Fujino, Takeo

    1988-06-01

    Thermodynamic data for the solid solutions M yU 1-yO 2+x ( x<0 and/or x ⩾ 0) were reviewed for M = Pu, Ce, La and Gd. Subsequently new experimental results were presented for M = Eu, Sr and Ba-Y. The oxygen potential of the plutonium-uranium solid solution was shown to be well-expressed by the Woodley equation. This type of equation was also obtained for Ce yU 1-yO 2+x ( x < 0) by least-squares fitting, which satisfactorily followed the experimental data. The lanthanum-uranium solid solution, with lanthanum concentrations as low as y = 0.01, shows a fairly wide range of hypostoichiometry. The increase of the oxygen potential due to changes of y from 0.01 to 0.05 is not very large. The gadolinium-uranium solid solutions, Gd yU 1-yO 2+x, exhibited the steepest increase in oxygen potential at x = 0 for both y = 0.14 and 0.27. The effect on the oxygen potential followed the order: La > Gd > Ce. Results for Eu yV 1-yO 2+x revealed a much larger effect of europium concentration together with the composition of the steepest increase in the oxygen potential at x < 0. The fluorite phase of Sr yU 1-yO 2+x, was found to exist up to x ~ 0.3 if the oxygen partial pressure is sufficiently high. The change of the lattice parameter was linear with the change in crystal radius. Following this rule, the valence state of cerium was between Ce 3+ and Ce 4+ in Ce yU 1-yO 2.00. Barium dissolves at least up to 5 mol% in the presence of yttrium, forming Ba 0.05Y 0.05U 0.9O 2+x. The oxygen potential is increased by the effect of barium. The steepest change in the oxygen potential also occurred at x < 0. Distinct peaks were not observed in either the partial molar enthalpy or the entropy curves of this solid solution. This behavior is in sharp contrast to that of the europium and gadolinium solid solutions.

  5. Tuning the Magnetic Properties of New Layered Iron Chalcogenides (BaF) 2 Fe 2–x Q 3 (Q = S, Se) by Changing the Defect Concentration on the Iron Sublattice

    SciTech Connect

    Sturza, Mihai; Allred, Jared M.; Malliakas, Christos D.; Bugaris, Daniel E.; Han, Fei; Chung, Duck Young; Kanatzidis, Mercouri G.

    2015-05-12

    Effecting and controlling ferromagnetic-like properties hi senticonductors has proven to be a complex problem, especially when approaching room temperature. Here, we demonstrate the important role of defects in the magnetic properties of semiconductors by reporting the structures and properties of the iron chalcogenides (BaF)(2)Fe2-x Q(3) (Q= S, Se), which exhibit anomalous Magnetic properties that are correlated' with detects in the Fe-sublattice, The compounds form in both long-range ordered and disordered polytypes of a new structure typified by the alternate stacking of fluorite (BaF)(2)(2+) and (Fe(2-x)Q(3))(2-) layers. The latter layers exhibit an ordered array of strong Pe-Pe dimers in edge-Sharing tetrahedra. Given the strong Fe-Fe interaction, it is expected that the Fe-Fe dimer is,antiferromagnetically coupled, yet crystals exhibit a Weak ferromagnetic moment that orders at relatively-high temperature: below 280-315 K and 240275 K for the sulfide and selenide analogues, respectively. This transition temperature positively correlates with the concentration of defect in the Fe-sublattice, as determined by single-crystal X-ray diffraction. Our results indicate that internal defects in Fe(2-x)Q(3) layers play an important role in dictating the magnetic properties of newly discovered (BaF)2Fe(2),Q-3, (Q= 5-, Se), which can yield switchable ferromagnetically ordered mother-its at or above room temperature.

  6. Assessment of the U3O7 Crystal Structure by X-ray and Electron Diffraction.

    PubMed

    Leinders, Gregory; Delville, Rémi; Pakarinen, Janne; Cardinaels, Thomas; Binnemans, Koen; Verwerft, Marc

    2016-10-03

    Polycrystalline U3O7 powder was synthesized by oxidation of UO2 powder under controlled conditions using in situ thermal analysis, and by heat treatment in a tubular furnace. The O/U ratio of the U3O7 phase was measured as 2.34 ± 0.01. The crystal structure was assessed from X-ray diffraction (XRD) and selected-area electron diffraction (SAED) data. Similar to U4O9-ε (more precisely U64O143), U3O7 exhibits a long-range ordered structure, which is closely related to the fluorite-type arrangement of UO2. Cations remain arranged identical to that in the fluorite structure, and excess anions form distorted cuboctahedral oxygen clusters, which periodically replace the fluorite anion arrangement. The structure can be described in an expanded unit cell containing 15 fluorite-like subcells (U15O35), and spanned by basis vectors A = ap - 2bp, B = -2ap + bp, and C = 3cp (lattice parameters of the subcell are ap = bp = 538.00 ± 0.02 pm and cp = 554.90 ± 0.02 pm; cp/ap = 1.031). The arrangement of cuboctahedra in U3O7 results in a layered structure, which is different from the well-known U4O9-ε crystal structure.

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

    NASA Astrophysics Data System (ADS)

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

    2017-06-01

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

  8. Modèles d'arrangements anioniques locaux dans les solutions solides à structure fluorine stabilisée par dopage Na 0.5- xY 0.5+ xF 2+2 x

    NASA Astrophysics Data System (ADS)

    Pontonnier, L.; Patrat, G.; Aléonard, S.

    1990-07-01

    Electron, X-ray, and neutron diffuse scattering measurements have been performed on single crystals of the Na 0.5- xY 0.5+ xF 2+2 x solid-solutions having a doped stabilized fluorite structure. The localization and the shape of the diffuse scattering agree with a description of the local structure using a tridimensional arrangement of microdomains. A model of the fluorine local order agreeing with the refinements of the average structure is proposed. It assumes that all the interstitial atoms are trapped in thermally stable [NaY 3F 12] 2- clusters diluted in a fluorite matrix. The high inelastic contribution to the diffuse scattering indicates that the fluorine sublattice of the fluorite structure is strongly affected by anharmonic vibrations.

  9. Concentrations of Fluorine in Magmatic and Mississippi Valley-type Ore Fluids in the Illinois-Kentucky Fluorite-Zn-Pb-Ba District: Implications for Ore Formation

    NASA Astrophysics Data System (ADS)

    Appold, M. S.; Kenderes, S. M.

    2016-12-01

    The Illinois-Kentucky (IL-KY) district is an anomalous occurrence of fluorite-dominant Mississippi Valley-type (MVT) mineralization. The close temporal and spatial association of the mineralization with ultramafic-carbonatitic igneous intrusive rocks has led to the long-standing hypothesis that the mineralization formed from sedimentary brines that had become enriched in F through mixing with local magmatic fluids. The purpose of the present study was to test this hypothesis by measuring the concentration of F in fluid inclusions hosted by sphalerite and by calculating the concentration of F in the magmatic fluid based on the concentration of F in apatite in the igneous rocks. Fluid inclusion F concentrations were determined by thermally decrepitating the fluid inclusions, measuring the F concentration of the resultant evaporative solute mounds using SEM-EDS, and calibrating to several standard solutions containing variable concentrations of F. The results showed the fluid inclusions to contain F concentrations on the order of 103 to 104 ppm, which are 2 to 3 orders of magnitude greater than the typical F concentrations of sedimentary brines. At fluorite saturation, these F concentrations correspond to pH values of about 1 to 2, which are much lower than the pH values of about 4 to 5.5 typical of MVT ore fluids. Thus, the high F concentrations of the IL-KY ore fluids would not only have promoted the precipitation of fluorite but would have suppressed the precipitation of metal sulfide minerals by increasing their solubility by lowering the ore fluids' pH. Apatite crystals in igneous intrusive rocks in the IL-KY district contain between about 2 to 3 weight percent F. Based on experimental partitioning relationships, the HF to water fugacity ratio of the exsolved magmatic fluid was calculated to be on the order of 10-3 at temperatures between 1200 and 1500° C. If this fugacity ratio was maintained as the magmatic fluid cooled to MVT ore-forming temperatures of about

  10. High-temperature X-ray diffraction measurements of fluorite-related rare earth antimonates Ln{sub 3}SbO{sub 7} (Ln=Nd, Tb) and their magnetic properties

    SciTech Connect

    Hinatsu, Yukio Doi, Yoshihiro

    2014-09-15

    Ternary rare-earth antimonates Ln{sub 3}SbO{sub 7} (Ln=rare earths) were prepared, and their structures were determined by X-ray diffraction measurements. They crystallize in an orthorhombic superstructure of cubic fluorite (space group Cmcm for Ln=La, Pr; Ccmm for Ln=Sm–Dy), in which Ln{sup 3+} ions occupy two different crystallographic sites (the 8-coordinated and 7-coordinated). For Ln=Nd, two phases with the Cmcm and Ccmm space groups coexist at room temperature. When the temperature was increased, the Nd{sub 3}SbO{sub 7} compound transformed into a single phase with the space group Cmcm. Through magnetic susceptibility measurements, an antiferromagnetic transition was observed at 3.0 K (Ln=Nd) and 7.8 K (Ln=Tb). Analysis of the magnetic specific heat for Tb{sub 3}SbO{sub 7} indicates that the 8-coordinated Tb ions magnetically orders at 7.8 K, and with furthermore decreasing temperature, the 7-coordinated Tb ions shows antiferromagnetic ordering at 3.0 K. - Graphical abstract: Temperature dependence of the specific heat divided by temperature (C{sub p}/T) and the magnetic entropy (S{sub mag}) for Tb{sub 3}SbO{sub 7}. Two-step magnetic transition has been observed. - Highlights: • The phase transition of Nd{sub 3}SbO{sub 7} is from the Ccmm space group to the Cmcm one. • Nd{sub 3}SbO{sub 7} shows an antiferromagnetic transition at 3.0 K. • For Tb{sub 3}SbO{sub 7}, two-step magnetic transition has been observed at 7.8 and 3.0 K.

  11. Taolin Zn-Pb-fluorite deposit, People's Republic of China: an example of some problems in fluid inclusion research on mineral deposits.

    USGS Publications Warehouse

    Roedder, E.; Howard, K.W.

    1988-01-01

    The ore in this large Zn-Pb-fluorite deposit in NE Hunan Province occurs as open space-filling in a major fault zone between granite and metasedimentary rocks. Following barren, pre-ore quartz, three stages of ore deposition are recognized. Studies on 400 fluid inclusions from all four stages show homogenization T of 120-200oC (av. approx 160o) and salinities of 0-14 wt.% equiv. NaCl (av. 7.7) . These results differ considerably from some previously published sulphur isotopic T (221-344oC), and data for five inclusions that are more saline (9.0-7.7 wt.% equiv. NaCl) and hotter (up to 345oC) (M.A. 85M/2835, 87M/0888).-R.A.H.

  12. Phase Transformation of U3O8 and Enhanced Structural Stability at Extreme Conditions

    NASA Astrophysics Data System (ADS)

    Zhang, Fuxiang; Lang, Maik; Ewing, Rodney

    2013-06-01

    A powder sample of β-U3O8 was pressurized at room temperature up to 37.5 GPa with a symmetric diamond anvil cell. XRD patterns clearly indicated that a phase transition occurred between 3-11 GPa. The high-pressure phase is a fluorite-like structure. The fluorite-like structure is stable up to 37.5 GPa. The high-pressure phase was then laser heated to over 1700 K in the diamond anvil cell at high pressure conditions. No phase transition was found at high pressure/ temperature conditions, and the fluorite-like structure of U3O8 is even fully quenchable. The lattice parameter of the fluorite-like high-pressure phase is 5.425 Å at ambient conditions, which is smaller than that of the stoichiometric UO2. Previous experiments have shown that the stoichiometric uranium dioxide (UO2) is not stable at high pressure conditions and starts to transform to a cotunnite structure at ~30 GPa. When heating the sample at high pressure, the critical transtion pressure is greatly reduced. However, the fluorite-like high-pressure phase of U3O8 is very stable at high pressure/high temperature conditions. The enhanced phase stability is believed to be related to the presence of extra oxygen (or U vacancies) in the structure. This work was supported by Materials Science of Actinides, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award No. DE-SC0001089.

  13. 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

  14. Fluorite-like phases in the BaF{sub 2}-BiF{sub 3}-Bi{sub 2}O{sub 3} system-synthesis, conductivity and defect clustering

    SciTech Connect

    Serov, T.V.; Dombrovski, E.N.; Ardashnikova, E.I. . E-mail: ard@inorg.chem.msu.ru; Dolgikh, V.A.; El Omari, Malika; El Omari, Mohamed; Abaouz, A.; Senegas, J.; Chaban, N.G.; Abakumov, A.M.; Van Tendeloo, G.

    2005-05-18

    A fluorite-like solid solution Ba{sub 1-x}Bi {sub x}O {sub z}F{sub 2+x-2z} on the basis of cubic BaF{sub 2} was synthesised in the BaF{sub 2}-Bi{sub 2}O{sub 3}-BiF{sub 3} system and the homogeneity range at 873 K was determined. The samples were studied by X-ray powder diffraction and electron diffraction, and their transport properties were measured by the complex impedance method at 300-623 K. Tendencies of variation of lattice parameters and transport properties were determined. These tendencies are discussed on the basis of a defect clustering hypothesis. Thermal treatment at 573 K of the solid solution, quenched from 873 K results in the formation of a new ordered tetragonal fluorite-like phase with lattice parameters a = 9.5355(4) A, c = 18.151(1) A.

  15. Brine history indicated by argon, krypton, chlorine, bromine, and iodine analyses of fluid inclusions from the Mississippi Valley type lead-fluorite-barite deposits at Hansonburg, New Mexico

    USGS Publications Warehouse

    Böhlke, J.K.; Irwin, J.J.

    1992-01-01

    Argon, krypton, chlorine, bromine, and iodine were measured in a homogeneous population of high-salinity hydrothermal fluid inclusions from the Tertiary-age Mississippi Valley-type (MVT) lead-fluorite-barite deposits at Hansonburg, New Mexico to establish new types of evidence for the history of both the fluid and the major dissolved salts. Noble gases and halogens in fluid inclusions containing 10−10–10−9 L of brine (Cl= 3 molal) were analyzed by laser microprobe noble-gas mass spectrometry (lmngms) on neutron-irradiated samples.The concentrations of36Ar (4.7 × 10−8 molal) and84Kr1.8 × 10−9 molal) in the fluid inclusions are equal to those of fresh surface waters in equilibrium with air at approximately20 ± 5°. The mole ratios ofBr/Cl (1.2 × 10−4) andI/Cl (1–2 × 10−6) are among the lowest measured in any natural waters, similar to those of modern brines formed by dissolution of Permian NaCl-bearing evaporites in southeast New Mexico.40Ar/36Ar ratios (600) are twice that of air, and indicate that the fluid inclusions had excess radiogenic40Ar (1.4 × 10−5 molal) when trapped. The amount of excess40Ar appears to be too large to have been acquired with Cl by congruent dissolution of halite-bearing evaporites, and possibly too small to have been acquired with Pb by congruent dissolution of granitic basement rocks with Proterozoic KAr ages.From thelmngms data, combined with published Pb and S isotope data, we infer the following sequence of events in the history of the Hansonburg MVT hydrothermal brine: (1) the brine originated as relatively dilute meteoric water, and it did not gain or lose atmospheric Ar or Kr after recharge; (2) the originally dilute fluid acquired the bulk of its Cl and sulfate in the subsurface after recharge by dissolving halite-bearing Permian? marine evaporites; (3) the high salinity brine then acquired most of its Pb and excess radiogenic40Ar from interactions with aquifer rocks other than evaporites, possibly clastic

  16. Pressure Effects on the Charge Carrier Transportation of BaF2 Nanocrystals

    NASA Astrophysics Data System (ADS)

    Cui, Xiao-Yan; Hu, Ting-Jing; Wang, Jing-Shu; Zhang, Jun-Kai; Li, Xue-Fei; Yang, Jing-Hai; Gao, Chun-Xiao

    2017-03-01

    Not Available Supported by the National Natural Science Foundation of China under Grant Nos 11374131, 11674404, 11404137 and 61378085, the Program for New Century Excellent Talents in University under Grant No NCET-13-0824, the Program for the Development of Science and Technology of Jilin Province under Grant Nos 201201079 and 20150204085GX, and the Twentieth Five-Year Program for Science and Technology of Education Department of Jilin Province under Grant No 20150221.

  17. On the dispersion management of fluorite whispering-gallery mode resonators for Kerr optical frequency comb generation in the telecom and mid-infrared range.

    PubMed

    Lin, Guoping; Chembo, Yanne K

    2015-01-26

    Optical whispering gallery mode (WGM) resonators have been very attracting platforms for versatile Kerr frequency comb generations. We report a systematic study on the material dispersion of various optical materials that are capable of supporting quality factors above 109. Using an analytical approximation of WGM resonant frequencies in disk resonators, we investigate the effect of the geometry and transverse mode order on the total group-velocity dispersion (GVD). We demonstrate that the major radii and the radial mode indices play an important role in tailoring the GVD of WGM resonators. In particular, our study shows that in WGM disk-resonators, the polar families of modes have very similar GVD, while the radial families of modes feature dispersion values that can differ by up to several orders of magnitude. The effect of these giant dispersion shifts are experimentally evidenced in Kerr comb generation with magnesium fluoride. From a more general perspective, this critical feature enables to push the zero-dispersion wavelength of fluorite crystals towards the mid-infrared (mid-IR) range, thereby allowing for efficient Kerr comb generation in that spectral range. We show that barium fluoride is the most interesting crystal in this regard, due to its zero dispersion wavelength (ZDW) at 1.93 μm and an optimal dispersion profile in the mid-IR regime. We expect our results to facilitate the design of different platforms for Kerr frequency comb generations in both telecommunication and mid-IR spectral ranges.

  18. Nanostructured crystals of Sr1- x R x F2+ x fluorite phases and their ordering: 6. Microindentation analysis of crystals

    NASA Astrophysics Data System (ADS)

    Gryaznov, M. Yu.; Shotin, S. V.; Chuvil'deev, V. N.; Marychev, M. O.; Sul'yanova, E. A.; Sul'yanov, S. N.; Sobolev, B. P.

    2012-01-01

    Hardness, crack resistance, brittleness, and effective fracture energy have been studied for crystals of 24 fluorite phases Sr1 - x R x F2 + x ( R are 14 rare earth elements (REEs); 0 < x ≤ 0.5) and SrF2 grown by the Bridgman method from a melt. These characteristics change nonlinearly with an increase in the REE content for Sr1 - x R x F2 + x (0 < x ≤ 0.5) with R = La, Nd, Sm, Gd, and Lu; it is maximum in the range x < 0.1 for all REEs. The changes in a number of REEs have been traced for an isoconcentration series of Sr0.90 R 0.10F2.10 crystals ( R = La, Nd, Sm, Gd, Ho, Er-Lu, or Y) and crystals (similar in composition) with R = Tb and Dy. The hardness of Sr1 - x R x F2 + x crystals is higher by a factor of ˜2-3 than that of SrF2. The effect of decrease in microstresses in SrF2 crystals is confirmed by the isomorphic introduction of R 3+ ions into this crystalline matrix.

  19. Understanding chemical expansion in perovskite-structured oxides.

    PubMed

    Marrocchelli, Dario; Perry, Nicola H; Bishop, Sean R

    2015-04-21

    In this work, chemical expansion in perovskite oxides was characterized in detail, motivated, inter alia, by a desire to understand the lower chemical expansion coefficients observed for perovskites in comparison to fluorite-structured oxides. Changes in lattice parameter and in local atomic arrangements taking place during compositional changes of perovskites, i.e., stoichiometric expansion, were investigated by developing an empirical model and through molecular dynamics and density functional theory atomistic simulations. An accurate empirical expression for predicting lattice constants of perovskites was developed, using a similar approach to previous reports. From this equation, analytical expressions relating chemical expansion coefficients to separate contributions from the cation and anion sublattices, assuming Shannon ionic radii, were developed and used to isolate the effective radius of an oxygen vacancy, rV. Using both experimental and simulated chemical expansion coefficient data, rV for a variety of perovskite compositions was estimated, and trends in rV were studied. In most cases, rV was slightly smaller than or similar to the radius of an oxide ion, but larger than in the fluorite structured materials. This result was in good agreement with the atomistic simulations, showing contractive relaxations of the closest oxide ions towards the oxygen vacancy. The results indicate that the smaller chemical expansion coefficients of perovskites vs. fluorites are largely due to the smaller change in cation radii in perovskites, given that the contraction around the oxygen vacancy appears to be less in this structure. Limitations of applicability for the model are discussed.

  20. Investigation of material property influenced stoichiometric deviations as evidenced during UV laser-assisted atom probe tomography in fluorite oxides

    NASA Astrophysics Data System (ADS)

    Valderrama, Billy; Henderson, Hunter B.; Yablinsky, Clarissa A.; Gan, Jian; Allen, Todd R.; Manuel, Michele V.

    2015-09-01

    Oxide materials are used in numerous applications such as thermal barrier coatings, nuclear fuels, and electrical conductors and sensors, all applications where nanometer-scale stoichiometric changes can affect functional properties. Atom probe tomography can be used to characterize the precise chemical distribution of individual species and spatially quantify the oxygen to metal ratio at the nanometer scale. However, atom probe analysis of oxides can be accompanied by measurement artifacts caused by laser-material interactions. In this investigation, two technologically relevant oxide materials with the same crystal structure and an anion to cation ratio of 2.00, pure cerium oxide (CeO2) and uranium oxide (UO2) are studied. It was determined that electronic structure, optical properties, heat transfer properties, and oxide stability strongly affect their evaporation behavior, thus altering their measured stoichiometry, with thermal conductance and thermodynamic stability being strong factors.

  1. Elastic stability and electronic structure of pyrite type PtN2: Ahard semiconductor

    SciTech Connect

    Yu, Rong; Zhan, Qian; Zhang, Xiao-Feng

    2006-08-29

    The elastic properties and electronic structure of PtN2 withthe pyrite structure (PtN2(C2)) were studied with first-principlescalculations. The crystal structure is demonstrated to be elasticallystable with a lower energy than the metastable fluorite structureproposed before. The calculated shear modulus of 214 GPa suggests thatPtN2(C2) is harder than some well known hard materials such as TiN andSiC. The high elastic moduli are attributed to a stacking ofcorner-shared PtN6 octahedra bonded by strong N-N covalent bonding. Incontrast to the metallic fluorite-type phase, PtN2(C2) is semiconductingwith an indirect band gap.

  2. Structural investigation of self-irradiation damaged AmO2

    NASA Astrophysics Data System (ADS)

    Prieur, Damien; Vigier, Jean-François; Wiss, Thierry; Janssen, Arne; Rothe, Jörg; Cambriani, Andrea; Somers, Joseph

    2014-04-01

    Studying self-irradiated materials is an ideal means to investigate the effect of the damage on material structure and to better understand the behavior of irradiated nuclear fuels. In this context, X-ray diffraction, X-ray absorption spectroscopy and transmission electron microscopy have been used to investigate self-irradiation damaged AmO2. Combining these techniques allows studying the microstructure and the variation of the fluorite structure at both short-range and long-range order. Thus, the increase of both interatomic distances and lattice parameter was shown, as well as the presence of nanometer sized He bubbles and dislocation loops. As confirmed by the observed high-level of crystallinity, the fluorite structure exhibits a high radiation tolerance, which is confirmed by the low increase of the lattice parameter. This could be explained by a self-annealing mechanism of the created defects at room temperature.

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

    SciTech Connect

    Gutiérrez–Llorente, Araceli; Joress, Howie; Woll, Arthur; Holtz, Megan E.; Ward, Matthew J.; Sullivan, Matthew C.; Muller, David A.; Brock, Joel D.

    2015-03-01

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

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

    DOE PAGES

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

    2015-03-01

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

  5. Computer-experimental synthesis of silica with the α-PbO2 structure

    NASA Astrophysics Data System (ADS)

    Matsui, Yoshito; Kawamura, Katsuyuki

    A series of molecular dynamics (MD) calculations on model SiO2 systems with the fluorite configuration have been carried out over the pressure range of 0-300 GPa. A pressure-controlling, noncubic cell MD program (XDORTHO) was used, assuming two-body central potential functions. We observed a very rapid transformation from cubic to orthorhombic lattice structure below pressures of dynamic instability of SiO2-fluorite, and found the "product" to have the α-PbO2 structure. The density (4.34 Mg/m3) and cohesive (lattice) energy (-13109 kJ/mol) of computer-synthesized SiO2 with the α-PbO2 structure are very close to the values for SiO2 with the futile structure (4.30 Mg/m3 and -13135 kJ / mol, respectively), with the IF-2 potential at 0 GPa and 300 K. In view of the absence of experimental evidence for existence of SiO2 with the α-PbO2 structure, we suppose that the polymorph, if it does exist, is metastable. However, if SiO2-fluorite exists at ultrahigh-pressure, we anticipate having only a reverted material in the α-PbO2structure.

  6. Local structural study of doped-ceria by EXAFS spectroscopy

    SciTech Connect

    Shirbhate, S. C.; Acharya, S. A.; Yadav, A. K.; Sagdeo, A. P.; Jha, S. N.

    2016-05-23

    In the present work, Structural and Local structural study of Sm, Gd doped and Sm-Gd co-doped ceria system has been studied by Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS). These ceria based systems are synthesized by hydrothermal synthesis route. Fluorite structure is confirmed by X-ray diffraction (XRD) and data is well fitted using Rietveld refinement by Full-Prof suite. Local structural changes in terms of coordination with surrounding, inter atomic distances and Debye Waller factor of nearest neighbor and next nearest neighbor has been discussed.

  7. Local structural study of doped-ceria by EXAFS spectroscopy

    NASA Astrophysics Data System (ADS)

    Shirbhate, S. C.; Yadav, A. K.; Acharya, S. A.; Sagdeo, A. P.; Jha, S. N.

    2016-05-01

    In the present work, Structural and Local structural study of Sm, Gd doped and Sm-Gd co-doped ceria system has been studied by Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS). These ceria based systems are synthesized by hydrothermal synthesis route. Fluorite structure is confirmed by X-ray diffraction (XRD) and data is well fitted using Rietveld refinement by Full-Prof suite. Local structural changes in terms of coordination with surrounding, inter atomic distances and Debye Waller factor of nearest neighbor and next nearest neighbor has been discussed.

  8. Kinetics of oxygen exchange over CeO2-ZrO2 fluorite-based catalysts.

    PubMed

    Sadovskaya, Ekaterina M; Ivanova, Yulia A; Pinaeva, Larisa G; Grasso, Giacomo; Kuznetsova, Tatiana G; van Veen, Andre; Sadykov, Vladislav A; Mirodatos, Claude

    2007-05-24

    The kinetics of 18O/16O isotopic exchange over CeO2-ZrO2-La2O3 and Pt/CeO2-ZrO2 catalysts have been investigated under the conditions of dynamic adsorption-desorption equilibrium at atmospheric pressure and a temperature range of 650-850 degrees C. The rates of oxygen adsorption-desorption on Pt sites, support surface, oxygen transfer (spillover) from Pt to the support as well as the amount of oxygen accumulated in the oxide bulk, and oxygen diffusion coefficient were estimated. The nanocrystalline structure of lanthana-doped ceria-zirconia prepared via the Pechini route with a developed network of domain boundaries and specific defects guarantees a high oxygen mobility in the oxide bulk (D = (1.5 / 2.0).10-18 m2 s-1 at 650 degrees C) and allows accumulation of over-stoichiometric/excess oxygen. For Pt/CeO2-ZrO2, oxygen transfer from Pt to support (characteristic time < 10-2 s) was shown to be responsible for the fast exchange between the gas-phase oxygen and oxygen adsorbed on the mixed oxide surface. The rate of direct exchange between the gas phase and surface oxygen is increased as well due to the increased concentration (up to 2 monolayers) of surface/near subsurface oxygen species accumulated on the oxygen vacancies (originated from the incorporation of highly dispersed Pt atoms). The characteristic time of diffusion of the oxygen localized in the subsurface layers is about 1 s. The overall quantity of over-stoichiometric oxygen and/or hydroxyl groups accumulated in the bulk can reach the equivalent of 10 monolayers, and characteristic time of oxygen diffusion within the bulk is about 20 s. All these kinetic data are required for the further step of modeling partial oxidation of hydrocarbons under steady- and unsteady-state conditions.

  9. Some electronic and magnetic properties of Fluoride ion in Fluoride structure nanocrystals

    NASA Astrophysics Data System (ADS)

    Imtani, Ali Nasir

    2012-01-01

    We have investigated the effects of the environment potential around Fluoride ion on some important electronic and magnetic properties such as dipole polarisability, moment of oscillator strengths S(k) and magnetic susceptibility. The theoretical procedure is based on the variational-perturbation theory with two parameter trial functions incorporated in an ionic model. We estimate these properties in four cases for Fluoride ion; free ion, ion under different potentials, ion in the crystals and ion in nanocrystal, CdF2, CaF2, PbF2, SrF2 and BaF2. Our results indicate that these properties vary with ion environments and the free state of Fluoride ion has higher values and there is linearity behaviour of these properties with lattice constant. For Fluoride ion in nanocrystal, we have found that there is an extra parameter that can also affect the dipole polarisability, the number of ions in the structure.

  10. Synthesis and crystal chemistry of two new fluorite-related bismuth phosphates, Bi(4.25)(PO4)2O(3.375) and Bi5(PO4)2O4.5, in the Series Bi4+x(PO4)2O3+3x/2 (0.175 < or = x < or = 1).

    PubMed

    Muktha, B; Guru Row, T N

    2006-06-12

    Two new phosphates, Bi(4.25)(PO4)2O(3.375) and Bi(5)(PO(4))(2)O(4.5), have been analyzed by single-crystal X-ray diffraction in the series Bi(4+x)(PO4)2O(3+3x/2) (0.175 < or = x < or = 1). The syntheses of the compositions ranging from x = 0.175 to 0.475 were carried out by the ceramic route. The compositions from x = 0.175 to 0.475 form a solid solution with a structure similar to that of Bi(4.25)(PO4)2O(3.375), while Bi(5)(PO4)2O(4.5) was isolated from a mixture of two phases. Both of the phases form fluorite-related structures but, nevertheless, differ from each other with respect to the arrangement of the bismuth atoms. The uniqueness in the structures is the appearance of isolated PO(4) tetrahedra separated by interleaving [Bi2O2] units. ac impedance studies indicate conductivity on the order of 10(-5) S cm(-1) for Bi(4.25)(PO4)2O(3.375). Crystal data: Bi(4.25)(PO4)2O(3.375), triclinic, space group P (No. 1), with a = 7.047(1) A, b = 9.863(2) A, c = 15.365(4) A, alpha = 77.604(4) degrees, beta = 84.556(4) degrees, gamma = 70.152(4) degrees, V = 980.90(4) A3, and Z = 4; Bi(5)(PO4)2O(4.5), monoclinic, space group C2/c (No. 15), with a = 13.093(1) A, b = 5.707(1) A, c = 15.293(1) A, beta = 98.240(2) degrees, V = 1130.95(4) A(3), and Z = 8.

  11. Influence of structural ordering on ion transport in BiO0.5F2.0 bismuth oxyfluoride

    NASA Astrophysics Data System (ADS)

    Sorokin, N. I.; Sobolev, B. P.

    2010-09-01

    The influence of structural defect ordering on ionic conductivity (σ) in the cubic (fluorite) modification of BiO0.5F2.0 oxyfluoride has been investigated. Upon cooling, the disordered fluorite BiO0.5F2.0 phase undergoes a reversible transition to an ordered form. This transition manifests itself as a jump in the temperature dependence σ( T) near 583 ± 6 K. The ordering of structural defects deteriorates the characteristics of ion transport in BiO0.5F2.0. At 500 K, the σ value for the ordered phase is 1 × 10-4 S/cm, whereas an extrapolation to this temperature for the disordered phase gives σ = 4 × 10-4 S/cm.

  12. Generalized adsorption isotherms for molecular and dissociative adsorption of a polar molecular species on two polar surface geometries: Perovskite (100) (Pm-3m) and fluorite (111) (Fm-3m)

    SciTech Connect

    Danielson, Thomas; Hin, Celine; Savara, Aditya

    2016-08-10

    Lattice based kinetic Monte Carlo (KMC) simulations have been used to determine a functional form for the second order adsorption isotherms on two commonly investigated crystal surfaces: the (111) fluorite surface and the (100) perovskite surface which has the same geometric symmetry as the NaCl (100) surface. The functional form is generalized to be applicable to all values of the equilibrium constant by a shift along the pressure axis. Functions have been determined for estimating the pressure at which a desired coverage would be achieved and for estimating the coverage at a certain pressure. The generalized form has been calculated by investigating the surface adsorbate coverage across a range of thermodynamic equilibrium constants that span the range 10-26 to 1013. Finally, the equations have been shown to be general for any value of the adsorption equilibrium constant.

  13. Generalized adsorption isotherms for molecular and dissociative adsorption of a polar molecular species on two polar surface geometries: Perovskite (100) (Pm-3m) and fluorite (111) (Fm-3m)

    NASA Astrophysics Data System (ADS)

    Danielson, Thomas; Hin, Celine; Savara, Aditya

    2016-08-01

    Lattice based kinetic Monte Carlo simulations have been used to determine a functional form for the second order adsorption isotherms on two commonly investigated crystal surfaces: the (111) fluorite surface and the (100) perovskite surface which has the same geometric symmetry as the NaCl (100) surface. The functional form is generalized to be applicable to all values of the equilibrium constant by a shift along the pressure axis. Functions have been determined for estimating the pressure at which a desired coverage would be achieved and, conversely, for estimating the coverage at a certain pressure. The generalized form has been calculated by investigating the surface adsorbate coverage across a range of thermodynamic equilibrium constants that span the range 10-26 to 1013. The equations have been shown to be general for any value of the adsorption equilibrium constant.

  14. Self-assembled three-dimensional epitaxial ionic fluorite Gd{sub 2}Zr{sub 2}O{sub 7} nanorods on (001) LaAlO{sub 3}

    SciTech Connect

    Jiang, J.C.; Meletis, E.I.; Asuvathraman, R.; Govindan Kutty, K.V.; Gnanasekar, K.I.; Chen, C.L.

    2005-04-18

    Self-organized, three-dimensional (3D), coherent fluorite Gd{sub 2}Zr{sub 2}O{sub 7} nanorods were synthesized with pulsed-laser ablation. Transmission electron microscopy studies reveal that the 3D nanorods are epitaxially grown on (001) LaAlO{sub 3} with (001){sub GZO} parallel (001){sub LAO}, [110]{sub GZO} parallel [100]{sub LAO} and [110]{sub GZO} parallel [010]{sub LAO}. The nanorods have in-plane dimensions of about 20-35 nm, and their length can extend for the entire film thickness. The 3D nanorods have a vertical prism shape with side surfaces exhibiting a serrated morphology composed of {l_brace}111{r_brace} planes. The formation of such self-organized nanorods is consistent with a preferential growth mechanism and surface energy minimization considerations.

  15. Generalized adsorption isotherms for molecular and dissociative adsorption of a polar molecular species on two polar surface geometries: Perovskite (100) (Pm-3m) and fluorite (111) (Fm-3m)

    DOE PAGES

    Danielson, Thomas; Hin, Celine; Savara, Aditya

    2016-08-10

    Lattice based kinetic Monte Carlo (KMC) simulations have been used to determine a functional form for the second order adsorption isotherms on two commonly investigated crystal surfaces: the (111) fluorite surface and the (100) perovskite surface which has the same geometric symmetry as the NaCl (100) surface. The functional form is generalized to be applicable to all values of the equilibrium constant by a shift along the pressure axis. Functions have been determined for estimating the pressure at which a desired coverage would be achieved and for estimating the coverage at a certain pressure. The generalized form has been calculatedmore » by investigating the surface adsorbate coverage across a range of thermodynamic equilibrium constants that span the range 10-26 to 1013. Finally, the equations have been shown to be general for any value of the adsorption equilibrium constant.« less

  16. Generalized adsorption isotherms for molecular and dissociative adsorption of a polar molecular species on two polar surface geometries: Perovskite (100) (Pm-3m) and fluorite (111) (Fm-3m)

    SciTech Connect

    Danielson, Thomas; Hin, Celine; Savara, Aditya

    2016-08-10

    Lattice based kinetic Monte Carlo (KMC) simulations have been used to determine a functional form for the second order adsorption isotherms on two commonly investigated crystal surfaces: the (111) fluorite surface and the (100) perovskite surface which has the same geometric symmetry as the NaCl (100) surface. The functional form is generalized to be applicable to all values of the equilibrium constant by a shift along the pressure axis. Functions have been determined for estimating the pressure at which a desired coverage would be achieved and for estimating the coverage at a certain pressure. The generalized form has been calculated by investigating the surface adsorbate coverage across a range of thermodynamic equilibrium constants that span the range 10-26 to 1013. Finally, the equations have been shown to be general for any value of the adsorption equilibrium constant.

  17. Synthesis and cation distribution in the new bismuth oxyhalides with the Sillén-Aurivillius intergrowth structures.

    PubMed

    Charkin, Dmitri O; Akinfiev, Victor S; Alekseeva, Anastasia M; Batuk, Maria; Abakumov, Artem M; Kazakov, Sergey M

    2015-12-21

    About 20 new compounds with the Sillén-Aurivillius intergrowth structure, Me(1)Me(2)Bi3Nb2O11X (Me(1) = Pb, Sr, Ba; Me(2) = Ca, Sr, Ba; X = Cl, Br, I), have been prepared. They are composed of stacking of [ANb2O7] perovskite blocks, fluorite-type [M2O2] blocks and halogen sheets. The cation distribution between the fluorite and perovskite layers has been studied for Ba2Bi3Nb2O11I, Ca1.25Sr0.75Bi3Nb2O11Cl, BaCaBi3Nb2O11Br and Sr2Bi3Nb2O11Cl. The smaller Me cations tend to reside in the perovskite block while the larger ones are situated in the fluorite-type block. The distribution of the elements was confirmed for BaCaBi3Nb2O11Br using energy dispersive X-ray analysis combined with scanning transmission electron microscopy (STEM-EDX). An electron diffraction study of this compound reveals a local symmetry lowering caused by weakly correlated rotation of NbO6 octahedra. Based on our findings, we suggest a new stability criterion for mixed-layer structures, which is that net charges of any two consecutive layers do not compensate for each other and only the whole layer sequence is electroneutral.

  18. Formation and Structure of AL(PO3)3 System Glasses,

    DTIC Science & Technology

    1982-08-06

    only the systems con- taining LiF or MgF2 that formed glass. 3) In the four-component systems that only contained NaF, KF, Ca?2 , SrF2 , or BaF2...a joining effect upon networks. Other single valued and dual valued chemicals such as NaF, KF, CaF2, SrF2 9 and BaF2 , belong to the out- er part of

  19. Structural investigation of self-irradiation damaged AmO{sub 2}

    SciTech Connect

    Prieur, Damien; Vigier, Jean-François; Wiss, Thierry; Janssen, Arne; Rothe, Jörg; Cambriani, Andrea; Somers, Joseph

    2014-04-01

    Studying self-irradiated materials is an ideal means to investigate the effect of the damage on material structure and to better understand the behavior of irradiated nuclear fuels. In this context, X-ray diffraction, X-ray absorption spectroscopy and transmission electron microscopy have been used to investigate self-irradiation damaged AmO{sub 2}. Combining these techniques allows studying the microstructure and the variation of the fluorite structure at both short-range and long-range order. Thus, the increase of both interatomic distances and lattice parameter was shown, as well as the presence of nanometer sized He bubbles and dislocation loops. As confirmed by the observed high-level of crystallinity, the fluorite structure exhibits a high radiation tolerance, which is confirmed by the low increase of the lattice parameter. This could be explained by a self-annealing mechanism of the created defects at room temperature. - Graphical abstract: The structure of damaged AmO2 (36 dpa) has been studied by XRD, XAS and TEM. Thus, the effects of the self-irradiation on the oxidation state, the lattice distances, the structural disorder, the radiation stability and the microstructure have been discussed. - Highlights: • The structure of highly self-irradiated AmO{sub 2} (36 dpa) was studied by XAS and TEM. • XRD, EXAFS and TEM confirm the high stability of the fluorite structure. • XRD and EXAFS show an increase of 0.3% of the structural distances. • XANES shows that Am is tetravalent as well as the Np recoil nucleus. • The presence of He bubbles was evidenced by TEM.

  20. Features of crystal and local structures of complex oxides formed in the binary system Dy2O3-ZrO2

    NASA Astrophysics Data System (ADS)

    Popov, V. V.; Menushenkov, A. P.; Yastrebtsev, A. A.; Zubavichus, Ya V.; Svetogorov, R. D.; Kolyshkin, N. A.

    2016-09-01

    The process of Dy2Zr2O7 (rDy3+/rZr4+ = 1.426) nanocrystals formation and evolution upon calcinations up to 1400 oC has been investigated by means of synchrotron radiation X-ray diffraction (XRD) and Raman spectroscopy. We observed that isothermal calcination of the X-ray amorphous precursor first leads to the formation of oxide nanocrystalline powders with a defect fluorite structure at 800 oC/3 h. The nucleation and growth of pyrochlore phase take place in nanodomains with pyrochlore-type superstructural ordering of cations and anions inside a well crystalline fluorite matrix at > 1200 oC. This is perhaps the first XRD study that reports the presence of a weak pyrochlore-type superstructure in the Dy2Zr2O7.

  1. An overview of DANCE: a 4II BaF[2] detector for neutron capture measurements at LANSCE.

    SciTech Connect

    Ullmann, J. L.

    2004-01-01

    The Detector for Advanced Neutron Capture experiments (DANCE) is a 162-element, 4{pi} BaF{sub 2} array designed to make neutron capture cross-section measurements on rare or radioactive targets with masses as little as 1 mg. Accurate capture cross sections are needed in many research areas, including stellar nucleosynthesis, advanced nuclear fuel cycles, waste transmutation, and other applied programs. These cross sections are difficult to calculate accurately and must be measured. Up to now, except for a few long-lived nuclides there are essentially no differential capture measurements on radioactive nuclei. The DANCE array is located at the Lujan Neutron Scattering Center at LANSCE, which is a continuous-spectrum neutron source with useable energies from below thermal to about 100 keV. Data acquisition is done with 320 fast waveform digitizers. The design and initial performance results, including background minimization, will be discussed.

  2. On the cosmic-ray induced background in neutral pion production measurements with a BaF 2 multidetector

    NASA Astrophysics Data System (ADS)

    Badalà, A.; Barbera, R.; Palmeri, A.; Pappalardo, G. S.; Riggi, F.; Russo, A. C.; Russo, G.; Turrisi, R.

    1995-02-01

    The problem of the cosmic-ray induced background in neutral pion production measurements with barium fluoride multidetectors is discussed. As a reference example, the response to cosmic rays of the MEDEA photon spectrometer is studied. The interaction of the cosmic radiation with the experimental filter has been treated by means of full Monte Carlo computer simulations with the GEANT3 code. The results of the simulations are compared with experimental data and general criteria to minimize the cosmic background are discussed.

  3. Structure and Properties of Some Layered U2O5 Phases: A Density Functional Theory Study.

    PubMed

    Molinari, Marco; Brincat, Nicholas A; Allen, Geoffrey C; Parker, Stephen C

    2017-04-05

    U2O5 is the boundary composition between the fluorite and the layered structures of the UO2→3 system and the least studied oxide in the group. δ-U2O5 is the only layered structure proposed so far experimentally, although evidence of fluorite-based phases has also been reported. Our DFT work explores possible structures of U2O5 stoichiometry by starting from existing M2O5 structures (where M is an actinide or transition metal) and replacing the M ions with uranium ions. For all structures, we predicted structural and electronic properties including bulk moduli and band gaps. The majority of structures were found to be less stable than δ-U2O5. U2O5 in the R-Nb2O5 structure was found to be a competitive structure in terms of stability, whereas U2O5 in the Np2O5 structure was found to be the most stable overall. Indeed, by including the vibrational contribution to the free energy using the frequencies obtained from the optimized unit cells we predict that Np2O5 structured U2O5 is the most thermodynamically stable under ambient conditions. δ-U2O5 only becomes more stable at high temperatures and/or pressures. This suggests that a low-temperature synthesis route should be tested and so potentially opens a new avenue of research for pentavalent uranium oxides.

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

    NASA Astrophysics Data System (ADS)

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

    2016-12-01

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

  5. High temperature crystal structures and superionic properties of SrCl{sub 2}, SrBr{sub 2}, BaCl{sub 2} and BaBr{sub 2}

    SciTech Connect

    Hull, Stephen; Norberg, Stefan T.; Ahmed, Istaq; Eriksson, Sten G.; Mohn, Chris E.

    2011-11-15

    The structural properties of the binary alkaline-earth halides SrCl{sub 2}, SrBr{sub 2}, BaCl{sub 2} and BaBr{sub 2} have been investigated from ambient temperature up to close to their melting points, using the neutron powder diffraction technique. Fluorite-structured SrCl{sub 2} undergoes a gradual transition to a superionic phase at 900-1100 K, characterised by an increasing concentration of anion Frenkel defects. At a temperature of 920(3) K, the tetragonal phase of SrBr{sub 2} undergoes a first-order transition to a cubic fluorite phase. This high temperature phase shows the presence of extensive disorder within the anion sublattice, which differs from that found in superionic SrCl{sub 2}. BaCl{sub 2} and BaBr{sub 2} both adopt the cotunnite crystal structure under ambient conditions. BaCl{sub 2} undergoes a first-order structural transition at 917(5) K to a disordered fluorite-structured phase. The relationship between the (disordered) crystal structures and the ionic conductivity behaviour is discussed and the influence of the size of the mobile anion on the superionic behaviour is explored. - Graphical abstract: Anomalous behaviour of the lattice expansion of SrCl{sub 2} at temperatures of {approx}1000 K is associated with the gradual transition to a superionic phase, whilst SrBr{sub 2} undergoes a first-order structural transition ({beta}{yields}{alpha}) to a fluorite-structured superionic phase at 920(3) K. Highlights: > Anomalous behaviour of the lattice expansion of SrCl{sub 2} occurs at temperatures {approx}1000 K. > Crystal structure of {beta}-SrBr{sub 2} is described in detail. > On heating, SrBr{sub 2} and BaCl{sub 2} transform to a fluorite-structured superionic phase. > Temperature dependence of the BaCl{sub 2} and BaBr{sub 2} structures is presented. > Nature of the superionic phases within the alkaline-earth halides is discussed.

  6. Propriétés électriques des solutions solides à structure fluorine excédentaire en anions Na 0.5- xY 0.5+ xF 2+2 x

    NASA Astrophysics Data System (ADS)

    Pontonnier, L.; Aléonard, S.; Roux, M. T.; Hammou, A.

    1987-07-01

    A study by impedance spectroscopy has been made of the ac conductivity of the Na 0.5- xY 0.5+ xF 2+2 x solid solutions which have a fluorite defect structure stabilized by doping. Although connected with the fluorine motion, the conductivity is almost independent of the anion excess. This behavior is different from that of CaF 2 or from that of βPbF 2 doped with trivalent cations.

  7. Interplay of structural, optical and magnetic properties in Gd doped CeO{sub 2}

    SciTech Connect

    Soni, S.; Dalela, S.; Kumar, Sudish; Meena, R. S.; Vats, V. S.

    2015-06-24

    In this research wok systematic investigation on the synthesis, characterization, optical and magnetic properties of Ce{sub 1-x}Gd{sub x}O{sub 2} (where x=0.02, 0.04, 0.06, and 0.10) synthesized using the Solid-state method. Structural, Optical and Magnetic properties of the samples were investigated by X-ray diffraction (XRD), UV-VIS-NIR spectroscopy and VSM. Fluorite structure is confirmed from the XRD measurement on Gd doped CeO{sub 2} samples. Magnetic studies showed that the Gd doped polycrystalline samples display room temperature ferromagnetism and the ferromagnetic ordering strengthens with the Gd concentration.

  8. Structural and dielectric properties of Cu doped CeO2

    NASA Astrophysics Data System (ADS)

    Kumar, Pawan; Chand, F.; Kumar, Parmod; Meena, R. C.; Kumar, Ashish; Asokan, K.

    2016-05-01

    In the present work, we report the structural and dielectric properties of Cu doped CeO2 compounds synthesized by solid state reaction method. X-ray diffraction study confirms the formation of cubic phase with Fm-3m space group corresponding to fluorite CeO2 structure. The crystallite size was found to decrease with Cu concentration. Furthermore, the dielectric measurements were carried out by varying frequency in the temperature range of 120 to 380 K. It has been found that the increase in Cu concentration in the host lattice leads to an enhancement in dielectric constant.

  9. Structural and dielectric properties of Cu doped CeO{sub 2}

    SciTech Connect

    Kumar, Pawan Chand, F.; Kumar, Parmod; Meena, R. C.; Kumar, Ashish; Asokan, K.

    2016-05-06

    In the present work, we report the structural and dielectric properties of Cu doped CeO{sub 2} compounds synthesized by solid state reaction method. X-ray diffraction study confirms the formation of cubic phase with Fm-3m space group corresponding to fluorite CeO{sub 2} structure. The crystallite size was found to decrease with Cu concentration. Furthermore, the dielectric measurements were carried out by varying frequency in the temperature range of 120 to 380 K. It has been found that the increase in Cu concentration in the host lattice leads to an enhancement in dielectric constant.

  10. The modulated structure of Co3Al4Si2: incommensurability and Co-Co interactions in search of filled octadecets.

    PubMed

    Fredrickson, Rie T; Fredrickson, Daniel C

    2013-03-18

    Incommensurate modulations are increasingly being recognized as a common phenomenon in solid-state compounds ranging from inorganic materials to molecular crystals. The origins of such modulations are often mysterious, but appear to be as diverse as the compounds in which they arise. In this Article, we describe the crystal structure and bonding of Co3Al4Si2, the δ phase of the Co-Si-Al system, whose modulated structure can be traced to a central concept of inorganic chemistry: the 18 electron rule. The structure is monoclinic, conforming to the 3 + 1D superspace group C/2m(0β0)s0. The basis of the crystal structure is a rod packing of columns of the fluorite (CaF2) type, a theme that is shared by the recently determined structure of Fe8Al(17.4)Si(7.6). The columns are arranged into sheets, within which the fluorite structure's primitive cubic network of Si/Al atoms continues uninterrupted from column to column. Between the sheets, layers of interstitial Si/Al atoms occur, some of which are arranged with a periodicity incommensurate with that of the fluorite-type columns. Strong modulations in the interstitial layers result. Electronic structure calculations, using a DFT-calibrated Hückel model on a commensurate approximate structure, reveal that the complex pattern of atoms within these interstitial layers serves to distribute Si/Al atoms around the Co atoms in order to reach 18 electron counts (filled octadecets). Central to this bonding scheme is the covalent sharing of electron pairs between Co atoms. The shared electron pairs occupy orbitals that are isolobal to classical Co-Co σ and π bonds, but whose stability is tied to multicenter character involving bridging Si/Al atoms. Through these features, Co3Al4Si2 expands the structural and electronic manifestations of the 18 electron rule in solid-state inorganic compounds.

  11. A Theoretical Study of Structural, Electronic and Vibrational Properties of Small Fluoride Clusters

    NASA Astrophysics Data System (ADS)

    Waters, Kevin; Pandey, Ratnesh; Nigam, Sandeep; He, Haiying; Pingle, Subhash; Pandey, Avinash; Pandey, Ravindra

    2014-03-01

    Alkaline earth metal fluorides are an interesting family of ionic crystals having a wide range of applications in solid state lasers, luminescence, scintillators, to name just a few. In this work, small stoichiometric clusters of (MF2)n (M = Mg, Ca Sr, Ba, n =1-6) were studied for structural, vibrational and electronic properties using first-principles methods based on density functional theory. A clear trend of structural and electronic structure evolution was found for all the alkaline earth metal fluorides when the cluster size n increases from 1 to 6. Our study reveals that these fluoride clusters mimic the bulk-like behavior at the very small size. Among the four series of metal fluorides, however, (MgF2)n clusters stands out to be different in its preference of equilibrium structures owing to the much smaller ionic radius of Mg and the higher degree of covalency in the Mg-F bonding. The calculated binding energy, highest stretching frequency, ionization potential, and HOMO-LUMO gap decrease from MgF2 to BaF2 for the same cluster size. These variations are explained in terms of the change in the ionic radius and the basicity of the metal ions.

  12. The Crystal Structure of Thorium and Zirconium Dihydrides by X-ray and Neutron Diffraction

    DOE R&D Accomplishments Database

    Rundle, R.E.; Shull, C.G.; Wollan, E.O.

    1951-04-20

    Thorium forms a tetragonal lower hydride of composition ThH{sub 2}. The hydrides ThH{sub 2}, ThD{sub 2}, and ZrD{sub 2} have been studied by neutron diffraction in order that hydrogen positions could be determined. The hydrides are isomorphous, and have a deformed fluorite structure. Metal-hydrogen distances in thorium hydride are unusually large, as in UH{sub 3}. Thorium and zirconium scattering amplitudes and a revised scattering amplitude for deuterium are reported.

  13. Hydrothermally synthesized barium fluoride nanocubes for thermoluminescence dosimetry

    NASA Astrophysics Data System (ADS)

    Bhadane, Mahesh S.; Dahiwale, S. S.; Bhoraskar, V. N.; Dhole, S. D.

    2016-05-01

    In this work, we report a hydrothermally synthesized Dy doped BaF2 (BaF2:Dy) nanocubes and its Thermoluminescence studies. The synthesized BaF2:Dy samples was found to posses FCC structure and having average size ~ 60-70 nm, as revealed through X-Ray Diffraction. Cubical morphology having size ~90 nm was observed from TEM analysis. The 60Co γ- ray irradiated BaF2:Dy TL dosimetric experiments shows a pre-dominant single glow peak at 153 °C, indicating a single level trap present as a metastable state. Furthermore, BaF2:Dy nanophosphor shows a sharp linear response from 10 Gy to 3 kGy, thus it can be applicable as a gamma dosimeter.

  14. Synthesis, crystal structure, and properties of new lead barium borate with B3O6 plane hexagonal rings

    NASA Astrophysics Data System (ADS)

    Zhao, Wenwu

    2017-08-01

    A new lead barium borate Ba8.02Pb0.98(B3O6)6 with B3O6 plane hexagonal rings was synthesized through spontaneous nucleation from a high-temperature solution utilizing PbO, H3BO3, and BaF2 as reagents. Its crystal structure was determined from single-crystal X-ray diffraction data and further characterized by FT-IR. It crystallizes in space group R32 and the crystallographic structure of Ba8.02Pb0.98(B3O6)6 can be described as a layer-like structure, there is stacking along the c-axis of B3O6 plane hexagonal rings with the Ba2 and Pb/Ba1 atoms alternately occupying sites between the B3O6 sheets. A comparison of the structures of Ba8.02Pb0.98(B3O6)6, PbBa2(B3O6)2 and α-BaB2O4 is presented. UV-Vis-NIR diffuse-reflectance spectrum indicates that the absorption edge of Ba8.02Pb0.98(B3O6)6 is about 399 nm.

  15. Structuralism.

    ERIC Educational Resources Information Center

    Piaget, Jean

    Provided is an overview of the analytical method known as structuralism. The first chapter discusses the three key components of the concept of a structure: the view of a system as a whole instead of so many parts; the study of the transformations in the system; and the fact that these transformations never lead beyond the system but always…

  16. Structuralism.

    ERIC Educational Resources Information Center

    Piaget, Jean

    Provided is an overview of the analytical method known as structuralism. The first chapter discusses the three key components of the concept of a structure: the view of a system as a whole instead of so many parts; the study of the transformations in the system; and the fact that these transformations never lead beyond the system but always…

  17. Site distortions created by the stereoactive lone pair of Tin(II) in highly symmetric structures

    NASA Astrophysics Data System (ADS)

    Dénès, Georges; Madamba, M. Cecilia; Merazig, Hocine; Muntasar, Abdualhafed; Zhu, Zhimeng

    2016-10-01

    Several fluoride compounds containing divalent tin that have a fluorite (CaF2-type) unit cell have been prepared and studied. Some are stoichiometric compounds while others are solid solutions. The cubic symmetry of the unit-cell (no lattice distortion and no superstructure) and the unique metal ion site of the fluorite structure make it that tin and the other metal have to be disordered on the normal metal site of the fluorite unit-cell. However, that site has the m3m-Oh point symmetry, and the metal ion is located in the center of a cube having fluoride ions in all its corners. Therefore, the same coordination should apply to tin. However, tin(II) possesses a non-bonding pair of electrons called a "lone pair", and in order for tin(II) to have a cubic symmetry, its lone pair has to be located on the unhybridized 5s orbital, that is spherical and thus does not distort the coordination. In such a case, the lone pair is said to be "non-stereoactive". This would make tin present in the form of the Sn2+ stannous ion, and therefore Sn-F bonding must be ionic. However, tin(II) fluorides are known to be always covalent with a hybridized lone pair on tin, which has therefore a reduced coordination number and therefore a highly distorted polyhedron of coordination. Such a hybridized lone pair is said to be "stereoactive". Tin-119 Mössbauer spectroscopy was used to probe the bonding type and it showed that bonding is covalent, the lone pair is hybridized and the tin coordination is dramatically distorted. A model based on a double disorder was made that accounts for the apparent contradiction between the crystallographic and the Mössbauer results.

  18. Anisotropy of bond projections in simple crystal structures

    NASA Astrophysics Data System (ADS)

    Šimůnek, Antonín

    2011-10-01

    The nearest-neighbor bond distances represented by the stick-and-ball model of a crystal are projected into planes in order to find the directions from where the projections have maximum or minimum values. The projection directions and their corresponding values of the maxima and minima are presented for simple cubic, body-centered-cubic, face-centered-cubic, sodium chloride, zinc sulfide, diamond, fluorite, cesium chloride, hexagonal close-packed, tungsten carbide, wurtzite, graphite, graphene, and aluminum boride structures. The purely geometrical considerations quantitatively reflect an anisotropy of the bond projections and provide data for a large amount of materials crystallizing in these structures. The presented results can be applied to the description, analysis, and understanding of anisotropic effects related to bond projection in 14 crystal structures. The application of hardness anisotropy for BN, SiC, and TiC is shown.

  19. Nanostructured crystals of fluorite phases Sr1 - x R x F2 + x ( R are rare-earth elements) and their ordering. I. Crystal growth of Sr1 - x R x F2 + x ( R = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu)

    NASA Astrophysics Data System (ADS)

    Sobolev, B. P.; Karimov, D. N.; Sul'Yanov, S. N.; Zhmurova, Z. I.

    2009-01-01

    Crystals of nonstoichiometric phases Sr1 - x R x F2 + x ( R are 14 rare-earth elements) and the ordered phase Sr4Lu3F17 with a trigonally distorted fluorite lattice were grown by the Bridgman method. Ten of 26 Sr1 - x R x F2 + x crystals, where R = La-Ho or Y, melt congruently. The isoconcentration series Sr0.90 R 0.10F2.10 includes four crystals with R = Er-Lu. The compositions corresponding to the maxima for the latter crystals were not determined. The concentration series, in which the mole fraction of RF3 varies from 10 to 50 mol %, were obtained for the crystals with R = La, Nd, and Gd. Most of the crystals are of good optical quality. To evaluate the composition changes in the course of crystal growth, the cubic unit-cell parameters were determined by X-ray powder diffraction. The line-broadening analysis revealed a nonmonotonic change of microdistortions as regards both the rare-earth content and rare-earth series. The changes in the lattice parameters and the congruent-melting points of the Sr1 - x R x F2 + x phases in the rare-earth series reflect the morphotropic transitions in the series of pure RF3 despite the fact that SrF2 dominates in nonstoichiometric fluorite crystals.

  20. A Hydrothermal System Associated with the Siljan Impact Structure, Sweden-Implications for the Search for Fossil Life on Mars

    NASA Astrophysics Data System (ADS)

    Hode, Tomas; von Dalwigk, Ilka; Broman, Curt

    2003-06-01

    The Siljan ring structure (368 +/- 1.1 Ma) is the largest known impact structure in Europe. It is a 65-km-wide, eroded, complex impact structure, displaying several structural units, including a central uplifted region surrounded by a ring-shaped depression. Associated with the impact crater are traces of a post-impact hydrothermal system indicated by precipitated and altered hydrothermal mineral assemblages. Precipitated hydrothermal minerals include quartz veins and breccia fillings associated with granitic rocks at the outer margin of the central uplift, and calcite, fluorite, galena, and sphalerite veins associated with Paleozoic carbonate rocks located outside the central uplift. Two-phase water/gas and oil/gas inclusions in calcite and fluorite display homogenization temperatures between 75°C and 137°C. With an estimated erosional unloading of ~1 km, the formation temperatures were probably not more than 10-15°C higher. Fluid inclusion ice-melting temperatures indicate a very low salt content, reducing the probability that the mineralization was precipitated during the Caledonian Orogeny. Our findings suggest that large impacts induce low-temperature hydrothermal systems that may be habitats for thermophilic organisms. Large impact structures on Mars may therefore be suitable targets in the search for fossil thermophilic organisms.

  1. A hydrothermal system associated with the Siljan impact structure, Sweden--implications for the search for fossil life on Mars.

    PubMed

    Hode, Tomas; von Dalwigk, Ilka; Broman, Curt

    2003-01-01

    The Siljan ring structure (368 +/- 1.1 Ma) is the largest known impact structure in Europe. It isa 65-km-wide, eroded, complex impact structure, displaying several structural units, including a central uplifted region surrounded by a ring-shaped depression. Associated with the impact crater are traces of a post-impact hydrothermal system indicated by precipitated and altered hydrothermal mineral assemblages. Precipitated hydrothermal minerals include quartz veins and breccia fillings associated with granitic rocks at the outer margin of the central uplift, and calcite, fluorite, galena, and sphalerite veins associated with Paleozoic carbonate rocks located outside the central uplift. Two-phase water/gas and oil/gas inclusions in calcite and fluorite display homogenization temperatures between 75 degrees C and 137 degrees C. With an estimated erosional unloading of approximately 1 km, the formation temperatures were probably not more than 10-15 degrees C higher. Fluid inclusion ice-melting temperatures indicate a very low salt content, reducing the probability that the mineralization was precipitated during the Caledonian Orogeny. Our findings suggest that large impacts induce low-temperature hydrothermal systems that may be habitats for thermophilic organisms. Large impact structures on Mars may therefore be suitable targets in the search for fossil thermophilic organisms.

  2. Percolating hierarchical defect structures drive phase transformation in Ce1-x Gd x O2-x/2: a total scattering study.

    PubMed

    Scavini, Marco; Coduri, Mauro; Allieta, Mattia; Masala, Paolo; Cappelli, Serena; Oliva, Cesare; Brunelli, Michela; Orsini, Francesco; Ferrero, Claudio

    2015-09-01

    A new hierarchical approach is presented for elucidating the structural disorder in Ce1-x Gd x O2-x/2 solid solutions on different scale lengths. The primary goal of this investigation is to shed light on the relations between the short-range and the average structure of these materials via an analysis of disorder on the mesocopic scale. Real-space (pair distribution function) and reciprocal-space (Rietveld refinement and microstructure probing) analysis of X-ray powder diffraction data and electron spin resonance (ESR) investigations were carried out following this approach. On the local scale, Gd- and Ce-rich droplets (i.e. small regions a few ångströms wide) form, exhibiting either a distorted fluorite (CeO2) or a C-type (Gd2O3) structure in the whole compositional range. These droplets can then form C-type nanodomains which, for Gd concentrations x Gd ≤ 0.25, are embedded in the fluorite matrix. At the site percolation threshold p C for a cubic lattice (x Gd = p C ≃ 0.311), C-type nanodomains percolate inside each crystallite and a structural phase transformation is observed. When this occurs, the peak-to-peak ESR line width ΔH pp shows a step-like behaviour, which can be associated with the increase in Gd-Gd dipolar interactions. A general crystallographic rationale is presented to explain the fluorite-to-C-type phase transformation. The approach shown here could be adopted more generally in the analysis of disorder in other highly doped materials.

  3. Percolating hierarchical defect structures drive phase transformation in Ce1−xGdxO2−x/2: a total scattering study

    PubMed Central

    Scavini, Marco; Coduri, Mauro; Allieta, Mattia; Masala, Paolo; Cappelli, Serena; Oliva, Cesare; Brunelli, Michela; Orsini, Francesco; Ferrero, Claudio

    2015-01-01

    A new hierarchical approach is presented for elucidating the structural disorder in Ce1−xGdxO2−x/2 solid solutions on different scale lengths. The primary goal of this investigation is to shed light on the relations between the short-range and the average structure of these materials via an analysis of disorder on the mesocopic scale. Real-space (pair distribution function) and reciprocal-space (Rietveld refinement and microstructure probing) analysis of X-ray powder diffraction data and electron spin resonance (ESR) investigations were carried out following this approach. On the local scale, Gd- and Ce-rich droplets (i.e. small regions a few ångströms wide) form, exhibiting either a distorted fluorite (CeO2) or a C-type (Gd2O3) structure in the whole compositional range. These droplets can then form C-type nanodomains which, for Gd concentrations x Gd ≤ 0.25, are embedded in the fluorite matrix. At the site percolation threshold p C for a cubic lattice (x Gd = p C ≃ 0.311), C-type nanodomains percolate inside each crystallite and a structural phase transformation is observed. When this occurs, the peak-to-peak ESR line width ΔH pp shows a step-like behaviour, which can be associated with the increase in Gd–Gd dipolar interactions. A general crystallographic rationale is presented to explain the fluorite-to-C-type phase transformation. The approach shown here could be adopted more generally in the analysis of disorder in other highly doped materials. PMID:26306193

  4. Superionic phase transition of doped fluorites

    NASA Astrophysics Data System (ADS)

    den Hartog, H. W.; van der Veen, J.

    1988-02-01

    In this paper we present new results of specific-heat experiments on superionic mixtures of cubic lead fluoride and some rare-earth fluorides. The results depend very strongly on the rare-earth ion; for samples doped with LaF3 we observe a peak in the specific heat as a function of T, which is located at an approximately fixed position. This peak, which is due to the superionic transition, increases in width with increasing concentrations of LaF3. If we add YbF3, however, the position of the peak varies. It appears that in samples doped with a few mol % YbF3 there are two peaks in the ``specific-heat spectrum'': one very similar to the peak observed in pure PbF2 and a second peak situated at significantly lower temperatures. The results are discussed in view of the experimental data on the clustering properties of the different solid solutions. In addition, we treat some of the specific-heat data with theoretical models, which have been proposed by Vlieg, den Hartog, and Winnink. This analysis suggests that due to the introduction of La impurities the formation of Frenkel pairs is more difficult. The introduction of Yb impurities, however, leads to additional Frenkel-pair formation, because Pb1-xYbxF2+x clusters, consisting of more than one Yb-F interstitial pair, trap extra interstitial F- ions. Because the energy of these extra trapped interstitial F- ions is lower than the energy of free anion interstitials, this leads to an enhancement of the formation of Frenkel pairs.

  5. Raman micro-spectroscopy of UOX and MOX spent nuclear fuel characterization and oxidation resistance of the high burn-up structure

    NASA Astrophysics Data System (ADS)

    Jegou, C.; Gennisson, M.; Peuget, S.; Desgranges, L.; Guimbretière, G.; Magnin, M.; Talip, Z.; Simon, P.

    2015-03-01

    Raman micro-spectroscopy was applied to study the structure and oxidation resistance of UO2 (burnup 60 GWd/tHM) and MOX (burnup 47 GWd/tHM) irradiated fuels. The Raman technique, adapted to working under extreme conditions, enabled structural information to be obtained at the cubic micrometer scale in various zones of interest within irradiated fuel (central and zones like the Rim for UOX60, and the plutonium-enriched agglomerates for MOX47 characterized by a high burn-up structure), and the study of their oxidation resistance. As regards the structural information after irradiation, the spectra obtained make up a set of data consistent with the systematic presence of the T2g band characteristic of the fluorite structure, and of a triplet band located between 500 and 700 cm-1. The existence of this triplet can be attributed to the presence of defects originating in changes to the fuel chemistry occurring in the reactor (presence of fission products) and to the accumulation of irradiation damage. As concerns the oxidation resistance of the different zones of interest, Raman spectroscopy results confirmed the good stability of the restructured zones (plutonium-enriched agglomerates and Rim) rich in fission products compared to the non-restructured UO2 grains. A greater structural stability was noticed in the case of high plutonium content agglomerates, as this element favors the maintenance of the fluorite structure.

  6. Crystal structure of α‴-(Zn1- x Cd x )3As2 ( x = 0.26)

    NASA Astrophysics Data System (ADS)

    Volodina, G. F.; Zakhvalinskii, V. S.; Kravtsov, V. Kh.

    2013-07-01

    Single crystals of the α‴-phase of (Zn1 - x Cd x )3As2 solid solution ( x = 0.26) have been prepared and investigated by X-ray diffraction analysis. The tetragonal unit-cell parameters are found to be a = b = 8.5377(2) Å, c = 24.0666(9) Å, sp. gr. I41/ amd, Z = 16. Zn and Cd atoms in the crystal statistically occupy three symmetrically independent positions in the mirror planes and are tetrahedrally coordinated by arsenic atoms. (Zn,Cd) tetrahedra share edges to form a three-dimensional structure framework. The α‴-phase is geometrically related to the fluorite structure. The character of arrangement of tetrahedral vacancies in fluorite-like unit cells is revealed. Chains of tetrahedral vacancies form microchannels oriented parallel the a and b axes, which pierce the three-dimensional structure framework at different levels along the c axis. The structure of α″-Cd3As2 crystals is found to be similar to that of α‴-(Zn0.74Cd0.26)3As2.

  7. Scheelite and coexisting F-rich zoned garnet, vesuvianite, fluorite, and apatite in calc-silicate rocks from the Mogok metamorphic belt, Myanmar: Implications for metasomatism in marble and the role of halogens in W mobilization and mineralization

    NASA Astrophysics Data System (ADS)

    Guo, Shun; Chen, Yi; Liu, Chuan-Zhou; Wang, Jian-Gang; Su, Bin; Gao, Yi-Jie; Wu, Fu-Yuan; Sein, Kyaing; Yang, Yue-Heng; Mao, Qian

    2016-03-01

    to a significant enrichment of F (up to 1.2 wt.%) and OH (up to 0.32 for nOH) and a negative correlation between F and Si in the garnet. Detailed petrographic observations show that the occurrence of scheelite in the MMB calc-silicate rocks is always associated with the growth of F-rich minerals such as garnet rims (0.8-1.2 wt.% F), vesuvianite (2.4-2.6 wt.% F), fluorite (48-49 wt.% F), apatite (3.9-4.1 wt.% F), and titanite (2.6-3.4 wt.% F). These textural characteristics, combined with the positive correlation of whole-rock F and W (as well as Sn, Mo) contents in the calc-silicate rocks, indicate that the elevated F contents increased the solubility of W in the infiltrating fluid, thereby allowing the W transfer in the hydrothermal-metasomatic system. The mineralization of scheelite was triggered by the crystallization of F-rich minerals during the formation of the calc-silicate rocks, which caused F depletion and consequent saturation of W in the metasomatic fluid. Our results suggest that, in the MMB metasomatic system, F rather than Cl is the key fluxing compound that facilitates the transfer of W and the mineralization of scheelite.

  8. Enhanced anisotropic ionic diffusion in layered electrolyte structures from density functional theory

    NASA Astrophysics Data System (ADS)

    Hirschfeld, J. A.; Lustfeld, H.

    2014-01-01

    Electrolytes with high ionic diffusivity at temperatures distinctively lower than the presently used ones are the prerequisite for the success of, e.g., solid oxide fuel cells. We have found a promising structure having an asymmetric but superior ionic mobility in the direction of the oxygen-ion current. Using a layering of zirconium and yttrium in the fluorite structure of zirconia, a high vacancy concentration and a low migration barrier in two dimensions are obtained, while the mobility in the third direction is basically sacrificed. According to our density functional theory calculations an electrolyte made of this structure could operate at a temperature reduced by ≈200∘C. Thus a window to a different class of electrolytes has been flung open. In our structure the price paid is a more complicated manufacturing method.

  9. Structure and stability of several high-pressure crystalline polymorphs of silica

    NASA Astrophysics Data System (ADS)

    Tse, John S.; Klug, Dennis D.; Allan, Douglas C.

    1995-06-01

    The structure and stability of several proposed high-pressure crystalline polymorphs of silica have been studied with a first-principles pseudopotential total-energy method within the local-density approximation. The monoclinic I2/a phase predicted recently from molecular-dynamics calculations on highly compressed amorphous silica or α quartz [J.S. Tse, D.D. Klug, and Y. LePage, Phys. Rev. Lett. 69, 3647 (1992)] is shown to be energetically competitive with the well-known stishovite structure and is substantially lower in energy than the cubic fluorite Pa3¯ structure up to 170 GPa. The results are in full accordance with previous molecular-dynamics calculations thus confirming that the empirical potential proposed by van Beest et al. is applicable to the study of the structure and dynamics of high-pressure phases of silica.

  10. Preparation and structural study from neutron diffraction data of Pr{sub 5}Mo{sub 3}O{sub 16}

    SciTech Connect

    Martinez-Lope, M.J.; Alonso, J.A.; Sheptyakov, D.; Pomjakushin, V.

    2010-12-15

    The title compound has been prepared as polycrystalline powder by thermal treatments of mixtures of Pr{sub 6}O{sub 11} and MoO{sub 2} in air. In the literature, an oxide with a composition Pr{sub 2}MoO{sub 6} has been formerly described to present interesting catalytic properties, but its true stoichiometry and crystal structure are reported here for the first time. It is cubic, isostructural with CdTm{sub 4}Mo{sub 3}O{sub 16} (space group Pn-3n, Z=8), with a=11.0897(1) A. The structure contains MoO{sub 4} tetrahedral units, with Mo-O distances of 1.788(2) A, fully long-range ordered with PrO{sub 8} polyhedra; in fact it can be considered as a superstructure of fluorite (M{sub 8}O{sub 16}), containing 32 MO{sub 2} fluorite formulae per unit cell, with a lattice parameter related to that of cubic fluorite (a{sub f}=5.5 A) as a{approx}2a{sub f}. A bond valence study indicates that Mo exhibits a mixed oxidation state between 5+ and 6+ (perhaps accounting for the excellent catalytic properties). One kind of Pr atoms is trivalent whereas the second presents a mixed Pr{sup 3+}-Pr{sup 4+} oxidation state. The similarity of the XRD pattern with that published for Ce{sub 2}MoO{sub 6} suggests that this compound also belongs to the same structural type, with an actual stoichiometry Ce{sub 5}Mo{sub 3}O{sub 16}. -- Graphical Abstract: Formerly formulated as Pr{sub 2}MoO{sub 6}, the title compound is a cubic superstructure of fluorite (a=11.0897(1) A, space group Pn-3n) due to the long-range ordering of PrO{sub 8} scalenohedra and MoO{sub 4} tetrahedral units, showing noticeable shifts of the oxygen positions in order to provide a tetrahedral coordination for Mo ions. A mixed valence Mo{sup 5+}-Mo{sup 6+} is identified, which could account for the excellent catalytic properties of this material. Display Omitted

  11. Pressure Effect on the Structural Transition and Suppression of the High-Spin State in the Triple-Layer T'-La₄Ni₃O₈

    SciTech Connect

    Cheng, J.-G.; Zhou, J.-S.; Goodenough, J. B.; Zhou, H. D.; Matsubayashi, K.; Uwatoko, Y.; Kong, P. P.; Jin, C. Q.; Yang, W. G.; Shen, G. Y.

    2012-06-08

    We report a comprehensive high-pressure study on the triple-layer T'-La₄Ni₃O₈ with a suite of experimental probes, including structure determination, magnetic, and transport properties up to 50 GPa. Consistent with a recent ab inito calculation, application of hydrostatic pressure suppresses an insulator-metal spin-state transition at Pc≈6 GPa. However, a low-spin metallic phase does not emerge after the high-spin state is suppressed to the lowest temperature. For P>20 GPa, the ambient T' structure transforms gradually to a T-type structure, which involves a structural reconstruction from fluorite La–O₂–La blocks under low pressures to rock-salt LaO-LaO blocks under high pressures. Absence of the metallic phase under pressure has been discussed in terms of local displacements of O²⁻ ions in the fluorite block under pressure before a global T† phase is established.

  12. Pressure Effect on the Structural Transition and Suppression of the High-Spin State in the Triple-Layer T'-La₄Ni₃O₈

    DOE PAGES

    Cheng, J.-G.; Zhou, J.-S.; Goodenough, J. B.; ...

    2012-06-08

    We report a comprehensive high-pressure study on the triple-layer T'-La₄Ni₃O₈ with a suite of experimental probes, including structure determination, magnetic, and transport properties up to 50 GPa. Consistent with a recent ab inito calculation, application of hydrostatic pressure suppresses an insulator-metal spin-state transition at Pc≈6 GPa. However, a low-spin metallic phase does not emerge after the high-spin state is suppressed to the lowest temperature. For P>20 GPa, the ambient T' structure transforms gradually to a T†-type structure, which involves a structural reconstruction from fluorite La–O₂–La blocks under low pressures to rock-salt LaO-LaO blocks under high pressures. Absence of the metallicmore » phase under pressure has been discussed in terms of local displacements of O²⁻ ions in the fluorite block under pressure before a global T† phase is established.« less

  13. Analysis of Mn2+ EPR spectral shapes for studies of the oxyfluoride glass ceramics

    NASA Astrophysics Data System (ADS)

    Fedotovs, A.; Berzins, Dz; Kiselova, O.; Sarakovskis, A.; Rogulis, U.

    2011-06-01

    We investigated the EPR superhyperfine structure of the Mn2+ ion in the disordered BaF2 crystalline media which tends to crystallize in the oxyfluoride glass-ceramics material. Obtained EPR spectra reveal explicit shf structure due to Mn2+ ion building into the BaF2 lattice showing its usefulness as a probe in orientationally disordered and amorphous structures. Two types with explicit shf structure of Mn2+ ion characteristic EPR spectra were obtained in BaF2 powder samples characteristic with broad (type 1) and narrow (type 2) hyperfine structure lines. Spectra of the ZnF2-BaF2 oxyfluoride glass-ceramics samples revealed explicit fine structure lines.

  14. Structural and catalytic properties of lanthanide (La, Eu, Gd) doped ceria

    NASA Astrophysics Data System (ADS)

    Hernández, W. Y.; Laguna, O. H.; Centeno, M. A.; Odriozola, J. A.

    2011-11-01

    Ce 0.9M0.1O 2- δ mixed oxides ( M=La, Eu and Gd) were synthesized by coprecipitation. Independent of the dopant cation, the obtained solids maintain the F-type crystalline structure, characteristic of CeO 2 (fluorite structure) without phase segregation. The ceria lattice expands depending on the ionic radii of the dopant cation, as indicated by X-ray diffraction studies. This effect also agrees with the observed shift of the F2g Raman vibrational mode. The presence of the dopant cations in the ceria lattice increases the concentration of structural oxygen vacancies and the reducibility of the redox pair Ce 4+/Ce 3+. All synthesized materials show higher catalytic activity for the CO oxidation reaction than that of bare CeO 2, being Eu-doped solid the one with the best catalytic performances despite of its lower surface area.

  15. Structural Refinement of the High Temperature Form of Bi 2MoO 6

    NASA Astrophysics Data System (ADS)

    Buttrey, D. J.; Vogt, T.; Wildgruber, U.; Robinson, W. R.

    1994-07-01

    The structure of the high temperature polymorph of Bi 2MoO 6 has been determined by Rietveld powder refinement of high-resolution neutron diffraction data from the H1A diffractometer at the Brookhaven HFBR (high flux beam reactor). The cation distribution forms a fluorite-related supercell with infinite channels of bismuth polyhedra surrounded by molybdenum tetrahedra, in agreement with previously proposed models. The structure is refined in the monoclinic space group P 2 1/ c to Rwp = 0.134 and R1 = 0.084 ( Rexp = 0.024) from 2559 reflections: a = 17.2627(1) Å, b = 22.4296(2) Å, c = 5.58489(5) Å, β = 90.4974(6)°, Z = 16. Bond valence sums and site potentials have been calculated which reveal evidence for significant charge transfer in this structure. The relationship of this phase to other catalytically-active bismuth molybdates is discussed.

  16. Topotactic oxidation pathway of ScTiO3 and high-temperature structure evolution of ScTiO3.5 and Sc4Ti3O12-type phases.

    PubMed

    Shafi, Shahid P; Hernden, Bradley C; Cranswick, Lachlan M D; Hansen, Thomas C; Bieringer, Mario

    2012-02-06

    The novel oxide defect fluorite phase ScTiO(3.5) is formed during the topotactic oxidation of ScTiO(3) bixbyite. We report the oxidation pathway of ScTiO(3) and structure evolution of ScTiO(3.5), Sc(4)Ti(3)O(12), and related scandium-deficient phases as well as high-temperature phase transitions between room temperature and 1300 °Cusing in-situ X-ray diffraction. We provide the first detailed powder neutron diffraction study for ScTiO(3). ScTiO(3) crystallizes in the cubic bixbyite structure in space group Ia3 (206) with a = 9.7099(4) Å. The topotactic oxidation product ScTiO(3.5) crystallizes in an oxide defect fluorite structure in space group Fm3m (225) with a = 4.89199(5) Å. Thermogravimetric and differential thermal analysis experiments combined with in-situ X-ray powder diffraction studies illustrate a complex sequence of a topotactic oxidation pathway, phase segregation, and ion ordering at high temperatures. The optimized bulk synthesis for phase pure ScTiO(3.5) is presented. In contrast to the vanadium-based defect fluorite phases AVO(3.5+x) (A = Sc, In) the novel titanium analogue ScTiO(3.5) is stable over a wide temperature range. Above 950 °C ScTiO(3.5) undergoes decomposition with the final products being Sc(4)Ti(3)O(12) and TiO(2). Simultaneous Rietveld refinements against powder X-ray and neutron diffraction data showed that Sc(4)Ti(3)O(12) also exists in the defect fluorite structure in space group Fm3m (225) with a = 4.90077(4) Å. Sc(4)Ti(3)O(12) undergoes partial reduction in CO/Ar atmosphere to form Sc(4)Ti(3)O(11.69(2)).

  17. Structural transition in rare earth doped zirconium oxide: A positron annihilation study

    SciTech Connect

    Chakraborty, Keka; Bisoi, Abhijit

    2012-11-15

    Graphical abstract: New microstructural analysis and phase transition of rare earth doped mixed oxide compounds such as: Sm{sub 2−x}Dy{sub x}Zr{sub 2}O{sub 7} (where x = 0.0 ≤ x ≥ 2.0) that are potentially useful as solid oxide fuels, ionic conductors, optoelectronic materials and most importantly as radiation resistant host for high level rad-waste disposal, structural transition in the system is reported through positron annihilation spectroscopy as there is an indication in the X-ray diffraction analysis. Highlights: ► Zirconium oxide material doped with rare earth ions. ► The method of positron annihilation spectroscopy suggests a phase transition in the system. ► The crystal structure transformation from pure pyrochlore to defect fluorite type of structure is shown by X-ray diffraction results. -- Abstract: A series of compounds with the general composition Sm{sub 2−x}Dy{sub x}Zr{sub 2}O{sub 7} (where 0 ≤ x ≥ 2.0) were synthesized by chemical route and characterized by powder X-ray diffraction (XRD) analysis. The rare earth ion namely Sm{sup +3} in the compound was gradually replaced with another smaller and heavier ion, Dy{sup +3} of the 4f series, there by resulting in order–disorder structural transition, which has been studied by positron annihilation lifetime and Doppler broadening spectroscopy. This study reveals the subtle electronic micro environmental changes in the pyrochlore lattice (prevalent due to the oxygen vacancy in anti-site defect structure of the compound) toward its transformation to defect fluorite structure as found in Dy{sub 2}Zr{sub 2}O{sub 7}. A comparison of the changes perceived with PAS as compared to XRD analysis is critically assayed.

  18. Characterization of the high-pressure structures and phase transformations in SnO2. A density functional theory study.

    PubMed

    Gracia, L; Beltrán, A; Andrés, J

    2007-06-14

    Theoretical investigations concerning the high-pressure polymorphs, the equations of state, and the phase transitions of SnO2 have been performed using density functional theory at the B3LYP level. Total energy calculations and geometry optimizations have been carried out for all phases involved, and the following sequence of structural transitions from the rutile-type (P42/mnm) driven by pressure has been obtained (the transition pressure is in parentheses): --> CaCl2-type, Pnnm (12 GPa) --> alpha-PbO2-type, Pbcn (17 GPa) --> pyrite-type, Pa (17 GPa) --> ZrO2-type orthorhombic phase I, Pbca (18 GPa) --> fluorite-type, Fmm (24 GPa) --> cotunnite-type orthorhombic phase II, Pnam (33 GPa). The highest bulk modulus values, calculated by fitting pressure-volume data to the second-order Birch-Murnaghan equation of state, correspond to the cubic pyrite and the fluorite-type phases with values of 293 and 322 GPa, respectively.

  19. Rare-earth doped sol-gel derived oxyfluoride glass-ceramics: Structural and optical characterization

    NASA Astrophysics Data System (ADS)

    Secu, C. E.; Secu, M.; Ghica, C.; Mihut, L.

    2011-09-01

    Glass-ceramics containing RE 3+-doped BaF 2 nanocrystals (RE = Eu, Sm, Dy, Ho and Pr) with the size below 10 nm size have been made by using the controlled crystallization at higher temperatures of the RE 3+-doped SiO 2-BaF 2 xerogels. Photoluminescence measurements have indicated the incorporation of the RE 3+ ions in both silica network and in the BaF 2 nanocrystals. Thermoluminescence measurements have shown a peak whose position depends on the nature of RE 3+-dopant as it follows: 140 °C (for Ho 3+, Dy 3+), 340 °C (for Sm 3+) and 370 °C (for Eu 3+); in Eu 3+-doped SiO 2 glass the TL peak is shifted to 383 °C. The peaks in glass-ceramics were assigned to the recombination of the electrons thermal released from the RE 3+-electron traps located in both glass-matrix and BaF 2 nanocrystals. Within the series the trivalent lanthanide ions act as increasingly deeper electron trapping centres.

  20. Simultaneous measurement of (n,{gamma}) and (n,fission) cross sections with the DANCE 4{pi} BaF2 array

    SciTech Connect

    Bredeweg, T. A.; Fowler, M. M.; Bond, E. M.; Chadwick, M. B.; Hunt, L. F.; O'Donnell, J. M.; Rundberg, R. S.; Schwantes, J. M.; Ullmann, J. L.; Vieira, D. J.; Wilhelmy, J. B.; Wouters, J. M.; Becker, J. A.; Clement, R. R. C.; Esch, E.-I.; Macri, R. A.; Wu, C.-Y.; Ethvignot, T.; Granier, T.; Yurkon, J. E.

    2006-03-13

    Neutron capture cross section measurements on many of the actinides are complicated by low-energy neutron-induced fission, which competes with neutron capture to varying degrees depending on the nuclide of interest. Measurements of neutron capture on 235U using the Detector for Advanced Neutron Capture Experiments (DANCE) have shown that we can partially resolve capture from fission events based on total photon calorimetry (i.e. total {gamma}-ray energy and {gamma}-ray multiplicity per event). The addition of a fission-tagging detector to the DANCE array will greatly improve our ability to separate these two competing processes so that improved neutron capture and (n,{gamma})/(n,fission) cross section ratio measurements can be obtained. The addition of a fission-tagging detector to the DANCE array will also provide a means to study several important issues associated with neutron-induced fission, including (n,fission) cross sections as a function of incident neutron energy, and total energy and multiplicity of prompt fission photons. We have focused on two detector designs with complementary capabilities, a parallel-plate avalanche counter and an array of solar cells.

  1. GEOMETRY AND VIBRATIONAL SPECTRA OF ALKALINE-EARTH DIHALIDES, II. CaF2, SrF2 AND BaF2.

    DTIC Science & Technology

    the shifts of the vibrational modes on isotopic substitution upper and lower limits for the apex angles of these molecules can be established. The...fundamental frequencies, as well as approximate values for the anharmonic corrections to these vibrational modes . The statistical entropies calculated

  2. The structures and crystal chemistry of magnesium chloride and cadmium chloride

    NASA Astrophysics Data System (ADS)

    Partin, D. E.; O'Keeffe, M.

    1991-11-01

    The crystal structures of MgCl 2 and CdCl 2 have been refined using powder X-ray diffraction data. They have space group R overline3m . For magnesium chloride a = 3.6363(1) Å, c = 17.6663(5) Å, V = 202.31 Å 3, Dx = 2.35 g cm -3, Mg 0,0,0; Cl 0,0,0.25784(8), and for cadmium chloride a = 3.8459(1) Å, c = 17.4931(4) Å, V = 224.07 Å 3, Dx = 4.08 g cm -3, Cd 0,0,0; Cl 0,0,0.2520(1). d(MgCl) = 2.4873(6) Å (6×), d(CdCl) = 2.637(1) (6×). The structures and their relationship to that of fluorite are discussed within the framework of a simple Born-Mayer model.

  3. Bi{sub 2} {sub n} {sub +4}Mo {sub n} O{sub 6(n+1)} with n=3, 4, 5, 6: A new series of low-temperature stable phases in the mBi{sub 2}O{sub 3} - MoO{sub 3} system (1.0Structural relationships and conductor properties

    SciTech Connect

    Vila, Eladio; Landa-Canovas, Angel R. Galy, Jean; Iglesias, Juan E.; Castro, Alicia

    2007-02-15

    Four low-temperature phases with compositions Bi{sub 10}Mo{sub 3}O{sub 24}, Bi{sub 6}Mo{sub 2}O{sub 15}, Bi{sub 14}Mo{sub 5}O{sub 36} and Bi{sub 8}Mo{sub 3}O{sub 21} have been prepared by the n-butylamine wet synthesis method. They have been characterized by powder X-ray diffraction and transmission electron microscopy, mainly by selected area electron diffraction. The four phases present a close structural relationship and a common basic fluorite-type structure and are members of a homologous series of phases with general formula Bi{sub 2n+4}Mo{sub n}O{sub 6(n+1)}, being n=3, 4, 5 and 6, respectively. The matrices relating their superstructures and the basic fluorite type unit cell are given, as well as a general one for the whole series. The conductor behavior of these phases is characterized by impedance spectroscopy being all these materials very good ionic conductors. - Graphical abstract: Four new phases Bi{sub 10}Mo{sub 3}O{sub 24}, Bi{sub 6}Mo{sub 2}O{sub 15}, Bi{sub 14}Mo{sub 5}O{sub 36} and Bi{sub 8}Mo{sub 3}O{sub 21} have been synthesized, characterized and described as consecutive members of a new family Bi{sub 2n+4}Mo{sub n}O{sub 6(n+1)}, with n=3, 4, 5 and 6. They are based on basic fluorite-type structure and their unit cells are depicted in the accompanying graphic.

  4. The local structure of Pd(x)Ce(1-x)O(2-x-δ) solid solutions.

    PubMed

    Gulyaev, R V; Kardash, T Yu; Malykhin, S E; Stonkus, O A; Ivanova, A S; Boronin, A I

    2014-07-14

    PdxCe1-xO2-x-δ solid solutions, which are highly efficient catalysts for the low-temperature oxidation of carbon monoxide, were examined using a set of structural (XRD-PDF, HRTEM, XRD) and spectral (XPS, Raman spectroscopy) methods in combination with quantum-chemical calculations. A comparison of the experimental results and pair distribution function (PDF) modeling data enabled reliable verification of the model of non-isomorphic substitution of Ce(4+) ions by Pd(2+) ions in PdxCe1-xO2-x-δ solid solutions. Palladium ions were shown to be in a near square planar environment with C4v symmetry, which is typical for Pd(2+) ions. Such a near square planar environment was revealed by Raman spectroscopy due to the appearance of the band at ω = 187 cm(-1), which corresponds to the A1 vibrational mode of Pd(2+) ions in [PdO4] subunits. The binding energy of Pd3d5/2 (Eb(Pd3d5/2)) for the Pd(2+) ion in the CeO2 lattice is 1 eV higher than that of Eb(Pd3d5/2) for PdO oxide due to a decrease in the Pd-O distances and the formation of more ionic bonds because of the displacement of Pd(2+) ions with respect to the position of Ce(4+) ions in the fluorite structure. Five structural models of solid solutions are considered in this work. As demonstrated by the DFT calculations, the most realistic model is based on the displacement of palladium ions leading to a near square planar PdO4 environment, which includes water molecules stabilizing the region of anion vacancies in their dissociated state as two hydroxyl groups. The introduction of water molecules in the composition of the PdxCe1-xO2-x-δ solution leads to a decrease in the formation energy and to additional stabilization of palladium in the CeO2 matrix. The formation of PdxCe1-xO2-x-δ solid solutions is accompanied by the dispersing effect caused by distortions of the fluorite structure induced by Pd(2+) ions. The coprecipitation method, which allows Pd(2+) ions to be introduced at the stage of fluorite structure

  5. Electronic structure and crystal phase stability of palladium hydrides

    NASA Astrophysics Data System (ADS)

    Houari, Abdesalem; Matar, Samir F.; Eyert, Volker

    2014-11-01

    The results of electronic structure calculations for a variety of palladium hydrides are presented. The calculations are based on density functional theory and used different local and semilocal approximations. The thermodynamic stability of all structures as well as the electronic and chemical bonding properties are addressed. For the monohydride, taking into account the zero-point energy is important to identify the octahedral Pd-H arrangement with its larger voids and, hence, softer hydrogen vibrational modes as favorable over the tetrahedral arrangement as found in the zincblende and wurtzite structures. Stabilization of the rocksalt structure is due to strong bonding of the 4d and 1s orbitals, which form a characteristic split-off band separated from the main d-band group. Increased filling of the formerly pure d states of the metal causes strong reduction of the density of states at the Fermi energy, which undermines possible long-range ferromagnetic order otherwise favored by strong magnetovolume effects. For the dihydride, octahedral Pd-H arrangement as realized, e.g., in the pyrite structure turns out to be unstable against tetrahedral arrangement as found in the fluorite structure. Yet, from both heat of formation and chemical bonding considerations, the dihydride turns out to be less favorable than the monohydride. Finally, the vacancy ordered defect phase Pd3H4 follows the general trend of favoring the octahedral arrangement of the rocksalt structure for Pd:H ratios less or equal to one.

  6. Electronic structure and crystal phase stability of palladium hydrides

    SciTech Connect

    Houari, Abdesalem; Matar, Samir F.; Eyert, Volker

    2014-11-07

    The results of electronic structure calculations for a variety of palladium hydrides are presented. The calculations are based on density functional theory and used different local and semilocal approximations. The thermodynamic stability of all structures as well as the electronic and chemical bonding properties are addressed. For the monohydride, taking into account the zero-point energy is important to identify the octahedral Pd-H arrangement with its larger voids and, hence, softer hydrogen vibrational modes as favorable over the tetrahedral arrangement as found in the zincblende and wurtzite structures. Stabilization of the rocksalt structure is due to strong bonding of the 4d and 1s orbitals, which form a characteristic split-off band separated from the main d-band group. Increased filling of the formerly pure d states of the metal causes strong reduction of the density of states at the Fermi energy, which undermines possible long-range ferromagnetic order otherwise favored by strong magnetovolume effects. For the dihydride, octahedral Pd-H arrangement as realized, e.g., in the pyrite structure turns out to be unstable against tetrahedral arrangement as found in the fluorite structure. Yet, from both heat of formation and chemical bonding considerations, the dihydride turns out to be less favorable than the monohydride. Finally, the vacancy ordered defect phase Pd{sub 3}H{sub 4} follows the general trend of favoring the octahedral arrangement of the rocksalt structure for Pd:H ratios less or equal to one.

  7. Comparative XRPD and XAS study of the impact of the synthesis process on the electronic and structural environments of uranium–americium mixed oxides

    SciTech Connect

    Prieur, D.; Lebreton, F.; Somers, J.; Delahaye, T.

    2015-10-15

    Uranium–americium mixed oxides are potential compounds to reduce americium inventory in nuclear waste via a partitioning and transmutation strategy. A thorough assessment of the oxygen-to-metal ratio is paramount in such materials as it determines the important underlying electronic structure and phase relations, affecting both thermal conductivity of the material and its interaction with the cladding and coolant. In 2011, various XAS experiments on U{sub 1−x}Am{sub x}O{sub 2±δ} samples prepared by different synthesis methods have reported contradictory results on the charge distribution of U and Am. This work alleviates this discrepancy. The XAS results confirm that, independently of the synthesis process, the reductive sintering of U{sub 1−x}Am{sub x}O{sub 2±δ} leads to the formation of similar fluorite solid solution indicating the presence of Am{sup +III} and U{sup +V} in equimolar proportions. - Graphical abstract: Formation of (U{sup IV/V},Am{sup III})O{sup 2} solid solution by sol–gel and by powder metallurgy. - Highlights: • Uranium–americium mixed oxides were synthesized by sol–gel and powder metallurgy. • Fluorite solid solutions with similar local environment have been obtained. • U{sup V} and Am{sup III} are formed in equimolar proportions.

  8. Fluorite Ce0.8Sm0.2O2- δ porous layer coating to enhance the oxygen permeation behavior of a BaCo0.7Fe0.2Nb0.1O3- δ mixed conductor

    NASA Astrophysics Data System (ADS)

    Wang, Tai-he; Song, Wei-jia; Li, Rong; Zhen, Qiang

    2016-06-01

    Fluorite Ce0.8Sm0.2O2- δ (SDC) nanopowder with a crystallite size of 15 nm was synthesized by a co-precipitation method. An SDC porous layer was coated onto a BaCo0.7Fe0.2Nb0.1O3- δ (BCFN) mixed conductor to improve its oxygen transport behavior. The results show that the SDC-coated BCFN membrane exhibits a remarkably higher oxygen permeation flux ({J_{{O_2}}}) than the uncoated BCFN in the partial oxidation of coke oven gas (COG). The maximum {J_{{O_2}}} value of the SDC-coated BCFN is 18.28 mL·min-1·cm-2 under a COG/air flux of 177 mL·min-1/353 mL·min-1 at 875°C when the thickness of the BCFN membrane is 1 mm; this {J_{{O_2}}} value is 23% higher than that of the uncoated BCFN membrane. This enhancement is likely because of the higher oxygen ionic conductivity of SDC, which supplies oxygen vacancies and accelerates oxygen exchange on the membrane/coating layer/gas three-phase boundary.

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

    DOE PAGES

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

    2015-01-21

    We study the chemical expansion for neutral and charged oxygen vacancies in fluorite, rocksalt, perovskite and pyrochlores materials using first principles calculations. We show that the neutral oxygen vacancy leads to lattice expansion whereas the charged vacancy leads to lattice contraction. In addition, we show that there is a window of strain within which an oxygen vacancy is stable; beyond that range, the vacancy can become unstable. Using CeO2|ZrO2 interface structure as an example, we show that the concentration of oxygen vacancies can be manipulated via strain, and the vacancies can be preferentially stabilized. Furthermore, these results could serve asmore » guiding principles in predicting oxygen vacancy stability in strained systems and in the design of vacancy stabilized materials.« less

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

    SciTech Connect

    Aidhy, Dilpuneet S.; Liu, Bin; Zhang, Yanwen; Weber, William J.

    2015-01-21

    We study the chemical expansion for neutral and charged oxygen vacancies in fluorite, rocksalt, perovskite and pyrochlores materials using first principles calculations. We show that the neutral oxygen vacancy leads to lattice expansion whereas the charged vacancy leads to lattice contraction. In addition, we show that there is a window of strain within which an oxygen vacancy is stable; beyond that range, the vacancy can become unstable. Using CeO2|ZrO2 interface structure as an example, we show that the concentration of oxygen vacancies can be manipulated via strain, and the vacancies can be preferentially stabilized. Furthermore, these results could serve as guiding principles in predicting oxygen vacancy stability in strained systems and in the design of vacancy stabilized materials.

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

    SciTech Connect

    Aidhy, Dilpuneet S.; Liu, Bin; Zhang, Yanwen; Weber, William J.

    2015-03-01

    We study the chemical expansion for neutral and charged oxygen vacancies in fluorite, rocksalt, perovskite and pyrochlores materials using first principles calculations. We show that the neutral oxygen vacancy leads to lattice expansion whereas the charged vacancy leads to lattice contraction. In addition, we show that there is a window of strain within which an oxygen vacancy is stable; beyond that range, the vacancy can become unstable. Using CeO2|ZrO2 interface structure as an example, we show that the concentration of oxygen vacancies can be manipulated via strain, and the vacancies can be preferentially stabilized. These results could serve as guiding principles in predicting oxygen vacancy stability in strained systems and in the design of vacancy stabilized materials.

  12. Phage-directed synthesis of copper sulfide: structural and optical characterization.

    PubMed

    Zaman, Mohammed Shahriar; Moon, Chung Hee; Bozhilov, Krassimir N; Haberer, Elaine D

    2013-08-16

    The growth of crystalline copper sulfide using a viral template was investigated using sequential incubation in CuCl2 and Na2S precursors. Non-specific electrostatic attraction between a genetically-modified M13 bacteriophage and copper cations in the CuCl2 precursor caused phage agglomeration and bundle formation. Following the addition of Na2S, polydisperse nanocrystals 2-7 nm in size were found along the length of the viral scaffold. The structure of the copper sulfide material was identified as cubic anti-fluorite type Cu1.8S, space group Fm3[overline]m. Strong interband absorption was observed within the ultraviolet to visible range with an onset near 800 nm. Furthermore, free carrier absorption, associated with the localized surface plasmon resonance of the copper sulfide nanocrystals, was seen in the near infrared with absorbance maxima at 1060 nm and 3000 nm, respectively.

  13. Molten salt synthesis of a mixed-valent lanthanide(III/IV) oxychloride with an unprecedented Sillen X(2)(4) structure: Ce1.3Nd0.7O3Cl.

    PubMed

    Vigier, Jean-François; Renard, Catherine; Henry, Natacha; Laplace, Annabelle; Abraham, Francis

    2012-04-02

    A new cerium neodymium oxychloride, Ce1.3Nd0.7O3Cl, has been synthesized by precipitation in a LiCl–CaCl2 molten salt by humid argon sparging. Chemical and structure characterization have been undertaken by powder X-ray diffraction, scanning electron microscopy, high-temperature X-ray diffraction, thermogravimetric analysis, and X-ray photoelectron scattering. This oxychloride crystallizes in space group P4/nmm, a = 3.9848(3) Å and c = 12.467(2) Å, in a new Sillen-type phase represented by the symbol X(2)(4) where “quadruple” fluorite-type layers [M4O6], containing Ce(IV) in “inner” sublayers and both CeIII and NdIII in “outer” sublayers, alternate with double-halide ion sheets. The structure is also described as a stacking of LnOCl and fluorite-type blocks and constitutes the term n = 2 of a possible series (MO2)n(NdOCl)2.

  14. Structural and luminescence behaviour of Er3+ doped telluro-fluoroborate glasses

    NASA Astrophysics Data System (ADS)

    Karthikeyan, P.; Suthanthirakumar, P.; Vijayakumar, R.; Marimuthu, K.

    2015-03-01

    The Er3+ doped telluro-fluoroborate glasses with the chemical composition (30 - x)B2O3 + 30TeO2 + 16ZnO + 10ZnF2 + 7CaF2 + 7BaF2 + xEr2O3 (x = 0.05, 0.1, 0.5, 0.75, 1.0 and 3 in wt%) have been prepared by melt quenching technique and characterized through XRD, SEM, FTIR, Raman, absorption and luminescence spectral analysis. The XRD and SEM measurements were made to examine the amorphous nature. The presence of various stretching and bending vibration modes of functional groups have been investigated through FTIR and Raman spectra. The bonding parameters (β bar and δ) were calculated from the absorption spectra to claim the covalent/ionic nature of the metal-ligand bond in the prepared glasses. From the absorption spectra, optical band gap energies (Eopt) corresponding to the direct and indirect allowed transitions were calculated to analyze the electronic band structure. The Urbach energy values have also been estimated and discussed. The Judd-Ofelt (JO) intensity parameters (Ωλ (λ = 2, 4, 6)) were determined from the absorption spectra in order to study the symmetry around the RE ion site and used to compute the radiative properties such as transition probability (AR), stimulated emission cross-section ( σPE) and branching ratios (βR) for the different emission transitions. The emission intensities of the prepared glasses were characterized through CIE 1931 chromaticity diagram and the results were discussed and compared with the reported literature.

  15. Impact-induced hydrothermal activity within the Haughton impact structure, arctic Canada: generation of a transient, warm, wet oasis

    NASA Astrophysics Data System (ADS)

    Osinski, Gordon R.; Spray, John G.

    2001-05-01

    Field studies and analytical scanning electron microscopy indicate that a hydrothermal system was created by the interaction of water with hot, impact-generated rocks following formation of the 24 km-diameter, 23 Ma Haughton impact structure. Hydrothermal alteration is recognized in two settings: within polymict impact breccias overlying the central portion of the structure, and within localized pipes in impact-generated concentric fault systems. The intra-breccia alteration comprises three varieties of cavity and fracture filling: (a) sulfide with carbonate, (b) sulfate, and (c) carbonate. These are accompanied by subordinate celestite, barite, fluorite, quartz and marcasite. Selenite is also developed, particularly in the lower levels of the impact breccia sheet. The fault-related hydrothermal alteration occurs in 1-7 m diameter subvertical pipes that are exposed for lengths of up 20 m. The pipes are defined by a monomict quartz-carbonate breccia showing pronounced Fe-hydroxide alteration. Associated sulfides include marcasite, pyrite and chalcopyrite. We propose three distinct stages in the evolution of the hydrothermal system: (1) Early Stage (>200 degC), with the precipitation of quartz (vapour phase dominated); (2) Main Stage (200-100 deg C), with the development of a two phase (vapour plus liquid) zone, leading to calcite, celestite, barite, marcasite and fluorite precipitation, and (3) Late Stage (<100 degC), with selenite and fibroferrite development through liquid phase-dominanted precipitation. We estimate that it took several tens of thousands of years to cool below 50 deg C following impact. During this time, Haughton supported a 14 km diameter crater lake and subsurface water system, providing a warmer, wetter niche relative to the surrounding terrain. The results also reveal how understanding the internal structure of impact craters is necessary in order to determine their plumbing and cooling systems.

  16. The effect of Gd doping on the atomic and electronic structure of HfO2 thin films.

    NASA Astrophysics Data System (ADS)

    Ketsman, Ihor; Sokolov, Andrei; Belashchenko, Kirill; Dowben, Peter; Losovyj, Yaroslav; Tang, Jinke; Wang, Zhenjun

    2008-03-01

    HfO2 is a promising oxide for many applications, including high-k gate dielectric for CMOS devices. In addition, Gd-doped HfO2 could lead to a dilute magnetic semiconductor and provide an efficient neutron detection medium due to huge neutron absorption cross section of Gd. Gd-doped HfO2 films deposited on both p-type and n-type silicon by PLD retain monoclinic phase at small doping levels, but can be stabilized in fluorite phase by increased doping [1]. At small doping levels, photoemission measurements indicate n-type character of the films as a result of overcompensation with oxygen vacancies. Depending on a doping level, the films form heterojunctions with good rectifying properties on n- or p-type silicon. Preliminary results show the potential ability of the formed diode structures to detect neutrons. [1] Ya.B.Losovyj, I.Ketsman et al.,APL, 91, 132908, (2007)

  17. Epitaxial crystals of Bi{sub 2}Pt{sub 2}O{sub 7} pyrochlore through the transformation of δ–Bi{sub 2}O{sub 3} fluorite

    SciTech Connect

    Gutiérrez–Llorente, Araceli Holtz, Megan E.; Muller, David A.; Joress, Howie; Woll, Arthur; Ward, Matthew J.; Sullivan, Matthew C.; Brock, Joel D.

    2015-03-01

    Bi{sub 2}Pt{sub 2}O{sub 7} pyrochlore is thought to be one of the most promising oxide catalysts for application in fuel cell technology. Unfortunately, direct film growth of Bi{sub 2}Pt{sub 2}O{sub 7} has not yet been achieved, owing to the difficulty of oxidizing platinum metal in the precursor material to Pt{sup 4+}. In this work, in order to induce oxidation of the platinum, we annealed pulsed laser deposited films consisting of epitaxial δ–Bi{sub 2}O{sub 3} and co-deposited, comparatively disordered platinum. We present synchrotron x-ray diffraction results that show the nonuniform annealed films contain the first epitaxial crystals of Bi{sub 2}Pt{sub 2}O{sub 7}. We also visualized the pyrochlore structure by scanning transmission electron microscopy, and observed ordered cation vacancies in the epitaxial crystals formed in a bismuth-rich film but not in those formed in a platinum-rich film. The similarity between the δ–Bi{sub 2}O{sub 3} and Bi{sub 2}Pt{sub 2}O{sub 7} structures appears to facilitate the pyrochlore formation. These results provide the only route to date for the formation of epitaxial Bi{sub 2}Pt{sub 2}O{sub 7}.

  18. Epitaxial crystals of Bi 2 Pt 2 O 7 pyrochlore through the transformation of δ –Bi 2 O 3 fluorite

    SciTech Connect

    Gutiérrez–Llorente, Araceli; Joress, Howie; Woll, Arthur; Holtz, Megan E.; Ward, Matthew J.; Sullivan, Matthew C.; Muller, David A.; Brock, Joel D.

    2015-03-01

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

  19. On the crystal structures of Ln{sub 3}MO{sub 7} (Ln=Nd, Sm, Y and M=Sb, Ta)-Rietveld refinement using X-ray powder diffraction data

    SciTech Connect

    Fu, W.T.; IJdo, D.J.W.

    2009-09-15

    We have investigated, using X-ray powder diffraction data, the crystal structures of some fluorite derivatives with the formula Ln{sub 3}MO{sub 7} (Ln=lanthanide or Y and M=Sb and Ta). In these compounds ordering of Ln and M occurs, leading to a parent structure in Cmmm. Tilting of the MO{sub 6} octahedra causes doubling of one of the cubic axes, leading to a number of non-isomorphic subgroups, e.g. Cmcm, Ccmm and Cccm. We have identified an alternative space group Ccmm instead of C222{sub 1} for those compounds containing a medium sized lanthanide or Y and M being Sb or Ta. Interestingly this is an alternative setting for the space group of the structure obtained when Ln is large (Cmcm). However, there tilting of the octahedra is around the a-axis of the parent structure, rather than around the b-axis as it is found in the compounds which we are reporting on here. In one compound, Nd{sub 3}TaO{sub 7}, both tilts occur. The phase transition between the two possible structures is a slow and difficult process above 80 K, allowing both phases to coexist. - Graphical abstract: (a) A projected view of Ln{sub 3}MO{sub 7} along the a-axis showing the ordering of Ln and M cations in the fluoride lattice. Note that the unit cells of the fluorite (dashed line), the parent Cmmm (dashed line) and the Cmcm/Ccmm structures (continuous line) are indicated. (b) Schematic representations of the crystal structures of Y{sub 3}SbO{sub 7} showing SbO{sub 6} octahedra and Y. Oxygens that do not bond to M cations are also shown.

  20. Structure, crystal chemistry and thermal evolution of the delta-Bi{sub 2}O{sub 3}-related phase Bi{sub 9}ReO{sub 17}

    SciTech Connect

    Sharma, Neeraj; Withers, Ray L.; Knight, Kevin S.; Ling, Chris D.

    2009-09-15

    The thermal evolution and structural properties of fluorite-related delta-Bi{sub 2}O{sub 3}-type Bi{sub 9}ReO{sub 17} were studied with variable temperature neutron powder diffraction, synchrotron X-ray powder diffraction and electron diffraction. The thermodynamically stable room-temperature crystal structure is monoclinic P2{sub 1}/c, a=9.89917(5), b=19.70356(10), c=11.61597(6) A, beta=125.302(2){sup o} (R{sub p}=3.51%, wR{sub p}=3.60%) and features clusters of ReO{sub 4} tetrahedra embedded in a distorted Bi-O fluorite-like network. This phase is stable up to 725 deg. C whereupon it transforms to a disordered delta-Bi{sub 2}O{sub 3}-like phase, which was modeled with delta-Bi{sub 2}O{sub 3} in cubic Fm3-barm with a=5.7809(1) A (R{sub p}=2.49%, wR{sub p}=2.44%) at 750 deg. C. Quenching from above 725 deg. C leads to a different phase, the structure of which has not been solved but appears on the basis of spectroscopic evidence to contain both ReO{sub 4} tetrahedra and ReO{sub 6} octahedra. - Abstract: The crystal structure of Bi{sub 9}ReO{sub 17} viewed along the [101] direction. Bi atoms and bonds are light gray, O atoms and bonds are black and ReO{sub 4} are represented by gray tetrahedra. Display Omitted

  1. 17O NMR studies of local structure and phase evolution for materials in the Y 2Ti 2O 7-ZrTiO 4 binary system

    NASA Astrophysics Data System (ADS)

    Palumbo, John L.; Schaedler, Tobias A.; Peng, Luming; Levi, Carlos G.; Grey, Clare P.

    2007-07-01

    17O MAS NMR and XRD studies of precursor-derived Y 1.6Zr 0.4Ti 2O 7.2 and Y 1.2Zr 0.8Ti 2O 7.4 have been performed to investigate the development of local and long-range order in these materials as they evolve from a metastable amorphous state upon heating. Zirconium titanate (ZrTiO 4) was also investigated to help interpret the 17O NMR spectra of the ternary compositions. Consistent with earlier studies, crystallization was observed at 800 °C to form a fluorite structure and a small amount of rutile; weak broad reflections were also observed which were ascribed to the presence of small pyrochlore-like ordered domains or particles within the fluorite phase. As the temperature was increased further, the sizes of these domains grew along with the concentration of rutile. At the highest temperature studied (1300 °C), the reflections of the thermodynamic phases, pyrochlore and zirconium titanate (ZrTiO 4), dominated the XRD pattern. The 17O NMR spectra revealed a series of different peaks that were assigned to different 3- and 4-coordinate O local environments. The data were consistent with the formation of a metastable phase Y 2-xZr xTi 2-yZr yO 7+x with pyrochlore-like ordering but with Zr substitution on both cation sites of the pyrochlore structure. At low temperatures, doping on the A (Y 3+) sites predominates (i.e., x> y), consistent with the fact that the pyrochlore develops out of a more disordered fluorite-like, phase. As the temperature is raised, the Zr doping on the A site decreases and the metastable phase at this temperature can now be written as Y 2-x'Zr x'Ti 2-y'Zr y'O 7+x' (i.e., x'< y'); TiO 2 is also observed, consistent with this suggestion. At high temperatures, doping on the B site decreases and the resonances due to the stoichiometric pyrochlore yttrium titanate (Y 2Ti 2O 7) dominate the NMR spectra. Weaker 17O NMR resonances due zirconium titanate (ZrTiO 4) are also observed.

  2. New Tl-based copper oxide containing double-MO 2-unit fluorite block: (Tl, Cu) Sr 2 (R, Ce) 3Cu 2O 11 (R: rare earth element)

    NASA Astrophysics Data System (ADS)

    Wada, Takahiro; Hamada, Kazuyuki; Ichinose, Ataru; Kaneko, Tetsuyuki; Yamauchi, H.; Tanaka, Shoji

    1991-05-01

    New Tl-based copper oxides. (Tl, Cu)Sr 2(R, Ce) 3Cu 2O 11 (R: rare earth element) (Tl-based “1232”), have been synthesized. These compounds have tetragonal unit cells with lattice constants approximately equal to a=3.8 Å and c=17.3 Å. A Rietveld analysis using X-ray powder diffraction data shows that the crystal structure of (Tl {4}/{5}Cu {1}/{5})Sr 2(Ho {1}/{3}Ce {2}/{3}) 3Cu 2O 11 consists of a double-MO 2-unit flouride block, i.e. [(Ho, Ce)O 2] 2 and a Tl-based “1212” block, i.e. (Tl, Cu)Sr 2(Ho, Ce)Cu 2O 7. The temperature dependences of the electrical resistivity for all of these compounds are semiconductive. However, these compounds are considered to be candidates for parent materials for new high- Tc superconductors.

  3. Uniform nanoparticles building Ce1- x Pr x O2-δ mesoarchitectures: structure, morphology, surface chemistry, and catalytic performance

    NASA Astrophysics Data System (ADS)

    Somacescu, Simona; Parvulescu, Viorica; Calderon-Moreno, Jose Maria; Suh, Soong-Hyuck; Osiceanu, Petre; Su, Bao-Lian

    2012-06-01

    Ce1- x Pr x O2-δ ( x = 0, 0.1, 0.5, 0.9) mesoarchitectures built from nanoparticles with crystalline framework have been synthesized by the self-assembly method assisted by surfactants and hydrothermal treatment. Cetyltrimethylammonium bromide (CTAB) was used as template, urea as hydrolyzing agent and tetraethylammonium hydroxide (TEAOH) as pH mediator to obtain pH 9. The inorganic precursors have been co-assembled with surfactant template to produce mesoarchitectures which have uniform pore size distribution, crystalline channel walls, high thermal stability, and high catalytic activity in the oxidation reaction of methane. The resulting powders, calcined at 550 °C, were characterized by X-ray diffraction (XRD), Raman spectroscopy, N2 adsorption/desorption isotherms (BET), thermogravimetric analysis (TG-DTG), scanning and transmission electron microscopy (SEM, TEM, and HRTEM), and X-ray photoelectron spectroscopy (XPS). The as-synthesized mesoporous nanoparticles are single-phase fluorite Ce1- x Pr x O2-δ solid solution without additional Ce- or Pr-based oxides, or secondary phases with different lattice symmetry or stoichiometry. A clear morphology of dispersed nanoparticles, with uniform grain size between 5 and 7 nm, and mean pore size around 5 nm, have been observed. The specific surface area of the as-synthesized mesoporous samples after calcination at 550 °C remains in the range 60-150 m2 g-1. All the spectroscopic methods clearly confirm the homogeneous incorporation of Pr into the CeO2 lattice of the nanocrystallites, to form a single-phase solid solution with fluorite structure, modifying the absorption spectra of the nanocrystallites. All the samples showed high catalytic activity in the oxidation reactions of methane.

  4. Structural reinvestigation of Li3FeN2: Evidence of cationic disorder through XRD, solid-state NMR and Mössbauer spectroscopy

    NASA Astrophysics Data System (ADS)

    Panabière, E.; Emery, N.; Lorthioir, C.; Sougrati, M. T.; Jumas, J.-C.; Bach, S.; Pereira-Ramos, J.-P.; Smith, R. I.; Willmann, P.

    2016-08-01

    A significant cationic disorder is evidenced on Li3FeN2 prepared through solid-state reaction under controlled atmosphere. This derivative anti fluorite type structure (orthorhombic, space group Ibam, a=4.870(1) Å, b=9.652(1) Å and c=4.789(1) Å), solved first through single crystal X-ray diffraction [7], is usually described by Li+ and Fe+3 ordered distribution in tetrahedral sites formed by the nitrogen network, leading to [FeN4/2]3- edge-sharing tetrahedral chains. From 7Li/6Li Nuclear Magnetic Resonance spectroscopy, 57Fe Mössbauer spectroscopy and powder X-ray and neutron diffraction, we demonstrate that about 4% of lithium sites are filled by iron and about 11% of iron sites are occupied by Li, which can explain the discrepancy within the Gudat's model observed on larger scale solid-state synthesis samples.

  5. Comparative XRPD and XAS study of the impact of the synthesis process on the electronic and structural environments of uranium-americium mixed oxides

    NASA Astrophysics Data System (ADS)

    Prieur, D.; Lebreton, F.; Martin, P. M.; Caisso, M.; Butzbach, R.; Somers, J.; Delahaye, T.

    2015-10-01

    Uranium-americium mixed oxides are potential compounds to reduce americium inventory in nuclear waste via a partitioning and transmutation strategy. A thorough assessment of the oxygen-to-metal ratio is paramount in such materials as it determines the important underlying electronic structure and phase relations, affecting both thermal conductivity of the material and its interaction with the cladding and coolant. In 2011, various XAS experiments on U1-xAmxO2±δ samples prepared by different synthesis methods have reported contradictory results on the charge distribution of U and Am. This work alleviates this discrepancy. The XAS results confirm that, independently of the synthesis process, the reductive sintering of U1-xAmxO2±δ leads to the formation of similar fluorite solid solution indicating the presence of Am+III and U+V in equimolar proportions.

  6. The response of the pyrochlore structure-type to ion-beam irradiation

    NASA Astrophysics Data System (ADS)

    Lian, Jie

    Pyrochlore with the general formula of A3+2B4+2O7 (Fd3m; Z = 8) has been proposed as the candidate waste form for the immobilization of actinides, particularly plutonium from dismantled nuclear weapons. Because actinides decay by alpha-decay events, radiation effects on the waste form are a concern. The effects of radiation on different pyrochlore compositions, A2B2O7 (A = La ˜ Lu, and Y; B = Ti, Sn, and Zr), have been investigated by 50 KeV He+, 600 KeV Ar+, 1.0 MeV Kr+, and 1.5 MeV Xe+ ion irradiations. Titanate pyrochlores are generally sensitive to ion beam damage and can be amorphized at a low damage level (˜0.2 dpa). The critical amorphization temperature, Tc, increases from ˜480 to ˜1120 K with increasing A-site cation size. A dramatically increasing radiation "resistance" to ion beam induced-amorphization has been observed with increasing Zr-content in the Gd2Ti2-xZrxO7 system. The pure end-member, Gd2Zr2O7, cannot be amorphized, even at doses as high as ˜100 dpa. Although zirconate pyrochlores are generally considered to be radiation "resistant", ion beam-induced amorphization occurs for La2Zr2O7 at a dose of ˜5.5 dpa at room temperature. Stannate pyrochlores A2Sn 2O7 (A = La, Nd, Gd) are readily amorphized by ion beam damage at a relatively low dose (˜1 dpa) at room temperature; while no evidence of amorphization has been observed in A2Sn2O7 (A = Er, Y, Lu) irradiated with 1 MeV Kr+ ions at a dose of ˜6 dpa at 25 K. The factors that influence the response of different pyrochlore compositions to ion irradiation-induced amorphization are discussed in terms of cation radius ratio, defect formation energies, and the tendency of the pyrochlore structure-type to undergo an order-disorder transition to the defect-fluorite structure. The "resistance" of the pyrochlore structure to ion beam-induced amorphization is not only affected by the relative sizes of the A- and B-site cations, but also the cation electronic configurations. Pyrochlore compositions

  7. Structural and catalytic properties of lanthanide (La, Eu, Gd) doped ceria

    SciTech Connect

    Hernandez, W.Y.; Laguna, O.H.; Centeno, M.A.; Odriozola, J.A.

    2011-11-15

    Ce{sub 0.9}M{sub 0.1}O{sub 2-{delta}} mixed oxides (M=La, Eu and Gd) were synthesized by coprecipitation. Independent of the dopant cation, the obtained solids maintain the F-type crystalline structure, characteristic of CeO{sub 2} (fluorite structure) without phase segregation. The ceria lattice expands depending on the ionic radii of the dopant cation, as indicated by X-ray diffraction studies. This effect also agrees with the observed shift of the F{sub 2g} Raman vibrational mode. The presence of the dopant cations in the ceria lattice increases the concentration of structural oxygen vacancies and the reducibility of the redox pair Ce{sup 4+}/Ce{sup 3+}. All synthesized materials show higher catalytic activity for the CO oxidation reaction than that of bare CeO{sub 2}, being Eu-doped solid the one with the best catalytic performances despite of its lower surface area. - Graphical abstract: In this work, Ce{sub 0.9}M{sub 0.1}O{sub 2-{delta}} mixed oxides (M=La, Eu and Gd) were synthesized by coprecipitation. Independent of the dopant cation, the obtained solids maintain the F-type crystalline structure, characteristic of CeO{sub 2} (fluorite structure) without phase segregation. The ceria lattice expands depending on the ionic radii of the dopant cation, as indicated by X-ray diffraction studies. This effect also agrees with the observed shift of the F{sub 2g} Raman vibrational mode. The presence of the dopant cations in the ceria lattice increases the concentration of structural oxygen vacancies and the reducibility of the redox pair Ce{sup 4+}/Ce{sup 3+}. All synthesized materials show higher catalytic activity for the CO oxidation reaction than that of bare CeO{sub 2}, being Eu-doped solid the one with the best catalytic performances despite of its lower surface area. Highlights: > Lanthanide doped ceria as catalytic supports for CO oxidation reaction. > A higher concentration of oxygen vacancies promotes a higher catalytic activity. > Eu-doped ceria shows

  8. Nanostructured crystals of fluorite phases Sr{sub 1−x}R{sub x}F{sub 2+x} (R Are Rare Earth Elements) and their ordering: 10. Ordering under spontaneous crystallization and annealing of Sr{sub 1−x}R{sub x}F{sub 2+x} Alloys (R = Tb-Lu, Y) with 23.8–36.1 mol % RF{sub 3}

    SciTech Connect

    Sulyanova, E. A. Karimov, D. N.; Sulyanov, S. N.; Zhmurova, Z. I.; Golubev, A. M.; Sobolev, B. P.

    2015-01-15

    The products of spontaneous crystallization (at a cooling rate of ∼200 K/min) of Sr{sub 1−x}R{sub x}F{sub 2+x} melts in the homogeneity range of the fluorite phase have been investigated. Thirty-two irrational compositions with 23.8–36.1 mol % RF{sub 3} and eight rational Sr{sub 2}RF{sub 7} compositions are obtained. With respect to the RF{sub 3} content, these compositions form five groups: (1) Sr{sub 0.762}R{sub 0.238}F{sub 2.238} (23.8% RF{sub 3}), (2) Sr{sub 0.744}R{sub 0.256}F{sub 2.256} (25.6%), (3) Sr{sub 0.718}R{sub 0.282}F{sub 2.282} (28.2%), (4) Sr{sub 2}RF{sub 7} (33.3%), and (5) Sr{sub 0.639}R{sub 0.361}F{sub 2.361} (36.1%). R = Tb-Lu, Y for all groups. Quenching melts of group 5 with R = Tb, Dy, and Ho leads to the formation of ordered phases with the trigonal distortion of the rhβ-Na{sub 7}Zr{sub 6}F{sub 31} type, while for melts of group 5 with R = Lu, quenching yields a phase of the trigonal rhα′-Sr{sub 4}Lu{sub 3}F{sub 17} type. In group 5 with R = Y, Er, Tm, or Yb and in groups 1–4 with all REEs, fluorite phases are formed. Annealing at 900 ± 20°C for 96 h with subsequent cooling at a rate of ∼200 K/min expands the variety of ordered phases: a phase with a new r type of orthorhombic distortion is formed in group 1 with R = Lu, in group 2 with R = Tm or Lu, and in group 3 with R = Ho-Lu, Y; a t-Sr{sub 2}RF{sub 7} phase with tetragonal distortion is formed in group 4 with R = Tb-Er, Y; and a phase of trigonal rhα′ type is formed in group 5 with R = Y, Yb, or Lu. A fluorite phase arises in group 1 with R = Tb-Lu, Y as a result of quenching and annealing. The tendency to ordering becomes more pronounced with an increase in the RF{sub 3} content and REE atomic number. The annealing conditions do not provide equilibrium or the completely ordered state of all alloys.

  9. On the local electronic and atomic structure of Ce1-xPrxO2-δ epitaxial films on Si

    SciTech Connect

    Niu, Gang Schubert, Markus Andreas; Zoellner, Marvin Hartwig; D'Acapito, Francesco; Boscherini, Federico

    2014-09-28

    The local electronic and atomic structure of (111)-oriented, single crystalline mixed Ce1-xPrxO2-δ (x = 0, 0.1 and 0.6) epitaxial thin films on silicon substrates have been investigated in view of engineering redox properties of complex oxide films. Non-destructive X-ray absorption near edge structure reveals that Pr shows only +3 valence and Ce shows only nominal +4 valence in mixed oxides. Extended x-ray absorption fine structure (EXAFS) studies were performed at K edges of Ce and Pr using a specially designed monochromator system for high energy measurements. They demonstrate that the fluorite lattice of ceria (CeO₂) is almost not perturbed for x = 0.1 sample, while higher Pr concentration (x = 0.6) not only generates a higher disorder level (thus more disordered oxygen) but also causes a significant reduction of Ce–O interatomic distances. The valence states of the cations were also examined by techniques operating in highly reducing environments: scanning transmission electron microscopy-electron energy loss spectroscopy and X-ray photoemission spectroscopy; in these reducing environments, evidence for the presence of Ce³⁺ was clearly found for the higher Pr concentration. Thus, the introduction of Pr³⁺ into CeO₂ strongly enhances the oxygen exchange properties of CeO₂. This improved oxygen mobility properties of CeO₂ are attributed to the lattice disorder induced by Pr mixing in the CeO₂ fluorite lattice, as demonstrated by EXAFS measurements. Thus, a comprehensive picture of the modifications of the atomic and electronic structure of Ce1-xPrxO2-δ epitaxial films and their relation is obtained.

  10. Calorimetric measurements of energetics of defect interactions in fluorite oxides.

    PubMed

    Navrotsky, Alexandra; Simoncic, Petra; Yokokawa, Harumi; Chen, Weiqun; Lee, Theresa

    2007-01-01

    Direct measurement by oxide melt solution calorimetry of energetics of mixing in rare earth and yttrium doped zirconia, hafnia, and ceria systems provides support for spectroscopic and computational studies of the location and clustering of vacancies in these systems. Strongly negative heats of mixing are seen when the vacancy is transferred from being nearest neighbor to Y or RE in the sesquioxide to being nearest neighbor to Zr or Hf in the cubic solid solution. In the absence of such redistribution, small positive enthalpies of mixing are seen in CeO2-YO1.5 and CeO2-REO.15 systems. Strongly positive enthalpies of mixing are seen in CeO2-ZrO2, which has a large difference in cation sizes and no vacancy formation. The system Ce0.8Y0.2O1.9-Zr0.8Y0.2O1.9 shows small positive heats of formation with less destabilization in the Ce-rich region, suggestive of "scavenging" of oxygen vacancies by Zr. The calorimetric data obtained in these studies offer direct comparison with the results of computations on defect clusters and their binding energies.

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

    PubMed

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

    2006-02-09

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

  12. Heterogeneous structure of the lithosphere of the Taimyr Peninsula

    NASA Astrophysics Data System (ADS)

    Litvinova, Tamara; Petrova, Alevtina

    2016-04-01

    Magnetic anomalies of the lower crust is well manifested in the satellite measurements and their reductions for the heights H = 100 and 400 km. Currently, however, a great interest is the area of negative magnetic anomalies, allocated to the same heights. They are confined to a special permeable zones of the crust and lithosphere, having increased geothermal activity and are associated with a variety of minerals. In digital magnetic anomalies and gravity anomalies circumpolar map of the Arctic Ocean (Total) was built geomagnetic and density sections along latitudinal and longitudinal cross sections of negative magnetic anomalies (n = 100 km). In the Taimyr Peninsula they capture the largest Fadyukudinsko Kotuiskaya-ring structure. In the north-central Siberia Fadyukudinsko Kotuiskaya ring structure is the "hub" articulation largest geoblocks (Anabar, Kureisko-Tunguska and North Kara). It is manifested in the gravity and magnetic field is also a ring structure. With Fadyukudinsko Kotui-ring structure formation associated injectors and high-carbonate metasomatic rocks tectonites controlling uranium and thorium-uranium-fluorite-barite-rare earth mineralization (VF Proskurnin, et al. 2010). It hypabyssal front of the hot spots. Fadyukudinsko-Kotuiskaya structure is defined posletrappovoe place in the north of the Eurasian plate, responding to a hot spot or a spot lower mantle plumes Triassic [Kravchenko SM, Hain VE 1996 Sazonov AM, Zvyagin EA, Leontiev SI et al., 2010]. Latitude and longitude revealed Profile permeable zones of low magnetic properties and density, confined to a weakened layer in the middle crust. Negative satellite magnetic anomalies (n = 100 km) at depths of 20 - 25-30 km weakly magnetic lens revealed a low density. The upper crust they overlap and dense magnetic rocks. At the bottom of the crust, these lenses are underlain by layers of dense and magnetic structures.

  13. The Structure of High Pressure Ca(OD) 2II from Powder Neutron Diffraction: Relationship to the ZrO 2and EuI 2Structures

    NASA Astrophysics Data System (ADS)

    Leinenweber, Kurt; Partin, Dan E.; Schuelke, Udo; O'Keeffe, Michael; Von Dreele, Robert B.

    1997-09-01

    The "unquenchable" high pressure form of Ca(OD) 2[Ca(OH) 2II] has been synthesized at 9 GPa and 400°C and recovered to ambient pressure at cryogenic temperatures. The structure was determined from powder neutron diffraction data using the Rietveld technique. The symmetry is monoclinic P2 1/ cwith a=5.3979(4) Å, b=6.0931(4) Å, c=5.9852(4) Å, β=103.581(6)°, Z=4 at 1 atm and 11 K. Rwp=2.8%, Rp=1.9%, reduced χ2=6.6. for 117 variables. The calcium and oxygen substructure is intermediate between that in α-PbO 2and that in fluorite; it was previously described as isostructural with baddeleyite (ZrO 2), but it is more accurately described as isostructural with EuI 2. This structure is distinguished by the presence of a 3 6anion net parallel to (100). Only one of the two kinds of D atoms in the structure shows appreciable hydrogen bonding to O, with a second neighbor D … O distance of 1.91 Å, and an O-D … O angle of 153.2°; the other D atom has 3 second-neighbor oxygens near 2.6 Å away.

  14. Structural, optical, and electrical properties of MgyTi1-yHx thin films

    NASA Astrophysics Data System (ADS)

    Borsa, D. M.; Gremaud, R.; Baldi, A.; Schreuders, H.; Rector, J. H.; Kooi, B.; Vermeulen, P.; Notten, P. H. L.; Dam, B.; Griessen, R.

    2007-05-01

    The structural, optical, and electrical transformations induced by hydrogen absorption and/or desorption in Mg-Ti thin films prepared by co-sputtering of Mg and Ti are investigated. Highly reflective in the metallic state, the films become highly absorbing upon H absorption. The reflector-to-absorber transition is fast, robust, and reversible over many cycles. Such a highly absorbing state hints at the coexistence of a metallic and a semiconducting phase. It is, however, not simply a composite material consisting of independent MgH2 and TiH2 grains. By continuously monitoring the structure during H uptake, we obtain data that are compatible with a coherent structure. The average structure resembles rutile MgH2 at high Mg content and is fluorite otherwise. Of crucial importance in preserving the reversibility and the coherence of the system upon hydrogen cycling is the accidental equality of the molar volume of Mg and TiH2 . The present results point toward a rich and unexpected chemistry of Mg-Ti-H compounds.

  15. Oxide-ion and proton conducting electrolyte materials for clean energy applications: structural and mechanistic features.

    PubMed

    Malavasi, Lorenzo; Fisher, Craig A J; Islam, M Saiful

    2010-11-01

    This critical review presents an overview of the various classes of oxide materials exhibiting fast oxide-ion or proton conductivity for use as solid electrolytes in clean energy applications such as solid oxide fuel cells. Emphasis is placed on the relationship between structural and mechanistic features of the crystalline materials and their ion conduction properties. After describing well-established classes such as fluorite- and perovskite-based oxides, new materials and structure-types are presented. These include a variety of molybdate, gallate, apatite silicate/germanate and niobate systems, many of which contain flexible structural networks, and exhibit different defect properties and transport mechanisms to the conventional materials. It is concluded that the rich chemistry of these important systems provides diverse possibilities for developing superior ionic conductors for use as solid electrolytes in fuel cells and related applications. In most cases, a greater atomic-level understanding of the structures, defects and conduction mechanisms is achieved through a combination of experimental and computational techniques (217 references).

  16. Crystalline structure of ceria particles controlled by the oxygen partial pressure and STI CMP performances.

    PubMed

    Kim, Ye-Hwan; Kim, Sang-Kyun; Kim, Namsoo; Park, Jea-Gun; Paik, Ungyu

    2008-09-01

    The effect of the crystalline structures of nano-sized ceria particles on shallow trench isolation (STI) chemical mechanical planarization (CMP) performance was investigated. The ceria particles were synthesized via a solid-state displacement reaction method, and their crystalline structure was controlled by regulating the oxygen partial pressure at the reaction site on the precursor. The crystalline structures of ceria particles were analyzed by the high-resolution TEM nano-beam diffraction pattern. In a calcination process with a high oxygen concentration, the synthesized ceria particles had a cubic fluorite structure (CeO(2)), because of the decarbonation of the cerium precursor. However, a low oxygen concentration results in a hexagonal phase cerium oxide (Ce(2)O(3)) rather than the cubic phase due to the insufficient oxidation of Ce(3+) to Ce(4+). In the STI CMP evaluation, the ceria slurry prepared with the cubic CeO(2) shows enhanced performances of the oxide-to-nitride removal selectivity.

  17. Room Temperature Pressure-induced Photoluminescence in MnF2

    NASA Astrophysics Data System (ADS)

    Hernandez, Ignacio; Hochheimer, Hans D.; Rodriguez, Fernando

    2003-10-01

    In previous works we have demonstrated the occurrence of photoluminescence (PL) in slightly Mn2+-doped systems depending only on the host site volume [1]. Thus, we have been able to induce room temperature (RT) PL in non-luminescent Mn2+ containing fluorites BaF2 and SrF2 by means of pressure [2]. The irrelevance of the coordination geometry for the Mn2+ complex is stressed by the fact that the occurrence of the fluorite to cotunnite phase transition (PT) only affects the PL transition probability rate in terms of the approx. 10% volume reduction and the loss of inversion centre that take place. On the other side, non-doped MnF2 is photoluminescent at low temperatures but PL is quenched at temperatures above 100 K [3] and it is not luminescent at RT. This is due to excitation migration mechanisms that take Frenkel excitons to non-radiative excitation traps. This migration is favoured by temperature so that at RT no luminescent Mn2+ complexes can retain excitation [4]. In this work we deal with the occurrence of PL in non-doped MnF2 at room temperature. Rutile-structure MnF2 is pressurized and undergoes two PTs. There exists a strong correlation between the occurrence of the PL and the induced structural changes. We observe two photoluminescent centres at P > 14 GPa placed at 2.34 eV and 1.87 eV, that are likely associated to Mn2+ intrinsic PL and perturbed Mn2+ complexes, respectively. Therefore, we demonstrate that pressure is an efficient tool to reduce the excitation migration so that excitons can be retained in Mn2+ traps and, thus, increase the PL efficiency of the material at a given temperature or induce PL beyond a critical volume. This is particularly interesting in cases such as the present, in which the PT show hysteresis. References: [1] to be published: F. Rodriguez, I. Hernandez, M. Moreno and R. Alcala, J. Chem. Phys., Vol. 119, No. 16, 22 October 2003 [2] I. Hernandez and F. Rodriguez, Phys. Rev. B 67, 012101 (2003) [3] W.W. Holloway, Jr, M

  18. An anti CuO2-type metal hydride square net structure in Ln2M2As2H(x) (Ln=La or Sm, M=Ti, V, Cr, or Mn).

    PubMed

    Mizoguchi, Hiroshi; Park, SangWon; Hiraka, Haruhiro; Ikeda, Kazutaka; Otomo, Toshiya; Hosono, Hideo

    2015-03-02

    Using a high pressure technique and the strong donating nature of H(-), a new series of tetragonal La2Fe2Se2O3-type layered mixed-anion arsenides, Ln2M2As2H(x), was synthesized (Ln=La or Sm, M=Ti, V, Cr, or Mn; x≈3). In these compounds, an unusual M2H square net, which has anti CuO2 square net structures accompanying two As(3-) ions, is sandwiched by (LaH)2 fluorite layers. Notably, strong metal-metal bonding with a distance of 2.80 Å was confirmed in La2Ti2As2H2.3, which has metallic properties. In fact, these compounds are situated near the boundary between salt-like ionic hydrides and transition-metal hydrides with metallic characters.

  19. A relationship between three-dimensional surface hydration structures and force distribution measured by atomic force microscopy

    NASA Astrophysics Data System (ADS)

    Miyazawa, Keisuke; Kobayashi, Naritaka; Watkins, Matthew; Shluger, Alexander L.; Amano, Ken-Ichi; Fukuma, Takeshi

    2016-03-01

    Hydration plays important roles in various solid-liquid interfacial phenomena. Very recently, three-dimensional scanning force microscopy (3D-SFM) has been proposed as a tool to visualise solvated surfaces and their hydration structures with lateral and vertical (sub) molecular resolution. However, the relationship between the 3D force map obtained and the equilibrium water density, ρ(r), distribution above the surface remains an open question. Here, we investigate this relationship at an interface of an inorganic mineral, fluorite, and water. The force maps measured in pure water are directly compared to force maps generated using the solvent tip approximation (STA) model and from explicit molecular dynamics simulations. The results show that the simulated STA force map describes the major features of the experimentally obtained force image. The agreement between the STA data and the experiment establishes the correspondence between the water density used as an input to the STA model and the experimental hydration structure and thus provides a tool to bridge the experimental force data and atomistic solvation structures. Further applications of this method should improve the accuracy and reliability of both interpretation of 3D-SFM force maps and atomistic simulations in a wide range of solid-liquid interfacial phenomena.Hydration plays important roles in various solid-liquid interfacial phenomena. Very recently, three-dimensional scanning force microscopy (3D-SFM) has been proposed as a tool to visualise solvated surfaces and their hydration structures with lateral and vertical (sub) molecular resolution. However, the relationship between the 3D force map obtained and the equilibrium water density, ρ(r), distribution above the surface remains an open question. Here, we investigate this relationship at an interface of an inorganic mineral, fluorite, and water. The force maps measured in pure water are directly compared to force maps generated using the solvent

  20. State of the Art and Future Trends in the Development of Thermal Barrier Coating Systems

    DTIC Science & Technology

    2006-05-01

    structure is similar to the fluorite structure assumed by YSZ but with one missing oxygen atom, and a large number of displaced oxygen atoms. As such...the pyrochlore crystal can be considered as an ordered, highly defective fluorite solid solution with reduced symmetry and more complicated structure...with the fluorite and pyrochlore structure”, Solid State Ionics, 9-10, 1983, 913-20. [36] Mori, M., Abe, T., Itoh, H., Yamamoto, O., Shen, G. Q

  1. Photo-Activated Synthesis of Functional Oxide Thin Films

    DTIC Science & Technology

    2010-03-01

    o (d200 = 2.53 Å), corresponding to lattice constant ~ 5.07 Å. In cubic fluorite structure, lattice parameter (d) can be calculated from ionic...noted that aliovalently introduced oxygen vacancies have an effect on anion ionic radius, therefore lattice constant in doped fluorite structure cannot...denotes the dopant content.[26] This relation indicates that oxygen deficiency decreases the lattice constant of fluorite -structured oxides. It is

  2. Structural, photoluminescence and radioluminescence properties of Eu3+ doped La2Hf2O7 nanoparticles

    NASA Astrophysics Data System (ADS)

    Wahid, Kareem; Pokhrel, Madhab; Mao, Yuanbing

    2017-01-01

    This study presents the structural, optical, and radioluminescent characterization of newly synthesized europium-doped lanthanum hafnate (La2Hf2O7:xmol%Eu3+, x=0 to 35) nanoparticles (NPs) for use as phosphors and scintillation materials. Samples prepared through a combined co-precipitation and molten salt synthetic process were found to crystalize in the pyrochlore phase, a radiation tolerant structure related to the fluorite structure. These samples exhibit red luminescence under ultraviolet and X-ray excitation. Under these excitations, the optical intensity and quantum yield of the La2Hf2O7:xmol%Eu3+ NPs depend on the Eu3+ concentration and are maximized at 5%. It is proposed that there is a trade-off between the quenching due to defect states/cross-relaxation and dopant concentration. An optimal dopant concentration allows the La2Hf2O7:5 mol%Eu3+ NPs to show the best luminescent properties of all the samples.

  3. Structural phase transitions in ionic conductor Bi2O3 by temperature dependent XPD and XAS

    NASA Astrophysics Data System (ADS)

    Zhu, Yingcai; An, Pengfei; Yu, Meijuan; Marcelli, Augusto; Liu, Yong; Hu, Tiandou; Xu, Wei

    2016-05-01

    The superionic behavior of cubic δ-phase Bi2O3, a metastable phase at high temperature, is of great interests from both scientific and technological perspectives. With the highest ionic conductivity among all known compounds, the δ-phase Bi2O3 possesses promising applications in solid-oxide fuel cells. Previous investigations pointed out the α to δ- phase transition occurs during the heating process, as supported by the X-ray and Neutron diffraction experiments. Through in situ measurements of the long-range order structure and the local structure by X-ray powder diffraction and X-ray absorption spectroscopy, we investigated the evolution of the structures under different temperatures. Both techniques provided ample evidence that the existence of meta-stable β-phase are crucial for forming the defective fluorite cubic δ phase. Our finding suggested that the phase transition from tetragonal β-phase to δ-phase is an influencing factor for the generation of the oxygen-ion pathways.

  4. Effect of Zn doping on structural, optical and thermal properties of CeO2 nanoparticles

    NASA Astrophysics Data System (ADS)

    Ramasamy, V.; Vijayalakshmi, G.

    2015-09-01

    The undoped and Zn doped CeO2 nanoparticles were synthesized by chemical precipitation method at room temperature. The undoped and Zn doped CeO2 nanoparticles have been characterized by X-ray powder diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), ultraviolet visible and photoluminescence (PL) spectroscopy, Fourier transform infrared spectroscopy (FTIR) and thermogravimetry and differential thermal analysis (TG-DTA). The cubic fluorite structures of the CeO2 nanoparticles were determined by XRD. The influence of particle size on structural parameters such as lattice parameter (a), inter planar distance (d), dislocation density (δ), microstrain (ε), lattice strain (η) and texture co-efficient (TC) were also determined. The lattice strains were determined by Williamson-Hall plot method. The effect of Zn doping with shifting of the bands were observed by UV-Vis spectroscopy and also their optical band gap were determined. The emission spectra and energy band diagram of the undoped and Zn doped samples were derived from PL spectroscopy. The structural bond vibrations of undoped and Zn doped CeO2 nanoparticles were analyzed by FTIR spectroscopy. The thermal property (weight loss and decomposition) of the sample is observed by TG-DTA curve.

  5. Structure and segregation of dopant-defect complexes at grain boundaries in nanocrystalline doped ceria.

    PubMed

    Dholabhai, Pratik P; Aguiar, Jeffery A; Wu, Longjia; Holesinger, Terry G; Aoki, Toshihiro; Castro, Ricardo H R; Uberuaga, Blas P

    2015-06-21

    Grain boundaries (GBs) dictate vital properties of nanocrystalline doped ceria. Thus, to understand and predict its properties, knowledge of the interaction between dopant-defect complexes and GBs is crucial. Here, we report atomistic simulations, corroborated with first principles calculations, elucidating the fundamental dopant-defect interactions at model GBs in gadolinium-doped and manganese-doped ceria. Gadolinium and manganese are aliovalent dopants, accommodated in ceria via a dopant-defect complex. While the behavior of isolated dopants and vacancies is expected to depend on the local atomic structure at GBs, the added structural complexity associated with dopant-defect complexes is found to have key implications on GB segregation. Compared to the grain interior, energies of different dopant-defect arrangements vary significantly at the GBs. As opposed to bulk, the stability of oxygen vacancy is found to be sensitive to the dopant arrangement at GBs. Manganese exhibits a stronger propensity for segregation to GBs than gadolinium, revealing that accommodation of dopant-defect clusters depends on the nature of dopants. Segregation strength is found to depend on the GB character, a result qualitatively supported by our experimental observations based on scanning transmission electron microscopy. The present results indicate that segregation energies, availability of favorable sites, and overall stronger binding of dopant-defect complexes would influence ionic conductivity across GBs in nanocrystalline doped ceria. Our comprehensive investigation emphasizes the critical role of dopant-defect interactions at GBs in governing functional properties in fluorite-structured ionic conductors.

  6. Crystal structures of orthorhombic, hexagonal, and cubic compounds of the Sm{sub (x)}Yb{sub (2−x)}TiO{sub 5} series

    SciTech Connect

    Aughterson, Robert D.; Lumpkin, Gregory R.; Reyes, Massey de los; Sharma, Neeraj; Ling, Christopher D.; Gault, Baptiste; Smith, Katherine L.; Avdeev, Maxim; Cairney, Julie M.

    2014-05-01

    A series of single phase compounds with nominal stoichiometry Sm{sub (x)}Yb{sub (2−x)}TiO{sub 5} (x=2, 1.4, 1, 0.6, and 0) have been successfully fabricated to generate a range of crystal structures covering the most common polymorphs previously discovered in the Ln{sub 2}TiO{sub 5} series (Ln=lanthanides and yttrium). Four of the five samples have not been previously fabricated in bulk, single phase form so their crystal structures are refined and detailed using powder synchrotron and single crystal x-ray diffraction, neutron diffraction and transmission electron microscopy. Based on the phase information from diffraction data, there are four crystal structure types in this series; orthorhombic Pnma, hexagonal P6{sub 3}/mmc, cubic (pyrochlore-like) Fd-3m and cubic (fluorite-like) Fm-3m. The cubic materials show modulated structures with variation between long and short range ordering and the variety of diffraction techniques were used to describe these complex crystal structure types. - Graphical abstract: A high resolution image of the compound Sm{sub 0.6}Yb{sub 1.4}TiO{sub 5} showing contrast from lattice fringes and the corresponding fast Fourier transform (FFT) of the HREM image with pyrochlore related diffraction spots marked “P” and fluorite marked “F”. The crystal is oriented down the [1 1 0] zone axis based on the Fd-3m structure. The ideal crystal structure (no vacancies) of the cubic, pyrochlore-like (Sm{sub 0.6}Yb{sub 1.4}TiO{sub 5}). - Highlights: • First fabrication of bulk single-phase material with stoichiometry Sm{sub 2}TiO{sub 5}. • Systematic study of crystal structure types within Ln{sub 2}TiO{sub 5} series (Ln=lanthanides). • A novel technique using IFFT of HREM images to study cubic structures.

  7. First-principles study of pressure-induced structural phase transitions in MnF2.

    PubMed

    López-Moreno, S; Romero, A H; Mejía-López, J; Muñoz, A

    2016-12-07

    In this work we report a complete structural and magnetic characterization of crystalline MnF2 under pressure obtained using first principle calculations. Density functional theory was used as the theoretical framework, within the generalized gradient approximation plus the Hubbard formalism (GGA+U) necessary to describe the strong correlations present in this material. The vibrational, the magnetic exchange couplings and the structural characterization of MnF2 in the rutile ground state structure and potential high pressure phases are reported. The quasiharmonic approximation has been used to obtain the free energy, which at the same time is used to evaluate the different structural transitions at 300 K. Based on previous theoretical and experimental studies on AF2 compounds, ten different structural candidates were considered for the high pressure regime, which led us to propose a path for the MnF2 structural transitions under pressure. As experimental pressure settings can lead to non-hydrostatic conditions, we consider hydrostatic and non-hydrostatic strains in our calculations. According to our results we found the following sequence for the pressure-driven structural phase transition in MnF2: rutile (P42/mnm) → α-PbO2-type (Pbcn) → dist. HP PdF2-type (Pbca) → dist. fluorite (I4/mmm) → cotunnite (Pnma). This structural path is correlated with other phase transitions reported on other metal rutile fluorides. In particular, we found that our proposed structural phase transition sequence offers an explanation of the different paths observed in the literature by taking into account the role of the hydrostatic conditions. In order to get a deep understanding of the modifications of MnF2 under pressure, we have analyzed the pressure evolution of the structural, vibrational, electronic, and magnetic properties for rutile and for each of the high pressure phases.

  8. Generality of the 18-n Rule: Intermetallic Structural Chemistry Explained through Isolobal Analogies to Transition Metal Complexes.

    PubMed

    Yannello, Vincent J; Fredrickson, Daniel C

    2015-12-07

    Intermetallic phases exhibit a vast structural diversity in which electron count is known to be one controlling factor. However, chemical bonding theory has yet to establish how electron counts and structure are interrelated for the majority of these compounds. Recently, a simple bonding picture for transition metal (T)-main group (E) intermetallics has begun to take shape based on isolobal analogies to molecular T complexes. This bonding picture is summarized in the 18-n rule: each T atom in a T-E intermetallic phase will need 18-n electrons to achieve a closed-shell 18-electron configuration, where n is the number of electron pairs it shares with other T atoms in multicenter interactions isolobal to T-T bonds. In this Article, we illustrate the generality of this rule with a survey over a wide range of T-E phases. First, we illustrate how three structural progressions with changing electron counts can be accounted for, both geometrically and electronically, with the 18-n rule: (1) the transition between the fluorite and complex β-FeSi2 types for TSi2 phases; (2) the sequence from the marcasite type to the arsenopyrite type and back to the marcasite type for TSb2 compounds; and (3) the evolution from the AuCu3 type to the ZrAl3 and TiAl3 types for TAl3 phases. We then turn to a broader survey of the applicability of the 18-n rule through a study of the following 34 binary structure types: PtHg4, CaF2 (fluorite), Fe3C, CoGa3, Co2Al5, Ru2B3, β-FeSi2, NiAs, Ni2Al3, Rh4Si5, CrSi2, Ir3Ga5, Mo3Al8, MnP, TiSi2, Ru2Sn3, TiAl3, MoSi2, CoSn, ZrAl3, CsCl, FeSi, AuCu3, ZrSi2, Mn2Hg5, FeS2 (oP6, marcasite), CoAs3 (skutterudite), PdSn2, CoSb2, Ir3Ge7, CuAl2, Re3Ge7, CrP2, and Mg2Ni. Through these analyses, the 18-n rule is established as a framework for interpreting the stability of 341 intermetallic phases and anticipating their properties.

  9. Synthesis of Ceramics from Solutions: Functionally Graded Composites, NanoComposites and Single Crystal Thin Films.

    DTIC Science & Technology

    2007-11-02

    deficiency and 0 <= phi <= 1 is the Zr/Ti molar ratio. Most compositions first crystallized as a metastable fluorite structure with varying degrees of...retention of fluorite and reduced the incidence of cation ordering. Compositions with off-stoichiometric amounts of PbO often yielded extended solid...compositions. The findings are discussed in light of structural relationships between the fluorite , pyrochlore and perovskite phases, as well as current understanding of the thermodynamics of the system.

  10. Structure and thermal expansion of Lu2O3 and Yb2O3 up to the melting points

    DOE PAGES

    Pavlik, Alfred; Ushakov, Sergey V.; Navrotsky, Alexandra; ...

    2017-08-24

    Knowledge of thermal expansion and high temperature phase transformations is essential for prediction and interpretation of materials behavior under the extreme conditions of high temperature and intense radiation encountered in nuclear reactors. We studied the structure and thermal expansion of Lu2O3 and Yb2O3 were studied in oxygen and argon atmospheres up to their melting temperatures using synchrotron X-ray diffraction on laser heated levitated samples. Both oxides retained the cubic bixbyite C-type structure in oxygen and argon to melting. In contrast to fluorite-type structures, the increase in the unit cell parameter of Yb2O3 and Lu2O33 with temperature is linear within experimentalmore » error from room temperature to the melting point, with mean thermal expansion coefficients (8.5 ± 0.6) · 10-6 K-1 and (7.7 ± 0.6) · 10-6 K-1, respectively. There is no indication of a superionic (Bredig) transition in the C-type structure or of a previously suggested Yb2O3 phase transformation to hexagonal phase prior to melting.« less

  11. Structure and thermal expansion of Lu 2 O 3 and Yb 2 O 3 up to the melting points

    DOE PAGES

    Pavlik, Alfred; Ushakov, Sergey V.; Navrotsky, Alexandra; ...

    2017-08-24

    Knowledge of thermal expansion and high temperature phase transformations is essential for prediction and interpretation of materials behavior under the extreme conditions of high temperature and intense radiation encountered in nuclear reactors. We studied the structure and thermal expansion of Lu2O3 and Yb2O3 were studied in oxygen and argon atmospheres up to their melting temperatures using synchrotron X-ray diffraction on laser heated levitated samples. Both oxides retained the cubic bixbyite C-type structure in oxygen and argon to melting. In contrast to fluorite-type structures, the increase in the unit cell parameter of Yb2O3 and Lu2O33 with temperature is linear within experimentalmore » error from room temperature to the melting point, with mean thermal expansion coefficients (8.5 ± 0.6) · 10-6 K-1 and (7.7 ± 0.6) · 10-6 K-1, respectively. There is no indication of a superionic (Bredig) transition in the C-type structure or of a previously suggested Yb2O3 phase transformation to hexagonal phase prior to melting.« less

  12. Crystal structure of the 'mixed-layer' Aurivillius phase Bi{sub 5}TiNbWO{sub 15}

    SciTech Connect

    Snedden, Alan; Charkin, Dmitri O.; Dolgikh, Valeriy A.; Lightfoot, Philip . E-mail: pl@st-and.ac.uk

    2005-01-15

    The crystal structure of the Aurivillius phase Bi{sub 5}TiNbWO{sub 15} has been analyzed in detail using powder X-ray and neutron diffraction. The structure can be described as a regular intergrowth of alternating single and double perovskite-like layers sandwiched between fluorite-like bismuth oxide layers, such that the layer sequence is ... [WO{sub 4}]-[Bi{sub 2}O{sub 2}]-[BiTiNbO{sub 7}]-[Bi{sub 2}O{sub 2}] .... There is complete ordering of tungsten within the B sites of the single perovskite layer, so that the structure can be described as a direct intergrowth of the 'component' Aurivillius phases Bi{sub 2}WO{sub 6} and Bi{sub 3}TiNbO{sub 9}. At 25 deg. C the structure adopts the polar orthorhombic space group I2cm, a=5.4231(2)A, b=5.4027(2)A, c=41.744(1)A.

  13. Structure and thermal expansion of Lu2O3 and Yb2O3 to the melting temperatures

    DOE PAGES

    Pavlik III, Alfred; Ushakov, Sergey V.; Navrotsky, Alexandra; ...

    2017-11-01

    Knowledge of thermal expansion and high temperature phase transformations is essential for prediction and interpretation of materials behavior under the extreme conditions of high temperature and intense radiation encountered in nuclear reactors. Structure and thermal expansion of Lu2O3 and Yb2O3 were studied in oxygen and argon atmospheres up to their melting temperatures using synchrotron X-ray diffraction on laser heated levitated samples. Both oxides retained the cubic bixbyite C-type structure in oxygen and argon to melting. In contrast to fluorite-type structures, the increase in the unit cell parameter of Yb2O3 and Lu2O3 with temperature is linear within experimental error from roommore » temperature to the melting point, with mean thermal expansion coefficients (8.5 ± 0.6) · 10−6 K−1 and (7.7 ± 0.6) · 10−6 K−1, respectively. There is no indication of a superionic (Bredig) transition in the C-type structure or of a previously suggested Yb2O3 phase transformation to hexagonal phase prior to melting.« less

  14. Structure and Optical Properties of CeO2 Nanoparticles Synthesized by Precipitation

    NASA Astrophysics Data System (ADS)

    Liu, I.-Tsan; Hon, Min-Hsiung; Teoh, Lay Gaik

    2013-08-01

    Cerium dioxide (CeO2) has special electrical and optical properties, and chemical and thermal stability. It has been used in semiconductor devices and as a luminescent material. In this work, CeO2 nanoparticles were synthesized by the precipitation method and the product annealed at various temperatures. Thermogravimetric analysis (TGA)/differential scanning calorimetry (DSC) results show that the optimum annealing temperature for fabrication of CeO2 nanoparticles is greater than 500°C. When the calcination temperature is increased from 550°C to 1050°C, Fourier-transform infrared (FTIR) results show that the water and impurities are almost completely removed, after calcination at 750°C. The x-ray diffraction (XRD) results suggest that the synthesized CeO2 exhibits a cubic fluorite structure. The crystallite size of the CeO2 increases from 8 nm to 75 nm when the calcination temperature is increased from 550°C to 1050°C. The absorption spectrum in the ultraviolet (UV) region from 372 nm to 395 nm demonstrates their applicability as UV-filter materials, and the shift of the estimated E g,eff from 3.21 eV to 3.65 eV demonstrates their applicability in photoelectric devices. CeO2 would be potentially important for applications such as insulator structures, stable capacitor devices, and light-emitting diodes (LEDs).

  15. Interlayer Communication in Aurivillius Vanadate to Enable Defect Structures and Charge Ordering.

    PubMed

    Zhang, Yaoqing; Yamamoto, Takafumi; Green, Mark A; Kageyama, Hiroshi; Ueda, Yutaka

    2015-11-16

    The fluorite-like [Bi2O2](2+) layer is a fundamental building unit in a great variety of layered compounds. Here in this contribution, we presented a comprehensive study on an unusual Aurivillius phase Bi3.6V2O10 with respect to its defect chemistry and polymorphism control as well as implications for fast oxide ion transport at lower temperatures. The bismuth oxide layer in Bi4V2O11 is found to tolerate a large number of Bi vacancies without breaking the high temperature prototype I4/mmm structure (γ-phase). On cooling, an orthorhombic distortion occurs to the γ-phase, giving rise to a different type of phase (B-phase) in the intermediate temperature region. Cooling to room temperature causes a further transition to an oxygen-vacancy ordered A-phase, which is accompanied by the charge ordering of V(4+) and V(5+) cations, providing magnetic (d(1)) and nonmagnetic (d(0)) chains along the a axis. This is a novel charge ordering transition in terms of the concomitant change of oxygen coordination. Interestingly, upon quenching, both the γ- and B-phase can be kinetically trapped, enabling the structural probing of the two phases at ambient temperature. Driven by the thermodynamic forces, the oxide anion in the γ-phase undergoes an interlayer diffusion process to reshuffle the compositions of both Bi-O and V-O layers.

  16. Structure reconstruction of TiO2-based multi-wall nanotubes: first-principles calculations.

    PubMed

    Bandura, A V; Evarestov, R A; Lukyanov, S I

    2014-07-28

    A new method of theoretical modelling of polyhedral single-walled nanotubes based on the consolidation of walls in the rolled-up multi-walled nanotubes is proposed. Molecular mechanics and ab initio quantum mechanics methods are applied to investigate the merging of walls in nanotubes constructed from the different phases of titania. The combination of two methods allows us to simulate the structures which are difficult to find only by ab initio calculations. For nanotube folding we have used (1) the 3-plane fluorite TiO2 layer; (2) the anatase (101) 6-plane layer; (3) the rutile (110) 6-plane layer; and (4) the 6-plane layer with lepidocrocite morphology. The symmetry of the resulting single-walled nanotubes is significantly lower than the symmetry of initial coaxial cylindrical double- or triple-walled nanotubes. These merged nanotubes acquire higher stability in comparison with the initial multi-walled nanotubes. The wall thickness of the merged nanotubes exceeds 1 nm and approaches the corresponding parameter of the experimental patterns. The present investigation demonstrates that the merged nanotubes can integrate the two different crystalline phases in one and the same wall structure.

  17. InP Transferred Electron Cathodes: Basic to Manufacturing Methods

    DTIC Science & Technology

    2007-08-29

    Figure 2.8 (a) Bulk Cs superoxide: calcium carbide structure (b) Bulk Cs peroxide: distorted calcium fluorite structure 46 ix...Figure 2.8 (a) Bulk Cs superoxide: calcium carbide structure (b) Bulk Cs peroxide: distorted calcium fluorite structure [34] 47

  18. Nature of the Transient BaF2-Related Phases in the "BaF2" Processing of Ba2YCu3O7-x Superconductors

    SciTech Connect

    Wong-Ng, W.; Levin, I.; Cook, L. P.; Feenstra, Roeland

    2006-01-01

    Transient BaF{sub 2}-based oxyfluoride phases are thought to play a critical role in the formation of the c-textured Ba{sub 2}YCu{sub 3}O{sub 7-x} layers of coated conductors. In situ high-temperature x-ray diffraction from the precursor films containing pure BaF{sub 2} as well as pseudobinary BaF{sub 2-Y}, BaF{sub 2-}Cu, and Y-Cu mixtures and heat treated in water vapor under reduced conditions revealed that the transient BaF{sub 2}-based superstructures, similar to those observed during formation of Ba{sub 2}Cu{sub 3}O{sub 7-x}, develop even from the pure BaF{sub 2} precursor. These superstructures results from the dissolution of oxygen in BaF{sub 2} leading to formation of the oxyfluoride phase, Ba(F{sub 2-2x}{open_square}{sub x})O{sub x}, with an ordered arrangement of O, F, and F vacancies.

  19. Measurement of the Np237(n,γ) cross section from 20 meV to 500 keV with a high efficiency, highly segmented 4π BaF2 detector

    NASA Astrophysics Data System (ADS)

    Esch, E.-I.; Reifarth, R.; Bond, E. M.; Bredeweg, T. A.; Couture, A.; Glover, S. E.; Greife, U.; Haight, R. C.; Hatarik, A. M.; Hatarik, R.; Jandel, M.; Kawano, T.; Mertz, A.; O'Donnell, J. M.; Rundberg, R. S.; Schwantes, J. M.; Ullmann, J. L.; Vieira, D. J.; Wilhelmy, J. B.; Wouters, J. M.

    2008-03-01

    The Np237(n,γ)Np238 cross section has been measured in the neutron energy range from 20 meV to 500 keV using the DANCE array at the Los Alamos National Laboratory. This new facility allows experiments with submilligram samples and is therefore well suited to investigate isotopes with half-lives as low as a few hundred days. In this benchmark measurement, only 0.42 mg of Np237 was sufficient to determine differential cross sections relative to the well-known resonance at 0.5 eV. The thermal cross section was measured to σ2200m/s=177±5 barn, σkT=25.3meV=167±4 barn and the resonance integral to RI=693±6 barn.

  20. Structure of delta-Bi{sub 2}O{sub 3} from density functional theory: A systematic crystallographic analysis

    SciTech Connect

    Aidhy, Dilpuneet S.; Sinnott, Susan B.; Wachsman, Eric D.; Phillpot, Simon R.; Nino, Juan C.

    2009-05-15

    A systematic crystallographic analysis of the <110> and <111> vacancy-ordered structure of cubic delta-Bi{sub 2}O{sub 3} obtained from electronic-structure calculations is presented. The ordering of vacancies leads to a doubling of the unit-cell that results in a 2x2x2 fluorite super-structure, with an associated reduction in its space group symmetry from Fm3-barm to Fm3-bar. The Bi atoms present inside the <111> vacancy-ordered oxygen sublattice have equal Bi-O bond lengths, whereas, those present inside the <110> vacancy-ordered oxygen sublattice have three different pairs of Bi-O bond lengths. The specific ionic displacements and electronic charge configurations also depend on the nature of vacancy ordering in the oxygen sub-lattice. - Graphical abstract: 1/8 of a 2x2x2 delta-Bi{sub 2}O{sub 3} superstructure having Fm3-bar space group. Every oxygen (black) has three possible positions, only one of which is filled either by O1 (red) or O{sub 2} (blue).

  1. Effect of annealing temperature on the structural and optical properties of CeO2:Ni thin films

    NASA Astrophysics Data System (ADS)

    Murugan, R.; Vijayaprasath, G.; Sakthivel, P.; Mahalingam, T.; Ravi, G.

    2016-05-01

    High quality Ni-doped CeO2 (CeO2:Ni) thin films were deposited on glass substrates at room temperature by using radio frequency magnetron sputtering. The effect of annealing temperature on structural and optical properties of the CeO2:Ni films was investigated. The structural, optical and vibrational properties of the films were determined using X-ray diffraction (XRD), photoluminescence spectrometer (PL) and Raman spectrometer. It was found that the as-deposited film has a fluorite cubic structure. By increasing annealing temperature from 100°C to 300°C, the crystalline quality of the thin films could be improved. The UV and visible band emissions were observed in the photoluminescence spectra, due to exciton, defect related emissions respectively. The micro-Raman results show the characteristic peak of CeO2 F2g at 465 cm-1 and 2L0 at 1142 cm-1. Defect peaks like D and 0 bands were observed at 641 cm-1 and 548 cm-1 respectively. It is found from the spectra that the peak intensity of the films increased with increase of annealing temperature.

  2. Structure and phase composition of nanocrystalline Ce{sub 1-x}Lu{sub x}O{sub 2-y}

    SciTech Connect

    Malecka, Malgorzata A.; Kepinski, Leszek Maczka, Miroslaw

    2008-09-15

    The microstructure and phase stability of nanocrystalline mixed oxide Lu{sub x}Ce{sub 1-x}O{sub 2-y} (x=0-1) are described. Nano-sized (3-4 nm) oxide particles were prepared by the reverse microemulsion method. Morphological and structural changes upon heat treatment in an oxidizing atmosphere were studied by transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman and Yb{sup 3+} emission spectroscopy, the latter ion being present as an impurity in the Lu{sub 2}O{sub 3} starting material. Up to 950 deg. C, the samples were single phase, with structure changing smoothly with Lu content from fluorite type (F) to bixbyite type (C). For the samples heated at 1100 deg. C phase separation into coexisting F- and C-type structures was observed for 0.35

  3. Influence of synthesis conditions on the crystal structure of the powder formed in the ZrO2 - Ce2O3/CeO2 system.

    NASA Astrophysics Data System (ADS)

    Popov, V. V.; Menushenkov, A. P.; Khubbutdinov, R. M.; Svetogorov, R. D.; Zubavichus, Ya V.; Sharapov, A. S.; Kurilkin, V. V.

    2016-09-01

    Influence of synthesis conditions (type of atmosphere, reduction and oxidation, annealing temperature) on the chemical composition and structure of the compounds formed in the “ZrO2 - Ce2O3 / CeO2” system has been investigated by X-ray absorption fine structure (XAFS) spectroscopy combined with X-ray diffraction (XRD) and thermogravimetric analysis (TGA). It is revealed that isothermal annealing of precursor at temperatures less than 1000 °C in air leads to formation of Ce0.5Zr0.5O2 powders with cubic fluorite-type structure (Fm-3m). Further increase of annealing temperatures above 1000 °C causes decomposition of formed crystal structure into two phases: cubic and tetragonal. Annealing in reduction hydrogen atmosphere causes formation of Ce4 + 2xCe3 + 2-2xZr2O7 + x compounds with intermediate valency of cerium, where value of x depends on the reducing conditions and treatment parameters of precursor. Annealing in vacuum at 1400 °C strongly reduces the content of Ce4+ in a powder samples and leads to formation of pyrochlore structure (space group Fd-3m) with practically Ce3+ valence state.

  4. Structure and bulk modulus of Ln-doped UO2 (Ln = La, Nd) at high pressure

    NASA Astrophysics Data System (ADS)

    Rittman, Dylan R.; Park, Sulgiye; Tracy, Cameron L.; Zhang, Lei; Palomares, Raul I.; Lang, Maik; Navrotsky, Alexandra; Mao, Wendy L.; Ewing, Rodney C.

    2017-07-01

    The structure of lanthanide-doped uranium dioxide, LnxU1-xO2-0.5x+y (Ln = La, Nd), was investigated at pressures up to ∼50-55 GPa. Samples were synthesized with different lanthanides at different concentrations (x ∼ 0.2 and 0.5), and all were slightly hyperstoichiometric (y ∼ 0.25-0.4). In situ high-pressure synchrotron X-ray diffraction was used to investigate their high-pressure phase behavior and determine their bulk moduli. All samples underwent a fluorite-to-cotunnite phase transformation with increasing pressure. The pressure of the phase transformation increased with increasing hyperstoichiometry, which is consistent with results from previous computational simulations. Bulk moduli are inversely proportional to both the ionic radius of the lanthanide and its concentration, as quantified using a weighted cationic radius ratio. This trend was found to be consistent with the behavior of other elastic properties measured for Ln-doped UO2, such as Young's modulus.

  5. Leucas aspera mediated multifunctional CeO2 nanoparticles: Structural, photoluminescent, photocatalytic and antibacterial properties.

    PubMed

    Malleshappa, J; Nagabhushana, H; Sharma, S C; Vidya, Y S; Anantharaju, K S; Prashantha, S C; Daruka Prasad, B; Raja Naika, H; Lingaraju, K; Surendra, B S

    2015-01-01

    Spherical shaped cerium dioxide (CeO2) nanoparticles (NPs) were synthesized via bio mediated route using Leucas aspera (LA) leaf extract. The NPs were characterized by PXRD, SEM, UV-Visible techniques. Photoluminescence (PL), photocatalysis and antibacterial properties of NPs were studied. PXRD patterns and Rietveld analysis confirm cubic fluorite structure with space group Fm-3m. SEM results evident that morphology of the NPs was greatly influenced by the concentration of LA leaf extract in the reaction mixture. The band gap energy of the NPs was found to be in the range of 2.98-3.4 eV. The photocatalytic activity of NPs was evaluated by decolorization of Rhodamine-B (RhB) under UVA and Sun light irradiation. CeO2 NPs show intense blue emission with CIE coordinates (0.14, 0.22) and average color coordinated temperature value ∼148,953 K. Therefore the present NPs quite useful for cool LEDs. The superior photocatalytic activity was observed for CeO2 NPs with 20 ml LA under both UVA and Sunlight irradiation. The enhanced photocatalytic activity and photoluminescent properties were attributed to defect induced band gap engineered CeO2 NPs. Further, CeO2 with 20 ml LA exhibit significant antibacterial activity against Escherichia coli (EC) and Staphylococcus aureus (SA). These findings show great promise of CeO2 NPs as multifunctional material for various applications.

  6. CONDENSED MATTER: ELECTRONICSTRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICALPROPERTIES: First-Principles Study of Structural Stabilities, Electronic and Optical Properties of SrF2 under High Pressure

    NASA Astrophysics Data System (ADS)

    Hao, Ai-Min; Yang, Xiao-Cui; Li, Jie; Xin, Wei; Zhang, Su-Hong; Zhang, Xin-Yu; Liu, Ri-Ping

    2009-07-01

    An investigation of structural stabilities, electronic and optical properties of SrF2 under high pressure is conducted using a first-principles calculation based on density functional theory (DFT) with the plane wave basis set as implemented in the CASTEP code. Our results predict that the second high-pressure phase of SrF2 is of a Ni2In-type structure, and demonstrate that the sequence of the pressure-induced phase transition of SrF2 is the fluorite structure (Fm3m) to the PbCl2-type structure (Pnma), and to the Ni2In-type phase (P63/mmc). The first and second phase transition pressures are 5.77 and 45.58 GPa, respectively. The energy gap increases initially with pressure in the Fm3m, and begins to decrease in the Pnma phases at 30 GPa. The band gap overlap metallization does not occur up to 210 GPa. The pressure effect on the optical properties is discussed.

  7. Self-irradiation induced structural changes in the transplutonium pyrochlores An{sub 2}Zr{sub 2}O{sub 7} (An=Am, Cf)

    SciTech Connect

    Sykora, Richard E. . E-mail: sykorare@ornl.gov; Raison, P.E.; Haire, Richard G.

    2005-02-15

    We have pursued the fundamental chemistry of actinide pyrochlore oxides, An{sub 2}Zr{sub 2}O{sub 7} (An=Am, Cm, Bk, and Cf), using X-ray diffraction as well as optical spectroscopy. One recent facet of our studies has been to observe the structural changes of these materials under self-irradiation as a function of time. It has been reported that both titanate and silicate materials transform from a crystalline to an amorphous state under irradiation. With the Zr-based actinide pyrochlores studied here, we have observed a phase change from a pyrochlore structure to a fluorite-type structure with the retention of crystallinity. We focus here on the impact of {alpha}-radiation ({sup 243}Am and {sup 249}Cf), rather than that from neutrons ({sup 248}Cm) or {beta}-radiation ({sup 249}Bk), on the An{sub 2}Zr{sub 2}O{sub 7} pyrochlore structures. As a result of this phase change, the local coordination environments of both the actinide and zirconium atoms are altered. We consider a defect/ion deficiency driven mechanism and also address the occurrence of oxidation of the trivalent actinides during the self-irradiation process as being potential mechanisms responsible for the observed phase change.

  8. Effect of heat treatment on the crystal structure and FTIR spectra of Sm doped cerium dioxide film

    NASA Astrophysics Data System (ADS)

    Liu, Xiaozhou; Liu, Xiaozhou; Xia, Letian; Chen, Jie; Wang, Xiaoyu

    2017-04-01

    The Sm doped cerium dioxide films were prepared with cerium foils as raw materials by anodization in Sm(NO3)3-Na2C2O4-NH3·H2O-H2O-(CH2OH)2 electrolyte. The anodic Sm doped cerium oxide films were heat treated in 100°C ~ 400°C. The heat treated anodic Sm doped cerium oxide films were characterized with X-ray diffraction (XRD) and Fourier transform infrared (FTIR) techniques respectively. The heat treated anodic Sm doped cerium oxide film at 100°C is the semi crystalline film. As heat treatment temperatures being in 200°C ~ 400°C, the heat treated anodic Sm doped cerium oxide films have a structure of cubic fluorite respectively. The doping of Sm can be achieved well by anodization method and be recognized as replacement doping or caulking doping. The crystal structure of Sm doped cerium dioxide films become more complete with the increase of heat treatment temperature in 200 ~ 400 °C. The doping of Sm can improve the crystallinity of the cerium dioxide film. The presence of adsorbing water, ethylene glycol and CO2 in the heat treated anodic Sm doped cerium oxide film at 100°C. The adsorbing ethylene glycols and water, CO2 in the anodic Sm doped cerium oxide film are removed at 200°C and 300°C respectively.

  9. DFT study on the electronic structure and chemical state of Americium in an (Am,U) mixed oxide

    NASA Astrophysics Data System (ADS)

    Suzuki, Chikashi; Nishi, Tsuyoshi; Nakada, Masami; Tsuru, Tomohito; Akabori, Mitsuo; Hirata, Masaru; Kaji, Yoshiyuki

    2013-12-01

    We investigated the electronic state of an (Am,U) mixed oxide with the fluorite structure using the all-electron full potential linear augmented plane wave method and compared it with those of Am2O3, AmO2, UO2, and La0.5U0.5O2. The valence of Am in the mixed oxide was close to that of Am2O3 and the valence of U in the mixed oxide was pentavalent. The electronic structure of AmO2 was different from that of Am2O3, particularly just above the Fermi level. In addition, the electronic states of Am and U in the mixed oxide were similar to those of trivalent Am and pentavalent U oxides. These electronic states reflected the high oxygen potential of AmO2 and the heightened oxygen potential resulting from the addition of Am to UO2 and also suggested the occurrence of charge transfer from Am to U in the solid solution process.

  10. Investigations of Shock Wave Synthesis of Metastable Compounds.

    DTIC Science & Technology

    1997-01-01

    that they crystallize into cubic structure of fluorite type with Fm3m space group. X - ray photographs of the compounds obtained are shown in Fig.2...wide range of homogeneity (- 30 + 70 mol %) crystallizes in cubic structure of fluorite type, the second - is stoichiometric compound of GeSiO4

  11. Structures and Crystal Chemistry of Layered Materials

    NASA Astrophysics Data System (ADS)

    Partin, Daniel Edward

    The crystal chemistry of several layered materials has been explored using a variety of methods, with an emphasis on their structural aspects. In the second part of this work, the structure of several copper oxides that are of significance to the study of superconductors are described. The crystal structures of MgCl_2 and CdCl_2 have been refined using powder X-ray diffraction data. They have the space group Roverline{3}m. For magnesium chloride the unit cell constants are a = 3.6363(1) A, c = 17.6663(5) A. For cadmium chloride they are a = 3.8459(1) A, c = 17.4931(4) A. The structures and their relationship to that of fluorite are discussed within the framework of a Born-Mayer model. The crystal structure of Mg(OD)_2 has been refined from time-of flight (TOF) neutron diffraction data and found to be trigonal, Poverline {3}m1, a = 3.1455(1) A, c = 4.7646(3) A. The data were collected at 305 K. The O-D bond length is 0.937 (1) A (corrected for "riding" motion 0.948 A). An infrared/Raman study of Mg(OH)_2 was conducted in a diamond anvil cell in the pressure range from room pressure up to 7 Gpa. For layered crystals, it was found that as the internally fixed layers are moved apart the Madelung energy of the system becomes constant after a very short distance, although not necessarily that of the given crystal's energy at ambient conditions. The crystal structure of Sr(OD)_2 has been refined from time-of-flight neutron diffraction data and the deuterium positions found. Strontium deuteroxide crystallizes in the space group Pnma, with the unit cell constants of a = 9.8269(5) A, b = 3.9051(2) A, and c = 6.0733(3) A. The crystal structures of SrCuO_2 and Sr_2CuO_3 have been refined by time-of-flight neutron diffraction. For SrCuO_2 the space group is Cmcm, a = 3.57002(2), b = 16.32268(8), c = 3.91100(2); for Sr _2CuO_3 the space group is Immm, a = 3.49900(5), b = 12.7009(2), c = 3.91120(5). In both structures the strontium atoms are coordinated by seven oxygen atoms

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

    SciTech Connect

    Aughterson, R. D.; Lumpkin, G. R.; de los Reyes, M.; Gault, B.; Baldo, P.; Ryan, E.; Whittle, K. R.; Smith, K. L.; Cairney, J. M.

    2016-04-01

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

  13. Synthesis, crystal structure and magnetic properties of new indium rhenium and scandium rhenium oxides, In 6ReO 12 and Sc 6ReO 12

    NASA Astrophysics Data System (ADS)

    Mikhailova, D.; Ehrenberg, H.; Fuess, H.

    2006-12-01

    The new complex indium rhenium and scandium rhenium oxides, In 6ReO 12 and Sc 6ReO 12, have been synthesized as single phases in sealed silica tubes and by high-pressure high-temperature syntheses, and their crystal structures have been determined by single crystal X-ray diffraction.The compounds crystallize in a rhombohedral structure related to the distorted fluorite structure like Ln 6ReO 12 for some rare earth elements, S. G.: R-3, Z=3, aH= 9.248(2) Å, cH=8.720(2) Å for Sc 6ReO 12 and aH=9.492(1) Å, cH=8.933(1) Å for In 6ReO 12. A maximum in magnetization is observed for Sc 6ReO 12 at T( Mmax)=1.89(2) K, whereas ferromagnetic ordering is found for In 6ReO 12 by a pronounced increase in the temperature dependence of magnetization at TC=7.5(5) K. The magnetic moment per rhenium ion in In 6ReO 12 and Sc 6ReO 12 is 0.84(1) and 0.65(1) μB, respectively, derived from the paramagnetic regions.

  14. Mg Doping Induced Effects on Structural, Optical, and Electrical Properties as Well as Cytotoxicity of CeO2 Nanostructures

    NASA Astrophysics Data System (ADS)

    Iqbal, Javed; Jan, Tariq; Awan, M. S.; Naqvi, Sajjad Haider; Badshah, Noor; ullah, Asmat; Abbas, Fazzal

    2016-04-01

    Here, Mg x Ce1- x O2 (where x = 0, 0.01, 0.02, 0.03, 0.04, and 0.05) nanostructures have been successfully synthesized by using a simple, easy, and cost-effective soft chemical method. X-ray diffraction (XRD) patterns substantiate the single-phase formation of a CeO2 cubic fluorite structure for all samples. Infrared spectroscopy results depict the presence of peaks only related to Ce-O bonding, which confirms the XRD results. It has been observed via ultraviolet (UV)-visible spectroscopy that Mg doping has tuned the optical band gap of CeO2 significantly. The electrical conductivity of CeO2 nanostructures has been found to increase with Mg doping, which is attributed to enhancement in carrier concentration due to the different valance states of dopant and host ions. Selective cytotoxic behavior of Mg x Ce1- x O2 nanostructures has been determined for neuroblastoma (SH-SY5Y) cancerous and HEK-293 healthy cells. Both doped and undoped CeO2 nanostructures have been found to be toxic for cancer cells and safe toward healthy cells. This selective toxic behavior of the synthesized nanostructures has been assigned to the different levels of reactive oxygen species (ROS) generation in different types of cells. This makes the synthesized nanostructures a potential option for cancer therapy in the near future.

  15. Structure and bulk modulus of Ln-doped UO2 (Ln = La, Nd) at high pressure

    DOE PAGES

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

    2017-04-10

    The structure of lanthanide-doped uranium dioxide, LnxU1-xO2-0.5x+y (Ln = La, Nd), was investigated at pressures up to ~50–55 GPa. Samples were synthesized with different lanthanides at different concentrations (x ~ 0.2 and 0.5), and all were slightly hyperstoichiometric (y ~ 0.25–0.4). In situ high-pressure synchrotron X-ray diffraction was used to investigate their high-pressure phase behavior and determine their bulk moduli. All samples underwent a fluorite-to-cotunnite phase transformation with increasing pressure. The pressure of the phase transformation increased with increasing hyperstoichiometry, which is consistent with results from previous computational simulations. Bulk moduli are inversely proportional to both the ionic radius ofmore » the lanthanide and its concentration, as quantified using a weighted cationic radius ratio. As a result, this trend was found to be consistent with the behavior of other elastic properties measured for Ln-doped UO2, such as Young's modulus.« less

  16. Local structure in solid solutions of stabilised zirconia with actinide dioxides (UO{sub 2}, NpO{sub 2})

    SciTech Connect

    Walter, Marcus; Somers, Joseph; Bouexiere, Daniel; Rothe, Joerg

    2011-04-15

    The local structure of (Zr,Lu,U)O{sub 2-x} and (Zr,Y,Np)O{sub 2-x} solid solutions has been investigated by extended X-ray absorption fine structure (EXAFS). Samples were prepared by mixing reactive (Zr,Lu)O{sub 2-x} and (Zr,Y)O{sub 2-x} precursor materials with the actinide oxide powders, respectively. Sintering at 1600 {sup o}C in Ar/H{sub 2} yields a fluorite structure with U(IV) and Np(IV). As typical for stabilised zirconia the metal-oxygen and metal-metal distances are characteristic for the different metal ions. The bond lengths increase with actinide concentration, whereas highest adaptation to the bulk stabilised zirconia structure was observed for U---O and Np---O bonds. The Zr---O bond shows only a slight increase from 2.14 A at 6 mol% actinide to 2.18 A at infinite dilution in UO{sub 2} and NpO{sub 2}. The short interatomic distance between Zr and the surrounding oxygen and metal atoms indicate a low relaxation of Zr with respect to the bulk structure, i.e. a strong Pauling behaviour. -- Graphical abstract: Metal-oxygen bond distances in (Zr,Lu,U)O{sub 2-x} solid solutions with different oxygen vacancy concentrations (Lu/Zr=1 and Lu/Zr=0.5). Display Omitted Research Highlights: {yields} EXAFS indicates high U and Np adaption to the bulk structure of stabilised zirconia. {yields} Zr---O bond length is 2.18 A at infinite Zr dilution in UO{sub 2} and NpO{sub 2}. {yields} Low relaxation (strong Pauling behaviour) of Zr explains its low solubility in UO{sub 2}.

  17. Ceramic planar waveguide structures for amplifiers and lasers

    NASA Astrophysics Data System (ADS)

    Konyushkin, V. A.; Nakladov, A. N.; Konyushkin, D. V.; Doroshenko, Maxim E.; Osiko, Vyacheslav V.; Karasik, Aleksandr Ya

    2013-01-01

    Ceramic and crystalline weakly guiding optical fibres with the core - cladding refractive index difference of 10-2 - 10-4 are fabricated by a hot pressing method. The waveguides with one or several cores for operation in the spectral range 0.2 - 5 μm are produced. The waveguides are based on CaF2, SrF2, and BaF2 ceramics and crystals and their solid solutions doped with trivalent Pr, Nd, Tb, Dy, Yb, Ho, Er, and Tm ions, as well as on LiF ceramics and crystals with colour centres. The first results of investigation of the lasing properties of ceramic SrF2 : NdF waveguides under diode pumping are presented, and the prospects of further investigation are discussed.

  18. Structure and cation ordering in La2UO6, Ce2UO6, LaUO4, and CeUO4 by first principles calculations

    DOE PAGES

    Casillas-Trujillo, Luis; Xu, H.; McMurray, Jake W.; ...

    2016-07-06

    In the present work, we have used density functional theory (DFT) and DFT+U to investigate the crystal structure and phase stability of four model compounds in the Ln2O3-UO2-UO3 ternary oxide system: La2UO6, Ce2UO6, LaUO4, CeUO4, due to the highly-correlated nature of the f-electrons in uranium. We have considered both hypothetical ordered compounds and compounds in which the cations randomly occupy atomic sites in a fluorite-like lattice. We determined that ordered compounds are stable and are energetically favored compared to disordered configurations, though the ordering tendencies are weak. To model and analyze the structures of these complex oxides, we have usedmore » supercells based on a layered atomic model. In the layer model, the supercell is composed of alternating planes of anions and cations. We have considered two different ordering motifs for the cations, namely single species (isoatomic) cation layers versus mixed species cation layers. Energy differences between various ordered cationic arrangements were found to be small. This may have implications regarding radiation stability, since cationic arrangements should be able to change under irradiation with little cost in energy.« less

  19. Structural characterization of nanosized CeO(2)-SiO(2), CeO(2)-TiO(2), and CeO(2)-ZrO(2) catalysts by XRD, Raman, and HREM techniques.

    PubMed

    Reddy, Benjaram M; Khan, Ataullah; Lakshmanan, Pandian; Aouine, Mimoun; Loridant, Stéphane; Volta, Jean-Claude

    2005-03-03

    Structural characteristics of nanosized ceria-silica, ceria-titania, and ceria-zirconia mixed oxide catalysts have been investigated using X-ray diffraction (XRD), Raman spectroscopy, BET surface area, thermogravimetry, and high-resolution transmission electron microscopy (HREM). The effect of support oxides on the crystal modification of ceria cubic lattice was mainly focused. The investigated oxides were obtained by soft chemical routes with ultrahighly dilute solutions and were subjected to thermal treatments from 773 to 1073 K. The XRD results suggest that the CeO(2)-SiO(2) sample primarily consists of nanocrystalline CeO(2) on the amorphous SiO(2) surface. Both crystalline CeO(2) and TiO(2) anatase phases were noted in the case of CeO(2)-TiO(2) sample. Formation of cubic Ce(0.75)Zr(0.25)O(2) and Ce(0.6)Zr(0.4)O(2) (at 1073 K) were observed in the case of the CeO(2)-ZrO(2) sample. Raman measurements disclose the fluorite structure of ceria and the presence of oxygen vacancies/Ce(3+). The HREM results reveal well-dispersed CeO(2) nanocrystals over the amorphous SiO(2) matrix in the cases of CeO(2)-SiO(2), isolated CeO(2), and TiO(2) (anatase) nanocrystals, some overlapping regions in the case of CeO(2)-TiO(2), and nanosized CeO(2) and Ce-Zr oxides in the case of CeO(2)-ZrO(2) sample. The exact structural features of these crystals as determined by digital diffraction analysis of HREM experimental images reveal that the CeO(2) is mainly in cubic fluorite geometry. The oxygen storage capacity (OSC) as determined by thermogravimetry reveals that the OSC of the mixed oxide systems is more than that of pure CeO(2) and is system dependent.

  20. In Situ Diffraction from Levitated Solids Under Extreme Conditions-Structure and Thermal Expansion in the Eu 2 O 3 -ZrO 2 System

    SciTech Connect

    Maram, Pardha S.; Ushakov, Sergey V.; Weber, Richard J. K.; Benmore, Chris J.; Navrotsky, Alexandra; Raveau, B.

    2014-12-31

    The accurate determination of structure and thermal expansion of refractory materials at temperatures above 1500 degrees C is challenging. Here, for the first time, we demonstrate the ability to reliably refine the structure and thermal expansion coefficient of oxides at temperatures to 2200 degrees C using in situ synchrotron diffraction coupled with aerodynamic levitation. Solid solutions in the Eu2O3-ZrO2 binary system were investigated, including the high-temperature order-disorder transformation in Eu2Zr2O7. The disordered fluorite phase is found to be stable above 1900 degrees C, and a reversible phase transition to the pyrochlore phase is noticed during cooling. Site occupancies in Eu2Zr2O7 show a gradual increase in disorder on both cation and anion sublattices with increasing temperature. The thermal expansion coefficients of all cubic solid solutions are relatively similar, falling in the range 8.6-12.0x10(-6)C(-1). These studies open new vistas for in situ exploration of complex structural changes in high-temperature materials.

  1. Is Geometric Frustration-Induced Disorder a Recipe for High Ionic Conductivity?

    PubMed

    Düvel, Andre; Heitjans, Paul; Fedorov, Pavel; Scholz, Gudrun; Cibin, Giannantonio; Chadwick, Alan V; Pickup, David M; Ramos, Silvia; Sayle, Lewis W L; Sayle, Emma K L; Sayle, Thi X T; Sayle, Dean C

    2017-04-26

    Ionic conductivity is ubiquitous to many industrially important applications such as fuel cells, batteries, sensors, and catalysis. Tunable conductivity in these systems is therefore key to their commercial viability. Here, we show that geometric frustration can be exploited as a vehicle for conductivity tuning. In particular, we imposed geometric frustration upon a prototypical system, CaF2, by ball milling it with BaF2, to create nanostructured Ba1-xCaxF2 solid solutions and increased its ionic conductivity by over 5 orders of magnitude. By mirroring each experiment with MD simulation, including "simulating synthesis", we reveal that geometric frustration confers, on a system at ambient temperature, structural and dynamical attributes that are typically associated with heating a material above its superionic transition temperature. These include structural disorder, excess volume, pseudovacancy arrays, and collective transport mechanisms; we show that the excess volume correlates with ionic conductivity for the Ba1-xCaxF2 system. We also present evidence that geometric frustration-induced conductivity is a general phenomenon, which may help explain the high ionic conductivity in doped fluorite-structured oxides such as ceria and zirconia, with application for solid oxide fuel cells. A review on geometric frustration [ Nature 2015 , 521 , 303 ] remarks that classical crystallography is inadequate to describe systems with correlated disorder, but that correlated disorder has clear crystallographic signatures. Here, we identify two possible crystallographic signatures of geometric frustration: excess volume and correlated "snake-like" ionic transport; the latter infers correlated disorder. In particular, as one ion in the chain moves, all the other (correlated) ions in the chain move simultaneously. Critically, our simulations reveal snake-like chains, over 40 Å in length, which indicates long-range correlation in our disordered systems. Similarly, collective

  2. Nd3+-substituted (Zr1-xCex)O2 (0.0 ≤ x ≤ 1.0) system: Synthesis, structural and thermophysical investigations

    NASA Astrophysics Data System (ADS)

    Nandi, Chiranjit; Grover, V.; Sahu, M.; Krishnan, K.; Guleria, A.; Kaity, Santu; Prakash, Amrit; Tyagi, A. K.

    2016-10-01

    In order to mimic co-loading of Pu and Am in zirconia, Nd0.20[Zr1-xCex]0.80O1.90 (0.0 ≤ x ≤ 1.0) system was synthesized and thoroughly characterized by X-ray diffraction (XRD) and Raman spectroscopy. The entire system was found to be single-phasic fluorite-type and most interesting result is stabilization of multi-phasic ceria-zirconia system in a single-phasic system by substituting Nd3+. Raman spectroscopy revealed entirely different nature of defects prevalent in the solid solutions possessing F-type structure across the composition range. The heat capacity of representative compositions was measured by heat flux-type differential scanning calorimeter. Specific heat capacity of the solid solutions was found to increase with decreasing CeO2 content. Different thermodynamic functions such as enthalpy increment, entropy and Gibbs energy functions were determined using heat capacity values. The lattice thermal expansion (298-1273 K) behaviour of the few selected compositions revealed a gradual increase in thermal expansion coefficient with increasing CeO2 content.

  3. HfO_2and ZrO2 : Comparison of Structures and Thermodynamic and Electronic Properties Based on Ab Initio Calculations and Experiment

    NASA Astrophysics Data System (ADS)

    Demkov, Alexander A.; Navrotsky, Alexandra

    2001-03-01

    The International Technology Roadmap for Semiconductors (ITRS) predicts that the strategy of scaling complementary metal-oxide-semiconductor (CMOS) devices will come to an abrupt end around the year 2012. The main reason for this will be the unacceptably high leakage current through the silicon dioxide gate with a thickness below 20 ÅFinding a gate insulator alternative to SiO2 has proven to be far from trivial. Hafnium and zirconium dioxides and silicates have been recently considered as gate dielectrics with intermediate dielectric constants. Hafnia and ziconia are important ceramic materials as well, and their phase relations are rather well studied. There is also interest in hafnia as a constituent of ceramic waste forms for plutonium, based on its refractory nature and high neutron absorption cross section. We use a combination of the ab-initio calculations and calorimetry to investigate thermodynamic and electronic properties of hafnia and zirconia. We describe the cubic to tetragonal phase transition in the fluorite structure by computing the total energy surface for zone-edge distortions correct to fourth order in the soft-mode displacement with the strain coupling renormalization included. We compare the two materials using some simple chemical concepts.

  4. STRUCTURAL ANALYSIS OF THE COMBUSTION SYNTHESIZED Y3+ DOPED CERIA (Ce0.9Y0.1O1.95)

    NASA Astrophysics Data System (ADS)

    Jeyanthi, C. Esther; Siddheswaran, R.; Kumar, Pushpendra; Mangalaraja, R. V.; Siva Shankar, V.; Rajarajan, K.

    2013-07-01

    Y3+ doped CeO2 nanopowders (Ce0.9Y0.1O1.95, abbreviated as YDC) were synthesized by citrate-nitrate-auto combustion process using cerium nitrate hexahydrate, yttrium nitrate hexahydrate and citric acid. The as-synthesized powders were calcined at 700°C and converted into dense bodies followed by sintering at 1200°C. The microstructure of the synthesized powders and sintered bodies were examined by scanning electron microscopy (SEM). The surface morphology of the nanoparticles and clusters were also analysed by transmission electron microscopy (TEM). The particles size of the YDC was found to be in the range from 10 to 30 nm, which is in good agreement with the crystallite size calculated from X-ray peak broadening method. Also, the X-ray diffraction confirmed that the Ce0.9Y0.1O1.95 crystallizes as the cubic fluorite structure of pure ceria. The optical absorption by functional molecules, impurities and oxygen vacancies were analysed by FTIR and Raman spectroscopic studies. From the FTIR spectrum, the absorption peak found at 530 cm-1 is attributed to the vibrations of metal-oxygen bonds. The characteristic Raman peak was found to be 468 cm-1, and the minute absorption of oxygen vacancies were observed in the region 500-640 cm-1.

  5. Local structure and ionic conductivity in the Zr2Y2O7-Y3NbO7 system

    NASA Astrophysics Data System (ADS)

    Norberg, Stefan T.; Ahmed, Istaq; Hull, Stephen; Marrocchelli, Dario; Madden, Paul A.

    2009-05-01

    The Zr0.5-0.5xY0.5+0.25xNb0.25xO1.75 solid solution possesses an anion-deficient fluorite structure across the entire 0<=x<=1 range. The relationship between the disorder within the crystalline lattice and the preferred anion diffusion mechanism has been studied as a function of x, using impedance spectroscopy measurements of the ionic conductivity (σ), powder neutron diffraction studies, including analysis of the 'total' scattering to probe the nature of the short-range correlations between ions using reverse Monte Carlo (RMC) modelling, and molecular dynamics (MD) simulations using potentials derived with a strong ab initio basis. The highest total ionic conductivity (σ = 2.66 × 10-2 Ω-1 cm-1 at 1473 K) is measured for the Zr2Y2O7 (x = 0) end member, with a decrease in σ with increasing x, whilst the neutron diffraction studies show an increase in lattice disorder with x. This apparent contradiction can be understood by considering the local structural distortions around the various cation species, as determined from the RMC modelling and MD simulations. The addition of Nb5+ and its stronger Coulomb interaction generates a more disordered local structure and enhances the mobility of some anions. However, the influence of these pentavalent cations is outweighed by the effect of the additional Y3+ cations introduced as x increases, which effectively trap many anions and reduce the overall concentration of the mobile O2- species.

  6. Electronic structures, hole-doping, and superconductivity of the s = 1, 2, 3, and 4 members of the (Cu, Mo)-12s2 homologous series of superconductive copper oxides.

    SciTech Connect

    Grigoraviciute, I.; Karppinen, M.; Chan, T.-S.; Liu, R.-S.; Chen , J.-M.; Chmaissem, O.; Yamauchi, H.; Materials Science Division; Tokyo Inst. of Tech.; Helsinki Univ. of Technology; National Taiwan Univ.; National Synchrotron Radiation Research Center; Northern Illinois Univ.

    2009-12-17

    We demonstrate that the T{sub c} value of superconductive copper oxides does not depend on the distance between two adjacent CuO{sub 2} planes as long as the hole-doping level and the immediate (crystal) chemical surroundings of the planes are kept the same. Experimental evidence is accomplished for the homologous series of (Cu,Mo)-12s2, the member phases of which differ from each other by the number (s) of cation layers in the fluorite-structured (Ce,Y)-[O{sub 2}-(Ce,Y)]{sub s-1} block between the CuO{sub 2} planes. X-ray absorption near-edge structure spectroscopy is employed as a probe for the hole states of these phases. The s = 1 member appears to be more strongly doped with holes than other phases of the series and accordingly to possess the highest T{sub c} value of 87 K. For s {ge} 2, unexpectedly, both the CuO{sub 2} plane hole concentration and the value of T{sub c} ({approx}55 K) remain constant, being independent of s.

  7. Electronic structures, hole-doping, and superconductivity of the s = 1, 2, 3, and 4 members of the (Cu,Mo)-12s2 homologous series of superconductive copper oxides.

    PubMed

    Grigoraviciute, Inga; Karppinen, Maarit; Chan, Ting-Shan; Liu, Ru-Shi; Chen, Jin-Ming; Chmaissem, Omar; Yamauchi, Hisao

    2010-01-20

    We demonstrate that the T(c) value of superconductive copper oxides does not depend on the distance between two adjacent CuO(2) planes as long as the hole-doping level and the immediate (crystal) chemical surroundings of the planes are kept the same. Experimental evidence is accomplished for the homologous series of (Cu,Mo)-12s2, the member phases of which differ from each other by the number (s) of cation layers in the fluorite-structured (Ce,Y)-[O(2)-(Ce,Y)](s-1) block between the CuO(2) planes. X-ray absorption near-edge structure spectroscopy is employed as a probe for the hole states of these phases. The s = 1 member appears to be more strongly doped with holes than other phases of the series and accordingly to possess the highest T(c) value of 87 K. For s > or = 2, unexpectedly, both the CuO(2) plane hole concentration and the value of T(c) (approximately 55 K) remain constant, being independent of s.

  8. Influence of grain growth on the structural properties of the nanocrystalline Gd2Ti2O7

    NASA Astrophysics Data System (ADS)

    Kulriya, P. K.; Yao, Tiankai; Scott, Spencer Michael; Nanda, Sonal; Lian, Jie

    2017-04-01

    The microstructural evolution and grain growth kinetics of the nanocrystalline Gd2Ti2O7 drastically affect its properties and functionalities as thermal barrier coatings and nuclear waste forms for actinide incorporation. Here, we report the synthesis of the dense nano-sized Gd2Ti2O7 by high energy ball milling (HEBM), and spark plasma sintering (SPS), and also investigated the isothermally annealing induced grain coarsening and structural properties variations. As-prepared nano powder (D∼60 nm) by HEBM exhibited an amorphous nature, which was consolidated to a dense single phase crystalline pyrochlore nano-ceramic (D∼120 ± 10 nm) by SPS sintering at 1200 °C. Isothermal annealing was performed at different temperatures (1300 °C - 1500 °C) with holding time varying from 0.5 to 8 h, and the pyrochlore phase is stable with no indication of a transformation into a defect fluorite structure. A rapid initial grain growth was observed which increased with temperature and annealing durations due to the large driving force of the curvature-driven grain coarsening of the nano-ceramics, and grain growth saturates at longer durations. The calculated value of the time constant and activation energy for the nanocrystalline Gd2Ti2O7 were 0.52 ± 0.02 and 240 ± 20 kJ/mol (∼2.48 eV), respectively. The enhanced grain growth kinetics with a lower value of activation energy can be explained by the effect of fast diffusion across the grain boundaries for dense nanoceramics.

  9. Superhyperfine structure in the EPR spectra and optical spectra of impurity f ions in dielectric crystals: A review

    NASA Astrophysics Data System (ADS)

    Aminov, L. K.; Kurkin, I. N.; Malkin, B. Z.

    2013-07-01

    The results of observation and simulation of the superhyperfine (ligand hyperfine) structure (SHFS) of the electron paramagnetic resonance (EPR) spectra of rare-earth and uranium impurity ions in dielectric crystals have been systematized. The resolved SHFS of the EPR spectra of doped cubic crystals (with the fluorite and perovskite structures) has been observed for orientations of a constant magnetic field along the crystallographic axes. Most attention has been paid to tetragonal double fluorides Li RF4 ( R = Y, Lu, Tm), in which the SHFS of the EPR spectra has also been found for intermediate orientations of the magnetic field. For the LiYF4: Nd3+ single crystal, the splitting of optical spectral lines due to the interaction of Nd3+ ions with nuclear magnetic moments of the nearest neighbor fluorine ions has been observed for the first time. The Van Vleck paramagnet LiTmF4: U3+ is characterized by the SHFS with clearly distinguishable components due to the interaction of uranium ions both with nuclei of the fluorine ions and with enhanced magnetic moments of the thulium nuclei. The SHFS envelopes of the EPR spectra of Yb3+, Ce3+, Nd3+, and U3+ ions in LiYF4 and LiLuF4 crystals are well reproduced by numerical calculations based on the microscopic model using only three fitting parameters: the width of transitions between the electron-nuclear sublevels of the complex containing the paramagnetic ion and nuclei of the ligands and two constants of covalent bonding of the f electrons with 2 s and 2 p electrons of the nearest neighbor fluorine ions.

  10. Structural, mineralogical, and paleoflow velocity constraints on Hercynian tin mineralization: the Achmmach prospect of the Moroccan Central Massif

    NASA Astrophysics Data System (ADS)

    Mahjoubi, El Mahjoub; Chauvet, Alain; Badra, Lakhlifi; Sizaret, Stanislas; Barbanson, Luc; El Maz, Abdelkader; Chen, Yan; Amann, Méderic

    2016-03-01

    The Achmmach tin mineralization (NE of the Moroccan Central Massif) is associated with tourmaline-rich alteration halos, veins, and faults hosted in sandstones and metapelites of the Upper Visean-Namurian. These deposits are reported to be late Hercynian in age and related to the emplacement of late-orogenic granite not outcropping in the studied area. Structural and paragenetic studies of the Achmmach tin deposit were conducted in order to establish a general model of the mineralization. From field constraints, the late Hercynian phase is marked by a transition from transpression to extension with deformation conditions evolving from ductile to brittle environments. The transpression (horizontal shortening direction roughly trending E-W) is coeval with the emplacement of the first tourmaline halos along several conjugated trends (N070, N020, and N120). Thereafter, a tourmaline-rich breccia formed in response to the fracturing of early tourmaline-altered rocks. Subsequently, during the extensional phase, these structures were reactivated as normal faults and breccias, allowing the formation of the main tin mineralization (cassiterite) associated with a wide variety of sulfides (arsenopyrite, chalcopyrite, sphalerite, galena, pyrrhotite, bismuthinite, pyrite, and stannite). This evolution ends with fluorite and carbonate deposition. The hydrothermal fluid flow velocity, calculated by applying statistical measures on the tourmaline growth bands, varies with the lithology. Values are lower in metapelites and higher in breccia. In the general evolution model proposed here, tourmaline alteration makes the rock more competent, allowing for brittle fracturing and generation of open space where the main Sn mineralization was precipitated.

  11. Nanostructured crystals of fluorite phases Sr{sub 1-x}R{sub x}F{sub 2+x} (R are rare earth elements) and their ordering: 5. A study of the ionic conductivity of as-grown Sr{sub 1-x}R{sub x}F{sub 2+x} crystals

    SciTech Connect

    Sorokin, N. I. Karimov, D. N.; Sulyanova, E. A.; Zhmurova, Z. I.; Sobolev, B. P.

    2010-07-15

    The ionic conductivity {sigma} of Sr{sub 1-x}R{sub x}F{sub 2+x} crystals (R = Y, La-Lu) has been measured in the temperature range of 324-933 K. The isomorphic introduction of R{sup 3+} ions into SrF{sub 2} is accompanied by an increase in conductivity up to four orders of magnitude, which makes these crystals superionic conductors. It is shown that the conduction mechanism in Sr{sub 1-x}R{sub x}F{sub 2+x} crystals changes when passing from R = La-Nd to R = Sm-Lu. A change in the type of cluster of structural defects between Nd and Sm is suggested. The concentration dependences of {sigma} and the activation energy of charge-carrier migration (E{sub a}) for Sr{sub 1-x}R{sub x}F{sub 2+x} are nonlinear. For crystals with R = La or Nd, these dependences are interpreted within the percolation model of 'defect regions,' the minimum size of which is estimated to be {approx}700 A{sup 3}. It is shown that the electrical properties of the crystals can be controlled by varying the RF{sub 3} type and concentration. The Sr{sub 1-x}R{sub x}F{sub 2+x} crystals (R = La-Nd, 0.3 {<=} x {<=} 0.5), for which {sigma} = (2-3) x 10{sup -2} S/cm at 673 K and E{sub a} = 0.6-0.7 eV, have the best electrolytic characteristics.

  12. Information Structure and Linguistic Structure.

    ERIC Educational Resources Information Center

    Zierer, Ernesto

    1972-01-01

    This document describes a format for analyzing the information content of sentences and the language patterns that accompany particular information content. The author writes in terms of information structures, each information structure having a corresponding linguistic structure composed of distinctive features. The information structure of a…

  13. Structural ceramics

    SciTech Connect

    Wachtman, J.B. Jr.

    1989-01-01

    The present work discusses opportunities for application of structural ceramics in heat engines, industrial-wear parts, prosthetics and bearings; conceptual and detailed design principles for structural ceramics; the processing, consolidation, and properties of members of the SiC family of structural ceramics; and the silicon nitride and sialon families of hot-pressed, sintered, and reaction-bonded, structural ceramics. Also discussed are partially-stabilized zirconia and zirconia-toughened ceramics for structural applications, the processing methods and mechanisms of fiber-reinforcement in ceramic-matrix fiber-reinforced composites, and the tribological properties of structural ceramics.

  14. Lattice thermal expansion and solubility limits of neodymium-doped ceria

    NASA Astrophysics Data System (ADS)

    Zhang, Jinhua; Ke, Changming; Wu, Hongdan; Yu, Jishun; Wang, Jingran

    2016-11-01

    NdxCe1-xO2-0.5x (x=0-1.0) powders were prepared by reverse coprecipitation-calcination method and characterized by XRD. The crystal structure of product powders transformed from single fluorite structure to the complex of fluorite and C-type cubic structure, and finally to trigonal structure with the increase of x-value. An empirical equation simulating the lattice parameter of neodymium doped ceria was established based on the experimental data. The lattice parameters of the fluorite structure solid solutions increased with extensive adoption of Nd3+, and the heating temperature going up. The average thermal expansion coefficients of neodymium doped ceria with fluorite structure are higher than 13.5×10-6 °C-1 from room temperature to 1200 °C.

  15. Effects of Eu3+ concentration on structural, optical and vibrational properties of multifunctional Ce(1-x)Eu(x)O2-delta) nanoparticles synthesized by thermolysis of 2,4-pentanedione complexes.

    PubMed

    Kremenovic, A; Bozanic, D K; Welsch, A M; Jancar, B; Nikolic, A S; Boskovic, M; Colomban, Ph; Fabian, Martin; Antic, B

    2012-12-01

    The 5-10 nm Ce(1-x)Eu(x)O(2-delta) (0 < or = x < or = 0.30) nanoparticles with fluorite structure were synthesized by thermal decomposition of Eu- and Ce-2,4-pentanedione complexes mixtures. X-ray line broadening analysis of mixed samples Ce(1-x)Eu(x)O(2-delta) (0.05 < or = x < or = 0.30) showed that the crystallite size was lower and root mean square strain higher than in pure ceria. However, within mixed samples Ce(1-x)Eu(x)O(2-delta) (0.05 < or = x < or = 0.30) crystallite size and root mean square strain were independent of Eu3+ concentration. Raman spectroscopy results indicated that europium ions yield disorder by breaking the phonon propagation and therefore making the non-centre Brillouin zone modes Raman active. The absorption bands in the spectra of mixed oxides were blue-shifted in comparison to pure CeO(2-delta) nanopowder. The samples show red emission typical for Eu ions. The biggest photoluminescent intensity was observed for the highest Eu3+ concentration (x = 0.30) and further enhanced with the increase in crystallinity.

  16. Website Structure

    ERIC Educational Resources Information Center

    Jackson, Larry S.

    2009-01-01

    This dissertation reports the results of an exploratory data analysis investigation of the relationship between the structures used for information organization and access and the associated storage structures within state government websites. Extending an earlier claim that hierarchical directory structures are both the preeminent information…

  17. Website Structure

    ERIC Educational Resources Information Center

    Jackson, Larry S.

    2009-01-01

    This dissertation reports the results of an exploratory data analysis investigation of the relationship between the structures used for information organization and access and the associated storage structures within state government websites. Extending an earlier claim that hierarchical directory structures are both the preeminent information…

  18. Unusual Physical and Chemical Properties of Ni in Ce1-xNixO2-y Oxides: Structural Characterization and Catalytic Activity for the Water Gas Shift Reaction

    SciTech Connect

    Rodriguez, J.A.; Barrio, L.; Kubacka, A.; Zhou, G.; Estrella, M.; Martınez-Arias, A.; Hanson, J.C.; Fernandez-Garcıa, M.

    2010-07-29

    The structural and electronic properties of Ce{sub 1-x}Ni{sub x}O{sub 2-y} nanosystems prepared by a reverse microemulsion method were characterized with synchrotron-based X-ray diffraction, X-ray absorption spectroscopy, Raman spectroscopy, and density functional calculations. The Ce{sub 1-x}Ni{sub x}O{sub 2-y} systems adopt a lattice with a fluorite-type structure with an acute local order where Ni displays a strongly distorted (oxygen) nearest-neighbor coordination and the presence of Ni atoms as first cation distances, pointing to the existence of Ni-O-Ni entities embedded into the ceria lattice. A Ni {leftrightarrow} Ce exchange within the CeO{sub 2} leads to a charge redistribution and the appearance of O vacancies. The Ni?O bonds in Ce{sub 1-x}Ni{sub x}O{sub 2-y} are more difficult to reduce than the bonds in pure NiO. The specific structural configuration of Ni inside the mixed-metal oxide leads to a unique catalyst with a high activity for the water gas shift (CO + H{sub 2}O {yields} H{sub 2} + CO{sub 2}) reaction and a simultaneous reduction of the methanation activity of nickel. Characterization results indicate that small particles of metallic Ni at the interface position of a ceria network may be the key for high WGS activity and that the formate?carbonate route is operative for the production of hydrogen.

  19. Unusual Physical and Chemical Properties of Ni in Ce1-xNixO2-y Oxides: Structural Characterization and Catalytic Activity for the Water Gas Shift Reaction

    SciTech Connect

    Barrio, L.; Kubacka, A; Zhou, G; Estrella, M; Martinez-Arias, A; Hanson, J; Fernandez-Garcia, M; Rodriguez, J

    2010-01-01

    The structural and electronic properties of Ce{sub 1-x}Ni{sub x}O{sub 2-y} nanosystems prepared by a reverse microemulsion method were characterized with synchrotron-based X-ray diffraction, X-ray absorption spectroscopy, Raman spectroscopy, and density functional calculations. The Ce{sub 1-x}Ni{sub x}O{sub 2-y} systems adopt a lattice with a fluorite-type structure with an acute local order where Ni displays a strongly distorted (oxygen) nearest-neighbor coordination and the presence of Ni atoms as first cation distances, pointing to the existence of Ni-O-Ni entities embedded into the ceria lattice. A Ni {leftrightarrow} Ce exchange within the CeO{sub 2} leads to a charge redistribution and the appearance of O vacancies. The Ni-O bonds in Ce{sub 1-x}Ni{sub x}O{sub 2-y} are more difficult to reduce than the bonds in pure NiO. The specific structural configuration of Ni inside the mixed-metal oxide leads to a unique catalyst with a high activity for the water gas shift (CO + H{sub 2}O {yields} H{sub 2} + CO{sub 2}) reaction and a simultaneous reduction of the methanation activity of nickel. Characterization results indicate that small particles of metallic Ni at the interface position of a ceria network may be the key for high WGS activity and that the formate-carbonate route is operative for the production of hydrogen.

  20. Electrical and structural properties of Ce{sub 0.8}(Sm{sub 0.5−x}Y{sub 0.5}Al{sub x}){sub 0.2}O{sub 2−δ} ceramics as an electrolyte for SOFC

    SciTech Connect

    Lee, Seung-Hwan; Kim, Hong-Ki; Kim, Seung-Hyun; Lee, Sung-Gap; Lee, Young-Hie

    2014-10-15

    Highlights: • Effect of Al doping on electrical and structural properties was investigated. • According to the contents of Al, the samples show each electrical property. • When Al is 0.4, the sample shows highest electrical properties. - Abstract: The effects of various contents of alumina co-doping on a ceria-based electrolyte were investigated. All samples were sintered at 1500 °C for 10 h. The X-ray diffraction (XRD) patterns showed a fluorite cubic structure. To investigate the surfaces of the Ce{sub 0.8}(Sm{sub 0.5−x}Y{sub 0.5}Al{sub x}){sub 0.2}O{sub 2−δ} (CSYA) ceramics by scanning electron microscopy (SEM), the electrical properties of the CSYA ceramics were measured by AC impedance. The optimal values of ionic conductivity and activation energy were reached at x = 0.4 measured at 600 °C. This result indicated that the alumina co-doping improved the electrical and microstructural properties of the ceria-based electrolyte.

  1. Structure and cation ordering in La2UO6, Ce2UO6, LaUO4, and CeUO4 by first principles calculations

    SciTech Connect

    Casillas-Trujillo, Luis; Xu, H.; McMurray, Jake W.; Shin, Dongwon; Baldinozzi, G.; Sickafus, Kurt

    2016-07-06

    In the present work, we have used density functional theory (DFT) and DFT+U to investigate the crystal structure and phase stability of four model compounds in the Ln2O3-UO2-UO3 ternary oxide system: La2UO6, Ce2UO6, LaUO4, CeUO4, due to the highly-correlated nature of the f-electrons in uranium. We have considered both hypothetical ordered compounds and compounds in which the cations randomly occupy atomic sites in a fluorite-like lattice. We determined that ordered compounds are stable and are energetically favored compared to disordered configurations, though the ordering tendencies are weak. To model and analyze the structures of these complex oxides, we have used supercells based on a layered atomic model. In the layer model, the supercell is composed of alternating planes of anions and cations. We have considered two different ordering motifs for the cations, namely single species (isoatomic) cation layers versus mixed species cation layers. Energy differences between various ordered cationic arrangements were found to be small. This may have implications regarding radiation stability, since cationic arrangements should be able to change under irradiation with little cost in energy.

  2. Containment Structures Versus Suppressive Structures

    DTIC Science & Technology

    1976-02-01

    may now be posed as follows: For a shell thickness h determined by the requirement for fragment containment,* how small can the midsurface radius R...notoriously inefficient. 2. Since structural weight of fragment containment structures varies approximately as the midsurface area, the structure

  3. Co-Determination of Crystal Structures at High Pressure: Combined Application of Theory and Experiment to the AuGa2 Intermetallic analog to High-Pressure SiO2

    NASA Astrophysics Data System (ADS)

    Godwal, B. K.; Stackhouse, S.; Yan, J.; Speziale, S.; Militzer, B.; Jeanloz, R.

    2012-12-01

    The intermetallic compounds AuX2 (X = In, Ga, Al) crystallize in the calcium fluorite (CaF2) structure, making them analogs for SiO2 at TPa pressures. A combination of high-pressure x-ray diffraction experiments and first-principles calculations reveals the sequence of crystal-structural phase transitions in AuGa2 from cubic (Fm3m) to orthorhombic (Pnma) at 10 (± 4) GPa, and then to monoclinic (P21/n) at 33 (± 6) GPa. The post-cotunnite (P1121/a) phase identified as the final step in the phase-transition sequence for other AX2 compounds is found to be unstable. As the structural sequence followed by AuGa2 is similar to that observed in ACl2 (A = Pb, Sn) compounds it implies that AX2 compounds iso-structural to PbCl2 will acquire the monoclinic (P21/n) structure at extreme pressures. Neither theory nor experiment would have been adequate, on their own, in documenting this sequence of phases, but together they confirm a sequence differing from the Fm3m → Pnma → P63/mmc transitions predicted for CaF2, and observed for the intermetallic analog compounds AuIn2 and AuAl2 under pressure. Experimental [and theoretical] values of zero-pressure volume and bulk modulus are 224.5 (± 0.2) [219.5] Å3 and 81 (± 5) [95] GPa; 204.0 (± 1) [204.8] Å3 and 112 (± 12) [96] GPa; and 192 (± 4) [201.1] Å3 and 157 (± 5) [99] GPa for the cubic, orthorhombic and monoclinic phases, respectively.

  4. In-situ integrated processing and characterization of thin films of high temperature superconductors, dielectrics and semiconductors by MOCVD

    NASA Technical Reports Server (NTRS)

    Singh, R.; Sinha, S.; Hsu, N. J.; Thakur, R. P. S.; Chou, P.; Kumar, A.; Narayan, J.

    1990-01-01

    In this strategy of depositing the basic building blocks of superconductors, semiconductors, and dielectric having common elements, researchers deposited superconducting films of Y-Ba-Cu-O, semiconductor films of Cu2O, and dielectric films of BaF2 and Y2O3 by metal oxide chemical vapor deposition (MOCVD). By switching source materials entering the chamber, and by using direct writting capability, complex device structures like three-terminal hybrid semiconductors/superconductors transistors can be fabricated. The Y-Ba-Cu-O superconducting thin films on BaF2/YSZ substrates show a T(sub c) of 80 K and are textured with most of the grains having their c-axis or a-axis perpendicular to the substrate. Electrical characteristics as well as structural characteristics of superconductors and related materials obtained by x-ray defraction, electron microscopy, and energy dispersive x-ray analysis are discussed.

  5. In-situ integrated processing and characterization of thin films of high temperature superconductors, dielectrics and semiconductors by MOCVD

    NASA Astrophysics Data System (ADS)

    Singh, R.; Sinha, S.; Hsu, N. J.; Thakur, R. P. S.; Chou, P.; Kumar, A.; Narayan, J.

    1990-04-01

    In this strategy of depositing the basic building blocks of superconductors, semiconductors, and dielectric having common elements, researchers deposited superconducting films of Y-Ba-Cu-O, semiconductor films of Cu2O, and dielectric films of BaF2 and Y2O3 by metal oxide chemical vapor deposition (MOCVD). By switching source materials entering the chamber, and by using direct writting capability, complex device structures like three-terminal hybrid semiconductors/superconductors transistors can be fabricated. The Y-Ba-Cu-O superconducting thin films on BaF2/YSZ substrates show a T(sub c) of 80 K and are textured with most of the grains having their c-axis or a-axis perpendicular to the substrate. Electrical characteristics as well as structural characteristics of superconductors and related materials obtained by x-ray defraction, electron microscopy, and energy dispersive x-ray analysis are discussed.

  6. In-situ integrated processing and characterization of thin films of high temperature superconductors, dielectrics and semiconductors by MOCVD

    NASA Technical Reports Server (NTRS)

    Singh, R.; Sinha, S.; Hsu, N. J.; Thakur, R. P. S.; Chou, P.; Kumar, A.; Narayan, J.

    1991-01-01

    In this strategy of depositing the basic building blocks of superconductors, semiconductors, and dielectrics having common elements, researchers deposited superconducting films of Y-Ba-Cu-O, semiconductor films of Cu2O, and dielectric films of BaF2 and Y2O3 by metal oxide chemical vapor deposition (MOCVD). By switching source materials entering the chamber, and by using direct writing capability, complex device structures like three terminal hybrid semiconductor/superconductor transistors can be fabricated. The Y-Ba-Cu-O superconducting thin films on BaF2/YSZ substrates show a T(sub c) of 80 K and are textured with most of the grains having their c-axis or a-axis perpendicular to the substrate. Electrical characteristics as well as structural characteristics of superconductors and related materials obtained by x-ray deffraction, electron microscopy, and energy dispersive x-ray analysis are discussed.

  7. Low-voltage cathodoluminescent properties of europium-activated anion-deficient fluorites

    SciTech Connect

    Anderson, M.T.; Phillips, M.L.F.; Walko, R.J.

    1994-05-01

    The authors examined the optical bandgap, particle size distribution, photoluminescence spectra, and cathodoluminescent response of fifteen phosphors that contain transition-metal and main-group sensitizers. They determined luminance versus applied voltage curves for electron energies from 5 to 1000 eV. Seven phosphors exhibit threshold voltages less than 110V. Y{sub 1.96} Eu{sub 0.04} O{sub 3} exhibits a threshold voltage of 13 V and, at 300 V, displays a luminance of 25 fL and a luminous efficiency of 0.02 lm/W.

  8. Highly selective defect-mediated photochemical CO2 conversion over fluorite ceria under ambient conditions.

    PubMed

    Jiang, Dong; Wang, Wenzhong; Gao, Erping; Sun, Songmei; Zhang, Ling

    2014-02-25

    A highly selective defect-mediated photochemical CO2 conversion to CO over defective CeO2 nanorods under ambient conditions (CO2 400 ppm) is presented. The local strain and surface oxygen vacancies embedded in the defect-rich CeO2 are suggested to play a decisive role in mediating the photochemical CO2 conversion.

  9. Existence of fluorite-type solid solutions in alkali metal-uranium-oxygen systems

    NASA Astrophysics Data System (ADS)

    Sali, S. K.; Sampath, S.; Venugopal, V.

    1996-09-01

    The solubilities of Li 2O and Na 2O in UO 2 + x have been established for the first time. They form cubic solid solutions, Li yU 1 - yO 2 + x and Na yU 1 - yO 2 + x ( y = 0-0.2 and x ⪋ 0). The lattice parameters of these solid solutions can be expressed, respectively, as a function of x and y: a0(nm) = 0.54704 - 0.0104x - 0.0495y and a0(nm) = 0.54704 - 0.0102 x - 0.0345 y. Oxidation of these solid solutions in air up to 773 K gave cubic MO 2 + x and the kinetics of oxidation was found to be controlled by oxygen chemical diffusion similar to oxidation of UO 2.

  10. Structural and chemical characterization of terbia thin films grown on hexagonally close packed metal substrates

    NASA Astrophysics Data System (ADS)

    Cartas, William

    Rare earth oxides (REOs) exhibit favorable catalytic performance for a diverse set of chemical transformations, including both partial and complete oxidation reactions. I will discuss our efforts to develop thin film systems of terbia for model surface science investigations of a REO that is effectively reducible, and which is thus expected to promote complete oxidation chemistry of adsorbed species. The growth of terbia on Cu(111) is shown to produce a complex surface that exhibits multiple phases of the oxide as well as exposed substrate. Growing the film on Pt(111) results in more uniform, single phase, and closed film. We used low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) to characterize the structural properties of terbia thin films grown on Pt(111) in ultrahigh vacuum (UHV) using physical vapor deposition. We find that the REO grows as a high quality Tb2O 3(111) film, and adopts oxygen-deficient fluorite structures wherein the metal cations form a hexagonal lattice in registry with the Pt(111) substrate, while oxygen vacancies are randomly distributed within the film. The Tb 2O3(111) films are thermally stable when heated to 1000 K in UHV. LEED and STM show that a fraction of the Tb2O3 forms hexagonal islands when first deposited, and further depositions typically result in three dimensional growth of the film. The Tb2O3 (111) / Pt(111) system produces a coincidence structure, seen very clearly in LEED images. We have also found that Tb2O3(111) films can be oxidized in UHV by exposure to plasma-generated atomic oxygen beams. The oxidized films have an estimated TbO2 stoichiometry and decompose to Tb2O3 during heating, with O2 desorption starting at about 500 K. Terbia films oxidized at 90 K show a weakly bound state of oxygen that is likely chemisorbed. Temperature programmed reaction spectroscopy (TPRS) studies using methanol show that increased oxygen in the film does not modify the chemical selectivity of the film; however

  11. Structure and Thermal Expansion of YSZ and La2Zr2O7 Above 1500°C from Neutron Diffraction on Levitated Samples

    DOE PAGES

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

    2015-07-28

    High-temperature time-of-flight neutron diffraction experiments were performed in this paper on cubic yttria-stabilized zirconia (YSZ, 10 mol% YO1.5) and lanthanum zirconate (LZ) prepared by laser melting. Three spheroids of each composition were aerodynamically levitated and rotated in argon flow and heated with a CO2 laser. Unit cell, positional and atomic displacement parameters were obtained by Rietveld analysis. Below ~1650°C the mean thermal expansion coefficient (TEC) for YSZ is higher than for LZ (13 ± 1 vs. 10.3 ± 0.6) × 10-6/K. From ~1650°C to the onset of melting of LZ at ~2250°C, TEC for YSZ and LZ are similar andmore » within (7 ± 2) × 10-6/K. LZ retains the pyrochlore structure up to the melting temperature with Zr coordination becoming closer to perfectly octahedral. Congruently melting LZ is La deficient. The occurrence of thermal disordering of oxygen sublattice (Bredig transition) in defect fluorite structure was deduced from the rise in YSZ TEC to ~25 × 10-6/K at 2350°C–2550°C with oxygen displacement parameters (Uiso) reaching 0.1 Å2, similar to behavior observed in UO2. Acquisition of powder-like high-temperature neutron diffraction data from solid-levitated samples is feasible and possible improvements are outlined. Finally, this methodology should be applicable to a wide range of materials for high-temperature applications.« less

  12. Organisational Structure

    ERIC Educational Resources Information Center

    National Centre for Vocational Education Research (NCVER), 2006

    2006-01-01

    An understanding of organisational structure can provide guidance for organisations that want to change and innovate. Many writers agree that this understanding allows organisations to shape how their work is done to ultimately achieve their business goals--and that too often structure is given little consideration in business strategy and…

  13. Reconfigurable structure

    NASA Technical Reports Server (NTRS)

    Curtis, Steven A. (Inventor)

    2010-01-01

    A reconfigurable structure includes a plurality of selectively extensible and retractable limbs, at least one node pivotably receiving respective ends of at least two limbs, and an actuator associated with each limb for extending and retracting the limb. The structure may further include an addressable module associated with each actuator to control the actuator.

  14. Protein Structure

    ERIC Educational Resources Information Center

    Asmus, Elaine Garbarino

    2007-01-01

    Individual students model specific amino acids and then, through dehydration synthesis, a class of students models a protein. The students clearly learn amino acid structure, primary, secondary, tertiary, and quaternary structure in proteins and the nature of the bonds maintaining a protein's shape. This activity is fun, concrete, inexpensive and…

  15. Protein Structure

    ERIC Educational Resources Information Center

    Asmus, Elaine Garbarino

    2007-01-01

    Individual students model specific amino acids and then, through dehydration synthesis, a class of students models a protein. The students clearly learn amino acid structure, primary, secondary, tertiary, and quaternary structure in proteins and the nature of the bonds maintaining a protein's shape. This activity is fun, concrete, inexpensive and…

  16. Structural Ceramics

    NASA Technical Reports Server (NTRS)

    1986-01-01

    This publication is a compilation of abstracts and slides of papers presented at the NASA Lewis Structural Ceramics Workshop. Collectively, these papers depict the scope of NASA Lewis' structural ceramics program. The technical areas include monolithic SiC and Si3N4 development, ceramic matrix composites, tribology, design methodology, nondestructive evaluation (NDE), fracture mechanics, and corrosion.

  17. Tau Structures

    PubMed Central

    Avila, Jesus; Jiménez, Juan S.; Sayas, Carmen L.; Bolós, Marta; Zabala, Juan C.; Rivas, Germán; Hernández, Felix

    2016-01-01

    Tau is a microtubule-associated protein that plays an important role in axonal stabilization, neuronal development, and neuronal polarity. In this review, we focus on the primary, secondary, tertiary, and quaternary tau structures. We describe the structure of tau from its specific residues until its conformation in dimers, oligomers, and larger polymers in physiological and pathological situations. PMID:27877124

  18. On Structure.

    ERIC Educational Resources Information Center

    Acland, Henry

    Structure as an aspect of educational setting is discussed and illustrated in the course of an argument indicating that efforts to change schools, including Project Follow Through, should aim to change structures rather than people. It is asserted that educational reform in America usually attempts to change people, especially children, and that…

  19. Hierarchical Heterogeneity at the CeO x –TiO 2 Interface: Electronic and Geometric Structural Influence on the Photocatalytic Activity of Oxide on Oxide Nanostructures

    DOE PAGES

    Luo, Si; Nguyen-Phan, Thuy-Duong; Johnston-Peck, Aaron C.; ...

    2015-01-13

    Mixed oxide interfaces are critical for delivering active components of demanding catalytic processes such as the photo-catalytic splitting of water. We have studied CeOxTiO₂ catalysts with low ceria loadings of 1 wt%, 3 wt% and 6 wt% that were prepared with wet impregnation methods to favor a strong interaction between CeOx and TiO₂. In these materials the interfaces between CeOx-TiO₂ have been sequentially loaded (1%, 3% and 6%), with and without Pt (0.5 wt%). The structure and properties of the catalysts were characterized using several X-ray and electron based techniques including XRD, XPS, UPS, NEXAFS, UV-Vis and HR-STEM/STEM-EELS, to unravelmore » the local morphology, bulk structure, surface states and electronic structure. The combination of all these techniques allow us to analyze in a systematic way the complete structural and electronic properties that prevail at the CeOx-TiO₂ interface. Fluorite structured nano crystallites of ceria on anatase-structured titania were identified by both XRD and NEXAFS. A sequential increasing of the CeOx loading led to the formation of clusters, then plates and finally nano particles in a hierarchical manner on the TiO₂ support. The electronic structures of these catalysts indicate that the interaction between TiO₂ and CeO₂ is closely related to the local morphology of nanostructured CeO₂. Ce³⁺ cations were detected at the surface of CeO₂ and at the interface of the two oxides. In addition, the titania is perturbed by the interaction with ceria and also with Pt. The photocatalytic activity for the splitting of H₂O using UV light was measured for these materials and correlated with our understanding of the electronic and structural properties. Optimal catalytic performance and photo response results were found for the 1 wt% CeOx-TiO₂ catalyst where low dimensional geometry of the ceria provided ideal electronic and geometrical properties. The structural and electronic properties of the interface

  20. Richat Structure

    NASA Image and Video Library

    2017-09-27

    Image taken 1/11/2001: The so-called Richat Structure is a geological formation in the Maur Adrar Desert in the African country of Mauritania. Although it resembles an impact crater, the Richat Structure formed when a volcanic dome hardened and gradually eroded, exposing the onion-like layers of rock. The Richat Structure can be found on Landsat 7 WRS Path 203 Row 45, center: 21.68, -11.94. To learn more about the Landsat satellite go to: landsat.gsfc.nasa.gov/ Credit: NASA/GSFC/Landsat 7/USGS

  1. Spacecraft Structures

    NASA Image and Video Library

    This activity challenges students to solve a real-world problem that is part of the space program using creativity, cleverness and scientific knowledge while learning about forces, structures and e...

  2. Structural Analysis

    NASA Technical Reports Server (NTRS)

    1991-01-01

    After an 800-foot-tall offshore oil recovery platform collapsed, the engineers at Engineering Dynamics, Inc., Kenner, LA, needed to learn the cause of the collapse, and analyze the proposed repairs. They used STAGSC-1, a NASA structural analysis program with geometric and nonlinear buckling analysis. The program allowed engineers to determine the deflected and buckling shapes of the structural elements. They could then view the proposed repairs under the pressure that caused the original collapse.

  3. Structures research

    NASA Technical Reports Server (NTRS)

    Abu-Saba, Elias; Mcginley, Williams; Shen, Ji-Yao

    1992-01-01

    The main objective of the structures group is to provide quality aerospace research with the Center for Aerospace Research - A NASA Center for Excellence at North Carolina Agricultural and Technical State University. The group includes dedicated faculty and students who have a proven record in the area of structures, in particular space structures. The participating faculty developed accurate mathematical models and effective computational algorithms to characterize the flexibility parameters of joint dominated beam-truss structures. Both experimental and theoretical modelling has been applied to the dynamic mode shapes and mode frequencies for a large truss system. During the past few months, the above procedures has been applied to the hypersonic transport plane model. The plane structure has been modeled as a lumped mass system by Doctor Abu-Saba while Doctor Shen applied the transfer matrix method with a piecewise continuous Timoshenko tapered beam model. Results from both procedures compare favorably with those obtained using the finite element method. These two methods are more compact and require less computer time than the finite element method. The group intends to perform experiments on structural systems including the hypersonic plane model to verify the results from the theoretical models.

  4. Structures Division

    NASA Technical Reports Server (NTRS)

    1997-01-01

    The NASA Lewis Research Center Structures Division is an international leader and pioneer in developing new structural analysis, life prediction, and failure analysis related to rotating machinery and more specifically to hot section components in air-breathing aircraft engines and spacecraft propulsion systems. The research consists of both deterministic and probabilistic methodology. Studies include, but are not limited to, high-cycle and low-cycle fatigue as well as material creep. Studies of structural failure are at both the micro- and macrolevels. Nondestructive evaluation methods related to structural reliability are developed, applied, and evaluated. Materials from which structural components are made, studied, and tested are monolithics and metal-matrix, polymer-matrix, and ceramic-matrix composites. Aeroelastic models are developed and used to determine the cyclic loading and life of fan and turbine blades. Life models are developed and tested for bearings, seals, and other mechanical components, such as magnetic suspensions. Results of these studies are published in NASA technical papers and reference publication as well as in technical society journal articles. The results of the work of the Structures Division and the bibliography of its publications for calendar year 1995 are presented.

  5. Centriole structure

    PubMed Central

    Winey, Mark; O'Toole, Eileen

    2014-01-01

    Centrioles are among the largest protein-based structures found in most cell types, measuring approximately 250 nm in diameter and approximately 500 nm long in vertebrate cells. Here, we briefly review ultrastructural observations about centrioles and associated structures. At the core of most centrioles is a microtubule scaffold formed from a radial array of nine triplet microtubules. Beyond the microtubule triplets of the centriole, we discuss the critically important cartwheel structure and the more enigmatic luminal density, both found on the inside of the centriole. Finally, we discuss the connectors between centrioles, and the distal and subdistal appendages outside of the microtubule scaffold that reflect centriole age and impart special functions to the centriole. Most of the work we review has been done with electron microscopy or electron tomography of resin-embedded samples, but we also highlight recent work performed with cryoelectron microscopy, cryotomography and subvolume averaging. Significant opportunities remain in the description of centriolar structure, both in mapping of component proteins within the structure and in determining the effect of mutations on components that contribute to the structure and function of the centriole. PMID:25047611

  6. Ni-doped (CeO2- δ )-YSZ mesoarchitectured with nanocrystalline framework: the effect of thermal treatment on structure, surface chemistry and catalytic properties in the partial oxidation of methane (CPOM)

    NASA Astrophysics Data System (ADS)

    Somacescu, Simona; Florea, Mihaela; Osiceanu, Petre; Calderon-Moreno, Jose Maria; Ghica, Corneliu; Serra, Jose Manuel

    2015-11-01

    Ni-doped (CeO2- δ )-YSZ (5 mol% Ni oxide, 10 mol% ceria) mesoarchitectures (MA) with nanocrystalline framework have been synthesized by an original, facile and cheap approach based on Triton X100 nonionic surfactant as template and water as solvent at a strong basic pH value. Following the hydrothermal treatment under autogenous pressure ( 18 bars), Ni, Ce, Y, and Zr were well ordered as MA with nanocrystalline framework, assuring thermal stability. A comprehensive investigation of structure, texture, morphology, and surface chemistry was performed by means of a variety of complementary techniques (X-Ray Diffraction, XRD; Raman Spectroscopy, RS; Brunauer—Emmett—Teller, BET; Temperature—Programmed Reduction, TPR; Transmission Electron Microscopy, TEM and DF-STEM; X-ray Photoelectron Spectroscopy, XPS; Catalytic activity and selectivity). N2 sorption measurements highlighted that the mesoporous structure is formed at 600 °C and remains stable at 800 °C. At 900 °C, the MA collapses, favoring the formation of macropores. The XRD and Raman Spectroscopy of all samples showed the presence of a pure, single phase with fluorite-type structure. At 900 °C, an increased tetragonal distortion of the cubic lattice was observed. The surface chemistry probed by XPS exhibits a mixture of oxidation states (Ce3+ + Ce4+) with high percentage of Ce3+ valence state 35 % and (Ni3+ and Ni2+) oxidation states induced by the thermal treatment. These nanoparticles assembled into MA show high stability and selectivity over time in catalytic partial oxidation of methane (CPOM). These promising performances suggest an interesting prospect for introduction as anode within IT-SOFC assemblies.

  7. Composite structural materials. [aircraft structures

    NASA Technical Reports Server (NTRS)

    Ansell, G. S.; Loewy, R. G.; Wiberley, S. E.

    1980-01-01

    The use of filamentary composite materials in the design and construction of primary aircraft structures is considered with emphasis on efforts to develop advanced technology in the areas of physical properties, structural concepts and analysis, manufacturing, and reliability and life prediction. The redesign of a main spar/rib region on the Boeing 727 elevator near its actuator attachment point is discussed. A composite fabrication and test facility is described as well as the use of minicomputers for computer aided design. Other topics covered include (1) advanced structural analysis methids for composites; (2) ultrasonic nondestructive testing of composite structures; (3) optimum combination of hardeners in the cure of epoxy; (4) fatigue in composite materials; (5) resin matrix characterization and properties; (6) postbuckling analysis of curved laminate composite panels; and (7) acoustic emission testing of composite tensile specimens.

  8. Adenovirus structure.

    PubMed

    Rux, John J; Burnett, Roger M

    2004-12-01

    Structural studies continue to play an essential role as the focus of adenovirus research shifts in emphasis from basic biology to adenovirus-based vector technologies. A crucial step in developing novel therapeutics for gene replacement, cancer, and vaccines is often to modify the virion. Such engineered changes are designed to retarget the virus, or to reduce the immunological responses to infection. These efforts are far more effective when they are based on detailed structural knowledge. This minireview provides a brief summary of the wealth of information that has been obtained from the combined application of X-ray crystallography and electron microscopy. This knowledge now includes a good working model for the architectural organization of the virion, and atomic resolution molecular structures for all the major capsid proteins, hexon, penton, and fiber. We highlight new developments, which include the structure of the penton base and the discovery that adenovirus has several relatives. We sketch how the structural information can be used to engineer novel virions and conclude with the prospects for future progress.

  9. Tension Structure

    NASA Technical Reports Server (NTRS)

    1978-01-01

    The fabric structure pictured is the Campus Center of La Verne College, La Verne, California. Unlike the facilities shown on the preceding pages, it is not air-supported. It is a "tension structure," its multi-coned fabric membrane supported by a network of cables attached to steel columns which function like circus tent poles. The spider-web in the accompanying photo is a computer graph of the tension pattern. The designers, Geiger-Berger Associates PC, of New York City, conducted lengthy computer analysis to determine the the best placement of columns and cables. The firm also served as structural engineering consultant on the Pontiac Silverdome and a number of other large fabric structures. Built by Birdair Structures, Inc., Buffalo, New York, the La Verne Campus Center was the first permanent facility in the United States enclosed by the space-spinoff fabric made of Owens-Corning Beta fiber glass coated with Du Pont Teflon TFE. The flexible design permits rearrangement of the interior to accommodate athletic events, student activities, theatrical productions and other recreational programs. Use of fabric covering reduced building cost 30 percent below conventional construction.

  10. Structural Diagnosis

    NASA Technical Reports Server (NTRS)

    1987-01-01

    All over the world, officials charged with preserving historical structures are faced with a problem of environmental contamination of building materials that cause structural deterioration. First requisite for preserving the structure is identification of the nature of contaminants. A "non-invasive"technique based on space technology has been developed and tested. It employs a neutron source and a gamma ray detector. Placed on one side of the wall to be examined, the source fires neutrons created by the decay of a radioactive isotope. As they pass through the wall, the neutrons collide with atoms and the atoms, depending on their type, emit various kinds of gamma rays. These rays are identified by the gamma ray detector on the other side of the wall. Energy of the rays shows the kind of element present. The intensity level indicates the quantity. Composition of the contaminants within the walls is determined by a multichannel analyzer.

  11. Effect of Mg doping and sintering temperature on structural and morphological properties of samarium-doped ceria for IT-SOFC electrolyte

    NASA Astrophysics Data System (ADS)

    Ahmad, Syed Ismail; Mohammed, Tasneem; Bahafi, Amal; Suresh, Madireddy Buchi

    2017-06-01

    Samples of Sm and Mg co-doped ceria electrolyte of Ce1- x Sm x- y Mg y O2- δ ( x = 0.2; y = 0.00, 0.05, 0.1, 0.15, and 0.175) were synthesized by sol-gel process. The prepared samples were sintered at 1100 and 1400 °C for 4 h. The bulk densities were measured by Archimedes method. XRD measurements indicate that the synthesized samples were in single-phase cubic fluorite structure (space group Fm3m). The cell parameters decrease with the concentration of Mg, and 2 θ values slightly shift towards right. The particle sizes obtained were between 7.14 and 17.44 nm. The sintered sample achieved 95% of theoretical density. FTIR spectra of samples sintered at 1400 °C indicates weak interactions between 3550-3400 cm-1 and 1600-1300 cm-1 are attributed to O-H stretching modes and strong bonds 850-450 cm-1 are assigned to characteristic Ce-O vibrations. The surface morphology and chemical composition were analyzed by SEM and EDS, SEM micrographs show spherical faceted grains, and the samples were crack free, dense material with some pores on surface which are inconsistent with density results. The average grain size obtained was 0.5 μm. Particle size obtained by TEM was in agreement with that obtained by XRD. The high-density ceria co-doped ceramic can be used as electrolyte in SOFC.

  12. Structures protection

    NASA Technical Reports Server (NTRS)

    1977-01-01

    Materials of which an aircraft is made and the methods used to hold these materials together forming the aircraft structure were studied as factors important in protecting a modern aircraft from hazardous natural environments. Since all-metal aircraft are being replaced by aircraft constructed partly of fiber reinforced plastics with desirable light weight and high strength properties but with poor electrical conductivity, the danger of lightning strikes has become more serious. Lightning effects on metal structures were reviewed and design protection was discussed. The expected lightning effects on nonmetallic materials such as fiberglass and advanced composites were also reviewed.

  13. Pressure-induced phase transition in SrF2: A Raman study

    NASA Astrophysics Data System (ADS)

    Kourouklis, G. A.; Anastassakis, E.

    1986-07-01

    Measurements of Raman scattering under hydrostatic pressure in SrF2 show that the β-->α phase transition, common to fluorite structures, occurs around 5.0 GPa at 300 K. Symmetry assignment of phonons in the α phase is based on a comparison to other fluorite crystals. Hysteresis phenomena are observed and discussed. Anharmonic processes are shown to be small but not negligible.

  14. Nanocrystal structures

    DOEpatents

    Eisler, Hans J.; Sundar, Vikram C.; Walsh, Michael E.; Klimov, Victor I.; Bawendi, Moungi G.; Smith, Henry I.

    2006-12-19

    A structure including a grating and a semiconductor nanocrystal layer on the grating, can be a laser. The semiconductor nanocrystal layer can include a plurality of semiconductor nanocrystals including a Group II–VI compound, the nanocrystals being distributed in a metal oxide matrix. The grating can have a periodicity from 200 nm to 500 nm.

  15. Structural Adaptation

    ERIC Educational Resources Information Center

    Crowley, Julianne; Titmus, Morgan

    2016-01-01

    This article explores an alternative conception held by high school and first-year university biology students regarding the structure of the left and right ventricles of the heart and the significance of the left ventricular wall being thicker than the right. The left ventricular wall of the heart is thicker than the right ventricular wall due to…

  16. Structural evolution

    SciTech Connect

    Burr, M.T.

    1993-03-01

    In this special report, financial executives discuss key trends in power project finance, new funding sources and evolving project structures. Industry wide, financial firms and developers are striving to improve the cost-effectiveness and efficiency of project financing, for projects in both greenfield development and the growing secondary market.

  17. Nanocrystal structures

    DOEpatents

    Eisler, Hans J [Stoneham, MA; Sundar, Vikram C [Stoneham, MA; Walsh, Michael E [Everett, MA; Klimov, Victor I [Los Alamos, NM; Bawendi, Moungi G [Cambridge, MA; Smith, Henry I [Sudbury, MA

    2008-12-30

    A structure including a grating and a semiconductor nanocrystal layer on the grating, can be a laser. The semiconductor nanocrystal layer can include a plurality of semiconductor nanocrystals including a Group II-VI compound, the nanocrystals being distributed in a metal oxide matrix. The grating can have a periodicity from 200 nm to 500 nm.

  18. Structured Observation.

    ERIC Educational Resources Information Center

    Johnson, Ellen; Borleske, Barbara; Gleason, Susan; Bailey, Bambi; Scantlebury, Kathryn

    1998-01-01

    Describes a tool that quantifies the number and kinds of interactions between students and teachers. Structured observation of classroom events indicates whether a teacher is interacting equitably with each student in the classroom. Science teachers were receptive to the relatively quantitative nature of the process. Both inservice and student…

  19. Structural Adaptation

    ERIC Educational Resources Information Center

    Crowley, Julianne; Titmus, Morgan

    2016-01-01

    This article explores an alternative conception held by high school and first-year university biology students regarding the structure of the left and right ventricles of the heart and the significance of the left ventricular wall being thicker than the right. The left ventricular wall of the heart is thicker than the right ventricular wall due to…

  20. Lightweight Structures.

    ERIC Educational Resources Information Center

    Shaver and Co., Michigan City, IN.

    One of the newest and most promising developments in architecture has been the use of lightweight structures for encapsulating space. Using this new technology, builders can enclose large and small areas at a fraction of the cost of conventional construction and at the same time provide interior space that is totally flexible. This brochure shows…

  1. Aluminosilicate melts and glasses at 1 to 3 GPa: temperature and pressure effects on recovered structural and density changes

    NASA Astrophysics Data System (ADS)

    Bista, S.; Stebbins, J. F.; Hankins, B.; Sisson, T. W.

    2013-12-01

    of pressure effects on melt structure. Samples were made in a piston-cylinder apparatus in 5mm platinum capsules, with 1 inch diameter calcium fluorite assemblies and graphite heaters. Structure was examined by Al-27 and Na-23 MAS NMR at 14.4 and 18.8 Tesla fields, and glass densities determined by high-precision sink-float methods.

  2. Structure/property relationships of the thermoelectric oxyselenides (Bi1-xAxCuOSe) (A=Ba and Ca)

    NASA Astrophysics Data System (ADS)

    Wong-Ng, Winnie; Yan, Yonggao; Kaduk, James A.; Tang, Xin F.

    2017-10-01

    The crystal structures, solid solution limit, and powder X-ray reference diffraction patterns for two 'natural superlattice' series Bi1-xBaxCuOSe (x = 0.05, 0.075, 0.1, 0.2, and 0.3), and Bi1-xCaxCuOSe (x = 0, 0.05, 0.075, 0.1, 0.2 and 0.3) have been determined. The structure/property relationships of these thermoelectric materials are summarized. As the ionic radius of Ba2+ is greater than that of Bi3+, the unit cell volume, V, of Bi1-xBaxCuOSe increases progressively from x = 0 to x = 0.2 (from 137.868 (5) Å3 to 141.194 (10) Å3, respectively). However, even though the ionic radius of Ca2+ is smaller than that of Bi3+, the unit cell volumes, V, of Bi1-xCaxCuOSe also show an increasing trend as a function of x (137.868 (5) Å3 to 139.295 (12) Å3 from x = 0 to 0.3, respectively) due to the relatively large increase in c parameter. The structure of Bi1-xAxCuOSe (A = Ba and Ca) can be considered as built from [Bi2(1-x)A2xO2]2(1-x)+ layers normal to the c-axis alternating with fluorite-like [Cu2Se2]2(1-x)- layers in the c-direction. The substitutions of Ba and Ca on the Bi site of Bi1-xAxCuOSe lead to the weakening of the 'bonding' between the [Bi2(1-x)A2xO2]2(1-x)+ and the [Cu2Se2]2(1-x)- layers (a decrease of Columbic force), resulting in an increase of the c-axis parameter and V. Powder X-ray diffraction patterns of Bi1-xAxCuOSe were submitted for inclusion in the Powder Diffraction File (PDF).

  3. Superconducting structure

    DOEpatents

    Kwon, Chuhee; Jia, Quanxi; Foltyn, Stephen R.

    2003-04-01

    A superconductive structure including a dielectric oxide substrate, a thin buffer layer of a superconducting material thereon; and, a layer of a rare earth-barium-copper oxide superconducting film thereon the thin layer of yttrium-barium-copper oxide, the rare earth selected from the group consisting of samarium, gadolinium, ytterbium, erbium, neodymium, dysprosium, holmium, lutetium, a combination of more than one element from the rare earth group and a combination of one or more elements from the rare earth group with yttrium, the buffer layer of superconducting material characterized as having chemical and structural compatibility with the dielectric oxide substrate and the rare earth-barium-copper oxide superconducting film is provided.

  4. Superconducting Structure

    DOEpatents

    Kwon, Chuhee; Jia, Quanxi; Foltyn, Stephen R.

    2005-09-13

    A superconductive structure including a dielectric oxide substrate, a thin buffer layer of a superconducting material thereon; and, a layer of a rare earth-barium-copper oxide superconducting film thereon the thin layer of yttrium-barium-copper oxide, the rare earth selected from the group consisting of samarium, gadolinium, ytterbium, erbium, neodymium, dysprosium, holmium, lutetium, a combination of more than one element from the rare earth group and a combination of one or more elements from the rare earth group with yttrium, the buffer layer of superconducting material characterized as having chemical and structural compatibility with the dielectric oxide substrate and the rare earth-barium-copper oxide superconducting film is provided.

  5. Lightweight Structures

    NASA Technical Reports Server (NTRS)

    Whittenberger, J. Daniel

    2001-01-01

    Present structural concepts for hot static structures are conventional "sheet & stringer" or truss core construction. More weight-efficient concepts such as honeycomb and lattice block are being investigated, in combination with both conventional superalloys and TiAl. Development efforts for components made from TiAl sheet are centered on lower cost methods for sheet and foil production, plus alloy development for higher temperature capability. A low-cost casting technology recently developed for aluminum and steel lattice blocks has demonstrated the required higher strength and stiffness, with weight efficiency approach- ing honeycombs. The current effort is based on extending the temperature capability by developing lattice block materials made from IN-718 and Mar-M247.

  6. Terminal structure

    DOEpatents

    Schmidt, Frank [Langenhagen, DE; Allais, Arnaud [Hannover, DE; Mirebeau, Pierre [Villebon sur Yvette, FR; Ganhungu, Francois [Vieux-Reng, FR; Lallouet, Nicolas [Saint Martin Boulogne, FR

    2009-10-20

    A terminal structure (2) for a superconducting cable (1) is described. It consists of a conductor (2a) and an insulator (2b) that surrounds the conductor (2a), wherein the superconducting cable (1) has a core with a superconducting conductor (5) and a layer of insulation that surrounds the conductor (5), and wherein the core is arranged in such a way that it can move longitudinally in a cryostat. The conductor (2a) of the terminal structure (2) is electrically connected with the superconducting conductor (5) or with a normal conductor (6) that is connected with the superconducting conductor (5) by means of a tubular part (7) made of an electrically conductive material, wherein the superconducting conductor (5) or the normal conductor (6) can slide in the part (7) in the direction of the superconductor.

  7. Structural Geology

    NASA Astrophysics Data System (ADS)

    Weber, John; Frankel, Kurt L.

    2011-05-01

    Structural geology and continental tectonics were ushered in to the modern quantitative age of geosciences with the arrival of the global plate tectonics paradigm (circa 1968), derived using new data from the oceans' depths, and John Ramsay's 1967 seminal work, Folding and Fracturing of Rocks. Fossen is to be applauded for crafting a unique, high-caliber, and accessible undergraduate textbook on structural geology that faithfully reflects this advance and the subsequent evolution of the discipline. This well-written text draws on Fossen's wealth of professional experience, including his broad and diverse academic research and experience in the petroleum industry. This book is beautifully illustrated, with excellent original color diagrams and with impressive color field photographs that are all keyed to locations and placed into geologic context.

  8. Structure and Thermal Expansion of YSZ and La2Zr2O7 Above 1500°C from Neutron Diffraction on Levitated Samples

    SciTech Connect

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

    2015-07-28

    High-temperature time-of-flight neutron diffraction experiments were performed in this paper on cubic yttria-stabilized zirconia (YSZ, 10 mol% YO1.5) and lanthanum zirconate (LZ) prepared by laser melting. Three spheroids of each composition were aerodynamically levitated and rotated in argon flow and heated with a CO2 laser. Unit cell, positional and atomic displacement parameters were obtained by Rietveld analysis. Below ~1650°C the mean thermal expansion coefficient (TEC) for YSZ is higher than for LZ (13 ± 1 vs. 10.3 ± 0.6) × 10-6/K. From ~1650°C to the onset of melting of LZ at ~2250°C, TEC for YSZ and LZ are similar and within (7 ± 2) × 10-6/K. LZ retains the pyrochlore structure up to the melting temperature with Zr coordination becoming closer to perfectly octahedral. Congruently melting LZ is La deficient. The occurrence of thermal disordering of oxygen sublattice (Bredig transition) in defect fluorite structure was deduced from the rise in YSZ TEC to ~25 × 10-6/K at 2350°C–2550°C with oxygen displacement parameters (Uiso) reaching 0.1 Å2, similar to behavior observed in UO2. Acquisition of powder-like high-temperature neutron diffraction data from solid-levitated samples is feasible and possible improvements are outlined. Finally, this methodology should be applicable to a wide range of materials for high-temperature applications.

  9. Microplastic Structures

    NASA Astrophysics Data System (ADS)

    Feely, Wayne E.

    1986-07-01

    Thick coatings (5-15μm) of a new dual image, aqueous developable photoresist can be exposed using a light attenuating photomask consisting of clear, opaque and grey areas and then processed to yield thermally stable 3-dimensional structures which are potentially useful as mechanical and optical components of devices. In the positive mode, relief and intaglio images are produced by processing similar to positive novolak based resists with 1- 2μm resolution and with the added feature that the images can be made thermally stable to temperatures >300° C. Negative mode processing of coated wafers imaged with the special mask produces thermally stable structures with tunnels or hollow chambers as well as cantilever beams. Because these structures are crosslinked at the time of development, processing in the negative mode shows much wider latitude than is the case in the positive mode. Images by negative mode processing are capable of submicron resolution, higher aspect ratio (>3 vs <1.6), and inward sloping wall profiles adjustable by exposure. The acid hardening resin compositions cover a broad range of compositions so that resins can be tailored to meet specific property requirements.

  10. Structural representation of data structures

    NASA Astrophysics Data System (ADS)

    Cantoni, Virginio; Gaggia, Alessandro; Gatti, Riccardo; Lombardi, Luca

    2014-06-01

    Study of the morphology of proteins, and their 3D structure, supports investigations of their functions and represents an initial step towards protein-based drug design. The goal of this paper is to define techniques, based on the geometrical and topological structure of protein surfaces, for the detection and analysis of sites of potential protein-protein and protein-ligand interactions. Two protein representation modalities based on the Concavity Tree (CT) and the Enriched Complex Extended Gaussian Image (EC-EGI) are considered. In particular, the concavity tree, in which the interface is usually extended and roughly planar, is considered to be better suited to protein-protein interaction studies. Instead, the EGI is more suited to protein-ligand interactions, in which the small ligand molecule usually has to fit into the protein cavity. In fact, the histogram of the orientations is better suited to representing a mainly convex object and its dual matching region (the cavity). Both these data structures are open, and can be easily integrated with biochemical features.

  11. Airfoil structure

    DOEpatents

    Frey, Gary A.; Twardochleb, Christopher Z.

    1998-01-01

    Past airfoil configurations have been used to improve aerodynamic performance and engine efficiencies. The present airfoil configuration further increases component life and reduces maintenance by reducing internal stress within the airfoil itself. The airfoil includes a chord and a span. Each of the chord and the span has a bow being summed to form a generally "C" configuration of the airfoil. The generally "C" configuration includes a compound bow in which internal stresses resulting from a thermal temperature gradient are reduced. The structural configuration reduces internal stresses resulting from thermal expansion.

  12. Airfoil structure

    DOEpatents

    Frey, G.A.; Twardochleb, C.Z.

    1998-01-13

    Past airfoil configurations have been used to improve aerodynamic performance and engine efficiencies. The present airfoil configuration further increases component life and reduces maintenance by reducing internal stress within the airfoil itself. The airfoil includes a chord and a span. Each of the chord and the span has a bow being summed to form a generally ``C`` configuration of the airfoil. The generally ``C`` configuration includes a compound bow in which internal stresses resulting from a thermal temperature gradient are reduced. The structural configuration reduces internal stresses resulting from thermal expansion. 6 figs.

  13. Hierarchical Heterogeneity at the CeO x –TiO 2 Interface: Electronic and Geometric Structural Influence on the Photocatalytic Activity of Oxide on Oxide Nanostructures

    SciTech Connect

    Luo, Si; Nguyen-Phan, Thuy-Duong; Johnston-Peck, Aaron C.; Barrio, Laura; Sallis, Shawn; Arena, Dario A.; Kundu, Shankhamala; Xu, Wenqian; Piper, Louis F. J.; Stach, Eric A.; Polyanskiy, Dmitry E.; Fujita, Etsuko; Rodriguez, José A.; Senanayake, Sanjaya D.

    2015-01-13

    Mixed oxide interfaces are critical for delivering active components of demanding catalytic processes such as the photo-catalytic splitting of water. We have studied CeOxTiO₂ catalysts with low ceria loadings of 1 wt%, 3 wt% and 6 wt% that were prepared with wet impregnation methods to favor a strong interaction between CeOx and TiO₂. In these materials the interfaces between CeOx-TiO₂ have been sequentially loaded (1%, 3% and 6%), with and without Pt (0.5 wt%). The structure and properties of the catalysts were characterized using several X-ray and electron based techniques including XRD, XPS, UPS, NEXAFS, UV-Vis and HR-STEM/STEM-EELS, to unravel the local morphology, bulk structure, surface states and electronic structure. The combination of all these techniques allow us to analyze in a systematic way the complete structural and electronic properties that prevail at the CeOx-TiO₂ interface. Fluorite structured nano crystallites of ceria on anatase-structured titania were identified by both XRD and NEXAFS. A sequential increasing of the CeOx loading led to the formation of clusters, then plates and finally nano particles in a hierarchical manner on the TiO₂ support. The electronic structures of these catalysts indicate that the interaction between TiO₂ and CeO₂ is closely related to the local morphology of nanostructured CeO₂. Ce³⁺ cations were detected at the surface of CeO₂ and at the interface of the two oxides. In addition, the titania is perturbed by the interaction with ceria and also with Pt. The photocatalytic activity for the splitting of H₂O using UV light was measured for these materials and correlated with our understanding of the electronic and structural properties. Optimal catalytic performance and photo response results were found for the 1 wt% CeOx-TiO₂ catalyst where low dimensional geometry of the ceria provided ideal electronic and geometrical properties

  14. Textural and structural evidence for a predeformation hydrothermal origin of the Tungsten Queen Deposit, Hamme District, North Carolina

    USGS Publications Warehouse

    Foose, M.P.; Slack, J.F.; Casadevall, T.

    1980-01-01

    The Hamme tungsten district is composed of a series of steeply dipping quartz-wolframite veins in the Piedmont of North Carolina. Veins are concentrated near the border of the lower Paleozoic Vance County pluton, along its western contact with green-schist-facies metapelites and metavolcanic rocks of the Carolina slate belt. One of these quartz veins, the Snead-Walker, hosts the Tungsten Queen deposit. The vein is 0 to 10 m thick and trends N 35 degrees E for approximately 3,500 m through slate belt rocks and the granitic pluton. The deposit has been worked to a depth of nearly 520 m and contains eight en echelon ore lodes that plunge 42 degrees to 65 degrees between S 10 degrees E and S 10 degrees W. Ore lodes commonly are encased in thin lenses of quartz-sericite greisen. The principal ore mineral is huebnerite and is accompanied by scattered occurrences of pyrite, sphalerite, galena, chalcopyrite, and tetrahedrite. The gangue is predominantly quartz with minor amounts of fluorite, sericite, and carbonate.Studies of minor structures and mineral textures indicate that both the wall rock and the ore and gangue minerals within the vein have been deformed by at least two events. The first event produced relatively gentle, open, and shallow-plunging folds; later, an intense episode of right-lateral shearing developed steeply plunging, tight folds and numerous northeast-trending shears. This latter deformation also developed a prominent alignment of ore and gangue minerals oblique to the vein walls and may have formed the en echelon distribution of ore lodes.In relatively undeformed parts of the vein, clusters of euhedral huebnerite crystals are oriented perpendicular to vein layering. Some prismatic crystals have terminations with cappings of sulfides and in polished thin section show concentric growth zones. These features are similar to textures found in unmetamorphosed tungsten-bearing hydrothermal vein deposits such as those at Pasto Bueno, Peru; Carrock Fell

  15. Structures of

    PubMed

    Kirik; Solovyov; Blokhin; Yakimov

    2000-06-01

    Crystal structures of [Pd(NH3)2X2] complexes, where X = Br or I, diamminediiodo-/-dibromopalladium(II), have been studied by X-ray powder diffraction. The series consists of five complexes: cis-[Pd(NH3)2Br2] (I) [a = 13.3202 (7), b = 12.7223 (6), c = 7.05854 (3) A, Z = 8, space group Pbca], trans-[Pd(NH3)2Br2] (II) [a = 6.7854 (3), b = 7.1057 (3), c = 6.6241 (2) A, alpha = 103.221 (3), beta = 102.514 (2), gamma = 100.386 (3) degrees, Z = 2, space group P1], beta-trans-[Pd(NH3)2Br2] (III) [a = 8.4315 (3), b = 8.4206 (3), c = 8.0916 (2) A, Z = 4, space group Pbca], cis-[Pd(NH3)2I2] (IV) [a = 13.9060 (8), b = 13.5035 (8), c = 7.5050 (4) A, Z = 8, space group Pbca], and beta-trans-[Pd(NH3)2I2] (V) [a = 8.8347 (5), b = 8.8410 (5), c = 8.6081 (2) A, Z = 4, space group Pbca]. Patterson synthesis and Rietveld refinement have been used for structural determination. Molecular structures with column- or parquet-type packing of flat complexes are characteristic of these substances. Corresponding cis- and beta-trans compounds are isostructural. The thermal transformations cis-->trans-->beta-trans (cis-->beta-trans in the case of iodine) are considered. Cl derivatives are also discussed. The transformations proceed irreversibly and are accompanied by decreasing specific volume. Owing to these features, they can be classified as chemical reactions. High-temperature X-ray powder diffraction was used to study the transformations in air. The set of data is consistent with a solid state transformation from cis to trans. According to this model, the columns of molecules remain intact during the process, and the transformation proceeds via the breaking of Pd...X and Pd...N intermolecular bonds. The powder diffraction data have been deposited in ICDD-JCPDS (45-0596, 46-0876, 46-0879, 47-1690, 48-1185).

  16. Armor structures

    DOEpatents

    Chu, Henry Shiu-Hung [Idaho Falls, ID; Lacy, Jeffrey M [Idaho Falls, ID

    2008-04-01

    An armor structure includes first and second layers individually containing a plurality of i-beams. Individual i-beams have a pair of longitudinal flanges interconnected by a longitudinal crosspiece and defining opposing longitudinal channels between the pair of flanges. The i-beams within individual of the first and second layers run parallel. The laterally outermost faces of the flanges of adjacent i-beams face one another. One of the longitudinal channels in each of the first and second layers faces one of the longitudinal channels in the other of the first and second layers. The channels of the first layer run parallel with the channels of the second layer. The flanges of the first and second layers overlap with the crosspieces of the other of the first and second layers, and portions of said flanges are received within the facing channels of the i-beams of the other of the first and second layers.

  17. Digital structural

    USGS Publications Warehouse

    Dohm, J.M.; Anderson, R.C.; Tanaka, K.L.

    1998-01-01

    Magmatic and tectonic activity have both contributed significantly to the surface geology of Mars. Digital structural mapping techniques have now been used to classify and date centers of tectonic activity in the western equatorial region. For example, our results show a center of tectonic activity at Valles Marineris, which may be associated with uplift caused by intrusion. Such evidence may help explain, in part, the development of the large troughs and associated outflow channels and chaotic terrain. We also find a local centre of tectonic activity near the source region of Warrego Valles. Here, we suggest that the valley system may have resulted largely from intrusive-related hydrothermal activity. We hope that this work, together with the current Mars Global Surveyor mission, will lead to a better understanding of the geological processes that shaped the Martian surface.

  18. Staggering Structure

    NASA Image and Video Library

    2017-09-06

    This view from NASA's Cassini spacecraft shows a wave structure in Saturn's rings known as the Janus 2:1 spiral density wave. Resulting from the same process that creates spiral galaxies, spiral density waves in Saturn's rings are much more tightly wound. In this case, every second wave crest is actually the same spiral arm which has encircled the entire planet multiple times. This is the only major density wave visible in Saturn's B ring. Most of the B ring is characterized by structures that dominate the areas where density waves might otherwise occur, but this innermost portion of the B ring is different. The radius from Saturn at which the wave originates (toward lower-right in this image) is 59,796 miles (96,233 kilometers) from the planet. At this location, ring particles orbit Saturn twice for every time the moon Janus orbits once, creating an orbital resonance. The wave propagates outward from the resonance (and away from Saturn), toward upper-left in this view. For reasons researchers do not entirely understand, damping of waves by larger ring structures is very weak at this location, so this wave is seen ringing for hundreds of bright wave crests, unlike density waves in Saturn's A ring. The image gives the illusion that the ring plane is tilted away from the camera toward upper-left, but this is not the case. Because of the mechanics of how this kind of wave propagates, the wavelength decreases with distance from the resonance. Thus, the upper-left of the image is just as close to the camera as the lower-right, while the wavelength of the density wave is simply shorter. This wave is remarkable because Janus, the moon that generates it, is in a strange orbital configuration. Janus and Epimetheus share practically the same orbit and trade places every four years. Every time one of those orbit swaps takes place, the ring at this location responds, spawning a new crest in the wave. The distance between any pair of crests corresponds to four years' worth of

  19. Asteroid structure

    NASA Astrophysics Data System (ADS)

    Asphaug, E.

    2014-07-01

    Even before the first space missions to asteroids, in the mid-1990s, it was known that asteroids have weird structures. Photometry indicated complicated shapes, and the pioneering radar investigations by Ostro and colleagues followed by adaptive optics campaigns and flybys showed odd binary forms, and confirmed the common presence of satellites, and indications of highly varying surface roughness. Some asteroids turned out to be dominated by a single major cratering event, while others showed no evidence of a major crater, or perhaps for global crater erasure. The first space mission to orbit an asteroid, NEAR, found a mixture of heavily cratered terrains and geomorphically active 'ponds', and indicated evidence for global seismicity from impact. The next mission to orbit an asteroid, Hayabusa, found what most agree is a rubble pile, with no major craters and an absence of fines. There is to date no direct evidence of asteroid interior geology, other than measurements of bulk density, and inferences made for mass distribution asymmetry based on dynamics, and inferences based on surface lineaments. Interpolating from the surface to the interior is always risky and usually wrong, but of course the answer is important since we are someday destined to require this knowledge in order to divert a hazardous asteroid from impact with the Earth. Even considering the near-subsurface, here we remain as ignorant as we were about the Moon in the early 1960s, whether the surface will swallow us up in dust, or will provide secure landing and anchoring points. Laboratory experimentation in close to zero-G is still in its early stages. Adventures such as mining and colonization will surely have to wait until we better know these things. How do we get from here to there? I will focus on 3 areas of progress: (1) asteroid cratering seismology, where we use the surface craters to understand what is going on inside; (2) numerical modeling of collisions, which predicts the internal

  20. Bi{sub 1−x}Nb{sub x}O{sub 1.5+x} (x=0.0625, 0.12) fast ion conductors: Structures, stability and oxide ion migration pathways

    SciTech Connect

    Tate, Matthew L.; Hack, Jennifer; Kuang, Xiaojun; McIntyre, Garry J.; Withers, Ray L.; Johnson, Mark R.; Radosavljevic Evans, Ivana

    2015-05-15

    A combined experimental and computational study of Bi{sub 1−x}Nb{sub x}O{sub 1.5+x} (x=0.0625 and 0.12) has been carried out using laboratory X-ray, neutron and electron diffraction, impedance measurements and ab-initio molecular dynamics. We demonstrate that Bi{sub 0.9375}Nb{sub 0.0625}O{sub 1.5625}, previously reported to adopt a cubic fluorite-type superstructure, can form two different polymorphs depending on the synthetic method: a metastable cubic phase is produced by quenching; while slower cooling yields a stable material with a tetragonal √2×√2×1 superstructure, which undergoes a reversible phase transition into the cubic form at ~680 °C on subsequent reheating. Neutron diffraction reveals that the tetragonal superstructure arises mainly from ordering in the oxygen sublattice, with Bi and Nb remaining disordered, although structured diffuse scattering observed in the electron diffraction patterns suggests a degree of short-range ordering. Both materials are oxide ion conductors. On thermal cycling, Bi{sub 0.88}Nb{sub 0.12}O{sub 1.62} exhibits a decrease in conductivity of approximately an order of magnitude due to partial transformation into the tetragonal phase, but still exhibits conductivity comparable to yttria-stabilised zirconia (YSZ). Ab-initio molecular dynamics simulations performed on Bi{sub 0.9375}Nb{sub 0.0625}O{sub 1.5625} show that oxide ion diffusion occurs by O{sup 2−} jumps between edge- and corner-sharing OM{sub 4} groups (M=Bi, Nb) via tetrahedral □M{sub 4} and octahedral □M{sub 6} vacancies. - Graphical abstract: Oxide ion migration in tetragonal Bi{sub 0.9375}Nb{sub 0.0625}O{sub 1.5625} occurs by O{sup 2−} jumps between edge- and corner-sharing OM{sub 4} groups (M=Bi, Nb) via tetrahedral M{sub 4} and octahedral M{sub 6} vacancies. - Highlights: • Bi{sub 0.9375}Nb{sub 0.0625}O{sub 1.5625} adopts a tetragonal √2×√2×1 fluorite superstructure. • Superstructure is due to ordering in the O-sublattice, with Bi

  1. Structural Biology Fact Sheet

    MedlinePlus

    ... Home > Science Education > Structural Biology Fact Sheet Structural Biology Fact Sheet Tagline (Optional) Middle/Main Content Area What is structural biology? Structural biology is a field of science focused ...

  2. Magnetic multilayer structure

    DOEpatents

    Herget, Philipp; O'Sullivan, Eugene J.; Romankiw, Lubomyr T.; Wang, Naigang; Webb, Bucknell C.

    2017-03-21

    A mechanism is provided for an integrated laminated magnetic device. A substrate and a multilayer stack structure form the device. The multilayer stack structure includes alternating magnetic layers and diode structures formed on the substrate. Each magnetic layer in the multilayer stack structure is separated from another magnetic layer in the multilayer stack structure by a diode structure.

  3. Magnetic multilayer structure

    SciTech Connect

    Herget, Philipp; O'Sullivan, Eugene J.; Romankiw, Lubomyr T.; Wang, Naigang; Webb, Bucknell C.

    2016-07-05

    A mechanism is provided for an integrated laminated magnetic device. A substrate and a multilayer stack structure form the device. The multilayer stack structure includes alternating magnetic layers and diode structures formed on the substrate. Each magnetic layer in the multilayer stack structure is separated from another magnetic layer in the multilayer stack structure by a diode structure.

  4. The Crystal Structure of Lanthanide Zirconates

    NASA Astrophysics Data System (ADS)

    Clements, Richard; Kennedy, Brendan; Ling, Christopher; Stampfl, Anton P. J.

    2010-03-01

    The lanthanide zirconates of composition Ln2Zr2O7 (Ln = La-Gd) are of interest for use in inert matrix fuels and nuclear wasteforms. The series undergoes a pyrochlore to fluorite phase transition as a function of the Ln atomic radii. The phase transition has been attributed to disordering of both the cation and the anion [1]. We have undertaken a synthesis of the lanthanide zirconate series Ln2Zr2O7 (Ln = La-Gd), Ln0.2Zr0.8O1.9 (Ln = Tb-Yb) and NdxHo2-xZr2O7 (0

  5. Common structure-balance between spacetime structure and massenergy structure

    NASA Astrophysics Data System (ADS)

    Cao, Daqing; Cao, Dayong

    2017-01-01

    According to Einstein field equation, there is a balance between spacetime structure and massenergy structure. nd the paper consider it as a common structurewhich was brought forward by Daqing Cao in 2011 ecause it is general structure in the universe and everything have the same model of structure in their one system. The Jovian planets is spacetime structure of solar system because they are gas-sphere and they have more density of spacetime (spacetime/massenergy) than the density of massenergy (massenergy/spacetime). The terrestrial planets is massenergy structure of solar system because they are rock-ball and they have more density of massenergy than the density of spacetime. That can explain of that the Jovian planets of big mass is far away from sun. With the idea that the wave is spacetime and the wave effect is spacetime structure, the planets have elliptic orbits and the same direction of their revolution. Because sun is like a massenergy center of the massenergy structure and the terrestrial planets, the paper supposes there is a dark sun-a dark hole who has a spacetime center of spacetime structure and influences on the orbits of the Jovian planets. http://meetings.aps.org/Meeting/APR16/Session/M13.8

  6. Growth and defect structure of CdF{sub 2} crystal and nonstoichiometric Cd{sub 1-x}R{sub x}F{sub 2+x} phases (R are rare earth elements and in): 6. Growth and ionic conductivity of Cd{sub 0.904}In{sub 0.096}F{sub 2.096} single crystal

    SciTech Connect

    Sorokin, N. I. Sul'yanova, E. A.; Buchinskaya, I. I.; Artyukhov, A. A.; Sobolev, B. P.

    2013-07-15

    Cd{sub 0.904}In{sub 0.096}F{sub 2.096} crystals with fluorite-type defect structures have been grown from melt in a fluorinating atmosphere by the Bridgman method, and their ionic conductivity is investigated. The fluorine-ion transport activation enthalpy in Cd{sub 0.904}In{sub 0.096}F{sub 2.096} ({Delta}H = 0.68 eV) is much smaller than the corresponding characteristic of the crystals belonging to the isoconcentration series Cd{sub 0.9}R{sub 0.1}F{sub 2.1}, R = La-Lu, Y ({Delta}H = 0.8-0.9 eV). The ionic conductivity of Cd{sub 0.904}In{sub 0.096}F{sub 2.096} is {sigma} = 2 Multiplication-Sign 10{sup -4} S/cm (at 467 K); this value exceeds the conductivity of the CdF{sub 2} crystal matrix and the highest conductivity Cd{sub 0.9}R{sub 0.1}F{sub 2.1} crystals with rare earth elements by factors of 3 Multiplication-Sign 10{sup 3} and {approx}10, respectively. Nonstoichiometric crystals of solid electrolyte Cd{sub 1-x}In{sub x}F{sub 2+x} have the highest conductivity out of all studied electrolytes based on the CdF{sub 2} matrix.

  7. Structural Health Monitoring of Large Structures

    NASA Technical Reports Server (NTRS)

    Kim, Hyoung M.; Bartkowicz, Theodore J.; Smith, Suzanne Weaver; Zimmerman, David C.

    1994-01-01

    This paper describes a damage detection and health monitoring method that was developed for large space structures using on-orbit modal identification. After evaluating several existing model refinement and model reduction/expansion techniques, a new approach was developed to identify the location and extent of structural damage with a limited number of measurements. A general area of structural damage is first identified and, subsequently, a specific damaged structural component is located. This approach takes advantage of two different model refinement methods (optimal-update and design sensitivity) and two different model size matching methods (model reduction and eigenvector expansion). Performance of the proposed damage detection approach was demonstrated with test data from two different laboratory truss structures. This space technology can also be applied to structural inspection of aircraft, offshore platforms, oil tankers, ridges, and buildings. In addition, its applications to model refinement will improve the design of structural systems such as automobiles and electronic packaging.

  8. Nonlinear Structural Analysis

    NASA Technical Reports Server (NTRS)

    1984-01-01

    Nonlinear structural analysis techniques for engine structures and components are addressed. The finite element method and boundary element method are discussed in terms of stress and structural analyses of shells, plates, and laminates.

  9. Study of Boundary Structures.

    DTIC Science & Technology

    1982-09-01

    THEORY OF ABC-CBA STACKING BOUNDARY IN fcc STRUCTURE .......... 11 - 4 TRANSITIONS AND PHASE EQUILIBRIA AMONG GRAIN BOUNDARY STRUCTURES...19 B THEORY OF ABC-CBA STACKING BOUNDARY IN fcc STRUCTURE .......... 37 C TRANSITIONS AND PHASE EQUILIBRIA AMONG GRAIN BOUNDARY...layer structure. 10 SECTION 3 THEORY OF ABC-CBA STACKING BOUNDARY IN fcc STRUCTURE The (111) planes of the fcc structure is stacked as ABCABC... as

  10. Thermoelectric properties of FeAs based superconductors, with thick perovskite- and Sm-O fluorite-type blocking layers

    NASA Astrophysics Data System (ADS)

    Singh, S. J.; Shimoyama, J.; Ogino, H.; Kishio, K.

    2015-11-01

    The transport properties (electrical resistivity, Hall and Seebeck coefficient, and thermal conductivity) of iron based superconductors with thick perovskite-type oxide blocking layers and fluorine-doped SmFeAsO were studied to explore their possible potential for thermoelectric applications. The thermal conductivity of former compounds depicts the dominated role of phonon and its value decreases rapidly below the Tc, suggesting the addition of scattering of phonons. Both the Seebeck coefficient (S) and Hall coefficient (RH) of all samples were negative in the whole temperature region below 300 K, indicating that the major contribution to the normal state conductivity is by electrons. In addition, the profile of S(T) and RH(T) of all samples have similar behaviours as would be expected for a multi-band superconductors. Although the estimated thermoelectric figure of merit (ZT) of these compounds was much lower than that of practically applicable thermoelectric materials, however its improvement can be expected by optimizing microstructure of the polycrystalline materials, such as densification and grain orientation.

  11. The Influence of Calcite, Fluorite, and Rutile on the Fusion-Related Behavior of Metal Cored Coated Electrodes for Hardfacing

    NASA Astrophysics Data System (ADS)

    Cruz-Crespo, Amado; Fuentes, Rafael Fernández; Scotti, Américo

    2010-07-01

    Coated electrodes for SMAW have exhibited their advantages for longer than one century. Means of boosting their performance, particularly for hardfacing, would be a raise in the alloying transfer efficiency and a lowering of the dilution with the substrate, without losing the production capacity. In this study, an evaluation about the operational behavior of a new conception of electrodes for hardfacing is described, i.e., metal cored coated electrodes. Experimental electrodes were produced using metal cored technique to obtain the rods. FeCrMn was used as alloying material at two grain sizes. Using the Mc Lean Anderson experimental design approach, the content ratio of CaCO3:CaF2:TiO2 in the coating was varied. The effect of the coating composition and granulometry of the filling alloying material on the formation a cannon-like end was assessed. Fusion and metal transfer behaviors were evaluated through measurements of fusion and deposition rates, deposition efficiency, and duration and frequencies of short-circuiting. Based on a balance of performances, the most appropriate composition for the coating was determined. It was also observed that a coarser FeCrMn presented better performance.

  12. Structural design of active precision structures

    NASA Astrophysics Data System (ADS)

    Charon, W.

    The independent structural design for stochastically disturbed active structures, as part of the integrated structure/control design methodology, is the subject of this paper. The methodology requires the consideration of a generic controller in order to guarantee the performances of the active structure resulting from the mechanical design. An elastic 20 m long Extendable and Retractable Mast fitted with piezoelectric local actuators and an offset rigid antenna illustrates the method. Configurations, values of modal dampings are investigated using modal analysis, analysis of disturbance and actuator effects, inclusion of modal shapes, choice of actuators and sensors, determination of final performances.

  13. Mechanosynthesis and structural characterization of nanocrystalline Ce{sub 1–x}Y{sub x}O{sub 2–δ} (x=0.1–0.35) solid solutions

    SciTech Connect

    Fabián, Martin; Antić, Bratislav; Girman, Vladimír; Vučinić-Vasić, Milica; Kremenović, Aleksandar; Suzuki, Shigeru; Hahn, Horst; Šepelák, Vladimír

    2015-10-15

    A series of nanostructured fluorite-type Ce{sub 1–x}Y{sub x}O{sub 2–δ} (0≤x≤0.35) solid solutions, prepared via high-energy milling of the CeO{sub 2}/Y{sub 2}O{sub 3} mixtures, are investigated by XRD, HR-TEM, EDS and Raman spectroscopy. For the first time, complementary information on both the long-range and short-range structural features of mechanosynthesized Ce{sub 1–x}Y{sub x}O{sub 2–δ}, obtained by Rietveld analysis of XRD data and Raman spectroscopy, is provided. The lattice parameters of the as-prepared solid solutions decrease with increasing yttrium content. Rietveld refinements of the XRD data reveal increase in microstrains in the host ceria lattice as a consequence of yttrium incorporation. Raman spectra are directly affected by the presence of oxygen vacancies; their existence is evidenced by the presence of vibration modes at ~560 and ~600 cm{sup –1}. The detailed spectroscopic investigations enable us to separate extrinsic and intrinsic origin of oxygen vacancies. It is demonstrated that mechanosynthesis can be successfully employed in the one-step preparation of nanocrystalline Ce{sub 1–x}Y{sub x}O{sub 2–δ} solid solutions. - Graphical abstract: Mechanosynthesis of nanocrystalline Ce{sub 1–x}Y{sub x}O{sub 2–δ} (x=0.1–0.35) solid solutions. - Highlights: • One-step mechanosynthesis of nanoscale Ce{sub 1–x}Y{sub x}O{sub 2–δ} (0≤x≤0.35) solid solutions. • Complementary information on the long-range and short-range structural features of mechanosynthesized Ce{sub 1–x}Y{sub x}O{sub 2–δ} is provided. • Structural variations as a response to the yttrium doping. • Separation of extrinsic and intrinsic origin of the induced oxygen vacancies.

  14. Crystal structure and prediction.

    PubMed

    Thakur, Tejender S; Dubey, Ritesh; Desiraju, Gautam R

    2015-04-01

    The notion of structure is central to the subject of chemistry. This review traces the development of the idea of crystal structure since the time when a crystal structure could be determined from a three-dimensional diffraction pattern and assesses the feasibility of computationally predicting an unknown crystal structure of a given molecule. Crystal structure prediction is of considerable fundamental and applied importance, and its successful execution is by no means a solved problem. The ease of crystal structure determination today has resulted in the availability of large numbers of crystal structures of higher-energy polymorphs and pseudopolymorphs. These structural libraries lead to the concept of a crystal structure landscape. A crystal structure of a compound may accordingly be taken as a data point in such a landscape.

  15. Crystal Structure and Prediction

    NASA Astrophysics Data System (ADS)

    Thakur, Tejender S.; Dubey, Ritesh; Desiraju, Gautam R.

    2015-04-01

    The notion of structure is central to the subject of chemistry. This review traces the development of the idea of crystal structure since the time when a crystal structure could be determined from a three-dimensional diffraction pattern and assesses the feasibility of computationally predicting an unknown crystal structure of a given molecule. Crystal structure prediction is of considerable fundamental and applied importance, and its successful execution is by no means a solved problem. The ease of crystal structure determination today has resulted in the availability of large numbers of crystal structures of higher-energy polymorphs and pseudopolymorphs. These structural libraries lead to the concept of a crystal structure landscape. A crystal structure of a compound may accordingly be taken as a data point in such a landscape.

  16. Distributed Structure Searchable Toxicity

    EPA Pesticide Factsheets

    The Distributed Structure Searchable Toxicity (DSSTox) online resource provides high quality chemical structures and annotations in association with toxicity data. It helps to build a data foundation for improved structure-activity relationships and predictive toxicology. DSSTox publishes summarized chemical activity representations for structure-activity modeling and provides a structure browser. This tool also houses the chemical inventories for the ToxCast and Tox21 projects.

  17. Ultrastiff cubic TiO2 identified via first-principles calculations.

    PubMed

    Swamy, Varghese; Muddle, Barry C

    2007-01-19

    The crystal structures and compressibilities of fluorite- and pyrite-structured TiO2 under varying hydrostatic pressures are calculated using gradient-corrected density functional as well as hybrid density functional-Hartree-Fock formulations. The results suggest that fluorite TiO2 is a highly incompressible solid with a large bulk modulus value (K(0) approximately 395 GPa), approaching that of ultrahard cotunnite TiO2 (K(0)=431 GPa). The bulk modulus obtained for pyrite TiO2 is considerably smaller (K(0) approximately 220-260 GPa), nonetheless larger than the value determined experimentally for cubic TiO2. Calculated shear modulus values indicate that fluorite TiO2 has the potential to be an ultrahard material, if it could be stabilized under ambient conditions.

  18. Theoretical study of Ti0 and Pb+ centers in alkali halide and alkaline earth fluoride type crystals

    NASA Astrophysics Data System (ADS)

    Andriessen, J.; Postma, H.

    1987-04-01

    A theoretical study has been carried out on the hfi of Pb+ defects in KCl, CaF2 and BaF2 using an earlier developed crystal field model. Experimental results can be explained in the same way as was done for Tl0 in KCl. However some parameters seem to have a less physical meaning than in the case of Tl0. A new method is in progress using the ASW band structure procedure in order to estimate genuine solid state effects. Preliminary results are encouraging.

  19. Optical properties of PbS/CdS superlattices grown by pulsed laser evaporation

    NASA Astrophysics Data System (ADS)

    Musikhin, Sergey F.; Bakueva, Ludmila G.; Il'in, Vladimir I.; Rabizo, Oleg V.; Sharonova, Larissa V.

    1994-06-01

    PbS/CdS superlattices have been grown on BaF 2, CaF 2, Si and glass subtrates. Each specimen contained 10 to 23 pairs of the PbS/CdS layers. Absorption spectra of the structures have shown specific steps. The energy distribution of the steps has been analysed using models of Tipe-I and Tipe-II superlattices. Experimental results have been shown to follow the theoretical values of the steps for Tipe-II superlattices. This gives a foundation for classifying PbS/CdS superlattices as Tipe-II.

  20. CdF2:Er(3+)/Si(111) Heterostructure For EL Displays

    DTIC Science & Technology

    1997-01-01

    temperature reduced epitaxy onto pre-made device structures. In principle , II-a and II-b group fluoride film growth on a semiconductor substrate (mainly, on...Review: Main Results on Growth of II-a and IT-b Fluorides on Semiconductor Substrates Recently, group II-a cubic fluorides, CaF2, SrF2 and BaF2, have...If such single-crystalline lattice-matched dielectric films can be grown on a semiconductor substrate, the interface trap densities, in principle