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At least 1,045 records · Page 58
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Theoretical study of the electronic, thermodynamic, and thermo-conductive properties of y-LiAlO2 with 6Li isotope substitutions for tritium production

Lithium aluminate has attracted researchers’ interests due to its wide applications. The structural, electronic, optical, lattice phonon thermodynamic, and thermo-conductive properties of ?-LiAlO2 with 6Li-isotope substitution are investigated by density functional theory and lattice phonon dynamics. The calculated results show that ?-LiAlO2 possesses a wide band-gap of 4.63 eV. The valence band is mainly formed by p orbitals of Li, O, and Al. The calculated lattice thermal conductivity of ?-LiAlO2 is found to be in good agreement over a wide range of temperatures with the available experimental data. The calculated dielectric matrix shows anisotropic behavior along z and x (or y) axes. Due to the wide band-gap, the optical conductivity of ?-LiAlO2 is mainly contributed from the electron hopping between valence-conduction bands. Substitution of 7Li with 6Li isotope in ?-LiAlO2 leads to observable differences in the lattice phonon frequencies at higher frequencies (>8THz) and to slight changes in corresponding thermal conductivity and the infrared and Raman spectra which are in good agreement with the measured data. The relevance of these properties for tritium production through 6Li absorbing neutron in nuclear reactors is discussed.

ab initio thermodynamics, ?-LiAlO2 and 6Li-isotope↗
DOE OSTI2013

Stability of Ruddlesden–Popper-structured oxides in humid conditions

Some of layered transition-metal oxides are known to react with atmospheric humidity to form through topotactic intercalation reactions new water-containing layered structures. Here we investigate the influence of oxygen content (7−δ) of the Ruddlesden–Popper-structured Sr{sub 3}FeMO{sub 7−δ} (M=Ni, Mn, Ti) oxides on the water-intercalation reaction. It is found that their oxygen contents influence greatly the reactivity of the phases with water. Other factors possibly affecting the reactivity are discussed on the basis of the present data in combination with a comprehensive review of previous works on Ruddlesden–Popper and related layered oxide phases. - Graphical abstract: Many of the Ruddlesden–Popper-structured A{sub 3}B{sub 2}O{sub 7−δ} oxides readily react with water via intercalation reactions. Three possible factors affecting the water intercalation are identified: oxygen content of the phase, ionic radius of cation A and valence state of cation B. The resultant layered water-derivative phases can be categorised into two groups, depending on the crystal symmetry of the phase. Highlights: • Ruddlesden–Popper oxides A{sub 3}B{sub 2}O{sub 7−δ} often accommodate water via intercalation reaction. • The lower the oxygen content 7−δ is the more readily the intercalation reaction occurs. • The second factor promoting the reaction is the large size of cation A. • The third possible factor is the high valence state of cation B. • Resultant water-derivatives can be categorised into two groups depending on symmetry.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Pressure-induced volume collapse and structural phase transitions in SrRuO{sub 3}

We report on the low temperature (6 K) structural properties of SrRuO{sub 3} under quasi-hydrostatic pressure studied by synchrotron X-ray powder diffraction in a diamond anvil cell. First principle calculations predict a first-order perovskite (Pv) to post-perovskite (pPv) phase transition at ∼40 GPa accompanied by a 1.9% volume collapse. Our results rule out the occurrence of a pPv phase to 54 GPa. Instead, we find a Pv to monoclinic to triclinic sequence of phase transitions. The monoclinic to triclinic phase transition at ∼38 GPa is accompanied by a 3.5% volume collapse. X-ray absorption spectroscopy indicates that this volume collapse is not accompanied by a change in Ru valence state. Our results should help guide improvements to theoretical treatments of this and other correlated d-electron systems based on density functional theory. - Graphical abstract: Unit cell volume as a function of pressure (T=6 K). Black squares and red circles correspond to Ne and He pressure media, respectively. Blue dashed lines are fit to the data before the volume collapse using a second-order Birch–Murnaghan equation of state excluding (lower curve) and including (upper curve) neutron diffraction data for the low temperature, ambient pressure volume. Dashed lines denote structural phase boundaries. Display Omitted - Highlights: • Crystal structure of SrRuO3 was studied under high pressure up to 54 GPa. • Pv to monoclinic to triclinic sequence of phase transitions was observed. • Transition to triclinic phase at ∼38 GPa is accompanied by a 3.5% volume collapse. • Volume collapse in SrRuO3 is not driven by a change in Ru valence state. • Pv-to-pPv phase transition predicted by DFT at 40 Gpa is not observed up to 54 GPa.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Hall mobility in tin iodide perovskite CH{sub 3}NH{sub 3}SnI{sub 3}: Evidence for a doped semiconductor

CH{sub 3}NH{sub 3}SnI{sub 3} is a metal halide perovskite that shows metallic conductivity over a wide temperature range, although ab initio calculations and optical absorption indicate that its band structure is consistent with that of an intrinsic semiconductor. Hall effect measurements of as-grown crystals give a hole concentration of about 9×10{sup 17} cm{sup −3} with rather high Hall mobility of about 200 cm{sup 2} V{sup −1} s{sup −1} at 250 K. Artificial hole doping enhances the electrical conductivity of the crystals without influencing mobility. These observations indicate that the electronic structure in stoichiometric CH{sub 3}NH{sub 3}SnI{sub 3} can be described as that of an intrinsic semiconductor with a wide valence band. This situation leads to metal-like conduction with even a trace amount of spontaneous hole doping in the as-grown crystal. - Graphical abstract: Hall effect measurements of as-grown CH{sub 3}NH{sub 3}SnI{sub 3} crystals give a hole concentration of about 9×10{sup 17} cm{sup −3}. Artificial hole doping enhances the electrical conductivity of the crystals without influencing mobility, indicating that the electronic structure of CH{sub 3}NH{sub 3}SnI{sub 3} can be described as that of an intrinsic semiconductor with a wide valence band. Display Omitted - Highlights: • Hole concentration in highly conducting CH{sub 3}NH{sub 3}SnI{sub 3} was determined. • Hall mobility was not affected by artificial hole doping. • The electronic structure can be described as that of an intrinsic semiconductor.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Structural, electronic and magnetic properties of the series of double perovskites (Ca,Sr){sub 2−x}La{sub x}FeIrO{sub 6}

Polycrystalline samples of the series of double perovskites Sr{sub 2−x}La{sub x}FeIrO{sub 6} were synthesized. Their structural, electronic and magnetic properties were investigated by X-ray powder diffraction, Mössbauer spectroscopy, magnetic susceptibility, heat capacity and electrical resistivity experiments. The compounds crystallize in a monoclinic structure and were fitted in space group P2{sub 1}/n, with a significant degree of Fe/Ir cationic disorder. As in Ca{sub 2−x}La{sub x}FeIrO{sub 6} the Sr-based system seems to evolve from an antiferromagnetic ground state for the end members (x=0.0 and x=2.0) to a ferrimagnetic order in the intermediate regions (x∼1). Since Mössbauer spectra indicate that Fe valence remains 3+ with doping, this tendency of change in the nature of the microscopic interaction could be attributed to Ir valence changes, induced by La{sup 3+} electrical doping. Upon comparing both Ca and Sr series, Sr{sub 2−x}La{sub x}FeIrO{sub 6} is more structurally homogenous and presents higher magnetization and transition temperatures. Magnetic susceptibility measurements at high temperatures on Sr{sub 1.2}La{sub 0.8}FeIrO{sub 6} indicate a very high ferrimagnetic Curie temperature T{sub C}∼700K. For the Sr{sub 2}FeIrO{sub 6} compound, electrical resistivity experiments under applied pressure suggest that this material might be a Mott insulator. - Graphical abstract: The Weiss constant as a function of La doping for the (Ca,Sr){sub 2−x}La{sub x}FeIrO{sub 6} series, indicating changes in Fe–Ir magnetic coupling on both families. - Highlights: • The double perovskite series (Ca,Sr){sub 2−x}La{sub x}FeIrO{sub 6} were synthesized. • Changes in the Fe-Ir magnetic coupling due to La doping on both series. • Evidence of high T{sub C} on Sr{sub 1.2}La{sub 0.8}FeIrO{sub 6}. • Indication of Mott insulator behavior on Sr{sub 2}FeIrO{sub 6}.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Structural and electronic properties of Sr{sub 1−x}Ca{sub x}Ti{sub 0.5}Mn{sub 0.5}O{sub 3}

The series of Sr{sub 1−x}Ca{sub x}Ti{sub 1/2}Mn{sub 1/2}O{sub 3} perovskites has been prepared using solid state methods and their structures determined using combination of synchrotron X-ray and powder neutron diffraction. At room temperature the crystal structure evolves with increasing Ca content from Pm3{sup ¯}m→tetragonal I4/mcm→orthorhombic Pbnm, as a consequence of the sequential introduction of cooperative tilting of the (Ti/Mn)O{sub 6} octahedra. X-ray absorption measurements show the Mn is tetravalent in all oxides. Magnetic susceptibility measurements demonstrate all the oxides to be paramagnetic at room temperature, with a transition to a spin glass arrangement occurring at low temperatures. Electrical measurements prove that all the members are semiconductors and their activation energy increase with Ca doping amount. - Graphical abstract: Changes in the bond valence sums for the A- and B-sites in Sr{sub 1−x}Ca{sub x}Mn{sub 0.5}Ti{sub 0.5}O{sub 3} reflect changes in the symmetry of the oxides. - Highlights: • Structures of the Sr{sub 1−x}Ca{sub x}Ti{sub 1/2}Mn{sub 1/2}O{sub 3} perovskites studies using synchrotron X-ray and powder neutron diffraction. • Tilting of the octahedra occurs as Ca content increases. • Changes in the bond valence sums for the A- and B-sites cations reflect changes in the crystal symmetry. • Magnetic properties dependent of Ca content.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Electronic structures, elastic properties, and minimum thermal conductivities of cermet M{sub 3}AlN

The electronic structures and elastic anisotropies of cubic Ti{sub 3}AlN, Zr{sub 3}AlN, and Hf{sub 3}AlN are investigated by pseudopotential plane-wave method based on density functional theory. At the Fermi level, the electronic structures of these compounds are successive with no energy gap between conduct and valence bands, and exhibit metallicity in ground states. In valence band of each partial density of states, the different orbital electrons indicate interaction of corresponding atoms. In addition, the anisotropy of Hf{sub 3}AlN is found to be significantly different from that of Ti{sub 3}AlN and Zr{sub 3}AlN, which involve the differences in the bonding strength. It is notable that Hf{sub 3}AlN is a desired thermal barrier material with the lowest thermal conductivity at high temperature among the three compounds. - Graphical abstract: 1.Young's moduli of anti-perovskite Ti{sub 3}AlN, Zr{sub 3}AlN, and Hf{sub 3}AlN in full space. 2.Electron density differences on crystal planes (1 0 0), (2 0 0), and (1 1 0) of anti-perovskite Zr{sub 3}AlN. - Highlights: • We calculated three anti-perovskite cermets with first-principles theory. • We illustrated 3D Young modulus and found the anomalous anisotropy. • We explained the anomaly and calculated the minimum thermal conductivities.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

First single crystal growth and structural analysis of superconducting layered bismuth oxyselenide; La(O,F)BiSe{sub 2}

Single crystals of La(O,F)BiSe{sub 2} were successfully grown for the first time by a CsCl flux method. Single crystal X-ray structural analysis clearly showed that La(O,F)BiSe{sub 2} crystallizes with space group P4/nmm (lattice parameters a=4.1408(2) Å, c=14.1096(6) Å). The structure is composed of alternating BiSe{sub 2} and La(O,F) layers. The bond distances and angles are similar to those of La(O,F)BiS{sub 2}. The bond valence sum calculation indicates the refined structure is slightly cation rich, suggesting the charge neutrality should be kept by modification of valence state of the cations. Magnetic susceptibility and resistivity measurements showed the compound is a superconductor with a transition temperature of 3.6 K, which is higher than that of as-grown La(O,F)BiS{sub 2}. - Graphical abstract: Single crystal La(O,F)BiSe{sub 2} has been prepared by a CsCl flux method. The structural analysis implies that La(O,F)BiSe{sub 2} is a potential superconductor having a higher T{sub c} under high pressure. - Highlights: • Single crystals of La(O,F)BiSe{sub 2} were grown by a CsCl flux method. • Crystal structure of La(O,F)BiSe{sub 2} was determined by single crystal X-ray analysis. • La(O,F)BiSe{sub 2} shows superconductivity with a transition temperature of 3.6 K. • Bonding nature and superconductivity of La(O,F)BiSe{sub 2} are discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Structural and magnetic properties of the La{sub 2−x}Ca{sub x}CoIrO{sub 6} double perovskite series

Polycrystalline samples of the series La{sub 2−x}Ca{sub x}CoIrO{sub 6} (0≤x≤1.2) have been synthesized and its structural and magnetic properties were investigated using experiments of X-ray powder diffraction, temperature dependent magnetic susceptibility and magnetization as a function of applied magnetic field. All grown polycrystalline compounds form in a monoclinic structure (space group P2{sub 1}/n) with a partially ordered distribution of transition metal cations Co/Ir over the six-coordinate sites. Our results indicate variation of the magnetic interactions between Co and Ir magnetic sublattices possibly due to valence changes of the transition metals, induced by doping. The undoped La{sub 2}CoIrO{sub 6} presents magnetic ordering at ∼97K, while for Ca-doped compounds a decrease of net magnetization and ordering temperatures was observed. For x=0.2 and 0.5 the emergence of a second magnetic interaction, due to the presence of high spin Co{sup 3+}, becomes evident. Interestingly, for x=0.8 the large effective magnetic moment indicates strong orbital contribution and spin-orbit coupling and/or the presence of Co{sup 4+} in high spin state. - Graphical abstract: Field cooling M vs.T curves for La{sub 2−x}Ca{sub x}CoIrO{sub 6} shows the suppression of magnetism with increasing La content. Zero field cooled curves indicate a second magnetic phase induced by the appearance of Co{sup 3+}. - Highlights: • Novel double perovskite series La{sub 2−x}Ca{sub x}CoIrO{sub 6} were synthesized by solid state reaction. • La{sup 3+} to Ca{sup 2+} substitution leads to Co valence and possibly spin state changes. • For some doped compounds, the emergence of a second magnetic phase, due to the presence of high spin Co{sup 3+}, becomes evident. • Physical properties of each compound can be well correlated to the electronic and magnetic evolution of the system.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

High-temperature, high-pressure hydrothermal synthesis, characterization, and structural relationships of mixed-alkali metals uranyl silicates

Three mixed-alkali metals uranyl silicates, Na{sub 3}K{sub 3}[(UO{sub 2}){sub 3}(Si{sub 2}O{sub 7}){sub 2}]·2H{sub 2}O (1), Na{sub 3}Rb{sub 3}[(UO{sub 2}){sub 3}(Si{sub 2}O{sub 7}){sub 2}] (2), and Na{sub 6}Rb{sub 4}[(UO{sub 2}){sub 4}Si{sub 12}O{sub 33}] (3), have been synthesized by high-temperature, high-pressure hydrothermal reactions at 550 °C and 1440 bar, and characterized by single-crystal X-ray diffraction, photoluminescence, and thermogravimetric analysis. Compound 1 and 2 are isostructural and contain layers of uranyl disilicate. The smaller cation, Na{sup +}, is located in the intralayer channels, whereas the larger cations, K{sup +} and Rb{sup +}, and water molecule are located in the interlayer region. The absence of lattice water in 2 can be understood according to the valence-matching principle. The structure is related to that of a previously reported mixed-valence uranium(V,VI) silicate. Compound 3 adopts a 3D framework structure and contains a unique unbranched dreier fourfold silicate chain with the structural formula {uB,4"1_∞}[{sup 3}Si{sub 12}O{sub 33}] formed of Q{sup 2}, Q{sup 3}, and Q{sup 4} Si. The connectivity of the Si atoms in the Si{sub 12}O{sub 33}{sup 18−} anion can be interpreted on the basis of Zintl–Klemm concept. Crystal data for compound 1: triclinic, P-1, a=5.7981(2) Å, b=7.5875(3) Å, c=12.8068(5) Å, α=103.593(2)°, β=102.879(2)°, γ=90.064(2)°, V=533.00(3) Å{sup 3}, Z=1, R1=0.0278; compound 2: triclinic, P-1, a=5.7993(3) Å, b=7.5745(3) Å, c=12.9369(6) Å, α=78.265(2)°, β=79.137(2)°, γ=89.936(2)°, V=546.02(4) Å{sup 3}, Z=1, R1=0.0287; compound 3: monoclinic, C2/m, a=23.748(1) Å, b=7.3301(3) Å, c=15.2556(7) Å, β=129.116(2)°, V=2060.4(2) Å{sup 3}, Z=2, R1=0.0304. - Graphical abstract: Three mixed-alkali metals uranyl silicates were synthesized under hydrothermal conditions at 550 °C and 1400 bar and structurally characterized by single-crystal X-ray diffraction. Two of them have a layer structure with the alkali metal cations within and between the layers. The third one adopts a 3D framework structure and contains a unique unbranched dreier fourfold silicate chain formed of Q{sup 2}, Q{sup 3}, and Q{sup 4} Si. - Highlights: • Three new mixed-alkali metals uranyl silicates were synthesized by high-T, high-P hydrothermal method and structurally. • Two compounds adopt a layer structure and the third one has a 3D framework structure. • The 3D framework structure contains a unique unbranched dreier fourfold silicate chain formed of Q{sup 2}, Q{sup 3}, and Q{sup 4} Si.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Electronic, optical and photocatalytic behavior of Mn, N doped and co-doped TiO{sub 2}: Experiment and simulation

The crystal phase structure, surface morphology, chemical states and optical properties of Mn, N mono-doped and co-doped TiO{sub 2} nanoparticles were investigated by X-ray powder diffractometry, Raman spectra, scanning electron microscopy, X-ray photoelectron spectroscopy and UV–vis diffuse reflectance spectroscopy. Meanwhile, geometry structures, formation energies, electronic and optical properties of all systems have been also analyzed by density functional theory. The results showed that the band gap values and the carrier mobility in the valence band, conduction band and impurity levels have a synergetic influence on the visible-light absorption and photocatalytic activity of the doped TiO{sub 2}. The number and the carrier mobility of impurity level jointly influence the photocatalytic activity of catalyst under visible-light. Especially, the photocatalytic activity of Mn-2N co-doped TiO{sub 2} beyond three-fold than that of pure TiO{sub 2} under visible-light. - Graphical abstract: The ILs formed by N-2p orbital in N single doped specimen lie above the VB, while the ILs formed by Mn-3d orbital in Mn single doped specimen appear below the CB. However, a large amount of ILs formed by N-2p orbital and Mn-3d orbital in N and Mn codoped specimens. The band gap values and the carrier mobility in the valence band, conduction band and impurity levels have a synergetic influence on the visible-light absorption and photocatalytic activity of the doped TiO{sub 2}. The number and the carrier mobility of impurity level jointly influence the photocatalytic activity of catalyst under visible-light.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Synthesis, crystal structure, electrical properties, and sodium transport pathways of the new arsenate Na{sub 4}Co{sub 7}(AsO{sub 4}){sub 6}

A new sodium cobalt (II) arsenate Na{sub 4}Co{sub 7}(AsO{sub 4}){sub 6} has been synthesized by a solid-state reaction and its crystal structure determined from single crystal X-ray diffraction data. It crystallizes in the monoclinic system, space group C2/m, with a=10.7098(9) Å, b=14.7837(9) Å, c=6.6845(7) Å, and β=105.545(9)°. The structure is described as a three-dimensional framework built up of corner-edge sharing CoO{sub 6}, CoO{sub 4} and AsO{sub 4} polyhedra, with interconnecting channels along [100] in which the Na{sup +} cations are located. The densest ceramics with relative density of 94% was obtained by ball milling and optimization of sintering temperature, and its microstructure characterized by scanning electron microscopy. The electrical properties of the ceramics were studied over a temperature interval from 280 °C to 560 °C using the complex impedance spectroscopy over the range of 13 MHz–5 Hz. The ionic bulk conductivity value of the sample at 360 °C is 2.51 10{sup −5} S cm{sup −1} and the measured activation energy is Ea=1 eV. The sodium migration pathways in the crystal structure were investigated computationally using the bond valence site energy (BVSE) model and classical molecular dynamics (MD) simulations. - Graphical abstract: Correlation between crystal structure, microstructure and ionic conductivity . Display Omitted - Highlights: • A new arsenate Na{sub 4}Co{sub 7}(AsO{sub 4}){sub 6} was prepared by solid state reaction. • Its crystal structure was determined by powder X-ray diffraction. • Na{sup +} ionic conductivity was probed by complex impedance spectroscopy. • Na{sup +} conduction pathways were modeled by bond-valence method and molecular dynamics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Identifying the sources of ferromagnetism in sol-gel synthesized Zn{sub 1−x}Co{sub x}O (0≤x≤0.10) nanoparticles

We have carefully investigated the structural, optical and electronic properties and related them with changes in the magnetism of sol-gel synthesized Zn{sub 1−x}Co{sub x}O (0≤x≤0.10) nanoparticles. Samples with x≤0.05 were free of spurious phases. Samples with x≤0.03 were found to be with only high spin Co{sup 2+} ions into ZnO structure, whereas sample with x=0.05, exhibited the presence of high spin Co{sup 2+} and low spin Co{sup 3+}. We found that the intensity of the main EPR peak associated with Co{sup 2+} varies with the nominal Co content in a similar manner as the saturation magnetization and coercive field do. These results point out that the ferromagnetism in these samples should directly be correlated with the presence of divalent cobalt ions. Bound magnetic polaron (BMP) model and the charge transfer model are insufficient to explain the ferromagnetic properties of Zn{sub 1−x}Co{sub x}O nanoparticles. The room temperature ferromagnetism (RTFM) may be originated from a combination of several factors such as the interaction of high spin Co{sup 2+} ions, perturbation/alteration and/or changes in the electronic structure of ZnO close to the valence band edge and grain boundary effects. - Graphical abstract: The intensity of the main EPR peak associated with Co{sup 2+} varies with the nominal Co content in a similar manner as the saturation magnetization and coercive field do. These results point out that the ferromagnetism in these samples should directly be correlated with the presence of Co{sup 2+} ions. Display Omitted - Highlights: • Systematic and carefully study of physical-chemical properties of Zn{sub 1−x}Co{sub x}O nanoparticles. • Samples with x=0.01 and 0.03 were found to be with only high spin Co{sup 2+}. • Sample with x=0.05, exhibited the presence of high spin Co{sup 2+} and low spin Co{sup 3+}. • The BMP and charge transfer models seem not explain the ferromagnetic properties. • RTFM: high spin Co{sup 2+} ions, defects close to the valence band and grain boundary effects.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Anion ordering, magnetic structure and properties of the vacancy ordered perovskite Ba{sub 3}Fe{sub 3}O{sub 7}F

This article describes a detailed investigation of the crystallographic and magnetic structure of perovskite type Ba{sub 3}Fe{sub 3}O{sub 7}F by a combined analysis of X-ray and neutron powder diffraction data. Complete ordering of vacancies within the perovskite lattice could be confirmed. In addition, the structure of the anion sublattice was studied by means of the valence bond method, which suggested partial ordering of the fluoride ions on two of the six crystallographically different anion sites. Moreover, the compound was found to show G-type antiferromagnetic ordering of Fe moments, in agreement with magnetometric measurements as well as previously recorded {sup 57}Fe Mössbauer spectroscopy data. - Graphical abstract: The vacancy and anion ordered structure of Ba{sub 3}Fe{sub 3}O{sub 7}F is described together with its magnetic properties. - Highlights: • Ba{sub 3}Fe{sub 3}O{sub 7}F possesses a unique vacancy order not found for other perovskite type compounds. • The valence bond method was used to locate oxide and fluoride ions. • Fluoride ions are distributed only on two of the six anion sites in Ba{sub 3}Fe{sub 3}O{sub 7}F. • The compound shows G-type antiferromagnetic ordering of magnetic moments. • The magnetic structure could be refined in one of the maximal magnetic subgroups of the nuclear structure.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Metal ion displacements in noncentrosymmetric chalcogenides La{sub 3}Ga{sub 1.67}S{sub 7}, La{sub 3}Ag{sub 0.6}GaCh{sub 7} (Ch=S, Se), and La{sub 3}MGaSe{sub 7} (M=Zn, Cd)

The quaternary Ga-containing chalcogenides La{sub 3}Ag{sub 0.6}GaS{sub 7}, La{sub 3}Ag{sub 0.6}GaSe{sub 7}, La{sub 3}ZnGaSe{sub 7}, and La{sub 3}CdGaSe{sub 7}, as well as the related ternary chalcogenide La{sub 3}Ga{sub 1.67}S{sub 7}, were prepared by reactions of the elements at 950 °C. They adopt noncentrosymmetric hexagonal structures (space group P6{sub 3}, Z=2) with cell parameters (a=10.2 Å, c=6.1 Å for the sulfides; a=10.6 Å, c=6.4 Å for the selenides) that are largely controlled by the geometrical requirements of one-dimensional stacks of Ga-centered tetrahedra separated by the La atoms. Among these compounds, which share the common formulation La{sub 3}M{sub 1–x}GaCh{sub 7} (M=Ga, Ag, Zn, Cd; Ch=S, Se), the M atoms occupy sites within a stacking of trigonal antiprisms formed by Ch atoms. The location of the M site varies between extremes with trigonal antiprismatic (CN6) and trigonal planar (CN3) geometry. Partial occupation of these sites and intermediate ones accounts for the considerable versatility of these structures and the occurrence of large metal displacement parameters. The site occupations can be understood in a simple way as being driven by the need to satisfy appropriate bond valence sums for both the M and Ch atoms. Band structure calculations rationalize the substoichiometry observed in the Ag-containing compounds (La{sub 3}Ag{sub 0.6}GaS{sub 7}, La{sub 3}Ag{sub 0.6}GaSe{sub 7}) as a response to overbonding. X-ray photoelectron spectroscopy supports the presence of monovalent Ag atoms in these compounds, which are not charge-balanced. - Graphical abstract: Partial occupation of metal atoms in multiple sites accounts for versatility in Ga-containing chalcogenides La{sub 3}M{sub 1–x}GaCh{sub 7} with noncentrosymmetric hexagonal structures. - Highlights: • La{sub 3}M{sub 1–x}GaCh{sub 7} (M =Ga, Ag, Zn, Cd; Ch =S, Se) adopt related hexagonal structures. • Large displacements of M atoms originate from partial occupation of multiple sites. • Bond valence sum arguments give a simple explanation for site preference. • XPS studies confirm presence of monovalent Ag in La{sub 3}Ag{sub 0.6}GaCh{sub 7}. • Substoichiometry in La{sub 3}Ag{sub 0.6}GaCh{sub 7} avoids occupation of Ag–Ch antibonding levels.

36 MATERIALS SCIENCE↗
DOE OSTI2017

From Ba{sub 3}Ta{sub 5}O{sub 14}N to LaBa{sub 2}Ta{sub 5}O{sub 13}N{sub 2}: Decreasing the optical band gap of a photocatalyst

Yellow LaBa{sub 2}Ta{sub 5}O{sub 13}N{sub 2} was successfully synthesized as phase-pure material crystallizing isostructurally to previously reported Ba{sub 3}Ta{sub 5}O{sub 14}N and mixed-valence Ba{sub 3}Ta{sup V}{sub 4}Ta{sup IV}O{sub 15}. The electronic structure of LaBa{sub 2}Ta{sub 5}O{sub 13}N{sub 2} was studied theoretically with the range-separated hybrid method HSE06. The most stable structure was obtained when lanthanum was placed on 2a and nitrogen on 4h sites confirming Pauling's second rule. By incorporating nitrogen, the measured band gap decreases from ∼3.8 eV for the oxide via 2.74 eV for Ba{sub 3}Ta{sub 5}O{sub 14}N to 2.63 eV for the new oxide nitride, giving rise to an absorption band well in the visible-light region. Calculated fundamental band gaps confirm the experimental trend. The atom-projected density of states has large contributions from N2p orbitals close to the valence band edge. These are responsible for the observed band gap reduction. Photocatalytic hydrogen formation was investigated and compared with that of Ba{sub 3}Ta{sub 5}O{sub 14}N revealing significantly higher activity for LaBa{sub 2}Ta{sub 5}O{sub 13}N{sub 2} under UV-light. - Graphical abstract: X-ray powder diffraction pattern of LaBa{sub 2}Ta{sub 5}O{sub 13}N{sub 2} with the results of the Rietveld refinements. Inset: Unit cell of LaBa{sub 2}Ta{sub 5}O{sub 13}N{sub 2} and polyhedral representation of the crystal structure. - Highlights: • Synthesis of a new oxide nitride LaBa{sub 2}Ta{sub 5}O{sub 13}N{sub 2}. • Refinement of the crystal structure. • Quantum chemical calculations provided band gap close to the measured value. • New phase shows a higher photocatalytic H{sub 2} evolution rate compared to prior tested Ba{sub 3}Ta{sub 5}O{sub 14}N.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

A family of rare earth uranium oxides, RE{sub 6}UO{sub 12-δ}, (RE = Rare Earth). Synthesis, structure and magnetic behavior

Highlights: • A series of rare earth urinates, RE{sub 6}UO{sub 12}, were synthesized using the solid state method. • The crystal structures of RE{sub 6}UO{sub 12} were refined using the Rietveld method. • Magnetic properties were measured and AF ordering was observed in RE = Gd, Tb, Dy. - Abstract: A series of rare earth uranates, RE{sub 6}UO{sub 12-δ} (RE = La, Nd, Sm – Dy, Er and Yb), were synthesized via the solid state method and their structures were determined using the Rietveld method. These compounds crystallize in the rhombohedral space group R-3H with the lattice parameters a = 10.47005(3) Å and c = 9.98034(3) Å for the La analogue. The temperature and field dependent magnetization are reported for each analogue. Antiferromagnetic ordering is observed for RE = Gd, Tb, and Dy with Néel temperatures of 2.1 K, 5.8 K, and 2.8 K, respectively. Magnetic measurements on the La{sub 6}UO{sub 12} analogue indicate the presence of some U{sup 5+}, and therefore, oxygen vacancies in the structure. The variability of the U valence is examined.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Electronic origins of the giant volume collapse in the pyrite mineral MnS 2

The pyrite mineral MnS 2 was recently shown to undergo a giant pressure-induced volume collapse at 12 GPa, via a disordered intermediate phase. The high pressure arsenopyrite phase is stabilised by metal-metal bonding, a mechanism now shown to be ubiquitous for Mn 2+ chalcogenides. Here we report a spectroscopic investigation of this transition up to pressures of 22 GPa. Using XANES we show that the transition does not involve a change in oxidation state, consistent with the arsenopyrite crystal structure proposed at high pressure. Notably, the XANES spectrum is almost identical in the pressure-induced disordered phase, and after crystallisation induced by laser-heating. The former is therefore a `valence bond glass', and is likely disordered due to kinetic hindrance of the phase transition. We also detect electronic changes in the compressed pyrite phase, and this is con rmed by Raman scattering which shows that the disulphide vibrations in the pyrite phase saturate before the volume collapse. Together with detailed DFT calculations, these results indicate that electronic changes precede valence bond formation between the Mn 2+ cations.

36 MATERIALS SCIENCE↗