Magnetic properties of barium uranate Ba2U2O7

Unique magnetic properties of a ternary uranate Ba₂U₂O₇ are reported. Magnetic susceptibility measurements reveal that this compound undergoes a magnetic transition at 19 K. Below this temperature, magnetic hysteresis was observed. The results of the low-temperature specific heat measurements below 30 K support the existence of the second-order magnetic transition at 19 K. Ba₂U₂O₇ undergoes a canted antiferromagnetic ordering at this temperature. The magnetic anomaly which sets in at 58 K may be due to the onset of one-dimensional magnetic correlations associated with the linear chains formed by U ions. The analysis of the experimental magnetic susceptibility data in the paramagnetic temperature region gives the effective magnetic moment μ_eff=0.73 μ_B, the Weiss constant θ=−10 K, and the temperature-independent paramagnetic susceptibility χ_TIP=0.14×10⁻³ emu/mole.The magnetic susceptibility results and the optical absorption spectrum were analyzed on the basis of an octahedral crystal field model. The energy levels of Ba₂U₂O₇ and the crystal field parameters were determined.     

Synthesis, structures, and phase transitions of barium bismuth iridium oxide perovskites Ba2BiIrO6 and Ba3BiIr2O9

The Ba-Bi-Ir-O system is found to contain two distinct perovskite-type phases: a rock-salt ordered double perovskite Ba₂BiIrO₆; and a 6H-type hexagonal perovskite Ba₃BiIr₂O₉. Ba₂BiIrO₆ undergoes a series of symmetry-lowering phase transitions on cooling , all of which are second order except the rhombohedral→monoclinic one, which is first order. The monoclinic phase is only observed in a 2-phase rhombohedral+monoclinic regime. The transition and 2-phase region lie very close to 300 K, making the room-temperature X-ray diffraction patterns extremely complex and potentially explaining why Ba₂BiIrO₆ had not previously been identified and reported. A solid solution Ba₂Bi_1+xIr_1−xO₆, analogous to Ba₂Bi_1+xRu_1−xO₆, 0≤x≤2/3, was not observed. The 6H-type phase Ba₃BiIr₂O₉ undergoes a clean second-order phase transition P6₃/mmc→C2/c at 750 K, unlike 6H-type Ba₃LaIr₂O₉, the P6₃/mmc structure of which is highly strained below ∼750 K but fails to distort coherently to the monoclinic phase.     

The crystal structure of Ba2V2O7

Ba₂V₂O₇ is triclinic with a = 13.571(3), b = 7.320(2), c = 7.306(2) Å, α = 90.09(1), β = 99.48(1), β = 99.48(1), γ = 87.32(1)°, V = 7.15.1 ų, Z = 4, and space group $\text{P}\text{1}$. The crystal structure was solved by Patterson and Fourier methods and refined by full-matrix least-squares analysis to a R_w of 0.034 (R = 0.034) using 2484 reflections measured on a Syntex $\text{P}\text{1}$ automatic four-circle diffractometer. The structure has two unique divanadate groups that are repeated by the b and c lattice translations to form sheets of divanadate groups parallel to (100). These sheets are linked by four unique Ba atoms that lie between these sheets. Ba(1) and Ba(3) are coordinated by eight oxygens arranged in a distorted biaugmented triangular prism and a distorted cubic antiprism, respectively. Ba(2) is coordinated by 10 oxygens arranged in a distorted gyroelongated square dipyramid and Ba(4) is coordinated by nine oxygens arranged in a distorted triaugmented triangular prism. These coordination numbers are substantiated by a bond strength analysis of the structure, and the variation in 〈BaO〉 distances is compatible with the assigned cation and anion coordination numbers. Both divanadate groups are in the eclipsed configuraton with 〈VO(br)〉 bond lengths of 1.821(4) and 1.824(4) Å and VO(br)V angles of 125.6(3) and 123.7(3)°, respectively. Examination of the divanadate groups in a series of structures allows certain generalizations to be made. Longer 〈VO(br)〉 bond lengths are generally associated with smaller VO(br)V angles. When VO(br)V < 140°, the divanadate group is generally in an eclipsed configuration; when VO(br)V > 140°, the divanadate group is generally in a staggered configuration. Nontetrahedral cations with large coordination numbers require more oxygens with which to bond, and hence O(br) is more likely to be three coordinate, with the divanadate group in the eclipsed configuration. In the eclipsed configuration, decrease in VO(br)V promotes bonding between O(br) and nontetrahedral cations, and hence smaller nontetrahedral cations are generally associated with smaller VO(br)V angles.     

A single-crystal study of eight-layer barium niobium lithium oxide,Ba4Nb3LiO12

A single-crystal study of a sample of Ba₄Nb₃LiO₁₂ provided by Dr. T. Negas has been carried out and confirms the |(4)|(4)| layer stacking scheme (Zhdanov notation) for the eight BaO₃ layers per unit cell. Of the eight MO₆ octahedra per cell (M = Nb or Li), four share faces in pairs, and these pairs are linked by pairs of corner-sharing MO₆ octahedra. The compound has an hexagonal cell of dimensions a = 5.777 ± 0.006 Å and c = 18.95 ± 0.03 Å, probable space group $\text{P6}{}_3\text{mmc}\text{, Z = 2}$. The theoretical density is 6.22 g/cm³; within the limit of error of the pycnometrically measured density, 6.08 ± 0.06 g/cm³. The study was carried out with 620 independent reflections, of which 437 were considered observed, collected by automated counter methods and refined by least-squares to a conventional R value of 0.076.     

Novel perovskite-related barium tungstate Ba11W4O23

Ba₁₁W₄O₂₃ was synthesized at 1300 °C, followed by quenching with liquid nitrogen. The crystal structure, which was known to be cryolite-related but has remained unclear, was initially determined by single-crystal X-ray diffraction for the isostructural Ru-substituted compound Ba₁₁(W_3.1Ru_0.9)O_22.5, which was discovered during exploratory synthesis in the Ba-Ru-O system. The structure of Ba₁₁W₄O₂₃ was refined by a combined powder X-ray and neutron Rietveld method (Fd-3m, a=17.1823(1) Å, Z=8, R_p=3.09%, R_wp=4.25%, χ²=2.8, 23 °C). The structure is an example of A-site vacancy-ordered 4×4×4 superstructure of a simple perovskite ABO₃, and it may be written as (Ba_1.75□_0.25)BaWO_5.75□_0.25, emphasizing vacancies on both metal and anion sites. The local structure of one of two asymmetric tungsten ions is the WO₆ octahedron, typical of perovskite. The other tungsten, however, is surrounded by oxygen and anionic vacancies statistically distributed over three divided sites to form 18 partially occupied oxygen atoms (∼30% on average), represented as WO_18/3. The A-site cation-vacancies are ordered at the 8a (, , ) site in between adjoining WO_18/3 polyhedra which form 1-D arrangements along [110] and equivalent directions. In situ high-temperature XRD data have shown that the quenched Ba₁₁W₄O₂₃ at room temperature is isostructural to the high-temperature phase at 1100 °C.     

The bonding in hexagonal Ba2/3Pt3B2 and CeCo3B2 type ternary metal borides

Tight-binding calculations with an extended Hückel Hamiltonian were performed on Ba_2/3Pt₃B₂ and LuOs₃B₂. Hypothetical linear metal boride chains present in these materials are analyzed with a three-dimensional model that contains a trigonal bipyramidal T₃B₂ (T = transition metal) building unit for the compounds. The geometrical structure for the T₃B₂ trigonal bipyramids depends on the number of electrons. For systems that have greater than 36 electrons in its trigonal bipyramidal building unit, a structural distortion is expected. Electron back donation from the electron-rich M₃ fragment to the empty e′ set on B₂ creates boron–boron interaction along the z-axis. Boron–boron pairing then participates as an electron sink and causes a trigonal distortion of the platinum Kagome net. On the other hand, a system with <35 electrons should have an undistorted, CeCo₃B₂ type structure. The electronic factors that create the breathing motion are discussed and analyzed with the aid of molecular and solid-state models. The metal–metal bonding associated with the structural properties also has been examined.     

The characterization of Ba3Re2O9 and Sr3Re2O9

The compounds Ba₃Re₂O₉ and Sr₃Re₂O₉ were prepared by the solid state reaction of the corresponding alkaline-earth oxide with ReO₃ at 750 to 900°C in sealed, evacuated, fused silica tubes. The two compounds are isostructural, having the nine-layer ABO₃ structure with vacant central octahedra. The unit cell parameters are given. The magnetic susceptibility for Ba₃Re₂O₉ indicates Curie-Weiss behavior with a Re⁶⁺ moment having localized electrons. The magnetic data for Sr₃Re₂O₉ suggest delocalized electron behavior from its temperature-independent susceptibility. Both compounds appear to have semiconducting properties, but the strontium analog is a better conductor. Both compounds are unstable when heated in air above 400°C. They are readily decomposed by chemical oxidizing agents.     

Non-centrosymmetric Ba3Ti3O6(BO3)2

The compound previously reported as Ba₂Ti₂B₂O₉ has been reformulated as Ba₃Ti₃B₂O₁₂, or Ba₃Ti₃O₆(BO₃)₂, a new barium titanium oxoborate. Small single crystals have been recovered from a melt with a composition of BaTiO₃:BaTiB₂O₆ (molar ratio) cooled between 1100°C and 850°C. The crystal structure has been determined by X-ray diffraction: hexagonal system, non-centrosymmetric space group, a=8.7377(11) Å, c=3.9147(8) Å, Z=1, wR(F²)=0.039 for 504 unique reflections. Ba₃Ti₃O₆(BO₃)₂ is isostructural with K₃Ta₃O₆(BO₃)₂. Preliminary measurements of nonlinear optical properties on microcrystalline samples show that the second harmonic generation efficiency of Ba₃Ti₃O₆(BO₃)₂ is equal to 95% of that of LiNbO₃.     

Synthesis, structure and physical properties of reduced barium titanate Ba2Ti13O22

Polycrystalline sample of the reduced barium titanate Ba₂Ti₁₃O₂₂ was synthesized by solid state reaction at 1523 K in Ar atmosphere for the first time. The Rietveld refinement using the powder X-ray diffraction data confirmed the sample to be main phase of Ba₂Ti₁₃O₂₂ having the orthorhombic crystal system, space group Bmab and the lattice parameters of a=11.67058(11) Å, b=14.12020(13) Å and c=10.06121(9) Å, and V=1657.995(20) ų. The valence state of Ti was evaluated by both the Ti–O bond distance analysis and the Ti K-edge XANES analysis. The magnetic susceptibility was nearly temperature independent in the range of 60–300 K, suggesting the Van Vleck Paramagnetism. The electrical conductivity at 300 K is approximately 320.50 S/cm, and a semiconducting behavior was observed below room temperature. The Seebeck coefficient showed a negative value of −1.25 μV/K at 300 K, indicating n-type behavior. These facts were confirmed by the results of the present theoretical calculations by the FLAPW method.     

A new promising scintillator Ba3InB9O18

We report on the synthesis, crystal structure and scintillation property of a new compound Ba₃InB₉O₁₈. This compound crystallizes in space group P6₃/m with unit cell of dimensions a=7.1359(3) Å, c=16.6151(8) Å and V=732.697 Å³ with two Ba₃InB₉O₁₈ molecular formula. Its crystal structure is made up of planar B₃O₆ groups parallel to each other along the 〈0001〉 direction, regular InO₆ octahedra, irregular BaO₆ hexagons and BaO₉ polyhedra to form an analog structure of Ba₃YB₉O₁₈. DTA and TGA curves for Ba₃InB₉O₁₈ show that it is a chemically stable and congruent melting compound. Its X-ray excited luminescence spectra show an intense emission band in the range of 360-500 nm with a maximum at 400 nm. Light yield for Ba₃InB₉O₁₈ is about 75% as large as that for BGO under the same measurement conditions. There may exist a correlation between the scintillation properties and the crystal structural features of Ba₃InB₉O₁₈.     

Sm2O3Ga2O3 and Gd2O3Ga2O3 phase diagrams

Four definite compounds exist in the Sm₂O₃Ga₂O₃ binary phase diagram, namely: Sm₃GaO₆, Sm₄Ga₂O₉, SmGaO₃, and Sm₃Ga₅O₁₂. The $\text{3}\text{1}$ compound is orthorhombic (space group Pnna - Z.4) with the cell parameters: a = 11.40₀Å, b = 5.51₅Å, c = 9.07Å and belongs to the oxysel family. Sm₃GaO₆ and SmGaO₃ melt incongruently at 1715 and 1565°C; Sm₄Ga₂O₉ and Sm₃Ga₅O₁₂ have a congruent melting point at 1710 and 1655°C. With regard to the Gd₂O₃Ga₂O₃ system three definite compounds have been identified: Gd₃GaO₆, Gd₄Ga₂O₉, and Gd₃Ga₅O₁₂. Only the garnet melts congruently at 1740°C with the following composition: Gd_3.12Ga_4.88O₁₂. Gd₃GaO₆, and Gd₄Ga₂O₉ melt incongruently at 1760 and 1700°C. GdGaO₃ is only obtained by melt overheating which may yield an equilibrium or a metastable phase diagram.     

Sr4PbPt4O11, the first platinum oxide containing Pt2 ions

We report the synthesis and crystal structure of the new compound Sr₄PbPt₄O₁₁, containing platinum in highly unusual square pyramidal coordination. The crystals were obtained in molten lead oxide. The structure was solved by X-ray single crystal diffraction techniques on a twinned sample, the final R factors are R=0.0260 and wR=0.0262. The symmetry is triclinic, space group P1¯, with , , , α=90.421(3)°, β=89.773(8)°, γ=90.140(9)° and Z=2. The structure is built from dumbell-shaped Pt₂O₉ entities formed by a dinuclear metal-metal bonded Pt₂⁶⁺ ion with asymmetric environments of the two Pt atoms, classical PtO₄ square plane and unusual PtO₅ square pyramid. Successive Pt₂O₉ entities deduced from 90° rotations are connected through the oxygens of the PtO₄ basal squares to form [Pt₄O₁₀⁸⁻]_∞ columns further connected through Pb²⁺ and Sr²⁺ ions. Raman spectroscopy confirmed the peculiar platinum coordination environment.     

Dopant substitution and oxygen migration in the complex perovskite oxide Ba3CaNb2O9: A computational study

Atomistic simulation methods have been used to study the defect chemistry of the complex perovskite oxide Ba₃CaNb₂O₉. Calculations were carried out for the hexagonal (P-3m1) phase and the cubic (Fm-3m) phase. The hexagonal structure is predicted to be energetically more stable at room temperature. In both structures the most favourable dopant for Nb⁵⁺ is found to be Ca²⁺ rather than Mg²⁺, in contrast to the generally accepted rule that size similarities govern such processes. The diffusion of oxygen vacancies in the hexagonal and cubic phases occurs within different networks of corner-sharing NbO₆ and CaO₆ octahedra. Irrespective of the arrangement of octahedra, however, migration of oxygen vacancies around NbO₆ octahedra takes place with lower activation energies than around the CaO₆ octahedra.     

Eu3+在Ba2TiSi2O8中的发光

The barium titano-silicate phosphors doped with Eu³⁺ were synthesized by high temperature solid state reaction. The structures of as-synthesized samples were characterized by powder XRD. The maximum peaks of emission spectra of Ba₂TiSi₂O₈ and Ba₂TiSi₂O₈∶Eu³⁺ were respectively located at 450 and 618 nm, coming from the transitions of charge-transfer bands of Ti⁴⁺-O^2- and forced electric-dipole transition ⁵D₀-⁷F₂ of Eu³⁺. The luminous mechanisms of Ba₂TiSi₂O₈ and Ba₂TiSi₂O₈∶Eu³⁺ were suggested. The effects of concentration of Eu³⁺ on the luminous performance of Ba₂TiSi₂O₈∶Eu³⁺ were also studied and the results showed that the optimum concentration of Eu³⁺ was 0.12 mol per mole of matrix.     

Crystal structure and luminescence properties of the barium europium tetragermanate Ba2Eu2Ge4O13

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High-pressure silicate pyrochlores,Sc2Si2O7 and In2Si2O7

The silicate compounds Sc₂Si₂O₇ and In₂Si₂O₇ have been converted from thortveitite type to pyrochlore type at 1000°C, 120 kbar, with resulting cell constants of 9.287(3) and 9.413(3) Å, respectively. Invariant reflection intensities in the X-ray powder diffraction patterns allowed precise absorption corrections to be made, and refinement of thermal parameters and of the single structural parameter x gave values of 0.4313(21) and 0.4272(15), respectively. The corresponding six-coordinate SiO distances were 1.761(7) and 1.800(5) Å, and the average eight-coordinate distances for ScO₈ and InO₈ were 2.267 and 2.275 Å. Values of structure-refined bond lengths for compounds containing six-coordinate silicon are surveyed, and overall weighted average octahedral distances of 1.782(14) Å for SiO and 2.520(18) Å for OO are derived. Pyrochlore phases were not produced from rare-earth disilicate or monosilicate phases subjected to the same reaction conditions as the Sc and In compounds.     

Garnet-perovskite transformations in the Sm2O3Ga2O3 and Gd2O3Ga2O3 systems

The effect of heating garnet melts to various temperatures has been investigated. The previously reported decomposition of the garnet phase due to loss of Ga₂O₃ was corroborated. However, it was also observed that when gallium oxide loss is prevented and the maximum temperature of the melt exceeds a critical value, phase separation of garnet to perovskite and β-gallium oxide occurs:

$\text{RE}{}_3\text{Ga}{}_5\text{O}{}_{12}\text{?3RE}\text{GaO}{}_3\text{+}\text{Ga}{}_2\text{O}{}_3$

.The reverse reaction will occur by reheating the two-phase mixture to the garnet melting point.     

Extension of the La7Mo7O30 structural type with La7Nb3W4O30 and La7Ta3W4O30 compounds

Two compounds of formula La₇A₃W₄O₃₀ (with A=Nb and Ta) were prepared by solid-state reaction at 1450 and 1490 °C. They crystallize in the rhombohedric space group R-3 (No. 148), with the hexagonal parameters: , and , . The structure of the materials was analyzed from X-ray, neutron and electronic diffraction. These oxides are isostructural of the reduced molybdenum compound La₇Mo₇O₃₀, which are formed of perovskite rod along [111]. An order between (Nb, Ta) and W is observed.