LiB3O5 crystal structure at 20, 227 and 377 °C

Crystal structure of LiB₃O₅ (a framework of [B₃O₅]⁻ rings and Li atoms located in interspaces) was refined at high temperatures using single-crystal X-ray diffraction, MoKα-radiation, anharmonic approximation, orthorhombic; Pna2₁; Z=4; 20 °C (a=8.444, b=7.378, c=5.146 Å, 1411 F(hkl), R=0.022); 227 °C (a=8.616, b=7.433, c=5.063 Å, 1336 F(hkl), R=0.026), 377 °C (a=8.746, b=7.480, c=5.013 Å, 1193 F(hkl), R=0.035). A high mobility of Li atoms and their highly asymmetric vibrations are revealed. Ellipsoid of Li thermal vibrations is oviform. Li is shifted on heating to 0.26 Å mainly along a-axis causing high thermal expansion in this direction; Li temperature factors are multiplied by 4 on heating. Rigid boron-oxygen groups in LiB₃O₅ remain practically stable on heating similar to α-Na₂B₈O₁₃ and α-CsB₅O₈. At the same time these groups rotate relative to each other like hinges leading to extremely anisotropic thermal expansion (α_a=101, α_b=31, α_c=−71, α_v=60×10⁻⁶ °C⁻¹, 20-530 °C, HTXRPD data).     

K2Mo4O13 phases prepared by hydrothermal synthesis

Hydrothermal synthesis in the K-Mo oxide system was investigated as a function of the pH of the reaction medium. Four compounds were formed, including two K₂Mo₄O₁₃ phases. One is a new low-temperature polymorph, which crystallizes in the orthorhombic, space group Pbca, with Z=8 and unit cell dimensions a=7.544(1) Å, b=15.394(2) Å, c=18.568(3) Å. The other is the known triclinic K₂Mo₄O₁₃, whose structure was re-determined from single crystal data; its cell parameters were determined as a=7.976(2) Å, b=8.345(2) Å, c=10.017(2) Å, α=107.104(3)°, β=102.885(3)°, γ=109.760(3)°, which are the standard settings of the crystal lattice. The orthorhombic phase converts endothermically into triclinic phase at ca. 730 K with a heat of transition of 8.31 kJ/mol.     

Subsolidus phase relations and crystal structures of the mixed-oxide phases in the In2O3-WO3 system

Subsolidus phase relationships in the In₂O₃-WO₃ system at 800-1400°C were investigated using X-ray diffraction. Two binary-oxide phases—In₆WO₁₂ and In₂(WO₄)₃—were found to be stable over the range 800-1200°C. Heating the binary-oxide phases above 1200°C resulted in the preferential volatilization of WO₃. Rietveld refinement was performed on three structures using X-ray diffraction data from nominally phase-pure In₆WO₁₂ at room temperature and from nominally phase-pure In₂(WO₄)₃ at 225°C and 310°C. The indium-rich phase, In₆WO₁₂, is rhombohedral, space group (rhombohedral), with Z=1, a=6.22390(4) Å, α=99.0338(2)° [hexagonal axes: a_H=9.48298(6) Å, c=8.94276(6) Å, a_H/c=0.9430(9)]. In₆WO₁₂ can be viewed as an anion-deficient fluorite structure in which 1/7 of the fluorite anion sites are vacant. Indium tungstate, In₂(WO₄)₃, undergoes a monoclinic-orthorhombic transition around 250°C. The high-temperature polymorph is orthorhombic, space group Pnca, with a=9.7126(5) Å, b=13.3824(7) Å, c=9.6141(5) Å, and Z=4. The low-temperature polymorph is monoclinic, space group P2₁/a, with a=16.406(2) Å, b=9.9663(1) Å, c=19.099(2) Å, β=125.411(2)°, and Z=8. The structures of the two In₂(WO₄)₃ polymorphs are similar, consisting of a network of corner sharing InO₆ octahedra and WO₄ tetrahedra.     

Structural studies of cerium tantalates

Structural data obtained from neutron diffraction studies of some cerium tantalate phases are presented, including the first report of the high temperature structure of a CeTaO₄ phase, Ce_0.85TaO_3.84 deduced from in situ data recorded at 900°C in vacuum. It was found that this material transformed from the low temperature LaTaO₄ type phase to the orthorhombic A2₁am structure reported here, with a unit cell of a=5.64062(2) Å, b=14.81609(6) Å and c=3.93482(1) Å. This data agrees well with the previously proposed structural transformations.     

Synthesis and structural investigation of the compounds containing HF2 anions: Ca(HF2)2, Ba4F4(HF2)(PF6)3 and Pb2F2(HF2)(PF6)

Three new compounds Ca(HF₂)₂, Ba₄F₄(HF₂)(PF₆)₃ and Pb₂F₂(HF₂)(PF₆) were obtained in the system metal(II) fluoride and anhydrous HF (aHF) acidified with excessive PF₅. The obtained polymeric solids are slightly soluble in aHF and they crystallize out of their aHF solutions. Ca(HF₂)₂ was prepared by simply dissolving CaF₂ in a neutral aHF. It represents the second known compound with homoleptic HF environment of the central atom besides Ba(H₃F₄)₂. The compounds Ba₄F₄(HF₂)(PF₆)₃ and Pb₂F₂(HF₂)(PF₆) represent two additional examples of the formation of a polymeric zigzag ladder or ribbon composed of metal cation and fluoride anion (MF⁺)_n besides PbF(AsF₆), the first isolated compound with such zigzag ladder. The obtained new compounds were characterized by X-ray single crystal diffraction method and partly by Raman spectroscopy. Ba₄F₄(HF₂)(PF₆)₃ crystallizes in a triclinic space group P1¯ with a=4.5870(2) Å, b=8.8327(3) Å, c=11.2489(3) Å, α=67.758(9)°, β=84.722(12), γ=78.283(12)°, V=413.00(3) Å³ at 200 K, Z=1 and R=0.0588. Pb₂F₂(HF₂)(PF₆) at 200 K: space group P1¯, a=4.5722(19) Å, b=4.763(2) Å, c=8.818(4) Å, α=86.967(10)°, β=76.774(10)°, γ=83.230(12)°, V=185.55(14) ų, Z=1 and R=0.0937. Pb₂F₂(HF₂)(PF₆) at 293 K: space group P1¯, a=4.586(2) Å, b=4.781(3) Å, c=8.831(5) Å, α=87.106(13)°, β=76.830(13)°, γ=83.531(11)°, V=187.27(18) Å³, Z=1 and R=0.072. Ca(HF₂)₂ crystallizes in an orthorhombic Fddd space group with a=5.5709(6) Å, b=10.1111(9) Å, c=10.5945(10) Å, V=596.77(10) Å³ at 200 K, Z=8 and R=0.028.     

Subsolidus phase relations and crystal structure of compounds in the PrOx-CaO-CuO system

The subsolidus phase relations of the PrO_x-CaO-CuO pseudo-ternary system sintered at 950-1000°C have been investigated by X-ray powder diffraction. In this system, there exist one compound Ca₁₀Pr₄Cu₂₄O₄₁, one Ca₂Pr₂Cu₅O₁₀-based solid solution, seven three-phase regions and two two-phase regions. The crystal structures of Ca₁₀Pr₄Cu₂₄O₄₁ and Ca₂Pr₂Cu₅O₁₀-based solid solution have been determined. Compound Ca₁₀Pr₄Cu₂₄O₄₁ crystallizes in an orthorhombic cell with space group D_2h²⁰−Cccm, Z=4. Its lattice parameters are a=11.278(2) Å, b=12.448(3) Å and c=27.486(8) Å. The crystal structure of Ca₂Pr₂Cu₅O₁₀-based solid solution is an incommensurate phase based on the orthorhombic NaCuO₂ type subcell. The lattice parameters of the subcell of the Ca_2.4Pr_1.6Cu₅O₁₀ are a₀=2.8246(7) Å, b₀=6.3693(5) Å, c₀=10.679(1) Å, and those of the orthorhombic superstructure are with a=5a₀, b=b₀, c=5c₀. The Ca_2.4Pr_1.6Cu₅O₁₀ structure can also be determined by using a monoclinic supercell with space group C_2h⁵−P2₁/c, Z=4, a=5a₀, b=b₀, and β=104.79(1)° or 136.60(1)°, V=5a₀b₀c₀.     

Crystal structure, thermal expansion and conductivity of anisotropic La1−xSrxGa1−2xMg2xO3−y (x=0.05, 0.1) single crystals

Crystal structure and anisotropy of the thermal expansion of single crystals of La_1−xSr_xGa_1−2xMg_2xO_3−y (x=0.05 and 0.1) were measured in the temperature range 300-1270 K. High-resolution X-ray powder diffraction data obtained by synchrotron experiments have been used to determine the crystal structure and thermal expansion. The room temperature structure of the crystal with x=0.05 was found to be orthorhombic (Imma, Z=4, a=7.79423(3) Å, b=5.49896(2) Å, c=5.53806(2) Å), whereas the symmetry of the x=0.1 crystal is monoclinic (I2/a, Z=4, a=7.82129(5) Å, b=5.54361(3) Å, c=5.51654(4) Å, β=90.040(1)°). The conductivity in two orthogonal directions of the crystals has been studied. Both, the conductivity and the structural data indicate three phase transitions in La_0.95Sr_0.05Ga_0.9Mg_0.1O_2.92 at 520-570 K (Imma-I2/a), 770 K (I2/a-R3c) and at 870 K (R3c-R-3c), respectively. Two transitions at 770 K (I2/a-R3c) and in the range 870-970 K (R3c-R-3c) occur in La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85.     

High-pressure synthesis and structural characterization of three new polyphosphides, α-SrP3, BaP8, and LaP5

Three novel metal polyphosphides, α-SrP₃, BaP₈, and LaP₅, were prepared in BN crucibles by the reaction of the respective stoichiometric mixtures under a high pressure of 3 GPa at 950-1000°C. Their crystal structures were determined from single-crystal X-ray data (α-SrP₃: space group C2/m, a=9.199(6) Å, b=7.288(3) Å, c=5.690(3) Å, β=113.45(4)°, Z=4, R₁/wR₂=0.0684/0.1180 for 471 observed reflections and 22 variables; BaP₈: space group P−1, a=6.762(2) Å, b=7.233(2) Å, c=8.567(2) Å, α=86.32(2)°, β=84.31(2)°, γ=70.40(2)°, Z=2, R₁/wR₂=0.0476/0.1255 for 2702 observed reflections and 82 variables; LaP₅: space group P2₁/m, a=4.885(1) Å, b=9.673(3) Å, c=5.577(2) Å, β=105.32(2)°, Z=2, R₁/wR₂=0.0391/0.1034 for 1272 observed reflections and 31 variables). α-SrP₃ is isostructural with SrAs₃ and the crystal structure consists of two-dimensional puckered polyanionic layers _∞²[P₃]²⁻ that stack along the c-axis yielding channels occupied by Sr²⁺ counterions. BaP₈ crystallizes in a new structure type which contains a three-dimensional infinite polyanionic framework _∞³[P₃]²⁻, with large channels hosting the barium cations. LaP₅ is a layered compound containing _∞²[P₅]³⁻ polyanionic layers separated by La³⁺ ions. All three compounds exhibit expected diamagnetic behaviors.     

Crystal chemistry of M[PO2(OH)2]2·2H2O compounds (M=Mg, Mn, Fe, Co, Ni, Zn, Cd): Structural investigation of the Ni, Zn and Cd salts

The compounds M[PO₂(OH)₂]₂·2H₂O (M=Mg, Mn, Fe, Co, Ni, Zn, Cd) were prepared from super-saturated aqueous solutions at room temperature. Single-crystal X-ray structure investigations of members with M=Ni, Zn, Cd were performed at 295 and 120 K. The space-group symmetry is P2₁/n, Z=2. The unit-cell parameters are at 295/120 K for M=Ni: a=7.240(2)/7.202(2), b=9.794(2)/9.799(2), c=5.313(1)/5.285(1) Å, β=94.81(1)/94.38(1)°, V=375.4/371.9 Å³; M=Zn: a=7.263(2)/7.221(2), b=9.893(2)/9.899(3), c=5.328(1)/5.296(2) Å, β=94.79(1)/94.31(2)°, V=381.5/377.5 Å³; M=Cd: a=7.356(2)/7.319(2), b=10.416(2)/10.423(3), c=5.407(1)/5.371(2) Å, β=93.85(1)/93.30(2)°, V=413.4/409.1 Å³. Layers of corner-shared MO₆ octahedra and phosphate tetrahedra are linked by three of the four crystallographically different hydrogen bonds. The fourth hydrogen bond (located within the layer) is worth mentioning because of the short O_h?O bond distance of 2.57-2.61 Å at room temperature (2.56-2.57 Å at 120 K); only for M=Mg it is increased to 2.65 Å. Any marked temperature-dependent variation of the unit-cell dimension is observed only vertical to the layers. The analysis of the infrared (IR) spectroscopy data evidences that the internal PO₄ vibrations are insensitive to the size and the electronic configuration of the M²⁺ ions. The slight strengthening of the intra-molecular P-O bonds in the Mg salt is caused by the more ionic character of the Mg-O bonds. All IR spectra exhibit the characteristic “ABC trio” for acidic salts: 2900-3180 cm⁻¹ (A band), 2000-2450 cm⁻¹ (B band) and 1550-1750 cm⁻¹ (C band). Both the frequency and the intensity of the A band provide an evidence that the PO₂(OH)₂ groups in M[PO₂(OH)₂]₂·2H₂O compounds form weaker hydrogen bonds as compared with other acidic salts with comparable O?O bond distances of about 2.60 Å. The observed shift of the O-H stretching vibrations of the water molecule in the order M=Mg>Mn≈Fe≈Co>Ni>Zn≈Cd has been discussed with respect to the influence of both the character and the strength of M↔H₂O interactions.     

Crystal growth, structure determination, and optical properties of new potassium-rare-earth silicates K3RESi2O7 (RE=Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu)

Single crystals of K₃RESi₂O₇ (RE=Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) were grown from a potassium fluoride flux. Two different structure types were found for this series. Silicates containing the larger rare earths, RE=Gd, Tb, Dy, Ho, Er, Tm, Yb crystallize in a structure K₃RESi₂O₇ that contains the rare-earth cation in both a slightly distorted octahedral and an ideal trigonal prismatic coordination environment, while in K₃LuSi₂O₇, containing the smallest of the rare earths, lutetium is found solely in an octahedral coordination environment. The structure of K₃LuSi₂O₇ crystallizes in space group P6₃/mmc with a=5.71160(10) Å and c=13.8883(6) Å. The structures containing the remaining rare earths crystallize in the space group P6₃/mcm with the lattice parameters of a=9.9359(2) Å, c=14.4295(4) Å, (K₃GdSi₂O₇); a=9.88730(10) Å, c=14.3856(3) Å, (K₃TbSi₂O₇); a=9.8673(2) Å, c=14.3572(4) Å, (K₃DySi₂O₇); a=9.8408(3) Å, c=14.3206(6) Å, (K₃HoSi₂O₇); a=9.82120(10) Å, c=14.2986(2) Å, (K₃ErSi₂O₇); a=9.80200(10) Å, c=14.2863(4) Å, (K₃TmSi₂O₇); a=9.78190(10) Å, c=14.2401(3) Å, (K₃YbSi₂O₇). The optical properties of the silicates were investigated and K₃TbSi₂O₇ was found to fluoresce in the visible.     

Synthesis and crystal structure of CuZrTiO5—A new crystal structure type

A new compound, CuZrTiO₅, was synthesized as strongly pleochroic green crystals from the oxides between 995 and 1010 °C, 1 atm. Its crystal structure was determined by single crystal XRD, resulting in R (F²>2σ(F²))=0.032 and wR (all data)=0.079). CuZrTiO₅ is orthorhombic, space group P2₁2₁2₁, a=3.5871(3) Å, b=6.6968(4) Å, c=14.6679(9) Å, V=352.35(4) Å³, Z=4. The structure is topologically similar to In₂TiO₅ but differs in space group and cation coordination. CuZrTiO₅ has relatively regular TiO₆ polyhedra, but coordination is 7+1 for Zr, and 4+2 for Cu due to the Jahn-Teller effect. Ordering of the long Cu-O bonds causes reduction in symmetry relative to In₂TiO₅. Layers of Cu alternate with Ti+Zr on (001), giving rise to a distinct cleavage. Bond valence sums on Ti and Zr are far from ideal, which appears due to the limited ability of this structural topology to avoid close next-nearest neighbour distances.     

Transition metal tetramolybdate dihydrates MMo4O13·2H2O (M=Co,Ni) having a novel pillared layer structure

Hydrothermal synthesis in the M/Mo/O (M=Co,Ni) system was investigated. Novel transition metal tetramolybdate dihydrates MMo₄O₁₃·2H₂O (M=Co,Ni), having an interesting pillared layer structure, were found. The molybdates crystallize in the triclinic system with space group P−1, Z=1 with unit cell parameters of a=5.525(3) Å, b=7.058(4) Å, c=7.551(5) Å, α=90.019(10)°, β=105.230(10)°, γ=90.286(10)° for CoMo₄O₁₃·2H₂O, and a=5.508(2) Å, b=7.017(3) Å, c=7.533(3) Å, α=90.152(6)°, β=105.216(6)°, γ=90.161(6)° for NiMo₄O₁₃·2H₂O The structure is composed of two-dimensional molybdenum-oxide (2D Mo-O) sheets pillared with CoO₆ octahedra. The 2D Mo-O sheet is made up of infinite straight ribbons built up by corner-sharing of four molybdenum octahedra (two MoO₆ and two MoO₅OH₂) sharing edges. These infinite ribbons are similar to the straight ones in triclinic-K₂Mo₄O₁₃ having 1D chain structure, but are linked one after another by corner-sharing to form a 2D sheet structure, like the twisted ribbons in BaMo₄O₁₃·2H₂O (or in orthorhombic-K₂Mo₄O₁₃) are.     

Order-disorder at Fe sites in SrFe2/3B″1/3O3 (B″=Mo, W, Te, U) tetragonal double perovskites

We have prepared SrFe_2/3B″_1/3O₃ (B″=Mo, U, Te, and W) double perovskites in polycrystalline form by ceramic methods. Phases with B″=U, Te and W have been studied by X-ray powder diffraction and the results have been compared with neutron diffraction data available for B″=Mo. At room temperature, the stoichiometric samples crystallize in the tetragonal crystal system (space group I4/m, Z=4). Cell parameters when B″=U, Te and W are a=5.6936(1) Å, c=8.0637(1)Å; a=5.5776(1) Å, c=7.9144(3) Å and a=5.5707(3) Å, c=7.9081(5) Å, respectively.The Mössbauer spectra at room temperature for all compounds show hyperfine parameters belonging to two Fe³⁺ sites located at lattice positions with different degrees of distortion. This is in agreement with diffraction data that indicate that the series of compounds display different degrees of Fe-site disorder, which increases in the following sequence: Mo<U<Te<W.     

Crystal chemistry and crystallography of the SrR2CuO5 (R=lanthanides) phases

The crystal chemistry and crystallography of the compounds SrR₂CuO₅ (Sr-121, R=lanthanides) were investigated using the powder X-ray Rietveld refinement technique. Among the 11 compositions studied, only R=Dy and Ho formed the stable SrR₂CuO₅ phase. SrR₂CuO₅ was found to be isostructural with the “green phase”, BaR₂CuO₅. The basic structure is orthorhombic with space group Pnma. The lattice parameters for SrDyCuO₅ are a=12.08080(6) Å, b=5.60421(2) Å, c=7.12971(3) Å, V=482.705(4) Å³, and Z=8; and for the Ho analog are a=12.03727(12) Å, b=5.58947(7) Å, c=7.10169(7) Å, V=477.816(9) Å³, and Z=8. In the SrR₂CuO₅ structure, each R is surrounded by seven oxygen atoms, forming a monocapped trigonal prism (RO₇). The isolated CuO₅ group forms a distorted square pyramid. Consecutive layers of prisms are stacked in the b-direction. Bond valence calculations imply that residual strain is largely responsible for the narrow stability of the SrR₂CuO₅ phases with R=Dy and Ho only. X-ray powder reference diffraction patterns for SrDy₂CuO₅ and SrHo₂CuO₅ were determined.     

Formation of Co octahedra and tetrahedra in YBaCo4O8.1

High-resolution neutron and synchrotron X-ray powder diffraction experiments were performed, at 300 and 10 K, for the determination of the structure of YBaCo₄O_8.1, which was prepared by controlled oxidation of the Kagomé lattice compound YBaCo₄O₇. Our diffraction data demonstrate that YBaCo₄O_8.1 crystallizes in the orthorhombic Pbc2₁ space group with the formation of a large superstructure (a=12.790 Å, b=10.845 Å, c=10.149 Å), with respect to the parent trigonal YBaCo₄O₇ material. The Co ions occupy both corner-sharing tetrahedral and edge-sharing octahedral sites, in contrast to YBaCo₄O₇, which has only corner-sharing tetrahedra. The octahedral sites form by the addition of two extra oxygen atoms and the drastic displacements of some of the original O atoms relative to the parent. The edge-sharing octahedra form isolated zigzag chains parallel to the c-axis linked to one another via tetrahedra. While found in a few phosphates, silicates and germanates, this motif appears unique to YBaCo₄O_8.1 among mixed-metal oxides. No structural phase transition or long range antiferromagnetic ordering are observed at 10 K.     

Phase transitions in K3AlF6

Phase transitions in the elpasolite-type K₃AlF₆ complex fluoride were investigated using differential scanning calorimetry, electron diffraction and X-ray powder diffraction. Three phase transitions were identified with critical temperatures , and . The α-K₃AlF₆ phase is stable below T₁ and crystallizes in a monoclinic unit cell with a=18.8588(2)Å, b=34.0278(2)Å, c=18.9231(1)Å, β=90.453(1)° (a=2a_c−c_c, b=4b_c, c=a_c+2c_c; a_c, b_c, c_c—the basic lattice vectors of the face-centered cubic elpasolite structure) and space group I2/a or Ia. The intermediate β phase exists only in very narrow temperature interval between T₁ and T₂. The γ polymorph is stable in the T₂<T<T₃ temperature range and has an orthorhombic unit cell with a=36.1229(6)Å, b=17.1114(3)Å, c=12.0502(3)Å (a=3a_c−3c_c, b=2b_c, c=a_c+c_c) at 250 °C and space group Fddd. Above T₃ the cubic δ polymorph forms with a_c=8.5786(4)Å at 400 °C and space group . The similarity between the K₃AlF₆ and K₃MoO₃F₃ compounds is discussed.     

Crystal growth and structural investigation of A2BReO6 (A=Sr, Ba; B=Li, Na)

Single crystals of the double perovskite rhenates A₂BReO₆ (A=Sr, Ba; B=Li, Na) were grown out of molten hydroxide fluxes. Single crystals of orange/yellow Ba₂LiReO₆, Ba₂NaReO₆ and Sr₂LiReO₆ were solved in the cubic, Fm-3m space group with a=8.1214(11) Å, 8.2975(3) Å, and 7.9071(15) Å, respectively, while Sr₂NaReO₆ was determined to be monoclinic P2₁/n with a=5.6737(6) Å, b=5.7988(6) Å, c=8.0431(8) Å, and β=90.02(6) °. The cubic structure consists of a rock salt lattice of corner-shared ReO₆ and MO₆ (M=Li, Na) octahedra which, in the monoclinic structure, are both tilted and rotated. A discrepancy exists between the symmetry of Sr₂LiReO₆ indicated by the single-crystal refinement of flux-grown crystals (cubic, Fm-3m) and the symmetry indicated by the powder diffraction data collected on polycrystalline samples prepared by the ceramic method (tetragonal, I4/m). It is possible that the cubic crystals are a kinetic product that forms in small quantities at low temperatures, while the powder represents the more stable polymorph that forms at higher reaction temperature.     

Structure and conductivity of strontium-doped cerium orthovanadates Ce1−xSrxVO4 (0≤x≤0.175)

A-site substituted cerium orthovanadates, Ce_1−xSr_xVO₄, were synthesised by solid-state reactions. It was found that the solid solution limit in Ce_1−xSr_xVO₄ is at x=0.175. The crystal structure was analysed by X-ray diffraction and it exhibits a tetragonal zircon structure of space group I4₁/amd (1 4 1) with a=7.3670 (3) and c=6.4894 (1) Å for Ce_0.825Sr_0.175VO₄. The UV-vis absorption spectra indicated that the compounds have band gaps at room temperature in the range 4.5-4.6 eV. Conductivity measurements were performed for the first time up to the strontium solid solution limit in air and in dry 5% H₂/Ar with conductivity values at 600 °C ranging from 0.3 to 30 mS cm⁻¹ in air to 30-45 mS cm⁻¹ in reduced atmosphere. Sample Ce_0.825Sr_0.175VO₄ is redox stable at a temperature below 600 °C although the conductivity is not high enough to be used as an electrode for solid oxide fuel cells.     

The non-centrosymmetric borate oxides, MBi2B2O7 (M=Ca, Sr)

Two novel noncentrosymmetric borates oxides, MBi₂B₂O₇ or MBi₂O(BO₃)₂ (MCa, Sr), have been synthesized by solid-state reactions in air at temperatures in the 600-700 °C range. Their crystal structures have been determined ab initio and refined using powder neutron diffraction data. CaBi₂B₂O₇ crystallizes in the orthorhombic Pna2₁ space group with a=8.9371(5) Å, b=5.4771(3) Å, c=12.5912(7) Å, Z=4, R_wp=0.118, χ²=2.30. SrBi₂B₂O₇ crystallizes in the hexagonal P6₃ space group with a=9.1404(4) Å, c=13.0808(6) Å, Z=6, R_wp=0.115, χ²=4.15. Large displacement parameters suggest the presence of disorder in SrBi₂B₂O₇ as also revealed by diffuse 2×a superstructure reflections in electron diffraction patterns. Both structures are built of identical (001) neutral layers of corner-sharing BO₃ triangles and MO₆ trigonal prisms forming six-membered rings in which Bi₂O groups are located. Adjacent layers are stacked in a staggered configuration and connected through weak Bi-O bonds. A moderate efficiency for second harmonic generation (SHG) has been measured for a powder sample of CaBi₂B₂O₇ (d_eff=2d_eff(KDP)).     

Contrasting oxide crystal chemistry of Nb and Ta:: The structures of the hexagonal bronzes BaTa2O6 and Ba0.93Nb2.03O6

The high-temperature hexagonal forms of BaTa₂O₆ and Ba_0.93Nb_2.03O₆ have P6/mmm symmetry with unit-cell parameters a=21.116(1) Å, c=3.9157(2) Å and a=21.0174(3) Å, c=3.9732(1) Å, respectively. Single crystal X-ray structure refinements for both phases are generally consistent with a previously proposed model, except for displacements of some Ba atoms from high-symmetry positions. The structures are based on a framework of corner- and edge-connected Nb/Ta-centred octahedra, with barium atoms occupying sites in four different types of [0 0 1] channels with hexagonal, triangular, rectangular and pentagonal cross-sections. The refinements showed that the non-stoichiometry in the niobate phase is due to barium atom vacancies in the pentagonal channels and to extra niobium atoms occupying interstitial sites with tri-capped trigonal prismatic coordination. The origin of the non-stoichiometry is attributed to minimisation of non-bonded Ba-Ba repulsions. The hexagonal structure is related to the structures of the low-temperature forms of BaNb₂O₆ and BaTa₂O₆, through a 30° rotation of the hexagonal rings of octahedra centred at the origin.