Formation of rutile-type Ta(IV)O2 by shock reduction and cation-deficient Ta0.8O2 by subsequent oxidation

Ta₂O₅ is reduced to Ta(IV)O₂ with the rutile structure by shock-loading to 50–60 GPa. Tetragonal unit cell parameters at room conditions are measured to be a = 4.7518(5)Å, c = 3.0878(4) Å, $\text{c}\text{a}\text{= 0.6498(1)}$, and V = 69.72(1) ų. The chemical composition is thermogravimetrically determined to be Ta_0.97±0.04O₂ by heating shock-reduced products in an oxygen gas flow to 1200°C. In the oxidation process a cation-deficient rutile-type compound Ta_0.8O₂ is found to be metastably formed.     

The crystal structure of vanadium ditelluride,V1+xTe2

Vanadium ditelluride, V_1.04Te₂, has a Cd(OH)₂-type structure with unit cell dimensions a_h = 3.638 Å and c_h = 6.582 Å above the transition temperature T_t of 482 K. Below T_t the structure is monoclinic, space group $\text{C2}\text{m}$, with cell dimensions a_m = 18.984 Å(≈3a_h√3), b_m = 3.5947 Å (≈a_h), c_m = 9.069 Å (≈√(3a²_h + c²_h)), β = 134.62°. This low-temperature form is isostructural with NbTe₂ and TaTe₂ (which do not show a phase transition); the vanadium atoms form double zigzag chains with VV distances of 3.316 Å, which distort the Te lattice. Complex diffraction patterns were observed due to the simultaneous occurrence of the distortion of the Cd(OH)₂-type structure of vanadium ditelluride in three equivalent directions. Similar patterns were found for the Nb and Ta ditellurides.     

Etude des equilibres solide-liquide des systems MIPO3Sm(PO3)3 (MI = Li,Na, Ag)

The M^IPO₃Sm(PO₃)₃(M^I = Li, Na, Ag) systems were studied. Differential thermal analysis and X-ray diffraction were used to investigate the liquidus and solidus relations. Three compounds LiSm(PO₃)₄, NaSm(PO₃)₄, and AgSm(PO₃)₄ were obtained which melt incongruently at 1248, 1143, and 1078 K, respectively. These compounds are isomorphous with their homologs LiLn(PO₃)₄, NaLn(PO₃)₄, AgLn(PO₃)₄ (Ln = Ce, La, Nd). They belong to the monoclinic system. The LiSm(PO₃)₄ unit cell parameters refined by least squares method are a = 16.43(3) Å, b = 7.16(1) Å, c = 9.65(3) Å, β = 125,9°(1), with the space group $\text{C2}\text{c}$ and Z = 4. NaSm(PO₃)₄ and AgSm(PO₃)₄ are isotypic; they cristallize in the $\text{P2}{}_1\text{c}$ space group, Z = 4; their unit cell parameters are, respectively, a = 12.18(1) Å, b = 13.05(1) Å, c = 7.25(5) Å, β = 126,53°(4), $\text{a = 12.25(1)}\text{A}\text{?}$, b = 13.06(1) Å, c = 7.201(9) Å, β = 126,57°(7). The ir spectra of the last two compounds indicate that these phosphates are chain phosphates.     

Neutron diffraction experiments on CsCrI3 at 300, 77, and 1.2 K

CsCrI₃ has been investigated by neutron powder diffraction at room temperature and 77 and 1.2 K. It undergoes a phase transition at 150 K due to the cooperative Jahn-Teller effect. The high-temperature form, α-CsCrI₃ (hexagonal, space group $\text{P6}{}_3\text{mmc}\text{, a = 8.127(1)}$Å, c = 6.944(1)Å, Z = 2), adopts the BaNiO₃ structure with a local Jahn-Teller distortion. The low-temperature form, β-CsCrI₃ (orthorhombic, space group Pbcn, a = 8.102(1)Å, b = 13.792(1)Å, c = 6.900(1)Å, Z = 4), has a structure not yet been reported for a Jahn-Teller distorted BaNiO₃ structure. It is shown that the low-temperature form can be derived from the BaNiO₃ structure by means of a canting of triangles, formed by the three common I^? ions of two adjacent CrI₆^4? octahedra. The magnetic structure of β-CsCrI₃ at 1.2 K is found to consist of an antiparallel sequence of ferromagnetic (0 0 1) planes with a magnetic moment in the ∥1 0 0∥ direction of 3.16 μ_B.     

Structural studies in the Li2MoO4MoO3 system: Part 1 the low temperature form of lithium tetramolybdate,LLi2Mo4O13

LLi₂Mo₄o₁₃ crystallizes in the triclinic system with unit-cell dimensions a = 8.578 Å, b = 11.450 Å, c = 8.225 Å, α = 109.24°, β = 96.04°, γ = 95.95° and space group $\text{P}\text{1}\text{, Z = 3}$. The calculated and measured densities are 4.02 g/cm³ and 4.1 g/cm³ respectively. The structure was solved using three-dimensional Patterson and Fourier techniques. Of the 2468 unique reflections collected by counter methods, 1813 with I ? 3σ(I) were used in the least-squares refinement of the model to a conventional R of 0.031 (ωR = 0.038). LLi₂Mo₄O₁₃ is a derivative of the V₆O₁₃ structure with oxygen ions arranged in a face-centred cubic type array with octahedrally coordinated molybdenum and lithium ions ordered into layers.     

Crystal growth and X-ray data of the lead phosphates Pb4P2O9 and Pb8P2O13

Single crystals of the title compounds have been grown by the Czochralski technique. Pb₄P₂O₉ crystallizes in the space group $\text{P2}{}_1\text{c}$ with the parameters a = 9.4812 Å, b = 7.1303 Å, c = 14.390 Å, β = 104.51° and Pb₈P₂O₁₃ in $\text{C2}\text{m}$ with a = 10.641 Å, b = 10.206Å c = 14.342 Å, β = 98.34°.     

Dimorphisme et structure du disulfure de lanthane LaS2

The stoichiometric lanthanum disulfide LaS₂ presents a reversible phase transition at about 750°C. The α low-temperature form is monoclinic with the LaSe₂ type. All the crystals are twinned with the same twin law (100). The cell parameters are a = 8.18, b = 8.13, c = 4.03Å, γ = 90°, space group $\text{P2}{}_1\text{a}$. The β high-temperature form has the orthorhombic structure previously described with the parameters a = 8.13, b = 16.34, c = 4.14 Å; space group Pnma. The two structures are compared.     

Crystal structures and magnetic properties of BaTiF5 and CaTiF5

Single crystals of BaTiF₅ and CaTiF₅ were obtained by the Czochralski and Bridgman techniques, respectively. The crystal structures were determined by X-ray diffraction; BaTiF₅: $\text{14}\text{m}\text{, a = 15.091(5)}$Å, c = 7.670(3)Å; CaTiF₅: $\text{I2}\text{c}\text{, a = 9.080(4)}$Å, b = 6.614Å, c = 7.696(3)Å, β = 115.16(3)°. Both structures are characterized by the presence of either branched or straight chains of TiF₆ octahedra. BaTiF₅ contains the unusual dimeric unit (Ti₂F₁₀)^4?. Magnetic susceptibility measurements were performed on both compounds in the temperature range 4.2 to 300 K, however, no evidence for magnetic interactions between the Ti³⁺ moments were observed.     

The crystal structure of niobium trisulfide,NbS3

The crystal structure of NbS₃ was determined from single-crystal diffractometer data obtained with MoKα radiation. The compound is triclinic, space group $\text{P}\text{1}$, with: a 4.963(2) Å; b = 6.730(2) Å; c = 9.144(4)Å; α = 90°; β = 97.17(1)°; γ = 90°. The structure is closely related to the ZrSe₃ structure type; it shows that the compound can be formulated as Nb⁴⁺(S₂)^2?S^2?, in agreement with XPS spectra. The main difference with ZrSe₃ is that the Nb atoms are shifted from the mirror planes of the surrounding bicapped trigonal prisms of sulfur atoms to form NbNb pairs (NbNb = 3.04 Å); this causes a doubling of the b axis relative to ZrSe₃ and a decrease of the symmetry to triclinic.     

Monoclinic-trigonal transition in some MI3M′III(XO4)3 compounds: The high temperature form of (NH4)3In(SO4)3

The high-temperature form of (NH₄)₃In(SO₄)₃ is rhombohedral, R3c, with a = 15.531 (12), c = 9.163 (8)Å, Z = 6. The structure was solved to R = 0.023 for 570 independent reflections measured at about 140°C. The structure is built up of [In(SO₄)₃]_∞ columns extending along the c axis and composed of InO₆ octahedra and SO₄ tetrahedra linked together; this arrangement is very similar to that found in the low-temperature form. To explain the transition mechanism, the existence of an intermediate phase of point symmetry $\text{3}$ is postulated and the whole sequence of possible forms would be $\text{2}\text{m}\text{→}\text{3}\text{→ 3m →}\text{3}\text{m}$. This last phase would be the prototypic structure of the possibly ferroelastic low-temperature modification, which can apparently exist only with nonspherical monovalent cations.     

New SbS2 strings in the BaSb2S4 structure

The new compound BaSb₂S₄ crystallizes in the monoclinic system (space group: $\text{P2}{}_1\text{c}$, No. 14) with a = 8.985(2) Å, b = 8.203(3) Å, c = 20.602(5) Å, β = 101.36(3)°. SbS₃ ψ tetrahedra and ψ-trigonal SbS₄ bipyramids are connected by common corners and edgers to infinite strings. These are arraged cross-wise in sheets perpendicular to the c axis.     

Neutron and X-ray powder diffraction study of LixRuO2 and LixIrO2: The crystal structure of Li0.9RuO2

Compounds formed by the insertion of lithium into the rutile structure hosts RuO₂ and IrO₂ were studied by X-ray and neutron powder diffraction techniques. Compositions in the range Li_xMO₂, M = Ru or Ir, 0 < x < 1 are two-phase materials consisting of unreacted host, x = 0, and limiting compositions x = 0.9 in both cases. Preparation of compounds with x > 1 was unsuccessful. Li_0.9RuO₂ and Li_0.9IrO₂ have orthorhombic cells with a = 5.062(3), b = 4.967(4), c = 2.771(4) and a = 4.962(4), b = 4.758(4), c = 3.108(6), respectively. Compared to the host rutile (tetragonal) cells those of the insertion compounds are greatly expanded along [100] and [010], ～0.5 Å for both, and contracted along [001], by ～0.3 Å for Li_0.9RuO₂ and 0.05 Å for Li_0.9IrO₂. The space group for both insertion phases appears to be Pnnm, a subgroup of the rutile space group $\text{P4}{}_2\text{mnm}$. The structure of Li_0.9RuO₂ was solved from neutron diffraction data. Lithium exists as Li⁺ in octahedral sites. The LiO coordination is highly regular with two bonds at 2.05(1) Å and four at 2.08(2) Å. The overall structure is essentially an ordered NiAs-type very similar to but more regular than the previously reported LiMoO₂. Attempts to solve the structure of Li_0.9IrO₂ from both X-ray and neutron powder data were unsuccessful due, presumably, to severe preferred orientation.     

Low-temperature structural determination of anhydrous Li2SeO4

Anhydrous Li₂SeO₄ crystallizes in the trigonal space group $\text{R}\text{3}$ with a = 13.931(2), c = 9.304(3) Å, V = 1563.7 ų, Z = 18, D_c = 2.988 g cm^?3. The unit cell transforms to the rhombohedral coordinate system as a = 8.620 Å, α = 107.81(2)°, V = 521.2 ų, Z = 6. The structure contains selenate anions bridged by Li in the phenacite structural type. Data collection was performed at low temperature for precise placement of the Li cations which are tetrahedrally surrounded by oxygen atoms. Some problems with secondary extinction were apparent and a correction was made. The structure refined to an R value of 0.034.     

Crystal structures of NaBaCr2F9 and NaBaFe2F9: Structural correlations with other enneafluorides,particularly with KPbCr2F9

NaBaCr₂F₉ and NaBaFe₂F₉ are monoclinic (SG $\text{P2}{}_1\text{n}$, No. 14). Lattice constants are found to be a = 7.318(2) Å, b = 17.311(4) Å, c = 5.398(1) Å, β = 91.14°(3) for chromium, and a = 7.363(2) Å, b = 17.527(4) Å, c = 5.484(1) Å, β = 91.50°(5) for iron. The structures were solved from 507 and 1113 X-ray reflections, respectively, for the Cr and Fe compounds; the corresponding R_w values are 0.025 and 0.037. The network is built from tilted double cis chains of octahedra (M₂F₉)^3n?_n [M = Cr, Fe], linked by Na⁺ and Ba²⁺ ions. The structures are compared to the previously described structures, particularly KPbCr₂F₉, whose symmetry and parameters are different. The difference is analyzed first in terms of tilted octahedra, but principally in terms of bond strengths and steric activity of the Pb²⁺ lone pair. A mechanism is proposed for the transformation between the structures of NaBaCr₂F₉ and KPbCr₂F₉.     

Crystal structures of V3S4 and V5S8

Crystal structures of the ordered phases of V₃S₄ and V₅S₈ were refined with single crystal data. Both are monoclinic. Chemical compositions, space groups and lattice constants are as follows: VS_1.47, $\text{I2}\text{m}$ (No. 12), a = 5.831(1), b = 3.267(1), c = 11.317(2)Å, β = 91.78(1)° and VS_1.64, $\text{F}\text{2}\text{m}$ (No. 12), a = 11.396(11), b = 6.645(7), c = 11.293(4), Å, β = 91.45(6)°. In both structures, short metal-metal bonds were found between the layers as well as within them. In comparison with the structure of Fe₇S₈, the stability of NiAs-type structure was discussed based on the detailed metal-sulfur distances.     

Crystal structures of the fluorite-related phases CaHf4O9 and Ca6Hf19O44

Crystal structures for the fluorite-related phases CaHf₄O₉$\text{ф}{}_1$) and Ca₆Hf₁₉O₄₄ ($\text{ф}{}_2$) have been determined from X-ray powder diffraction data. qf₁ is monoclinic, $\text{C2}\text{c}$, with a = 17.698 Å, b = 14.500Å, c = 12.021 Å, β = 119.47° and Z = 16. qf₂ is rhombohedral, $\text{R}\text{3}\text{c}$, with a = 12.058 Å, α = 98.31° and Z = 2.Both phases are superstructures derived from the defect fluorite structure by ordering of the cations and of the anion vacancies. The ordering is such that the calcium ions are always 8-coordinated by oxygen ions, while the hafnium ions may be 6-, 7-, or 8-coordinated. The closest approach of anion vacancies is a $\text{1}\text{2}\text{〈111〉}$ fluorite subcell vector, and in each structure vacancies with this separation form strings.     

Crystal growth and properties of lead pyrophosphate,Pb2P2O7

Single crystals of Pb₂P₂O₇ have been grown by the Czochralski technique. They have the triclinic space group $\text{P}\text{1}$ with cell dimensions a = 6.9627 Å, b = 6.9754Å, c = 12.764 Å, α = 96.78°, β = 91.16°, γ = 89.68°. There are four molecules per unit cell. Dielectric properties for this compound have been measured and are discussed.     

Synthese,structure cristalline et analyse vibrationnelle de l'hexathiohypodiphosphate de lithium Li4P2S6

The crystalline compound Li₄P₂S₆ is obtained either by devitrification of Li₄P₂S₇ glass at 450°C with sulfur formation or by crystallisation at 450°C of a Li₂S, P and S melt. The structure determination has been solved by X-ray diffraction on a monocrystal. The unit cell is hexagonal $\text{P6}{}_3\text{mcm}$ with a = 6.070(4), c = 6.577(4) Å, V = 209 ų, Z = 1. Intensities were collected at 293°K with Kα (λ = 0.71069 Å) Mo radiation on an automatic Nonius CAD-4 diffractometer. The structure was solved under the assumption of random disorder of P atoms over two sites (occupancy factor of 0.5). Anisotropic least-squares refinement with W = 1 gave R = 0.047 for 90 independent reflections and 9 variables. The structure is built according to an ABAB sequence sulfur packing. Per unit cell, four out of six octahedral sites are occupied by Li ions, and the other two are statistically filled (0.5) by PP pairs. The PP central bond (2.256(13) Å) links two staggered PS₃ groups (PS = 2.032(5) Å) to form the D_3d symmetry P₂S^4?₆ anion. Infrared and Raman spectra show features very similar to those of Na₄P₂S₆, 6H₂O and M^IIPS₃ compounds. A new assignment in terms of symmetry species is proposed for the P₂S₆ internal modes, which is confirmed by a normal coordinate calculation using a valence force field; the stretching force constants f_PP and f_PS are equal to 1.6 and 2.7 mdyne Å^?1, respectively.     

L'oxyde double TeVO4 II. Structure cristalline de TeVO4-β-relations structurales

β-TeVO₄ crystallizes in the monoclinic system with the space group $\text{P2}{}_1\text{c}$ and the parameters: a = 4.379 Å, b = 13.502 Å, c = 5.446 Å, and β = 91.72°. Vanadium occupies the center of a square pyramid of oxygens, an extra oxygen is at VO = 2.77 Å. These distorted octahedra share corners forming puckered sheets parallel to (010). The sheets are held together by [Te₂O₆]^4? groups in which tellurium is one-side coordinated by four oxygen atoms.     

High pressure and high temperature investigations in the system MgOSiO2H2O

The system MgOSiO₂H₂O was investigated at pressures between 40 and 95 kbar and at temperatures between 500 and 1400°C. The reaction products were examined by X-ray, optical and thermal analysis techniques and the density of phase A discovered by Ringwood and Major was also measured. It was found that phase A was hydrated and its chemical formula was H₆Mg₇Si₂O₁₄. When the $\text{Mg}\text{Si}$ ratio of the system is 2, phase A + clinoenstatite, and forsterite are stable at temperatures lower and higher than a boundary curve T (°C) = 10P (kbar), respectively. When the $\text{Mg}\text{Si}$ ratio of the system is 3, phase A + phase D (which is completely different from the phases, A, B and C discovered by Ringwood and Major, and any other known phases of magnesium silicate) and phase D + brucite are stable at temperatures lower and higher than a boundary curve T(°C) = 10P (kbar) + 200. Phase A has approximately an hexagonal symmetry and the space group and the lattice parameters are determined as P6₃ or $\text{P6}{}_3\text{m}$ and a = 7.866(2) Å and c = 9.600(3) Å, respectively. The measured density is 2.96 ± 0.02 g/cm³. The optical observations show that phase A is biaxial positive crystal with refractive indices α = 1.638 ± 0.001, β = 1.640 ± 0.002, and γ = 1.649 ± 0.001. Some interpretation is given on the inconsistency between the symmetry determined by the X-ray diffraction and the optical observation. The new phase D belongs to the space group $\text{P2}{}_1\text{c}$ with lattice parameters a = 7.914(2)Å, b = 4.752(1) Å, c = 10.350(2) Å and β = 108.71(5)° and is a biaxial crystal with refractive indices α = 1.630 ± 0.002, β = 1.642 ± 0.002 and γ = 1.658 ± 0.001.