Structure and crystal chemistry of fluorite-related Bi38Mo7O78 from single crystal X-ray diffraction and ab initio calculations

The floating-zone furnace method was used to synthesize single crystals of the fluorite-related δ-Bi₂O₃-type phase Bi₃₈Mo₇O₇₈ 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 5×3×3 commensurate superstructure of fluorite-type δ-Bi₂O₃ in Pbcn (a=28.7058(11) Å, b=16.8493(7) Å and c=16.9376(6) Å, Z=4, R_F=11.26%, wR_I=21.67%). The structure contains stepped channels of Mo⁶⁺ in tetrahedral environments along the b axis and chains of Mo⁶⁺ 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 Mo⁶⁺, something not previously observed in Mo⁶⁺-doped δ-Bi₂O₃-type phases, is supported by charge balance considerations in addition to the results of crystallographic and ab initio analysis.     

Chiolite-like Ca5Te3O14: An X-ray and neutron diffraction study

The crystal structure of Ca₅Te₃O₁₄ at room temperature was studied by the Rietveld method using combined X-ray and neutron powder diffraction data. The compound crystallizes in the space group Cmca with the lattice parameters a=10.4268(2) Å, b=10.3908(2) Å and c=10.4702(2) Å. The structure of Ca₅Te₃O₁₄ is chiolite-like and consists of a framework of corner-linked TeO₆ octahedral layers in which a linear TeO₂ group of every fourth octahedron is substituted by a Ca atom. This type of structure was previously observed in BaSr₄U₃O₁₄. The relationship between the chiolite-like structure and the fluorite structure is discussed.     

Superstructure of a phosphor material Ba3MgSi2O8 determined by neutron diffraction data

Ba₃MgSi₂O₈, a phosphor host examined for use in white-light devices and plant-growth lamps, was synthesized at 1225 °C in air. Its crystal structure has been determined and refined by a combined powder X-ray and neutron Rietveld method (, Z=3, a=9.72411(3) Å, c=7.27647(3) Å, V=595.870(5) Å³; R_p/R_wp=3.79%/5.03%, χ²=4.20). Superstructure reflections, observed only in the neutron diffraction data, provided the means to establish the true unit cell and a chemically reasonable structure. The structure contains three crystallographically distinct Ba atoms—Ba1 resides in a distorted octahedral site with S₆ () symmetry, Ba2 in a nine-coordinate site with C₃ (3) symmetry, and Ba3 in a ten-coordinate site with C₁ (1) symmetry. The Mg atoms occupy distorted octahedral sites, and the Si atom occupies a distorted tetrahedral site.     

Structure of H(D)TaO3 determined by powder X-ray and neutron diffraction

Pure crystalline samples of HTaO₃ and DTaO₃ have been prepared. The crystal structure has been solved using powder X-ray and neutron diffraction (at 2 K and room temperature) and has been shown to consist of Ta(O,OH)₆ octahedra sharing vertices with the oxygen atoms displaced toward the vacant perovskite A sites. The compound is isomorphous with HNbO₃, D_xWO₃, and D_xReO₃ and contains hydrogen atoms as hydroxide groups.     

The crystal structure determinations and refinements of K2S2O7, KNaS2O7 and Na2S2O7 from X-ray powder and single crystal diffraction data

The crystal structures of K₂S₂O₇, KNaS₂O₇ and Na₂S₂O₇ have been solved and/or refined from X-ray synchrotron powder diffraction data and conventional single-crystal data. K₂S₂O₇: From powder diffraction data, monoclinic C2/c, Z=4, a=12.3653(2), b=7.3122(1), , β=93.0792(7)°, R_Bragg=0.096. KNaS₂O₇: From powder diffraction data; triclinic , Z=2, a=5.90476(9), b=7.2008(1), , α=101.7074(9), β=90.6960(7), γ=94.2403(9)°, R_Bragg=0.075. Na₂S₂O₇: From single-crystal data; triclinic , Z=2, a=6.7702(9), b=6.7975(10), , α=116.779(2), β=96.089(3), γ=84.000(3)°, R_F=0.033. The disulphate anions are essentially eclipsed. All three structures can be described as dichromate-like, where the alkali cations coordinate oxygens of the isolated disulphate groups in three-dimensional networks. The K-O and Na-O coordinations were determined from electron density topology and coordination geometry. The three structures have a cation-disulphate chain in common. In K₂S₂O₇ and Na₂S₂O₇ the neighbouring chains are antiparallel, while in KNaS₂O₇ the chains are parallel. The differences between the K₂S₂O₇ and Na₂S₂O₇ structures, with double-, respectively single-sided chain connections and straight, respectively, corrugated structural layers can be understood in terms of the differences in size and coordinating ability of the cations.     

Thermodynamic studies on SrFe12O19(s), SrFe2O4(s), Sr2Fe2O5(s) and Sr3Fe2O6(s)

The citrate-nitrate gel combustion route was used to prepare SrFe₂O₄(s), Sr₂Fe₂O₅(s) and Sr₃Fe₂O₆(s) powders and the compounds were characterized by X-ray diffraction analysis. Different solid-state electrochemical cells were used for the measurement of emf as a function of temperature from 970 to 1151 K. The standard molar Gibbs energies of formation of these ternary oxides were calculated as a function of temperature from the emf data and are represented as (SrFe₂O₄, s, T)/kJ mol⁻¹ (±1.7)=−1494.8+0.3754 (T/K) (970?T/K?1151). (Sr₂Fe₂O₅, s, T)/kJ mol⁻¹ (±3.0)=−2119.3+0.4461 (T/K) (970?T/K?1149). (Sr₃Fe₂O₆, s, T)/kJ mol⁻¹ (±7.3)=−2719.8+0.4974 (T/K) (969?T/K?1150).Standard molar heat capacities of these ternary oxides were determined from 310 to 820 K using a heat flux type differential scanning calorimeter (DSC). Based on second law analysis and using the thermodynamic database FactSage software, thermodynamic functions such as Δ_fH°(298.15 K), S°(298.15 K) S°(T), C_p°(T), H°(T), {H°(T)-H°(298.15 K)}, G°(T), free energy function (fef), Δ_fH°(T) and Δ_fG°(T) for these ternary oxides were also calculated from 298 to 1000 K.     

The crystal structure of the interrupted framework silicate K9.6Ca1.2Si12O30 determined from laboratory X-ray diffraction data

The crystal structure of a potassium calcium silicate with composition K_9.6Ca_1.2Si₁₂O₃₀ (or K₈CaSi₁₀O₂₅) has been solved by direct methods aided by distance least squares optimization from laboratory X-ray powder diffraction data. The trigonal compound adopts the non-centrosymmetric space group R3c with the following basic crystallographic data: a=11.13623(5) Å, c=21.9890(2) Å, V=2361.63(2) Å³, Z=3, D_calc=2.617 g cm⁻³. The crystal structure can be classified as an interrupted framework with exclusively Q³-units. It can be thought of as being built from layers parallel to (001) containing isolated six-membered tetrahedral rings in UDUDUD conformation. Corner sharing of tetrahedra belonging to adjacent sheets results in a tetrahedral framework. The framework density of the structure is 15.2 T-atoms/1000 Å³. The coordination sequences are identical for both silicon atoms in the asymmetric unit: 3-6-11-20-32-46-60-80-102-122. The vertex symbols for the two tetrahedral centers are 10₂·10₂·6₁. Topologically, the structure can be described as an Archimedean three-dimensional 3-connected net. It can be derived from the diamond or cristobalite net by removing 20% of the knots. Charge compensation in the structure is achieved by the incorporation of mono- and divalent M-cations (M: K, Ca). These extra-framework ions are coordinated by six to nine oxygen ligands. Ca/K distributions for the five symmetrically independent M-sites were obtained from a combination of bond distance considerations, site occupancy refinements and the bulk chemical composition. The structural characterization is completed by a detailed Raman spectroscopic study. Furthermore, possible implications of the structural chemistry of interrupted framework silicates for the field of silicate glass research are addressed.     

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.     

Electron microscopy of the perovskite-related phases 4H Ba0.1Sr0.9MnO2.96,5H Ba5Nb4O15 and 6H BaFeO2.79

Lattice images of 4H, 5H, and 6H perovskite polytypes have been obtained. With the electron beam parallel to 10 , the images are correlated directly with the projected structures of the polytypes. Stacking faults were found only in the 6H compound, and consisted of additional cubic close-packed AO₃ layers. Ordering of cation vacancies in the 5H material was evident in the lattice image as an array of white dots.     

Mild hydrothermal synthesis and ferrimagnetism of Pr3Fe5O12 and Nd3Fe5O12 garnets

Two pure light rare earth iron garnets Pr₃Fe₅O₁₂ and Nd₃Fe₅O₁₂ single crystals were synthesized under mild hydrothermal conditions and structurally characterized by single crystal and powder X-ray diffraction methods. Both compounds crystallize in cubic space group Ia3?d with lattice parameters a=12.670(2) Å for Pr₃Fe₅O₁₂ and a=12.633(2) Å for Nd₃Fe₅O₁₂, respectively. The synthesis of compounds was studied with regard to phase evolution and morphology development with hydrothermal conditions. We proposed the formation mechanisms and formulated a reasonable explanation for their growth habits. Ferrimagnetic Curie temperatures which have been inferred from thermo-magnetization curves were 580 K for Pr₃Fe₅O₁₂ and 565 K for Nd₃Fe₅O₁₂, and the transitions of long range order were also evidenced by differential scanning calorimetry method. The result of magnetic properties has shown that moments of the large radius Pr³⁺ and Nd³⁺ ions are parallelly coupled with net moments of iron ions.     

Neutron diffraction and X-ray absorption study of Ag5Pb2O6

Neutron diffraction and X-ray absorption measurements were carried out for a silver-lead oxide Ag₅Pb₂O₆. The powder neutron diffraction patterns could be fitted to the trigonal structure, as was found by X-ray diffraction. From the bond-valence-sum (BVS) analysis, the valences of the Ag and Pb ions were estimated to be about 1+ and 3.7+, respectively. The X-ray absorption measurements indicated that the ionic state of Ag is close to 1+, while that of Pb stands between 3+ and 4+. The deviation of the valence of the Pb ion from 4+ suggests a contribution of Pb orbitals to the metallic conduction as well as the possible superconductivity of this material, consistently with a recent band-structure calculation.     

Synthesis and crystal structure determination of yttrium ultraphosphate YP5O14

The crystal structure of monoclinic YP₅O₁₄ (space group C2/c, a=12.919(2) Å, b=12.796(4) Å, c=12.457(2) Å, β=91.30(1)°, Z=8) has been refined from single-crystal X-ray diffraction data. Full-matrix least-squares refinement on F² using 2249 independent reflections for 183 refinable parameters results in a final R value of 0.027 (ωR=0.069). The structure is isotypic with HoP₅O₁₄. This structure is built up from infinite layers of PO₄ tetrahedra linked through isolated YO₈ polyhedra. The three-dimensional cohesion of the framework results from Y-O-P bridges. This crystal structure refinement leads to the calculated X-ray diffraction powder pattern of this monoclinic polymorph, which has been the starting point of a thorough study of the solid-state synthesis of this ultraphosphate. This investigation further leads to a better outstanding of features observed during the synthesis of powdered samples. The thermal behavior of this ultraphosphate has been studied by DTA and TGA analyses. The infrared and Raman spectroscopic characterizations have been carried out on polycrystalline samples. The luminescence properties of the Eu³⁺ ion incorporated in the monoclinic C2/c polymorph of YP₅O₁₄ as local structural probe show that in YP₅O₁₄: 5% Eu³⁺ sample, the Eu³⁺ ions are distributed over the two Y³⁺ crystallographic sites of C₂ symmetry of this structure.     

The crystal structure of “Ba2Fe4S5”: A two-dimensional array of FeS4 tetrahedra

The crystal structure of a compound with a nominal composition Ba₂Fe₄S₅ was determined from 432 observed structure amplitudes. The lattice constants are a = 4.016(2), Å, b = 9.616(4), Å, c = 6.514(4), Å, Pmmn, Z = 1. The structure consists of BaS₆ trigonal prisms that share triangular faces to form infinite columns parallel to a. Zig-zag chains of Fe are formed parallel to b by filling the tetrahedral interstices. The structure can also be viewed as an infinite two-dimensional plane of FeS₄ tetrahedra formed by the sharing of two edges of a face in the ±a directions and connecting by corner sharing in the ±b directions. The Ba are in the trigonal prismatic interstices. The apical sulfur atom shows an anomalously large temperature factor and has a 50% site occupancy. On the basis of full occupancy of that site the composition corresponds to BaFe₂S₃. This structure is closely related to the previously reported BaFe₂S₃ and may be an averaged structure due to twinning of a polymorph of BaFe₂S₃.     

X-ray single crystal structures of Hg(AuF6)2 and AgFAuF6

Hg(AuF₆)₂ crystallizes at 200 K in the orthorhombic space group Pbcn (No. 60) with a = 917.67(7) pm, b = 971.59(8) pm, c = 962.04(8) pm, and Z = 4. Mercury atoms are coordinated by eight fluorine atoms with six short and two long Hg-F contacts. HgF₈ polyhedra share their four vertices and two edges with six AuF₆ units forming a tridimensional framework.The results of X-ray diffraction analysis on single crystals of AgFAuF₆ are in agreement with previously known powder X-ray diffraction data (Casteel et al, J. Solid State Chem. 96 (1992) 84-96). AgFAuF₆ crystallizes orthorhombic in the space group Pnma (No. 62), a = 717.06(7) pm, b = 761.67(7) pm, c = 1013.61(10) pm at 200 K, Z = 4.     

Low-temperature single crystal X-ray diffraction and high-pressure Raman studies on [(CH3)2NH2]2[SbCl5]

The structure of bis(dimethylammonium) pentachloroantimonate(III), [(CH₃)₂NH₂]₂[SbCl₅], BDP, was studied at 15 K and ambient pressure by single-crystal X-ray diffraction as well as at ambient temperature and high pressures up to 4.87(5) GPa by Raman spectroscopy. BDP crystallizes in the orthorhombic Pnma space group with a=8.4069(4), b=11.7973(7), c=14.8496(7) Å, and Z=4; R₁=0.0381, wR₂=0.0764. The structure consists of distorted [SbCl₆]³⁻ octahedra forming zig-zag [{SbCl₅}_n]²ⁿ⁻ chains that are cross-linked by dimethylammonium [(CH₃)₂NH₂]⁺ cations. The organic and inorganic substructures are bound together by the N-H…Cl hydrogen bonds. The distortions of [SbCl₆]³⁻ units increase, partly due to the influence of the hydrogen bonds which became stronger, with decreasing temperature. The preliminary room temperature, high-pressure X-ray diffraction experiments suggest that BDP undergoes a first-order phase transition below ca. 0.44(5) GPa that destroys single-crystal samples. The transition is accompanied by changes in the intensities and positions of the Raman lines below 400 cm⁻¹.     

Layered perovskite-related ruthenium oxychlorides: crystal structure of two new compounds Ba5Ru2Cl2O9 and Ba6Ru3Cl2O12

Single crystals of the title compounds were prepared using a BaCl₂ flux and investigated by X-ray diffraction methods using MoKα radiation and a charge coupled device (CCD) detector. The crystal structures of these two new compounds were solved and refined in the hexagonal symmetry with space group P6₃/mmc, a=5.851(1) Å, c=25.009(5) Å, ρ_cal=4.94 g cm⁻³, Z=2 to a final R₁=0.069 for 20 parameters with 312 reflections for Ba₅Ru₂Cl₂O₉ and space group , a=5.815(1) Å, c=14.915(3) Å, ρ_cal=5.28 g cm⁻³, Z=1 to a final R₁=0.039 for 24 parameters with 300 reflections for Ba₆Ru₃Cl₂O₁₂. The structure of Ba₅Ru₂Cl₂O₉ is formed by the periodic stacking along [001] of three hexagonal close-packed BaO₃ layers separated by a double layer of composition Ba₂Cl₂. The BaO₃ stacking creates binuclear face-sharing octahedra units Ru₂O₉ containing Ru(V). The structure of Ba₆Ru₃Cl₂O₁₂ is built up by the periodic stacking along [001] of four hexagonal close-packed BaO₃ layers separated by a double layer of composition Ba₂Cl₂. The ruthenium ions with a mean oxidation degree +4.67 occupy the octahedral interstices formed by the four layers hexagonal perovskite slab and then constitute isolated trinuclear Ru₃O₁₂ units. These two new oxychlorides belong to the family of compounds formulated as [Ba₂Cl₂][Ba_n+1Ru_nO_3n+3], where n represents the thickness of the octahedral string in hexagonal perovskite slabs.     

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.     

Structure de La2Fe2S5 et de La2Fe1.87S5

The compound La₂Fe₂S₅ is orthorhombic. Cell parameters are: a = 3.997(2)Å; b = 16.485(5)Å; c = 11.394(4)Å. Space group is Cmc2₁ (Z = 4. In the cell, chains of polyedra comprised of sulfur atoms tetrahedrally or octahedrally coordinating centrally located iron atoms give a monodimensional character to the structure. This one is refined to R = 0.037. To complete the study of these chains, in the La₂Fe_2?xS₅ system, vacancies are introduced on iron atom sites. The ordered compound, La₂Fe_1.87S₅, having such vacancies, is an orthorhombic superstructure of the stoechiometric compound. Cell parameters are: a = 3.9996(5)Å; b = 49.508(3)Å; c = 11.308(3)Å. Space group is Cmc2₁ and Z = 12. The structure is refined to R = 0.068. Only two iron atom sites have vacancies. One is tetrahedral, the other octahedral. In this last case the chain deformations are the more important. The chain becomes a sort of tunnel made of atoms of sulfur, with in its center the short iron-iron separation of 2.82 Å.     

Synthesis, crystal structure and characterization of iron pyroborate (Fe2B2O5) single crystals

Single crystals of iron(II) pyroborate, Fe₂B₂O₅, were prepared at 1000–1050 °C under an argon atmosphere. The crystals were transparent, yellowish in color and needle-like or columnar. The crystal structure of Fe₂B₂O₅ was analyzed by single-crystal X-ray diffraction. Refined triclinic unit cell parameters were a=3.2388(2), b=6.1684(5), c=9.3866(8) Å, α=104.613(3)°, β=90.799(2)° and γ=91.731(2)°. The final reliability factors of refinement were R1=0.020 and wR2=0.059 [I > 2σ(I)]. Transmittance over 50% in the visible light region from 500 to 750 nm was observed for a single crystal of Fe₂B₂O₅ with a thickness of about 0.3 mm. The light absorption edge estimated from a diffuse reflectance spectrum was at around 350 nm (3.6 eV). Magnetic susceptibility was measured for single crystals at 4–300 K. Fe₂B₂O₅ showed antiferromagnetic behavior below the Néel temperature, T_N≈70 K, and the Weiss temperature was T_W=36 K. The effective magnetic moment of Fe was 5.3μ_B.