Double (n=2) and triple (n=3) [M4Bi2n−2O2n] polycationic ribbons in the new Bi∼3Cd∼3.72M∼1.28O5(PO4)3 oxyphosphate (M=Co, Cu, Zn)

The crystal structure of the new Bi_∼3Cd_∼3.72Co_∼1.28O₅(PO₄)₃ has been refined from single crystal XRD data, R₁=5.37%, space group Abmm, a=11.5322(28) Å, b=5.4760(13) Å, c=23.2446(56) Å, Z=4. Compared to Bi_∼1.2M_∼1.2O_1.5(PO₄) and Bi_∼6.2Cu_∼6.2O₈(PO₄)₅, this compound is an additional example of disordered Bi³⁺/M²⁺ oxyphosphate and is well described from the arrangement of double [Bi₄Cd₄O₆]⁸⁺ (=D) and triple [Bi₂Cd_3.44Co_0.56O₄]⁶⁺ (=T) polycationic ribbons formed of edge-sharing O(Bi,M)₄ tetrahedra surrounded by PO₄ groups. According to the nomenclature defined in this work, the sequence is TT/DtDt, where t stands for the tunnels created by PO₄ between two subsequent double ribbons and occupied by Co²⁺. The HREM study allows a clear visualization of the announced sequence by comparison with the refined crystal structure. The Bi³⁺/M²⁺ statistic disorder at the edges of T and D entities is responsible for the PO₄ multi-configuration disorder around a central P atom. Infrared spectroscopy and neutron diffraction of similar compounds (without the highly absorbing Cadmium) even suggests the long range ordering loss for phosphates. Therefore, electron diffraction shows the existence of a modulation vector q^*=1/2a^*+(1/3+ε)b^* which pictures cationic ordering in the (001) plane, at the crystallite scale. This ordering is largely lost at the single crystal scale. The existence of mixed Bi³⁺/M²⁺ positions also enables a partial filling of the tunnels by Co²⁺ and yields a composition range checked by solid state reaction. The title compound can be prepared as a single phase and also the M=Zn²⁺ term can be obtained in a biphasic mixture. For M=Cu²⁺, a monoclinic distortion has been evidenced from XRD and HREM patterns but surprisingly, the orthorhombic ideal form can also be obtained in similar conditions.     

On the characterization of BiMO2NO3 (M=Pb, Ca, Sr, Ba) materials related with the Sillén X1 structure

The compounds BiMO₂NO₃, with M=Pb, Ca, Sr, and Ba, were obtained as single-phase products from solid-state reactions in an atmosphere of nitrous gases. The oxide nitrates with Pb and Ca crystallize in the tetragonal space group I4/mmm with two formula units per unit cell; the oxide nitrates with Sr and Ba crystallize in the orthorhombic space group Cmmm with four formula units per unit cell. Lattice parameters at room temperature are a=397.199(4), c=1482.57(2) pm for M=Pb; a=396.337(5), c=1412.83(3) pm for M=Ca; a=1448.76(3), b=567.62(1), c=582.40(1) pm for M=Sr and a=1536.50(8), b=571.67(3), c=597.55(3) pm for M=Ba. The structures, which were refined by powder X-ray diffraction, consist of alternating [BiMO₂]⁺ and [NO₃]⁻ layers stacked along the direction of the long axis. IR and thermogravimetric data are also given. The various M²⁺ cations in BiMO₂NO₃ are compatible with each other; therefore and because of their layer-type structure, these compounds are interesting precursors for oxide materials, e.g., the HTSC compounds (Bi,Pb)₂Sr₂Ca_n−1Cu_nO_x.     

Ab initio structure determination and Rietveld refinement of Bi10Mo3O24 the member n=3 of the Bi2n+4MonO6(n+1) series

Bi₂O₃-MoO₃ system shows a large panoply of phases depending on Bi/Mo ratio, among them, the low temperature phases of the homologous series Bi_2(n+2)Mo_nO_6(n+1) with n=3, 4, 5 and 6. They exhibit, alike most of the phases of this system, strong fluorite sub-network. Nevertheless, a multitechnique approach has been followed in order to solve the crystal structure of the n=3 member, i.e. Bi₁₀Mo₃O₂₄. From ab initio indexing X-ray powder pattern cell parameters were derived. It belongs to the monoclinic system, space group C2, with cell parameters: a=23.7282(2) Å, b=5.64906(6) Å, c=8.68173(9) Å, β=95.8668(7)° with Z=2. The matrix relating this cell with the fluorite one is 4 0 1/0 1 0/ 0  and a cationic localization was derived. HRTEM allowed the cationic Bi and Mo order to be modified and specified, as well as to build up a full structural ab initio model on the basis of crystal chemistry considerations. Simultaneous Rietveld refinement of multipattern X-ray and neutron powder diffraction data taking advantage of the neutron scattering length for O location have been performed. The goodness of the model was ascertained by low reliability factors, weighted R_b=4.97% and R_f=3.21%. This complex Bi₁₀Mo₃O₂₄ structure, with 5Bi, 2Mo and 13O in different crystallographic positions of the asymmetric unit, shows good agreement between observed and calculated patterns within the data resolution. Moreover, the determination of this structure sets the basis for the crystallographic characterization of the complete family Bi_2(n+2)Mo_nO_6(n+1), whose guidelines are also evidenced in this paper.     

Crystal structures and phase transitions of Sr2CrSbO6

Sr₂CrSbO₆ was synthesized by the conventional solid-state reaction process. X-ray powder diffraction (XRPD) and neutron powder diffraction (NPD) has been used to reinvestigate the structure at room temperature and to study the phase transitions at high- and low-temperature. Rietveld analysis revealed that Sr₂CrSbO₆ crystallizes at room temperature in a monoclinic system having a space group I2/m, with a=5.5574(1) Å; b=5.5782(1) Å; c=7.8506(2) Å and β=90.06(2), no P2₁/n space group as was previously reported. The high-temperature study (300-870 K) has shown that the compound presents the following temperature induced phase-transition sequence: I2/m-I4/m-Fm-3m. The low-temperature study (100-300 K) demonstrated that the room-temperature I2/m monoclinic symmetry seems to be stable down to 100 K.     

Synthesis, crystal structure and magnetic properties of a new pillared perovskite La5Mo2.75V1.25O16

A new pillared perovskite compound La₅Mo_2.76(4)V_1.25(4)O₁₆, has been synthesized by solid-state reaction and its crystal structure has been characterized using powder X-ray and neutron diffraction. The magnetic properties of this compound have been investigated using SQUID magnetometry, and the magnetic structure has been studied using neutron diffraction data. A theoretical calculation of relative strengths of spin interactions among different magnetic ions and through different pathways has been performed using extended Hückel, spin dimer analysis. The crystal structure of this material contains perovskite-type layers that are connected through edge-sharing dimeric units of octahedra. The structure is described in space group C2/m with unit cell parameters a=7.931(2) Å, b=7.913(2) Å, c=10.346(5) Å and β=95.096(5)°. The material shows both short-range ferrimagnetic correlations from ∼200 to 110 K and long-range antiferromagnetic order below T_c∼100 K. The magnetic structure was investigated by neutron diffraction and is described by k=(0 0 ) as for other pillared perovskites. It consists of a ferrimagnetic arrangement of Mo and V within the layers that are coupled antiferromagnetically between layers. This is the first magnetic structure determination for any Mo-based pillared perovskite.     

Crystal structures and chemistry of double perovskites Ba2M(II)M′(VI)O6 (M=Ca, Sr, M′=Te, W, U)

Structures of the double perovskites Ba₂M(II)M ′(VI)O₆ (M=Ca, Sr, M′=Te, W, U) at room temperature have been investigated by the Rietveld method using X-ray and neutron powder diffraction data. For double perovskites with M=Sr, the observed space groups are I2/m (M′ =W) and (M′=Te), respectively. In the case of M=Ca, the space groups are either monoclinic P2₁/n (M′=U) or cubic (M′=W and Te). The tetragonal and orthorhombic symmetry reported earlier for Ba₂SrTeO₆ and Ba₂CaUO₆, respectively, were not observed. In addition, non-ambient X-ray diffraction data were collected and analyzed for Ba₂SrWO₆ and Ba₂CaWO₆ in the temperature range between 80 and 723 K. It was found that the rhombohedral structure exists in Ba₂SrWO₆ above room temperature between the monoclinic and the cubic structure, whereas the cubic Ba₂CaWO₆ undergoes a structural phase transition at low temperature to the tetragonal I4/m structure.     

Crystal structure and properties of the new vanadyl(IV)phosphates Na2MVO(PO4)2, M=Ca and Sr

Two new complex vanadyl(IV)phosphates Na₂MVO(PO₄)₂ (M=Ca, Sr) were synthesized in evacuated quartz ampoules and investigated by means of X-ray diffraction, electron microscopy, DTA, ESR and magnetic susceptibility measurements. The crystal structure of Na₂SrVO(PO₄)₂ was solved ab initio from X-ray powder diffraction data. Both compounds are isostructural: a=10.5233(3) Å, b=6.5578(2) Å, c=10.0536(3) Å and a=10.6476(3) Å, b=6.6224(2) Å, c=10.2537(3) Å for Ca and Sr, respectively; S.G. Pnma, Z=4. The compounds have a three-dimensional structure consisting of V⁴⁺O₆ octahedra connected by PO₄ tetrahedra via five of the six vertexes forming a framework with cross-like channels. The strontium and sodium atoms are located in the channels in an ordered manner. Electron diffraction as well as high-resolution electron microscopy confirmed the structure solution. The new vanadylphosphates are Curie-Weiss paramagnets in a wide temperature range down to 2 K with θ=12 and 5 K for Ca and Sr phases, respectively.     

Magnetic ordering and spin structure in Ca-bearing clinopyroxenes CaM(Si, Ge)2O6, M=Fe, Ni, Co, Mn

The compounds CaFeSi₂O₆ (hedenbergite), CaNiGe₂O₆, CaCoGe₂O₆ and CaMnGe₂O₆ have been synthesized by hydrothermal or ceramic sintering techniques and were subsequently characterized by SQUID magnetometry and powder neutron diffraction in order to determine the magnetic properties and the spin structure at low temperature. All four compounds reveal the well-known clinopyroxene structure-type with monoclinic symmetry, space group C2/c, Z=4 at all temperatures investigated. Below 35 K hedenbergite shows a ferromagnetic (FM) coupling of spins within the infinite M1 chains of edge-sharing octahedra. This FM coupling dominates an antiferromagnetic (AFM) coupling between neighbouring chains. The magnetic moments lie within the a-c plane and form an angle of 43° with the crystallographic a-axis. Magnetic ordering in CaFeSi₂O₆ causes significant discontinuities in lattice parameters, Fe-O bond lengths and interatomic Fe-Fe distances through the magnetic phase transition, which could be detected from the Rietveld refinements of powder neutron diffraction data. CaCoGe₂O₆ and CaNiGe₂O₆ show magnetic ordering below 18 K, the spin structures are similar to the one in hedenbergite with an FM coupling within and an AFM coupling of spins between the M1 chains. The moments lie within the a-c plane. The paramagnetic Curie temperature, however, decreases from CaFeSi₂O₆ (+40.2 K) to CaCoGe₂O₆ (+20.1 K) and CaNiGe₂O₆ (−13.4 K), suggesting an altered interplay between the concurring AFM and FM interaction in and between the M1 chains. CaMnGe₂O₆ finally shows an AFM ordering below 11 K. Here the magnetic moments are mainly oriented along the a-axis with a small tilt out from the a-c plane.     

Structural phase transition and magnetic properties of double perovskites Ba2CaMO6 (M=W, Re, Os)

Structures and magnetic properties for double perovskites Ba₂CaMO₆ (M=W, Re, Os) were investigated. Both Ba₂CaReO₆ and Ba₂CaWO₆ show structural phase transitions at low temperatures. For Ba₂CaReO₆, the second order transition from cubic to tetragonal I4/m has been observed near 120 K. For Ba₂CaWO₆, the space group of the crystal structure is I4/m at 295 K and the transition to monoclinic I2/m has been observed between 220 K. Magnetic susceptibility measurements show that Ba₂CaReO₆ (S=1/2) and Ba₂CaOsO₆ (S=1) transform to an antiferromagnetic state below 15.4 and 51 K, respectively. Anomalies corresponding to their structural phase transition and magnetic transition have been also observed through specific heat measurements.     

Metallic Re-Re bond formation in different MRe2O6 (MFe, Co, Ni) rutile-like polymorphs: The role of temperature in high-pressure synthesis

Different polymorphs of MRe₂O₆ (MFe, Co, Ni) with rutile-like structures were prepared using high-pressure high-temperature synthesis. For syntheses temperatures higher than ∼1573 K, tetragonal rutile-type structures (P4₂/mnm) with a statistical distribution of M- and Re-atoms on the metal position in the structure were observed for all three compounds, whereas rutile-like structures with orthorhombic or monoclinic symmetry, partially ordered M- and Re-ions on different sites and metallic Re-Re-bonds within Re₂O₁₀-pairs were found for CoRe₂O₆ and NiRe₂O₆ at a synthesis temperature of 1473 K. According to the XPS measurements, a mixture of Re⁺⁴/Re⁺⁶ and M²⁺/M³⁺ is present in both structural modifications of CoRe₂O₆ and NiRe₂O₆. The low-temperature forms contain more Re⁺⁴ and M³⁺ than the high-temperature forms. Tetragonal and monoclinic modifications of NiRe₂O₆ order with a ferromagnetic component at ∼24 K, whereas tetragonal and orthorhombic CoRe₂O₆ show two magnetic transitions: below ∼17.5 and 27 K for the tetragonal and below 18 and 67 K for the orthorhombic phase. Tetragonal FeRe₂O₆ is antiferromagnetic below 123 K.     

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)).     

Phase formation, structural and microstructural characterization of novel oxynitride-perovskites synthesized by thermal ammonolysis of (Ca,Ba)MoO4 and (Ca,Ba)MoO3

Reactions of AMoO₄ and AMoO₃ (A=Ca²⁺, Ba²⁺) with ammonia were investigated at 873 K<T<1123 K with the particular intention to synthesize novel oxynitride-perovskites of the general composition AMo(O,N)₃ and to study their crystal structure. CaMo(O,N)₃ and BaMo(O,N)₃ were prepared by thermal ammonolysis of the corresponding CaMoO₃ and BaMoO₃ precursors at T=898 and 998 K, respectively. The structural parameters of the oxynitrides were obtained from Rietveld refinements of X-ray and neutron powder diffraction data. CaMo(O,N)₃ crystallizes in the GdFeO₃ distorted perovskite structure with orthorhombic space group Pbnm and a=5.5029(1) Å, b=5.5546(1) Å, c=7.8248(1) Å as determined by X-ray powder diffraction. Its O/N content refined from the neutron diffraction data corresponds to the composition CaMoO_1.7(1)N_1.3(1). BaMo(O,N)₃ crystallizes in the cubic perovskite structure with space group Pm3¯m and a=4.0657(1) Å as determined by X-ray powder diffraction. Transmission electron microscopy reveals a complex microstructure for both CaMoO₃ and CaMoO_1.7(1)N_1.3(1) represented by twin domains of different orientation.     

Two new titanium molybdenum arsenides: Ti2MoAs2 and Ti3MoAs3, ternary substitution variants of V3As2 and β-V4As3

The title compounds were prepared by arc-melting pre-annealed mixtures of Ti, Mo, and As. Both Ti₂MoAs₂ and Ti₃MoAs₃ adopt structures formed by the corresponding binary vanadium arsenides, V₃As₂ and β-V₄As₃. Ti₂MoAs₂ crystallizes in the tetragonal space group P4/m, with a=9.706(4) Å, c=3.451(2) Å, V=325.1(3) Å³ (Z=4), and Ti₃MoAs₃ in the monoclinic space group C2/m, with a=14.107(3) Å, b=3.5148(7) Å, c=9.522(2) Å, β=100.66(3)°, V=464.0(2) Å³ (Z=4). In both cases, the metal atoms form infinite chains of trans edge-condensed octahedra, and the As atoms are located in (capped) trigonal prismatic voids. While most metal atom sites exhibit mixed Ti/Mo occupancies, the Mo atoms prefer the sites with more metal atom and fewer As atom neighbors. Ti₂MoAs₂ and Ti₃MoAs₃ are metallic entropy-stabilized materials that decompose upon annealing at intermediate temperatures.     

Preparation and crystal structure of a new bismuth chromate: Bi8(CrO4)O11

Single crystals of a new bismuth chromate, Bi₈(CrO₄)O₁₁, were prepared by hydrothermal reaction of NaBiO₃·nH₂O in K₂CrO₄ solution. The bismuth chromate crystallizes in the monoclinic space group P2₁/m with a=9.657(3), b=11.934(3), c=13.868(2)Å and β=104.14(1)°, Z=4 and the final R factors are R=0.038 and R_w=0.041 for 3541 unique reflections. The crystal structure has a slab built up by (CrO₄)²⁻ tetrahedra and distorted bismuth polyhedra which are five-fold pyramids, six-fold trigonal prisms and octahedra. The distance of lone pair from nucleus for bismuth atoms ranges from 0.29 to 1.12 Å, depending on the coordination environment. Bi₈(CrO₄)O₁₁ decomposes to Bi₁₄CrO₂₄ and a small amount of an unknown phase at 796 °C.     

Synthesis and crystal structure of new uranyl tungstates M2(UO2)(W2O8) (M=Na, K), M2(UO2)2(WO5)O (M=K, Rb), and Na10(UO2)8(W5O20)O8

The solid-state reactions of UO₃ and WO₃ with M₂CO₃ (M=Na, K, Rb) at 650°C for 5 days result, accordingly the starting stoichiometry, in the formation of M₂(UO₂)(W₂O₈) (M=Na (1), K (2)), M₂(UO₂)₂(WO₅)O (M=K (3), Rb (4)), and Na₁₀(UO₂)₈(W₅O₂₀)O₈ (5). The crystal structures of compounds 2, 3, 4, and 5 have been determined by single-crystal X-ray diffraction using Mo(Kα) radiation and a charge-coupled device detector. The crystal structures were solved by direct methods and Fourier difference techniques, and refined by a least-squares method on the basis of F² for all unique reflections. For (1), unit-cell parameters were determined from powder X-ray diffraction data. Crystallographic data: 1, monoclinic, a=12.736(4) Å, b=7.531(3) Å, c=8.493(3) Å, β=93.96(2)°, ρ_cal=6.62(2) g/cm³, ρ_mes=6.64(1) g/cm³, Z=4; 2, orthorhombic, space group Pmcn, a=7.5884(16) Å, b=8.6157(18) Å, c=13.946(3) Å, ρ_cal=6.15(2) g/cm³, ρ_mes=6.22(1) g/cm³, Z=8, R1=0.029 for 80 parameters with 1069 independent reflections; 3, monoclinic, space group P2₁/n, a=8.083(4) Å, b=28.724(5) Å, c=9.012(4) Å, β=102.14(1)°, ρ_cal=5.83(2) g/cm³, ρ_mes=5.90(2) g/cm³, Z=8, R1=0.037 for 171 parameters with 1471 reflections; 4, monoclinic, space group P2₁/n, a=8.234(1) Å, b=28.740(3) Å, c=9.378(1) Å, β=104.59(1)°, ρ_cal=6.13(2) g/cm³,  g/cm³, Z=8, R1=0.037 for 171 parameters with 1452 reflections; 5, monoclinic, space group C2/c, a=24.359(5) Å, b=23.506(5) Å, c=6.8068(14) Å, β=94.85(3)°, ρ_cal=6.42(2) g/cm³,  g/cm³, Z=8, R1=0.036 for 306 parameters with 5190 independent reflections. The crystal structure of 2 contains linear one-dimensional chains formed from edge-sharing UO₇ pentagonal bipyramids connected by two octahedra wide (W₂O₈) ribbons formed from two edge-sharing WO₆ octahedra connected together by corners. This arrangement leads to [UW₂O₁₀]²⁻ corrugated layers parallel to (001). Owing to the unit-cell parameters, compound 1 probably contains similar sheets parallel to (100). Compounds 3 and 4 are isostructural and the structure consists of bi-dimensional networks built from the edge- and corner-sharing UO₇ pentagonal bipyramids. This arrangement creates square sites occupied by W atoms, a fifth oxygen atom completes the coordination of W atoms to form WO₅ distorted square pyramids. The interspaces between the resulting [U₂WO₁₀]²⁻ layers parallel to plane are occupied by K or Rb atoms. The crystal structure of compound 5 is particularly original. It is based upon layers formed from UO₇ pentagonal bipyramids and two edge-shared octahedra units, W₂O₁₀, by the sharing of edges and corners. Two successive layers stacked along the [100] direction are pillared by WO₄ tetrahedra resulting in sheets of double layers. The sheets are separated by Na⁺ ions. The other Na⁺ ions occupy the rectangular tunnels created within the sheets. In fact complex anions W₅O₂₀¹⁰⁻ are built by the sharing of the four corners of a WO₄ tetrahedron with two W₂O₁₀ dimmers, so, the formula of compound 5 can be written Na₁₀(UO₂)₈(W₅O₂₀)O₈.     

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.     

Two isotypic diphosphates LiM2H3(P2O7)2 (M=Ni, Co) containing ferromagnetic zigzag MO6 chains

Two new isotypic phosphates LiNi₂H₃(P₂O₇)₂ (1) and LiCo₂H₃(P₂O₇)₂ (2) have been hydrothermally synthesized and structurally characterized by the single-crystal X-ray diffraction technique. They crystallize in the monoclinic space group C2/c with the lattice: a=10.925(2) Å, b=12.774(3) Å, c=8.8833(18) Å, β=123.20(3)° for 1 and a=10.999(2) Å, b=12.863(3) Å, c=8.9419(18) Å, β=123.00(3)° for 2. The transition metal atoms are octahedrally coordinated, whereas the lithium and phosphorus atoms are all tetrahedrally coordinated. As the lithium-induced derivatives of MH₂P₂O₇ (M=Ni, Co), 1 and 2 possess the same structure with MH₂P₂O₇ in terms of topology, comprising the MO₆ zigzag chains and P₂O₇ as the interchain groups. The magnetisms of 1 and 2 could be interpreted by adopting a quasi-one-dimensional (1D) zigzag chain model as that in their parent compounds: both 1 and 2 have ferromagnetic (FM) NiO₆/CoO₆ chains; 1 shows a FM cluster glass behavior at low temperatures, which is originated from the possible antiferromagnetic (AFM) next-nearest-neighbour intrachain interactions; 2 shows a AFM ordering at T_N=2.6 K and a metamagnetic transition at H_C=4.2 kOe at 1.8 K.     

Synthesis and Structure of New BiMn2MO6 Compounds Where M=P, As, or V

The new compounds BiMn₂PO₆, BiMn₂AsO₆, and BiMn₂ VO₆ have been prepared and shown to be structurally related to several other BiA₂MO₆ compounds. The structure of BiMn₂PO₆ was refined from neutron powder diffraction data in space group Pnma with a=12.04 Å, b=5.37 Å, c=8.13 Å, and Z=4. It contains (BiO₂)¹⁻ chains and (PO₄)³⁻ tetrahedra. The observed fivefold coordination for the Mn²⁺ cations is unusual for Mn in this oxidation state.     

Synthesis, structure and magnetic properties of the new mixed-valence vanadate Na2SrV3O9

The new complex oxide Na₂SrV₃O₉ was synthesized and investigated by means of X-ray diffraction, electron microscopy and magnetic susceptibility measurements. This oxide has a monoclinic unit cell with parameters a=5.416(1) Å, b=15.040(3) Å, c=10.051(2) Å, β=97.03(3)°, space group P2₁/c and Z=4. The crystal structure of Na₂SrV₃O₉, as determined from X-ray single-crystal data, is built up from isolated chains formed by square V⁴⁺O₅ pyramids. Neighboring pyramids are linked by two bridging V⁵⁺O₄ tetrahedra sharing a corner with each pyramid. The Na and Sr atoms are situated between the chains. Electron diffraction and HREM investigations confirmed the crystal structure. The temperature dependence of the susceptibility indicates low-dimensional magnetic behavior with a sizeable strength of the magnetic intra-chain exchange J of the order of 80 K, which is very likely due to superexchange through the two VO₄ tetrahedra linking the magnetic V⁴⁺ cations.     

Synthesis crystal structure and ionic conductivity of Ca0.5Bi3V2O10 and Sr0.5Bi3V2O10

Two new compounds Ca_0.5Bi₃V₂O₁₀ and Sr_0.5Bi₃V₂O₁₀ have been synthesized in the ternary system: MO-Bi₂O₃-V₂O₅ system (M=M²⁺). The crystal structure of Sr_0.5Bi₃V₂O₁₀ has been determined from single crystal X-ray diffraction data, space group and Z=2, with cell parameters a=7.1453(3) Å, b=7.8921(3) Å, c=9.3297(3) Å, α=106.444(2)°, β=94.088(2)°, γ=112.445(2)°, V=456.72(4) Å³. Ca_0.5Bi₃V₂O₁₀ is isostructural with Sr_0.5Bi₃V₂O₁₀, with, a=7.0810(2) Å, b=7.8447(2) Å, c=9.3607(2) Å, α=106.202(1)°, β=94.572(1)°, γ=112.659(1)°, V=450.38(2) Å³ and its structure has been refined by Rietveld method using powder X-ray data. The crystal structure consists of infinite chains of (Bi₂O₂) along c-axis formed by linkage of BiO₈ and BiO₆ polyhedra interconnected by MO₈ polyhedra forming 2D layers in ac plane. The vanadate tetrahedra are sandwiched between these layers. Conductivity measurements give a maximum conductivity value of 4.54×10⁻⁵ and 3.63×10⁻⁵ S cm⁻¹ for Ca_0.5Bi₃V₂O₁₀ and Sr_0.5Bi₃V₂O₁₀, respectively at 725 °C.