Bi2n+4MonO6(n+1) with n=3, 4, 5, 6: A new series of low-temperature stable phases in the mBi2O3 - MoO3 system (1.0<m<1.7): Structural relationships and conductor properties

Four low-temperature phases with compositions Bi₁₀Mo₃O₂₄, Bi₆Mo₂O₁₅, Bi₁₄Mo₅O₃₆ and Bi₈Mo₃O₂₁ have been prepared by the n-butylamine wet synthesis method. They have been characterized by powder X-ray diffraction and transmission electron microscopy, mainly by selected area electron diffraction. The four phases present a close structural relationship and a common basic fluorite-type structure and are members of a homologous series of phases with general formula Bi_2n+4Mo_nO_6(n+1), being n=3, 4, 5 and 6, respectively. The matrices relating their superstructures and the basic fluorite type unit cell are given, as well as a general one for the whole series. The conductor behavior of these phases is characterized by impedance spectroscopy being all these materials very good ionic conductors.     

Structural and magnetic properties of the solid solution () YMn1−x(Cu3/4Mo1/4)xO3

Recently, the ferroelectromagnet YMnO₃ has been the focus of interest because it exhibits both antiferromagnetism (Néel temperature 80 K) and ferroelectricity (Curie temperature 914 K). There have been no reports of complete YMn_1−xM_xO₃ solid solutions in which substitution of the foreign M cation preserves the hexagonal P6₃cm structure. In contrast there exist several homeotypic phases with the general formula, Ln_1+nCu_nMO_3+3n (n=1 (M=Ti), 2 (M=V) and 3 (M=Mo); Ln: lanthanide). Several YMn_1−x(Cu_3/4Mo_1/4)_xO₃ compounds have been synthesized. The solid solution, from YMnO₃ (x=0) to YCu_3/4Mo_1/4O₃ (x=1) has been characterized by X-ray diffraction and transmission electron microscopy study. For 0<x<0.9, the compounds are found to crystallize in the non-centrosymmetric structure, space group P6₃cm, of YMnO₃. The Mn-free end member, x=1, crystallizes in a complex multiple cell, the superstructure being associated to Cu³⁺/Mo⁶⁺ cationic ordering. Dilution of the Mn³⁺ magnetic array by the paramagnetic (Cu²⁺) and diamagnetic (Mo⁶⁺) cations is found to decrease the antiferromagnetic ordering temperature and it becomes undetectable for x0.5 compositions.     

Crystal and magnetic structures of Sr4MMn2O9 (M=Cu or Zn)

The crystal and magnetic structures of Sr₄MMn₂O₉ (M=Cu, Zn) have been refined from neutron powder diffraction data. These trigonal compounds (space group P321, a=9.5918(1), c=7.8114(1) Å (Cu); a=9.5894(1), c=7.5039(1) Å (Zn)) are n=3 members of the series A_3n+3M_nB_n+3O_6n+9, with each unit cell containing three offset [001] polyhedral chains, each of which ideally contains a 1:1 ratio of B₂O₉ units and MO₆ trigonal prisms. In fact anti-site disorder between Mn and M is observed, and for M=Cu the cations are disordered off the center of the prism towards a rectangular face. Both compositions show 3D anti-ferromagnetic order at 1.6 K, with an ordered magnetic moment of 1.91(6) (M=Cu) or 1.8(1) (M=Zn) μ_B per Mn. No ordered magnetic moment was detected on the trigonal prismatic site in either compound, consistent with the observed temperature dependence of the magnetic susceptibility.     

Intergrowth in tunnel structures: The titanates A2Ti6O13)n·A′Ti4Og

A new structural family, (A₂M₆O₁₃)_n·A′M₄O₉, was isolated and studied by means of X-ray diffraction, electron diffraction, and electron microscopy. The structure consists of an ordered intergrowth of two types of structural units: A₂Ti₆O₁₃ and hypothetical A′M₄O₉, both characterized by zigzag ribbons of, respectively, 2 × 3 and 2 × 2 edge-sharing octahedra, joined by corner sharing to form a series of open tunnels containing A and A′ cations. The monoclinic unit-cell parameters can be deduced, for an “n” term, from those of A₂Ti₆O₁₃.     

La6Mo8O33: a new ordered defect scheelite superstructure

The structure of La₆Mo₈O₃₃ has been determined from a triple pattern powder diffraction analysis. Two high-resolution neutron diffraction patterns collected at 1.594 and 2.398 Å and one X-rays were used. This molybdate crystallizes in a non-centrosymmetric monoclinic space group P2₁(N°4), Z=2,a=10.7411(3) Å, b=11.9678(3) Å, c=11.7722(3) Å, β=116.062 (1)°. La₆Mo₈O₃₃ is an unusual ordered defect Scheelite. Hence, it should be described with cation vacancies and an extra oxygen atom following the formula: La₆□₂Mo₈O₃₂₊₁. This extra oxygen atom leads to a pyramidal environment, whereas the other molybdenum atoms present tetrahedral environment. A molybdenum tetrahedral is connecting to the pyramid, forming an [Mo₂O₉] unit.     

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.     

Unusual valences and fast electron exchange in MoFe2O4

The distribution of d electrons over the cations in MoFe₂O₄, which is represented by the formal valence assignment, is shown to be complicated by the equilibrium reactionsFe²⁺_B+Fe³⁺_A+Mo³⁺Fe³⁺_B+Fe²⁺_A+Mo⁴⁺We have used thermal treatment to confirm that the Mo are primarily on octahedral sites; Fe_A[Mo_BFe_B]O₄. K-shell absorption and Mössbauer data at T = 423 K > T_c demonstrate that the iron has an average valence near 2.5+ with fast electron transfer (τ_h < 10⁻⁸ sec) on both octahedral and tetrahedral sites. Paramagnetic susceptibility data give a Curie constant C_M = 7.95 ± 0.2 emu/mole and a Weiss constant θ_p = −445 K; magnetometer measurements confirm a compensation point near 160 K. Transport data give a surprisingly high electronic conductivity, but also give an activated mobility similar to that found in AlFe₂O₄ and CrFe₂O₄ where mixed Fe^3+/2+ valences on both A and B sites have been demonstrated. However, a positive Seebeck coefficient and a preexponential factor one order of magnitude higher in MoFe₂O₄ point to involvement of a fraction of the Mo atoms in electronic transport, which would be consistent with the observation of a τ_h < 10⁻⁸ sec on the A sites of a spinel. An energy diagram consistent with these data and other information about the relative redox potentials of these ions in oxides are proposed for this system.     

A Mixed-Valent Molybdenotungsten Monophosphate with a Tunnel Structure: Nax(Mo, W)2O3(PO4)2

A mixed-valent molybdenotungstophosphate, Na_x(Mo, W)₂O₃(PO₄)₂ (x 0.75) has been isolated for the first time. It crystallizes in the space group P 2₁/m with a = 7.200(1) Å, b = 6.369(1) Å, c = 9.123(1) Å, and β = 106.29(1)°. Its structure consists of M₂PO₁₃ units built up of two M O₆ octahedra (M = Mo, W) and one PO₄ tetrahedron sharing their apices as already observed in several molybdenum phosphates. These units share their apices with PO₄ tetrahedra forming [M₂P₂O₁₅]_∞ chains running along . The host lattice [(Mo, W)₂P₂O₁₁]_∞ can be described by the assemblage of such chains or by the assemblage of [MPO₈]_∞ chains running along , in which one PO₄ tetrahedron alternates with one MO₆ octahedron. The tridimensional framework [Mo, WP₂O₁₁]_∞ delimits tunnels running along , occupied by sodium with two kinds of coordination, 6 and 5. The distribution of the different species, in the octahedral sites according to the formulation Na_0.75(Mo^VI_0.42W^VI_0.58)_M1 (Mo^V_0.75W^VI_0.25)₂O₃(PO₄)₂, is discussed.     

Structures of three polymorphs of the complex oxide K5Yb(MoO4)4

Polymorphous modifications (γ-, β- and α-) of the double potassium ytterbium molybdenum oxide K₅Yb(MoO₄)₄ were synthesized by the solid-state method and their structures were studied by X-ray powder diffraction, electron diffraction and high-resolution electron microscopy. DSC analysis shows that the γ→β↔α phase transitions are not accompanied with a significant reconstruction of the palmierite-type structure. All modifications of K₅Yb(MoO₄)₄ are related to the mineral palmierite—K₂Pb(SO₄)₂. The palmierite-type structure is made up of isolated AO₄ tetrahedra, which connect the MO_n polyhedra into a 3-D framework via common vertices. Cations occupy two crystallographic positions M1 and M2. The γ-phase crystallizes in a monoclinic system (space group C2/c) with unit-cell parameters: a=14.8236(1) Å, b=12.1293(1) Å, c=10.5151(1) Å, β=114.559(1)°, Z=4. The α-phase has space group with unit-cell parameters: a=6.0372(1) Å, c=20.4045(2) Å. The structures of the γ- and α-modification were refined by the Rietveld method (R_wp=6.25%, R_I=2.16% and R_wp=9.09%, R_I=5.80% for γ- and α-, respectively). In K₅Yb(MoO₄)₄ ytterbium cations occupy M1 while K⁺ cations occupy M2 and M1 positions of the palmierite-type structure. In the high-temperature (α-) modification the Yb³⁺ and K⁺ occupy the M1 site in a statistical manner (M1=0.5Yb³⁺+0.5K⁺) while in the low-temperature (γ-) modification these cations occupy this site in an ordered way. The intermediate β-phase shows an incommensurate modulated structure.     

Fabrications of some kinds of 2-D frameworks consisting of nanosized polyoxomolybdate anion [Mo36O112(H2O)16] via condensation processes

We succeeded to prepare novel [Mo₃₆O₁₁₂(H₂O)₁₆]⁸⁻ ({Mo₃₆}) compounds by using 1,3-diamino-2-propanol (βOHC₃-DA) and 1,3,5-tris(aminomethyl)benzene (MES-TA)+1,3-diaminopropane (C₃-DA) as linkers, and determined their crystal structures. We have confirmed they have unique two-dimensional (2-D) molybdenum oxide frameworks, which are formed by condensation of {Mo₃₆}s. Side-staggered arrays of {Mo₃₆}s, connected in lying position by eight bridges per a {Mo₃₆}, are formed in the compound with βOHC₃-DA, while herringbone arrays of {Mo₃₆}s, connected in standing position by four bridges per a {Mo₃₆}, are built in the compound with MES-TA+C₃-DA. The latter compound exhibited non-stoichiometric property, and its composition and cell parameters varied depending on the relative concentration of MES-TA in the mother solution.     

Mo_2C/γ-Al_2O_3催化剂上苯加氢反应的原位红外光谱研究

采用程序升温反应法制备了钝化态、还原钝化态和新鲜态Mo2C/γ-Al2O3催化剂,结合原位红外光谱表征技术和反应性能评价,考察、比较了三种催化剂苯加氢反应活性.原位红外光谱结果表明,新鲜态Mo2C/γ-Al2O3催化剂在室温就显示了较好的苯加氢反应活性,表现了类贵金属的催化活性.CO吸附在反应前后新鲜态Mo2C/γ-Al2O3催化剂上的对比结果表明,低价态的Mo位(Moδ+(0δ2))是苯加氢反应活性中心.三种催化剂的反应活性结果表明,新鲜态Mo2C/γ-Al2O3催化剂反应活性最好,催化剂寿命最长,失活之后在500°C下H2处理即可恢复原有活性.     

Preparation and structural study from neutron diffraction data of Pr5Mo3O16

The title compound has been prepared as polycrystalline powder by thermal treatments of mixtures of Pr₆O₁₁ and MoO₂ in air. In the literature, an oxide with a composition Pr₂MoO₆ has been formerly described to present interesting catalytic properties, but its true stoichiometry and crystal structure are reported here for the first time. It is cubic, isostructural with CdTm₄Mo₃O₁₆ (space group Pn-3n, Z=8), with a=11.0897(1) Å. The structure contains MoO₄ tetrahedral units, with Mo-O distances of 1.788(2) Å, fully long-range ordered with PrO₈ polyhedra; in fact it can be considered as a superstructure of fluorite (M₈O₁₆), containing 32 MO₂ fluorite formulae per unit cell, with a lattice parameter related to that of cubic fluorite (a_f=5.5 Å) as a≈2a_f. A bond valence study indicates that Mo exhibits a mixed oxidation state between 5+ and 6+ (perhaps accounting for the excellent catalytic properties). One kind of Pr atoms is trivalent whereas the second presents a mixed Pr³⁺-Pr⁴⁺ oxidation state. The similarity of the XRD pattern with that published for Ce₂MoO₆ suggests that this compound also belongs to the same structural type, with an actual stoichiometry Ce₅Mo₃O₁₆.     

Synthesis and Characterization of a Novel Organic/Inorganic Hybrid Based on Octamolybdates and Benzimidazole Molecules [Hbenzimi]4 [(benzimi)2Mo8O26] · 2H2O (benzimi?=?benzimidazole)

A novel organic/inorganic compound [Hbenzimi]₄[(benzimi)₂Mo₈O₂₆] · 2H₂O (1) has been prepared hydrothermally and characterized by elemental analyses, i.r., x.p.s., t.g. and single crystal X-ray diffraction. The single crystal X-ray diffraction analysis reveals that compound (1) consists of the [Mo₈O₂₆]⁴⁻ cluster as the structural motif covalently linked by benzimidazole molecules and protonated benzimidazole molecules as charge compensation cations. It is interesting that the benzimidazole molecules were synthesized from 1,2-phenylenediamine and oxalic acid. The [Mo₈O₂₆]⁴⁻ polyoxoanions and organic ligands in compound (1) interact with each other via extensive hydrogen bonds to form a three-dimensional supramolecular framework.     

Crystal structure of the parent misfit-layered cobalt oxide [Sr2O2]qCoO2

The crystal structure of our newly discovered Sr-Co-O phase is investigated in detail through high-resolution electron microscopy (HREM) techniques. Electron diffraction (ED) measurement together with energy dispersive X-ray spectroscopy (EDS) analysis show that an ampoule-synthesized sample contains an unknown Sr-Co-O ternary phase with monoclinic symmetry and the cation ratio of Sr/Co=1. From HREM images a layered structure with a regular stacking of a CdI₂-type CoO₂ sheet and a rock-salt-type Sr₂O₂ double-layered block is observed, which confirms that the phase is the parent of the more complex “misfit-layered (ML)” cobalt oxides of [M_mA₂O_m+2]_qCoO₂ with the formula of [Sr₂O₂]_qCoO₂, i.e. m=0. It is revealed that the misfit parameter q is 0.5, i.e. the two sublattices of the CoO₂ sheet and the Sr₂O₂ block coexist to form a commensurate composite structure. We propose a structural model with monoclinic P2₁/m symmetry, which is supported by simulations of ED patterns and HREM images based on dynamical diffraction theory.     

Synthesis and structural characterization of Ba(Ln2/3B1/3)O3 (Ln=Dy, Gd and Sm; B=Mo or W) complex perovskites

We describe in this work the synthesis and crystal structure of five rare earth and Mo(VI) or W(VI) containing complex perovskites. The compounds studied are Ba(Dy_2/3Mo_1/3)O₃, Ba(Dy_2/3W_1/3)O₃, Ba(Gd_2/3Mo_1/3)O₃, Ba(Gd_2/3W_1/3)O₃ and Ba(Sm_2/3W_1/3)O₃ and were prepared starting from solutions, by the polymeric precursors method. Structural characterization by HREM, SAED and powder XRD revealed the five compounds to be ordered cubic perovskites, SG Fm-3m (225), with a cell parameter double of that of a simple perovskite cell and increasing as the size of the trivalent lanthanide ion increases (Dy<Gd<Sm).     

Mechanism of the Formation of Molybdenum Metallocarbene Complexes in the Gas Phase

The reactions of Mo⁺ ions and Mo _x O _y ⁺ oxygen-containing molybdenum cluster ions (x = 1-3; y = 1-9) with methane, ethylene oxide, and cyclopropane were studied using ion cyclotron resonance. The formation of a number of organometallic ions, including the metallocarbene MoCH₂ ⁺ , as well as molybdenum oxometallocarbenes Mo _x O _y CH₂ ⁺ (x = 1-3; y = 2, 4, 5, or 8) and Mo _x O _y (CH₄)⁺ ions (x = 1-3; y = 2, 5, or 8), was detected. The upper and lower limits of bond energies in oxometallocarbene complexes were evaluated: 111 > D ⁰ (Mo _x O _y ⁺-CH₂) > 82 kcal/mol (x = 1-3; y = 2, 5, 8).     

Structural relations between weberite and zirconolite polytypes—refinements of doped 3T and 4M Ca2Ta2O7 and 3T CaZrTi2O7

New weberite-type Ca₂Ta₂O₇ and zirconolite-type CaZrTi₂O₇ polytypes have been prepared by doping with Nd/Zr and Th/Al, respectively, and their structures have been refined using single-crystal X-ray diffraction intensity data. The 3T zirconolite polytype, Ca_0.8Ti_1.35Zr_1.3Th_0.15Al_0.4O₇, has a=7.228(1), c=16.805(1) Å. The 3T weberite-type polytype, Ca_1.92Ta_1.92Nd_0.08Zr_0.08O₇, has a=7.356(1), c=18.116(1) Å. Both 3T polytypes have space group P3₁21, Z=6. The 4M Ca₂Ta₂O₇ polytype has the same composition, from electron microprobe analyses, as the 3T polytype, and has cell parameters: a=12.761(1), b=7.358(1), c=24.565(1) Å, β=100.17(1)°, space group C2, Z=16. The structural relationships between the different zirconolite and weberite polytypes are discussed. A consideration of the structures from the viewpoint of anion-centered tetrahedral arrays shows that zirconolite can be considered as an anion-deficient fluorite derivative phase. However, the fluorite-type topology of edge-shared OM₄ tetrahedra is not maintained in the Ca₂Ta₂O₇ weberite-type polytypes, even though they have a fluorite-like fcc packing of metal atoms. One of the oxygen atoms moves from a tetrahedral Ta₃Ca interstice to an adjacent Ta₂Ca₄ octahedral interstice in the weberite polytypes.     

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.     

Cryoscopic studies in molten salts. Dissociation state of some alkali isopolymolybdates and some related molybdenum(VI) compounds in molten K2Cr2O7and KNO3

The dissociation state of the solutes M₂MoO₄, M₂Mo₃O₁₀, M₂Mo₄O₁₃, M₂Mo₅O₁₆ (MRb or Cs), Na₂CrO₄·MoO₃, K₂CrO₄·2 MoO₃, Cr₂Mo₃O₁₂ and V₂MoO₈ was studied cryoscopically in molten K₂ Cr₂O₇ and KNO₃ solvents. The freezing point depression, ΔT, of the solvents was obtained by measuring the cooling curves of the binary salt mixtures over unlimited range of solute concentration. The number of foreign ions obtained ν, showed that the solutes were either simply dissociated in the melt into the probable stable species (MoO₄)^2?, (Mo₃O₁₀)^2?, (Mo₄O₁₃)^2? and (Mo₅O₁₆)^2? or, in some cases after reactions and rearrangements, into (CrMo₂O₁₀)^2? heteropolyions. The solute V₂MoO₈, on the other hand, was found to dissolve without any apparent dissociation. An agreement between the experimental and calculated values of activity, a, based on the Temkin and Random Mixing models and that of Van't Hoff's equation support the proposed simple dissocia- tion scheme for K₂Cr₂O₇Cs₂MoO₄ system.     

Cluster intergrowth of perovskite and defect sodium chloride type structures in the K---Nb---O system: The structure of KNb4O6

A new reduced potassium niobate (KNb₄O₆) of intergrowth type structure containing condensed Nb₆O₁₂ clusters has been found. The structure has been determined from HREM images. The atomic positions have been refined with the Rietveld technique using X-ray powder diffraction data. The space group of KNb₄O₆ is P4/mmm; Z = 1, and its unit cell parameters are a = 4.1393(1) and c = 8.2537(2). KNb₄O₆ consists of alternating slabs of KNbO₃ (perovskite) and NbO (ordered deficient NaCl-type) both being a single unit thick. The structure is closely related to that of A₂Nb₅O₉ (A = Ba, Sr). Both phases can be considered as members (n = 1 and 2 respectively) of a homologous series A_nNb_3+nO_3+3n. Electron microscopy studies show the presence of defects, both as extra perovskite layers and missing NbO slabs, together with areas of more disordered intergrowth. The profile refinement and microanalysis of individual crystal fragments both indicate the structure to be niobium deficient according to the formula K_1+x/2Nb_4−xO₆.