Syntheses and characterization of zero-dimensional molybdoantimonites, A2(Mo4Sb2O18) (A=Y, La, Nd, Sm, Gd and Dy)

Six new isostructural A₂(Mo₄Sb₂O₁₈) (A=Y, La, Nd, Sm, Gd and Dy) compounds have been synthesized by solid-state reactions and characterized by single crystal X-ray diffraction and spectroscopic techniques. They crystallize in C2/c space group with 4 formula units and contain A³⁺ cations and discrete centrosymmetric anionic (Mo₄Sb₂O₁₈)⁶⁻ aggregates, made of tetrahedral MoO₄ and disphenoidal SbO₄ moieties. They exhibit characteristic Sb³⁺ photoluminescence.     

New alkali-metal-molybdenum(VI)-selenium(IV) oxides: syntheses, structures, and characterization of A2SeMoO6 (A=Na, K, or Rb)

Three new quaternary selenites, A₂SeMoO₆ (A=Na⁺, K⁺, or Rb⁺), were synthesized through the solid-state reaction of A₂MoO₄ with SeO₂ at 400°C. Although the reported materials are ‘stoichiometrically equivalent’, the compounds exhibit strikingly different crystal structures. Whereas Na₂SeMoO₆ has a three-dimensional crystal structure, K₂SeMoO₆ and Rb₂SeMoO₆ are molecular and uni-dimensional, respectively. However, all of the new materials have structures containing Mo⁶⁺ octahedra linked to Se⁴⁺ trigonal pyramids. Although the Mo⁶⁺ and Se⁴⁺ cations are in local asymmetric environments in all three materials, only Na₂SeMoO₆ is non-centrosymmetric. Single crystal X-ray data: Na₂SeMoO₆, cubic, space group, P2₁3 (no. 198), a=8.375(5) Å, Z=4, R(F)=0.0143; K₂SeMoO₆, monoclinic, space group, P2₁/c (no. 14), a=6.118(8) Å, b=15.395(2) Å, c=7.580(9) Å, β=112.39(4)°, Z=4, R(F)=0.0281; Rb₂SeMoO₆, orthorhombic, space group, Pnma (no. 62), a=7.805(9) Å, b=6.188(7) Å, c=14.405(4) Å, Z=4, R(F)=0.0443.     

Expanding the Hierarchy of Metal-Oxide Building Blocks from Fragments via Clusters to Networks: A ∞2 [({Mo17(NO)2}3{MoV2}3{FeIII}6)(FeII1.5)]-Type Layer Compound

A planar network consisting of {Mo₁₇(NO)₂}₃{Mo^V ₂}₃{Fe₆^III} cluster entities that are interlinked to layers via {Fe^II(H₂O)₄}²⁺ groups is formed stepwise from building units. The corresponding mixed-valence compound exhibits a variety of different formal oxidation states: {MoNO}³⁺, Mo^V, Mo^VI, Fe^II, and Fe^III. This compound also represents an extension of building-block hierarchy from the molecular level to extended networks.     

Investigations of molybdenum redox phenomenon in Li2O-MoO3-P2O5 phosphate glasses

Two series of glasses have been prepared and characterized. One with varying Li₂O/P₂O₅ ratio and the other with varying Mo/P ratio. The relationship between the formation of the reduced state of molybdenum in phosphate glasses and the type of gases released in heating batch materials has been investigated. Effect of temperature on the valence state of molybdenum is also studied. Oxidation-reduction (redox) equilibrium of Mo⁵⁺/Mo⁶⁺ and environment of molybdenum (V) in these series of lithium-molybdenum-phosphate glasses are related to the glass composition and the possible structural units formation in the glasses.     

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.     

Paramagnetic defects in α-WxV2O5

Paramagnetic defects in α-W_xV₂O₅ have been studied by ESR. A model is proposed where the unpaired electron arising from a valence induction effect remains localized on a single vanadium ion near the W⁶⁺ along the b direction. Introducing W⁶⁺ leads to a lattice distortion which is more important than that in the case of Mo⁶⁺. A slight displacement of vanadium along the a direction is observed in the defect, V⁴⁺ showing a stronger tendency toward octahedral coordination than V⁵⁺.     

Syntheses, structure and rare earth metal photoluminescence of new and known isostructural A2Mo4Sb2O18 (A=Ce, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) compounds

Nine new A₂Mo₄Sb₂O₁₈ (A=Ce, Pr, Eu, Tb, Ho, Er, Tm, Yb, Lu) compounds have been synthesized by solid-state reactions. They are isostructural with six reported analogues of yttrium and other lanthanides and the monoclinic unit cell parameters of all fifteen of them vary linearly with the size of A³⁺ ion. Single crystal X-ray structures of eight A₂Mo₄Sb₂O₁₈ (A=Ce, Pr, Eu, Gd, Tb, Ho, Er, Tm) compounds have been determined. Neat A₂Mo₄Sb₂O₁₈ (A=Pr, Sm, Eu, Tb, Dy, Ho, Er, Tm) compounds exhibit characteristic rare earth metal photoluminescence.     

Hydrothermal Synthesis and Characterization of (H3N(CH2)4NH3)[V6O14]

A new layered vanadium oxide [H₃N(CH₂)₄NH₃](V₆O₁₄) was synthesized hydrothermally under autogenous pressure at 180°C for 48 h from a mixture of H₂N(CH₂)₄NH₂ and V₂O₅ in aqueous solution. Its structure was determined from single-crystal X-ray diffraction at room temperature with final R=0.0774 and R_w=0.0893. It crystallizes in the monoclinic system (space group P2₁/n with a=9.74(2) Å, b=6.776(5) Å, c=12.60(2) Å, β=96.1(1)°, V=827(2) ų and Z=2). This compound contains mixed-valence V⁵⁺/V⁴⁺ vanadium oxide layers built from [V^VO₄] tetrahedra and pairs of edge-sharing [V^IVO₅] square pyramids with protonated organic amines occupying the interlayer space.     

Different [Bi12O14]n Columnar Structural Types in the Bi-Mo-Cr-O System: Synthesis, Structure, and Electrical Properties of the Solid Solution Bi26Mo10−xCrxO69

In order to search for new ionic conductor materials exhibiting a columnar [Bi₁₂O₁₄] structural type, the syntheses of the solid solutions Bi₂Mo_1−xCr_xO₆ and Bi₂₆Mo_10−xCr_xO₆₉ have been undertaken. Single phases were obtained for the last composition with 0≤x≤5 homogeneity range. Moreover, a new oxide with Bi₆Cr₂O₁₅ composition has been obtained from the limit nominal stoichiometries Bi₆CrO₆ and Bi₂₆Cr₁₀O₆₉. X-ray powder diffraction studies have shown that this oxide crystallizes in the orthorhombic system, space group Ccc2 or Cccm, with unit-cell parameters a=19.8986(9) Å, b=12.2756(6) Å, c=5.8868(3) Å, and V=1437.96 ų. Impedance spectroscopy measurements carried out on the representative Bi₂₆Mo₈Cr₂O₆₉ phase, showed that this material is a good oxygen ion conductor, in fact the best one belongs to the columnar structural type, with a conductivity as high as 1.7×10⁻³Scm⁻¹ at 425°C.     

Synthesis, crystal structure, and physical properties of the monoclinic form of the R4Mo4O11 compounds (R=Yb and Lu) containing infinite chains of trans-edge-shared octahedral clusters

Polycrystalline samples and single crystals of the R₄Mo₄O₁₁ compounds (R=Yb and Lu) were synthesized by solid-state reactions at high temperature in sealed Mo crucibles. The structure of Lu₄Mo₄O₁₁ (a=10.5611(1), b=5.61930(5), c=15.6877 (2), β=99.5131(4) and Z=4) was determined by single crystal X-ray diffraction and refined by least squares on F² converging to R₁=0.0425, wR₂=0.0980 for 3508 intensities. Contrary to the R₄Mo₄O₁₁ compounds with lighter rare earths, which crystallize in the orthorhombic space group Pbam, the Yb and Lu compounds crystallize in the monoclinic space group P2/m. The R₄Mo₄O₁₁ compounds contain distorted infinite oxide-molybdenum chains of trans-edge-sharing Mo₆ octahedra diluted with the rare earths. Magnetic susceptibility measurements indicate that the oxidation state of the Yb atoms is +3, affording 14 metallic valence electrons per Mo₄ fragment and, the absence of localized moments on the Mo network. Resistivity measurements on single crystals show that the Yb₄Mo₄O₁₁ and Lu₄Mo₄O₁₁ compounds are small band-gap semi-conductors.     

Mechanisms of molybdenum substitution in vanadium antimonate

The formation of a solid solution containing the three elements V, Sb and Mo, which are key-elements in the design of light alkane oxidation catalysts, has been studied by incorporating molybdenum into the pure VSbO₄ compound as obtained in air at 700°C (V³⁺_0.28V⁴⁺_0.64□_0.16Sb⁵⁺_0.92O₄). Monophasic compounds with a rutile-type structure have been obtained and characterized by X-ray diffraction, electron microscopy, Infrared Fourier transform, X-ray absorption and electron spin resonance spectroscopies. At low molybdenum content, Mo⁶⁺ substitute V⁴⁺ in the cationic-deficient structure. The charge balance is maintained by an increase of the cationic vacancy number. This leads to the formation of a solid solution corresponding to the formula V³⁺_0.28V⁴⁺_0.64−3xMo⁶⁺_2x□_0.16+xSb⁵⁺_0.92O₄ with 0<x<0.09. At higher molybdenum content, Mo⁵⁺ are stabilized and substitute Sb⁵⁺ in the rutile structure: V³⁺_0.28V⁴⁺_0.37Mo⁶⁺_0.18□_0.25Mo⁵⁺_ySb⁵⁺_0.9−yO₄ with 0<y<0.06. At higher molybdenum content the rutile phase is no longer stable and two new phases are formed: Sb₂O₄ and a new mixed vanadium molybdenum antimonate.     

Subsolidus phase diagram of Cu2OCuOMoO3 system

Five chemical compounds, CuMoO₄, Cu₃Mo₂O₉, Cu₂Mo₃O₁₀, Cu₆Mo₄O₁₅, and Cu_4?x Mo₃O₁₂ (0.10 ? x ? 0.40), were identified in the system Cu₂OCuOMoO₃ and characterized by DTA, X-ray powder patterns, ir spectra, and magnetic properties. Cupric molybdates CuMoO₄ and Cu₃Mo₂O₉ are stable in air up to 820 and 855°C, respectively, melting at these temperatures with simultaneous decomposition (oxygen loss). Congruent mp of cuprous molybdates Cu₂Mo₃O₁₀ and Cu₆Mo₄O₁₅, in argon, are 532 and 466°C, respectively. Nonstoichiometric phase Cu_4?x Mo₃O₁₂ = Cu²⁺₃Cu⁰_1?xMo⁶⁺₃O₁₂, melts in argon between 630 and 650°C depending on the value of x and at 525–530°C undergoes polymorphic transformation. Areas of coexistence of the above-mentioned phases are determined. The μ_eff of Cu²⁺ ions and θ values are: 1.80 B.M. and 28°K for CuMoO₄, 1.71 B.M. and ? 12°K for Cu₃Mo₂O₉, and 1.74 B.M. and ? 93°K for Cu_4?xMo₃O₁₂. Below 200°K CuMoO₄ becomes antiferromagnetic. Cu₂Mo₃O₁₀ and Cu₆Mo₄O₁₅ show weak temperature-independent paramagnetism.     

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.     

Magnetic order in AVX3 (A = Rb,Cs, (CD3)4 N; X = Cl,Br, I): A neutron diffraction study

AVX₃ (A = Rb, Cs, (CD₃)₄ N; X = Cl, Br, I) crystallize in the hexagonal system, space group $\text{P6}{}_3\text{mmc}$, with chains of face-sharing VX₆ octahedra along the c-axis. This leads to a pronounced one-dimensional character of their magnetic properties with a strong antiferromagnetic exchange interaction J between nearest neighbor V²⁺ ions along these chains. All compounds except [(CD₃)₄N]VCl₃ order three-dimensionally with ordering temperatures T_c between 13 and 32 K. In the ordered phase the magnetic moments, μ, lie in the basal plane in a triangular arrangement typical for antiferromagnetic interchain interaction J′.     

Cluster compounds [Ca(DMF)6][Mo6Cl14] and [{Ca(OPPh3)4}{Mo6Cl14}]∞: Synthesis, crystal structure, and properties

The [Ca(DMF)₆][Mo₆Cl₁₄] complex (I) was synthesized by reacting octahedral cluster Mo complex (H₃O)₂Mo₆Cl₁₄·6H₂O with the Ca²⁺ cation in DMF. Analogous reaction carried out in the presence of triphenylphosphine oxide Ph₃PO in acetonitrile resulted in [{Ca(OPPh₃)₄}{Mo₆Cl₁₄}] (II). According to X-ray diffraction data, complex I has ionic structure, while the structure of complex II consists of infinite linear chains of the cluster anionic complexes {Mo₆Cl₁₄}²⁻ bonded to the cationic Ca complexes {Ca(OPPH₃)₄}²⁺. Both complexes exhibit intensive luminescence in red and near-IR regions of the spectrum. Original Russian Text ? Zh.S. Kozhomuratova, Yu.V. Mironov, M.A. Shestopalov, Ya.M. Gaifulin, N.V. Kurat’eva, E.M. Uskov, V.E. Fedorov, 2007, published in Koordinatsionnaya Khimiya, 2007, Vol. 33, No. 1, pp. 3–8.     

Untersuchungen zum ternären System V/Mo/O

Investigation on the Ternary System V/Mo/O During chemical transport reactions mixtures of pseudo-binary line of intersection V₂O₅/MoO₃ are separated into two phases, the V₂O₅-(α) phase, which MoO₃-content depends on the oxygen partial pressure during the deposition and the MoO₃ phase which contains no more than 1% (n/n) V₂O₅. Ternary compounds do not exist on the pseudo-binary line. V₉Mo₆O₄₀ is formed by the reaction of V₂O₅ and MoO₃ (3:2) under exclusion of oxygen. The compound may be chemically transported under the own oxygen coexistence pressure. It was shown by total pressure measurements of V₂O₅/MoO₃ starting mixtures that the insertion of MoO₃ in the α-phase and the formation of the V₉Mo₆O₄₀ phase respectively is connected with elimination of oxygen and the reduction of V^V to V^IV in equivalence of the quantity of incorporated MoO₃.     

Octahedral Mo6 clusters supported by methoxide ligands

The structures of the [Mo₆(μ₃-Cl)₈(OMe)₆]^2? and [Mo₆(μ₃-OMe)₈(OMe)₆]^2? anions have been determined and reveal interesting comparisons of MM and MO distances with related Mo₆(μ₃-X)₈⁴⁺ containing compounds and polynuclear alkoxides of molybdenum.     

The crystal structure of the monohydrate R2Mo6O21·H2O (R=Pr, Nd, Sm, and Eu): a layer structure containing disordered [Mo2O7] groups

Although R₂O₃:MoO₃=1:6 (R=rare earth) compounds are known in the R₂O₃-MoO₃ phase diagrams since a long time, no structural characterization has been achieved because a conventional solid-state reaction yields powder samples. We obtained single crystals of R₂Mo₆O₂₁·H₂O (R=Pr, Nd, Sm, and Eu) by thermal decomposition of [R₂(H₂O)₁₂Mo₈O₂₇]·nH₂O at around 685-715 °C for 2 h, and determined their crystal structures. The simulated XRD patterns of R₂Mo₆O₂₁·H₂O were consistent with those of previously reported R₂O₃:MoO₃=1:6 compounds. All R₂Mo₆O₂₁·H₂O compounds crystallize isostructurally in tetragonal, P4/ncc (No. 130), a=8.9962(5), 8.9689(6), 8.9207(4), and 8.875(2) Å; c=26.521(2), 26.519(2), 26.304(2), and 26.15(1) Å; Z=4; R₁=0.026, 0.024, 0.024, and 0.021, for R=Pr, Nd, Sm, and Eu, respectively. The crystal structure of R₂Mo₆O₂₁·H₂O consists of two [Mo₂O₇]²⁻-containing layers (A and B layers) and two interstitial R(1)³⁺ and R(2)³⁺ cations. Each [Mo₂O₇]²⁻ group is composed of two corner-sharing [MoO₄] tetrahedra. The [Mo₂O₇]²⁻ in the B layer exhibits a disorder to form a pseudo-[Mo₄O₉] group, in which four Mo and four O sites are half occupied. R(1)³⁺ achieves 8-fold coordination by O²⁻ to form a [R(1)O₈] square antiprism, while R(2)³⁺ achieves 9-fold coordination by O²⁻ and H₂O to form a [R(2)(H₂O)O₈] monocapped square antiprism. The disorder of the [Mo₂O₇]²⁻ group in the B layer induces a large displacement of the O atoms in another [Mo₂O₇]²⁻ group (in the A layer) and in the [R(1)O₈] and [R(2)(H₂O)O₈] polyhedra. A remarkable broadening of the photoluminescence spectrum of Eu₂Mo₆O₂₁·H₂O supported the large displacement of O ligands coordinating Eu(1) and Eu(2).     

Spin glass transitions in the absence of chemical disorder for the pyrochlores A2Sb2O7 (A=Mn, Co, Ni)

The pyrochlores in the series A₂Sb₂O₇ have been synthesized and characterized as exhibiting spin glass transitions at T_SG=41, 4.5, and 2.6 K (for A=Mn²⁺, Co²⁺ and Ni²⁺S=1, respectively) despite the lack of chemical disorder. Since the Curie-Weiss temperature remains essentially constant for all members in the series (), the frustration index for these materials increases significantly as the moment size is reduced from f=|θ|/T_SG=1.1 (Mn²⁺), to 9.3 (Co²⁺) to 14.6 (Ni²⁺). There is also a corresponding change in the spin dynamics measured by the shift in the AC susceptibility signal as a function of frequency. These new materials provide an avenue to investigate the effect of quantum fluctuations on the Heisenberg pyrochlore lattice in the low spin limit, and show there is a dramatic change in the spin dynamics as the quantum regime is approached.     

Synthesis and Eu luminescence in new oxysilicates, ALa3Bi(SiO4)3O and ALa2Bi2(SiO4)3O [ACa, Sr and Ba] with apatite-related structure

New oxysilicates with the general formula ALa₃Bi(SiO₄)₃O and ALa₂Bi₂(SiO₄)₃O [ACa, Sr and Ba] are synthesized and characterized. Powder X-ray diffraction of these silicates show that they are isostructural with BiCa₄(VO₄)₃O which has an apatite-related structure. Eu³⁺ luminescence in the newly synthesized oxysilicates show broad emission lines due to disorder of cations. The relatively high intense magnetic dipole transition ⁵D₀→⁷F₁ points to a more symmetric environment. The photoluminescence results confirm that the compounds have apatite-related crystal structure.