Investigations on the crystallisation of potassium molybdenum bronzes from flux melts

Results are presented to identify the fields of primary crystallisation of red and blue potassium molybdenum oxide bronzes K_0.30MoO₃ and K_0.33MoO₃ in the ternary system K₂MoO₄–MoO₃–MoO₂. Starting from crystal growth experiments for the preparation of bronze crystals by electrolytic reduction the binary marginal systems K₂MoO₄–MoO₃ as well as MoO₃–MoO₂ and further pseudo‐binary sections have been investigated by differential thermo‐analysis, hot‐stage microscopy, and by quenching experiments under inert gas. We succeeded in determining phase transition temperatures in the binary section Mo₄O₁₁–K₂Mo₂O₇. Temperature and composition ranges have been found in this section to crystallise red and blue potassium molybdenum oxide bronzes. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal Structure of pyridinium isopolymolybdate (C5H6N)2n[Mo4O13]n

Synthesis and crystal structure are described for pyridinium isopolymolybdate of chemical composition (C₅H₆N)_2n[Mo₄O₁₃]_n. The crystals are triclinic, space group P1, with the following unit‐cell parameters: a =8.2695(11) Å, b =10.544(4) Å, c =11.177(4) Å, α = 71.76(5)°, β = 89.68(3)°, γ = 78.79(3)°, V =906.4(4) ų, Z = 2 (chemical formula (C₅H₆N)₂[Mo₄O₁₃]), D _calcd = 2.755 g·cm^–3. Crystal structure was solved by Patterson methods and refined to a final R value 0.085 for 4045 independent reflections. The studied compound, considered in analogy to triclinic (NH₄)₂Mo₄O₁₃ as pyridinium polyoctamolybdate, is proposed to be better described as pyridinium isopolytetramolybdate (C₅H₆N)_2n[Mo₄O₁₃]_n. It seems that the proper coordination number of molybdenum (VI) ions is five, resulting in pyramidal coordination polyhedra [MoO₅]. Coordination polyhedra joined by common edges form tetramolybdate monomeric unit [Mo₄O₁₃]. The mers are connected by oxygen bridges Mo ‐ O ‐ Mo into infinite ribbon chains. Each two infinite chains are hold together by weaker intermolecular interactions. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Flux growth of straw‐like rutile monocrystals

Millimetric straw‐like rutile monocrystals were grown by the flux growth technique. A suitable mixture of flux (MoO₃, V₂O₅, Li₂CO₃) and amorphous TiO₂ gel was slowly cooled down to 750°C from 1250°C or 1350°C. The best yields of straw‐like rutile were obtained with a nutrient/flux ratio and a cooling rate in the range 0.015‐0.006 and 1.8‐1.9 K h^‐1, respectively. The hollowed crystals were characterized by powder and single‐crystal X‐ray diffraction, scanning electron microscopy, microthermometry, and µ‐Raman spectroscopy. As for skeletal crystal, the formation of axial canals in rutile is attributed to a lack of nutrient due to the viscosity of the melt and the high growth rate along [001]. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Flux growth and characterization of Ti‐ and Ni‐doped forsterite single crystals

Forsterite monocrystals doped with Ti and Ni were grown by the flux growth technique. A suitable mixture of flux (MoO₃, V₂O₅, Li₂CO₃) and nutrient was slowly cooled down to 750 °C from 1250 °C or 1350 °C. The crystals were then characterized by powder and single‐crystal X‐ray diffraction, scanning electron microscopy and differential scanning calorimetry (DSC). Variations observed in crystal size were attributed by both the varying experimental conditions in which they had been obtained, and to the amount of Ni substituted for Mg in the structure. High abundances of doped forsterite required a cooling rate of 1.8 K h^‐1. These synthetic, well‐characterized Ti and Ni doped forsterite crystals may have potential for exploitation in industrial fields. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The crystal structure of hexaammonium diacetyl‐octa‐molybdate tetrahydrate

The crystal structure of hexaammonium diacetyl‐octa‐molybdate tetrahydrate, (NH₄)₄₂[Mo₁₃₂O₃₇₂ (CH₃COO)₃₀(H₂O)₇₂] is documented. The crystals are triclinic, space group P‐1, with a=8.1018(16) Å, b=10.334(2) Å, c=12.238(2) Å, α=68.20(3)°, β=74.98(3)°, γ=67.25(3)°, V=869.3(3) ų, Z=1. The structure was solved by direct methods and refined by least squares methods to a Final R1 = 0.0374 and wR2 = 0.1074 for 3805 observed reflections with I > 2σ(I). The structure contains ammonium cations and isolated acetyl octamolybdate(6‐) anions, [Mo₈O₂₈(CH₃CO)₂]^6‐. The crystallographic data of the structure was deposited with the Cambridge Data Center as No. CCDC 249565. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magnetic resonance of the B2O3Na2OMoO3 vitreous system

The structure and properties of molybdenum-doped soda-borate glasses were studied by means of electron paramagnetic resonance as functions of the molybdenum oxide concentration in the range 1–25 mol% MoO₃, the keeping time of the samples at the preparation temperature in the range of 15–180 min, the preparation temperature in the range of 900–1300°C, and the soda-borate matrix composition in the range 0–34 mol% Na₂O.The EPR measurements gave evidence of the existence of three types of site for the Mo⁵⁺ ions, their relative fraction as well as the paramagnetic center density and the EPR lineshape symmetry being dependent on the above-mentioned factors. The evolution of the ¹¹B NMR lineshape in the studied samples also showed that the fourfold-coordinated boron atoms fraction is greatly influenced by the modification of these factors. The good agreement between the structural information obtained by ¹¹B NMR and Mo⁵⁺ EPR studies recommends the EPR spectra of Mo⁵⁺ ions as an excellent sensor for the structure of the borate glasses.     

Effect of V2O5 addition on the crystallisation of glasses belonging to the CaO–ZrO2–SiO2 system

The crystallisation of CaO–ZrO₂–SiO₂ glasses doped with V₂O₅ (0.1–5 mol%) has been investigated in terms of microstructure and thermal parameters. Results indicate that crystallisation is predominantly controlled by a surface nucleation mechanism, even though a partial bulk nucleation has been encountered in compositions containing more than 2 mol% of doping oxide. As detected from differential thermal analysis curves, glass transition temperature and crystallisation temperature, are strongly dependent upon V₂O₅ content varying from 0.0 to 2.0 mol%, while the crystallisation activation energy values decrease with a parabolic trend from B-glass (0.0 mol% V₂O₅ content, 495±7) to V-0.7 (0.7 mol% V₂O₅ content, 420±6) composition, increasing again to 442±5 kJ/mol K with higher amount of V₂O₅. The microstructure of the glass-ceramic materials clearly showed a marked dependence upon the amount of V₂O₅, also due to the presence of phase separation for content higher than 0.7 mol%. Wollastonite, CaO·SiO₂, and a calcia–zirconia–silicate, 2CaO·4SiO₂·ZrO₂, are the main crystalline phases whose ratio slightly varies with vanadium oxide content. The glass ceramics obtained from the studied materials are greenish and bluish coloured, so it is possible to use the studied glasses as coloured frits for tile glazes.     

Growth and crystal structure of binary molybdate CsFe(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript>

CsFe(MoO₄)₂ single crystals have been grown by solution-melt crystallization with a charge-to-solvent ratio of 1: 3 (with Cs₂Mo₃O₁₀ used as a solvent). The crystal structure of this compound has been refined by X-ray diffraction (X8 APEX automatic diffractometer, MoK _α radiation, 356 F(hkl), R = 0.0178). The trigonal unit cell has the following parameters: a = b = 5.6051(2) Å, c = 8.0118(4) Å, V = 217.985(15) ų, Z = 1, ρ_calc = 3.875 g/cm³, and sp. gr. P \(\bar 3\) m1. The structure is composed of alternating layers of FeO₆ octahedra (with MoO₄ tetrahedra attached by sharing vertices) and CsO₁₂ icosahedra.     

Synthesis and crystal structure of Li3.17(P0.69Ge0.24Mo0.07)O4

The crystal structure of Li_3.17(P_0.69Ge_0.24Mo_0.07)O₄, which was synthesized by the flux method under the action of an electric field with a potential difference of 0.8 V, was studied by X-ray diffraction on a Nonius Kappa CCD diffractometer (MoK _α radiation, λ = 0.71073 Å, R = 0.0137). The crystal structure is based on a tetrahedral framework typical of γ-Li₃PO₄. The framework of the new compound is formed by (P,Ge,Mo)O₄ and LiO₄ tetrahedra. The positions of additional Li atoms in the structure of Li_3.17(P_0.69Ge_0.24Mo_0.07)O₄ are found to be different from those in the structure of Li_3.31(P_0.69Ge_0.31)O₄, which is synthesized in the absence of an electric field and also belongs to the γ-Li₃PO₄ structure type.     

Growth and characterization of pure and Co2+‐doped Li2B4O7 single crystals

Pure and Co‐doped Li₂B₄O₇ (LBO) single crystals were grown by the Czochralski method. Starting concentrations of Co₂O₃ in the melt were: 0.5, 0.85 and 1 mol% relative to Li₂CO₃. Technological factors affecting the quality of both crystals were discussed. Optical absorption and EPR spectra were analyzed to define the oxidation states and lattice sites of cobalt ions. It was shown that Co²⁺ ions enter LBO crystal at octahedral Li⁺ site positions. Low‐temperature EPR measurements revealed that two types of Co²⁺ complexes can be distinguished in the Li₂B₄O₇:Co crystals. Additional absorption calculated for γ‐irradiated crystals showed V_k type defects suggesting the creation of cation vacancies during growth. The concentration of the defects decreases with an increase of intentional Co concentration. Introduction of cobalt ions to LBO crystal is limited probably by the formation of cobalt ion pairs or by the entrance of cobalt as Co⁺. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure of Gd<Subscript>0.95</Subscript>Bi<Subscript>0.05</Subscript>Fe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> single crystals at 293 and 90 K

The structure of GdFe₃(BO₃)₄ single crystals has been studied by X-ray diffraction at 293 and 90 K. The crystals are grown from a flux in the Bi₂Mo₃O₁₂–B₂O₃–Li₂MoO₄–Gd₂O₃–Fe₂O₃ system. The results of chemical analysis and structural study show that these crystals contain bismuth as an impurity. It is found that bismuth atoms are located at gadolinium sites in the structure. A decrease in the temperature is accompanied by a lowering of the symmetry from sp. gr. R32 (at 293 K) to sp. gr. P3₁21 (at 90 K). The presence of two types of iron chains with different geometries at 90 K promotes a change in the magnetic properties of these crystals with a decrease in the temperature.     

EPR study of molybdenum-lead-phosphate glasses

The local symmetry and interaction between paramagnetic ions in xMoO₃(1 ? x)[2 P₂O₅PbO] glasses with 0.5 ? x ? 50 mol% are investigated by EPR spectroscopy. For x ? 10 mol% the isolated Mo⁵⁺ ions surrounded by five oxygen ligands in a square-pyramidal form (C_4v symmetry) prevail. The short range disorder in the environment of Mo⁵⁺ ions is not significantly (ΔR/R ≈ 2%). At high molybdenum content (x > 20 mol%) the dipole–dipole and superexchange coupled Mo⁵⁺ ions appear and their number increases with the MoO₃ content. These two aspects are also correlated with the network modifier and former role of molybdenum oxide in function of its concentration. Thus a strong depolymerization of the phosphate structure and the formation of P–O–Mo or Mo–O–Mo bonds in studied glasses appear.     

Real Structure of LiB3O5 (LBO) Crystals Grown in Li2O‐B2O3‐MoO3 System

The liquidus surface structure and field of LiB₃O₅ (LBO) primary crystallization have been revealed in Li₂O‐B₂O₃ ‐MoO₃ ternary system. The optimization of charge composition and growth conditions results in large volume optical quality LBO single crystals yielding. Crystallographic properties and real defect structure of grown LBO single crystals have been investigated by X‐ray powder diffraction method and X‐ray reflection topography. The volume of the crystals is partly free of any structural imperfections.     

Growth of near‐stoichiometric LiNbO3 crystals and Li2O contents determination

Two kinds of near‐stoichiometric LiNbO₃ crystals (SLN11 and SLN19) were grown by a flux pulling method from stoichiometric melt with addition of 11mol%K₂O and 19mol%K₂O, respectively. Compared with the congruent melting LiNbO₃, the ultraviolet absorption edges of two crystals shift towards shorter wavelengths, and the locations of the OH^‐ infrared absorption band have obvious change and the bandwidths become greatly narrower. From these experimental results, the Li₂O contents are determined indirectly to be about 49.6mol% for SLN11 and 49.9mol% for SLN19, respectively. The Li₂O content in SLN19 is very close to the ideal value of 50mol%. The coercive fields of two crystals were measured by the poling method at room temperature. A linear relationship between the Li₂O content and the coercive field was fitted. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Contribution to the tin‐rich region of the Sn‐Zn‐Ti system

The tin‐rich region of the system Sn‐Zn‐Ti system has been studied by diffusion couples, differential scanning calorimetry and electron microprobe analyses. Ternary eutectic reaction occurs at 193.7°C near to the binary tin‐zinc eutectic point and titanium content less than 0.9 at.% Ti. Three ternary compounds with approximate formulae: Ti₈Sn₅Zn₂ to Ti₅Sn₃Zn, TiSn₄Zn₅ and Ti₂Sn₄Zn₃ have been observed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

XAFS study of six-coordinated silicon in R2O–SiO2–P2O5 (R = Li,Na, K) glasses

The local structures around silicon and phosphorous atoms in R₂O–SiO₂–P₂O₅ (R = Li, Na and K) glasses have been investigated using Si and P K-edge XAFS spectroscopy by transmission mode at BL-4 and BL-3 at the synchrotron facility in Ritsumeikan University. As a result of XANES and EXAFS analyzes, six-coordinated silicon atoms were observed in the glasses. The fraction of six-coordinated silicon atom changed with increasing of the concentration of alkali oxide and P₂O₅. For the change of concentration of alkali oxide, it takes maximum values which are 60% in Li₂O, 90% in Na₂O and 85% in K₂O system at 20 mol% alkali oxide. It gradually increased up for the increase of the concentration of P₂O₅ to 55% in Li₂O, 80% in Na₂O and 90% in K₂O system. The Si–O inter-atomic distance in the glasses changes from 1.63 to 1.79 Å with increasing the fraction of six-coordinated silicon atom. On the other hand, it was not observed the local structural change around the phosphorous atom.     

The crystal and molecular structure of bis(tetramethylammonium) hexamolybdate(VI)

The crystal structure of the title compound, C₈H₂₄N₂Mo₆O₁₉ was determined by single crystal X‐ray diffraction technique. The crystals are trigonals, space group P‐3c1, with a = b = 10.0176(11) Å, c = 14.089(2) Å, γ = 120°, V = 1224.4(3) Å₃, Z = 2. The structure was solved by direct methods and refined by least‐squares methods to a Final R = 0.0316 for 1114 observed reflections with I > 2σ(I). In hexamolybdate anion, six MoO₆ distorted octahedra are fused together so that they all share a common vertex. O atoms are of three types: central, terminal and bridging, bonded to six, one and two Mo atoms, respectively. The crystallographic data of the structure was deposited with the Cambridge Data Center as No. CCDC 199679. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural characterization of alumina-supported nickel molybdena catalysts

The object of our studies was a NiMo/Al₂O₃ catalyst thermally activated in oxygen, hydrogen, and in a mixture of hydrogen and hydrogen sulfide. On the surface of the oxide form of the catalyst there are different species, among them dimeric and polymeric octahedrally coordinated Mo⁶⁺ ions containing oxygen bridges are found. The presence of structures resembling α- and β-NiMoO₄ and 6-heteropolymolybdenates was determined. Reduction with hydrogen leads to formation of two nuclear structures with single and double bridges of [Mo₂⁵⁺ O₃] and [Mo⁵⁺ O₄] type, respectively, and partially reduced 6-heteropolymolybdenates. On the other hand, when the reduction and sulfiding take place at the same time, then terminal oxide ions are partially replaced by sulfide ones. The presence of [MoO₂S₂]²⁻ species has been observed, however, there is no evidence of the exchange —OH groups for —SH ones and exchange of oxygen bridged ions for sulfur ions. It has been also noted that the subsurface structures remained intact after reduction and sulfiding.     

Glass formation in the MoO3-CuO-PbO system

Glasses in the MoO₃-CuO-PbO system are obtained at high cooling rates (10⁴-10⁵ K/s) and characterized using X-ray diffraction (XRD), differential thermal analysis (DTA), infrared (IR) and X-ray photoelectron spectroscopy (XPS). Two glass formation regions are determined: one with compositions having a high MoO₃ content (50-80 mol%) and the other in the PbO-rich compositions (65-80 mol%). In the region of MoO₃-rich compositions the building units of the amorphous network are МоО₆, МоО₄ and CuO₄ groups. For these high MoO₃ contents and respectively low PbO concentrations, the lead oxide is supposed to act as a network modifier while at high content PbO is found to be the main glass network former. In latter case the structure of glasses is formed by chains of PbO_n (n = 3, 4) polyhedra, between which there are isolated MoO₄ and CuO₄ complexes. IR and XPS data reveal the existence of Mo-O-Mo, Mo-O-Me(Me’) (where Me = Cu²⁺, Cu¹⁺ and Me’ = Pb) and Me(Me’)-O-Me(Me’) bonds in the amorphous network. Surprising result is found for low PbO content (10 mol%) where the lead oxide acts as glass network modifier: the actual MoO₃ content drops strongly which is accompanied with a significant increase of the actual CuO content with respect to their nominal MoO₃-CuO composition. Such effect is not observed in PbO-rich composition (70 mol%) where PbO has a role of network former.     

Infrared spectra and structure of superionic conducting glasses in the system AgIAg2OMoO3

Thin blown films of glasses with the mole ratio Ag₂O/MoO₃ = 1 in the system AgIAg₂OMoO₃ (or the pseudobinary system AgIAg₂MoO₄) show three absorption bands in the range 4000-200 cm^?1; 875 cm^?1 (w), 780 cm^?1 (s), and 320 cm^?1 (m, b), which are characteristic of tetrahedral MoO₄^2? ions. The glasses with the ratio Ag₂O/MoO₃ < 1 have two additional bands at 600 cm^?1 (w) and 450 cm^?1 (vw), which are characteristic of condensed ions of MoO₄ tetrahedra, probably Mo₂ O₇^2? ions. These glasses are thus composed of Ag⁺, I^?, MoO₄^2?, and probably Mo₂O₇^2? ions, and classified as “ionic” glasses containing one type of cations. The presence of partial covalency in the Ag⁺… ^?OMo link and the influence of ion exchange of Ag⁺ with K⁺ on IR spectra are discussed. The molar volume of the glasses with the ratio Ag₂O/MoO₃ = 1 is primarily determined by a fairly dense packing of the constituent anions, I^? and MoO₄^2?.