Search

The phase formation in the Na2MoO4-Cs2MoO4-NiMoO4 system is studied using the solid-state synthesis and flux crystallization. The formation of a cesium-containing solid solution based on double molybdate Na4-2xNi1+x(MoO4)3 of the alluaudite type and new triple molybdate of a related structure is established. Their structures contain layers of МоО4 tetrahedra and coupled (Ni, Na)O6 octahedra bridged by МоО4 tetrahedra into three-dimensional open frameworks with tunnels along the c axis filled by sodium cations. By the example of isostructural Na3.39Ni1.31(MoO4)3 (I) and Na3.18Cs0.29Ni1.26(MoO4)3 (II), it is shown that in the alluaudite-type solid solution, cesium ions partially replace sodium cations in the framework tunnels, which provides the sodium ion conductivity. The structure of triple molybdate Na2.64Cs0.64Ni1.36(MoO4)3 (III) is a new type of the alluaudite superstructure deformed and tripled in volume. Its formation is caused by a significant differentiation of cations at (Ni, Na) and (Na, Cs) positions. In terms of the degree of deformation, this structure is intermediate between alluaudite and (pseudo)orthorhombic Na10Cs4M5(MoO4)12 (M = Mn, Co) and Na25Cs8R5(MoO4)24 (R = Fe, Sc, In). Among the crystallization products in the Na2MoO4-Cs2MoO4-NiMoO4 system, crystals with a similar incommensurate modulation of the alluaudite structure along the c axis are found. Calculations of bond valence sum maps for sodium ions in the structures of I–III confirm the possibility of the one-dimensional sodium-ion conductivity for them at its lowest threshold for double molybdate for which the two-dimensional transport is also probable at elevated temperatures. [ABSTRACT FROM AUTHOR]

The tungsten substitution for molybdenum in cesium dimolybdate Cs2Mo2O7 with the formation of an α-Cs2Mo2−xWxO7 solid solution is studied, and its homogeneity region at 540 °C is found to reach 0 ≤ x ≤ 0.6. It is established that the volume of the monoclinic (pseudo-orthorhombic) unit cell and the solid solution melting point monotonically increase with increasing tungsten content, whereas the phase transition temperature to the orthorhombic phase remains practically constant and is ∼390 °C. The crystals of solid solutions with several compositions are obtained, their structure is determined, and the tungsten atoms are found to be mainly located in octahedral positions of the α-Cs2Mo2O7 structure. As a result of the phase transition at 390 °C the electrical conductivity of Cs2Mo2O7 increases approximately by two orders of magnitude reaching 1.2·10−4S/cm at 460 °C. Due to a large size of Cs+ ions and a high charge of molybdenum cations it is supposed that the charge carriers are oxygen anions. The calculation of bond valence sum (BVS) maps for possible positions of oxygen atoms in the α-Cs2Mo2O7 cell shows the formation of a 3D system of oxygen conductivity channels at boundary BVS isosurface values ≥ 2.13. [ABSTRACT FROM AUTHOR]

The phase transition temperatures of chromates and molybdates of certain alkali metals, and the melting temperature and enthalpy of polymorphic transformations for tungstates, are determined by means of thermal analysis. Enthalpies of dissolution of rubidium and cesium chromates in water and enthalpies of dissolution of alkali metal tungstates in a melt at 923 K are measured via calorimetry. Standard enthalpies of formation of sought chromates are calculated. The linear correlations between the enthalpies of formation of sulfates, selenates, chromates, tungstates, and molybdates are established, and a linear correlation within (ΔG°ox ) - ((ΔMV)ox)-1 coordinates is found for isopolymolybdates. [ABSTRACT FROM AUTHOR]

In the samples of the Na 2MoO 4-MgMoO 4 system quenched in the air at above 600°C, by powder X-ray diffraction two double molybdates of variable composition are detected: monoclinic alluaudite-like Na 4−2xMg 1+x(MoO 4) 3 (0.05 ≤ x ≤ 0.35) and triclinic Na 2−2yMg 2+y(MoO 4) 3 (0.10 ≤ y ≤ 0.40) isostructural to previously studied Na 2Mg 5(MoO 4) 6. Sodium-magnesium molybdate of the Li 3Fe(MoO 4) 3 structure type is not revealed in this system. By spontaneous flux crystallization, the crystals are obtained and the structures of two triclinic double molybdates of the Na 2Mg 5(MoO 4) 6 structure type (space group $P\bar 1$ , Z = 1) containing magnesium and manganese are determined. The results of the refinement of site occupancies made it possible to determine the composition of the studied crystals: for the compound with magnesium (Na) 0.5(Na 0.255□ 0.745)(Na 0.755Mg 0.245)Mg 2(MoO 4) 3 or Na 1.51Mg 2.245(MoO 4) 3 ( a = 6.9577(1) Å, b = 8.6330(2) Å, c = 10.2571(2) Å, α = 106.933(1)°, β = 104.864(1)°, γ = 103.453(1)°, R = 0.0188); for the compound with manganese (Na) 0.5(Na 0.33□ 0.67)(Na 0.83Mn 0.17)Mn 2(MoO 4) 3 or Na 1.64Mn 2.17(MoO 4) 3 ( a = 7.0778(2) Å, b = 8.8115(2) Å, c = 10.4256(2) Å, α = 106.521(1)°, β = 105.639(3)°, Γ = 103.233(1)°, R = 0.0175). The Na 2Mg 5(MoO 4) 6 structure is redetermined and it is shown that actually it corresponds to the composition Na 1.40Mg 2.30(MoO 4) 3. [ABSTRACT FROM AUTHOR]

We have studied tetragonal scheelite-like solid solutions in the ternary system Na2MoO4-CaMoO4-Ce2/3MoO4: Na 0.7Cay Ce1.1 − 2 y/3 (MoO4)2 (0 ≤ y ≤ 0.6) and Na0.3 CazCe1.23− 2 z/3 (MoO4)2 (0 ≤ z ≤ 1.4). The solid solutions melt congruently at temperatures from 1100 to 1200°C. Their lattice parameters have been determined. Using reflection spectra, we evaluated the color parameters of all the samples studied. [ABSTRACT FROM AUTHOR]

Phase relations have been studied for the subsolidus region of the Dy2O3-HfO2-MoO3 system; binary molybdates, Dy2Hf(MoO4)5 and Dy2Hf2(MoO4)7, were found to form. The crystal structure of the latter has been solved (a= 20.661(3) Å,b= 9.816(1) Å,c= 13.796(3) Å, ß = 113.47(1)°,Z= 4, space groupC2/c,R= 0.023) and found to be a new type of structure. In the structure, MoO4 tetrahedra are linked by their vertices with HfO6 octahedra and DyO8 tetragonal antiprisms to form a 3D open-work, in which one can isolate double-row tetrahedral and octahedral chains extended down thecaxis. [ABSTRACT FROM AUTHOR]