Structure and Properties of the α-Cs2Mo2−xWxO7 Solid Solution

Abstract

The tungsten substitution for molybdenum in cesium dimolybdate Cs₂Mo₂O₇ with the formation of an α-Cs₂Mo_2−xW_xO₇ 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.