Ternary Molybdate Rb5(Ag1/3Hf5/3)(MoO4)6: Synthesis, Structure, Thermal Expansion and Ionic Conductivity

Abstract

This study aims to characterize the properties of the ternary molybdate Rb5(Ag1/3Hf5/3)(MoO4)6, which was previously identified during phase equilibriumvinvestigations in the Ag2MoO4-Rb2MoO4-Hf(MoO4)2 system. A ternary molybdate Rb5(Ag1/3Hf5/3)(MoO4)6 was obtained through a solid-state reaction. It was found that the compound crystallizes in the trigonal space group R3̅ c and melts at 596 °C with decomposition. Its structure was refined using the Rietveld method. The crystal structure consists of a mixed framework composed of isolated (Ag/Hf)O6 octahedra and MoO4 tetrahedra, interconnected through shared oxygen vertices. Large voids within the framework accommodate two types of rubidium atoms. Using the powder XRD data recorded over 30-400 °C, the principal components of the thermal expansion tensor were determined. Rb5(Ag1/3Hf5/3)(MoO4)6 can be classified as a material with high thermal expansion coefficient (αV = 36.7∙10-6 °C-1 at 400 °C). At elevated temperatures, the compound exhibited significant ionic conductivity, reaching 1.7∙10-3 S/cm at 480 °C with an activation energy Ea = 0.8 eV with oxygen ions as the probable charge carriers. Energy barriers for one-, two-, and three-dimensional transport in the compound were theoretically evaluated using the softBV program and bond valence sum maps (BVS).