Ternary molybdate Rb₅(Ag_1/3Hf_5/3)(MoO₄)₆: synthesis, structure, thermal expansion and ionic conductivity

This study aims to characterize the properties of the ternary molybdate Rb5(Ag1/3Hf5/3)(MoO4)6, which was previously identified during phase equilibrium investigations 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 R 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 °С–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 Еа = 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).