Ho2Zr(MoO4)5 — A novel double molybdate with negative thermal expansion

Abstract

In this study we investigated the subsolidus phase equilibria in the Ho2O3–ZrO2–MoO3 system and discovered the double molybdate Ho2Zr(MoO4)5. The Rietveld method was used to refine the structures of the low- and high-temperature phases. The title compound is isostructural to Er2Zr(MoO4)5 double molybdate and crystallizes in the monoclinic space group P21, but rearranges to rhombic (space group Cmc21) at temperatures above 100°С. Ho2Zr(MoO4)5 exhibits negative thermal expansion in the high temperature region (αV = −3.9 × 10−6 °C−1). Theoretical calculations using bond valence sum maps method suggest that this compound may have three-dimensional O2− conductivity with relatively low energy barriers (0.588 eV).