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

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 P2(1), but rearranges to rhombic (space group Cmc2(1)) at temperatures above 100(degrees)& Scy;. Ho2Zr(MoO4)(5) exhibits negative thermal expansion in the high temperature region (alpha V = -3.9 x 10(-6) C-degrees(-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).