Enhanced negative thermal expansion by solid solution of HfMgMo_1.5W_1.5O₁₂

HfMgMo₃O₁₂ exhibits a low positive thermal expansion and HfMgW₃O₁₂ displays a low negative thermal expansion (NTE). It is found that the solid solution of HfMgMo_1.5W_1.5O₁₂ has an enhanced NTE with lower anisotropy with respect to HfMgMo₃O₁₂ and HfMgW₃O₁₂. The coefficients of thermal expansion (CTEs) for HfMgMo_1.5W_1.5O₁₂ are α _a = –6.78 × 10^–6 K^–1, α _b = 6.59 × 10^–6 K^–1, α _c = –5.23 × 10^–6 K^–1, α _v = –6.19 × 10^6– K^–1 for the a-, b- and c-axes and volume, respectively. This gives rise to a linear CTE of α _l = –2.06 × 10^–6 K^–1. The alternative arrangements of W and Mo around each octahedra and differences in electronegativity and cation radius between W and Mo account probably for the enhanced NTE and decreased anisotropy with respect to HfMgMo₃O₁₂.