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Near-Zero Thermal Expansion in In(HfMg)0.5Mo3O12


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In(HfMg)0.5Mo3O12, which can be considered as a 1:1 mole ratio solid solution of the low-positive thermal expansion material HfMgMo3O12 and the low-negative thermal expansion (NTE) material In2Mo3O12 was prepared. From DSC and XRPD results, we show that In(HfMg)0.5Mo3O12 exists in a monoclinic (P21/a) structure at low temperature and undergoes a phase transition at ~425 K to an orthorhombic phase (Pnma), with an associated enthalpy change of 0.89 kJ mol−1. Thermal expansion is large and positive in the low-temperature monoclinic phase (average α = 16 × 10−6 K−1 and 20 × 10−6 K−1, from dilatometry and XRPD, respectively). Remarkably, this material has a near-zero thermal expansion (ZTE) coefficient over the temperature range ~500 to ~900 K in the high-temperature orthorhombic phase, both intrinsically and for the bulk sample. The average linear intrinsic (XRPD) value is α = −0.4 × 10−6 K−1, and the average bulk (dilatometric) value is α = 0.4 × 10−6 K−1 with an uncertainty of ± 0.2 × 10−6 K−1. The slight difference between intrinsic and bulk thermal expansion is attributed to microstructural effects. XRPD results show that the thermal expansion is more isotropic than for the parent compounds HfMgMo3O12 and In2Mo3O12.