Near‐Zero Thermal Expansion in In ( HfMg ) 0.5 Mo 3 O 12

In ( HfMg ) 0.5 Mo 3 O 12 , which can be considered as a 1:1 mole ratio solid solution of the low‐positive thermal expansion material HfMgMo 3 O 12 and the low‐negative thermal expansion (NTE) material In 2 Mo 3 O 12 was prepared. From DSC and XRPD results, we show that In ( HfMg ) 0.5 Mo 3 O 12 exists in a monoclinic ( P 2 1 / 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 ℓ   = 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 ℓ  = −0.4 × 10 −6  K −1 , and the average bulk (dilatometric) value is α ℓ  = 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 HfMgMo 3 O 12 and In 2 Mo 3 O 12 .