Low-temperature investigations of the open-framework material HfMgMo3O12

The A2Mo3O12 family, where A3+ is a large trivalent cation, can show interesting thermal properties such as negative thermal expansion. One member of this family, HfMgMo3O12, where the two A3+ cations have been replaced by Hf4+ and Mg2+, has been shown to have a low positive coefficient of thermal expansion above room temperature. This property makes HfMgMo3O12 an attractive candidate as a component for solid solutions with near-zero thermal expansion. However, its properties below room temperature were unexplored. In this work we report the phase transition from orthorhombic Pnma to monoclinic P21/a at T∼175 K with an enthalpy change of 0.27 kJ mol−1. Relaxation calorimetry, from 5 K to 300 K, show only the small anomaly associated with this transition. The thermal conductivity, determined from 2 K to 300 K, was low, but not as low as some other materials exhibiting negative thermal expansion. Analysis of the low-temperature heat capacity indicates the presence of low-energy phonon modes in HfMgMo3O12, consistent with the low thermal conductivity. The upper bound of the Youngʹs modulus, estimated from the effective Debye temperature derived from the low-temperature heat capacity, is 20 GPa, a relatively low value due to the flexibility of the framework structure.