Heat capacity, lattice dynamics, and thermodynamic stability of the negative thermal expansion material HfMo2O8

Abstract

We explore the lattice dynamics of the negative thermal expansion material, cubic HfMo2O8, through analysis of its heat capacity (measured from 0.5 to 300 K) and its room-temperature Raman spectrum. Its heat capacity is quantitatively very similar to that of ZrW2O8, as is its Raman spectrum. The heat capacity of HfMo2O8 can be well represented by the present lattice dynamical assignment and by C-P(HfW2O8)-C-P(ZrW2O8)+C-P(ZrMo2O8), but not by C-P(HfO2)+2C(P)(MoO3), likely because the AB(2)O(8) compounds have low-frequency optic modes, not present in HfO2 and MoO3. The present thermodynamic data also allow an analysis of the thermodynamic stability of cubic HfMo2O8, and it is shown to be unstable with respect to MoO3 and HfO2 at room temperature.