Heat capacity and Grüneisen function of negative thermal expansion compound HfW2O8

Heat capacity of HfW2O8 was measured from 1.8 to 330 K. Grüneisen function of HfW2O8 was obtained between 80 and 330 K from the heat capacity and other thermal data. Effective phonon density of states was determined through an analysis of the heat capacity of HfW2O8 assuming one Debye, two Einstein and two rectangular type functions. The mode Grüneisen parameters were estimated from these phonon distribution. It is found that two low-energy Einstein modes at 3–6 meV have large negative model Grüneisen parameters, resulting in the negative thermal expansion.