Anomalies in the specific heat of PbF2

We have measured the heat capacity, C_p, of PbF₂ between 400 K and 800 K using a cubic, fluorite-structure, crystal as well as material recovered in the orthorhombic form at room temperature after a fluorite-structure crystal had been pressurized to 4.5 kbar. C_p of each behaved normally up to somewhat above 600 K. However C_p of the recovered material revealed that orthorhombic PbF₂ undergoes a sharp, endothermic, nonreversible transition at T = 633 K, which seems to be a transformation back to the cubic fluorite structure. The C_p of each sample exhibited a λ-like anomaly in the temperature range where a transition to the superionic state is known to occur in cubic PbF₂, thus confirming the second order nature suggested for the transition.