Measurements of ultrasonic velocity in single-crystal cubic ${\mathrm{PbF}}_2$ have been made over a temperature range from 300 to 850 K. Values of the three independent, adiabatic elastic constants over this temperature range are obtained from $C_{11}$, $\frac{(C_{11}+C_{12}+{2C}_{44})}{2}$, $\frac{(C_{11}-C_{12})}{2}$, and $C_{44}$. The results are anomalous at the Faraday temperature of 711 K for ${\mathrm{PbF}}_2$, which is well below its melting temperature of 1158 K. A large decrease in the elastic constant $C_{11}$ and much smaller decrease in $C_{12}$ and $C_{44}$ are observed near 711 K in addition to the nearly linear decrease in the elastic constants with temperature.

• Received 6 September 1983

DOI:https://doi.org/10.1103/PhysRevB.30.3482