Pressure and Temperature Derivatives of the Low-Frequency Dielectric Constants of the Alkaline-Earth Fluorides

The first- and second-order 308^∘{}K pressure derivatives of the low-frequency dielectric constants of CaF₂, SrF₂, and BaF₂ are determined to about 0.1% and 2%, respectively. The first- and second-order 308^∘{}K temperature derivatives are determined to about 0.2% and 20%. The results are used to calculate the associated volume derivatives. It is found that Srinivasan's calculation of v(∂εₛ∂v)T based on Axe's shell model agrees well with experiment. The results are also used to show that in the alkaline-earth fluorides, as in the alkali halides, the question as to the sign of the temperature-independent volume derivative of the Szigeti effective charge remains open.