Pressure and Temperature Derivatives of the Low-Frequency Dielectric Constants of LiF, NaF, NaCl, NaBr, KCl, and KBr

Abstract

The first-, second-, and third-order 308°K pressure derivatives of the low-frequency dielectric constants for LiF, NaF, NaCl, NaBr, KCl, and KBr have been determined to an accuracy of about 0.2%, 2%, and 20%, respectively. In addition, the first- and second-order temperature derivatives are established at approximately the 0.2% and 20% level. The volume-independent temperature derivative of the low-frequency dielectric constant is found to be negative for LiF and positive for the remaining crystals. This trend may be understood in terms of the interaction between the acoustic and optic modes using Szigeti's formalism for anharmonic crystals as interpreted by Fuchs. The results are also used to show that the question as to the interpretation of the Szigeti effective charge remains open for the alkali halides.