Pressure derivatives of the elastic constants of LiF and NaF

Abstract

The pressure derivatives of the elastic stiffness constants of LiF and NaF have been measured by the ultrasonic pulse echo technique. The values found are: dC dP dC' dP dB S dP LiF 1.38 3.62 5.14 NaF 0.205 4.79 5.18 The notation C = C 44, C' = (C 11 − C 12) 2 , and B S = (C 11 + 2C 12) 3 has been used. These pressure results have been related to the temperature dependence of the elastic constants and to the thermal expansion. The data for the elastic constants and their pressure variations have also been analyzed in terms of the classical Born model. The theoretical Coulomb contributions have been subtracted from the data and the resulting short-range terms interpreted in terms of nearest and next-nearest neighbor interactions. Particularly useful in this analysis was the elastic stiffness C and its pressure derivative dC dP . The analysis indicates that next-nearest-neighbor interactions are definitely necessary in order to account for the observed experimental values for LiF but not for NaF.