Pressure Derivatives of the Elastic Constants of NaBr and KF

Abstract

The isothermal pressure derivatives of the adiabatic second order elastic constants at 25 °C of NaBr and KF have been measured by the ultrasonic interferometric technique. The results are: magnified image Here c′_αβ = ―ϱ where ―ϱ is the density and V_αβ is the velocity at the given pressure and temperature. These pressure derivatives are analyzed in the framework of the Born model of ionic solids. The pressure derivatives of the second order elastic constants can be expressed in terms of the second and third order elastic constants. The theoretical values of the second and third order elastic constants used in this article were obtained by considering the coulombic interactions of point ions, short range repulsive interactions up to second nearest neighbors, and van der Waals terms. Even though there is qualitative agreement between theory and experiment quantitative agreement is less satisfactory than desired.