The pressure dependence of the elastic constants of the alkali chlorides

Abstract

The pressure derivatives of the elastic stiffness constants of LiCl and RbCl have been measured using the ultrasonic pulse-echo technique. The values at room temperature and zero pressure are: (table convertion page 2363)where the notation B=(C 11+2C 12)/3 and C′=(C 11-C 12) /2 has been used. These results in combination with previous results for NaCl and KCl allow for a systematic study of the elastic properties of all alkali chlorides having the NaCl structure. Such a study has been made using a Born model with the cohesive energy consisting of two parts: a Coulomb interaction between all ions and a short-range repulsive interaction between nearest neighbors only. In most cases the experimental results are predicted within 10 per cent, the exceptions being associated with the shear constant C 44 and its pressure derivative. The model is least successful for LiCl, and it is shown that the addition of next-nearest-neighbor interactions can improve the situation for LiCl but not for the other three materials.