Shell Model Calculation of Microscopic Grüneisen Parameters for Rocksalt‐Type Materials

The microscopic Grüneisen parameters have been calculated for the two optical modes in the Brillouin zone center and for the four modes at the Brillouin zone edge in [100] for nine alkali halides and MgO. A six parameter shell model which takes into account anion polarizability, non‐central first nearest neighbor interaction and central forces between second neighbors has been used, and the pressure dependence of the frequencies is obtained from the assumption of a pressure independent core‐shell force constant. The parameters of the model are determined from experimental data of the elastic constants, from the high and low frequency dielectric constants, from the pressure derivatives of these quantities, and from the zero pressure value of the transverse optical frequency in the zone center. Good to fair agreement with the available experimental data is found. It is suggested that the discrepancies are mainly due to the limited experimental accuracy. The results confirm that the B1 → B2 phase transformation in alkali halides is not due to an instability of the TA mode at the [100] zone boundary.