Simple Shell-Model Calculation of Lattice Dynamics and Thermal Expansion of Alkali Halides

A six-parameter shell model which takes into account anion polarizability, noncentral nearest-neighbor interactions, central second-neighbor interactions, and the effective charge of the core and shell of the anion has been utilized for the determination of the lattice dynamics of LiF, NaCl, KCl, and RbI. The model parameters are determined from experimental data on the long-wavelength optical-mode phonon frequencies, the three elastic constants, and the high- and low-frequency dielectric constants. By the incorporation of the pressure derivatives of these quantities into the lattice-dynamical model, the individual-mode Gr\"uneisen parameters and the temperature dependence of the thermal-expansion coefficient are calculated. Reasonable agreement is found between the calculated values and the available experimental data on these quantities.