Lattice dynamics of zinc chalcogenides under compression: Phonon dispersion, mode Grüneisen, and thermal expansion

A comprehensive phenomenological calculation is reported to understand the temperature- and pressure-dependent phonon properties in ZnS, ZnSe, and ZnTe. Using the pressure dependence of optical data established recently by Weinstein, we have constructed an 11-parameter rigid-ion model, which in the pressure p→0 limit goes over to the fitting of neutron scattering results of phonon dispersions. Model calculations for the lattice dynamics, frequency spectrum, mode gammas, Gr\"uneisen constant, and the linear thermal-expansion coefficient are shown to be in good agreement with the experiments. The present results not only compare well with the low-temperature experimental data, but also predict accurately the temperature region for which the Gr\"uneisen constant [γ(T)] and the linear thermal-expansion coefficient [α(T)] will be negative. We believe that the variation of αₘᵢₙ and γTO is not simply related to the ionicity but also to the physical fact that ionicity is affected by the bond charge and the bond length in zinc-blende-type crystals.