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

Abstract

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 $p\ensuremath{\rightarrow}0$ 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 [$\overline{\ensuremath{\gamma}}(T)$] and the linear thermal-expansion coefficient [$\ensuremath{\alpha}(T)$] will be negative. We believe that the variation of ${\ensuremath{\alpha}}_{min}$ and ${\ensuremath{\gamma}}_{\mathrm{TO}}$ 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.