THERMAL-EXPANSION AND ANHARMONIC PHONON PHONON INTERACTION EFFECTS ON AN EFFECTIVE CHARGE IN GAAS AND KCL

The effects of anharmonic phonon-phonon interactions on an effective charge Z(c)*(T)e in a typical semiconductor GaAs and an ionic compound KCl are investigated by dividing the frequency shifts of TO and LO phonons into two parts depending on the volume and the anharmonic phonon-phonon interaction. The Z(c)*(T) value in GaAs is mainly decreased by the increasing volume but that of KCl is decreased dominantly by the phonon-phonon interaction, though the increase of volume enhances the charge. This difference between these compounds is understood by considering the contribution of crystal anharmonicity to the ionic charge. Such a consideration can also interpret curious temperature dependence of effective charge which has been observed in several semiconductors and a few superionic conductors.