Temperature Dependence of the Linear Thermal Expansion Coefficient and Phonon Curves at the Covalent‐Ionic Transition Pressure of ZnS, ZnSe, and ZnTe

Abstract

The phonon curves of tetrahedrally covalent compounds have been obtained by inelastic neutron scattering experiments, and recently, with the improvement of the diamond-anvil high pressure cell, the pressure effect on the phonon frequencies has been energetically studied. Experimental data on the elastic stiffness constants related to long-wave phonons were previously obtained by the ultrasonic-pulse technique, and their pressure dependence has been reported from the variation of ultrasonic wave velocities under hydrostatic pressure. The negative thermal expansion at low temperatures in covalent crystals has been reported experimentally. Useful experimental data on ZnS, ZnSe, and ZnTe a r e those for phonon dispersion curves /1 to 3/, the pressure derivatives of the elastic stiffness constants /4, 5/, the pressure dependence of phonon frequencies /6 to 9/, the linear expansion coefficients /lo, 11/, and the specific heat capacities /12, 13/.Ge, we /14/ have proposed a phenomenological theory reproducing the experimentally observed temperature dependence of the thermal expansion coefficient o~, i. e. the Griineisen constant r . Then, we /15/ have extended our previous treatment from the diamond-type crystals to the zinc-blende-type ones, and estimated the phonon dispersion curves at the covalent-metallic transition pressure of GaAs, GaSb, and InSb using Griineisen parameters T,C $ ) of the individual modes of phonons. In the present work, we apply our previous treatment /14, 15/ to the lattice dynamics of If-VI covalent compounds ZnS, ZnSe, and ZnTe near the covalent-ionic transition pressure.Recently, in order to sthdy the lattice dynamics under pressure of.Si and 1) Akita 010, Japan.