Effects of pressure and temperature on the dielectric constant of GaS, GaSe, and InSe:: Role of the electronic contribution

In this work we report on direct measurements of the temperature and pressure dependences of the low-frequency dielectric constant along c axis (epsilon(parallel to)) of GaS, GaSe, and InSe. The temperature dependence of both the ordinary and extraordinary refractive indexes is also presented. A large increase of epsilon(parallel to) under pressure has been observed. In the framework of a rigid ion model, the lattice contribution to epsilon(parallel to) is shown to increase slightly under pressure, due to the change of the angle between the anion-cation bond and the layer plane. Consequently, the pressure behavior of epsilon(parallel to) is proposed to arise from a large increase of the electronic contribution to epsilon(parallel to). This fact is explained through a decrease of the Penn gap for polarization parallel to the c axis, whose energy and pressure coefficient are shown to scale with those of the indirect band gap in these compounds. A supplementary and reversible step increase of epsilon(parallel to) is observed at 1.6 GPa in GaS, which is associated with a phase transition that has been already observed by other authors. [S0163-1829(99)11547-9].