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 (ε_∥) of GaS, GaSe, and InSe. The temperature dependence of both the ordinary and extraordinary refractive indexes is also presented. A large increase of ε_∥ under pressure has been observed. In the framework of a rigid ion model, the lattice contribution to ε_∥ 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 ε_∥ is proposed to arise from a large increase of the electronic contribution to ε_∥. 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 ε_∥ is observed at 1.6 GPa in GaS, which is associated with a phase transition that has been already observed by other authors.