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Abstract
In this work we report on the pressure dependence of the low-frequency dielectric constant parallel to the c-axis (ε∥) in GaS, GaSe, and InSe as obtained from direct capacitance measurements. A large increase of ε∥ with pressure has been observed. The pressure change of the lattice polarizability along the c-axis is calculated in the framework of a rigid-ion model from the change of the angle of the anion–cation bond with respect to the layer plane, which results in a slight increase of the lattice contribution. Consequently, the pressure behaviour of ε∥ is proposed to arise from the large increase of the electronic polarizability along the c-axis. This is explained through a decrease of the Penn gap, whose energy and pressure coefficients are shown to scale with those of the indirect gap in these compounds. A supplementary and reversible step-increase of ε∥ has been observed at 1.6 GPa in GaS. This increase has been associated to a phase transition that was reported by other authors.
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