Results of investigations of absorption and photoluminescence spectra of layered crystals GaSe, InSe, and GaS under uniaxial (parallel to crystal c-axis) pressure in the temperature range 4.2 to 150 K are presented. Pressure does not exceed 1 kbar. It is shown that the deformation potentials describing the variation of direct energy gap under uniaxial (parallel to c-axis) pressure in layered crystals of GaSe type change their signs near T = 80 K. This peculiarity is connected with the existence of two strongly different forces (covalent and van der Waals type) binding the atoms within the unit cell of layered crystals.
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Belenkii et al. (1981) studied this question.
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