Elastic Constants of the Layered Compounds GaS, GaSe, InSe, and Their Pressure Dependence. II. Theoretical Part

The block diagonalization of the dynamical matrix of β‐GaS is reported in the case of propagation perpendicular to the layers. A linear chain model that includes intralayer interactions between any atoms is introduced, the principal frequencies and the elastic constants C 33 and C 44 are calculated. Within the model, the existence of real coupling parameters is discussed, that leads to an evaluation of the intralayer forces contribution to the elastic constants. The evolution of the interlayer interactions with pressure is studied, using previously published experimental results. Finally, as far as possible, the theoretical formulas are applied to the similar structures of ϵ‐GaSe and γ‐InSe.