Splitting and Coupling of Lattice Modes in the Layer Compounds GaSe, GaS, andGaSexS1−x

The $\overrightarrow{\mathrm{k}}\approx 0$ phonon spectra of the layer crystals GaSe, GaS, and their solid solutions are presented. All phonon bands in GaSe ($D_{3h}$ symmetry) and GaS ($D_{6h}$ symmetry) are assigned and good agreement prevails between experimental results and group-theory predictions. It is shown that GaSe behaves approximately like a two-dimensional crystal; judging by the display of its phonons the interaction between layers given rise to Davydov doublets. It is demonstrated that these splittings in GaSe may be accurately measured. Owing to lack of inversion symmetry both components of some of the doublets are Raman active, and thus the splitting is yielded directly by a single Raman experiment. The largest splitting was found to be about 7 ${\mathrm{cm}}^{-1\mathrm{}}$. A correspondence between GaSe doublets and GaS single Raman bands is proven. A detailed study of $\overrightarrow{\mathrm{k}}\approx 0$ phonons in $\mathrm{Ga}{\mathrm{Se}}_x{\mathrm{S}}_{1-x}$ solid solutions is presented. It shows that these solid solutions yield an exceptional behavior of the phonon modes; some of the bands are of the one-mode type, while the other are of the two-mode type. In the two GaSe $A$ doublets, mode coupling between the two conjugate lines occurs, and both repulsion and intensity transfer are revealed and discussed.