Lattice dynamics of tetrahedrally bonded semiconductors containing ordered vacant sites

Infrared and Raman spectra of defect-structure semiconductors ${\mathrm{In}}_2$ ${\mathrm{Te}}_3$, ${\mathrm{Ga}}_2$ ${\mathrm{Se}}_3$, and ${\mathrm{Ga}}_2$ ${\mathrm{S}}_3$ with ordered vacant sites were measured on polycrystalline samples and analyzed. The interpretation of the observed spectra is based on the existence of uniterrupted chains of tetrahedra along the directions interconnecting the two next-nearest neighbors (the [110] direction in the cubic lattice). Arguments are presented to show that the phonons propagating along this direction dominate the spectra, and that in the first approximation their frequencies can be obtained by a sixfold folding of the Brillouin zone of the simple zinc-blende structure in the [110] direction. The observed spectra were fitted by a rigid-ion model of the corresponding zinc-blende structure, and the dispersion relations for this structure were obtained. It is shown that the defects affect the central forces between the nearest neighbors only slightly while the noncentral forces are changed more significantly. The softening of a mode in ${\mathrm{In}}_2$ ${\mathrm{Te}}_3$ may point out an instability of its ordered structure.