Optical Phonons in Mixed Crystals ofCdSeyS1−y

The infrared reflectivity spectra of mixed crystals of $\mathrm{Cd}{\mathrm{Se}}_{y}{\mathrm{S}}_{1\ensuremath{-}y}$ have been measured at 15 and 300\ifmmode^\circ\else\textdegree\fi{}K. The infrared-active phonons occur in two main bands with frequencies near those of pure CdS and pure CdSe. Some features of the spectra are similar to the lattice vibrations in the $\mathrm{Ga}{\mathrm{As}}_{y}{\mathrm{P}}_{1\ensuremath{-}y}$ system. The model used for that system is extended to the wurtzite structure. The main features of this model are the dependence of the phonon frequencies and strengths on concentration $y$ and on the short-range clustering of the cations around the anions. Compared with $\mathrm{Ga}{\mathrm{As}}_{y}{\mathrm{P}}_{1\ensuremath{-}y}$, the clustering behavior is found to be significantly less for the present system.