Optically Active Phonon Processes in CdS and ZnS

Transmission spectra of thin (10 to 30 μ) CdS single-crystal samples have been recorded in the hitherto uninvestigated region of 250 to 400 ${\mathrm{cm}}^{-1\mathrm{}}$. A number of new absorption maxima are located which necessitated a new phonon assignment with considerably lower values of the acoustic modes. Some 30 absorption peaks are attributed to multiphonon combinations of the following Brillouin zone-edge frequencies: LO=295 ${\mathrm{cm}}^{-1\mathrm{}}$, T${\mathrm{O}}_1$=261 ${\mathrm{cm}}^{-1\mathrm{}}$, T${\mathrm{O}}_2$=238 ${\mathrm{cm}}^{-1\mathrm{}}$, LA=149 ${\mathrm{cm}}^{-1\mathrm{}}$, T${\mathrm{A}}_1$=79 ${\mathrm{cm}}^{-1\mathrm{}}$, and T${\mathrm{A}}_2$=70 ${\mathrm{cm}}^{-1\mathrm{}}$. The transmission spectrum of hexagonal ZnS is obtained for the first time in the region 300 to 750 ${\mathrm{cm}}^{-1\mathrm{}}$. Again some 27 absorption maxima are assigned to multiphonon combinations of LO=346 ${\mathrm{cm}}^{-1\mathrm{}}$, T${\mathrm{O}}_1$=318 ${\mathrm{cm}}^{-1\mathrm{}}$, T${\mathrm{O}}_2$=297 ${\mathrm{cm}}^{-1\mathrm{}}$, LA=181 ${\mathrm{cm}}^{-1\mathrm{}}$, T${\mathrm{A}}_1$=92 ${\mathrm{cm}}^{-1\mathrm{}}$, and T${\mathrm{A}}_2$=73 ${\mathrm{cm}}^{-1\mathrm{}}$. The phonon assignment for the optical lattice absorption in cubic ZnS due to Deutsch is shown to be inconsistent with the requirements of the Brout sum rule and other regularities expected for the zincblende-type semiconductors. Two new assignments (LO=339 ${\mathrm{cm}}^{-1\mathrm{}}$, TO=298 ${\mathrm{cm}}^{-1\mathrm{}}$, LA=155 ${\mathrm{cm}}^{-1\mathrm{}}$, and TA=93 ${\mathrm{cm}}^{-1\mathrm{}}$, and LO=339 ${\mathrm{cm}}^{-1\mathrm{}}$, TO=298 ${\mathrm{cm}}^{-1\mathrm{}}$, LA=190 ${\mathrm{cm}}^{-1\mathrm{}}$, and TA=115 ${\mathrm{cm}}^{-1\mathrm{}}$) only slightly different from each other are proposed. The characteristic phonon frequencies are discussed in terms of structure and effective ionic charge.