Lattice Dynamics of Wurtzite: CdS

Abstract

A mixed valence-Coulomb force field has been constructed and used to calculate phonon frequencies for waves propagating in three principal directions in CdS wurtzite, and in two directions of a CdS sphalerite structure. Using calculated phonon frequencies, the observed two-phonon Raman spectrum in CdS wurtzite is interpreted. Phonon symmetries and energy shifts predicted by the model have been verified by infrared and Raman experiments. The qualitative and quantitative importance of both valence and Coulomb forces in materials like CdS is demonstrated. This has implications for interpreting phonon studies in mixed crystals with wurtzite, and with sphalerite structure. The valence-Coulomb force-field model may have general applicability to other crystals, for which a shell model cannot be applied.