Infrared and Raman study of lattice vibrations of CdAl2Se4 and CdAl2S4 single crystals

Infrared and Raman spectra of oriented CdAl2Se4 single crystals and of as-grown CdAl2S4 single crystals have been measured in the wavenumber range from 600 to 25 cm-1. Both ordered vacancy compounds grown by the chemical vapour transport method crystallize in the thiogallate structure with space group I. All 13 optical phonon modes are Raman active; ten of these are in addition infrared active. We report for the first time the combined infrared and Raman study of the optical phonon modes of both compounds. With the complementary information from infrared and Raman spectroscopy all optical phonon modes have been identified and assigned to their corresponding symmetry classes. The fact that all five modes of E symmetry exhibit a counterpart of B symmetry in the close spectral environment is unambiguous evidence of space group I. The measured frequencies of the nonpolar modes are in a very good agreement with data calculated in the framework of a simplified Keating model. For an oriented CdAl2Se4 single crystal the directional dispersion of the polar phonons has been studied.