Directional Dispersion of the Phonon Modes in Optically Uniaxial Solids, Far‐Infrared Reflection Spectra, Dielectric and Optic Constants, Dynamic Effective Ionic Charges of the Defect Chalcopyrites CdGa2S4, CdGa2Se4, HgGa2S4, and HgGa2Se4

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Abstract
The polarized far‐infrared reflection spectra of single crystals of the defect chalcopyrites CdGa2S4 CdGa2Se4, HgGa2S4, and HgGa2Se4 grown by vapour phase transport technique are presented in the range from 40 to 600 cm−1. The spectra reveal ten reststrahlen bands as predicted by group theoretical treatment. The oscillator parameters εα∞, ωαf, ωαf, and γαf, the transverse and longitudinal optical phonon frequencies ωTO and ωLO, and the dynamic effective ionic charges are calculated using classical oscillator fit method. The high‐frequency dielectric constants εα∞ show an anisotropic behaviour, the effective charges (Szigeti charges), however, are approximately scalar quantities. From the obtained oscillator parameters the directional dispersion of the extraordinary polar phonons is determined. The reflection spectra calculated for oblique phonons are discussed and compared to those of pellets hot‐pressed from polycrystalline samples.