RAMAN-SPECTRA OF TETRAHEDRAL VIBRATIONS IN CRYSTALLINE GERMANIUM DICHALCOGENIDES, GES2 AND GESE2, IN HIGH AND LOW-TEMPERATURE FORMS

Raman spectra of GeX2 (X = S and Se) crystals in high (HT) and low (LT) temperature forms have been studied on the view point of the connectivity of GeX4 tetrahedra. The characteristic Raman bands are understood by a simple valence-force-field model combined with a bond polarizability model. The most intense Raman bands in HT-GeX2 have been assigned to tetrahedral A1 vibrations extended in the chain structures along the [100] direction. Quasi-localized A1 vibrations in the bridge structures in HT-GeX2 are responsible for a higher side-band of the main band. The main bands in the spectra of LT-GeX2 have been assigned to nearly degenerated A1 vibrations linearly extended in the two chains along the [001] direction. The difference between the spectra of GeS2 and GeSe2 has been roughly explained to be due to the difference between atomic masses.