Raman-active modes of a-GeSe2 and a-GeS2:: A first-principles study

We have used a recently developed computational technique based on density-functional theory to study the Raman-active modes of amorphous GeSe2 and GeS2. Vibrational modes and the associated Raman activities for three cluster building blocks of the glasses are calculated directly from first principles. The positions of the calculated symmetric-stretch modes in the cluster models ape in excellent agreement with sharp features in the observed spectra. Moreover, simulated spectra based on the cluster results are in good agreement with experiment, accounting for all the observed features in the bond-stretch region of the spectra. The cluster results suggest a new interpretation for the 250 cm(-1) mode appearing in the spectra of Ge-rich samples in the GexS1-x family. [S0163-1829(99)51846-8].