Ab initio calculation of vibrational frequencies of GexSe1-x glass

We have used the density functional theory to make the models of GexSe1-x glass for which the energy is a minimum. The clusters, Ge2Se2, Ge2Se3, Ge3Se, Ge3Se2, Ge4Se, GeSe3, GeSe4, chain mode zig-zag Ge4Se3, corner sharing GeSe4, and edge sharing Ge2Se6, have been made successfully and their vibrational spectra have been calculated from the first principles. We are able to optimize the bond distances as well as the bond angles. The calculated values of the frequencies of vibrations of the various clusters have been compared with those obtained from the experimental Raman spectra of actual glasses, GexSe1-x(0 < x < 0.3). The local concentration, x within 0.25 nm is nonuniform in the amorphous material. When the same cluster occurs in two stable configurations, low frequency vibrations of frequency, v < 100 cm(-1), are found. The corner sharing GeSe4 has low frequency modes at 54 cm(-1) and 93 cm(-1) whereas these modes disappear in the pyramidal configuration. The low frequency modes are therefore associated with the breaking of C-4 symmetry of the pyramidal configuration. The computed vibrational frequencies of clusters Ge-3, Ge4Se3, Ge2Se3, GeSe3 and Ge3Se2 are actually present in the Raman spectra of the glass, GexSe1-x(0 < x < 0.3). (c) 2007 Elsevier B.V. All rights reserved.