VIBRATIONAL-SPECTRA OF RINGS IN SILICATE-GLASSES

Isotropic, HH and HV Raman as well as infrared spectra of isolated n-membered rings (n = 3, 4, 5, 6, 7) of ideal silicon-oxygen tetrahedra are calculated. The dependences of the spectral line frequencies and intensities on the number of tetrahedra in the ring as well as on the type and the degree of structural disorder are studied. The results show that the ring order influences the mode localization, thus changing the spectral line positions and intensities, while the structural disorder (ring puckering and topology) affects mainly the vibrations of the bridging oxygen atoms changing the frequencies and intensities of the peaks corresponding to the modes localized in this type of atom. The effect of the surrounding matrix upon vibrational properties of the rings is briefly discussed and a better, but by no means full, agreement with experiment is obtained by taking into account this effect.