EVIDENCE FOR RINGS IN THE DYNAMICS OF TETRAHEDRAL GLASSES

Following Galeener, the experimental evidence and qualitative arguments for the existence of small regular rings in amorphous SiO2 and similar AX(2) tetrahedral glasses are reviewed. The vibrations of planar three-rings and puckered four-rings are calculated using central and non-central forces in a born potential, while the rest of the network is modelled by Bethe lattices. It is shown that the symmetric oxygen motions in the rings, which are expected to have a large Raman polarisability, are nearly decoupled from the rest of network and are in quantitative agreement with the sharp 'defect' lines D-1 and D-2 observed in the Raman spectra of SiO2 at 495 and 606 cm(-1) respectively.