REGULAR RING DYNAMICS IN AX2 TETRAHEDRAL GLASSES

We review the experimental evidence and qualitative arguments for the existence of small highly regular rings of bonds in amorphous (a-)SiO2 and selected other AX2 tetrahedral glasses. The structure and vibrations of planar 3-rings and regular puckered 4-rings in a-SiO2 are then modeled using Born central and noncentral forces. The vibrational coupling of these rings to the more disordered glass network is modeled by attaching a Bethe lattice at each connection. The calculated vibrational properties of the breathing modes of the rings are found to be quite consistent with the observed frequencies, linewidths, and isotope shifts of the sharp lines D1 and D2 seen in the Raman spectra of a-SiO2. The results support the previous assignment of D2 (606 cm-1) to a planar 3-ring and D1 (495 cm-1) to a regular ring. Similar calculations for a-GeO2 are consistent with the suggestion that this material contains a substantial concentration of nearly planar 3-rings. Our methods can be generalized to treat similar forms of intermediate range order in glasses having other network connectivities.