RAMAN BAND ASSIGNMENTS OF SILICATE AND GERMANATE GLASSES USING HIGH-PRESSURE AND HIGH-TEMPERATURE SPECTRAL DATA

The assignments of Raman bands in the spectra of silicate and germanat e glasses, including SiO2, GeO2, Rb2Si4O9, NaFe3+Si3O8, anhydrous and hydrous albite (NaAlSi3O8) and anhydrous and deuterated Ge-albite (NaA lGe3O8) and Ge-jadeite (NaAlGe2O6), were re-examined in the light of n ew information from Raman measurements at high pressures and temperatu res. On the basis of polarized Raman spectra of single-component (e.g. SiO2 and GeO2) glasses, it is demonstrated that the nu(as)(T-O-T) mod e (where T represents tetrahedrally coordinated cations, Si or Ge) giv es rise to LO-TO splitting. The depolarized nature of the nu(as)(T-O-T ) LO-TO doublet in the Raman spectra of SiO2 and GeO2 glasses reflects the fact that the local site symmetry of T cations is close to T-d. P ersistence of the nu(as)(T-O-T) LO-TO doublet slightly above the melti ng temperature demonstrates that significant intermediate range order exists even in the high-temperature liquids. Likewise, in the spectra of NaFe3+Si3O8 and NaFe3+Si2O6 glasses, the high-wavenumber nu(as)(T-O -T) bands exhibit a depolarized character characteristic of a three-di mensional network. The depolarized nature of the nu(as)(T-O-T) bands i ndicates that the local site symmetry of the T cation is close to T-d and that the triply degenerate (F) character of the nu(as)(T-O-T) is p reserved. Additionally, a significant shift in the position of the nu( as)(T-O-T) mode on substituting NaFe3+ for Si4- reflects strong vibrat ional coupling of FeO4 and SiO4 tetrahedra in the glass. In the hydrat ed and deuterated aluminosilicate and -germanate glasses, it is propos ed that the degeneracy of the nu(as)(T-O-T) bands is lifted because of structural distortion caused by the presence of H3O+ (D3O+) ions. (C) 1997 John Wiley & Sons, Ltd.