High-pressure Raman spectroscopic study of sodium tetrasilicate (Na2Si4O9) glass

The Raman spectrum of sodium tetrasilicate (Na2Si4O9) glass has been obtained as a function of pressure at 298 K to approximately 50 GPa. The scattering intensity of bands associated with silica tetrahedra containing nonbridging oxygens (Q3 species) decreases with increasing pressure and these bands are absent above 20 GPa. The spectral changes observed over this pressure interval are consistent with the formation of high-coordinate Si species at the expense of nonbridging oxygens. Above 33 GPa, the mode of change of the Raman spectrum with pressure changes abruptly. The main Raman band markedly broadens and weakens, accompanied by a twofold increase in the pressure derivative of the peak maximum. These spectral changes indicate the onset of a second densification mechanism operative at higher pressures. We conjecture that above 33 GPa, high coordinate silicon species are formed through the involvement of bridging oxygens (i.e., formation of IIIO species). The changes in the Raman spectrum associated with this latter mechanism are found to be reversible on decompression. However, spectral changes observed at lower pressures (<20 GPa) are not fully reversible and decompressed glass samples do not completely relax to their normal 1 atm state. Although silica tetrahedral species containing nonbridging oxygens hysteretically reform at low pressures on the decompression path, a comparison of the Raman spectra of pressure-quenched glass samples with that of normal sodium tetrasilicate glass indicates a change in distribution of these species. In particular, there is an increase in the relative abundance of Q2 species (silica tetrahedra containing two non-bridging oxygens) in samples decompressed from high pressures. We emphasize that Q2 species are not formed under pressurization but are formed on the decompression path and may be due to the reversion of high-coordinate silicon species to tetrahedral species under pressure release.