Effect of pressure on the absolute Raman scattering cross section of SiO₂ and GeO₂ glasses

Quantitative measurements of the absolute Raman scattering intensity for the tetrahedral oxide glasses SiO₂ and GeO₂ show a dramatic decrease in the scattering cross section of the main symmetric stretching band in the pressure regions of 10-23 GPa for silica and 4-7 GPa for germania. This pressure region also corresponds to the pressure interval in which permanent structural changes occur for both glasses as observed in the Raman spectra of pressure-quenched samples when compressed in a hydrostatic medium. The noted relative enhancement of the D₂ defect peak of the pressure-quenched samples of both silica and germania begins at 10 and 4 GPa and reaches saturation by 15 and 6.5 GPa, respectively. Decompression from higher peak pressures does not result in a significantly altered spectrum. These observations are in agreement with the formation of both five- and six-coordinated metal species via the formation of ^IIIO species during the four- to six-fold coordination change of the metal atom with increasing pressure.