Raman spectroscopic study of the pressure-induced coordination change in ${\mathrm{GeO}}_2$ glass

Raman spectra of ${\mathrm{GeO}}_2$ glass are recorded in situ as a function of pressure to 56 GPa at room temperature. Under initial compression to 6 GPa the main 419-${\mathrm{cm}}^{\mathrm{-}1}$ Raman band shifts to higher frequency and broadens with a gradual loss of intensity. These spectral changes are consistent with an increase in distortion of ${\mathrm{GeO}}_4$ tetrahedra and a decrease in the intertetrahedral bond angle with pressure. Between 6 and 13 GPa (the pressure range of the reported fourfold- to sixfold-coordination change of Ge in germania glass) the main Raman band broadens, and the scattering intensity is dramatically reduced with little shift in peak frequency. This pressure interval is also marked with the appearance and growth of a broad low-frequency band near 240 ${\mathrm{cm}}^{\mathrm{-}1}$. The inferred pressure-induced coordination change occurs without the formation of nonbridging oxygens. Above 13 GPa no further major structural changes are indicated by the Raman data taken with pressures up to 56 GPa. On decompression the back transformation of octahedral Ge to tetrahedral coordination is complete but exhibits a large hysteresis. The Raman data indicate that the high-coordinate germanium species are retained down to pressures of at least 2.3 GPa. In samples decompressed from high pressures, the intensity of the 520-${\mathrm{cm}}^{\mathrm{-}1}$ ‘‘defect’’ band is considerably enhanced relative to that in normal germania glass, consistent with an increase in three-membered-ring population. It is proposed that a large component of this increase in three-membered rings is a result of the reversion of ${}_{}{}^{\mathrm{III}}{}_{}{}^{}\mathrm{O}$ species to tetrahedra-bridging ${}_{}{}^{\mathrm{II}}{}_{}{}^{}\mathrm{O}$ species under decompression.