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In the past decade numerous crystalline phases have been reported to undergo a crystalline to amorphous transition as a function of pressure at room temperature. For quartz and hexagonal GeO₂, disordering occurs at a length scale of less than 100 Å as a result of failure of the oxygen sublattice to achieve a cubic body-centered symmetry upon densification. While many studies have focussed on the structural details of pressure-induced amorphization, nothing about the energetics of this process has yet been determined. Particularly important in this respect is the comparison of pressure-amorphized materials with densified glasses. To investigate these aspects, we have measured enthalpies of solution in hydrofluoric acid of GeO₂ glass and hexagonal crystals compressed statically up to pressures of 15 GPa in a multi-anvil apparatus. For both kinds of materials, we have found that the same linear relationship obtains between the density and the enthalpy of solution (Fig. 3.3-5). Increasing the density
 

Fig. 3.3-5: The relationship between enthalpy of solution and density for various forms of GeO2. Solid symbols are glasses, open symbols amorphised crystals, crosses crystalline polymorphs.

from 3.6 g/cm³ (glass at 1 atm) to 5.0 g/cm³ (crystal amorphized at 15 GPa) leads to an increase of 60 kJ/mol. By comparison, the enthalpy of the transition between the hexagonal and tetragonal forms is only 20 kJ/mol for an increase in the density from 4.3 to 6.3 g/cm³. Consistent with available EXAFS, infrared and Raman data, the measured densities indicate that Ge is essentially four-fold coordinated in the decompressed samples. The energy stored in densified materials through straining of bonds is thus three times as high as that needed for the reconstructive crystalline phase transition.