Enthalpy and density measurements of pressure-amorphized GeO2 quartz

Quartz-type GeO2 was compressed at room temperature up to pressures of 11, 15 and 18 GPa, and the samples recovered to 1 atm were mostly amorphized. Measured densities of the amorphous phases increased with increasing synthesis pressure: the amorphous phase synthesized at 18 GPa was 32 % denser than GeO2 quartz. Enthalpies of the amorphous phases relative to GeO2 quartz and glass prepared at 1 atm were measured by lead berate solution calorimetry. The enthalpies increased almost linearly with decreasing molar volumes from GeO2 quartz to the 18 GPa-amorphous phase. The enthalpy of the 18GPa-amorphous phase was about 18 kJ/mol higher than GeO2 quartz, and about 4 kJ/mol higher than the 1 atm-glass. The higher enthalpy relative to GeO2 quartz is interpreted as the energy associated with disordering of the quartz structure and collapse of the tetrahedral framework which lead to octahedral coordination of germanium atoms.