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 amo rphized. Measured densities of the amorphous phases increased with inc reasing synthesis pressure: the amorphous phase synthesized at 18 GPa was 32 % denser than GeO2 quartz. Enthalpies of the amorphous phases r elative to GeO2 quartz and glass prepared at 1 atm were measured by le ad berate solution calorimetry. The enthalpies increased almost linear ly with decreasing molar volumes from GeO2 quartz to the 18 GPa-amorph ous phase. The enthalpy of the 18GPa-amorphous phase was about 18 kJ/m ol higher than GeO2 quartz, and about 4 kJ/mol higher than the 1 atm-g lass. The higher enthalpy relative to GeO2 quartz is interpreted as th e energy associated with disordering of the quartz structure and colla pse of the tetrahedral framework which lead to octahedral coordination of germanium atoms.