Equation of state of the α-Pb O2 and Pa 3 -type phases of Ge O2 to 120 GPa

Abstract

The compression behavior of crystalline and amorphous germania holds considerable interest as an analog for silica and for understanding the structural response of A X2 compounds generally. In this paper, the α -Pb O2 -type and P a 3 ̄ -type polymorphs of Ge O2 were investigated under high pressure using angle-dispersive synchrotron x-ray diffraction in the laser-heated diamond anvil cell. Theoretical calculations based on density functional theory were also performed. The experimental pressure-volume data were fitted to third-order Birch-Murnaghan equations of state. The fit parameters for the α -Pb O2 type are V0=53.8 (2 ) Å3,K0 T=293 (7 ) GPa with fixed K0T '=4 , where V ,KT, and KT' are the volume, isothermal bulk modulus, and pressure derivative of the bulk modulus and the subscript zero refers to ambient conditions. The corresponding parameters for the P a 3 ̄ -type phase are V0=50.3 (3 ) Å3,K0 T=342 (12 ) GPa with fixed K0T '=4 . The theoretical calculations are in good agreement with the experimental results with slight underestimation and overestimation of V0 and K0 T, respectively. A theoretical Hugoniot was calculated from our data and compared to shock equation of state data for vitreous and rutile-type Ge O2 . The high-pressure phase observed on the Hugoniot is most consistent with either the α -Pb O2 -type or CaC l2 -type phase. Finally, we have compared our data on crystalline germania with existing studies on the corresponding phases of Si O2 to better understand the effects of cation substitution on phase transformations and equations of state in group 14 dioxides.