Equation of state of the rutile and CaCl2-type phases of GeO2 to 70 GPa

Abstract

The high-pressure behavior of rutile- and CaCl2-type phases of GeO2 was studied using laser-heated diamond anvil cell experiments coupled with in situ synchrotron x-ray diffraction and theoretical calculations based on density functional theory. Both the experimental and theoretical data were fit to third-order Birch–Murnaghan equations of state. The resulting experimental fit parameters for the rutile-type phase are V0=55.33 Å3, K0T=267±10 GPa, and K0T′=5.6±1.2, where V, KT, and KT′ are the volume, isothermal bulk modulus, and pressure derivative of the bulk modulus, respectively, and the subscript 0 refers to ambient conditions. The CaCl2-type phase could be fit using V0=55.96±0.16 Å3, K0T=238±6 GPa, and K0T′=4 (fixed). Fit parameters from theoretical calculation for a rutile-type phase are V0=52.15 Å3, K0T=261 GPa, and K0T′=5.0, whereas for the CaCl2-type phase, V0=52.44 Å3, K0T=248 GPa, and K0T′=4. The theoretical fit parameters for both phases are in good agreement with the experiments. The rutile-CaCl2 phase transition is second order in nature driven by an orthorhombic distortion of the tetragonal phase. To understand the rutile-to-CaCl2-type phase transition in further detail, we have used a Landau-type model. The Landau-type model improves the quality of the fit, as evidenced by the significant decrease in the difference between measured and calculated pressures obtained from the fits. We have also compared the density of crystalline SiO2 and GeO2 from x-ray diffraction with their amorphous counterparts to evaluate the relative compression behavior of crystalline and glassy materials for the two compositions. The compression behavior of four phases of GeO2 (rutile-, CaCl2-, α-PbO2-, and Pa3¯-type polymorphs) now has been constrained on compression up to 120 GPa using a consistent pressure-transmitting medium (Ne) and a pressure calibrant (Pt).