Saturation curve of SiO₂ component in rutile-type GeO₂: A recoverable higherature pressure standard from 3 GPa to 10 GPa

The accuracy and precision of pressure measurements and the pursuit of reliable and readily available pressure scales at simultaneous high temperatures and pressures are still topics in development in high pressure research despite many years of work. In situ pressure scales based on x-ray diffraction are widely used but require x-ray access, which is lacking outside of x-ray beam lines. Other methods such as fixed points require several experiments to bracket a pressure calibration point. In this study, a recoverable higherature pressure gauge for pressures ranging from 3 GPa to 10 GPa is presented. The gauge is based on the pressure-dependent solubility of an SiO₂ component in the rutile-structured phase of GeO₂ (argutite), and is valid when the argutite solid solution coexists with coesite. The solid solution varies strongly in composition, mainly in pressure but also somewhat in temperature, and the compositional variations are easily detected by x-ray diffraction of the recovered products because of significant changes in the lattice parameters. The solid solution is measured here on two isotherms, one at 1200°C and the other at 1500°C, and is developed as a pressure gauge by calibrating it against three fixed points for each temperature and against the lattice parameter of MgO measured in situ at a total of three additional points. A somewhat detailed thermodynamic analysis is then presented that allows the pressure gauge to be used at other temperatures. This provides a way to accurately and reproducibly evaluate the pressure in high pressure experiments and applications in this pressureerature range, and could potentially be used as a benchmark to compare various other pressure scales under high temperature conditions.