A Complete Solid Solution with Rutile-Type Structure in SiO₂-GeO₂ System at 12 GPa and 1600°C

High pressure and temperature synthesis of compositions made of (Si_1-x,Ge_x)O₂ where x is equal to 0, 0.1, 0.2, 0.5, 0.7, and 1 was performed at 7-12 GPa and 1200-1600°C using a Kawai-type high-pressure apparatus. At 12 GPa and 1600°C, all the run products were composed of a single phase with a rutile structure. The lattice constants increase linearly with the germanium content (x), which indicates that the rutile-type phases in the SiO₂-GeO₂ system form a complete series of solid solutions at these pressure and temperature conditions. Our experimental results show that thermodynamic equilibrium state was achieved in this system at 12 GPa and 1600°C, but not at 1200°C. At lower pressures (7 and 9 GPa) and 1600°C, we observed the decomposition of (Si_0.5,Ge_0.5)O₂ into SiO₂-rich coesite and GeO₂-rich rutile phases. The silicon content in the rutile structure increases sharply with pressure in the vicinity of the coesite-stishovite phase transition pressure in SiO₂.