Stishovite single-crystal growth and application to silicon self-diffusion measurements

Large single crystals of stishovite were successfully synthesized at 11 GPa from a silica solution in water. The potential of both slow cooling and thermal gradient methods were examined. The thermal gradient method provided crystals of 0.8 × 0.8 × 1.3 mm in size grown at 1350 °C and a thermal gradient of 50 °C/mm using stishovite as a silica source. The use of quartz: as a source resulted in the appearance of numerous stishovite crystals in the solution interior resulting in diminished space for the growth of large crystals. This can be explained by a significant difference in the solubility of metastable quartz and stishovite in water, estimated to be 85.3 and 5.6 wt% SiO₂ at 1000 °C and 11 GPa, respectively. Crystals up to 0.8 × 1.3 × 1.5 mm were grown by the slow cooling method in the system SiO₂ + 14.7 wt% H₂O as temperature was decreased from 1600 to 1000 °C with a cooling rate of 2 °C/min. The size of single crystals obtained was large enough to carry out silicon self-diffusion experiments, which were performed at a pressure of 14 GPa and temperatures from. 1400 to 1800 °C. The lattice diffusion coefficients along the [110] and [001] directions can be expressed as D_[110] (m²/s) = 4.10 × 10 ^-12 exp [-322 (kJ/mol)/RT] and D_[001] (mVs) = 5.62 × 1.0^-12 exp [-334 (kJ/mol)/RT], respectively, where R is the gas constant and T is the absolute temperature.