Silicon coordination in rutile and TiO₂-II at ambient and high pressures: Si-29 NMR

The structural environment of silicon dissolved in rutile and α-PbO₂-structured TiO₂ (TiO₂-II) was probed using ²⁹Si MAS NMR on ²⁹Si-enriched samples. At 1 atm, about 0.01 wt% SiO₂ is incorporated into TiO₂ as IVSi, presumably in interstitial sites. Rutile recovered from 6 GPa, 1600 °C contains about 0.6 wt% SiO₂, incorporated both as ^VISi (∼90%) and ^IVSi (∼10%). TiO₂-II, synthesized at 12 GPa, 1200 °C, contains only ^VISi. The chemical shift for ^VISi in TiO₂-II is slightly less negative than that for rutile, and the peak is split, suggesting either a more complex mechanism of substitution or a different response to quenching or decompression in the lower-symmetry structure. Future thermodynamic studies of the TiO ₂-SiO₂ solid solution will have to take into account the mixed coordination environment of the Si in TiO₂, at low pressures.