Silicon coordination in rutile and TiO_2-II at ambient and high pressures: Si-29 NMR

The structural environment of silicon dissolved in rutile and α-PbO_2-structured TiO_2 (TiO_2-II) was probed using ^(29)Si MAS NMR on ^(29)Si-enriched samples. At 1 atm, about 0.01 wt% SiO_2 is incorporated into TiO_2 as ^(IV)Si, presumably in interstitial sites. Rutile recovered from 6 GPa, 1600 °C contains about 0.6 wt% SiO_2, incorporated both as ^(VI)Si (~90%) and ^(IV)Si (~10%). TiO_2-II, synthesized at 12 GPa, 1200 °C, contains only ^(VI)Si. The chemical shift for ^(VI)Si in TiO_2-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-SiO_2 solid solution will have to take into account the mixed coordination environment of the Si in TiO_2, at low pressures.