In order to gain new structural insights into phase D and superhydrous B, the two highest-pressuredense-hydrous-magnesium-silicate(DHMS) phasesand potential lower mantle water reservoirs, we have applied a range of one- and two- dimensional1H and 29Si NMR techniques, as well as Raman spectroscopy, to samples synthesized at 24 GPa and 900~1100 °C. These data have revealed that phase D is characterized by disordered and varying local structures around both Si and H. The observed 29Si NMR peak shape and varying Mg/Si ratios may be explained by a combination of Si-Mg substitution and Si site vacancy. The high-resolution 1H CRAMPS spectra of phase D contain three components, suggesting a distribution of hydrogen bonding distances;their average valuesare significantly shorter than those previously inferred from X-ray diffraction and Raman studies. The diffraction studies likely over-estimated the hydrogen-bonding distances, because O•••O distances without hydrogen bonding were also includedas a result of partial H site occupancy. Previously Raman studies clearly failed to detect the lower-frequency portion of the OH stretching bands, and our high-quality Raman spectra are in good agreement with the 1H CRAMPS results. For superhydrous B, three different space groups have been proposed thus far. Our comprehensive 2D 1H and 29Si NMR results have clearly revealed that it contains dissimilar hydrogen (H1-H2) pairs and one tetrahedral (and one octahedral) Si site, consistent only with space group Pnn2. The 1H chemical shifts of the two hydrogen sites differ by only 0.1 ppm, suggesting that the structure deviates only slightly from the higher-symmetry space group (Pnnm). This may explain the difficulty in unambiguously resolving the space group by X-ray diffraction. This study has demonstrated the usefulness of advanced solid-state NMR in providing detailed information about both the local structures around each elements and their connectivities, complementing diffraction techniques. (3, Jul., 2008)
Xue, X., Kanzaki, M., Shatskiy, A., Dense hydrous magnesium silicates, phase D, and superhydrous B:New structural constratints from one- and two-diminsional 29Si and 1H NMR, Am. Min., 93, 1099 – 1111, 2008., Abstract.
(Left) 1H-29Si heteronuclear correlation (HETCOR) MAS NMR spectrum of a sample synthesized at 24 GPa and 900°C. The spectrum contains two sharp 29Si peaks, attributable to tetrahedral and octahedral Si, both correlatedto the same pair of 1H peaks, and a broader peak attributable to octahedral Si in phase D. The much broader peak width of the latter is due to local structural disorder.
(Right) 1H CRAMPS-MAS NMR spectrum of the same sample. The CRAMPS dimension is free from dipolar interaction and gives the true 1H chemical shift. The broad, elongated peak is attributable to phase D, with the distribution of intensities in the CRAMPS dimension corresponding to a disordered proton distribution with a range of hydrogen-bonding distances. The two narrower peaks are attributable to two H sites in superhydrous B with slightly different chemical shifts; the much larger peak separation in the MAS dimension is due to combined effects of dipolar interaction and chemical shift anisotropy, a feature characteristic of nonequivalent H1-H2 pairs with short distances.