A single-crystal X-ray diffraction study of the 3.65 Å-phase MgSi(OH)6, a high-pressure hydroxide perovskite

The structure of the 3.65 A-phase MgSi(OH)6 has been determined by single-crystal X-ray diffraction for a crystal synthesized at 10 GPa, 450 °C, 205 h from MgO·SiO2 gel and H2O. The crystal structure at 300 K was solved and refined in space group P21/n to R 1 = 0.038, wR 2 = 0.112, GoF = 1.331. Unit-cell parameters are: a = 5.1171(1) A, b = 5.1932(1) A, c = 7.3364(1) A, β = 90.017(2)°, V = 194.960(6) A3. Several crystals were examined and consistent results were obtained in refinements. Despite the metrically orthorhombic unit cell, reflection merging in point group mmm conclusively excludes orthorhombic symmetry, for which R int values of 0.23–0.27 were obtained for various crystals. A few violators of the n-glide were observed in two of the larger crystals examined. An attempt was made to solve and refine in space group P21 in order to test the theoretical structure that Wunder et al. (2012) derived from Density Functional Theory. It was possible to refine the structure in space group P21, but there were strong correlations between atoms that are equivalent in P21/n, and also unsatisfactory behaviour of the Flack parameter that did not allow the absolute structure to be determined. Overall, the X-ray data do not allow convincing derivation of a P21 model. The basic framework motif of alternating corner-sharing Mg(OH)6 and Si(OH)6 octahedra identified by Wunder et al. (2012) is confirmed. The octahedral tilts with reference to the pseudo-cubic axes of the aristotype are − − + , corresponding to one in-phase and two out-of-phase tilts. The IR spectra reported by Wunder et al. (Am Mineral 96:1207–1214, 2011), which contain ≥6 OH absorption peaks are consistent with symmetry lower than P21/n, at least at the local level. As such the non-H structure, and thereby X-ray diffraction, may be quite insensitive to local H behaviour.