The unique cation-disordered crystal structures of two samples of phase E, a non-stoichiometric, hydrous silicate synthesized in a uniaxial, split-sphere, multi-anvil apparatus at conditions above 13 GPa and 1000-degrees-C, have been solved and refined in space group R3mBAR. The compositions and unit cells for the two materials, assuming six oxygens per cell, are Mg2.08Si1.16H3.20O6, a = 2.9701(1) angstrom, c = 13.882(1) angstrom, V = 106.05(4) angstrom3 for sample 1, and Mg2.17Si1.01H3.62O6, a = 2.9853(6) angstrom, c = 13.9482(7) angstrom, V = 107.65(4) angstrom3 for sample 2. The structure contains layers with many features of brucite-type units, with the layers stacked in a rhombohedral arrangement. The layers are cross linked by silicon in tetrahedral coordination and magnesium in octahedral coordination, as well as hydrogen bonds. Interlayer octahedra share edges with intralayer octahedra. Interlayer tetrahedra would share faces with intralayer octahedra. To avoid this situation, there are vacancies within the layers. There is, however, no long-range order in the occupation of these sites, as indicated by the lack of a superstructure. Selected-area electron diffraction patterns show walls of diffuse intensity similar in geometry and magnitude to those observed in short-range-ordered alloys and Hagg phases. Phase E thus appears to represent a new class of disordered silicates, which may be thermodynamically metastable.

The unit-cell dimensions and crystal structure of sillimanite at various pressures up to 5.29 GPa have been refined from single-crystal X-ray diffraction data. As pressure increases, a and b decrease linearly, whereas c decreases nonlinearly with a slightly positive curvature. The axial compression ratios at room pressure are beta(a):beta(b):beta(c) = 1.22:1.63:1.00. Sillimanite exhibits the least compressibility along c, but the least thermal expansivity along a (Skinner et al. 1961; Winter and Ghose 1979). The bulk modulus of sillimanite is 171(1) GPa with K' = 4 (3), larger than that of andalusite (151 GPa), but smaller than that of kyanite (193 GPa). The bulk moduli of the [Al1O(6)], [Al2O4], and [SiO4] polyhedra are 162(8), 269(33), and 367(89) GPa, respectively. Comparison of high-pressure data for Al2SiO5 polymorphs reveals that the [SiO4] tetrahedra are the most rigid units in all these polymorphic structures, whereas the [AlO6] octahedra are most compressible. Furthermore, [AlO6] octahedral compressibilities decrease from kyanite to sillimanite, to andalusite, the same order as their bulk moduli, suggesting that [AlO6] octahedra control the compression of the Al2SiO5 polymorphs. The compression of the [Al1O(6)] octahedron in sillimanite is anisotropic with the longest AII-OD bond shortening by similar to 1.9% between room pressure and 5.29 GPa and the shortest AII-OB bond by only 0.3%. The compression anisotropy of sillimanite is primarily a consequence of its topological anisotropy coupled with the compression anisotropy of the Al-O bonds within the [Al1O(6)] octahedron.

Crystal structures of HgBa2CaCu2O6+delta (Hg-1212) and HgBa2Ca2Cu3O8+delta (Hg-1223) have been determined on single crystals at room temperature conditions by X-ray diffraction techniques. Both tetragonal specimens are isomorphous with compounds in the Tl-Ba-Ca-Cu-O system. We observe excess oxygen in both samples on two sites near the z = 0 plane: at (1/2 00) and close to (1/2, 0.4, 0) - but no oxygen at (1/2 1/2 0) as reported in earlier studies. Refined Hg occupancies for both samples indicate a substitution of Cu for Hg of more than ten percent. If the substitution of Cu for Hg and excess 0 at (1/2 00) are coupled, then the effect is to insert a perovskite-like six-coordinated copper site at (000). No evidence was found for a split atom at the barium sites in these compounds.