The electron distribution in corundum. A study of the utility of merging single-crystal and powder diffraction data

CaAl12Si4O27, a New High-Pressure Phase Containing Al6O19 Clusters

“…On the basis of a closepacked anion model for the HP phase the a parameter of 7.223 A s can then be assigned as '7;O}O, corresponding to an anion}anion separation of 2.73 A s in the close-packed layers. This compares closely with an anion}anion separation of 2.75 A s in corundum (11).…”

“…By analogy we considered that the HP phase had a four-layer stacking of close-packed layers, (ch) with the stacking periodicity equal to c"8.614 A s . This gives a separation between close-packed layers of c/4" 2.15 A s which compares closely with the close-packed layer repeat of 2.16 A s in corundum (11). On the basis of a closepacked anion model for the HP phase the a parameter of 7.223 A s can then be assigned as '7;O}O, corresponding to an anion}anion separation of 2.73 A s in the close-packed layers.…”

Relationship between Crystallite Size and Bond Lengths in Boehmite

“…Considering atom ordering in its crystalline structure, this oxide can be classi-"ed into two groups: the one represented by alpha alumina with its atoms in a hexagonal arrangement (7) and the group of the transitional aluminas ( -, -, -, -, and -alumina), where atoms are ordered in a spinel-based structure (8). Alpha alumina, which is free of hydroxyls, is the most stable phase and is obtained after annealing samples at temperatures higher that 8003C.…”

Accurate Charge-Density Studies as an Extension of Bayesian Crystal Structure Determination

“…The main sources of model bias in multipolar density studies are: the choice of exponents appearing in the radial parts of the deformation functions, still`more of an art than a science' (Flensburg et al, 1995); the insuf®cient radial¯exibility in modelling valence-charge density in metals, minerals (Nowack et al, 1991;Brown et al, 1993) and coordination complexes (Iversen, Larsen, Figgis & Reynolds, 1997);² and the limited order of the spherical harmonics used, which do not usually extend past the hexadecapolar level l 4. The latter limitation is not always imposed by the quality of the data, and can be due to the need to preserve an adequately high observations/parameters ratio.…”