Band 126, Heft 4

MoO 3 whiskers grown in a gas atmosphere show x-ray diagrams with diffuse reflections h + k = 2 n + 1. This phenomenon can be explained by twin lamellae where the contact zones consist of disordered layers with statistical distribution in the crystal.

The basic principles of a possible polarity standard for non-centrosymmetrical materials of adamantine structure are discussed. In view of the obvious lack of a standard convention for the designation of polar planes for these materials, placement of the “metal” A atom in 000 position, and a positive value of the displacement parameter fixing the “non-metal” partial B lattice are suggested for a general standard. The absolute polarity has been determined for InP, GaP, GaSb, AlSb, and ZnSe and related to specific etch patterns. Etch-pit microphotographs of the major polar planes of these materials, and their respective light figures are given and discussed.

An improved method of locating the heavy atoms from the intensity data of a pair of isomorphous crystals is suggested. This is based on statistical considerations and can be useful in a typical case of a parent protein and its heavy-atom derivative. Results have been tested on a hypothetical model in projection.

The cells and space-group symmetries of damascenine acid, its trihydrate, and the tri-acid dihydrate have been determined. The results are: a b c β damascenine acid: P 2 1 2 1 2 1 9.74 10.91 7.95 Å damascenine acid trihydrate: P 2 1 / c 6.89 8.16 20.89 Å 91.49° tri-damascenine acid dihydrate: P 2 1 / c 11.51 31.91 8.53 Å 114.89° The existence of the tri-acid dihydrate was established on the basis of density and the size of the cell. A fourth damascenine-acid modification was predicted by a set of d values measured with a diffractometer.

The Buerger algorithm can also be used to obtain the Delaunay reduced cell. The possible ambiguities in conventional cell settings are enumerated.

It is proposed that the concept of ‘special positions’ in unit cells should be extended to include positions with higher translational symmetry than the general position. An example is given to demonstrate the practical relevance of this proposition in structure determination. Symmetry combinations giving rise to face-centred positions in non-face-centred orthorhombic unit cells are tabulated. Another table lists all primitive and base-centred orthorhombic space groups which can accommodate face-centred positions, together with the coordinates of these positions.

The β and the γ ′ class of syntheses proposed earlier from this laboratory for the recovery of the electron densities from the Patterson function have been tested on the non-centrosymmetric projections of a known structure. Effects of positional errors in the known group of atoms, and observational errors in the intensity data on the strengths of the peaks in the beta and the heavy-atom-phased syntheses ( γ ′), are separately considered. It is shown that, in either case, the beta synthesis yields larger peak-strength ratios at the unknown atoms than the heavy-atom synthesis, as expected by theory. An important result is obtained in the use of the beta synthesis when one or more of the atoms in the known part of the structure are incorrectly placed. In this case, the beta synthesis is found to be more discriminating than the heavy-atom synthesis in showing the incorrect atoms both in the non-centrosymmetric and the centrosymmetric cases, while the peak strengths at the sites of the unknown atoms, as well as at the correct sites of the wrongly placed atoms, develop appropriate strengths.

A least-squares refinement of the structure of CoGeO 3 , which is isotypic with the diopsidic pyroxenes (space group C 2/ c ), has been carried out. The arrangement of oxygen atoms very closely approximated cubic closest packing, with both Co(1) and Co(2) being octahedrally coordinated, and with the chain of Ge tetrahedra having a significantly different geometry than equivalent chains in the isotypic pyroxenes. The polytypism shown by this compound is explained by the approach of the structure to geometrically ideal closest packing.

The crystals of bis (hydrazinecarboxylato-N′,O) cadmium, Cd(N′H 2 –NH–COO) 2 , are orthorhombic, space group P 2 1 2 1 2, with unit-cell constants a = 5.372 ± 0.007 Å, b = 9.116 ± 0.007 Å, c = 6.444 ± 0.005 Å. Two stoichiometric units are contained in the unit cell. The structure has been determined by threedimensional data. The structure consists of distorted octahedral complexes having Cd at the center and atoms N′ and O of the anion N′H 2 –NH–COO at the corners. The O atoms of each anion belong to distinct octahedra, which consequently are bound in layers parallel to (001).