Showing papers in "Acta Crystallographica in 1957"

The scattering of electrons by atoms and crystals. I. A new theoretical approach

Abstract: The scattering of electrons by three-dimensional potential fields, and, in particular, the potential fields associated with a crystal lattice, is considered in terms of the new approach to physical optics proposed by Cowley & Moodie.

Structure and properties of CaTiO3

Abstract: A new determination of the atomic structure of CaTiOa has been made. It is orthorhombic, with space group _Pcmn; atomic coordinates are given. Synthetic crystals are identical with the majority of natural perovskite specimens. Two of the modes of twinning reported by Bowman from optical analysis have been confirmed by both X-ray and optical methods. With increasing temperature in the range --200 ° C. to 900 ° C. changes in cell dimensions are such as to approximate more closely to those characteristic of higher symmetry, but the substance is still orthorhombic at 900 ° C.

The crystal structures of copper dipyridine dichloride and the violet form of cobalt dipyridine dichloride

Abstract: The structures of copper dipyridine dichloride and a-cobalt dipyridine dichloride have been determined by X-ray analysis. CuPy2C12 is monoclinic, a ---17.00, b --8.59, c -3.87 A, fl = 91 ° 52', space group P21/n, two centrosymmetric molecules in the cell. The structure consists of square coplanar Units (Cu-C1, 2.28/~; Cu-:N, 2.02 A) aggregated into chains running parallel to c by weaker Cu-Cl bonds (3"05 A) which complete distorted octahedra about each copper. a-CoPy2C12 is monoclinic and is referred to a pseudo-orthorhombic cell centred on the oblique face C, a = 34.42, b = 17.38, c = 3.66 A, }, ---90 °, containing eight molecules. The corresponding primitive cell has space group P2/b. The cobalt atoms lie on twofold rotation axes and the crystal contains two formally independent sets of molecules. The structure consists of polymeric chains, running parallel to c, in which the coordination about the cobalt is octahedral (Co-Cl, 2.49 /~; Co-N, 2.14 A). The two structures are closely related and appear virtually identical when viewed down the respective c axes.

Crystal and magnetic structure of copper chromite

Abstract: Copper chromite, CuCr~O 4, is a tetragonally distorted normal spinel with a c/a ratio, as referred to a face-centered unit cell, of about 0.91. A structure which is physically reasonable, belonging to the space group D~-I42d, is based on an environment for the copper ions which is distorted from a tetr~hedron toward a square, and also slightly rotated about the c axis. The chromium ions are slightly displaced from their ideal positions. The magnetic neutron diffraction pattern, below a Curie point at about 135 ° K., indicates a triangular arrangement of moments such as one of those proposed by Yafet & Kittel.

X‐ray diffraction effects of atomic size in alloys

Abstract: A modification of the diffraction theory for a disordered substitutional solid solution containing atoms of different sizes gives a more accurate and simpler expression for the size effect diffuse scattering. A comparison of the theory with measurements of the diffuse scattering for Cu/sub 3/Au near hkl = 200 shows good agreement, even though the specimen contained some short-range order. (auth)