Showing papers in "Acta Crystallographica in 1956"

The crystal structure of ferrocene

Abstract: Detailed X-ray analysis of crystals of ferrocene by Dunitz, Orgel & Rich confirms that the molecule has a 'sandwich' structure with approximate D5d symmetry, and Fe-C, 2.045±0.01 A; C-C, 1.403±0.02 A.

Über die Verfeinerung der Kristallstrukturbestimmung einiger Vertreter des Rutiltyps: TiO2, SnO2, GeO2 und MgF2

Abstract: The lattice constants an(t atomic parameters of TiO 2 (rutile), SnO2, GeO2 and ~¢1gF 2 have been redetermined. Ihk0 of TiO2, SnO2 and MgF~ were obtained by careful photometry of Weissenberg photographs (Mo Kc~ radiation) and were used for Fourier and (Fo--Fc) projections. The atomic parameter of GeO2 was determined from a three-dimensional Fourier section using intensities from powder diagrams, because single crystals were not available. Results:

A detailed refinement of the crystal and molecular structure of anthracene

Abstract: A detailed refinement of Mathieson, Robertson & Sinclair's X-ray crystallographic data for anthracene is described. The many calculations needed were carried out on the Manchester University electronic computer. The difference maps indicate some channelling of the electrons along the lines of the bonds. The anisotropie vibrations of the carbon atoms are interpreted in terms of anisotropic translational and rotational oscillations of the molecules. The magnitudes of the angular oscillations agree well with those inferred from Raman spectra. C-C bond lengths with e.s.d.'s of about 0.004 A and bond angles with e.s.d.'s of about 12' are obtained. Close intermolecular approaches explain a very slight buckling of the molecules.

Neutron diffraction studies of hafnium-hydrogen and titanium-hydrogen systems

Abstract: Results of neutron and X-ray diffraction studies on I-If and Ti hydrogen systems are reported. It is shown that the first single-phase hydride formed is a non-stoichiometric compound with a CaF2-type structure. I t deforms to a face-centered tetragonal structure as the composition approaches M X 2. From the crystal-structure data of these hydrides the positions of hydrogen atoms, the nearest-neighbor atoms and their interatomic distances are determined. It is indicated that the size of the 'hole' for the hydrogen atom in the hydride is such that it must enter as an ion. The effect of metal-hydrogen bonds on the physical properties of hydrides, such as characteristic temperature and vibration frequency, is given. Evidence and explanation for the embrittlement of metals by hydrogen are offered.

The theory of the method of isomorphous replacement for protein crystals. I

Abstract: The average change of intensity due to adding a heavy atom to a protein crystal is calculated for both the centrie and the acentric reflections. The amount by which small shifts of the molecules change the intensities is obtained for the monoclinic and orthorhombie systems, and formulae are given for the changes caused by small translations and rotations of the molecules, by alterations of the lattice parameters, and by 'breathing' movements. The effects always increase linearly with 1/d. It is shown that quite small molecular shifts would interfere with the isomorphous-replacement method for proteins at the higher values of 1/d, but not at the lower values.

The structure of chrysotile. II. Clino-chrysotile

Abstract: Clino-chrysotile is based on a cylindrical lattice of type c 2. In the case of the specimen studied in most detail the lattice is monoclinic of the 3rd kind with lattice parameters a = 14.65, b ~9.25, c ---5.34 .~, fl = 93 ° 16'. The atomic positions have been refined by trial and error, and by Fourier methods. It is confirmed that the structure m based on kaolin-like layers which are stacked in a manner consistent with a cylindrical structure.

The measurement of growth and deformation faulting in hexagonal cobalt

Abstract: the use of Fourier and line-breadth methods for the determination of stacking-fault parameters in hexagonal close-packed metals has been investigated. Greatest accuracy is probably obtained by Fourier analysis of the { 10i 1 } line, but analyses of other lines axe required in addition when more than one type of faulting is present. Line-breadth methods usually give satisfactory results, and are valuable in determining the predominant type of faulting. Spontaneously transformed cobalt powders often contain mainly growth faults, whereas specimens which have been deformed to complete the martensitic transformation contain mainly deformation faults. Specimens with mixed faulting have also been obtained; an alternative hypothesis, that the faults are 'clustered', will explain the line-breadth results, but not those obtained by Fourier analysis.

Isomorphous replacement and phase determination in non-centrosymmetric space groups

Abstract: below, for the a tom 09 is a t y = 1⁄4 , and it would therefore lie over a t rough if the second layer were undisplaced with respect to the first. In these circumstances the h06 reflexions would indicate a spacing of 7.3 A instead of twice this. I f 01 lies over the troughs, however, a l te rnate layers must suffer a relative disp lacement of b/12 so t ha t a c~ type of latt ice will be produced, in accordance with the observations. The fur ther al ternat ive, t ha t the 01 and 02 a toms might both lie a t the same level and at positions midway between the crests and t roughs of the layer below, can also be ruled out. This would introduce the possibility of equivalent relat ive shifts of +b/24 in each layer, and would therefore lead to extinction of the h/c0 reflexions except for k = 12n. Since the chrysotile layers m a y be expected to have a plane of s y m m e t r y perpendicular to the b axis, there is no possibility of a fur ther polymorph related to para-chrysoti le in the same way as clino-chrysotile is related to ortho-chrysotile.