Showing papers in "Acta Crystallographica in 1965"

The crystal structure of 1,3,5-triamino-2,4,6-trinitrobenzene

Abstract: The compound 1,3, 5triamino2, 4, 6trinitrobenzene (TATB) is unusual in its thermal and solubility properties. The compound sublimes and decomposes above 300 °C. Concentrated sulfuric acid is the best of the few known solvents for TATB. These facts indicated tha t the compound would be of interest because of its hydrogen bonding system in which both interand intra-molecular hydrogen bonds are likely between amine groups and adjacent nitro groups. Another compound, 1,3-diamino-2,4,6-trinitrobenzene, form I (Holden, 1962), tha t has a similar hydrogen bond system has been reported.

A ternary alloy with pbcl2-type structure: tinisi(e),

Abstract: : Beck and coworkers have found E phases in several ternary systems of transition elements with either silicon or germanium at the composition 1:1:1. The crystal structure of TiNiSi(E) has been determined and refined by least squares with (limited) three-dimensional single-crystal data to a final R value of 0.086 (excluding 002 due to apparent extinction, and all nonobserved reflexions). The lattice parameters for the primitive orthorhombic cell are: a sub 0 = 6.1484=0.0012, b sub 0 = 7.0173=0.0014, c sub 0 = 3.6698=0.0007 A. The E phase is isotypic with PbCl2(C23), space group Pnam. All near-neighbor distances are within 0.06 A of the following average values: Ti-Ti 3.18, Ti-Ni 2.83, Ti-Si 2.61, Ni-Ni 2.67, Ni-Si 2.33 A. The numbers of nearneighbors are compared with those in Co2Si, theta Ni2Si and U3Si2. (Author)

A neutron diffraction study of uranyl nitrate hexahydrate

Abstract: The crystal structure of uranyl nitrate hexahydrate has been determined by neutron diffraction. The structure is orthorhombic with a = 13.197 + 0.003, b = 8-035 + 0.002, c = 11.467 + 0-003 /~, space group Cmc21. Intensities of 1136 independent reflections with a 2-0< 105 ° were obt~ned with 2= 1"065 A. The structure was solved from the three-dimensional neutron Patterson and Fourier syntheses, and refined by a full-matrix least squares, using a statistical weighting scheme. The final weighted R index was 3.9 Vo. The uranyl group is surrounded equatorially by a near-planar oxygen hexagon of four oxygen atoms from two non-equivalent bidentate nitrate groups and two equivalent water oxygens. In the nitrate groups, the N-O bonds involving coordinated oxygens are 0.03--0.05/~ longer than the N-O bonds involving the non-coordinated oxygen atoms; also the O-N-O angles differ significantly from 120 °. All hydrogens are involved in hydrogen bonding and these may be divided into (a) O (water) O (water) hydrogen bonds of length 2"68-2"75 A and (b) weaker O (water)O (nitrate)hydrogen bonds of length 2.93 and 2.99/1,. The water molecules associate into sheets perpendicular to the a axis.

The combination of isomorphous replacement and anomalous scattering data in phase determination of non-centrosymmetric reflexions

Abstract: Earl ier a t t empts to combine anomalous scat ter ing da ta and isomorphous replacement da ta in phase de termina t ion are discussed. The relat ive weights given to the two types of observation have not been on a sat isfactory basis; e i ther a n a rb i t ra ry me thod has been used or the intrinsic accuracy of the anomalous scat ter ing da ta has not been realized. A new me thod is proposed, which allows appropriate weighting to be applied.