Showing papers in "Acta Crystallographica in 1991"

The structure of urea-oxalic acid (2/1) determined by neutron diffraction at 100 K

Abstract: 2CH 4 N 2 O•C 2 H 2 O 4 cristallise dans P2 1 /c avec a=5,048, b=12,374, c=6,876 A, β=94,40°, Z=2; affinement jusqu'a R=0,045. La structure cristalline contient des molecules d'uree et d'acide oxalique, liees par un reseau de liaisons hydrogene

On the Refinement on Profile, Background and Net Intensities

Abstract: The raw data obtained with a single-crystal diffractometer are typically integrated profile and background intensities. All linear combinations of these data items, and in particular net intensities and total backgrounds, are equivalent observations. However, net intensities may be negative, in apparent contradiction with basic physics. Backgrounds are generally discarded from the list of observations once they have been subtracted from the profile intensities. In a crystal structure refinement on all observations, structural parameters including scale, extinction and possibly others, as well as individual background intensities, are refined on the observed profile intensities and on the observed background intensities. It is shown that, in such a refinement, the structural parameters are determined exclusively by the net intensities as in traditional least squares, whereas calculated backgrounds depend on the difference between observed and calculated net intensities. Negative net intensities are therefore not in contradiction with physics or with the structural model, and should be retained unaltered in the data set.

Charge Densities in CoS2 and NiS2 (Pyrite Structures)

Abstract: The electron-density distributions in the pyrite-type structures of CoS2 and NiS2 have been determined from high-resolution single-crystal diffraction data [Ag Ka radiation; resolution and temperature (sin0/A)max = 1\"49 /~-I at 295 K for COS2, 1.63 ~-1 at 135 K for NiS2]. The charge densities were refined using a multipolar deformation model [R(IFI) = 0.0119 and 0.0136, respectively]. The X-ray diffraction data of FeS2 [Stevens, DeLucia & Coppens (1980). lnorg. Chem. 19, 813-820; Ag Ka radiation, (sin0/a)m.x = 1.46 A-i, room temperature] were refined using the same deformation model and program in order to facilitate comparison of the results [R(IFI) = 0.0176]. The main features of the resulting deformation maps agree well for all three structures. They consist of important maxima in the immediate vicinity of the metal atoms pointing towards the faces of the coordination octahedron. The heavier the metal atom, the smaller is the distance of the maxima from the atomic centre. These features are interpreted by a preferential occupation of the metal d orbitals which correspond to the cubic t2e orbitals. An analysis of the d-orbital populations indicates that the symmetry of the electron distribution around the metal atom is in all cases very close to cubic, the site symmetry being 3; the t2g orbitals appear to be fully occupied by six electrons while the occupation of the eg orbitals increases in the series Fe, Co, Ni and indicates covalent overlap with the S ligands.

Structure du [4-phénylacétylimino-1-(1,2,4-triazolio)]acétate de sodium dihydrate

Abstract: C 12 H 11 N 4 O 3 − •Na + •2H 2 O cristallise dans P2 1 /n avec a=5,382, b=32,005, c=8,285 A, β=100,78 o , Z=4; affinement jusqu'a R=0,049. Le cycle triazolium est plan. C(2) et N(4) sont significativement hors du plan de 0,141 et 0,137 A respectivement. La coordination autour de l'atome Na implique six atomes O avec des longueurs comprises entre 2,282 et 2,507 A. Tous les atomes H des molecules d'eau participent aux liaisons hydrogene O-O