Band 226, Heft 5 -

The crystal structure of NaY was re-determined in space group Fd -3 m from 87 unique electron diffraction amplitudes obtained at 1250 kV by A. Carlsson, et al. (Chem. Eur. J. 5 (1999) 244), where a = 24.7 Å. The determination, based on maximum entropy and likelihood (computer program MICE ), leads to a clearly resolved structure, in contrast to the use of crystallographic phases derived from the Fourier transforms of experimental high-resolution images cited in the above paper. Potential maps from a trial solution contain density sites that correspond closely to the known T-site locations of this FAU framework. Although only framework T-site positions and not linkage oxygens are observed in the map, they can be linked with theoretical oxygens. Subsequent distance least squares ( DLS ) refinement of the linked model converges to a complete framework structure very close to the one found in earlier X-ray diffraction studies. Contrasting to the earlier study, where no counterion positions were identified, two possible Na positions were clearly visible in the direct phasing map One of these corresponds to a site postulated in previous studies. The other is near a site proposed in a separate electron microscopic/electron diffraction study of Na,FeY. Least squares refinement verifies the former position while eliminating the latter. On the other hand, a separate direct analysis of a dataset from the Y zeolite containing both Na and Fe (Carlsson et al. , loc. cit.) was not successful, owing to the smaller number of recorded amplitudes.

Crystal structures of low temperature phases III, IV and V of Rubidium Zinc Bromide have been determined by single crystal X-ray diffraction at 90 K, 85 K, and 30 K, respectively. The analysis of the structures in terms of symmetry-adapted modes of the high temperature prototype phase permits the identification of the modes responsible for each phase and to assess its thermal evolution. The primary mode responsible for the incommensurate and lock-in phases remains dominant throughout the whole transition sequence, maintaining its structure and slightly increasing its amplitude as temperature decreases. The two monoclinic phases IV and V are presumably due to the instability of a second mode associated with a single active irreducible representation. However, the presence with similar weights of several symmetry-adapted distortions of different symmetry indicates a strong hybridization in phase III of the softer normal modes. A symmetry-mode description of the structures of phase IV and V fully consistent with a simple Landau model, can then only be obtained if phase III is taken as effective parent phase. Besides the insight on the microscopic origin of the different phases, the symmetry modes were used as crystallographic coordinates in the refinement of the structure of phase V. The result shows the advantages of using these non-conventional collective structural parameters instead of individual atomic coordinates. The refinable parameters can be reduced to the amplitudes of those symmetry modes with significant weight in the distortions.

The crystallographic orientation relationship of a fluocerite (Ce,La)F 3 crystal overgrown with bastnaesite (Ce,La)[CO 3 ]F from the Pikes Peak area in Colorado, USA was investigated by transmission electron microscopy. The epitaxial overgrowth of bastnaesite on fluocerite was confirmed by evaluating the electron diffraction patterns of both minerals. During the examination, a third mineral phase, which was unexpected in this phase assemblage, was detected and identified as cerianite, CeO 2 , by EDS analysis in conjunction with electron diffraction. Its orientation relationship to the two main mineral phases can be described as a syntactical growth. The La-concentration of bastnaesite changes with distance from the cerianite interface, as confirmed by EDS. The reduction in Ce-content in close proximity to the interface of cerianite yielded a model rationalizing the local precipitation of cerianite. For all three mineral phases, a special orientation relationship was established.

The crystal structure of acetolone (5-(acetoacetylamino)benzimidazolone, C 11 H 11 N 3 O 3 ), was determined from X-ray powder data. Despite strong preferred orientation effects, the structure could be solved with real-space methods and refined by the Rietveld method using restraints. The resulting structure gave a good Rietveld fit with reasonable confidence values; the structure looked chemically sensible and passed all tests including a CSD check and the checkCIF procedure. But dispersion-corrected density functional theory (DFT) calculations revealed that this structure was actually wrong, and further work showed that the terminal acetyl group had to be rotated by 180°. The correct crystal structure led to a better Rietveld refinement with improved R -values. This structure was confirmed by dispersion-corrected DFT calculations. The compound crystallises in P -1 with two molecules per unit cell. The molecules are connected by a 2-dimensional hydrogen bond network.

The molecular structures of tert -butyl ( S )-2-hydroxy-[ N -(substituted-benzylidene)hydrazinyl-carbonyl]-ethylcarbamates, C 15 H 20 N 3 O 4 X ( 3 : X = H, o -OH, o -NO2, m -NO 2 and p -CN), derived from L -serine, are formally rather similar. Despite the variation in substituent, X, tricälinic ( 3 : X = H), ( 3 : X = o -OH), ( 3 : X = m -NO 2 ) and ( 3 : X = p -CN) are isostructural and exhibit, with minor variations, the same fundamental intermolecular O—H···O and N—H···O hydrogen-bond connectivity in two-dimensions in layers of molecules. In monoclinic ( 3 : X = o -NO 2 ), however, the oxygen atoms of the nitro group function as additional O—H···O and N—H···O hydrogen-bond acceptors resulting in a more complex form of two-dimensional connectivity.

The crystal structure of the molecular salt (hexamethylenetetraminium) · ( p -iodobenzoate) is the first example of a complex using the hexamethylene (HMTA) molecule that has both halogen and hydrogen bonding. The desired supramolecular packing arrangement of 1-D zig-zag chains was observed. A detailed breakdown of related HMTA complexes in the literature was used to show that zig-zag chains are a common packing feature of HMTA complexes with bifunctional molecules. The crystal structure determination of the molecular salt shows that the two intermolecular interactions N—H + ···OOC – and N···I are positionally disordered around a centre of inversion.