Volume 215, Issue 8

The diffuse scattering of two single crystals in the quaternary system Zr-Y-O-N (Y 0.04 Zr 0.96 O 1.64 N 0.223 and Y 0.17 Zr 0.83 O 1.68 N 0.158 ) has been measured by neutron scattering in different layers of the [1-10] zone to study short range order originating from the vacancies. While trying to fit the diffuse scattering with models given in the literature remarkable discrepancies occurred. In contrast to the existing models which explain the diffuse scattering by correlated clusters of ordered vacancies, a new model is proposed based on longitudinal displacements of rigid (O-)Zr-O-units within the lattice where the distance between Zr and O (¼ of the cubic body diagonal) is kept fixed.

Diffuse scattering is usually associated with some disorder in the analyzed material. Different kinds of disorder may produce different diffuse scattering - or not. In this letter, we demonstrate some aspects of the variety of diffuse scattering that occurs even in very simple examples, and how unawareness may lead astray.

The mixed-valent indium chalcogenides InTe, In 4 Se 3 , In 4 Te 3 , In 6 S 7 and In 6 Se 7 were studied together with b-InS 3 and TlInS 2 by In and Te K-edge EXAFS spectroscopy. These well-characterized crystalline solids were chosen as model systems to assess the general applicability of EXAFS for structural studies of first order mixed-valent solids containing indium. The question was whether the interatomic distances and coordination numbers derived from EXAFS data could be used for a basic structural characterization in the sense of "structural fingerprints" to identify the indium species present in a given sample. Indium species in the samples comprised In + , In 3+ , (In 2 ) 4+ and (In 3 ) 5+ , with up to 3 of them being simultaneously present. While the analysis of compounds with only one indium species was possible with high accuracy, it was impossible to interpret the EXAFS data of complex mixed-valent solids containing three different indium species (e.g. In + , In 3+ , (In 2 ) 4+ ) without prior knowledge of the crystal structures. Intermediate cases are those compounds in which two indium species are present, e.g. In 3+ in tetrahedral and octahedral coordination or In+ in a highly regular coordination accompanied by In 3+ . In conclusion, EXAFS measurements alone can be a reliable basis for structural discussions beyond the first coordination sphere only for compounds of moderate complexity. In most cases prior information of structural data from single crystals or other sources is necessary. This is particularly relevant for future experiments with amorphous or poly-crystalline samples that cannot be studied with standard diffraction techniques.

Lipid systems, which provide valuable model systems for biological membranes, display a variety of polymorphic phases, depending on their molecular structure and environmental conditions. By use of X-ray and neutron diffraction the temperature- and pressure-dependent structure and phase behavior of lipid systems, differing in chain configuration and headgroup structure, have been studied. Besides lamellar phases also nonlamellar phases have been investigated. Hydrostatic pressure has been used as a physical parameter for studying the stability and energetics of lyotropic lipid mesophases, but also because high pressure is an important feature of certain natural membrane environments (e.g., marine biotopes) and because the high pressure phase behavior of biomolecules is of biotechnological interest (e.g., high pressure food processing). We demonstrate that temperature and pressure have noncongruent effects on the structural and phase behavior. By using the pressure-jump relaxation technique in combination with time-resolved synchrotron X-ray diffraction, the kinetics of different lipid phase transformations was also investigated. The time constants for completion of the transitions depend on the direction of the transition, the symmetry and topology of the structures involved, and also on the pressure-jump amplitude. In addition, the effect of incorporating ions, steroids and polypeptides into bilayers on the temperature- and pressure-dependent phase behavior of the lipid systems is discussed.

The crystal structure of Cu 2.31 TeS 0.32 , a sulfur-poor member of the solid solution series Cu 61.3 Te 18.3 S 20.5 - Cu 61.9 Te 35.7 S 2.36 prepared at 675°C, was determined and refined from single-crystal diffractometer X-ray data (R = 0.044). The quenched statistical cubic structure (a = 14.836(4)Å, space group F4 1 32) consists at room temperature of two interpenetrating tetrahedral frameworks of icosahedral tellurium cages which enclose icosahedral clusters of half-occupied Cu positions centred by additional, fully occupied central copper sites. In one subset, statistically occupied sulfur tetrahedra occur in the tellurium cages, centred on this central Cu site. The Cu 2.31 TeS 0.32 structure is a stuffed derivative of Cr 3 Si (structure type A13) and shows affinities to the structures of Cu 9.1 TeSb 3 , the Ag 8 GeTe 6 family and some metal clusters, especially [Cu 26 Se 13 (PEt 2 Ph) 14 ] and [Au 13 C l2 (PMe 2 Ph) 10 ].

Bis (3-hydroximinobut-2-one thiosemicarbazone) nickel(II), C 10 H 22 Cl 2 N 8 NiO 3 S 2 , exhibits a considerably distorted octahedral NiN 4 S 2 core with two tridentate ligands approximately oriented along their respective meridional planes, forming two fused five - membered rings while binding the Ni 2+ ion. The complex crystallised as the monohydrate of its chloride, space group P2 1 /a Z = 4, a = 15.907(3)Å, b = 7.135(2)Å, c = 19.380(3)Å, β= 102.55(2)0. The only intramolecular association in the structure constitutes an interligand oximino proton – oximino nitrogen interaction. Water molecules and chloride anions are found to occupy lattice cavities. The protons attached to the water molecules and the primary amine groups display bifurcated H-bonding interactions. The chloride ions are interactionally tetracoordinated, forming two kinds of extremely distorted H-bonding systems, one pyramidal and the other tetrahedral. Thus, the molecular packing is stabilised by a complicated interactional network.

Single crystals of 1,1-difluoroallene and tetrafluoroallene (mp 136 K and 108 K) were grown in situ on a four-circle diffractometer from the liquid substance in a cold N 2 gas stream. The crystal and molecular structures of the two compounds were determined by X-ray crystallography at 95(2) and 100(2) K, respectively. The molecule of 1,1-difluoroallene has an almost linear C=C=C framework, while for tetrafluoroallene exact linearity is provided by the 222 crystallographic site symmetry. In agreement with theoretical calculations, the C=C bond lengths decrease with increasing number of fluorine substituents. For difluoroallene a collection of high order data (sinθ/λ max = 1.0 Å –1 ) was possible allowing for the determination of its charge density. The topological analysis of this function was performed and the results were compared to those obtained by quantum-chemical calculations.