Band 227, Heft 4 -

A crystal chemical approach to superconductivity based upon a bond-valence sum analysis of inorganic compounds is extended by an analysis of local bond-valence distribution. The notions of stoichiometric and structural valence together with Pauling’s rule of equal distribution of valences among the ligands of a given atom allow to provide a characterization for the strength of bonds which is used to define structurally distinguished bond paths. Combinations of these paths represent infinite units of strongly overlapping orbitals. Such units are assumed to be the conducting units in superconductors. A large number of crystal-structure data determined at ambient temperature and pressure has been analyzed including data of chalcogenides and pnictides as well as cuprates. It is shown that infinite units of strongly overlapping orbitals always exist in compounds which become superconducting on cooling with or without applied pressure and/or by doping. As a consequence, a systematic search for potential unconventional superconductors in inorganic crystallographic databases seems to be promising.

This paper focuses on statistical methods and formulas that solve the constraints Ȓ q 2 ( x 1 , …, x N ) = R q 2 for all q . To this end we invented several non trivial techniques. We introduce a functional measure that uses the Patterson function and show that this functional measure is equivalent to using a non uniform prior distribution of the x i given by a product of delta functions δ ( Ȓ q 2 ( x 1 , …, x N ) – R q 2 . We then use a representation of the delta function for calculating probabilities. In this paper we don‘t aim to give a “final” formula (this we hope to do in a future paper) but we emphasize instead on the method. We also show in this paper that SAD gives almost the same conditional j.p.d. of the phases given the magnitudes of the structure factors. Finally we show that by using plausible prior densities of the x i the principal part of the probabilities of phases given some neighborhood does not depend anymore on the number of atoms which is a completely new and unexpected result. The Bessel functions will play a dominant role in all our formulas.

Quantitative analysis of grain boundaries relies on proper boundary parameterizations. One of the approaches to identification of boundaries is the ‘interface-plane scheme’. A rigorous examination reveals that this approach is deficient. It is demonstrated that the ‘interface-plane scheme’ fails to satisfy necessary conditions for a parameterization. Moreover, the difference between boundaries symmetric in the conventional sense and those termed as symmetric in the ‘interface-plane scheme’ is clarified. Also the relationship of these boundaries to twist and tilt boundaries is analyzed. Revealing the unsuitability of the ‘interface-plane scheme’ for boundary parameterization and eliminating the confusion caused by the incompatible definitions of symmetric boundaries is of importance for the progress in analysis of homophase interfaces.

X-ray powder-diffraction data recorded using different wave lengths as well as neutron powder diffraction data on Hägg carbide, χ -Fe 5 C 2 , were evaluated by Rietveld or Pawley refinements, respectively. Likewise, employing different starting models, first-principles calculations using density functional theory (DFT) involving structure optimisation with respect to energy were performed for χ -Fe 5 C 2 . The results from diffraction and DFT imply a crystal structure having a monoclinic C 2/ c symmetry with a quite regular (monocapped) trigonal-prismatic coordination of C by Fe atoms. The anisotropy of the microstrain broadening observed in the powder-diffraction patterns agrees with the anisotropy of the reciprocal Young’s module obtained from elastic constants calculated by DFT. The anisotropic microstrain broadening can to some degree, be modelled allowing for a triclinic distortion of the metric of χ -Fe 5 C 2 (deviation of the lattice angle γ from 90°) involving reflection spitting, which mimics the hkl -dependently broadened reflections. This distortion corresponds to the most compliant shear direction of the monoclinic χ -Fe 5 C 2 . The anisotropic microstrain broadening results from microstress induced e.g. by anisotropic thermal expansion inducing misfit between the grains, in association with the intrinsic anisotropic elastic compliance of χ -Fe 5 C 2 . This anisotropic microstrain broadening was likely the origin of previous proposals of triclinic P -1 space-group symmetry for the crystal structure of χ -Fe 5 C 2 , which is rejected in the present work.

A route of solving crystal structures from complicated pseudo-merohedric twinning crystals was described. The structure of a [3Fe2S] complex was solved and refined in the space group of P 42/ n to R 1 factor of 0.1789. Consequently, by deleting one of the two disordered groups in the structure, a space group of Aea 2 for the absolute structure was found. The new absolute structure with four twinning components was refined to R 1 about 0.1171. At the final stage, the disorder was again added to the structure. The structure in space group Aea 2 with both twinning and disorder was refined to R 1 of 0.0722, which implies the special structure feature.

Co-crystals formed between anthranilic acid and each of 4,4′-bipyridine ( 1 ), 1,2-trans-1,2-bis(4-pyridyl)ethene ( 2 ) and 1,2-bis(4-pyridyl)ethane ( 3 ) have been formed from solutions containing 2:1 stoichiometric quantities of the reagents. Each structure features a centrosymmetric three-molecule aggregate mediated by O—H…N(pyridyl) hydrogen bonds indicating the robustness of the seven-membered {…HOCO…HCN} heterosynthon. The structures of ( 1 )–( 3 ) are isomorphous whereby the three-molecule aggregates are connected into flattened helical chains by amino-N—H…O(carbonyl) hydrogen bonds; the chains are consolidated into the three-dimensional structure by π…π interactions. The magnitude of the O—H…N hydrogen bonds in ( 1 )–( 3 ) are correlated with the basicity of the pyridine residue.