Showing papers in "Acta Crystallographica in 1964"
1,249 citations
TL;DR: In this paper, the formation of the well ordered super spinel structure is suppressed by obstructing the diffusion of the A1 ions either by the surface of numerous pores or by incorporating large ions such as Na ions (y-alumina from crystalline boehmite) and in these cases the disordered spinel type aluminas are transformed at a high temperature to 0and a-Al~O 3 by complete recrystallization to small particles which still have some preferred orientation.
Abstract: Bayerite, fl-AI(OH)3, and boehmite, 7-A1OOH, both with crystallite sizes of some tenths of a micron, were used for this investigation. The transformation of bayerite to T-alumina and of boehmite to 7and a for 7-alumina the oxygen sublattice is fairly well ordered, whereas the tetrahedral A1 lattice is strongly disordered. The crystallographic texture of 7and 7-alumina could be related to their microporous texture and this in its turn to the length of the OH bonds in the hydroxides. The formation of the well ordered super spinel structure ~-A120 3 is suppressed by obstructing the diffusion of the A1 ions either by the surface of numerous pores (T-alumina from bayerite or 7-alumina from gelatinous boehmite) or by incorporating large ions such as Na ions (y-alumina from crystalline boehmite). In these cases the disordered spinel type aluminas are transformed at a high temperature to 0and a-Al~O 3 by complete recrystallization to small particles which still have some preferred orientation.
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TL;DR: In this article, an adaptation of the method of least squares to the refinement of molecular structures, based on gaseous electron-diffraction sector-microphotometer intensity data, is presented.
Abstract: ; ~ ', An adaptation of the method of least squares to the refinement of molecular structures, based on gaseous electron-diffraction sector-microphotometer intensity data, is presented. The method has the advantages of permitting simultaneous refinement of many more parameters than can be handled by trial-and-error methods and of opening the possibility of obtaining realistic standard errors and error correlations in place of the more subjective 'error limits'. Although the problem of weighting of the observations is not solved, it is felt that the method provides a more straightforward way to take account of factors known to introduce uncertainties than does the alternative approach based on radial distribution functions. A brief account of general experience with the method is given.
TL;DR: HAL as discussed by the authors is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not, which may come from teaching and research institutions in France or abroad, or from public or private research centers.
Abstract: HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Neutron diffraction investigation of U3O8 B.O. Loopstra
TL;DR: The main chain of the carotene molecule tends to be planar, but the chain is curved in its course, and the flionone ring is s-cis about the C(7)-C(6) bond, thus interrupting the all-trans structure of the chain this article.
Abstract: The crystals of fl-carotene (C40H56) are monoclinic (P21/c), with the unit-cell dimensions a =7-55, b =9.51, c =24.8 A, and fl = 105.3 °, and with two molecules in the unit cell. The main chain of the molecule tends to be planar, but the chain is curved in its course. The fl-ionone ring is s-cis about the C(7)-C(6) bond, thus interrupting the all-trans structure of the chain. ~Tith photographically registered intensities, refinement of parameters by a least-squares method yielded a discrepancy index, R, of 0.19.
TL;DR: In this article, the distribution of diffuse X-ray scattering in two planes in reciprocal space for the substitutional solid solution copper-16 at. % aluminum was measured and the symmetry of the distribution showed that there is not only short-range order but also small static atomic displacements from the expected sites.
Abstract: The distribution of diffuse" X-ray scattering in two planes in reciprocal space for the substitutional solid solution copper-16 at. % aluminum was measured. The symmetry of the distribution showed that there is not only short-range order but as well small static atomic displacements from the expected sites. The short-range order intensity was separated from that associated with the static displacements and the two were treated separately. It was fotmd that the order parameters were satisfactorily reproduced by the following model. The A1 atoms are distributed on the atomic sites in sets of four so that they occupy the corners of a distorted tetrahedron. The tetrahedra are distributed at random except that no two may be so close that they form an A1-A1 nearest neighbor pair. All other sites are occupied by Cu atoms. Measurements after neutron irradiation showed that such exposure caused a small but significant increase in the state of order.