1,720,965 research outputs found
A cluster-modeling study of the structure of yttria-stabilized zirconias
No full textThe Zr and Y coordination polyhedra, as found in the room-temperature phases of their pure oxides, are used as building blocks in modeling the structures of yttria-stabilized zirconias (YSZ). It is found that, in order to obtain the right coordination geometry, three clusters of four Zr- and Y-based building blocks can be constructed. When Y building blocks are present in a cluster they form a couple, characterized by a common structural vacancy, a neutral defect which can be defined when an oxygen is missing with respect to the case of the Zr seven coordination in monoclinic zirconia. It is shown that this model fairly well justifies the experimental results obtained by others. In conclusion, this kind of cluster modeling is proposed as a general method for solving structural problems in other mixed oxide
The Crystal Structure of the Cesium Molybdenum Bronze CS0.14MoO3
No full textJournal of Solid State Chemistry
Volume 104, Issue 2, June 1993, Pages 209-214
The Crystal Structure of the Cesium Molybdenum Bronze CS0.14MoO3 (Article)
Depero, L.E.,
Zocchi, M.,
Zocchi, F.,
Demartin, F.
Dipartimento di Ingegneria Meccanica dell'Università, Via Branze, 25123 Brescia, Italy; IMAI-CNR Area della Ricerca di Roma, 00016 Monterotondo Stazione, Rome, Italy and Istituto di Chimica Strutturistica Inorganica dell'Università degli Studi, Via G. Venezian 21, 20133, Milan, Italy
Abstract
The crystal of the title compound is hexagonal, space group P 63/m, with a = 10.620(9), c = 3.722(2) Å, and Z = 6. Couples of MoO6 distorted octahedra, linked by edges, constitute, by repetitive edge sharing along the c axis, ribbons kept together in bundles by lateral corner sharing. These bundles, by being packed together, generate tunnels where delocalized Cs atoms are inserted. The results confirmed the previously found correlation between the Mo oxidation state and the distortion of the coordination "octahedra" and allow one to extend the correlation to the possible conduction properties of the title compound and to those of structurally related oxides and bronzes. © 1993 Academic Press. All rights reserved.
Reaxys Database Information
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ISSN: 00224596 CODEN: JSSCBSource Type: Journal Original language: English
DOI: 10.1006/jssc.1993.1155Document Type: Article
Depero, L.E.; Dipartimento di Ingegneria Meccanica dell'Università, Via Branze, 25123 Brescia, Italy; IMAI-CNR Area della Ricerca di Roma, 00016 Monterotondo Stazione, Rome, Italy and Istituto di Chimica Strutturistica Inorganica dell'Università degli Studi, Via G. Venezian 21, 20133, Milan, Italy,
© Copyright 2008 Elsevier B.V., All rights reserved
Structure and properties of lanthanide catalysts: the crystal and molecular structure of SmBr3·4iPr(OH)
No full textThe crystal of the title compound is monoclinic (space group C2/c), with a = 22,067(17), b = 10.111 (10), c = 16.886(11) Å, β=142.95(5)°, and Z = 4. Its structure is based on the packing of SmBr3·4iPr(OH) (iPr = isopropyl) molecules. The metal atom is coordinated by three Br ions and by the oxygen atoms of the four iPr(OH) ligands. The coordination polyhedron is a distorted pentagonal bipyramid of C2 symmetry, with Sm and one of the Br atoms lying on the twofold axis of crystallographic symmetry. Polymerization experiments are reported showing that the activity of the catalytic system based on the title complex is much lower than that shown by systems based on similar Nd complexes. A possible rationalization of this fact is given in terms of the oxidation capacity of the rare-earth ion
Study of the anatase-rutile transformation in TiO2 powders obtained by laser-induced synthesis
No full textPowder samples of pure anatase were produced using laser-induced pyrolysis of titanium alkoxides, and the catalysts were prepared using conventional wet impregnation methods. The diffraction patterns were interpreted in microstructural terms by Fourier analysis of their peak profiles. The transition temperature for the anatase-rutile transition in these catalysts was found between 500° and 550 °C. For the reflections of the anatase phase, a decrease of their Bragg (2θ) positions was observed up to 550 °C when the presence of the rutile phase becomes important. The response of the anatase structure to the thermal treatment is anisotropic with the c-axis showing the highest sensitivity to the observed expansion of the lattice. The rutile Bragg reflections are sharper than those of the anatase phase. The corresponding microstructural parameters indicate that, in all cases, the transformation is accompanied by an increase of the crystallites and/or of the lattice perfection. The evolution of these parameters is influenced by the presence of vanadium. The V-treated surface layer must be particularly distorted and apparently act as a restraint to perfecting by thermal treatments. Only the transition to rutile is capable of overcoming that restraint by allowing crystallite growth at the expense of the smaller and distorted anatase crystallites
Butadiene polymerization with lanthanide catalysts: activity and X-ray diffraction study of neodymium bromide complexes with basic ligands
No full textThis paper reports on the study of five neodymium bromide complexes of general formula NdBr3·nL prepared according to the scheme NdBr3·6H2O + 6HC(OCH3)3 → NdBr3·nCH3OH + 6HCO(OCH3) + (12 - n)CH3OH, followed by ligand exchange, leading to neodymium tribromide complexes NdBr3·nL with nL = 4iPrOH (I), 4THF (II), 4Py (III), 2THF (IV) and 2iPrOH (V) where iPr = isopropyl, THF = tetrahydrofuran and Py = pyridine. The catalytic activity of all the above complexes in the polymerization of butadiene is very high and it increases from I to III. The diffraction study of I and II shows that both molecules are monomeric (coordination number 7), indicates that the nature of the NdBr bonds is influenced by ligand substitution, and reveals that the covalent contribution is more important in II than in I. The correlation of these findings with the results obtained from the study on the activity is discusse
Derivation of structural models for two intercalated compounds of α-zirconium phosphate
No full texteasonable structural models are proposed for two intercalated compounds of α-Zr(HPO4)2·H2O (α-ZrP), one with 2,2′-bipyridyl (A) and another one with 1,10-phenanthroline (B). The models are derived by considerations of the unit-cell geometry changes undergone by α-ZrP upon insertion of the organic molecules and on a set of general criteria of structural analysis, already developed by us and called the comparison method. If it is assumed that the water molecules in A and B keep the positions they have in α-ZrP, as fixed by their links to the phosphate-OH groups, the inserted molecules, while obeying the general conditions of optimum packing, establish interactions of the hydrogen-bonding type with the water molecules. This modeling gives for A and B a possible structural explanation to the maximum number of inserted molecules of the two species, as fixed by their stoichiometr
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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