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Structural Distortion (structural + distortion)
Selected AbstractsChemInform Abstract: Polar Hexagonal Tungsten Oxide (HTO) Materials: (1) Synthesis, Characterization, Functional Properties, and Structure,Property Relationships in A2(MoO3)3(SeO3) (A: Rb+ and Tl+) and (2) Classification, Structural Distortions, and Second-Harmonic Generating Properties of Known Polar HTOs.CHEMINFORM, Issue 33 2010Hong Youn Chang Abstract Crystals of the title compounds are prepared from mixtures of MoO3, A2CO3 (A: Rb, Tl), SeO2, and H2O (autoclave, 230 °C, 3 d, 50% and 80% yields for the Rb and Tl compounds, respectively). [source] Flame-Synthesized Ceria-Supported Copper Dimers for Preferential Oxidation of COADVANCED FUNCTIONAL MATERIALS, Issue 3 2009Richard Kydd Abstract Rapid synthesis of CuCeO2 catalysts by flame spray pyrolysis produces highly active Cu dimer morphologies without the need for additional catalyst pretreatment. The active Cu component is enriched onto the CeO2 surface at concentrations higher than the nominal loading with no evidence of amorphous or crystalline CuO phase. Increasing the Cu content results in a morphological transition from isolated Cu monomers to oxygen-bridged dimers and an associated increase in oxygen vacancy concentration. Dimer-containing CuCeO2 catalysts display high levels of activity and selectivity in the low-temperature preferential oxidation of CO. Experimental measurements and simulations suggest that the geometry of the dimer presents a comparatively ionic CuO bond at the catalyst surface. Further studies indicate that these ionic dimer species promote preferential CO oxidation at lower temperatures than observed for monomeric Cu species. This is the first report to explicitly propose and demonstrate that the structural distortion associated with the formation of Cu dimers directly induces increased bond ionicity at the catalyst surface and that these changes are responsible for improved catalytic activity. [source] Effects of Structural Variation on the Photocatalytic Performance of Hydrothermally Synthesized BiVO4,ADVANCED FUNCTIONAL MATERIALS, Issue 16 2006J. Yu Abstract Highly crystalline monoclinic scheelite BiVO4 powders are synthesized from aqueous Bi(NO3)3 and NH4VO3 solutions over a wide range of pH by a hydrothermal process. BiVO4 powders with various morphologies, surface textures, and grain shapes are selectively synthesized by adjusting the pH. The dependence of the Raman peak position and intensity on the synthesis conditions indicates that the symmetry distortions in the local structure of the synthesized BiVO4 are affected by the preparation conditions. These variations in the local structure result in the modification of the electronic structure of BiVO4, which results in a blue-shift in the UV-vis absorption spectrum of hydrothermally synthesized BiVO4 in comparison with a well-crystallized sample prepared by homogeneous coprecipitation. The photocatalytic activities for O2 evolution from an aqueous AgNO3 solution under visible-light irradiation are strongly dependent on the pH used in the synthesis. The differences in the photocatalytic activities between BiVO4 samples prepared under various conditions is attributed to the degree of structural distortion, leading to differences in the mobility of photogenerated holes formed in the valence band, which consists of Bi,6s and O,2p orbitals. [source] Raman spectroscopy on carbon nanotubes at high pressureJOURNAL OF RAMAN SPECTROSCOPY, Issue 7-8 2003I. Loa Abstract Raman spectroscopy has been the most extensively employed method to study carbon nanotubes at high pressures. This review covers reversible pressure-induced changes of the lattice dynamics and structure of single- and multi-wall carbon nanotubes and irreversible transformations induced by high pressures. The interplay of covalent and van der Waals bonding in single-wall nanotube bundles and a structural distortion near 2 GPa are discussed in detail. Attempts to transform carbon nanotubes into diamond and other ,superhard' phases are reviewed critically. Copyright © 2003 John Wiley & Sons, Ltd. [source] Relationship between the structural distortion and the Mn electronic state in La1,xCaxMnO3: a Mn K -edge XANES studyJOURNAL OF SYNCHROTRON RADIATION, Issue 4 2009Jesús Chaboy A theoretical study of the X-ray absorption near-edge structure (XANES) spectra at the Mn K -edge in the La1,xCaxMnO3 series is reported. The relationship between the edge shift, the Ca,La substitution and the distortion of the MnO6 octahedra in these systems has been studied. It is shown that, by correctly considering these effects simultaneously, the experimental XANES data are consistent with the presence of two different Mn local environments in the intermediate La1,xCaxMnO3 compounds. By taking into account the energy shift associated with the modification of the MnO6 distortion as Ca substitutes for La, it is possible to reproduce the XANES spectra of the intermediate-doped compounds starting from the experimental spectra of the end-members LaMnO3 and CaMnO3. These results point out the need to re-examine the conclusions derived in the past from the simple analysis of the Mn K -edge XANES edge-shift in these materials. In particular, it is shown that the modification of the Mn K -edge absorption through the La1,xCaxMnO3 series is well reproduced by considering the simultaneous presence of both distorted and undistorted octahedra and, consequently, that the existence of charge-ordering phenomena cannot be ruled out from the XANES data. [source] Octahedral tilting in Pb-based relaxor ferroelectrics at high pressureACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2010Bernd J. Maier We have employed a combination of powder neutron diffraction and single-crystal synchrotron X-ray diffraction to characterize the pressure-induced phase transitions that occur in the perovskite-type relaxor ferroelectric PbSc0.5Ta0.5O3 (PST) and Pb0.78Ba0.22Sc0.5Ta0.5O3 (PST-Ba). At ambient pressure the symmetry of the average structure for both compounds is as a result of partial ordering of the Sc and Ta cations on the octahedral sites. At pressures above the phase transition both the neutron and X-ray diffraction patterns exhibit an increase in the intensities of h,k,l = all odd reflections and no appearance of additional Bragg reflections. Synchrotron single-crystal X-ray diffraction data show that the intensity of hhh peaks, h,=,2n,+,1, does not change with pressure. This indicates that the structural distortion arising from the phase transition has a glide-plane pseudo-symmetry along the ,111, cubic directions. Rietveld refinement to the neutron powder data shows that the high-pressure phase has either or symmetry, depending on whether the presence of 1:1 octahedral cation ordering is neglected or taken into account, and comprises octahedral tilts of the type a,a,a, that continuously evolve with pressure. The cubic-to-rhombohedral transition is also marked by a large increase in the anisotropy of the displacement ellipsoids of the Pb cations, indicating larger displacements of Pb cations along the rhombohedral threefold axis rather than within the perpendicular plane. For PST the anisotropy of the Pb displacement parameters decreases at approximately 3,GPa above the phase-transition pressure. For both PST and PST-Ba the average magnitudes of Pb-cation displacements expressed in terms of isotropic displacement ellipsoids gradually decrease over the entire pressure range from ambient to 7.35,GPa. [source] Structural study of the low-temperature phase of TlH2PO4 at 180,KACTA CRYSTALLOGRAPHICA SECTION B, Issue 5 2002E. Álvarez-Otero The low-temperature phase of TlH2PO4 has been studied by X-ray diffraction. A structural model is proposed with space group P. This model is compared with the structure of the antiferroelectric phase of TlD2PO4 at room temperature to analyze the expected isomorphism at low temperature. Given the structural distortion of TlH2PO4, such isomorphism present in the common high-temperature phase is not recovered in this phase. Through the analysis of the displacements of the PO4 groups there is some evidence that the ordering of the H atoms responsible for the appearance of antiferroelectricity seems to be incomplete. An exhaustive study of the detected ferroelastic domains is also performed. [source] Anisotropic crystal structure distortion of the monoclinic polymorph of acetaminophen at high hydrostatic pressuresACTA CRYSTALLOGRAPHICA SECTION B, Issue 2 2000E. V. Boldyreva The anisotropy of structural distortion of the monoclinic polymorph of acetaminophen induced by hydrostatic pressure up to 4.0,GPa was studied by single-crystal X-ray diffraction in a Merrill,Bassett diamond anvil cell (DAC). The space group (P21/n) and the general structural pattern remained unchanged with pressure. Despite the overall decrease in the molar volume with pressure, the structure expanded in particular crystallographic directions. One of the linear cell parameters (c) passed through a minimum as the pressure increased. The intramolecular bond lengths changed only slightly with pressure, but the changes in the dihedral and torsion angles were very large. The compressibility of the intermolecular hydrogen bonds NH,O and OH,O was measured. NH,O bonds were shown to be slightly more compressible than OH,O bonds. The anisotropy of structural distortion was analysed in detail in relation to the pressure-induced changes in the molecular conformations, to the compression of the hydrogen-bond network, and to the changes in the orientation of molecules with respect to each other in the pleated sheets in the structure. Dirichlet domains were calculated in order to analyse the relative shifts of the centroids of the hydrogen-bonded cycles and of the centroids of the benzene rings with pressure. [source] 4,6-Dimethyl-2-(3-pyridyl)quinolin-5-amineACTA CRYSTALLOGRAPHICA SECTION C, Issue 4 2007José Antonio Henao The title compound, C16H15N3, shows a hindrance effect between adjacent amino and methyl groups that leads to a structural distortion, which is reflected in the non-planarity of the quinoline entity and in the bond angles and distances. The crystal packing consists of chains along the b axis sustained by an intermolecular hydrogen bond between the amino group and the N atom of the pyridyl ring. [source] Bis(fluoromesityl) Palladium Complexes, Archetypes of Steric Crowding and Axial Protection by ortho Effect , Evidence for Dissociative Substitution Processes , Observation of 19F,19F Through-Space CouplingsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 11 2004Camino Bartolomé Abstract Bisarylated complexes trans -[Pd(Fmes)2(SR2)2] [Fmes = 2,4,6-tris(trifluoromethyl)phenyl (fluoromesityl); SR2 = SMe2, tht; tht = tetrahydrothiophene] are precursors for various bisarylated fluoromesityl palladium(II) complexes by ligand-substitution reactions. Boiling under reflux in acetonitrile gives the mixed complexes trans -[Pd(Fmes)2(NCMe)(SR2)], whereas boiling under reflux in toluene leads to trans -[PdCl2L2] (L = PMe3, tBuNC, pTol-NC, 4-MePy), in the presence of neutral monodentate ligands, or to (NnBu4)[trans -Pd(Fmes)2I(SR2)] when treated with (NnBu4)I. trans -[Pd(Fmes)2(SMe2)2] reacts with bidentate ligands, also boiling under reflux in toluene, to give [Pd(Fmes)2(L,L)] [L,L = Me2bipy, 2,2, - biquinolyl, ,2N,N, -OCPy2, dppm (Ph2PCH2PPh2), dppe (Ph2PCH2CH2PPh2), pte (PhSCH2CH2SPh), ,2S,N -SPPh2Py, ,2O,N -OPPhPy2], or the bimetallic complex [Pd(Fmes)2(,-1,N:1,2,O:2,N -Py2MeCO)Pd(Fmes)(SMe2)] (characterized by X-ray diffractometry) when treated with (OH)(CH3)CPy2. The crowding associated with two Fmes groups produces several interesting features: (1) trans complexes are preferred over cis complexes, against the expected electronic preferences; (2) the low-temperature NMR spectra of several complexes, or the X-ray diffraction structure of [Pd(Fmes)2(2,2, - biquinolyl)], reveal significant structural distortions associated with steric crowding; (3) the need for boiling under reflux in the synthesis suggests a dissociative substitution mechanism, which is unknown so far for Pd; (4) some of the complexes show 19F,19F through-space couplings. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] A Theoretical Study of the Exchange Coupling in Hydroxo- and Alkoxo-Bridged Dinuclear Oxovanadium(IV) CompoundsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 1 2004Antonio Rodríguez-Fortea Abstract A density functional study of exchange coupling in hydroxo- and alkoxo-bridged dinuclear oxovanadium(IV) compounds is presented. Coupling constants calculated for full unmodeled structures are in good agreement with experimentally reported values, confirming the ability of the computational strategy used in this work to predict the exchange coupling in dinuclear VIV compounds. The influence of the configuration of the [VO(,-OR)2VO]2+ core in compounds with two edge-sharing octahedrally coordinated oxovanadium(IV) centers, of the nature of the terminal ligands and of structural distortions on the coupling constant have been analyzed through model calculations. The results indicate that the coupling constant is less affected by these factors than for hydroxo- and alkoxo-bridged CuII compounds. The calculations support the orbital models usually employed in qualitative interpretations of magneto-structural correlations, showing good correlations between the calculated coupling constants and the overlap between the two magnetic orbitals or the square of the orbital gap. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] The changes of magnetic properties of the perovskite-type titanium oxides by the structural distortions: In the case of LaTiO3 and YTiO3INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 15 2007Taku Onishi Abstract In the perovskite-type titanium oxides, the changes of magnetic properties by GdFeO3 -type lattice distortion are observed. In this study, we have performed cluster model calculations based on the density functional theory method, and have obtained the effective exchange integral (Jab) in order to elucidate the magnetic change by the GdFeO3 -type lattice distortion. The components of deciding the orbital ordering and magnetic property have also been discussed. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Adsorption of water in mordenite,An ab initio studyINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 1 2001Th. Demuth Abstract The adsorption of water in mordenite has been investigated using density functional theory using gradient corrections to the exchange,correlation functional. In the neutral complex water is strongly physisorbed through two different hydrogen bonds, the stronger between the acid site and the water oxygen atom, the weaker between a hydrogen atom of the adsorbed molecule and a framework oxygen atom. Strong polarizations and structural distortions of both the acid site and the molecule have been observed. To elucidate the question if a chemisorbed complex (creation of a hydroxonium ion) is possible, ab initio molecular dynamics have been performed, indicating that a protonation of water occurs even for the low coverage of one water molecule per unit cell. However, this ionic configuration cannot be stabilized by the electrostatic field of the zeolite framework and is therefore not a minimum of the potential energy surface. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 84: 110,116, 2001 [source] | ||||||||||