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Orientational Ordering (orientational + ordering)
Selected AbstractsOrientational ordering, tilting and lone-pair activity in the perovskite methylammonium tin bromide, CH3NH3SnBr3ACTA CRYSTALLOGRAPHICA SECTION B, Issue 4 2010Ian Swainson Synchrotron powder diffraction data from methylammonium tin bromide, CH3NH3SnBr3, taken as a function of temperature, reveal the existence of a phase between 230 and 188,K crystallizing in Pmc21, a = 5.8941,(2), b = 8.3862,(2), c = 8.2406,(2),Å. Strong ferroelectric distortions of the octahedra, associated with stereochemical activity of the Sn 5s2 lone pair, are evident. A group analysis and decomposition of the distortion modes of the inorganic framework with respect to the cubic parent is given. The primary order parameters driving this upper transition appear to be an in-phase tilt (rotation) of the octahedra coupled to a ferroelectric mode. The precise nature of the lower-temperature phase remains uncertain, although it appears likely to be triclinic. Density-functional theory calculations on such a triclinic cell suggest that directional bonding of the amine group to the halide cage is coupled to the stereochemical activity of the Sn lone pair via the Br atoms, i.e. that the bonding from the organic component may have a strong effect on the inorganic sublattice (principally via switching the direction of the lone pair with little to no energy cost). [source] Molecular crystal global phase diagrams.ACTA CRYSTALLOGRAPHICA SECTION A, Issue 6 2004A method is described to produce global phase diagrams for single-component molecular crystals with separable internal and external modes. The phase diagrams present the equilibrium crystalline phase as a function of the coefficients of a general intermolecular potential based on rotational symmetry-adapted basis functions. It is assumed that phase transitions are driven by orientational ordering of molecules with a fixed time-averaged shape. The mean-field approximation is utilized and the process begins in a high-temperature disordered reference state, then spontaneous symmetry-breaking phase transitions and phase structure information at lower temperature are sought. The information is mapped onto phase diagrams using the intermolecular expansion coefficients as independent variables. This is illustrated by global phase diagrams for molecules having tetrahedral symmetry (e.g. carbon tetrachloride, adamantane and white phosphorus). Uses of global phase diagrams include crystal structure data mining, guidance for crystal design and enumeration of likely or missing polymorphic structures. [source] Cluster approach to the Ti2Ni structure typeACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2006D. Rodi It has been established that the 16(c) first coordination clusters in the Ti2Ni structure type (space group Fd3m) follow icosahedral-face orientational ordering along regular tetrahedron edge directions. The actual crystal structure appears due to the prevalence of the face-centred cubic translational ordering over the cluster assembling. This way, the competition of the `regular' crystal phase and its icosahedral analogue is reconstructed at the atomic level. The model accounts for the markedly different electronic characteristics at the different crystallographic positions obtained by hyperfine interaction measurements, and other curious experimental facts help to create an exact physical definition of the first coordination in the solid state and to distinguish between various structure types on fundamental principles. [source] Two-dimensionally modulated structure of the rare-earth polysulfide GdS2,x (x = 0.18 , 13/72)ACTA CRYSTALLOGRAPHICA SECTION B, Issue 6 2003Rafael Tamazyan The crystal structure of GdS2,x is determined by single-crystal X-ray diffraction as a 144-fold superstructure of the ZrSSi structure type. The superstructure is described as a two-dimensional, commensurately modulated structure with the superspace group P4/n(,,½)(00)(ss) and with , = 1/4 and , = 1/3. Structure refinements within the classical approach, employing the 144-fold supercell, fail because most of the superlattice reflections have zero intensities within the experimental resolution. Within the superspace approach the absent superlattice reflections are systematically classified as higher-order satellite reflections. Accordingly, the superspace approach has been used to refine the structure model comprising the basic structure positions and the amplitudes of the modulation functions of the three crystallographically independent atoms. The quality of fit to the diffraction data and the values of the refined parameters are independent of the assumption on the true symmetry (incommensurate or a 12,×,12,×,2, I -centred superlattice with different symmetries). Arguments of structural plausibility then suggest that the true structure is a superstructure with space group I, corresponding to sections of superspace given by (t1, t2) equal to [(4n, 1)/48, (4m, 3)/48] or [(4n, 3)/48, (4m, 1)/48] (n and m are integers). Analysis of the structure, employing both superspace techniques (t plots) and the supercell structure model all show that the superstructure corresponds to an ordering of vacancies and an orientational ordering of S dimers within the square layers of the S2 atoms. [source] Structural Evolution of Self-Assembled Alkanephosphate Monolayers on TiO2,CHEMPHYSCHEM, Issue 14 2008Hong-Bo Liu Conformation and orientation: The formation of self-assembled monolayers (SAM) of hexadecyl phosphate on macroscopically flat TiO2 surfaces is studied by using IR spectroscopy (see picture). The evolution of a well-organized monolayer structure can be readily monitored from the conformational and orientational ordering of the alkyl chains. 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