Browsing by Author "Welberry, TR"

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    Investigating short-range order in triglycine sulphate using x-ray and neutron diffuse scattering
    (Australian Institute of Physics, 2012-02-02) Hudspeth, JM; Goossens, DJ; Gutmann, MJ; Studer, AJ; Welberry, TR
    Triglycine sulphate (TGS) [(NH2CH2COOH)3H2SO4] is a hydrogen-bonded ferroelectric with a phase transition temperature of 322K [1]. The phase transition is reversible and second order, order disorder type, making TGS of fundamental interest in the field of phase transitions [2]. Above the critical temperature, one of the glycine molecules is disordered across a mirror plane. Below the critical temperature, it chooses a side, breaking the symmetry. The ferroelectric state is obtained through the ordering of the glycine orientations on neighbouring sites, but the mechanism for the phase transition is not well understood. We have investigated the short-range order in TGS by collecting single crystal x-ray and neutron diffuse scattering data on hydrogenous and fully deuterated TGS respectively. Data was collected at temperatures from well below to well above TC. We have also developed a model for the short-range order using the program ZMC [3]. This has given us some new insight into the behavior of the disordered glycine in TGS. For example, above TC, the orientations of the disordered glycine appear to be correlated over short range rather than being completely random as suggested by the average structure.
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    Neutron diffuse scattering in deuterated para-terphenyl, C18D14
    (Institute of Physics, 2009-03-25) Goossens, DJ; Beasley, AG; Welberry, TR; Gutmann, MJ; Piltz, RO
    Neutron diffuse scattering is used to explore the short-range order (SRO) in deuterated para-terphenyl, C18D14. The crystal shows SRO because the central of the three phenyl groups of each molecule can twist positively or negatively and these twists are correlated over the local scale. The presence of incipient Bragg peaks at (1/2 1/2 0) at 200 K shows that these flips are negatively correlated along the a direction (nearest neighbour correlation coefficient of similar to-0.3) and b direction (nearest neighbour correlation coefficient of similar to-0.87) and appear essentially uncorrelated along c. Diffuse peak anisotropy indicates that the range of the correlations along b is found to be similar to 3 times that along a. These correlations persist, although weaker, at room temperature. A Monte Carlo simulation was used to impose a correlation structure on the population of central ring twists that was deduced from Bragg scattering. By then allowing displacive relaxation of the structure, the observed diffuse scattering was well reproduced. Modelling the displacive motions of molecules showed that the positions of nearest ab-plane neighbour molecules are strongly positively correlated, particularly for motions approximately parallel to a, while the displacive correlations are weaker between molecules stacked along c. The apparent contradiction that the displacements are most strongly correlated along a while the occupancies are most strongly correlated along b is explained in terms of the connectivity of molecular interactions. © 2009, Institute of Physics