Browsing by Author "McBride, BR"

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    Rigid unit modes dynamics in the molecular sieve material AlPO4-5
    (Australian Institute of Nuclear Science and Engineering, 2016-11-29) Cortie, DL; McBride, BR; Mole, RA; Withers, RL; Yu, DH; McIntyre, GJ; Liu, Y
    The web-like ring structure composed of corner-sharing tetrahedra in AlPO4-5 has received considerable attention over the years because its unusual topology enables entrapment and diffusion of guest molecules within the extended micropores1. Recent work on this molecular sieve reported clear evidence for strong sub-picosecond tetrahedral dynamics attributed to rigid-unit modes (RUMS). In order to preserve the bond-lengths during these motions, some chemical species undergo restricted rotational diffusion within relatively large volumes around their average lattice positions. These motions drive fluctuations in the effective pore-window sizes, potentially acting to filter absorption and diffusion of guest species. Here we report a quasi-elastic neutron scattering investigation of the dynamics over the temperature range 1.5 – 300 K using the PELICAN instrument. This is compared with ab initio molecular dynamics to gain a clear microscopic picture of the timescales of the entangled motions within the tetrahedra network.
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    Time-disordered crystal structure of AlPO4-5
    (American Chemical Society, 2017-08-03) Cortie, DL; McBride, BR; Narayanan, N; de Souza, NR; Avdeev, M; Mole, RA; McIntyre, GJ; Kearley, GJ; Withers, RS; Yu, DH; Liu, Y
    Modern ab initio calculations have become increasingly accurate for predicting the symmetry of crystal structures; however, the standard methods usually only determine the 0 K static configuration and therefore misrepresent structures where dynamics play a key role. In this work, we demonstrate a clear experimental example of this phenomenon in the AlPO4-5 molecular sieve where the average high-symmetry P6cc structure emerges from local dynamics involving corner-sharing tetrahedra. Quasielastic and inelastic neutron scattering experiments were conducted to clarify the thermally activated motions between 1.5 and 300 K. Through comparison with ab initio molecular dynamics, we explain why the theoretically predicted structure is not observed in diffraction experiments. Instead, the results indicate this material is an inorganic analogue of a plastic crystal where thermal dynamics dictate the average symmetry. © 2017 American Chemical Society