Browsing by Author "Valiente, R"

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    Phase transition sequences in tetra­methyl­ammonium tetra­chloro­metallates by x-ray diffraction and spectroscopic measurements
    (International Union of Crystallography, 2017-01) Binns, J; McIntyre, GJ; Barreda-Argüeso, JA; González, J; Aguado, F; Rodríguez, F; Valiente, R; Parsons, S
    The phase transition sequences of two members of the tetra­methyl­ammonium tetra­chloro­metallate(III) family of hybrid organic–inorganic salts have been determined and structurally characterized as a function of temperature for the first time. Unusually, a reduction in point-group symmetry with increasing temperature until reaching a cubic prototype phase is observed. Two additional intermediate phases are observed for Fe3+. First-principles calculations and the presence of short Cl...Cl contacts for Ga3+ suggest the [GaCl4]− anion to be conformationally hindered due to stronger lone-pair–σ-hole interactions. The conformationally more flexible Fe3+ structures show sublattice melting with the onset of rotational disorder in the [NMe4]+ cations occurring 40 K below the corresponding onset of rotational disorder in the [FeCl4]− sublattice. © International Union of Crystallography
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    Structural studies of phase transitions in hybrid organic-inorganic salts with temperature and pressure
    (Australian Institute of Physics, 2014-02-04) Binns, J; Parsons, S; Moggach, S; Valiente, R; McIntyre, GJ; Kamenev, KV
    The alkylammonium tetrachlorometallates have attracted significant attention for the numerous phase transitions observed in a relatively narrow range of temperatures and pressures as well as ferroelectric,-elastic and -magnetic behaviours. [1,2] Such simple organic salts could find possible applications as thin-film functional materials in low cost ferroelectric capacitors and RAM. With the exception of bis(tetramethylammonium) tetrachlorozincate(II) this class of materials has been subject to relatively little structural investigation, with a number of general phase sequences being determined from calorimetric and polarisation measurements. [3,4] While there are known to be ferroelectric phase transitions in many of these materials, the exact mechanism by which these simple organic salts exhibit such behaviour is unknown. We report on the phase sequences observed in two related materials: tetramethylammonium tetrachloroferrate(III) (TCF), and the previously unknown tetramethylammonium tetrachlorogallate(III) (TCG) which display re-entrant as well as plastic crystalline phases.