Browsing by Author "Moggach, S"

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    Development of high-pressure single-crystal neutron diffraction on the Laue diffractometer, KOALA, at OPAL
    (Australian Institute of Physics, 2016-02-04) Binns, J; McIntyre, GJ; Kamenev, KV; Moggach, S; Parsons, S
    Hydrogen bonds are one of the most important classes of intermolecular interaction, and accurate H-atom positions are critical for analysis of the energy terms which determine the thermodynamic stability of molecular crystals. At ambient pressure and low temperatures, H atoms can often be located by X-ray diffraction, and X-ray data can provide an accurate picture of the intermolecular contacts. High-pressure experiments do not afford this luxury. The high systematic errors introduced by the pressure cell and low completeness mean that H-atom positions are not revealed in X-ray Fourier maps. In some compounds H-atom positions can be inferred from the positions of other atoms, but this is not possible in all cases. Neutron diffraction data are much more sensitive to H than are X-ray data, and they are essential in cases where accurate H-atom location is important. Neutron powder patterns of complex molecular systems suffer from extensive peak overlap, and single-crystal diffraction therefore has a huge advantage; there is also no need to deuterate. The main disadvantage of neutron diffraction is that a large sample is usually required, which is at odds with the decreasing volumes possible with increasing pressure with existing pressure-cell materials. Modern neutron Laue diffraction and large moissanite anvil cells offer some respite 1, but complementing high-pressure X-ray data with high-pressure neutron data is still fraught with technical challenges to obtain identical conditions. Initial developmental experiments using a miniature diamond-anvil cell with a single crystal of size typical for X-ray diffraction on the KOALA Laue diffractometer at OPAL have shown the feasibility of the Laue technique for single-crystal neutron studies at high pressure. Remarkably, data completeness is similar to ambient-pressure measurements, despite the presence of the pressure cell. It is now possible to perform joint X-ray and neutron studies on the same sample under identical conditions.
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    [Mn-6] under pressure: a combined crystallographic and magnetic study
    (Wiley-VCH Verlag Berlin, 2008-03) Prescimone, A; Milios, CJ; Moggach, S; Warren, JE; Lennie, AR; Sanchez-Benitez, J; Kamenev, KV; Bircher, R; Murrie, M; Parsons, S; Brechin, EK
    Folding under pressure: High-pressure crystallography of an Mn6 single-molecule magnet reveals dramatic changes in the intramolecular geometry of the magnetic core. These structural changes effect the magnetic properties of the molecule: the magnitude of the ferromagnetic exchange between the metals is decreased, and under extreme pressure switches to antiferromagnetic. © 2008, Wiley-VCH Verlag Berlin
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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.
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    Towards joint high-pressure x-ray and neutron single-crystal diffraction
    (International Union of Crystallography, 2017-01) McIntyre, GJ; Binns, J; Kamenev, KV; Moggach, S; Parsons, S
    Diffraction methods can provide the highest-quality structural information about a crystal on the atomic scale and much work has been carried out to adapt X-ray and neutron diffraction techniques to a variety of challenging sample environments, including high-pressure. The ability to influence directly intermolecular distances makes high pressure one of the most important tools at our disposal for answering one of the big questions in chemistry - the prediction and control of solid-state structure. Modern neutron Laue diffractometers with large image-plate detectors permit extensive continuous sampling of reciprocal space with high resolution in the two-dimensional projection and a wide dynamic range with negligible bleeding of intense diffraction spots, qualities that are highly suited to high-pressure crystallography [1]. Here we show that high-pressure single-crystal neutron diffraction data can be collected using Laue diffraction from a sample of hexamine in a miniature diamond-anvil cell (mini-DAC) with no significant reductions in completeness or resolution [2]. The data are of similar quality, as judged by R-factors, geometric parameters, and estimated standard deviations, to those obtained at ambient pressures. This is achieved by the ability to measure diffracted intensity directly through the body of the mini-DAC. Joint high-pressure experiments using both X-ray and neutron diffraction on the same sample are now feasible using the mini-DAC and modern neutron Laue diffractometers like KOALA on the OPAL reactor. © International Union of Crystallography