Helen E. Maynard-Casely

The magnetic structures and transitions of a potential multiferroic orthoferrite ErFeO3

Rare-earth orthoferrites are very interesting due to their appealing optical and multiferroic properties. In this study, the magnetic structures and transitions of a typical rare-earth orthoferrite, ErFeO3, have been reinvestigated in detail. The spin-reorientation transition of the Fe3+ magnetic phase and the low-temperature magnetic ordering of Er3+ were observed by neutron powder diffraction. The corresponding magnetic structures have been solved anew by symmetry analysis and refinement of the diffraction results. The magnetic moments of Fe3+ align in an antiferromagnetic way along the c axis with a weak ferromagnetic component along the b axis below the Neel temperature and above the spin-reorientation transition. Below the spin-reorientation transition, the Fe3+ moments rotate into an antiferromagnetic ordering state along the b axis with weak ferromagnetic alignment along the c axis. The spin-reorientation takes place in the bc plane. The Er3+ moments align antiferromagnetically with a Cy mode below...

Deformation-resembling microstructure created by fluid-mediated dissolution–precipitation reactions

Deformation microstructures are widely used for reconstructing tectono-metamorphic events recorded in rocks. In crustal settings deformation is often accompanied and/or succeeded by fluid infiltration and dissolution–precipitation reactions. However, the microstructural consequences of dissolution–precipitation in minerals have not been investigated experimentally. Here we conducted experiments where KBr crystals were reacted with a saturated KCl-H2O fluid. The results show that reaction products, formed in the absence of deformation, inherit the general crystallographic orientation from their parents, but also display a development of new microstructures that are typical in deformed minerals, such as apparent bending of crystal lattices and new subgrain domains, separated by low-angle and, in some cases, high-angle boundaries. Our work suggests that fluid-mediated dissolution–precipitation reactions can lead to a development of potentially misleading microstructures. We propose a set of criteria that may help in distinguishing such microstructures from the ones that are created by crystal-plastic deformation.

Structure and thermal expansion of sulfuric acid octahydrate

Synchrotron X-ray powder diffraction has been used to structurally characterize crystallization products from 37.8 and 40.5 wt% aqueous sulfuric acid solutions. It is confirmed that, despite speculation in the literature, the structure that predominately crystallized from these solutions is sulfuric acid octahydrate (SAO). The existence of an uncharacterized phase is also noted. It was found that existing models proposed for the crystal structure of SAO did not satisfactorily fit to the data acquired here, and hence a new structure solution was sought. It is reported here that the structure of SAO is contained within a unit cell with I2 symmetry with a = 7.44247 (11), b = 7.4450 (1), c = 26.1168 (3) A, β = 125.0428 (7)°, V = 1184.78 (3) A3 at 80 K. Data were collected at temperatures between 80 and 198 K, which enabled determination of the thermal expansion of SAO.

Copper(II) coordination polymers of imdc− (H2imdc+ = the 1,3-bis(carboxymethyl)imidazolium cation): unusual sheet interpenetration and an unexpected single crystal-to-single crystal transformation

The monoanion of 1,3-bis(carboxymethyl)imidazolium (H2imdc+) combines with Cu(II) to produce an undulating 2D coordination polymer of composition [Cu2(imdc)2(CH3OH)2](BF4)2·(CH3OH)(H2O) (1) in which copper acetate-like dimers, linked by imdc− ligands, act as 4-connecting centres. Cationic sheets stack on top of each other in an A, B, A, B… fashion and produce a structure that contains channels running parallel to the plane of network. Tetrafluoroborate anions are located in channels between sheets. Upon removal of coordinated and non-coordinated solvent molecules a single crystal-to-single crystal transformation occurs to yield a similar compound but with BF4− anions now coordinated. CO2 isotherms measured at 258 and 273 K show only modest uptake of CO2 but provide an indication that the sheets move apart at elevated pressures in order to accommodate the guest molecules. A compound of composition [Cu3(OH)2(imdc)2]·SiF6·2H2O·2MeOH (3), which possesses a 3D network, is formed by the combination of copper(II) acetate, copper(II) hexafluorosilicate and Himdc. In this structure infinite parallel Cu3(OH)2 chains are linked by bridging imdc− ligands to form channels that have an approximately triangular cross-section. These channels are occupied by SiF62− anions in addition to solvent molecules. When copper(II) acetate is combined with Himdc in the appropriate ratio, a 1D coordination polymer of composition Cu(imdc)2 (4) is formed in which pairs of imdc− anions bridge Cu(II) centres. When the reaction is performed in the presence of NaBF4 a minor crystalline product with tetragonal symmetry is isolated in addition to the 1D coordination polymer. This compound of composition Cu2(imdc)4NaBF4·7H2O (5) consists of 2D Cu(imdc)2 networks and features an unusual mode of interpenetration.

Structural and Magnetic Properties of the Osmium Double Perovskites Ba2–xSrxYOsO6

The crystal and magnetic structures of double perovskites of the type Ba2-xSrxYOsO6 were studied by synchrotron X-ray and neutron powder diffraction methods, bulk magnetic susceptibility measurements, and X-ray absorption spectroscopy. The structures were refined using combined neutron and synchrotron data sets based on an ordered array of corner-sharing YO6 and OsO6 octahedra, with the Ba/Sr cations being completely disordered. The structure evolves from cubic to monoclinic Fm3̅m (x ≈ 0.6) → I4/m (x ≈ 1.0) → I2/m (x ≈ 1.6) → P21/n as the Sr content is increased, due to the introduction of cooperative tilting of the octahedra. Bulk magnetic susceptibility measurements demonstrate the oxides are all anti-ferromagnets. The decrease in symmetry results in a nonlinear increase in the Neel temperature. Low-temperature neutron diffraction measurements of selected examples show these to be type-I anti-ferromagnets. X-ray absorption spectra collected at the Os L3- and L2-edges confirm the Os is pentavalent in all cases, and there is no detectable change in the covalency of the Os cation as the A-cation changes. Analysis of the L3/L2 branching ratio shows that the spin-orbit coupling is constant and insignificant across the series.

The next dimension of structural science communication: simple 3D printing directly from a crystal structure

Communicating science is hard. This is particularly true for a lot of structural science concepts which are inherently three dimensional in nature such as molecular geometry, symmetry, intermolecular interactions and the packing of crystal structures. One of the most effective ways to get around this difficulty is to use physical 3D models for communication, whether it is in an outreach setting, through classroom education or even presenting research results at a conference. Recent studies have shown how to generate instruction files to 3D print experimentally accurate models. Here we present for the first time how scientists can do this from any standard structural model file (incl. MOL2, XYZ, SDF, PDB, CIF, RES) easily using the well-known, freely available structure visualisation program, Mercury.

Prospects for mineralogy on Titan

Abstract Saturn’s moon Titan has a surface that is dominated by molecular materials, much of which are photochemically produced in the moon’s atmosphere. This outlook reviews the potential minerals that would be expected to form on the surface and subsurface of Titan from these molecular solids. We seek to classify them and look toward how the future study of these minerals will enhance our understanding of this planetary body. The classification uses the basis of intermolecular interactions, with the materials grouped into “Molecular solids,” “Molecular co-crystals,” and “Hydrates” classes alongside speculation on other possible classes of potential Titan minerals.

Crossover between liquid-like and gas-like behaviour in CH4 at 400 K

We report experimental evidence for a crossover between a liquidlike state and a gaslike state in fluid methane (CH_{4}). This crossover is observed in all of our experiments, up to a temperature of 397 K, 2.1 times the critical temperature of methane. The crossover has been characterized with both Raman spectroscopy and x-ray diffraction in a number of separate experiments, and confirmed to be reversible. We associate this crossover with the Frenkel line-a recently hypothesized crossover in dynamic properties of fluids extending to arbitrarily high pressure and temperature, dividing the phase diagram into separate regions where the fluid possesses liquidlike and gaslike properties. On the liquidlike side the Raman-active vibration increases in frequency linearly as pressure is increased, as expected due to the repulsive interaction between adjacent molecules. On the gaslike side this competes with the attractive van der Waals potential leading the vibration frequency to decrease as pressure is increased.

‘Peaks in space’ – crystallography in planetary science: past impacts and future opportunities

ABSTRACT Crystallography has done much to underpin and enhance our understanding of our planetary neighbours. From analysis of the samples plucked from space by audacious sample-return missions, to the detailed study of meteorites that have landed on our planets surface; investigations of the crystal structures, the growth of components and the physical properties of these precious samples has been key to teasing out the story of these fragments and their parent bodies. To prove that there are no limits to the ends that a crystallographer will take to collect planetary data, there is now one diffractometer working another planet, Mars. Not to be daunted by the lack of samples from beyond the terrestrial planets, crystallography has also been at the forefront of in situ experiments that recreate the extremes of planetary environments. This review will capture how crystallography has been used to all of these ends and reveal a number of opportunities for future application of the technique.

Making crystal structure an everyday thing – Crystallography365 and beyond

To the public, the concept of crystal structures can be very abstract. Despite the fact that we are surrounded by crystalline materials, most people struggle to see the value in the work we crystallographers do. However, kick started by the International Year of Crystallography 2014, a number of projects have been undertaken to show the value of crystallography to public audiences, such as the world’s largest crystal structure [1] and the series of video’s produced by the UK’s Royal Institution [2].