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Abstract: W-type hexaferrites with varied Co/Zn ratios were synthesized and the magnetic order was investigated using neutron powder diffraction. In SrCo2Fe16O27 and SrCoZnFe16O27 a planar (Cm′cm′) magnetic ordering was found, rather than the uniaxial ordering (P63/mm′c′) found in SrZn2Fe16O27 which is common in most W-type hexaferrites. In all three studied samples, non-collinear terms were present in the magnetic ordering. One of the non-collinear terms is common to the planar ordering in SrCoZnFe16O27 and uniaxial ordering in SrZn2Fe16O27, which could be a sign of an imminent transition in the magnetic structure. The thermomagnetic measurements revealed magnetic transitions at 520 and 360 K for SrCo2Fe16O27 and SrCoZnFe16O27, and Curie temperatures of 780 and 680 K, respectively, while SrZn2Fe16O27 showed no transition but a Curie temperature at 590 K. This leads to the conclusion that the magnetic transition can be adjusted by fine-tuning the Co/Zn stoichiometry in the sample.

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Abstract: Rozenite (FeSO4·4H2O) is a candidate mineral component of the polyhydrated sulfate deposits on the surface and in the subsurface of Mars. To better understand its behavior at temperature conditions prevailing on the Martian surface and aid its identification in ongoing and future Rover missions, we have carried out a combined experimental and computational study of the mineral’s structure and properties. We collected neutron powder difraction data at temperatures ranging from 21–290 K, room-temperature synchrotron X-ray data and Raman spectra. Moreover, first-principles calculations of the vibrational properties of rozenite were carried out to aid the interpretation of the Raman spectra. We found, in contrast to a recent Raman spectroscopic study, that there are no phase transitions between 21 and 290 K. We confirm the heavy atom structure reported in the literature (space group P21/n) to be correct and present, for the first time, an unconstrained determination of the H atom positions by means of high-resolution neutron powder diffraction, and report the complete crystal structure at 290 and 21 K. The anisotropy of the thermal expansion of the lattice vectors is αa:αb:αc = 1.00:2.19:1.60 at 285 K. Subsequent analysis of the thermal expansion tensor revealed highly anisotropic behavior as reflected in negative thermal expansion approximately ||〈101⟩ and ratios of the tensor eigenvalues of α1:α2:α3 = −1:3.74:5.40 at 285 K. Lastly, we demonstrated how combining Raman spectroscopy and X-ray difraction of the same sample sealed inside a capillary with complementary first-principles calculations yields accurate reference Raman spectra. This workflow enables the construction of a reliable Raman spectroscopic database for planetary exploration, which will be invaluable to shed light on the geological past as well as in identifying resources for the future colonization of planetary bodies throughout the solar system.

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Abstract: In this work, we report the synthesis, structural characterisation and sorption properties of an 8-fold interpenetrated diamondoid (dia) metal–organic framework (MOF) that is sustained by a new extended linker ligand, [Cd(Imibz)2], X-dia-2-Cd, HImibz or 2 = 4-((4-(1H-imidazol-1-yl)phenylimino)methyl)benzoic acid. X-dia-2-Cd was found to exhibit reversible single-crystal-to-single-crystal (SC–SC) transformations between four distinct phases: an as-synthesised (from N,N-dimethylformamide) wide-pore phase, X-dia-2-Cd-α; a narrow-pore phase, X-dia-2-Cd-β, formed upon exposure to water; a narrow-pore phase obtained by activation, X-dia-2-Cd-γ; a medium-pore CO2-loaded phase X-dia-2-Cd-δ. While the space group remained constant in the four phases, the cell volumes and calculated void space ranged from 4988.7 Å3 and 47% (X-dia-2-Cd-α), respectively, to 3200.8 Å3 and 9.1% (X-dia-2-Cd-γ), respectively. X-dia-2-Cd-γ also exhibited a water vapour-induced structural transformation to the water-loaded X-dia-2-Cd-β phase, resulting in an S-shaped sorption isotherm. The inflection point occurred at 18% RH with negligible hysteresis on the desorption profile. Water vapour temperature-humidity swing cycling (60% RH, 300 K to 0% RH, 333 K) indicated hydrolytic stability of X-dia-2-Cd and working capacity was retained after 128 cycles of sorbent regeneration. CO2 (at 195 K) was also observed to induce a structural transformation in X-dia-2-Cd-γ and in situ PXRD studies at 1 bar of CO2, 195 K revealed the formation of X-dia-2-Cd-δ, which exhibited 31% larger unit cell volume than X-dia-2-Cd-γ.

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Abstract: Magnesium sulfate hydrates are considered important rock-forming minerals on the outer three Galilean moons of Jupiter (i.e., Europa, Ganymede, Callisto) and, alongside ferrous sulfate hydrates, are promising candidate minerals for the widespread sulfate deposits that occur in the equatorial region of Mars. In such extraterrestrial environments, these minerals experience extreme high-pressure conditions in the interiour of the Galilean moons and low temperature conditions on the surface of these moons and Mars. The aim of this thesis is to understand the structural stability, compressibility, and thermal expansion of these compounds in such extreme environments and aid their identification in ongoing and future space missions. Most magnesium sulfate hydrates lack accurate reference elastic tensors, which hinders their seismological identification in lander missions on the icy moons of the outer solar system, as envisioned for the near future. In this thesis, the accuracy of recent advancements in density functional theory to predict the compressibility and elastic constants of icy satellite candidate minerals (i.e., epsomite (MgSO₄·7H₂O), gypsum (CaSO₄·2H₂O), carbon dioxide (CO₂), and benzene (C₆H₆)) was assessed by benchmarking them against experimental reference data from the literature. Key findings are that density functional theory calculations do not yield elastic constants accurate enough to be used as a reference for the seismic exploration of icy moons. However, the bulk compressibility of such materials is very accurately reproduced by density functional theory, which was therefore used to predict the compressibility of the icy satellite candidate minerals starkeyite (MgSO₄·4H₂O) and cranswickite (MgSO₄·4H₂O). Knowledge of the compressibility of such minerals is critical to model mantle processes (e.g., salt diaprisim, plate tectonics, subduction) and the density structure of the outer three Galilean moons. The thermal expansion and structural stability of three sulfate minerals (i.e., rozenite (FeSO₄·4H₂O), starkeyite, and cranswickite) was characterised for the first time using neutron diffraction. Cranswickite transforms to starkeyite at 330 K, well above the maximum surface temperature of 308 K hitherto reported on Mars. Starkeyite likely undergoes a structural phase transition at around 245 K. The structure of this proposed low-temperature polymorph could not be determined but would be of great interest since the temperature drops below 245 K on equatorial Mars at night-time. Starkeyite was also studied by means of synchrotron X-ray diffraction but suffered radiation damage. No phase transition was observed in rozenite from 290 – 21 K, which contrasts with Raman data reported in the literature, where sharpening of vibrational modes upon cooling was misinterpreted as mode splitting and evidence for two phase transitions at temperatures relevant to the Martian surface. First-principles phonon frequency calculations provide evidence supporting the absence of vibrational mode splitting. A workflow to obtain reliable reference Raman spectra for space exploration was proposed and an optical centre stick for the simultaneous acquisition of neutron diffraction and Raman spectroscopy data at the HRPD instrument was commissioned. Lastly, the structure of a polymorph of hexahydrite (MgSO₄·6H₂O), most recently proposed in the literature, was shown to be unambiguously wrong.

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Abstract: Phase transitions in crystalline molecular solids have important implications in the fundamental understanding of materials properties and in the development of materials applications. Herein, we report the solid-state phase transition behavior of 1-iodoadamantane (1-IA) investigated using a multi-technique strategy [synchrotron powder X-ray diffraction (XRD), single-crystal XRD, solid-state NMR, and differential scanning calorimetry (DSC)], which reveals complex phase transition behavior on cooling from ambient temperature to ca. 123 K and on subsequent heating to the melting temperature (348 K). Starting from the known phase of 1-IA at ambient temperature (phase A), three low-temperature phases are identified (phases B, C, and D); the crystal structures of phases B and C are reported, together with a re-determination of the structure of phase A. Remarkably, single-crystal XRD shows that some individual crystals of phase A transform to phase B, while other crystals of phase A transform instead to phase C. Results (from powder XRD and DSC) on cooling a powder sample of phase A are fully consistent with this behavior while also revealing an additional transformation pathway from phase A to phase D. Thus, on cooling, a powder sample of phase A transforms partially to phase C (at 229 K), partially to phase D (at 226 K) and partially to phase B (at 211 K). During the cooling process, each of the phases B, C, and D is formed directly from phase A, and no transformations are observed between phases B, C, and D. On heating the resulting triphasic powder sample of phases B, C, and D from 123 K, phase B transforms to phase D (at 211 K), followed by the transformation of phase D to phase C (at 255 K), and finally, phase C transforms to phase A (at 284 K). From these observations, it is apparent that different crystals of phase A, which are ostensibly identical at the level of information revealed by XRD, must actually differ in other aspects that significantly influence their low-temperature phase transition pathways. This unusual behavior will stimulate future studies to gain deeper insights into the specific properties that control the phase transition pathways in individual crystals of this material.