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Abstract: The Na3PnS4 (Pn = P, Sb) solid electrolytes are promising candidates for sodium solid-state batteries due to their potential high ionic conductivities. Structural modifications of these materials can induce a tetragonal-to-cubic phase transition, either by increasing temperature or by aliovalent substitutions. In this study, we introduce pressure as an alternative approach to observe the tetragonal-to-cubic phase transition in these materials. In situ synchrotron high-pressure powder X-ray diffraction shows a tetragonal-to-cubic phase transition at pressures of 2.9 GPa for Na3SbS4 and 14.6 GPa for Na3PS4. Rietveld refinements and symmetry analysis provide insights into the displacive phase transition mechanism related to the motion of Na+ and the rotation of the SbS43– tetrahedra. Density functional theory calculations confirm that the cubic phase becomes thermodynamically favorable under high pressure compared to the tetragonal phase. These findings highlight the importance of high-pressure considerations in tailoring the properties of ionic conductors, an area that remains underexplored.

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Abstract: Through laser-heated diamond anvil cell experiments, we synthesize a series of rubidium superhydrides and explore their properties with synchrotron x-ray powder diffraction and Raman spectroscopy measurements, combined with density functional theory calculations. Upon heating rubidium monohydride embedded in H2 at a pressure of 18 GPa, we form RbH9−I, which is stable upon decompression down to 8.7 GPa, the lowest stability pressure of any known superhydride. At 22 GPa, another polymorph, RbH9−II is synthesised at high temperature. Unique to the Rb-H system among binary metal hydrides is that further compression does not promote the formation of polyhydrides with higher hydrogen content. Instead, heating above 87 GPa yields RbH5, which exhibits two polymorphs (RbH5−I and RbH5−II). All of the crystal structures comprise a complex network of quasimolecular H2 units and H− anions, with RbH5 providing the first experimental evidence of linear H− 3 anions.

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Abstract: NaNiO$_2$ is a Ni$^{3+}$-containing layered material consisting of alternating triangular networks of Ni and Na cations, separated by octahedrally-coordinated O anions. At ambient pressure, it features a collinear Jahn--Teller distortion below $T^\mathrm{JT}_\mathrm{onset}\approx480$\,K, which disappears in a broad first-order transition on heating to $T^\mathrm{JT}_\mathrm{end}\approx500$\,K, corresponding to the increase in symmetry from monoclinic to rhombohedral. It was previously studied by variable-pressure neutron diffraction [ACS Inorganic Chemistry 61.10 (2022): 4312-4321] and found to exhibit an increasing $T^\mathrm{JT}_\mathrm{onset}$ with pressure up to $\sim$5\,GPa. In this work, powdered NaNiO$_2$ was studied \textit{via} variable-pressure synchrotron x-ray diffraction up to pressures of $\sim$67\,GPa at 294\,K and 403\,K. Suppression of the collinear Jahn--Teller ordering is observed \textit{via} the emergence of a high-symmetry rhombohedral phase, with the onset pressure occurring at $\sim$18\,GPa at both studied temperatures. Further, a discontinuous decrease in unit cell volume is observed on transitioning from the monoclinic to the rhombohedral phase. These results taken together suggest that in the vicinity of the transition, application of pressure causes the Jahn--Teller transition temperature, $T^\mathrm{JT}_\mathrm{onset}$, to decrease rapidly. We conclude that the pressure-temperature phase diagram of the cooperative Jahn--Teller distortion in NaNiO$_2$ is dome-like.

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Abstract: Nancyrossite, ideally FeGeO6H5, is a new hydroxyperovskite from Tsumeb. It likely formed by the oxidation and partial dehydrogenation of stottite, FeGe(OH)6, with which it is intimately associated. The structure of nancyrossite has been determined in tetragonal space group P42/n: a = 7.37382(12) Å, c = 7.29704(19) Å, V = 396.764(16) Å3, Z = 4, R1(all) = 0.034, wR2(all) = 0.051, GoF = 1.057. Empirical formulae of two crystals have almost end- member compositions, Fe3+1.01Zn0.03Ge0.98O6H5 and Fe3+1.01Zn0.04Ge0.98O6H5. Structure determination indicates that 88% Fe is ferric. The chemical formula proposed here for nancyrossite recognizes that although H atoms form OH groups, writing the formula as FeGeO(OH)5 implies that one of the six oxygen atoms is very underbonded, with a bond- valence sum of only ~1.2 v.u. As such, H in nancyrossite may have novel crystal chemistry. For example, the five H atoms may be distributed dynamically over the six O atoms, a phenomenon that would be averaged by X-ray diffraction, and so go undetected. Nancyrossite is the Ge-analogue of jeanbandyite. By analogy with nancyrossite, we propose revision of the ideal formula of jeanbandyite from FeSnO(OH)5 (Welch and Kampf, 2017) to FeSnO6H5.

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Abstract: Metal-organic frameworks (MOFs) are a versatile class of hybrid inorganic-organic materials known for their adjustable chemical and physical properties, as well as their exceptional porosity. These characteristics render MOFs particularly valuable for applications requiring extensive surface areas, such as gas storage and catalysis. Despite being advantageous in many applications, the high porosity and specific bonding characteristics of crystalline MOFs can, however, make them susceptible to pore collapse and amorphisation under pressure. This limits the practical effectiveness of MOFs in their most commonly synthesised form of crystalline powders, as large-scale production and shaping of powders for industrial use often involves pressure and heating. This thesis examines how a range of MOFs behave under various high pressure and high pressure-temperature conditions to examine both their amorphisation mechanisms and amorphous phases formed. The MOFs were selected to fall within two groups: zirconium-based (UiO-66, MOF-808 and NU-1000) and zinc-based (ZIF-8, ZIF-4 and ZIF-62). The methods chosen for investigations were hydrostatic compression, non-hydrostatic compression, and ball-milling, as they are all used for industrial processing of powders: The former two are methods for shaping, and the latter for mixing. Hydrostatic compression of these MOFs is investigated in depth through in situ high pressure-temperature crystallographic and spectroscopic measurements, allowing real-time analysis on the MOFs’ collapse mechanisms. Both groups display partially reversible amorphisation under hydrostatic compression to certain pressures, indicating a displacive amorphisation transition into an amorphous phase topologically similar to the crystalline. Penetration of the pressure-transmitting media into the framework’s pores was also indicated in each MOF, with clear negative volume compressibility shown in the zinc-based MOFs. Ex situ investigations into non-hydrostatic compression then introduce the effect of shear stress so its effect on the MOFs can be highlighted, where the two groups demonstrate quite different behaviour attributed to differences in the connectivity of their inorganic components. Ball-milling is finally examined as a non-compression form of amorphisation with a high shear component. In both shear-based pressure states, decoordination of the organic components from the inorganic is seen as a driving factor of amorphisation. Understanding the collapse mechanisms and resultant amorphous phases from various amorphisation methods in these MOFs gives insight into trends in mechanical properties and stability within this class of materials, and is essential for the future industrialisation of MOFs.