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Abstract: The structural behaviour of homoleptic xenon difluoride (XeF2) com­plexes [M(XeF2)6][SbF6]2 (M = Cu, Zn) under varying tem­per­a­ture and pressure has been investigated, aiming to resolve the disordered Jahn–Teller distortions in the copper com­plex (CuSb). At 200 K, both CuSb and its zinc analogue (ZnSb) crystallize in a layered CdCl2-type structure with the space group R3. Upon cooling below 170 (CuSb) and 160 K (ZnSb), both systems transition to iso­structural phases in P1, with CuSb assuming an ordered Jahn–Teller distortion. The transformation is driven by the shortening and optimization of the Xe⋯F inter­molecular contacts, forming stronger and more directional inter­actions, rather than by Jahn–Teller effects alone. This is supported by the observation of similar transitions in the Jahn–Teller-inactive Zn system. High-pressure ex­peri­ments up to ∼2.8 GPa at room tem­per­a­ture show the structural stability of the high-symmetry phases, implicating kinetic barriers to further transformation. Additionally, the synthesis and structural characterization of a novel arsenic analogue, [Zn(XeF2)6][AsF6]2 (ZnAs), reveal similar layered motifs but distinct phase behaviour. Symmetry-mode analyses relate all observed phases through distortions of a common CdCl2 aristotype.

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Abstract: Advances in ceramic additive manufacturing have enabled fabrication of piezoelectric metamaterials in which architecture can be used as a design parameter to expand and independently tune the different anisotropic electromechanical coupling modes. A critical step toward rational design of architected piezoceramics is the determination of complete piezoelectric tensor matrices for the differently oriented elements. In principle, this can be accomplished if the magnitude and direction of local electric fields are known, and it is assumed that the various elements reach their full potential for poling. The former aspect, that is, the local electric fields, can be readily computed using advanced finite element simulation tools. However, direct measurements of microstructural changes within architected piezoceramics also reveal that the spatial distribution of local poling levels does not exactly correlate to the local electric field distributions and instead are modulated by various material-specific parameters. This necessitates computing the spatial map of piezoelectric tensors based on direct knowledge of microstructural changes within the various elements. The current article expounds on a methodology for realizing the same by combining in situ x-ray microdiffraction experiments with a micromechanical model. The methodology is demonstrated for a 3D printed structure of BaTiO3 piezoceramic with periodic octagonal pattern.

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Abstract: The atomic-scale structure and melting curve of liquid mercury was measured using in situ synchrotron x-ray diffraction (SXRD) at pressure and temperature (𝑝−𝑇) conditions up to 9.44(2) GPa and 651(1) K. Ab initio molecular dynamics (AIMD) simulations were employed to obtain a detailed atomistic model of the liquid structure. The results reveal a pronounced flattening, and potential maximum, in the measured melting curve between 6 and 9 GPa. The structure factors 𝑆HgHg⁡(𝑄) and pair distribution functions 𝑔HgHg⁡(𝑟)calculated from the AIMD simulations are in good overall agreement with the SXRD measurements under comparable reduced densities and temperatures, indicating that the atomistic structure of liquid Hg is well captured by AIMD. With increasing pressure, the principal peak in 𝑆HgHg⁡(𝑄) shifts to higher 𝑄, with the subsidiary peak at 𝑄=2⁢𝑘Fexperiencing a concomitant shift consistent with the increased electron density. Considering the Evans 𝑡-matrix formulation of the Ziman theory of liquid metals, the structural 𝑆⁡(2⁢𝑘F) term is expected to have only a weak influence on the electrical resistivity under compression. In contrast, the pressure-induced broadening and shift of the 𝑑-projected density of states towards the Fermi level is consistent with enhanced near-resonant 𝑑-electron scattering, and a corresponding increase in resistivity, analogous to the behavior of first-row transition metals. Analysis of the measured 𝑔HgHg⁡(𝑟) functions, and AIMD trajectories in real space, indicates that the liquid structure experiences a progressive development towards simple hard-sphere-like behavior at increasing 𝑝−𝑇along the melting curve. However, topological cluster classification analysis shows that while the structural fingerprint of liquid Hg strongly resembles an effective hard-sphere system, even at the highest pressures investigated it contains more many-body motifs than expected for this simple model.

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Abstract: Crystalline compounds are a class of solid material, distinguished from glasses and amorphous substances, by the presence of long-range periodic order within their structures. This ordering causes these compounds to have sharp diffraction patterns, enabling elucidation of their structural features. The factors that impact how a crystal is structured has been the subject of much research in crystal engineering, which studies how intermolecular and non-covalent interactions, such as hydrogen bonding, impacts molecular packing and physical properties. Distinct structural forms of a compound, known as phases, can interconvert to adapt these intermolecular interactions in response to external thermodynamic stimuli, such as pressure and temperature. The response of those interactions to these conditions provides an avenue into investigating their importance in determining a crystal’s structure. Whilst there has been a significant body of work investigating the response of many organic, metal-organic and inorganic crystals towards both pressure and temperature, noble gas chemistry is a severely under investigated field as regards the response to these conditions. The reasons for this are multifold: the synthesis of noble gas compounds is highly hazardous and non-trivial, they tend to be highly air and moisture sensitive, making handling difficult, and very few research groups have the capabilities to both synthesise and analyse their structures. This thesis outlines work carried out aiming to address this, and investigates the structures, and intermolecular interactions taking place, in select noble gas compounds at both high pressure and low temperature. Chapter 1 serves as an introduction to synthetic noble gas chemistry, and the key structural features of the known compounds. Chapter 2 outlines the experimental techniques used and developed in this thesis to investigate the structural behaviour of noble gas compounds at high pressure. Chapter 3 contains work determining a thermal pressure equation of state for the simple salt sodium fluoride (NaF). NaF serves as a useful pressure marker due to its unreactive nature, it is unable to be oxidised and is expected to be entirely unreactive towards fluorinated noble gas compounds. This is a benefit over some typical pressure markers such as ruby, lead, gold or tungsten, which can be potentially oxidised by noble gas compounds or fluorine at high pressure. The EoS that is developed in this chapter relies on a thermal pressure model, the pressure needed to supress the effects of thermal expansion, to determine a PVT relationship. This model comprises data collected at Edinburgh University and ISIS neutron and muon source between 12 and 350 K, and up to pressures of 5 GPa, as well as literature PVT data up to 950 K and 25 GPa, and adiabatic bulk modulus data between 4 K and 650 K. This provides a precise and consistent EoS between 12 and 950 K and up to 25 GPa for NaF that can be used to measure the third variable when any two of pressure, volume or temperature are known. This is one of only a few examples where an EoS has been determined combining high pressure and low temperature. Chapter 4 continues this theme and develops a multiphonon equation of state for the simplest noble gas compound, XeF2. Previous work in this area has largely dealt with the very high-pressure regime, up to 116 GPa, with a recent study also laser heating the sample. Our work presents an in-depth investigation up to 5 GPa, at temperatures down to 120 K, and measures the thermal expansion of XeF2 between 12 K and 300 K. The simple Debye oscillator used to model thermal pressure in the NaF model proves unsuitable to adequately reproduce the experimentally determined thermal expansion of XeF2, and thus necessitates a multiphonon model, where the vibrational modes of XeF2 in the solid state are explicitly addressed.

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Abstract: BiFeO3-BaTiO3 (BF-BT) ceramics are important lead-free ferroelectric materials, which are attracting attention due to their high Curie temperatures. In the present study, the effects of annealing at intermediate temperatures on the structure and ferroelectric hardening effects in Ti-doped BF-BT ceramics are investigated. It is shown that enhanced ferroelectric and piezoelectric properties are obtained by air-quenching. After subsequent annealing for 200 hours at temperatures in the range from 500 to 600 °C, the ferroelectric hysteresis loops became constricted due to a strong domain stabilisation effect. The ferroelectric internal bias field increased to an outstanding value of 5 kV mm-1 in a poled-annealed specimen. Analysis of the extrinsic (domain switching) and intrinsic (lattice strain) contributions to electro-strain by in-situ x-ray diffraction indicated that domain switching is dominant in the quenched BF-BT ceramic, but the domain orientation fraction under high electric field was reduced dramatically, from approximately 80% to 25%, after annealing.