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Transition metal nitrides, particularly those of 5d metals, are known for their outstanding properties, often relevant for industrial applications. Among these metal elements, tungsten is especially attractive given its low cost. In this high-pressure investigation of the W-N system, two novel ultra-incompressible tungsten nitride superconductors, namely W2N3 and W3N5, are successfully synthesized at 35 and 56 GPa, respectively, through a direct reaction between N2 and W in laser-heated diamond anvil cells. Their crystal structure is determined using synchrotron single-crystal X-ray diffraction. While the W2N3 solid's sole constituting nitrogen species are N3- units, W3N5 features both discrete N3- as well as N24- pernitride anions. The bulk modulus of W2N3 and W3N5 is experimentally determined to be 380(3) and 406(7) GPa, and their ultra-incompressible behavior is rationalized by their constituting WN7 polyhedra and their linkages. Importantly, both W2N3 and W3N5 are recoverable to ambient conditions and stable in air. Density functional theory calculations reveal W2N3 and W3N5 to have a Vickers hardness of 30 and 34 GPa, and superconducting transition temperatures at ambient pressure (50 GPa) of 11.6 K (9.8 K) and 9.4 K (7.2 K), respectively. Additionally, transport measurements performed at 50 GPa on W2N3 corroborate with the calculations. Two recoverable tungsten nitrides, namely W2N3 and W3N5, are synthesized using laser-heated diamond anvil cells. Both compounds exhibit a high bulk modulus, hardness, and superconducting transition temperature. image, Funding Agencies|UKRI Future Leaders Fellowship; European Synchrotron Radiation Facility (ESRF) [ID15B]; Deutsche Forschungsgemeinschaft [DFG Emmy-Noether, BY112/2-1]; Friedrich Naumann Foundation for Freedom; Federal Ministry of Education and Research (BMBF); European Research Council (ERC) under the European Union [101002868]; National Science Foundation [EAR-1634415]; DOE Office of Science by Argonne National Laboratory [DE-AC02-06CH11357]; Knut and Alice Wallenberg Foundation (Wallenberg Scholar grant) [KAW-2018.0194, Mrc-Mr/T043733/1]; Guangdong Basic and Applied Basic Research Foundation [2022a1515110404]; Swedish Research Council [2022-06725, 2018-05973]; [MR/V025724/1]

Despite the additional experimental difficulty required for high pressure measurements, the ability to tune materials with applied pressure opens up a remarkable phase space of novel structures, stoichiometries and exploration through the rich phase diagrams of condensed matter systems. Exploring the interplay of these ordered phases is vital in understanding the underlying superconductivity present in many of these such materials. For the field of high temperature superconductivity, the hydride family of superconductors have the highest experimentally verified critical temperatures, but are only currently stable and accessible with experiments in the megabar pressure range. As the current best candidates for room-temperature superconductivity, research is vital in characterising these hydride compounds at extreme conditions. This thesis covers a variety of compounds and experimental techniques, but grouped under the topic of superconductors under pressure. In particular, these measurements focus on characterising the pairing gap, a key parameter in understanding the nature of superconductivity in all superconductors. Firstly, hydrostatic pressure up to 8 GPa has been used to suppress the charge-density wave phase in the transition metal dichalchogenide NbSe2, where measurements of the magnetic susceptibility track the superconducting critical temperature with applied pressure. Here, the onset temperature of the charge density wave (CDW) and superconducting phases (SC) show a clear anticorrelation, revealing a competition for electronic density of states between these two phases. Magnetisation measurements also show a sharp increase in the lower critical field with applied pressure that cannot be solely explained by a SC-CDW competition, but instead indicates unconventional superconducting behaviour in NbSe2. Fits to the temperature-dependent superfluid density are used to measure a superconducting gap magnitude as a function of press

Cadmium arsenide Cd3As2 hosts massless Dirac electrons in its ambient-conditions tetragonal phase. We report X-ray diffraction and electrical resistivity measurements of Cd3As2 upon cycling pressure beyond the critical pressure of the tetragonal phase and back to ambient conditions. We find that at room temperature the transition between the low- and high-pressure phases results in large microstrain and reduced crystallite size both on rising and falling pressure. This leads to non-reversible electronic properties including self-doping associated with defects and a reduction of the electron mobility by an order of magnitude due to increased scattering. Our study indicates that the structural transformation is sluggish and shows a sizable hysteresis of over 1 GPa. Therefore, we conclude that the transition is first-order reconstructive, with chemical bonds being broken and rearranged in the high-pressure phase. Using the diffraction measurements we demonstrate that annealing at ~200 deg. C greatly improves the crystallinity of the high-pressure phase. We show that its Bragg peaks can be indexed as a primitive orthorhombic lattice with a_HP = 8.68 Å, b_HP = 17.15 Å and c_HP = 18.58 Å. The diffraction study indicates that during the structural transformation a new phase with another primitive orthorhombic structure may be also stabilized by deviatoric stress, providing an additional venue for tuning the unconventional electronic states in Cd3As2.