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Title: High pressure studies of hydrogen-chalcogen systems
Author: Pace, Edward John
ISNI:       0000 0004 7654 7459
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2018
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Binary element-hydride systems have become a pertinent topic for high pressure research, following the measurement of record high temperature superconductivity in the dense hydrogen-sulfur system. The experimental study followed predictions of superconductivity with high transition temperature (Tc) in (H2S)2H2 at high pressures, leading to the current consensus that the high Tc phase is H3S, produced from the decomposition and recombination of H2S at high pressures. However, conjecture over the behaviour of hydrogen sulfide upon compression, and experimental limitations, cast significant ambiguity over interpretations of the structure and mechanism of the superconducting phase. Nonetheless, theory also predicts high Tc superconductivity in the dense hydrogen selenide and telenide systems; both experimentally uncharted at high pressures prior to this study. This thesis explores and maps the phase diagrams of hydrogen-chalcogen (S, Se, Te) systems using a combination of high pressure Raman spectroscopy and x-ray diffraction techniques. Gaining a comprehensive understanding of the behaviour of these systems under pressure is crucial to the eventual elucidation of the true nature of high Tc superconductivity. Hydrogen sulfide (H2S) and hydrogen selenide (H2Se) are appreciably toxic. A simple in situ synthesis technique is reported for producing hydrogen-chalcogenides directly from their constituent elements within diamond anvil cells, circumventing the need to condense toxic gases. This technique is also utilised to provide excess hydrogen, in order to produce the hydrogen-rich cocrystals thought to be vital to the formation of the high Tc phase. The hydrogen-sulfur system is most thoroughly investigated, and first presented. High quality Raman spectroscopic data provides an experimental review of pure H2S. Studies of (H2S)2H2 evaluate the current known ambient temperature phases and reveal three novel low temperature phases. Phase II0 is identified on cooling of phase I to 173 K (10 GPa), via splitting of both the single S-H stretching mode and low-frequency H2 vibron; sharp stretching modes indicate a significant reduction in orientational disorder. Successive splitting of the low-frequency H2 vibrons indicates two additional phase changes at 29 GPa (phase III0) and 53 GPa (phase- IV0) respectively, at 80 K. Phase IV0 is associated with an overall increase in symmetry. Evidence is also presented for a tentative fourth novel low temperature phase at ~160 GPa (20 K) and for the formation of an exceptionally stable hydrogen-sulfur compound with potentially novel stoichiometry. The behaviour of the H2S and (H2S)2H2 mixed molecular system is also reported; demonstrating that the coexistence of (H2S)2H2 and H2S can influence the hydrogen-bonding within both systems at high pressures. The first high pressure studies of the hydrogen-selenium system at ambient temperature are reported. The high pressure phase sequence of H2Se (I { I0 - IV) is identified by Raman spectroscopy, mirroring that of H2S. The isothermal boundaries for phases I0 and IV are found at 7 and 12 GPa respectively, at 300 K. Phase IV may have higher symmetry than phase IV H2S. X-ray diffraction and Raman spectroscopy demonstrate that the H2Se:H2 mixtures form cocrystals of (H2Se)2H2 from 4.2 GPa, with tetragonal space group I4=mcm, analogous to (H2S)2H2. Both H2Se and (H2Se)2H2 are shown to decompose into their constituent elements above 24 GPa. Attempts to synthesise the elusive H2Te directly from hydrogen and tellurium are reported. No reaction occurs upon heating Te in H2 at 0.2 GPa to 573 K. No visible reaction occurs between H2 and the high-pressure phases of Te, upon laser-heating. No photoreaction occurs upon exposure of tellurium in hydrogen to intense laser light (532 nm) at 0.2 GPa and 300 K, but formation may be stabilised at lower temperatures.
Supervisor: Attfield, John ; Gregoryanz, Eugene Sponsor: Engineering and Physical Sciences Research Council (EPSRC)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: high pressure ; chalcogen