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Probing the local and average structure of functional materials
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Herlihy, Anna (2022) Probing the local and average structure of functional materials. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b3860966
Abstract
Studying the average structure of functional materials as a function of temperature or chemical substitution can provide insight into structure-property relationships. Pressure provides another external control parameter, now routinely used in conjunction with diffraction. Local symmetry-breaking, or order-disorder type behaviours sometimes must be considered, for example, using total scattering, however, this is challenging for high-pressure measurements. This Thesis adds insight into structure-property relationships using this conventional paradigm and develops high pressure total scattering techniques, demonstrating applicability to ferroelectric phase transitions in BaTiO3.
Variable-pressure neutron total scattering is complicated by contaminant scattering from pressure-transmitting media. This Thesis details an approach to account for the presence of a methanol:ethanol medium in PDFs, measured on PEARL (ISIS). Proof-of-principle measurements are demonstrated with Ni and MgO, in addition to the first neutron local structure measurements and analysis of α-quartz under hydrostatic pressure.
The local structure of ferroelectric BaTiO3 under hydrostatic pressure has been interrogated using neutron total scattering, across its tetragonal to cubic transition. Small-box modelling, using local rhombohedral, tetragonal, and cubic distortions, shows that rhombohedral-type distortions become suppressed with pressure. The transition in high-temperature BaTiO3 is best described with an order—disorder model, whereas the high-pressure results indicate that the structure is better described by the harmonic approximation.
The average and local structures of the superconductor La1.875Ba0.125CuO4 have been investigated using temperature, pressure, and chemical substitution of JT-inactive Mg2+ for JT-active Cu2+ cations. The anomalous suppression of superconductivity coincides with a transition to a low-temperature tetragonal phase. Increased Mg2+ concentration is found to stabilise this phase, but prevents its coexistence with a superconductivity-hindering low-temperature orthorhombic phase, observed in the parent LBCO compound. X-ray total scattering indicates dynamic rotation of the Cu/MgO6 octahedra at 400 K, and average structure measurements show a transition to a ‘high-temperature’ tetragonal phase occurs almost immediately with applied pressure.
Item Type: | Thesis (PhD) | ||||
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Subjects: | Q Science > QC Physics Q Science > QD Chemistry T Technology > TA Engineering (General). Civil engineering (General) |
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Library of Congress Subject Headings (LCSH): | Materials -- Structure, Crystals -- Structure, Ferroelectricity, Barium compounds -- Structure-activity relationships, Materials at high pressures, High pressure crystallography, Neutrons -- Scattering, Materials science | ||||
Official Date: | March 2022 | ||||
Dates: |
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Institution: | University of Warwick | ||||
Theses Department: | Department of Chemistry | ||||
Thesis Type: | PhD | ||||
Publication Status: | Unpublished | ||||
Supervisor(s)/Advisor: | Senn, Mark S. ; Funnell, Nick | ||||
Format of File: | |||||
Extent: | xxxi, 226 pages : illustrations | ||||
Language: | eng |
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