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Title: Local structure analysis of solid state ionic conductors, perovskite-derived structures by NMR and computational studies
Author: Dervisoglu, Riza
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2013
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In this work, local environments of ions in solid oxide fuel cell (SOFC) electrolyte materials with perovskite and perovskite-derived crystallographic structures, i.e. Ba₂In₂O₅, Ba₂(In₁₋ₓGaₓ)₂O₅ and Ba₂In₂O₄(OH)₂, were investigated for their high ionic (O²⁻ and H⁺) mobility at elevated temperatures. Two general methods were employed in this investigation; first, computational methods, such as density functional theory (DFT), gauge including projector augmented wave (GIPAW), cluster expansion (CE) and Monte Carlo simulations (MC); second, experimental methods, such as nuclear magnetic resonance (NMR), X-ray scattering (both powder diffraction and pair distribution function (PDF) analysis) and thermo-gravimetric analysis (TGA). The parent material, Ba₂In₂O₅, has inherent oxygen vacancies which allow for fast O₂₋ ion mobility at elevated temperatures and for hydration of the material needless of doping. We improve a previous NMR study of Ba₂In₂O₅ by Adler et al. [1], assigning all three oxygen crystallographic sites to their relevant NMR peaks and investigate the high temperature structure. We then study the iso-valent doping of Ga into the In site resulting in Ba₂(In₁₋ₓGaₓ)₂O₅. While Yao et al. [2] find that Ga doping levels higher than 20% form a stable cubic structure, our findings indicate that Ga doping results in a phase segregation. However our findings for quenched samples are no different than those of Yao et al. [2]. Lastly we study the hydrated form of the parent material, Ba₂In₂O₄(OH)₂, which has high H⁺ ion mobility above 180°C. We observe at least three possible hydrogen sites with local environments slightly different from the previous neutron diffraction study by Jayaraman et al. [3]. In contrast to the observation by Jayaraman et al. [3] of the hydrogen presence in all O2 layers we find an alternating occupancy of hydrogens in those layers.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral