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Local structure analysis of solid state ionic conductors, perovskite-derived structures by NMR and computational studies


Type

Thesis

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Authors

Dervisoglu, Riza 

Abstract

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 180C. 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.

Description

The full text of this thesis is embargoed until end November 2015 for publication reasons.

Supporting data is held on a separate record at https://www.repository.cam.ac.uk/handle/1810/244979

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Qualification

Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge