Masina,Sikhumbuzo Mfanawemphi2024-10-292024-10-292023Masina,Sikhumbuzo Mfanawemphi. (2023). Fast oxide ion conductors for solid oxide fuel cells: average and local structure – property correlations in solid solutions of bi2o3 [PhD thesis, University of the Witwatersrand, Johannesburg]. WireDSpace.https://hdl.handle.net/10539/42076A thesis submitted to the Faculty of Science in fulfillment of the requirements for the degree of Doctor of Philosophy, Faculty of Science, School of Chemistry University of the Witwatersrand, Johannesburg, 2024In this thesis, substituted Bi2O3 systems were fabricated and characterized. W, Dy, Erand Nb were used as substituents in a goal to stabilise the highly conductive δ-Bi2O3like phases (hence forth referred to as the δ-phases) to ambient temperatures. Changes in both the average and local structures of the substituted Bi2O3 systems were correlated with the physical property conductivity. In the first part of the thesis, powder X-ray diffraction and Raman spectroscopy were used to show that WO3 on its own did not stabilise the δ-phase at ambient temperatures. The true equilibrium phase in the Bi2O3- WO3 system was a mixture of two tetragonal phases 7Bi2O3·2WO3 and 7Bi2O3·WO3. The co-doping strategy was used to fabricate the Bi2O3-Dy2O3-WO3 system (DWSB, where D =Dy, SB = stabilised Bi2O3). The δ-phase was stabilised with a minimum of 15 mol% total substituent concentration. Powder X-ray diffraction indicated that the δ- phases obtained in this system were metastable and degraded after isothermal annealing at ~ 500 °C for 100 hours. Addition of Er to the DWSB system to create the novel system Bi2O3-Dy2O3-Er2O3-WO3 (DEWSB, where E=Er) was found to significantly improve the stability of the δ-phase when annealed at virtually identical conditions as DWSB. The rest of the thesis is focused on the effect of each substituent cation on phase stability, local structure and the ageing phenomenon–the decrease in ionic conductivity upon isothermal annealing without any observable changes in average structure under powder X-ray diffraction. X-ray pair distribution function, X-ray absorption spectroscopy and photoluminescence were used to probe the local structure around the host Bi cations and some of the substituent cations (Dy, Er, W). The results indicated that some of the Bi cations are displaced away from the 4a site of the defect fluorite structure (Fm-3m) and that at the local level, the Bi cations assume an arrangement similar to that found in the monoclinic α-Bi2O3 phase. Dy and Er were also found to prefer local environments similar to those in their parent oxides. The resemblance increased as the material aged and might explain why the conductivity decreases upon ageingen© 2023 University of the Witwatersrand, Johannesburg. All rights reserved. The copyright in this work vests in the University of the Witwatersrand, Johannesburg. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of University of the Witwatersrand, Johannesburg.Solid Oxide Fuel CellsTotal ScatteringX-ray Absorption SpectroscopyUCTDSDG-17: Partnerships for the goalsThesisUniversity of the Witwatersrand, Johannesburg