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Experimental and Computational Studies of Oxide Ion Conductors

SCHWAIGHOFER, BETTINA (2023) Experimental and Computational Studies of Oxide Ion Conductors. Doctoral thesis, Durham University.

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Abstract

The work in this thesis focuses on the study of oxide ion dynamics with the aim to develop improved oxide ion conductors. As the main techniques used to achieve this were ab initio molecular dynamics (AIMD) and quasielastic neutron scattering (QENS), this combined approach is also the focus of the literature review in Chapter 1.

Chapter 2 introduces the methods used for synthesis, characterisation, and further study of the materials studied.

Chapter 3 investigates the effect of the dopant on the oxide ion dynamics in two doped $\delta$ -Bi $_2$ O $_3$ oxide ion conductors: Bi $_{0.852}$ V $_{0.148}$ O $_{1.648}$ and Bi $_{0.852}$ P $_{0.148}$ O $_{1.648}$ . QENS allowed observation of nanosecond dynamics, corresponding to the diffusion of the oxide ions in the Bi-O sublattice via vacancy-hopping, and picosecond dynamics, corresponding to localised motion within the dopant sublattices. AIMD gave further insight into the different oxide ion dynamics in Bi $_{0.852}$ V $_{0.148}$ O $_{1.648}$ and Bi $_{0.852}$ P $_{0.148}$ O $_{1.648}$ , showing that the flexibility of the V coordination environment plays an important role, creating additional vacancies in the Bi-O sublattice, consistent with the superior conductivity of the vanadate.
Chapter 4 describes the systematic study of conductivity of the complex scheelite-type materials: Bi $_{3}$ (BO $_{4}$ )(B'O $_{4}$ ) $_{2}$ (B = Fe, Ga, Fe $_{0.9}$ Ti $_{0.1}$ ; B' = Mo) as well as Bi $_3$ (B $_{2}$ O $_{8}$ ) $_{1/2}$ (B'O $_{4}$ ) $_{2}$ (B = Sc, In; B' = Mo). Impedance measurements indicate that interstitial oxide ions are responsible for conductivity in these materials, and the conductivity of Bi $_{3}$ (Fe $_{0.9}$ Ti $_{0.1}$ O $_{4.05}$ )(MoO $_{4}$ ) $_{2}$ was found to be 1.5 $\times$ 10 $^{-3}$ S cm $^{-1}$ at 800 $^\circ$ C, which is comparable to the scheelite-type oxide ion conductor LaNb $_{0.92}$ W $_{0.08}$ O $_{4.04}$

Chapter 5 discusses the study of two hexagonal perovskites: Ba $_3$ NbMoO $_{8.5}$ and Ba $_7$ Nb $_4$ MoO $_{20}$ . Using variable temperature powder X-ray diffraction, the reversibility of the phase transition in Ba $_3$ NbMoO $_{8.5}$ was demonstrated for the first time. QENS showed that oxide ion dynamics in both compounds are too slow to be observed on a nanosecond timescale. In Ba $_7$ Nb $_4$ MoO $_{20}$ , AIMD revealed a continuous oxide ion migration pathway in the $ab$ plane, and moreover showed an important out-of-plane contribution to the long-range diffusion. This allowed suggestion of a new doping strategy to further enhance oxide ion conductivity.

Chapter 6 discusses results obtained from the first AIMD simulations on a Dion-Jacobson phase oxide ion conductor, CsBi $_{2}$ Ti $_{2}$ NbO $_{10-\delta}$ , revealing an important contribution of the O2 site to the long-range diffusion. This suggests that oxide ion migration occurs predominantly via an O1-O2-O1 pathway, demonstrating the importance of rotationally flexible octahedra for high ionic conductivity in this new family of oxide ion conductors.

Item Type:	Thesis (Doctoral)
Award:	Doctor of Philosophy
Keywords:	Oxide-ion conductor, QENS, AIMD, neutron scattering, molecular dynamics
Faculty and Department:	Faculty of Science > Chemistry, Department of
Thesis Date:	2023
Copyright:	Copyright of this thesis is held by the author
Deposited On:	05 Jun 2023 12:01

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Experimental and Computational Studies of Oxide Ion Conductors

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