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Title: Atomic structures and properties of oxide interfaces
Author: Zhu, Bonan
ISNI:       0000 0004 9354 1315
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2020
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This thesis uses computational approaches, mainly first-principles methods, to study interfaces in oxide thin films. One of the difficulties in interface studies is the lack of definitive atomistic models, yet they are essential input for any calculations. Here, this problem is tackled by ab initio random structure searching (AIRSS), or more broadly speaking, random structure searching (RSS). The initial work studies the interfaces in vertically aligned nanocomposites (VANs) that consist of CeO2 pillars embedded in a SrTiO3 matrix. Enhanced ionic conductivity has been found in these VANs in prior studies, but the role of vertical interfaces is not explained. The initial interface searches are performed with interatomic potentials due to the large size of the interface, followed by refinement first-principles calculations. Based on the obtained structures, it is shown that the majority interfaces are unlikely to directly enhance ionic conductivity. However, a parallel solid-state O17 NMR study by our collaborators later obtained interface signals that suggest fast ionic conduction. First-principles NMR calculations show the observed signals are not consistent with the majority interface initially studied; instead, they can be assigned to the minority interfaces that are in different orientations. The following work studies the planar interfaces between epitaxial films of CeO2 and STO substrates. A significant amount of research has been devoted to fluorite-perovskite interfaces since the controversial report of colossal ionic conductivity enhancement in YSZ/STO heterostructures. However, the exact atomic structures of these interfaces are not well understood. AIRSS is used for finding stable CeO2/STO planar interfaces taking account of different terminations and local stoichiometries. When the STO terminates with a TiO2 layer, a rock salt structured CeO layer emerges at the interface. On the other hand, with SrO termination, the stable structure contains a partially occupied anion lattice, which gives rise to lateral diffusion of oxygen anions in molecular dynamics simulations. In both cases, the interfaces are found to attract oxygen vacancies, which hinders ionic transport in the perpendicular direction. The subsequent work starts with addressing the perovskite-perovskite interfaces between La0.9Ba0.1MnO3 (LBMO) and STO. LBMO is a ferromagnetic insulator with a relatively high ferromagnetic transition temperature, which makes it an ideal material for spintronics applications. However, thin films of LBMO are conductive except when the thickness is less than eight unit cells. This has been attributed to the octahedral proximity effects, as electron microscopy reveals that octahedral tilting in LBMO is suppressed near the interfaces. Whist some experimental observations are successfully accounted for by the first-principles calculations, the predicted tilt angle suppression is much weaker than that observed. By studying the response of octahedral networks to corner perturbations, it is shown that a competing LBMO phase with an alternative tilt configuration is stable as a result of interface coupling.
Supervisor: Driscoll, Judith Sponsor: Cambridge Commonwealth ; European and International Trust ; China Scholarship Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: materials science ; oxides ; first-principles ; density functional calculation ; interface ; thin film ; ionic conduction ; crystal structure prediction