Shenton, John Kane; (2018) First-principles investigations of structure-function relationships in bismuth ferrite. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The simplicity of the basic perovskite structure belies a seemingly boundless potential for novel phenomena and technological applications. The ferroelectric perovskites, in particular, can have complex and subtle relationships between their crystal structures and functional properties. Understanding such structure-function relationships is a central theme of this work. In this work, advanced computational approaches are used to gain insight into key structure–function relationships in the multiferroic perovskite, bismuth ferrite (BFO). Density functional theory (DFT) has proven to be an immensely valuable tool in the study of condensed matter. However, techniques beyond standard DFT are often required to deal with the strongly localised electronic states in materials like BFO. In the first part of this thesis, we conduct a systematic study of a commonly used correction to DFT: DFT+U. We focus on the effect of the U parameter on the conduction band minimum (CBM) and valence band maximum (VBM). We find drastic changes to the location and curvature of the CBM in particular. Specifically, we find a surprising inversion in the ordering of the Fe t2g and eg manifolds at the CBM when U exceeds 4 eV. We therefore suggest caution when employing large values of U to calculate optoelectronic properties. In the second part of this work, motivated by the prospects of BFO-based photovoltaics, we investigate the influence of crystal structure on charge carrier effective masses. We begin by comparing the effective masses of several known phases of BFO, finding orders of magnitude differences between them. The strain-induced tetragonal phase emerged from this comparison as having the promising combination of a large spontaneous polarisation and relatively light charge carriers. Through a systematic decomposition of geometric relationships between different phases of BFO, we identify key physical influences on effective masses. We suggest that these insights could be exploited to improve photovoltaic efficiency. In the final part of this thesis, we enter the realm of designer materials. The creation and characterisation of atomically sharp interfaces between different complex oxides has proven to be an exciting and fruitful area of research in recent years. We investigate superlattices made up of repeating lanthanum aluminate (LAO) and BFO layers. By simply varying the thickness of the BFO layer, we found that one could tune the tetragonality, spontaneous polarisation and band gap of the superlattices. We also predict the formation of two-dimensional (2D) electron and hole gases at opposing interfaces above a critical thickness of BFO. The ferroelectric origin of the 2D gases, together with an emergent magnetism, suggests that this system may be a promising source of novel multiferroic functionality. In particular, we suggest the possibility of switching the 2D electron and hole gases via an external electric field.

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