Understanding the Structure-Composition Relationship in Layered Perovskite-Related Materials

Abstract

Complex oxides, particularly ABO3 perovskite oxides, have garnered the interest of solid-state scientists because of their fascinating physical properties such as ferroelectricity, piezoelectricity, optical properties, and technological applications. The relationship between their structure, composition, and properties has been very well studied. This understanding allows for the precise tailoring of ABO3 perovskites to meet specific application requirements. The layered perovskite-related materials share many features with the three-dimensional perovskites but are more complex. This thesis focuses on such layered systems - Dion-Jacobson phases, Aurivillius phases and Ruddlesden-Popper phases to understand the relationship between structure and composition to help design property optimised materials.
Chapter one provides an introduction to perovskites and layered perovskite-related materials with particular emphasis on the mechanisms of ferroelectricity within these materials. In chapter two, the experimental and computational methodology used throughout the project are introduced.
The Dion-Jacobson (DJ) phases are known to adopt polar ground states stabilised by either proper or hybrid improper mechanisms. Chapter three investigates the structure-composition-property relationships in the n = 3 A’A2B3O10 (A’ = Rb, Cs; A = Ca, Sr, Ba; B = Nb, Ta) Dion-Jacobson phases, showing how both geometric as well as electronic factors play a role in determining the structures of these materials. This chapter also demonstrates the advantages of neutron powder diffraction and single-crystal diffraction over conventional X-ray diffraction. In addition to their ferroelectric behaviour, these n = 3 DJ phases have recently attracted attention for their optical properties and applications such as photocatalytic activity, X-ray detectors, solar cells, photo detectors. The study shows the dependence of bandgap on the composition of the material.
Chapter four uses both X-ray and neutron diffraction to explore the structure-composition relation in the n = 2 Bi2A0.5La0.5B2O9 (A = Na, K; B = Nb, Ta) Aurivillius phases. The study shows how tuning the composition can tune the balance between polar and anti-polar states. Density Functional Theory (DFT) calculations help to validate experimental findings of the Aurivillius phases and to study composition–structure relationship in the n = 2 Li2AB2O7 (A = Ca, Sr, Ba; B = Nb, Ta) Ruddlesden–Popper (RP) phases. The findings of this study offer valuable insights into how composition can be tuned to manipulate the polar behaviour in Aurivillius and Ruddlesden-Popper phases.
Chapter five offers insights into the fluorination of n = 2 Ruddlesden-Popper phases. Inserting fluoride ions into RP phases can help modify the tilting and twisting of octahedra. This provides an additional route to break inversion symmetry and stabilise the polar structures in n = 2 RP phases.
Chapter six is a preliminary study using X-ray diffraction on A-site ordered A0.5Nd0.33TaO3. It shows that the structure undergoes a change when the vacancies in the structure are occupied by A = Li+, Na+ and K+. It suggests that the difference between the size of the A-cations can influence the structure of perovskites. To complete the study and determine the structure of A0.5Nd0.33TaO3 (A = Li, Na, K) neutron powder diffraction, solid-state NMR, DFT calculations are required.
Chapter seven summarises the key conclusions of this thesis and outlines the future work.