Symmetry Methods for Understanding Structures of Inorganic Functional Materials

Abstract

The aim of this thesis is to develop and apply new techniques for structural characterisation of inorganic functional materials undergoing a phase transition. We develop robust, reliable and fast techniques that are demonstrated for existing known structures and used for new structure determinations. We hope they will be used well into the future as new structure characterisation needs arise.
Chapter 1 explains the concepts of symmetry and phase transitions in crystalline materials. It introduces experimental characterisation methods and the challenges for robust structure determination from polycrystalline samples.
Chapter 2 develops new structure determination techniques including Genetic Algorithms, exhaustive searches of subgroup trees and inclusion and exclusion type searches. WO3 is used as a test example to illustrate the techniques and give detailed analysis of the advantages and disadvantages of the different methodologies.
Chapter 3 reports new structure determinations for α- and β-Bi2Sn2O7. The β-Bi2Sn2O7 structure is the first structural model that accounts fully for high resolution diffraction data while the α-Bi2Sn2O7 model is considerably simpler than previously thought. Both structures are robustly determined using our new exhaustive search algorithms.
Chapter 4 discusses application of our algorithms to the known but highly complex ZrP2O7 structure. The structure is solved with our methods using displacive symmetry-adapted distortion modes. The new method of applying rotational distortion modes to rigid bodies allows us to simplify the structural description. An extremely complex search is undertaken which identifies the important rotational distortions.
Chapter 5 describes the development of new routines to model the effect of stacking faults in layered materials on powder diffraction data, allowing unprecedented model complexity and speed in Rietveld refinement. The new stacking routines are applied to La2O2Cu0.667Cd0.667Se2 and La2O2Cu0.667Mn0.667Se2 layered oxychalcogenides, allowing the observed powder diffraction data to be fitted to extract both structural and microstructural information. The chapter also tests our new search algorithms on another oxychalcogenide,
Abstract
2
(Ce0.78La0.22)2O2MnSe2 with a known low temperature magnetic structure. The important magnetic modes are extracted using our search algorithms allowing the magnetic structure to be determined. The techniques are then applied to the previously unknown magnetic structure of Ce2O2Fe0.75Zn0.25Se2 in order to extract important magnetic modes and try to determine the magnetic ordering in the low temperature phase.
Chapter 6 summarises the work in this thesis and explains how the new techniques may continue to be used in the future.