A Study of the Relationship Between Correlated Displacive Disorder, Metastability and Lattice Dynamics using the Epitaxially Stabilised Alloy System γ^s-(U_1−xMo_x)

Abstract

Understanding the role of disorder and the correlations that exist within it, is one of the defining challenges in contemporary materials science. However, there are few material systems, devoid of other complex interactions, which can be used to systematically study the effects of crystallographic conflict on correlated disorder. In this way, the metastable alloy system γ^s-(U_1−xMo_x) represents a rare opportunity. The system is both a simple binary alloy and exhibits the required crystallographic conflict, specifically between the preferred local symmetry of uranium and the higher, bcc symmetry, of the global lattice.

Firstly, this thesis demonstrates the effectiveness of epitaxial matching as an improvement over rapid cooling techniques, allowing high quality, single crystal thin films of γ^s-(U_1−xMo_x)  to be synthesised across a large molybdenum range. Subsequently, extensive diffuse x-ray scattering studies show a novel form of correlated displacive disorder arises as a natural resolution to the crystallographic conflict, significantly lowering local symmetry while preserving the average bcc structure. Furthermore, both alloy content and heavy ion irradiation are shown to be effective, intrinsic and extrinsic, tuning parameters, through which the correlated disorder strength may be controlled. Finally, by combining grazing incidence inelastic x-ray scattering with state-of-the-art ab initio molecular dynamics simulations strong disorder-phonon coupling is discovered. This breaks global symmetry and dramatically suppresses phonon-lifetimes compared to alloying alone, providing an additional design strategy for phonon engineering.

In conclusion, having demonstrated epitaxial matching techniques can be leveraged to synthesise high quality, single crystal thin films of metastable γ^s-(U_1−xMo_x) over a large range in molybdenum content. This thesis then explores the deep links between local crystallographic conflict, metastability, order-disorder relations and the coupling between correlated disorder and other periodic phenomena, specifically lattice dynamics. These findings have implications wherever crystallographic conflict can be accommodated and, among other things, may be exploited in the development of future functional materials.

Date of Award	24 Jun 2021
Original language	English
Awarding Institution	The University of Bristol
Supervisor	Ross S Springell (Supervisor) & Chris Bell (Supervisor)