Phase behavior of correlated disorder states

Disorder does not necessarily stipulate randomness as solid materials can exhibit short-range order and long-range disorder. Such phases are described as having disorder correlations and are becoming increasingly important. In fact these phases may even be more common than crystalline phases in simple systems such as the dense packings of simple polyhedra. Disorder plays a key role in the chemistry of functional materials. Studying disorder, however, remains challenging from theoretical, computational, and experimental perspectives as structural complexity increases when structural periodicity is lost. As such, there is much to be gained by establishing a greater understanding of disorder, not only by studying its experimental signatures as a mechanism for characterising disordered states, but also potentially controlling physical properties by manipulating the correlations within disordered states. For now, the language of crystallography is not designed for talking about even the simplest of systems containing disorder (e.g. water ice) as configurations with correlated disorder have an absence of translational periodicity and therefore no space group or unit cell can be used to properly describe them. Ice is not the only exception, and any have noted the inadequacy of classical crystallography to describe important families of materials containing disorder. The interplay between strongly correlated structural disorder and bulk properties (e.g.
polarisation, excitations, specific heat) is of particular interest in this project. In general, condensed matter chemistry has paid little attention to disorder as conventional diffraction techniques have been developed to determine the average structure of the sample. Consequently, correlated disorder is hard to identify as it requires characterisation of the local structure, which is not obtainable from conventional crystallographic refinements. Our aim is to explore the link between correlated disorder and bulk properties of materials by creating generalised computational models and analysing their physical behaviour over phase transitions. By analysing the nature of bulk properties, such as heat capacity, experimental signatures of "hidden order" transitions where the average structure remains
unchanged can be explored. This project also aims to study the link between correlated disorder and the emergence of polarization in proper ferroelectrics to try determine if local ordering mechanisms may be combined to break inversion symmetry in a manner conceptually similar to that operating in the hybrid improper ferroelectrics. Finally, we aim to explore the extent to which one dimensional statistical mechanical models can map onto the structural behavior of simple bulk phases which contain correlated disorder. This could result in being able to use less experimentally demanding techniques to probe information, such as structure and behaviour, on more complex systems. This project falls within the EPSRC Physical Sciences research area.