All materials are disordered at finite temperature. Sometimes disorder is random. More frequently it’s not. We’re interested in cases where the deviations away from order, and away from randomness, are crucial for material function.
Order
We have to learn the rules to learn how to break them. While our conceptual focus is on disordered systems, we have an ongoing interest in order of a variety of different types.
For example, we study systems with unconventional or ‘hidden’ order, which involve crystallisation of collective degrees of freedom whilst supporting fluctuations of individual components. The multipolar order in gadolinium gallium garnet (GGG) is one such example.
Another interest is in understanding and exploiting the landscape of symmetry-breaking distortions accessible to hybrid or molecular frameworks. Here there is particular scope for combining different distortions to drive emergent order of another kind; hybrid improper ferroelectricity is a topical case.
And at the same time we maintain a synthetic programme aimed at developing the structural chemistry of some (ordered) framework materials — especially systems that can support unusual microscopic degrees of freedom.
Disorder
The heart of our research programme lies in the study of disordered materials. We are agnostic with respect to the type of material involved, and so we investigate a wide range of systems: from frustrated magnets to metal–organic frameworks and pharmaceuticals to supramolecular assemblies.
A particular focus is on the development and application of scattering-based methods for characterising disordered states. Historically we exploited pair distribution function (PDF) methods, and we even led the bid for the I15-1 PDF beamline at Diamond. We still use PDF, but increasingly focus on its single-crystal variants of diffuse-scattering and 3D-ΔPDF. We also develop approaches and software for interpreting PDF / diffuse scattering data.
One of our key current goals is to identify systems where there is clear scope for chemical control over the type of disorder present, and also a clear link between disorder and material function.
Flexibility
Flexible materials are systems that can adopt any one of a number of different states, amongst which they then switch in response to external stimuli — e.g. changes in temperature or pressure or through host/guest interactions.
Historically we were interested in ‘wine-rack’ systems, where flexibility drove counterintuitive mechanical phenomena such as negative thermal expansion or negative compressibility. (The Hoberman sphere shown here has a negative Poisson ratio, for example). But increasingly we have focussed on systems with more complex configurational landscapes, and in particular those with rigorous mappings to the statistical mechanics of specific correlated disordered states. The phenomenology of negative hydration expansion in ZrW2O8 is one recent example, and there are others in the pipeline. The interplay between geometry and collective behaviour is what threads this aspect of our work to that of disordered materials more generally.
Function
Disorder is important in a materials chemistry context only if it drives function. Here we are working on a number of different angles.
Many aspects of mass transport appear to be optimised in disordered states. This is of particular relevance to battery materials (e.g. the disordered rocksalts) but is key also for porous materials more generally.
Systems that support correlated disorder often exhibit order/disorder transitions under very small external perturbations that give rise to very large entropy changes . These are obvious candidates for exploitation in energy-efficient cooling technologies.
We are particularly interested in the interplay between correlated disorder and collective phenomena such as polarisation, or vibrational or electronic band structure. In this respect we’re looking into new routes to multiferroics, relaxors, and thermoelectrics.
And finally we’re exploring different ways of exploiting disorder in data storage and manipulation.