DISORDER, WHAT IS IT GOOD FOR?

Is disorder necessarily a bad thing?

Ask most crystallographers, and the answer is probably a resounding ‘yes’. Order means Bragg peaks; Bragg peaks mean structure solutions; structure solutions mean understanding. Disorder, on the other hand, means diffuse scattering. Pretty pictures, perhaps, but the journey from these to an understanding is a great deal trickier (if possible at all).

But difficult shouldn’t mean useless. Instead, we ask the awkward question whether disorder might actually be a way into properties that simply aren’t normally possible?

Our approach has been to study the systems that contain elements of both perfect order and random disorder. The idea is that this should allow us to retain some of the periodicity that helps us understand and control structure, while also incorporating the diversity and anomalous behaviour associated with disorder. In precisely these cases at the borderline of order and disorder, diffraction patterns contain highly structured diffuse scattering patterns. These patterns act like a fingerprint for the particular type of disorder present, helping us work out relationships between disordered systems that recur across apparently disparate fields of science. Who knew, for example, that the electronic states in cuprate superconductors resemble domino packings? Or that rhombuses arrange themselves in the same semi-random way as some organic acids?

Characterising disorder is one thing, but making it useful is something else. What we really want to know is whether disorder can do something new and exciting that isn’t possible in ordered systems? The short answer is yes! In fact we show that disorder provides a new way to control the extent to which a material vibrates when heated. And, it turns out, this is exactly what is needed to make solids that can convert waste heat into electricity.

Design of crystal-like aperiodic solids with selective disorder-phonon coupling
A R Overy, A B Cairns, M J Cliffe, A Simonov, M G Tucker, and A L Goodwin
Nature Communications 7, 10445 (2016)