Seminar Abstract:

Crystalline material bulk properties are directly related to the disorder inherent in all crystals. Often
thought to be purely random, disorder tends to follow strict rules based on short-range environments.
Traditional crystallographic methods probe long range order and so fail to describe the short-range
correlations in disordered materials that often contribute observed phenomena. However, recent advances in the detection and refinement of scattering analysis techniques allow for the long theorized
correlations in short range order to be probed more directly. Ergo, attempts to control hidden order to tune material properties accordingly are prudent. To this end, Nikolaj Roth and Andrew L.
Goodwin examine hidden order in crystals in their Nature Communications paper “Tuning electronic
and phononic states with hidden order in disordered crystals.” In this work, computational models
of perfectly ordered two- and three-dimensional A2B and A3B crystals, where A is hydrogen and B
is sulfur, are compared to crystal models with identical long-range order but differing short-range
order induced by distortions in the lattice created by varying local bonding environments. Compared
to the fully ordered case, the disordered cases exhibit an expected band gap widening. In surprising
contrast, band delocalization matches or exceeds that of the fully ordered model when square- and
cube-ice bonding rules are applied. These findings are posited to influence the behavior of several
important classes of material, including thermoelectrics, topological insulators, and photovoltaics.
Furthermore, simulated X-ray diffraction enforces the assertion that the implemented bonding rules
do not alter the long range order; Instead, short-range order produces bond rule dependent diffuse
scattering patterns. The authors suggest that diffuse scattering can now provide leverage for crystallographers to describe the correlated disorder present in many crystalline materials where traditional
crystallography fails. In concert with their suggestion and historical precedent, recent and ongoing
work attempts to meaningfully tie physical phenomena in disordered crystals to patterns in X-ray
diffuse scattering.