Atomic vibrations in crystals ("phonons") affect many physical properties: e.g. phonons are an important part of thermal transport. At the same time, phonons are interesting in their own right: e.g. anharmonic crystals can host novel structural dynamics. Understanding phonons is critical to understanding many phenomena. Probably the most direct and powerful method to study phonons is neutron scattering combined with modelling. In this talk, I will briefly summarize how atomic vibrations can be studied with neutrons+modelling before discussing two recent projects that these tools made possible:
(i) In the hybrid perovskite CH3NH3PbI3, we discovered local, two-dimensional fluctuations of the lattice within the "cubic" phase. Neutron and X-ray diffuse scattering revealed non-trivial features and comparison to classical molecular dynamics simulations identified the real-space fluctuations as ``pancakes" formed from correlated rotations of the PbI_6 octahedra.
(ii) In the thermoelectric clathrate Ba8Ga16Ge30, we combined inelastic neutron scattering with the results of ab-initio calculations to understand the role of occupational disorder in the dispersions. We found that "hidden" anti-crossings may be present, decreasing group velocities and reducing the thermal conductivity by ~30%. I will conclude by discussing some ongoing projects where both neutron scattering and modelling are playing a significant role.