Abstract
Organolead halide perovskites are a family of hybrid organic-inorganic compounds whose remark-
able optoelectronic properties have been under intensive scrutiny in recent years. Here we use inelas-
tic X-ray scattering to study low-energy lattice excitations in single crystals of methylammonium
lead iodide and bromide perovskites. Our ndings con rm the displacive nature of the cubic-to-
tetragonal phase transition, which is further shown, using neutron and x-ray di raction, to be close
to a tricritical point. The experimental sound speed, around 100-200 m/s, suggests that electron-
phonon scattering is likely a limiting factor for further improvements in carrier mobility. Lastly, we
detect quasistatic symmetry-breaking nanodomains persisting well into the high-temperature cubic
phase, possibly stabilized by local defects. These ndings reveal key structural properties of these
materials, but also bear important implications for carrier dynamics across an extended temperature
range relevant for photovoltaic applications.