  Organolead halide perovskites are a family of hybrid organic-inorganic
compounds whose remarkable optoelectronic properties have been under intensive
scrutiny in recent years. Here we use inelastic X-ray scattering to study
low-energy lattice excitations in single crystals of methylammonium lead iodide
and bromide perovskites. Our findings confirm the displacive nature of the
cubic-to-tetragonal phase transition, which is further shown, using neutron and
x-ray diffraction, to be close to a tricritical point. Lastly, we detect
quasistatic symmetry-breaking nanodomains persisting well into the
high-temperature cubic phase, possibly stabilized by local defects. These
findings reveal key structural properties of these materials, and also bear
important implications for carrier dynamics across an extended temperature
range relevant for photovoltaic applications.
