Much recent attention has been devoted towards unraveling the microscopic optoelectronic properties of hybrid organic-inorganic perovskites. Here we investigate by coherent inelastic neutron scattering spectroscopy and Brillouin light scattering, low frequency acoustic phonons in four different hybrid perovskite single crystals: ${\mathrm{MAPbBr}}_3$, ${\mathrm{FAPbBr}}_3$, ${\mathrm{MAPbI}}_3$, and $\alpha \text{−}{\mathrm{FAPbI}}_3$ (MA: methylammonium, FA: formamidinium). We report a complete set of elastic constants characterized by a very soft shear modulus $C_{44}$. Further, a tendency towards an incipient ferroelastic transition is observed in ${\mathrm{FAPbBr}}_3$. We observe a systematic lower sound group velocity in the technologically important iodide-based compounds compared to the bromide-based ones. The findings suggest that low thermal conductivity and hot phonon bottleneck phenomena are expected to be enhanced by low elastic stiffness, particularly in the case of the ultrasoft $\alpha \text{−}{\mathrm{FAPbI}}_3$.

Low frequency dynamics has been studied in a CH₃NH₃PbBr₃ hybrid perovskite single crystal by using four different spectroscopy techniques: coherent inelastic neutron, Raman and Brillouin scatterings, and ultrasound measurements. Sound velocities were measured over five decades in energy to yield the complete set of elastic constants in a hybrid halide perovskite crystal in the pseudocubic plastic phase. The C₄₄ shear elastic constant is very small, leading to a particularly low resistance to shear stress. Brillouin scattering has been used to study the relaxation dynamics of methylammonium cations and to evidence translation–rotation coupling associated with the cubic to tetragonal phase transition at T_c ≈ 230 K. Low frequency and highly damped optical phonons observed using both Raman and inelastic neutron below 18 meV, do not present softening close to T_c. The critical dynamics at T_c ≈ 230 K is compatible with an order–disorder character, dominated by relaxational motions of the molecules.