Elastic Constants, Optical Phonons, and Molecular Relaxations in the High Temperature Plastic Phase of the CHNHPbBr Hybrid Perovskite.

Low frequency dynamics has been studied in a CHNHPbBr 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 C 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 ≈ 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. The critical dynamics at T ≈ 230 K is compatible with an order-disorder character, dominated by relaxational motions of the molecules.