Résumé
The low-frequency lattice vibrations and relaxations are investigated in single crystals of the four 3D hybrid organolead perovskites, MAPbBr3, FAPbBr3, MAPbI3, and α-FAPbI3, using Brillouin, Raman, and inelastic neutron scattering [1].The temperature dependence of the PbX6 lattice modes in the four compounds can be renormalized into universal curves, highlighting a common vibrational dynamics at the cubic to tetragonal transition. In particular, no soft vibration is observed in Raman and neutron scattering excluding a displacive-like transitional dynamics.The reorientational motions of the molecular cations exhibit a seemingly order-disorder character recalling that of plastic crystals, but they do not necessarily drive the phase transitions.At ultralow frequency, a quasielastic component evidenced by Brillouin scattering and associated to the unresolved central peak observed in neutron scattering, is attributed to translational-like modes of the molecular cations in the perovskite cavities. Its partially unexpressed critical behavior at the transition might point toward the general importance of defects in HOPs preventing the net divergence of order parameter correlations at the critical temperatures.