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Résumé:

The low frequency lattice vibrations and relaxations are investigated in single crystals of the four 3Dhybrid organolead perovskites, MAPbBr3, FAPbBr3, MAPbI3, and FAPbI3, at the Brillouin zone centerusing Raman and Brillouin scattering and at the zone boundary using inelastic neutron scattering. Thetemperature dependence of the PbX6 lattice modes in the four compounds can be renormalized intouniversal curves and no soft vibration is observed excluding a displacive-like transitional dynamics. Thereorientational (pseudospin) motions of the molecular cations exhibit a seemingly order-disordercharacter recalling that of plastic crystals, but attributed to a secondary order-parameter. At ultra-lowfrequency, a quasi-elastic component evidenced by Brillouin scattering and associated to the unresolvedcentral peak observed in neutron scattering, is attributed to center of mass anharmonic motions andrattling of the molecular cations in the perovskite cavities.