Physics

Formamidinium

Laser

Crystallography

Optics

Perovskite (structure)

Halide

Inorganic chemistry

Chemistry

Materials science

Phonon

Condensed matter physics

Brillouin Spectroscopy

Shear modulus

Brillouin scattering

Elastic modulus

Thermodynamics

Physical Review Letters

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 $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{FAPbI}}_{3}$ (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 $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{FAPbI}}_{3}$.

Elastic Softness of Hybrid Lead Halide Perovskites