Elastic softness of hybrid lead halide perovskites

TL;DR

This study investigates the elastic properties of hybrid lead halide perovskites, revealing their softness and potential ferroelastic behavior, which impact their thermal and optoelectronic performance.

Contribution

It provides a comprehensive analysis of the elastic constants and phonon dynamics in four hybrid perovskites, highlighting their ultrasoft nature and ferroelastic tendencies.

Findings

Very soft shear modulus C44 observed

FAPbBr3 shows incipient ferroelastic transition

Lower sound velocities in iodide-based compounds

Abstract

Much recent attention has been devoted towards unravelling the microscopic optoelectronic properties of hybrid organic-inorganic perovskites (HOP). Here we investigate by coherent inelastic neutron scattering spectroscopy and Brillouin light scattering, low frequency acoustic phonons in four different hybrid perovskite single crystals: MAPbBr$_3$, FAPbBr$_3$, MAPbI$_3$ and $\alpha$-FAPbI$_3$ (MA: methylammonium, FA: formamidinium). We report a complete set of elastic constants caracterized by a very soft shear modulus C$_{44}$. Further, a tendency towards an incipient ferroelastic transition is observed in 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 $\alpha$-FAPbI$_3$.