Speed of sound and elastic properties of single crystals of inorganic and hybrid lead-free iodide perovskites obtained by femtosecond transient optical reflectivity

Optoelectronic devices operate under continuous thermal stress that may influence both long-term stability and optical properties of the active material. It is therefore useful to know the elastic properties of the active materials. In general, the elastic constants of a material may be deduced by the measurement of the speed of sound in that material. In this work, we report on the measurements of the speed of sound in three lead-free halide perovskites, namely, the inorganic ${\mathrm{Cs}}_{3}{\mathrm{Bi}}_{2}{\mathrm{I}}_{9}$ and the hybrid ${\mathrm{MA}}_{3}{\mathrm{Bi}}_{2}{\mathrm{I}}_{9}$ and ${\mathrm{MA}}_{3}{\mathrm{Sb}}_{2}{\mathrm{I}}_{9}$, by means of femtosecond transient optical reflectivity that shows oscillations of the signal intensity that are related to the strain field caused by the photoexcitation of electron-hole pairs. The values of the speed of sound thus obtained have allowed us to extract the elastic constant, ${c}_{11}$ , along the propagation direction of the light. The ${c}_{11}$ values indicate a lower stiffness of the hybrid materials, an important aspect when designing optoelectronic devices.