Acoustic Anomalies and the Critical Slowing-Down Behavior of MAPbCl Single Crystals Studied by Brillouin Light Scattering.

Inelastic light scattering spectra of organic-inorganic halide perovskite MAPbCl single crystals were investigated by using Brillouin spectroscopy. Sound velocities and acoustic absorption coefficients of longitudinal and transverse acoustic modes propagating along the cubic [100] direction were determined in a wide temperature range. The sound velocities exhibited softening upon cooling in the cubic phase, which was accompanied by the increasing acoustic damping. The obtained relaxation time showed a critical slowing-down behavior, revealing the order-disorder nature of the phase transition, which is consistent with the growth of strong central peaks upon cooling toward the phase transition point. The temperature dependences of the two elastic constants and were obtained in the cubic phase for the first time. The comparison of and with those of other halide perovskites showed that of MAPbCl is larger and is slightly smaller compared to the values of MAPbBr and MAPbI. It suggests that MAPbCl has a more compact structure (smaller lattice constant) along with stronger binding forces, causing larger and bulk modulus in this compound, and that the shear rigidity is exceedingly small similar to other halide perovskites. The reported elastic constants in this study may serve as a testbed for theoretical and calculational approaches for MAPbCl.