Acoustic Anomalies and the Critical Slowing-Down Behavior of MAPbCl₃ Single Crystals Studied by Brillouin Light...

Inelastic light scattering spectra of organic-inorganic halide perovskite MAPbCl₃ 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 C₁₁ and C₄₄ were obtained in the cubic phase for the first time. The comparison of C₁₁ and C₄₄ with those of other halide perovskites showed that C₁₁ of MAPbCl₃ is larger and C₄₄ is slightly smaller compared to the values of MAPbBr₃ and MAPbI₃. It suggests that MAPbCl₃ has a more compact structure (smaller lattice constant) along with stronger binding forces, causing larger C₁₁ 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 MAPbCl₃.