Elastic and vibrational properties of Formamidinium based mixed halide perovskites

Halide mixing effects on the elastic and vibrational properties were investigated for FAPbBr1.5Cl1.5 single crystals. The elastic and vibrational characteristics of the synthesized single crystal were examined using temperature-dependent Brillouin and Raman spectroscopies. Elastic constants C11 and C44 were derived from the longitudinal and transverse sound velocities, respectively. The softening and subsequent hardening of the elastic constants upon cooling was accompanied by substantial increase of the acoustic damping peaks indicating significant interaction between the acoustic waves and reorientational dynamics of the FA cations. Especially, FAPbBr1.5Cl1.5 exhibited extremely low shear rigidity compared to other MA-based perovskites. Raman spectroscopy was used to study the vibrational modes of the PbX6 octahedral lattice and the internal motions of FA cations. Raman modes did not reveal any drastic changes within the investigated temperature range. The combined elastic and vibrational properties showed that the structural phase transitions observed from pure compounds were suppressed in FAPbBr1.5Cl1.5 due to the local heterogeneous environment for the FA cations caused by random substitution of two halogen ions. These findings are expected to enhance the understanding of the material's behavior under varying thermal conditions, which is crucial for optimizing the performance of perovskite-based devices.