Lattice dynamics and phase transitions in FAPbBr(3) single crystals: Temperature- and pressure-dependent Raman spectroscopy

The rapidly emerging perovskites hold immense potential for revolutionizing optoelectronic and photovoltaic applications. However, understanding the complex structural dynamics and their response at extreme conditions in robust environments is crucial for optimizing the practical performance of perovskite-based devices. Previous X-ray diffraction studies have shown great potential for studying the average structures of these materials, but for more complex analysis at the local atomic level, Raman spectroscopy is a powerful tool. In this study, temperature- and pressure-dependent Raman spectroscopy was used to elucidate the intricate lattice dynamics, phase transitions, and amorphization of FAPbBr(3) single crystals at extreme conditions such as low temperature or high pressure. Temperature-dependent Raman spectra unveiled significant variations in wavenumbers, full width at half maximum, and the vanishing of specific modes at higher temperatures. These were ascribed to symmetry changes caused by two crystallographic phase transitions at -127 & DEG;C from orthorhombic to tetragonal and at -33 & DEG;C from tetragonal to cubic phase. In addition, some of the Raman modes disappeared upon heating at -90 & DEG;C, which was associated with a crystallographically unresolved transition. Pressure-dependent Raman scattering revealed two phase transitions at 0.3 GPa and 2.2 GPa, which corresponded to the contraction of the PbBr6 octahedra and their tilting distortion, respectively. Further compression uncovered amorphization at 3.6 GPa, characterized by the vanishing of crystalline Raman modes together with the emergence of broad new modes due to the disorder within the crystal structure. Upon pressure release, the original Raman modes were recovered, indicating the reversibility of the pressure-induced phase transitions. The changes in the Raman spectra were the most significant, especially in the low-wavenumber lattice modes, when the FAPbBr(3) underwent orthorhombic phase transition. It indicates that the inorganic sublattice is affected by the structural change into the orthorhombic phase caused either by temperature or pressure variation.