Temperature-Dependent Raman Studies of FAPbBr3 and MAPbBr3 Perovskites: Effect of Phase Transitions on Molecular Dynamics and Lattice Distortion

Three-dimensional hybrid organic–inorganic lead halide perovskites are promising photovoltaic and light-emitting materials. A key phenomenon relevant for their optoelectronic applications is electron–phonon coupling. Since it can be strongly modified by structural deformation changes in the dynamics of molecular cations, great importance to study temperature dependence phonon properties perovskites. In this work, temperature-dependent Raman scattering studies FAPbBr3 MAPbBr3 single crystals reported 1800–22 cm−1 300–90 K ranges. The data showed clear anomalies near 236, 155 148 K, which were attributed Pm3¯m→I4/mcm→P4/mmm (or Imma)→Pnma phase transitions. They also provided strong evidence that crystal structure stable 155–148 range very similar I4/mcm phase, not lowest-temperature Pmna as suggested some reports. Therefore, symmetry seems more likely P4/mmm rather than Imma. An analysis evolution modes revealed transitions 236 associated with weak distortion inorganic subnetwork MA+ ions. Very pronounced lattice region a narrowing bands below indicated transition Pnma triggered freezing motions, turn leads subnetwork. material behaves different way MAPbBr3. First all, FA+ cations frozen even phase. Moreover, small observation confirmed, however, two crystallographically resolved (Pm3¯m→P4/mbm 260 P4/mbm→Pnma 150 K) On other hand, an internal these transitions, well unresolved 120 180 change dynamics.