Phase transitions in CsPbBr3: evaluating perovskite behavior over different time scales

Abstract

Halide perovskites have gained relevance in the field of solar cells due to their remarkable electro-optical properties, which enable efficient conversion of solar energy into electricity. Despite their promising characteristics, challenges such as long-term stability and structural complexity demand exceptional attention and dedication in the research on these materials. Their inherent soft nature, high atom mobility (especially of the halides) and the unconventional dynamics of structural motifs (halide octahedra) make them interesting from a fundamental point of view as well. The study focuses on understanding phase transitions in CsPbBr₃ perovskite, considering the importance of the dynamic properties it exhibits. The phase transitions of the CsPbBr₃ perovskite were studied through ab initio NPT molecular dynamics simulations considering several different temperatures. By taking into account the average structures over a simulation time of 45 ps after thermalization, we predict phase transitions between 300 and 325 K, as well as between 400 and 450 K, in line with previous experimental findings reported in the literature. Furthermore, through the analysis of the angles within the octahedron (Br–Pb–Br) and between octahedra (Pb–Br–Pb), the mechanism underlying the phase transitions is understood, and the structural anomalies previously reported [Svirskas et al., J. Mater. Chem. A, 2020, 8, 14015–14022] near 220 K are identified. In addition to the results obtained by performing long-time averages, we also conducted an analysis of the implications of using different time windows when calculating the average properties of interest. In this case, we observed a more complex pattern, where the material exhibits various structures depending on the exposure time and temperature, which aligns with the polymorphic nature of these materials. Our results show that, depending on the type of experiment that is being performed, different analysis, with averages considered over different times, must be performed. Long-time averages can be compared to X-ray diffraction experiments, while short-time averages should be compared to experiments that track the local structure of the material, such as PDF or Raman. Our results also indicate that phase transitions in CsPbBr₃ are not as abrupt as previously considered, posing new challenges for the experimental observation of these features.