Refined GFN1-xTB Parameters for Engineering Phase-Stable CsPbX₃Perovskites

Halide perovskites are a promising class of materials for optoelectronic applications due to their excellent optoelectronic performance. However, they suffer from several dynamical degradation problems, the characterization of which is challenging. Atomic scale simulations can provide valuable insights; however, the high computational cost of first-principles methods such as DFT makes it difficult to model dynamical processes in large perovskite systems. In this work, we present refined GFN1-xTB parameters for the accurate description of structural and dynamical properties of CsPbBr3, CsPbI3, and CsPb(I1-xBrx)3. Our molecular dynamics simulations show that the phase stability is strongly correlated to the displacement of ions in the perovskites. In the low temperature orthorhombic phase, directional movement of the Cs cations increases their distance to the surrounding halides, initiating a transition to the nonperovskite phase. However, once enough thermal energy is available, the increased Cs-halide distance can be compensated by increased halide fluctuations, resulting in a transition to the phase-stable tetragonal or cubic phases. Furthermore, we find the mixing of halides increases halide displacement over a significant range of temperatures, resulting in lower phase transition temperatures and therefore improved phase stability.