A Reparameterized Density Functional Tight-Binding Method for Engineering phase-stable CsPbX\textsubscript{3} Perovskites

Halide perovskites are a promising class of materials for optoelectronic applications, due to their excellent optoelectronic performance. However, they suffer several dynamical degradation problems, the characterization of which is challenging in experiments. Atomic scale simulations can provide valuable insights, however, the high computational cost of traditional quantum mechanical methods such as DFT makes it difficult to model dynamical processes in large perovskite systems. In this work, we present a re-parameterized GFN1-xTB method for the accurate description of structural and dynamical properties of CsPbBr\textsubscript{3}, CsPbI\textsubscript{3}, and CsPb(I\textsubscript{1-x}Br\textsubscript{x})\textsubscript{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, the directional movement of the Cs cations decreases contact with the surrounding halides, initiating a transition to the non-perovskite phase. However, this loss of contact can be compensated by increased halide displacement, once enough thermal energy is available, resulting in a transition to the tetragonal or cubic phases. Furthermore, we find the mixing of halides increase halide displacement over a significant range of temperatures, resulting in lower phase transition temperatures and therefore improved phase stability.