  Halide perovskites make efficient solar cells due to their exceptional
optoelectronic properties, but suffer from several stability issues. The
characterization of the degradation processes is challenging because of the
limitations in the spatio-temporal resolution in experiments and the absence of
efficient computational methods to study the reactive processes. Here, we
present the first effort in developing reactive force fields for large scale
molecular dynamics simulations of the phase instability and the defect-induced
degradation reactions in inorganic CsPbI$_{3}$. We find that the phase
transitions are driven by a combination of the anharmonicity of the perovskite
lattice with the thermal entropy. At relatively low temperatures, the Cs
cations tend to move away from the preferential positions with good contacts
with the surrounding metal halide framework, potentially causing its conversion
to a non-perovskite phase. Our simulations of defective structures reveal that,
although both iodine vacancies and interstitials are very mobile in the
perovskite lattice, the vacancies have a detrimental effect on the stability,
initiating the decomposition reactions of perovskites to PbI$_{2}$. Our work
puts ReaxFF forward as an effective computational framework to study reactive
processes in halide perovskites.
