crystals*

Halide perovskites have been one of the most heavily investigated solar cell materials since their efficacy was first demonstrated in 2009. One of the most studied inorganic halide perovskites is cesium lead iodide (CsPbI₃), which has shown high efficiencies and decreased volatility compared to its organic-inorganic hybrid counterparts. However, one major issue that CsPbI₃ still faces is that it is unstable at room temperature. This problem is fascinating as most tolerance-factor calculations predict that CsPbI₃ should be stable in its perovskite phase. One explanation for this instability is that the Cs atom rattles between two sites near room temperature, one with a much lower coordination number than the other. We study this rattling using density functional theory and Car-Parrinello molecular dynamics. We show how the rattling evolves for temperatures between 100K to 295K, how that affects the other thermodynamic properties of the materials, and how it compares back to experimental results. These results can be used to elucidate further the importance of Cs rattling on the stability of inorganic halide perovskites.