Atomistic and Electronic Origin of Phase Instability of Metal Halide Perovskites

The excellent optoelectronic properties of metal halide perovskites (MHPs) have attracted extensive scientific interests and boosted their application in optoelectronic devices. Despite their attractive optoelectronic properties, their poor stability under ambient conditions remains the major challenge, hindering their large-scale practical applications. In particular, some MHPs undergo spontaneous phase transitions from three-dimensional perovskites (3DP) to two-dimensional non-perovskites (2DNP). Compositional engineering via mixing cations or anions has been widely reported to be effective in suppressing such unwanted phase transition. However, the atomistic and electronic origins of the stabilization effect remain unexplored. Here, by combining Density Functional Theory (DFT) calculations and Crystal Orbital Hamilton Population (COHP) analysis, we provide insights for the undesired phase transition of pristine perovskites (FAPbI3, CsPbI3, and CsSnI3) and reveal the mechanisms of the improved phase stability of the mixed compounds (CsxFA1-xPbI3, CsSnyPb1-yI3, and CsSn(BrzI1-z)3). We identify that the phase transition is determined by the relative strength of the M-X bonds as well as that of the hydrogen bonds (for hybrid compositions) in 3D and 2D phases. The phase transition can be suppressed by mixing ions, giving rise to either increased bond strength for the 3DP or decreased bond strength in their 2DNP counterparts. Our results present a comprehensive understanding of the mechanisms for the phase instability of metal halide perovskites and provide design rules for engineering phase-stable perovskite compositions.