Iodine-based lead halide perovskites exhibit numerous potential phase transition configurations that are challenging to be identified experimentally. This study utilized density functional theory (DFT) to evaluate around 740 structural prototypes, identifying 12 possible crystal structures for CsPbI3 and 2 for RbPbI3, with the Cmcm structure of RbPbI3 reported for the first time. Frozen phonon analysis reveals that CsPbI3 does not possess any dynamically stable configurations within these low-energy structures, while the Cmcm structure of RbPbI3 exhibits adequate dynamical stability. The variation in Gibbs free energy indicates that the Pnma structure of RbPbI3 will transform to the Cmcm structure at a temperature of 120 K. Furthermore, we calculated the band structure and optical properties. Band structure calculations indicate that CsPbI3 possesses five direct bandgap configurations, whereas RbPbI3 displays indirect bandgap properties. The analysis of optical property analysis reveals a positive relationship between the bandgap and photon energy.While structures with larger bandgaps show superior optical qualities while those with smaller bandgaps respond optically more quickly.

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