Effects of bromine substitution for iodine on structural stability and phase transition of CsPbI3

First-principles density functional theory (DFT) and the variable-cell nudged-elastic-band (VC-NEB) method were used to investigate the roles of bromine substitution for iodine in perovskite to non-perovskite phase transition of CsPbI3. CsPb(I1-xBrx)3 mixed halide alloys with x = 0, 1/6, 1/3, and 1 have been inspected to understand the effects of this bromine substitution. From the VC-NEB simulation results, the phase transition mechanism has been elucidated. The perovskite cubic to non-perovskite orthorhombic phase transition of CsPbI3 could be described based on the following processes: the cubic symmetry breaking, which leads to monoclinic intermediate states; distortion and deformation of octahedral cages in response to the cubic symmetry breaking, which differentiate the length of PbI bond in PbI6 octahedra; loss of 3D corner-sharing octahedral connectivity due to some PbI bond becomes too long; and the formation of 1D edge-sharing octahedral chain, which is the type of octahedral connectivity for the non-perovskite yellow phase. From the results of mixed halide alloys, using bromine to replace iodine was found to increase the perovskite to non-perovskite phase's energy barrier. Thus, the bromine substitution has been clarified to be useful to prevent phase transition to the yellow phase. Surprisingly, the non-perovskite to perovskite phase's energy barrier of CsPbI2Br is anomalously low with large deviation from the linear trend. This indicates that the non-perovskite to perovskite phase's transformation is significantly easier to occur for CsPbI2Br. Therefore, the results from this study' calculation suggest that bromine substitution not only prevents the formation of non-perovskite yellow phase but also exhibits capability to transform the yellow phase back to the desirable black phase.