Figure : Observed structures for the phase transition of the supercell of 1296 atoms. All figures are made with the structures from AIMD simulations using VESTA software. Pb-I configurations are depicted as octahedra with iodine at corners. FA + cations are shown with balls and sticks configuration. ulations of the phase transitions from face-sharing to corner-sharing phase via a seeding approach. All computational details are provided in the methods section. An initial configuration is built where a few corner-sharing structures are placed in a large face-sharing structure of FAPbI 3 . In order to observe the crystallization of black-FAPbI 3 , I perform MD simulations at accelerated temperatures. This is also practically relevant as annealing is an important step during the fabrication of PSCs, and temperature gradients could occur in thin-films during the annealing process. Moreover, simulations are carried out for different supercells from 1000 to 5000 atoms. In these simulations, I find that different polytypes and hexagonal stacking faults 34 can form while heating hexagonal-FAPbI 3 . Figure shows the observed intermediate structures for the supercell of 1296 atoms, where all structures are arranged based on their energy ranking increasing from (a) to (i). Internal energies are calculated from ab-initio MD (AIMD) simulations using density functional theory (DFT), see methods section. Atomic view of the phase transitions is shown in supplementary movies. Similar intermediate phases are also observed for other supercells. Furthermore, structures of 4H, 6H, 8H, and other different polytypes are extracted from the simulated trajectories, therefore observing new polytypes of FAPbI 3 . I notice that the different point defects can form on the face-sharing and corner-sharing octahedra boundary during the phase transition. These defects may act as degradation centers during the operation of PSCs. Therefore, a direct heating of yellow-phase can produce multiple polytypes, stacking faults, and defects across the thin-films: therefore lower efficiency and stability. Now moving on to the formation of grain boundaries, I perform computational experiments with the seeded crystallization of FAPbI 3 from a homogeneous mixture of Pb 2+ , I -and FA + . I start with the different sizes and configurations of initial seeds. I note here that I do not take solvents in our simulations. This fundamental assump-tion is considered because the solution's large-scale crystallization is beyond our current computational architectures. Within these approximations, I now discuss our observations. I start from the growth simulations of ∼11000 atoms with seed from our simulations mentioned above, meaning exposing the 〈111〉-plane of perovskite seed to the mixture of FAI and PbI 2 . Starting configuration is shown in Figure During the growth process, I also detect the formation of point defects: formation of Pb 2+ , FA + and I -vacancies, interstitial as well as substitutional contaminations. All of the simulated trajectories and geometries are provided in the supplementary material. To conclude, the present study reveals the formation of stacking faults and predicts new polytypes of FAPbI 3 . Within approximations, I attempted to comprehend the atomic details of morphology and growth process of halide perovskites. I anticipate these fundamental details are helpful in better understanding the complex synthesis process of halide perovskites.