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Polytypism in Lead Halide Perovskites: An Evolutionary Investigation

Joseph Millan

Close-packed AX3 layers piled along the h111i axis make up the standard ABX3 cubic perovskite structure. In addition to a succession of intermediate polytype sequences, an equivalent hexagonal close-packed network can be created. Internally, there are combinations of face- and cornersharing octahedral chains that can drastically affect the material's physical properties. The thermodynamics of polytypism in CsPbI3 and CsPbBr3 are investigated here. Density functional theory total energies are utilised to paramaterize an axial Ising-type model Hamiltonian with linear and cubic correlation terms of the pseudo-spin. The polytype phase space, which expands exponentially with the number of layers, is explored using a genetic algorithm. The ground-state structures of CsPbX3 polytypes are investigated in order to identify polytypism traits such as unique layer configurations and symmetry prohibited sequences.