Macroscopic and Microscopic Structures of Cesium Lead Iodide Perovskite from Atomistic Simulations

A first-principles-based effective Hamiltonian is developed and employed to investigate finite-temperature structural properties of a prototype of perovskite halides, that is CsPbI3. Such simulations, when using first-principles-extracted coefficients, successfully reproduce the existence of an orthorhombic Pnma state and its iodine octahedral tilting angles around room temperature. However, they also yield a direct transformation from Pnma to cubic Pm3 over bar m upon heating, unlike measurements that reported the occurrence of an intermediate long-range-tilted tetragonal P4/mbm phase in-between the orthorhombic and cubic phases. Such disagreement, which may cast some doubts about the extent to which first-principle methods can be trusted to mimic hybrid perovskites, can be resolved by only changing one short-range tilting parameter in the whole set of effective Hamiltonian coefficients. In such a case, some reasonable values of this specific parameter result in the predictions that i) the intermediate P4/mbm state originates from fluctuations over many different tilted states; and ii) the cubic Pm3 over bar m phase is highly locally distorted and develops strong transverse antiphase correlation between first-nearest neighbor iodine octahedral tiltings, before undergoing a phase transition to P4/mbm under cooling.