Intermolecular Interactions in Hybrid Perovskites Understood from a Combined Density Functional Theory and Effective Hamiltonian Approach

We show how the complex molecular structure of hybrid perovskites can be understood simply in terms of a few important intermolecular interactions. We deduce structural rules and coupling constants from an extensive density functional theory study of the structural energy landscape of methylammonium lead iodide. We have generated an unbiased structure–energy database, taking into account the orientations of molecular dipoles and inorganic lattice distortions. Analysis of this database shows that room-temperature structures are heavily dominated by a few hydrogen bonding patterns and lattice distortion modes. The molecule–molecule interactions, mediated by lattice distortions and hydrogen bonding, are shown to favor alignment of molecules at right angles to each other. We develop an effective Hamiltonian which shows that this interaction drives the cubic–tetragonal phase transition, explaining the temperature dependence of the tetragonal distortion observed in experiment.