Spontaneous bidirectional ordering of CH3NH3+ in lead iodide perovskites at room temperature: The origins of the tetragonal phase

CH3NH3PbI3 is a hybrid organic-inorganic material with a perovskite structure and a temperature-dependent polymorphism whose origins are still unclear. Here we perform ab initio molecular dynamics simulations in order to investigate the structural properties and atom dynamics of CH3NH3PbI3 at room temperature. Starting from different initial configurations, we find that a single-crystalline system undergoes a spontaneous ordering process which brings the CH3NH3+ ions to alternately point towards the center of two out of the six faces of the cubic PbI3- framework, i.e. towards the < 100 > and < 010 > directions. This bidirectional ordering gives rise to a preferential distortion of the inorganic lattice on the a-b plane, shaping the observed tetragonal symmetry of the system. The process requires tens of picoseconds for CH3NH3PbI3 supercells with just eight CH3NH3+ ions.