Structural Evolution in Methylammonium Lead Iodide CH3NH3PbI3

The organic inorganic hybrid perovskite, in particular, methylammonium lead iodide (MAPbI(3)), is currently a subject of intense study due to its desirability in making efficient photovoltaic devices economically. It is known that MAPbI3 undergoes structural phase transitions from orthorhombic Pnma to tetragonal I4/mcm at similar to 170 K and then to cubic Pm (3) over barm at similar to 330 K. A tetragonal P4mm phase is also reported at 400 K considering total cation disorder is not appealing due to its hydrogen-bonding capabilities. Resolving this ambiguity of phase transition necessitates the study of the structural evolution across these phases in our work using ab initio methods. In this work, we show that the structural phase evolves from Pnma to I4/mcm to P4mm to Pm (3) over barm with increasing volume. The P4mm phase is a quasi-cubic one with slight distortion in one direction from cubic Pm (3) over barm due to the rotation of MA cations. Biaxial strain on MAPbI3 reveals that only the Pnma and P4mm phases are energetically stable at a < 9.14 angstrom and a> 9.14 angstrom, respectively. The Pnma, I4/mcm, P4mm, and Pm (3) over barm phases can be stable under various uniaxial strain conditions. Our study provides a clear understanding of the structural phase transitions that occur in MAPbI(3) and provides a guide for the epitaxial growth of specific phases under various strain conditions.