Structural Evolution in Methylammonium Lead Iodide CH3NH3PbI3.

The organic-inorganic hybrid perovskite, in particular, methylammonium lead iodide (MAPbI3), 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 ∼170 K and then to cubic Pm3̅m at ∼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 Pm3̅m with increasing volume. The P4mm phase is a quasi-cubic one with slight distortion in one direction from cubic Pm3̅m 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 Å and a > 9.14 Å, respectively. The Pnma, I4/mcm, P4mm, and Pm3̅m phases can be stable under various uniaxial strain conditions. Our study provides a clear understanding of the structural phase transitions that occur in MAPbI3 and provides a guide for the epitaxial growth of specific phases under various strain conditions.