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 <i>Pnma</i> to tetragonal <i>I</i>4/<i>mcm</i> at ∼170 K and then to cubic <i>Pm</i>3̅<i>m</i> at ∼330 K. A tetragonal <i>P</i>4<i>mm</i> 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 <i>Pnma</i> to <i>I</i>4/<i>mcm</i> to <i>P</i>4<i>mm</i> to <i>Pm</i>3̅<i>m</i> with increasing volume. The <i>P</i>4<i>mm</i> phase is a quasi-cubic one with slight distortion in one direction from cubic <i>Pm</i>3̅<i>m</i> due to the rotation of MA cations. Biaxial strain on MAPbI₃ reveals that only the <i>Pnma</i> and <i>P</i>4<i>mm</i> phases are energetically stable at <i>a</i> < 9.14 Å and <i>a</i> > 9.14 Å, respectively. The <i>Pnma</i>, <i>I</i>4/<i>mcm</i>, <i>P</i>4<i>mm</i>, and <i>Pm</i>3̅<i>m</i> 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