27. Prediction of a stable hexagonal phase of CH3NH3PbI3

First-principles prediction of a stable hexagonal phase of CH₃NH₃PbI₃.
A. S. Thind, X. Huang, J. Sun, R. Mishra. Chemistry of Materials29, 6003 (2017).

Methylammonium lead iodide (CH₃NH₃PbI₃ or MAPbI₃) perovskite is a promising new photovoltaic material with high power conversion efficiency. However, its perovskite phase with corner-connected PbI₆ octahedra shows poor environmental stability. More recently, MAPbI₃ has been shown to be thermodynamically unstable with a positive formation enthalpy. Here, using first-principles density functional theory calculations, we predict a layered hexagonal phase of MAPbI₃consisting of infinite chains of face-shared PbI₆ octahedra with P6₃mc space-group symmetry to be thermodynamically the most stable phase for a wide range of volume and temperature compared to any of the experimentally observed perovskite phases with a different tilt pattern of the corner-connected octahedra. The predicted hexagonal phase is also dynamically stable without any soft phonon modes. The change from corner to face-shared connectivity in the hexagonal phase leads to a predicted band gap of 2.6 eV and a band structure that favors highly anisotropic charge transport.