Fig. 2

Structural relaxation in DFT. Lattice parameters (labelled a, b, c) as a function of relaxation step for cubic (a) and tetragonal (b) MAPbI₃ structures. The out-of-plane lattice parameter is denoted as c in the tetragonal structure. The relaxation results in both structures becoming orthorhombic, as seen by the different values of the three lattice parameters at the end of the relaxation. The cubic (c) and tetragonal (d) structures obtained after full relaxation without symmetry constraints exhibit considerable distortions of the PbI framework. The MA-relaxed cubic (e) and tetragonal (f) structures with symmetry consistent with experiment are also shown. All structures consist of 4-formula-unit supercells, and the atoms are represented with spheres: Pb is grey, I is purple, C is brown, N is blue, and H is pink