Phase behaviour and composition in the formamidinium–methylammonium hybrid lead iodide perovskite solid solution

Abstract

The phase behaviour of mixed A-site cation methylammonium (MA)/formamidinium (FA) lead iodide hybrid perovskites FA_xMA_1−xPbI₃ has been investigated using powder and variable temperature single crystal X-ray diffraction, with A site composition determined by ¹H solution NMR. At room temperature, the crystal class is cubic across the composition range 0.2 ≤ x ≤ 1 but a tetragonal phase is observed for x = 0 (MAPbI₃) and x = 0.1. Cooling cubic FA_xMA_1−xPbI_3, 0.2 ≤ x ≤ 1, phases below room temperature results in a phase change to a larger unit cell with tilted [PbI₆] octahedra and the temperature at which this occurs, T_C→T, decreases sharply until x = 0.2 (T_C→T = 257 K) before steadily increasing to T_C→T = 283 K for FA_0.9MA_0.1PbI₃. The lattice parameters and optical band gap of cubic FA_xMA_1−xPbI₃, 0.2 ≤ x ≤ 1 at 298 K have been shown to vary in accordance with Vegard's law, though a larger band gap is observed for the tetragonal phases, 0.0 ≤ x ≤ 0.1.