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

The phase behaviour of mixed A-site cation methylammonium (MA)/formamidinium (FA) lead iodide hybrid perovskites FAxMA1-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 less than or equal to x less than or equal to 1 but a tetragonal phase is observed for x = 0 (MAPbI3) and x = 0.1. Cooling cubic FAxMA1-xPbI3, 0.2 less than or equal to x less than or equal to 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, TC arrow right T, decreases sharply until x = 0.2 (TC arrow right T = 257 K) before steadily increasing to TC arrow right T = 283 K for FA0.9MA0.1PbI3. The lattice parameters and optical band gap of cubic FAxMA1-xPbI3, 0.2 less than or equal to x less than or equal to 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 less than or equal to x less than or equal to 0.1.