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 FA(x)MA(1-x)PbI(3) has been investigated using powder and variable temperature single crystal X-ray diffraction, with A site composition determined by H-1 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 (MAPbI(3)) and x = 0.1. Cooling cubic FA(x)MA(1-x)PbI(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(0.9)MA(0.1)PbI(3). The lattice parameters and optical band gap of cubic FA(x)MA(1-x)PbI(3), 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.