Phase Diagram of Methylammonium/Formamidinium Lead Iodide Perovskite Solid Solutions from Temperature Dependent Photoluminescence and Raman Spectroscopy

The complete phase diagram of organic-cation solid-solutions of lead iodide perovskites (FAxMA1-xPbI3, where MA stands for methylammonium, CH3NH3, and FA for formamidinium, CH(NH2)2 with compositions x ranging from 0 to 1 in steps of 0.1 was constructed in the temperature range from 10 to 365 K by combining Raman scattering and photoluminescence (PL) measurements. The occurrence of phase transitions was inferred from both the temperature-induced changes in the optical emission energies and/or the phonon frequencies and linewidths, complementing X-ray and neutron scattering literature data. For MA-rich perovskites (x≤0.2) the same structural behavior as for MAPbI3 is observed with decreasing temperature: cubic Pm-3m → tetragonal-I I4/mcm → orthorhombic Pnma. Being the FA molecule larger, more symmetric but less polar than MA, a tetragonal crystal structure is favored at low temperatures and FA compositions x>0.4, in detriment of the orthorhombic phase. As a consequence, with decreasing temperature the phas...