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

The complete phase diagram of organic-cation solid solutions of lead iodide perovskites [FA(x)MA(1-x)PbI(3), 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 line widths, complementing X-ray and neutron scattering literature data. For MA-rich perovskites (x <= 0.2), the same structural behavior as for MAPbI(3) was observed with decreasing temperature: cubic Pm (3) over barm -> tetragonal-I I4/mcm -> orthorhombic Pnma. As the FA molecule is larger and more symmetric but less polar than MA, a tetragonal crystal structure is favored at low temperatures and FA compositions x > 0.4, to the detriment of the orthorhombic phase. As a consequence, with decreasing temperature, the phase transition sequence for FA- rich compounds is cubic Pm (3) over barm -> tetragonal-II P4/mbm -> tetragonal-III. The latter presumably belongs to the P4bm symmetry group, according to neutron scattering data. Strikingly, the isostructural (tetragonal-totetragonal) transformation, which occurs between 200 and 150 K, exhibits a kind of critical point for x = 0.7. For intermediate FA contents, the perovskite solid solution transforms close to 250 K directly from the cubic phase to the tetragonal-III phase. The latter is characterized by a nonmonotonic dependence of the band-gap energy on temperature. We ascribe such behavior to a substantial tilting of the PbI6 octahedra in the tetragonal-III phase. In this way, we established important links between crystal-phase stability and the electronic as well as vibrational properties of mixed organic-cation halide perovskites, which might impact the current search for more stable best-performing optoelectronic materials.