Phase Behavior and Substitution Limit of Mixed Cesium-Formamidinium Lead Triiodide Perovskites

The mixed cation lead iodide perovskite photovoltaics show improved stability following site substitution of cesium ions (Cs⁺) onto the formamidinium cation sites (FA⁺) of (CH­(NH₂)₂PbI₃ (FAPbI₃) and increased resistance to formation of the undesirable ∂-phase. The structural phase behavior of Cs_0.1FA_0.9PbI₃ has been investigated by neutron powder diffraction (NPD), complemented by single crystal and power X-ray diffraction and photoluminescence spectroscopy. The Cs-substitution limit has been determined to be less than 15%, and the cubic α-phase, Cs_0.1FA_0.9PbI₃, is shown to be synthesizable in bulk and stable at 300 K. On cooling the cubic Cs_0.1FA_0.9PbI₃, a slow, second-order cubic to tetragonal transition is observed close to 290 K, with variable temperature NPD indicating the presence of the tetragonal β-phase, adopting the space group P4/mbm between 290 and 180 K. An orthorhombic phase or twinned tetragonal phase is formed below 180 K, and the temperature for further transition to a disordered state is lowered to 125 K compared to that seen in phase pure α-FAPbI₃ (140 K). These results demonstrate the importance of understanding the effect of cation site substitution on structure–property relationships in perovskite materials.