Determination of the miscibility gap in the solid solutions series of methylammonium lead iodide/chloride

Perovskites are widely known for their enormous possibility of elemental substitution, which leads to a large variety of physical properties. Hybrid perovskites such as CH₃NH₃PbI₃ (MAPbI₃) and CH₃NH₃PbCl₃ (MAPbCl₃) are perovskites with an A^[XII]B^[VI]X^[II]₃-structure, where A is an organic molecule, B is a lead(II) cation and X is a halide anion of iodine or chlorine. Whereas MAPbCl₃ crystallizes in the cubic space group Pm{\overline 3}m, MAPbI₃ is in the tetragonal space group I4/mcm. The substitution of I by Cl leads to an increased tolerance against humidity but is challenging or even impossible due to their large difference in ionic radii. Here, the influence of an increasing Cl content in the reaction solution on the miscibility of the solid solution members is examined systematically. Powders were synthesized by two different routes depending on the I:Cl ratio. High-resolution synchrotron X-ray data are used to establish values for the limits of the miscibility gap which are 3.1 (1.1) mol% MAPbCl₃ in MAPI and 1.0 (1) mol% MAPbI₃ in MAPCl. The establishment of relations between average pseudo-cubic lattice parameters for both phases allows a determination of the degree of substitution from the observed lattice parameters.