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

Abstract

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 anA[XII]B[VI]X[II]3-structure, whereAis an organic molecule,Bis a lead(II) cation andXis a halide anion of iodine or chlorine. Whereas MAPbCl3crystallizes in the cubic space group Pm{\overline 3}m, MAPbI3is in the tetragonal space groupI4/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% MAPbCl3in MAPI and 1.0 (1) mol% MAPbI3in 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.