Modulation of Mixed-Cation Lead Halide Crystal Structures and Properties by Changing Small Size Cation

Mixing of organic cations is a promising strategy for extending the diversity of hybrid lead halides and tuning their properties. Most famous examples of this approach are multilayered Ruddlesden-Popper and Dion-Jacobson perovskites and three-dimensional (3D) MPDA2APb4Br13 perovskitoids (MPDA = N-methylpropanediammonium, A = methylammonium, formamidinium, ethylammonium, and dimethylammonium). We show that the 3D perovskitoid structure is no longer stable when imidazolium (IM+) and guanidinium (GA+) are used, but instead two- and one-dimensional structures are formed, respectively, and the stoichiometry changes to MPDA2IM3Pb4Br15 and MPDA6GA6Pb7Br30Br2. Most interestingly, MPDA2IM3Pb4Br15 comprises unprecedented layers composed of the corner-sharing PbBr6 octahedra organized into tetrameric units with IM+ located in the perovskite cages. It undergoes a transition from the high-temperature P21/c phase to the low-temperature P21/n phase associated with the dielectric anomaly. The MPDA6GA6Pb7Br30Br2 structure is composed of corner-sharing PbBr6 octahedra arranged into chains, and its crystal structure remains trigonal, P-3C1, in the entire temperature range studied. Optical studies show that different structural motifs strongly affect band gaps and emission properties of these compounds. In particular, MPDA6GA6Pb7Br30Br2 exhibits very stable broadband greenish-yellow emission, while MPDA2IM3Pb4Br15 exhibits both narrowband emission (attributed to free excitons (FEs) and bound excitons (BEs)) and self-trapped (STE) broadband emission, a thermally activated population transfer from STE to FE + BE, and strong thermochromism.