Mixed Halide Ordering as a Tool for the Stabilization of Ruddlesden-Popper Structures

While the constraints on the choice of organic cations are greatly relaxed for layered two-dimensional perovskites compared to three-dimensional perovskites, the shape of the spacer cation is still subject to limitations due to the size of the inorganic pocket between four adjacent corner-sharing octahedra. To investigate the effect of the spacer cation branching on the formation of Ruddlesden-Popper (RP) structures, we performed a comprehensive investigation of structures formed using tert-butyl ammonium (t-BA). We demonstrate that in contrast to pure bromides and pure iodides, the use of mixed halides enables the formation of the t-BA2PbBr2I2 RP perovskite structure with the specific ordering of the bromide and iodide anions. The t-BA spacer, despite its branched and bulky shape that prevents its deeper penetration, is able to form significant H-bonds that lead to the stabilization of the RP assembly if the inorganic pocket is designed in such a way that the bromide anions occupy terminal axial positions, while the iodides occupy equatorial positions. We obtain excellent agreement between experimentally determined and theoretically predicted structures using global optimization via a minima hopping algorithm for layered perovskites, illustrating the ability to predict the structure of RP perovskites and to manipulate the perovskite structure by the rational design of the inorganic pocket.