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.