Are Mixed-Halide Ruddlesden–Popper Perovskites Really Mixed?

Mixing bromine and iodine within lead halide perovskites
is a common
strategy to tune their optical properties. This comes at the cost
of instability, as illumination induces halide segregation and degrades
device performances. Hence, understanding the behavior of mixed-halide
perovskites is crucial for applications. In 3D perovskites such as
MAPb(BrxI1–x)3 (MA = methylammonium), all of the halide crystallographic
sites are similar, and the consensus is that bromine and iodine are
homogeneously distributed prior to illumination. By analogy, it is
often assumed that Ruddlesden–Popper layered perovskites such
as (BA)2MAPb2(BrxI1–x)7 (BA = butylammonium)
behave alike. However, these materials possess a much wider variety
of halide sites featuring diverse coordination environments, which
might be preferentially occupied by either bromine or iodine. This
leaves an open question: are mixed-halide Ruddlesden–Popper
perovskites really mixed? By combining powder and single-crystal diffraction
experiments, we demonstrate that this is not the case: bromine and
iodine in RP perovskites preferentially occupy different sites, regardless
of the crystallization speed.