Axial-equatorial halide ordering in layered hybrid perovskites from isotropic-anisotropic ${}^{207}$Pb NMR

Bandgap-tuneable mixed-halide 3D perovskites are of interest for multi-junction solar cells, but suffer from photoinduced spatial halide segregation. Mixed-halide 2D perovskites are more resistant to halide segregation and are promising coatings for 3D perovskite solar cells. The properties of mixed-halide compositions depend on the local halide distribution, which is challenging to study at the level of single octahedra. In particular, it has been suggested that there is a preference for occupation of the distinct axial and equatorial halide sites in mixed-halide 2D perovskites. ${}^{207}$Pb NMR can be used to probe the atomic-scale structure of lead-halide materials, but although the isotropic ${}^{207}$Pb shift is sensitive to halide stoichiometry, it cannot distinguish configurational isomers. Here, we use 2D isotropic-anisotropic correlation ${}^{207}$Pb NMR and relativistic DFT calculations to distinguish the [PbX${}_6$] configurations in mixed iodide-bromide 3D FAPb(Br${}_{1-x}$I${}_x$)${}_3$ perovskites and 2D BA${}_2$Pb(Br${}_{1-x}$I${}_x$)${}_4$ perovskites based on formamidinium (FA${}^{+}$) and butylammonium (BA${}^{+}$), respectively. We find that iodide preferentially occupies the axial site in BA-based 2D perovskites, which may explain the suppressed halide mobility. (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH)