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 [PbX₆ ] configurations in mixed iodide-bromide 3D FAPb(Br_1-x I_x )₃ perovskites and 2D BA₂ 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.