Resolving ²⁰⁹Bi sites in mixed-halide double perovskites at 28 T

Ag-Bi double perovskites are of interest as lead-free alternatives to halide perovskite optoelectronic materials. The properties can be tuned by halide mixing or dimensional reduction, but to understand how this changes the atomic structure requires a local structural probe. ²⁰⁹Bi NMR spectroscopy is extremely sensitive to the local environment but suffers from severe quadrupolar broadening. Here, we show that the combination of ultra-high field (28.2 T), fast magic angle spinning (50 kHz), and a sideband separation pulse sequence enables all seven local [BiX₆] configurations to be distinguished in the ²⁰⁹Bi NMR spectra of mixed chloride-bromide Cs₂AgBi(Cl_1-xBr_x)₆ (0 ≤ x ≤ 1) double perovskites. The ⁸¹Br NMR spectrum of Cs₂AgBiBr₆ was further measured at 28.2 T using ultra-wideline methods. Finally, variable field experiments (11.7, 20.0, and 28.2 T) enabled the ²⁰⁹Bi CSA and quadrupolar parameters to be determined for the lower symmetry BA₄AgBiBr₈ layered double perovskite (BA⁺ = n-butylammonium). This work demonstrates the promise of ultra-high field NMR spectroscopy for challenging nuclei such as ²⁰⁹Bi in complex contemporary materials.