Automated wide-line nuclear quadrupole resonance of mixed-cation lead-halide perovskites

Nuclear quadrupole resonance (NQR), a technique related to nuclear magnetic resonance, is extremely sensitive to local crystal composition and structure. Unfortunately, in disordered materials, this sensitivity also leads to very large linewidths, presenting a technical challenge and requiring a serious time investment to get a full spectrum. Here, we describe our newly developed, automated NQR set-up to acquire high-quality wide-line spectra. Using this set-up, we carried out ¹²⁷I NQR on three mixed-cation lead-halide perovskites (LHPs) of the form MA _x FA_1-x PbI₃ (where MA denotes methylammonium; FA denotes formamidinium; and $x=$ 0.25, 0.50 and 0.75) at various temperatures. We achieve a signal-to-noise ratio of up to $\sim 400$ for lineshapes with a full width at half maximum of $\sim 2.5\mathrm{MHz}$ acquired with a spectral width of 20 MHz in the course of 2-3 d. The spectra, which at least partially exhibit features encoding structural information, are interpreted using a statistical model. This model finds a degree of MA-MA and FA-FA clustering ( $0.2\le S\le 0.35$ ). This proof-of-principle for both the wide-line NQR set-up and the statistical model widens the applicability of an underutilised avenue of non-invasive structural research.