Fig. 2. (a) ¹⁴N MAS NMR spectra of MAPbI_3−xBr_x (x = 0, 1, 1.5, 2, 3) recorded at spinning speeds of 5 kHz (x = 0, 3) and 10 kHz (x = 1, 1.5, 2). Fits of the spectra to extract the average quadrupolar coupling constants are depicted in Fig. S3.† (b) Normalized ¹H DQ build-up curves of MAPbI_3−xBr_x (x = 0, 1, 1.5, 2, 3) and corresponding fits (dashed lines) following the approach of Saalwächter et al.^68,69 to extract the average dipolar coupling constants. (c) Average quadrupolar coupling (blue) and average dipolar coupling D̄ (orange) as a function of Br content x in MAPbI_3−xBr_x. The data points (cyan; magenta) at x = 0.45 corresponds to a double-mixed MA_0.15FA_0.85PbI_2.55Br_0.45 composition with additional MA/FA mixing on the A site, which was previously analyzed with respect to the ¹H dipolar coupling,²⁶ while the corresponding ¹⁴N MAS spectrum is depicted in Fig. S4.† Both, the average quadrupolar and the average dipolar coupling, show a parabolic behavior as a function of halide composition.