Nuclear\nQuadrupolar Resonance Structural Characterization\nof Halide Perovskites and Perovskitoids: A Roadmap from Electronic\nStructure Calculations for Lead–Iodide-Based Compounds

Metal\nhalide perovskites, including some of their related perovskitoid\nstructures, form a semiconductor class of their own, which is arousing\never-growing interest from the scientific community. With halides\nbeing involved in the various structural arrangements, namely, pure\ncorner-sharing MX<sub>6</sub> (M is metal and X is halide) octahedra,\nfor perovskite networks, or alternatively a combination of corner-,\nedge-, and/or face-sharing for related perovskitoids, they represent\nthe ideal probe for characterizing the way octahedra are linked together.\nWell known for their inherently large quadrupolar constants, which\nis detrimental to the resolution of nuclear magnetic resonance spectroscopy,\nmost abundant halide isotopes (<sup>35/37</sup>Cl, <sup>79/81</sup>Br, <sup>127</sup>I) are in turn attractive for magnetic field-free\nnuclear quadrupolar resonance (NQR) spectroscopy. Here, we investigate\nthe possibility of exploiting NQR spectroscopy of halides to distinctively\ncharacterize the various metal halide structural arrangements, using\ndensity functional theory simulations. Our calculations nicely match\nthe available experimental results. Furthermore, they demonstrate\nthat compounds with different connectivities of their MX<sub>6</sub> building blocks, including lower dimensionalities such as 2D networks,\nshow distinct NQR signals in a broad spectral window. They finally\nprovide a roadmap of the characteristic NQR frequency ranges for each\noctahedral connectivity, which may be a useful guide to experimentalists,\nconsidering the long acquisition procedures typical of NQR. We hope\nthis work will encourage the incorporation of NQR spectroscopy to\nfurther our knowledge of the structural diversity of metal halides.