Supporting the Structural Characterization of Halide Perovskite and Perovskitoids via Halide Nuclear Quadrupole Resonance

Halide Perovskites (HP) and perovskitoids are currently at the center of a feverish research activity, following their record-breaking performances in photovoltaics.[1,2] Besides their inherent excellent semiconducting properties, however, these solution-processed compounds offer an unparalleled compositional and structural tailorability, paving the way for new functionalities, and fields of applications.[3] They sustain, for instance, different chemical compositions, as in the case of halide substitution (I→Br→Cl) and mixing.[4] But most notably, they allow for the formation of very diverse atomistic nano-architectures. Considering the MX₆ (M=metal, X=halide) octahedra as fundamental building blocks, these may be organized in corner-shared, edge-shared and face-shared fashion, or even a combination of these.[5]
Such a wide structural variety still raises a fundamental concern, on how to efficiently discriminate among these different structures. How to characterize freshly synthesized compounds with a brand-new composition or samples containing phase mixtures? X-Ray Diffraction (XRD) is the spearhead technique to access the atomic structure of materials. It may however greatly benefit from the back-up from other techniques with complementary characteristics, able to probe materials at a more local scale. Within the stream of the so called Nuclear Magnetic Resonance crystallography,[6] we propose here the Nuclear Quadrupolar Resonance (RQN), as an ideal technique to characterize new perovskite-derived materials, in particular, focusing on the halides, which are actually involved in the most diverse chemical environment, as related to different octahedral connectivities, dimensionality, etc. The inherently large nuclear quadrupolar moment of halides, in fact, which largely hinders their NMR characterization, is ideal for NQR spectroscopy.
Aiming to verify the suitability of NQR of halides as a means to characterize of halide perovskites and perovskitoids, we performed a thorough set of periodic Density Functional Theory (DFT) calculations. These allow, first, to determine whether different chemical environments (halide in face-sharing octahedral connectivity, rather than corner-sharing) actually present a distinguishable NQR response; second, with current NQR experimental set-up able to sample only narrow frequency windows at the time, our calculations may guide experimentalists in the future, by providing some reasonable guesses.
Our calculations nicely parallel the (few) experimental data available from the literature for CsPbX₃ compounds (X=Cl, Br, I), as well as for MAPbX₃ (X=Cl, Br, I) and FAPbI₃. In addition, they anticipate that in two-dimensional (2D) lead-iodide perovskite the response of those “terminal iodines”, chemically disconnecting the lead-iodide layers, have characteristic frequencies lying at half those of their corner-sharing counterparts. We also suggest that iodines involved in a three PbI₆ edge-sharing fashion, as in PbI₂, have very mall characteristic frequencies (in the domain of the MHz). To provide a roadmap for future research, we investigate a wide set of model systems composed by corner-, edge- and face-sharing PbI6 octahedra, with known crystalline structures. This work highlights that NQR has all the potential to support more established techniques in structurally characterizing halide perovskites.[7]

[1] E. Aydin, , et al., Science 2024, 383, eadh3849.
[2] H. Chen., et al., Science 2024, 384, 189.
[3] Z. Yuan, et al., Nat Commun 2017, 8, 14051
[4] J. H., Noh, et al., Nano Lett. 2013, 13, 1764.
[5] M. E. Kamminga, Chem. Mater. 2016, 28, 4554
[6] C. Quarti, et al., Helv. Chim. Acta 2021, 104, e2000231
[7] C. Quarti, manuscript under review.