Alkali Tin Halides: Exploring the Local Structure of A2SnX6 (A = K, Rb; X = Cl, Br, I) Compounds Using Solid-State NMR and DFT Computations

Metal-halide perovskites have both interesting structural characteristics and strong potential for applications in devices such as solar cells and light-emitting diodes. While not true perovskites, A ₂SnX ₆ materials are relatives of traditional ABX ₃ perovskites that commonly adopt the K ₂PtCl ₆ structure type. Herein, we use solid-state nuclear magnetic resonance (NMR) spectroscopy to explore the influence of group 1 (alkali metal) and group 17 (halogen) substitutions on octahedral tilting and spin-orbit (SO) coupling in A ₂SnX ₆ (A = K ⁺, Rb ⁺; X = Cl ^-, Br ^-, or I ^-) materials. For the monoclinic K ₂SnBr ₆ and tetragonal Rb ₂SnI ₆ compounds, the impact of static octahedral tilting on A-site environments is evident in the form of chemical shift anisotropy (CSA) and sizeable quadrupole coupling constants (C _Qs) for ³⁹K and ⁸⁷Rb. Ultrahigh-field NMR analysis combined with periodic density functional theory (DFT) calculations enables successful determination of the relative orientation between the electric field gradient (EFG) and CSA tensors for ³⁹K in K ₂SnBr ₆. The B-site polyhedral environments are probed throughout the compositional range via ¹¹⁹Sn NMR spectroscopy, demonstrating that the ¹¹⁹Sn chemical shift follows a normal halogen dependence (NHD). Quantum chemical modeling using scalar relativistic and SO DFT on cluster models shows that the NHD is driven by the SO term of the magnetic shielding. Consistent with SO heavy atom effects on NMR chemical shifts, the NHD can be explained in terms of the frontier molecular orbitals and the involvement of Sn and X atomic orbitals in Sn-X bonds. The importance of proper relativistic treatment of heavy atoms is also highlighted by comparing calculations of ¹¹⁹Sn chemical shifts at different levels of theory.