Uncovering Halide Mixing and Octahedral Dynamics in Vacancy-Ordered Double Perovskites by Multinuclear Magnetic Resonance Spectroscopy

Vacancy-ordered double perovskites Cs₂SnX₆ (X = Cl, Br, I) have emerged as promising lead-free and ambient-stable materials for photovoltaic and optoelectronic applications. To advance these promising materials, it is crucial to determine the correlations between physical properties and their local structure and dynamics. Solid-state NMR spectroscopy of multiple NMR-active nuclei (¹³³Cs, ¹¹⁹Sn and ³⁵Cl) in these cesium tin(IV) halides has been used to decode the structure, which plays a key role in the materials’ optical properties. The ¹¹⁹Sn NMR chemical shifts span approximately 4000 ppm and the ¹¹⁹Sn spin-lattice relaxation times span three orders of magnitude when the halogen goes from chlorine to iodine in these diamagnetic compounds. Moreover, ultrawideline ³⁵Cl NMR spectroscopy for Cs₂SnCl₆ indicates an axially symmetric chlorine electric field gradient tensor with a large quadrupolar coupling constant of ca. 32 MHz, suggesting a chlorine that is directly attached to Sn(IV) ions. Variable temperature ¹¹⁹Sn spin lattice relaxation time measurements uncover the presence of hidden dynamics of octahedral SnI₆ units in Cs₂SnI₆ with a low activation energy barrier of 12.45 kJ/mol (0.129 eV). We further show that complete mixed-halide solid solutions of Cs₂SnCl_xBr_6−x and Cs₂SnBr_xI_6−x (0 ≤ x ≤ 6) form at any halogen compositional ratio. ¹¹⁹Sn and ¹³³Cs NMR spectroscopy resolve the unique local SnCl_nBr_6−nand SnBr_nI_6−n (n = 0−6) octahedral and CsBr_mI_12−m (m = 0−12) cuboctahedral environments in the mixed-halide samples. The experimentally observed ¹¹⁹Sn NMR results are consistent with magnetic shielding parameters obtained by density functional theory computations to verify random halogen distribution in mixed-halide analogues. Finally, we demonstrate the difference in the local structures and optical absorption properties of Cs₂SnI₆ samples prepared by solvent-assisted and solvent-free synthesis routes.