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

Vacancy-ordered double perovskites Cs 2 SnX 6 (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 ( 133 Cs, 119 Sn and 35 Cl) in these cesium tin(IV) halides has been used to decode the structure, which plays a key role in the materials’ optical properties. The 119 Sn NMR chemical shifts span approximately 4000 ppm and the 119 Sn spin-lattice relaxation times span three orders of magnitude when the halogen goes from chlorine to iodine in these diamagnetic compounds. Moreover, ultrawideline 35 Cl NMR spectroscopy for Cs 2 SnCl 6 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 119 Sn spin lattice relaxation time measurements uncover the presence of hidden dynamics of octahedral SnI 6 units in Cs 2 SnI 6 with a low activation energy barrier of 12.45 kJ/mol (0.129 eV). We further show that complete mixed-halide solid solutions of Cs 2 SnCl x Br 6−x and Cs 2 SnBr x I 6−x (0 ≤ x ≤ 6) form at any halogen compositional ratio. 119 Sn and 133 Cs NMR spectroscopy resolve the unique local SnCl n Br 6− n and SnBr n I 6− n ( n = 0−6) octahedral and CsBr m I 12− m ( m = 0−12) cuboctahedral environments in the mixed-halide samples. The experimentally observed 119 Sn 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 Cs 2 SnI 6 samples prepared by solvent-assisted and solvent-free synthesis routes.