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

Abstract

Vacancy-ordered double perovskites Cs<sub>2</sub>SnX<sub>6</sub> (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 (<sup>133</sup>Cs, <sup>119</sup>Sn and <sup>35</sup>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 <sup>119</sup>Sn NMR chemical shifts span approximately 4000 ppm and the <sup>119</sup>Sn spin-lattice relaxation times span three orders of magnitude when the halogen goes from chlorine to iodine in these diamagnetic compounds. Moreover, ultrawideline <sup>35</sup>Cl NMR spectroscopy for Cs<sub>2</sub>SnCl<sub>6</sub> indicates an axially symmetric chlorine electric field gradient tensor with a large quadrupolar coupling constant of <i>ca.</i> 32 MHz, suggesting a chlorine that is directly attached to Sn(IV) ions. Variable temperature <sup>119</sup>Sn spin lattice relaxation time measurements uncover the presence of hidden dynamics of octahedral SnI<sub>6</sub> units in Cs<sub>2</sub>SnI<sub>6</sub> 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<sub>2</sub>SnCl<sub>x</sub>Br<sub>6−x</sub> and Cs<sub>2</sub>SnBr<sub>x</sub>I<sub>6−x</sub> (0 ≤ x ≤ 6) form at any halogen compositional ratio. <sup>119</sup>Sn and <sup>133</sup>Cs NMR spectroscopy resolve the unique local SnCl<i><sub>n</sub></i>Br<sub>6−<i>n</i></sub>and SnBr<i><sub>n</sub></i>I<sub>6−<i>n</i></sub> (<i>n</i> = 0−6) octahedral and CsBr<i><sub>m</sub></i>I<sub>12−<i>m</i></sub> (<i>m</i> = 0−12) cuboctahedral environments in the mixed-halide samples. The experimentally observed <sup>119</sup>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<sub>2</sub>SnI<sub>6</sub> samples prepared by solvent-assisted and solvent-free synthesis routes.