Uncovering Halogen Mixing and Octahedral Dynamics in Cs₂SnX₆ by Multinuclear Magnetic Resonance Spectroscopy

Abstract

Cs2SnX6 (X = Cl, Br, and I) compounds 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 reveal the atomic 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 3 orders of magnitude when the halogen goes from chlorine to iodine in these diamagnetic compounds. Moreover, ultrawideline 35Cl NMR spectroscopy of Cs2SnCl6 indicates an axially symmetric chlorine electric field gradient tensor with a large quadrupolar coupling constant of ca. 32 MHz, suggesting the presence of a chlorine moiety that is directly attached to Sn­(IV) ions. Variable-temperature 119Sn spin-lattice relaxation time measurements support the presence of dynamics of octahedral SnI6 units in Cs2SnI6. We further show that complete mixed-halide solid solutions of Cs2SnCl x Br6–x and Cs2SnBr x I6–x (0 ≤ x ≤ 6) form at any halogen compositional ratio. 119Sn and 133Cs NMR spectroscopy resolve the unique local SnCl n Br6–n and SnBr n I6–n (n = 0–6) octahedral and CsBr m I12–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 that were obtained to model the random halogen distribution in mixed-halide analogues. Finally, we demonstrate the difference in the local structures and the optical absorption properties of Cs2SnI6 samples prepared by solvent-assisted and solvent-free synthesis routes.

Metal Halide Perovskite and Perovskite-like Materials through the Lens of Ultra-wideline 35/37Cl NMR Spectroscopy

“… a Sign of C Q was not determined experimentally. b Error representation is given as, e.g., 17.46 ± 0.05 MHz as 17.46(5) ppm. c Reference . …”

“…Though standard characterization tools, such as diffraction techniques, are traditionally used to characterize the average long-range structures of MHPs, complementary techniques are often needed to provide further insight into local structure, vacancies, dynamics, and nanodomains. − Solid-state nuclear magnetic resonance (NMR) spectroscopy is a robust analytical method that may be used to inform short- to medium-range structure and ion dynamics and has been shown to be a highly sensitive probe to understand the structure-property relationships displayed by MHPs. − Typically, all the constituents of MHPs have sufficiently abundant and NMR-sensitive isotopes, which make them perfect candidates for NMR analyses. Among all NMR-active nuclei in Cs-based MHPs, 133 Cs (nuclear spin, I = 7/2, quadrupolar ( I > 1/2) but with a small quadrupole moment such that quadrupolar effects are often negligible) and 207 Pb ( I = 1/2) are the most intensely studied because of their relative accessibility compared to more difficult nuclei and have contributed significantly to the understanding of phase transitions, , chemical exchange reactions, , and phase segregation. , On the other hand, quadrupolar halogen nuclei ( 35/37 Cl, 79/81 Br, 127 I) are extremely useful in characterizing unique chemical environments, however challenging, as resolution and sensitivity are often hampered due to second-order quadrupolar-broadened line shapes .…”

“…The resulting quadrupolar interactions can be described by the nuclear quadrupole coupling constant ( C Q ) and the asymmetry parameter (η). Recent halogen NMR and nuclear quadrupolar resonance (NQR) studies have demonstrated their utility in probing chemical structure and dynamics in solids. ,,− Amidst the quadrupolar halogen nuclei, 35 Cl has a moderate quadrupole moment ( Q m ) and high natural abundance (N.A. ), making it a less challenging halogen nucleus for NMR studies compared to 79/81 Br and 127 I (Table ).…”

NMR Response of the Tetrel Bond Donor

“…To conclude, the present results contribute to the elucidation of important analogies and differences between tetrel bonds and related classes of noncovalent interactions such as hydrogen bonds and halogen bonds. The methods and findings presented here may also hold value in the ongoing characterization via J and quadrupolar couplings of perovskites and related photovoltaic and optoelectronic materials, which feature tin–halogen and lead–halogen bonds. , …”

Mixing, Domains, and Fast Li-Ion Dynamics in Ternary Li–Sb–Bi Battery Anode Alloys

“…Solid-state nuclear magnetic resonance (NMR) spectroscopy is a highly sensitive method to interrogate local atomic chemical environments and is therefore suitable to study subtle compositional variations on short length scales (<10 Å) in a multicomponent alloy. Such so-called “incipient” phase segregation can be detected by NMR more readily than by other techniques that represent an average long-range crystalline structure as is the case for X-ray diffraction (XRD). − The lithiation potential of antimony is approximately 200 mV more positive than that of bismuth, indicating higher thermodynamic stability, which could result in the formation of Sb-rich clusters. Therefore, diffraction and NMR methods are complementary techniques that, together, provide a detailed and nuanced view of a material’s structure.…”