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Article Dans Une Revue Journal of the American Chemical Society Année : 2023

Dynamic Lone Pairs and Fluoride-Ion Disorder in Cubic-BaSnF 4

Samuel Coles
Monique Body
Benjamin Morgan

Résumé

Introducing compositional or structural disorder within crystalline solid electrolytes is a common strategy for increasing their ionic conductivity. (M,Sn)F2 fluorites have previously been proposed to exhibit two forms of disorder within their cationic host frameworks: occupational disorder from randomly distributed M and Sn cations and orientational disorder from Sn(II) stereoactive lone pairs. Here, we characterize the structure and fluoride-ion dynamics of cubic BaSnF4, using a combination of experimental and computational techniques. Rietveld refinement of the X-ray diffraction (XRD) data confirms an average fluorite structure with {Ba,Sn} cation disorder, and the 119Sn Mössbauer spectrum demonstrates the presence of stereoactive Sn(II) lone pairs. X-ray total-scattering PDF analysis and ab initio molecular dynamics simulations reveal a complex local structure with a high degree of intrinsic fluoride ion disorder, where 1/3 of fluoride ions occupy octahedral “interstitial” sites: this fluoride-ion disorder is a consequence of repulsion between Sn lone pairs and fluoride ions that destabilizes Sn-coordinated tetrahedral fluoride-ion sites. Variable-temperature 19F NMR experiments and analysis of our molecular dynamics simulations reveal highly inhomogeneous fluoride-ion dynamics, with fluoride ions in Sn-rich local environments significantly more mobile than those in Ba-rich environments. Our simulations also reveal dynamical reorientation of the Sn lone pairs that is biased by the local cation configuration and coupled to the local fluoride-ion dynamics. We end by discussing the effect of host-framework disorder on long-range diffusion pathways in cubic BaSnF4.

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hal-04250709 , version 1 (19-10-2023)

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Briséïs Mercadier, Samuel Coles, Mathieu Duttine, Christophe Legein, Monique Body, et al.. Dynamic Lone Pairs and Fluoride-Ion Disorder in Cubic-BaSnF 4. Journal of the American Chemical Society, 2023, 145 (43), pp.23739-23754. ⟨10.1021/jacs.3c08232⟩. ⟨hal-04250709⟩
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