Dynamic Lone Pairs and Fluoride-Ion Disorder in Cubic-BaSnF4

01 August 2023, Version 1
This content is a preprint and has not undergone peer review at the time of posting.


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 characterise the structure and fluoride-ion–dynamics of cubic-BaSnF4, using a combination of experimental and computational techniques. Rietveld refinement of XRD data confirms an average fluorite structure with {Ba,Sn} cation disorder, and the 119Sn Mo ̈ssbauer 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 destabilises 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 is 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 .


solid electrolytes
stereoactive lone pairs
host-framework disorder
lone-pair dynamics

Supplementary materials

Supporting Information for "Dynamic Lone Pairs and Fluoride-Ion Disorder in Cubic-BaSnF4"
Supporting information, including: Full EDX analysis data; Temperature-dependent ionic-conductivity data; X-ray PDF data 12 ̊A ≤ r ≤ 50 ̊A; Mössbauer hyperfine parameters; 19F MAS NMR data; Sn-dipole orientational autocorrelation function data (PDF).


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