Steroactivity and disorder cause the fluoride ion conducting fluorite BaSnF4 to be stranger than it seems Samuel Coles 1 and Benjamin Morgan 1,2 1 University Of Bath, Claverton Down, BA2 7AY, 2 The Faraday Institution, Didcot,OX11 0RA Fluoride ion conductors are perhaps most notable in the simplicity of the crystallographic archetypes from which they arise. Simple structures such as the fluorite cubic BaSnF 4 have far greater simplicity than structures like the garnet and nasicon types we observe for alkali metal conductors. However, are they really that simple? This talk will present ab initio molecular dynamics simulations, supported by experimental study, to revisit the assumed fluorite cubic-phase of BaSnF 4 [1,2]; it is far more complex than it first appears, largely due to the presence of stereoactive tin lone pairs and site disorder. We show that Sn 2+ stereoactive lone pairs are dynamic, but feature a bias in orientation towards tin-rich regions. This preferential orientation has the effect of raising tin rich tetrahedral sites in energy above “interstitial” octahedral sites leading to substantial site-occupation disorder. This curious effect leads to cubic BaSnF 4 , which has historically been viewed as somewhat mundane, being a curious outlier, with the coupled effect of stereoactivity and cation mixing observed in the fluorine diffusion mechanism. References 1. Ahmad, M. M.; Yamane, Y.; Yamada, K. Structure, Ionic Conduction, and Giant Dielectric Properties of Mechanochemically Synthesized BaSnF4. Journal of Applied Physics 2009 , 106 (7), 074106. https://doi.org/10.1063/1.3234393. 2. Birchall, T.; Dénès, G.; Ruebenbauer, K.; Pannetier, J. A Neutron Diffraction And119Sn Mössbauer Study of PbSnF4 and BaSnF4. Hyperfine Interact 1986 , 29 (1), 1331–1334. https://doi.org/10.1007/BF02399479.
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