Lone electron pair (E) role on the crystal structures and the mechanism of high ionic conductivity of PbSnF4E2. Stereochemical and ab initio investigations

• An alternative mechanism of F– motion in fast anionic conductor PbSnF4 is provided.
• Within integrated crystal chemical – DFT analysis major role is assigned to the electron lone pair E.
• E stereochemistry illustrated by the ELF isosurface plots and crystal analysis.
• A rapid exchange between different F– is enhanced within changing polyhedra enclosing E.

The F– anion mobility of archetype fast ionic conductor PbSnF4 formerly investigated by neutron diffraction with temperature is revisited based on a joint stereochemical and DFT investigation. It is mainly shown that a rapid exchange between F anions at the different tetragonal lattice sites is enhanced within the polyhedra enclosing the lone pair E in a dynamic change of coordination from octahedral to square pyramidal as for Sn(II). E stereoactivity in the interspaces along c direction is illustrated by the electron localization function ELF isosurface representations and followed by the non linear change of the c lattice constant with temperature.

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