This rapid decay of the Sn-dipole orientational autocorrelation funcion when the fluoride positions are fixed suggests that fluoride-ion dynamics and lone pair dynamics are strongly coupled. When the fluoride ions are fixed to their 8c lattice positions, the lone pair moves freely-no matter which direction it points in there is a strong lone-pair-fluoride repulsion. When the fluoride ions are free to move, however, a number of these fluoride ions move from unstable tetrahedral sites into more favourable octahedral sites, leaving vacant tetrahedral sites next to tin. The Sn lone pair preferentially orients towards these vacant sites to minimise the lone-pairfluoride-ion repulsion (Fig. ). As fluoride ions move between sites, the Sn lone pairs dynamically reorient in concert with the changing local fluoride ion configuration, giving strong coupling between the fluoride-ion dynamics and the lone-pair reorientation dynamics.