In Fig. (b) we show the proportion of time a lone pair points towards a given face of the enclosing cubic site, as a function of the number of nearest-neighbour Sn (out of a maximum of 4) that coordinate the frontface 8c sites and the number of nearest-neighbour Sn (again out of a maximum of 4) that coordinate the backface 8c sites, with the data presented as a heat map. Data on the diagonal where n(Sn) front = n(Sn) back correspond to lone pair orientations with symmetric frontface-back-face nearest-neighbour cation environments. These data all show relatively low values, indicating that lone pair orientations with balanced cation coordination are weakly or negligibly biased. In contrast, lone pair orientations with more front-face Sn neighbours than backface Sn neighbours show a strong bias. As a consequence, the stereoactive Sn lone pairs in c-BaSnF 4 , on average, tend to point towards other nearby tins. Clusters of Sn cations are therefore expected to have all their lone pairs preferentially oriented towards the interior of the cluster, giving a cooperative effect where these Sn lone pairs all disrupt any fluoride ion occupation of mutually coordinated tetrahedral sites. This model is consistent with the increasing tetrahedral site free energy with increasing Sn-coordination (Fig. ) and provides an explanation for the extreme disruption of the fluoride substructure in Snrich regions, as observed in the fluoride-ion time-average density (Fig. ).