The presence of a stereoactive tin lone pair requires an asymmetric tin coordination environment, which is inconsistent with the structural model implied by the diffraction data, in which Ca and Sn both have cubic MF 8 coordination (cf. Fig. ). To reconcile these apparently contradictory data, Dénès and coworkers proposed a structural model wherein each tin is displaced towards one face of the enclosing [F8] cube to give squarepyramidal SnF 4 E coordination, where E denotes the stereoactive lone pair, with this lone pair oriented towards the more distant [F8] cube face. These tin lone pairs then are assumed to orient randomly along each possible ⟨100⟩ direction to give an average structure with cubic symmetry, consistent with the experimental diffraction data.