Significantly higher ionic conductivities are found in fluorite-structured materials that contain cations with stereoactive lone pairs, such as β-PbF 2 . This effect has been suggested to be a possible consequence of the high polarisability of the cation facilitating diffusion of the mobile anions , or that the negative charge of the Pb lone pairs might electrostatically destabilise adjacent fluoride ions in tetrahedral sites, thereby promoting the formation of anti-Frenkel pairs . Neutron diffraction data and AIMD simulations of β-PbF 2 , however, show no evidence for octahedral site occupation by fluoride ions, bringing into question the hypothesis that the presence of stereoactive lone pairs in fluorite-structured materials promotes Frenkel pair formation .