Anion diffusion within highly conducting layered fluorides

Neutron diffraction has played an important role in the study of solids which possess high values of ionic conductivity, ranging from 'model' superionic compounds such as AgI and β-PbF2 to more technologically relevant materials for use within lightweight Li+ batteries and oxide fuel cells. An understanding of the structural properties of these systems is essential to identify those factors which promote high ionic conductivity and underpin the quest for new materials. In recent years, attention has focussed on the role of cations characterized by lone-pair electrons (such as Bi3+, Pb2+, Sn2+ and Tl+) within highly conducting oxides and halides, with differing views concerning their importance in promoting extensive anion disorder. In this context, the results of powder neutron diffraction studies of two layered compounds within the MSnF4 (M=Pb, Sr and Ba) and MSn2F5 (M=K, Rb, Cs and Tl) families are presented, to illustrate the structural role of Sn2+ ions within these anion conductors