The conductivities and crystal structures of material which was doped with high concentrations of KF, RbF and CsF were investigated at high temperatures using complex impedance spectroscopy and powder neutron diffraction. In the case of (PbF2)1-x(KF)x, a high-temperature superionic phase with a body-centered cubic cation sub-lattice was observed for x-values of between 0.333 and about 0.68; in which the anions occupied tetrahedral interstices predominantly. The corresponding phase in the (PbF2)1-x(RbF)x system was observed for x-values of between 0.333 and 0.500; although there was a gradual tendency towards cation ordering over the 000 and ½½½ sites as x increased. This process was associated with a change in the preferred anion sites; in favor of a sub-set of the octahedral cavities, such that the mean structure tended towards a partially ordered perovskite-type arrangement. Only a single phase at x = 0.50 was observed in the (PbF2)1-x(CsF)x system. This phase adopted a fully-ordered perovskite structure and exhibited no sign of superionic behavior up to its melting point.

S.Hull, P.Berastegui: Journal of Physics - Condensed Matter, 1999, 11[27], 5257-72