The structures and conductivities of polycrystalline samples of (PbF2)1-x-(KF)x (x = 0 to 0.333) were investigated by using neutron diffraction and impedance spectroscopic techniques. The maximum solid solubility of KF in the fluorite-structured β-phase was found to be x = 0.013. The introduction of K+, and associated charge-compensating anion vacancies, had a marked effect upon the room-temperature ionic conductivity; which was increased by a factor of 5.6 x 103 for an x = 0.01 sample at 350K. At higher dopant levels, the room-temperature conductivity decreased steadily for x-values ranging from 0.05 to 0.333. This was attributed to the presence of an increasing volume fraction of relatively less-conductive phases which seemed to include orthorhombic α-PbF2 and possibly PbKF3. The ionic conductivity of these samples exhibited a sharp increase at 520K, as the 2 minority phases reacted to form a single superionic phase. Within this high-temperature modification, the anions were dynamically disordered. This disordering occurred mainly over the tetrahedral interstices and, to a lesser extent, over the octahedral interstices that were created by a body-centered cubic cation sub-lattice made up of Pb2+ and K+. This was an example of an anion-conducting body-centered cubic superionic phase, and its structure was opposite to that adopted by the archetypal superionic phase, α-AgI. A model was proposed which described the structural relationship, between these 2 opposite types, in terms of the short-range order between mobile ions within the body-centered cubic (Pb1-xKx)F2-x phase.

Crystal Structure and Superionic Conductivity of PbF2-Doped with KF. S.Hull, P.Berastegui, S.G.Eriksson, N.J.G.Gardner: Journal of Physics - Condensed Matter, 1998, 10[38], 8429-46