The conductivity of polycrystalline samples of (PbF2)1-x(KF)x, where x ranged from 0 to 0.333, was investigated by using neutron diffraction and impedance spectroscopic techniques. The maximum solid solubility of KF in the fluorite-structured β-PbF2 was found to occur at x = 0.014. However, the introduction of K+ and the associated charge-compensating anion vacancies, had a marked effect upon the room-temperature ionic conductivity; which increased by a factor of 5620 in the case of an x = 0.01 sample at 350K. At higher dopant levels, the room-temperature conductivity fell steadily for x-values ranging from 0.05 to 0.333; due to the presence of an increasing volume fraction of relatively poorly conducting additional phases. These appeared to include orthorhombic α-PbF2, and another phase with the probable composition, PbKF3. However, the ionic conductivity of these samples exhibited an abrupt 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 over the tetrahedral and, to a lesser extent, the octahedral interstices which were created by a body-centered cubic cation sub-lattice of Pb2+ and K+. This phase was an example of an anion-conducting body-centered cubic superionic material, and its structure was opposite to that adopted by the archetypal superionic phase, α-AgI.

S.Hull, P.Berastegui, S.G.Eriksson, N.J.G.Gardner: Journal of Physics - Condensed Matter, 1998, 10[38], 8429-46