Superionic phases in the (PbF2)1-x-(MF)x, M = K, Rb and Cs, systems

Abstract

The ionic conductivity and crystal structure of PbF2 doped with high concentrations of KF, RbF and CsF have been investigated at elevated 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-centred-cubic cation sublattice is observed for 0.333 x0.68, in which the anions predominantly occupy the tetrahedral interstices. The corresponding phase in the (PbF2)1-x(RbF)x system is observed for 0.333 x0.500, though there is a gradual tendency towards cation ordering over the 0,0,0 and 1/2,1/2,1/2 sites as x increases. This process is accompanied by a change in the preferred anion sites in favour of a subset of the octahedral cavities, such that the mean structure tends towards a partially ordered perovskite-type arrangement. In the (PbF2)1-x-(CsF)x system only a single phase at a composition x = 0.500 is observed. This phase adopts a fully ordered perovskite structure and shows no evidence of superionic behaviour up to its melting point. The structural behaviour of the three (PbF2)1-x(MF)x systems with M = K+, Rb+ and Cs+ is discussed in relation to the changes in the measured ionic conductivity with temperature and dopant concentration. The implications of these results for the wider question of possible superionic behaviour within halide perovskites is also discussed.