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

The ionic conductivity and crystal structure of PbF2 doped with high concen trations of KF, RbF and CsF have been investigated at elevated temperatures using complex-impedance spectroscopy and powder neutron diffraction. In th e case of (PbF2)(1-x)-(KF)(x), a high-temperature superionic phase with a b ody-centred-cubic cation sublattice is observed for 0.333 less than or equa l to x less than or similar to 0.68, in which the anions predominantly occu py the tetrahedral interstices. The corresponding phase in the (PbF2)(1-x)( RbF)(x) system is observed for 0.333 less than or equal to x less than or e qual to 0.500, though there is a gradual tendency towards cation ordering o ver the 0, 0, 0 and 1/2, 1/2, 1/2 sites as x increases. This process is acc ompanied 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 parti ally ordered perovskite-type arrangement. In the (PbF2)(1-x)-(CsF)(x) syste m only a single phase at a composition x = 0.500 is observed. This phase ad opts a fully ordered perovskite structure and shows no evidence of superion ic 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 relati on to the changes in the measured ionic conductivity with temperature and d opant concentration. The implications of these results for the wider questi on of possible superionic behaviour within halide perovskites is also discu ssed.