Crystal structure and superionic conductivity of PbF2 doped with KF

The crystal structure and ionic conductivity of polycrystalline samples of (PbF2)(1-x)-(KF)(x) with 0 less than or equal to x less than or equal to 0.333 are investigated using neutron diffraction and impedance spectroscopy techniques. The maximum solid solubility of KF in the fluorite structured beta-PbF2 is found to be x = 0.013(6). However, the introduction of KC and the associated charge compensating anion vacancies has a dramatic effect on the ambient temperature ionic conductivity, which increases by a factor 5.6(2) x 10(3) for an x = 0.01 sample at 350 K. At higher dopant levels the ambient temperature conductivity falls steadily in the range 0.05 less than or equal to x less than or equal to 0.333 due to the presence of an increasing volume fraction of relatively poorly conducting additional phases. apparently comprising orthorhombic alpha-PbF2 and a further phase of probable composition PbKF3. However, the ionic conductivity of these samples show an abrupt increase at T = 520(5) K, as the two minority phases react to form a single, superionic phase, Within this high-temperature modification the anions are dynamically disordered over the tetrahedral and, to a lesser extent, the octahedral interstices created by a body centred cubic (bcc) cation sublattice formed by Pb2+ and K+. This phase is, therefore, an example of an anion conducting bcc superionic and its structure is 'anti-' to that adopted by the archetypal superionic phase alpha-AgI. A plausible model describing the structural relationship between these two antitypes, including the short-range order between mobile ions within the bcc-(Pb1-xKx)F2-x phase, is given.