| dc.description |
The crystal structure and ionic conductivity of polycrystalline samples of with are investigated using neutron diffraction and impedance spectroscopy techniques. The maximum solid solubility of KF in the fluorite structured is found to be x = 0.013(6). However, the introduction of and the associated charge compensating anion vacancies has a dramatic effect on the ambient temperature ionic conductivity, which increases by a factor for an x = 0.01 sample at 350 K. At higher dopant levels the ambient temperature conductivity falls steadily in the range due to the presence of an increasing volume fraction of relatively poorly conducting additional phases, apparently comprising orthorhombic and a further phase of probable composition . 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 and . 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 . A plausible model describing the structural relationship between these two anti-types, including the short-range order between mobile ions within the bcc- phase, is given. |
