Anisotropy of the ionic conductivity and dielectric response in orthorhombic and monoclinic inorganic fluorides

The anisotropy of the ionic conductivity and permittivity of (1) BaR₂F₈ (R=rare earth element) single crystals with monoclinic BaTm₂F₈ structure and of (2) (β-YF₃ structured) rare earth trifluorides is studied. Single crystals, eutectic composites and ceramics are investigated in a broad temperature range. In the monoclinic BaR₂F₈ crystals, a pronounced anisotropy of the ionic conductivity and a slight anisotropy of the permittivity are found. The fastest ionic transport with the lowest activation energy (0.563 eV) and the highest value of the permittivity are observed along thea axis. The temperature dependencies of the fluoride ion conductivities of various orthorhombic rare earth trifluorides differ only slightly from one another. For the bulky single crystals, the conductivity at 500 °C and the conduction activation enthalpy are equal to 1.1(4)×10⁻⁵ S/cm and 0.75(3) eV, respectively. The ionic conductivity is almost isotropic, but the anisotropy of the permittivity is significant. For the fluorides of both structural types, plausible conduction mechanisms are proposed, networks of most probable conduction paths are presented and the origin of the observed anisotropy of the ionic conductivity is elucidated. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997