Increase in the Fluorine-Ion Conductivity of Single Crystals of Tysonite-type CeF3 Superionic Conductor by Substituting Polarized Cd2+ Ions for Ce3+ Ions

The fluorine-ion conductivity of single crystals with a tysonite (LaF 3 ) structure with heterovalent isomorphic substitutions of highly polarizable Cd 2+ cations with a 18-electron shell for rare earth ions Ce 3+ have been studied for the first time. Ce 0.995 Cd 0.005 F 2.995 single crystals have been grown from melt by the Bridgman technique in a fluorinating atmosphere. The fluorine-ion conductivity of single crystal is measured by impedance spectroscopy in the temperature range from 153 to 1073 K, where it increases by a factor of 10 9 , approaching the value σ dc = 5 × 10 –2 S/cm at 1073 K. At T 0 = 450 ± 20 K, the dependence σ dc ( T ) is split into two portions with the ion-transport activation enthalpy Δ H σ = 0.39 ± 0.01 eV ( T < T 0 ) and Δ H σ = 0.23 ± 0.02 eV ( T > T 0). It is found that at T = 293 K the conductivity σ dc = 3 × 10 –5 S/cm of Ce 0.995 Cd 0.005 F 2.995 crystal is higher by a factor of 10 than the conductivity of the tysonite matrix CeF 3 and close to the σ dc value for Ce 0.995 Sr 0.005 F 2.995 crystal. This finding indicates a significant effect of the substitutions of Cd 2+ ions for Ce 3+ on the σdc value and the advantage of Cd 2+ ions over Ca 2+ and Ba 2+ from the viewpoint of increasing σ dc .