IONIC-CONDUCTIVITY OF FLUORITE-STRUCTURED SOLID-SOLUTIONS OF COMPOSITION - MF2-UF4-CEF3 (M = CA, SR, BA)

The ionic conductivity of single crystals of the fluorite-structured solid solutions M1-x-yUxCeyF2x+y (M = Ca, Sr, Ba, and 0.027 < 2x + y < 0.265) where UF4 constitutes the predominant dopant has been studied as a function of temperature in the region 300°-550°K. The ionic conductivity of these solid solutions increases superlinearly upon doping, while simultaneously the conductivity activation enthalpy decreases. These results are in line with literature data on fluoride-conducting solid solutions based on CaF2 and ;3-PbF2 with trivalent cation dopants. The increase of the conductivity in all these solid solutions is accounted for by an increase of the mobility of the mobile species due to interactions with defect clusters which are present in this concentration regime. It is shown that lattice relaxations in close proximity to the clusters lead to a distribution of activation enthalpies for fluoride-ion motion via the interstitialcy mechanism. The shape of the distribution function and its dependence on the dopant concentration has been estimated on the basis of a simple model. The results are in line with the experimental data. In addition it is shown that the fluorite lattice with defect clusters can accommodate some maximum number of mobile interstitial fluoride ions. © 1979, The Electrochemical Society, Inc. All rights reserved.