Ionic Conductivity Enhancement of the Fluoride Conductor CaF2 by Grain Boundary Activation Using Lewis Acids

Following the concept of "heterogeneous doping," fine powder and films were treated with the Lewis acids and to create a strong driving force to draw F − ions out of the boundary regions according to The powder has been vacuum‐treated with and vapor and then processed to pellets. The conductivity results indicate a significant enhancement of the ionic conductivity with a slope that is characteristic for F − ‐vacancy migration. A simple homogeneous doping by Sb and B impurities would lead to a conductivity via interstitials and thus to a different activation energy. The enhancement should not be due to oxygen impurities introduced by the heat‐treatment: this is shown by preparation in different gas atmospheres. A quantitative estimation of the enhanced conductivity assuming space‐charge effects is presented. Conductivity results on thin films also confirmed the assumption of space‐charge regions as a consequence of the attractive forces of the adsorbed Lewis acid.