Combined homo-hetero doping for enhancement of ionic conductivity

Electrical conductivity of Na_0.005Ca_0.995F_1.995 and Y_0.008Ca_0.992F_2.008 samples containing 2 mol% CeO_2−x as the dispersed phase has been measured in the range 630 to 1030 K using an ac bridge at 1 kHz. The presence of the dispersed phase enhanced the conductivity of CaF₂ homogeneously doped with NaF. However, heterogeneous doping with CeO_2−x decreased the conductivity of YF₃-doped CaF₂. This suggests that preferential adsorption of F− ions to the CeO_2−x interface and the consequent creation of a space charge region with increased fluorine vacancies near the interface is the primary mechanism responsible for the enhancement of the conductivity of CaF₂ heterogeneously doped with CeO_2−x. By the synergetic effect of homogeneous and heterogeneous doping, the conductivity of polycrystalline CaF₂ can be enhanced by 4.5 orders of magnitude at 630 K