High temperature protonic conduction in LaPO₄ doped with alkaline earth metals

Electrical conduction in the monazite-type LaPO₄ doped with 1 mol% of alkaline earth metals (Mg, Ca, Sr, and Ba) was investigated by using conductivity measurements under 0.4-5 kPa of p(H₂ O) and 0.01-100 kPa of p(O₂) at 500-925°C. The Sr-doped LaPO₄ exhibited highest conductivities among the four samples. Conductivities of the Ca-doped LaPO₄ were in same levels as those of the Sr-doped sample, but the Ba-doped LaPO₄ showed conductivities in almost half magnitude compared with the Sr- and Ca-doped samples. Conductivities of the Mg-doped LaPO₄ were much lower than those of the other doped samples, and were comparable with those of the undoped LaPO₄. From humidity dependence and H/D isotope effect of the conductivity, it was found that the Ca-, Sr-, and Ba-doped LaPO₄ conducted protons dominantly under moisturized conditions, whereas the Mg-doped LaPO₄ did not show significant protonic conduction. The differences in conduction properties depending on the doped alkaline earth metals could be basically interpreted by the differences in the solubility of the alkaline earth metals into LaPO₄. The p(H ₂O) and p(O₂) dependencies of the conductivities of the Ca-, Sr-, and Ba-doped LaPO₄ were finally discussed in terms of defect equilibria.