High temperature protonic conduction in LaPO 4 doped with alkaline earth metals

Abstract

Electrical conduction in the monazite-type LaPO 4 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(H 2O) and 0.01–100 kPa of p(O 2) at 500–925 °C. The Sr-doped LaPO 4 exhibited highest conductivities among the four samples. Conductivities of the Ca-doped LaPO 4 were in same levels as those of the Sr-doped sample, but the Ba-doped LaPO 4 showed conductivities in almost half magnitude compared with the Sr- and Ca-doped samples. Conductivities of the Mg-doped LaPO 4 were much lower than those of the other doped samples, and were comparable with those of the undoped LaPO 4. From humidity dependence and H/D isotope effect of the conductivity, it was found that the Ca-, Sr-, and Ba-doped LaPO 4 conducted protons dominantly under moisturized conditions, whereas the Mg-doped LaPO 4 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 LaPO 4. The p(H 2O) and p(O 2) dependencies of the conductivities of the Ca-, Sr-, and Ba-doped LaPO 4 were finally discussed in terms of defect equilibria.