Electrical conduction in the monazite-type LaPO4 doped with 1mol% of alkaline earth metals (Mg, Ca, Sr and Ba) was investigated by using conductivity measurements under 0.4–5kPa of PH2O and 0.01–100kPa of PO2 at 500 to 925C. 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 exhibited 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 PH2O and PO2 dependencies of the conductivities of the Ca-, Sr-, and Ba-doped LaPO4 were analyzed in terms of defect equilibria.

High Temperature Protonic Conduction in LaPO4 Doped with Alkaline Earth Metals. K.Amezawa, Y.Tomii, N.Yamamoto: Solid State Ionics, 2005, 176[1-2], 135-41