High temperature protonic conduction in Sr2P2O7–LaPO4 system

(x/2)Sr2P2O7–(1−x)LaPO4 (x=0–0.60) was prepared with a solid state reaction between LaPO4 and SrHPO4, and its electrical conduction was investigated by using conductivity measurements under 0.4–5 kPa of p(H2O) and 0.01–100 kPa of p(O2) at 500–930 °C. From humidity dependence and H/D isotope effect of the conductivity, it was found that (x/2)Sr2P2O7–(1−x)LaPO4 with x>0 conducted protons dominantly under moisturized conditions, whereas LaPO4 (x=0) did not exhibit protonic conduction. The conductivity of (x/2)Sr2P2O7–(1−x)LaPO4 showed a maximum when x=0.02–0.03, which was nearly equal to the solubility limit of Sr into LaPO4. These results suggested that substitution of Sr for La was essential to induce protonic conduction in Sr2P2O7–LaPO4 system and that existence of Sr2P2O7 as a secondary phase deteriorated the conductivity. Finally, p(H2O) and p(O2) dependencies of the conductivity were discussed in terms of defect equilibrium.