Electrical conduction properties of Sr-doped LaPO₄ and CePO ₄ under oxidizing and Reducing conditions

Electrical conduction properties of Sr-doped LaPO₄ and Sr-doped CePO₄ under H₂O/O₂ and H₂O/H ₂ conditions were investigated with conductivity measurements. Conductivities of 1 mol % Sr-doped LaPO₄ were 10^.2 - 10^3.5 S cm^-1 at 500-925°C under wet reducing conditions and were close to those under wet oxidizing conditions. It was found from the H/D isotope effect on conductivity that the material showed dominant protonic conduction under wet reducing conditions. As for 1 mol % Sr-doped CePO₄, conductivities were 10^2.8-10^2.0 S cm^-1 at 500-925°C under wet oxidizing conditions and much higher than those of 1 mol % Sr-doped LaPO₄ under the same conditions. Such high conductivities of Sr-doped CePO₄ seemed attributable to electronic conduction due to partial oxidation of Ce³⁺ to Ce ⁴⁺ caused by substituting Sr²⁺ for Ce³⁺. Under wet reducing conditions, however, conductivities of the material decreased to 10^-5.2-10^-3.43.4 S cm^-1 at 500-925°C, which was almost comparable with those of 1 mol % Sr-doped LaPO₄. It was concluded that Sr-doped CePO₄ showed mixed protonic and p-type electronic conductions under wet reducing conditions. Based on the results of conductivity measurements, defect structures in Sr-doped LaPO₄ and CePO₄ under wet oxidizing and reducing conditions are discussed.