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Electrical conduction properties of Sr-doped LaPO 4 and Sr-doped CePO 4 under H 2 O/O 2 and H 2 O/H 2 conditions were investigated with conductivity measurements. Conductivities of 1 mol % Sr-doped LaPO 4 were 10 −5.2 -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 CePO 4 , 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 LaPO 4 under the same conditions. Such high conductivities of Sr-doped CePO 4 seemed attributable to electronic conduction due to partial oxidation of Ce 3+ to Ce 4+ caused by substituting Sr 2+ for Ce 3+ . Under wet reducing conditions, however, conductivities of the material decreased to 10 −5.2 -10 −3.4 S cm −1 at 500-925°C, which was almost comparable with those of 1 mol % Sr-doped LaPO 4 . It was concluded that Sr-doped CePO 4 showed mixed protonic and p-type electronic conductions under wet reducing conditions. Based on the results of conductivity measurements, defect structures in Sr-doped LaPO 4 and CePO 4 under wet oxidizing and reducing conditions are discussed.

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