High-temperature protonic conduction in La₇P₃O ₁₈

Electrical conduction in La₇P₃O₁₈ was investigated by using conductivity measurements under 0.4-5 kPa of p(H ₂O), and 0.01-100 kPa of p(O₂) at 500-930°C. The conductivity of La₇P₃O₁₈ was considerably enhanced by Sr substitution for La. The Sr-doped La₇P ₃O₁₈ had higher conductivities in wet than dry conditions, and exhibited the apparent H/D isotope effect on conductivity. From these results, it was found that La₇P₃O₁₈ conducted protons dominantly by substituting Sr for La and that conducting protons were introduced through equilibrium between the material and the water vapor in ambient atmosphere. Results of conductivity measurements under various p(H ₂O) and p(O₂) suggested that p-type electronic conduction became significant in addition to protonic conduction with increasing temperature, increasing p(O₂) and decreasing p(H₂O). The conductivity behavior of Sr-doped La₇P₃O₁₈ was rather similar to that of acceptor-doped LaPO₄, although the conductivity of Sr-doped La₇P₃O₁₈ was lower than that of acceptor-doped LaPO₄.