High temperature protonic conduction in Sr-doped LaP3O9

Electrical conduction of Sr-doped LaP3O9 ([Sr]/{[La] + [Sr]} = 2–10 mol%) was investigated under 0.4–5 kPa of p(H2O) and 0.01–100 kPa of p(O2) or 0.3–3 kPa of p(H2) at 573–973 K. Sr-doped LaP3O9 showed apparent H/D isotope effect on conductivity regardless of the Sr-doping level under both H2O/O2 oxidizing and H2/H2O reducing conditions at investigated temperatures. Conductivities of the material were almost independent of p(O2) and p(H2O). These results demonstrated that the Sr-doped LaP3O9 exhibited protonic conduction under wide ranges of p(O2), p(H2O) and temperature. The conductivity of the Sr-doped LaP3O9 increased with increasing Sr concentration up to its solubility limit, ca. 3 mol%, while the further Sr-doping slightly degraded the conductivity. These indicate that Sr2+ substitution for La3+ leads to proton dissolution into the material and induced protonic conduction. Conductivities of the 3 mol% Sr-doped sample were 2 × 10- 6–5 × 10− 4 S cm− 1 at 573–973 K.