Enhanced Low-Temperature Proton Conduction in Sr_0.02La_0.98NbO_4−δ by Scheelite Phase Retention

The high-temperature scheelite phase of Sr 0.02 La 0.98 NbO 4 materials has been retained to room temperature via vanadium substitution for the Sr 0.02 La 0.98 Nb 1− x V x O 4−δ compositional range of 0.25 ≤ x ≤ 0.325. Such structural stabilization avoids the characteristic break in the thermal expansion coefficient (TEC) that may otherwise be deleterious for the application of LaNbO 4 -based materials as proton-conducting electrolytes. The transport properties of composition Sr 0.02 La 0.98 Nb 0.7 V 0.3 O 4−δ have been extensively characterized. The composition offers pure proton conduction with higher conductivity than the base composition Sr 0.02 La 0.98 NbO 4−δ in the low-temperature range under wet oxidizing conditions. A wide ionic domain is observed, which increases as the temperature decreases. Suggested operational limits have been documented. Under dry oxidizing conditions, vanadium substitution is shown to suppress p -type conductivity, in comparison to the base composition Sr 0.02 La 0.98 NbO 4−δ . In contrast, under wet conditions, Sr 0.02 La 0.98 Nb 0.7 V 0.3 O 4−δ is observed to be a pure proton conductor in oxygen and argon atmospheres at temperatures lower than ∼700 °C. The total ionic conductivity equals that of the proton conductivity, within experimental error, suggesting negligible oxide-ion conduction in this material under these conditions.