Enhanced Low-Temperature Proton Conduction in Sr_(0.02)La_(0.98)NbO_(4-δ) by Scheelite Phase Retention

摘要

The high-temperature scheelite phase of Sr_(0.02)La_(0.98)NbO4 materials has been retained to room temperature via vanadium substitution for the Sr_(0.02)La_(0.98)Nb_(1-x)V_xO_(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 LaNbCvbased materials as proton-conducting electrolytes. The transport properties of composition Sr_(0.02)La_(0.98)NbO_(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 ℃. The total ionic conductivity equals that of the proton conductivity, within experimental error, suggesting negligible oxide-ion conduction in this material under these conditions.