X-ray and Infrared Studies of the B-Site Effect in La_0.9Sr_0.1<i>M</i>O_3-δ (M = Al, Ga, In, and Yb) on Surface Chemistry

This study observes the interfacial dehydration and rehydration of La0.9Sr0.1MO3-δ (M = Al, Ga, In, and Yb) using X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FTIR) spectroscopy. The goal is to understand the hydration behavior at the gas/solid interface of La-based perovskites, functioning as oxygen-ion conductors (La0.9Sr0.1MO3-δ (M = Al and Ga)) and proton conductors (La0.9Sr0.1MO3-δ (M = In and Yb)). After dehydration, rehydration is performed under a N2 atmosphere to observe whether the ionic conductivity has increased and to elucidate the relationship between the conductivity and hydration reaction at the gas/solid interface. The results indicate that the ionic conductivity of La0.9Sr0.1MO3-δ (M = Al and Ga) does not show significant changes under humidification at 600 °C. However, the ionic conductivity of La0.9Sr0.1MO3-δ (M = In and Yb) increases and plateaus under humidification, after which no further increase occurs within 10 h. The XPS O 1s results indicate that dehydration reduces the O1– fraction in all materials. However, in La0.9Sr0.1MO3-δ (M = In and Yb), the O1– fraction increases after humidification owing to hydration reactions. This indicates that the pronounced hydration reactions at the interface significantly increase the ionic conductivity of the proton conductors. In La0.9Sr0.1AlO3-δ, the O1– fraction increases only slightly after humidification, which is consistent with the conductivity results. The FTIR OH– results also show a trend similar to that observed in the XPS O1– results. The results of this study indicate that the interfaces of proton conductors facilitate hydration reactions, thereby increasing the concentration of protons and improving the conductivity of the materials. However, although protons are observed in oxygen-ion conductors, their lattice sizes are unsuitable for proton hopping, and their interface is unfavorable for hydration reactions. Therefore, their ionic conductivity exhibits only slight changes.