La2Ce2O7 doped with alkaline earth elements: Phase behavior, hydration and electrical properties

Abstract

La2Ce2O7 is known to be a disordered fluorite, i.e., 50% La-doped CeO2, exhibiting oxide ion conduction with contributions from protons in wet atmospheres and minority electrons under reducing conditions. Here, we report the effect of additional doping by partial substitution of La3+ with divalent alkaline earth cations A2+ (A = Ca, Sr, Ba) on the hydration behavior and electrical conductivity of La2Ce2O7. A single-phase cubic fluorite structure was obtained for La2Ce2O7, (La1-xCax)2Ce2O7-δ (x = 0–0.3), (La1-xSrx)2Ce2O7-δ (x = 0–0.2), and (La1-xBax)2Ce2O7-δ (x = 0–0.05). Substitution by A2+ enhanced the degree of hydration, which may be attributed to increased concentration of hydratable oxygen vacancies and coordination of oxide ions by A2+ in addition to La3+, while the basicity of the different A2+ seemed unimportant. The doping increased ionic conductivity slightly, while the n-type electronic conductivity under reducing atmospheres increased considerably. XPS analyses indicated that the concentration and binding energy of Ce 3d electrons decreased with increasing doping level of Ca, implying an enhanced mobility of electrons which may be attributed to an observed decrease in lattice parameter.