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

• Ca has the highest solubility in La2Ce2O7 among alkaline earth elements. • Proton concentration increases with increasing doping level. • Doping alkaline earth elements enhances mobility and conductivity of electrons. La 2 Ce 2 O 7 is known to be a disordered fluorite, i.e., 50% La-doped CeO 2 , 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 A 2+ (A = Ca, Sr, Ba) on the hydration behavior and electrical conductivity of La 2 Ce 2 O 7. A single-phase cubic fluorite structure was obtained for La 2 Ce 2 O 7 , (La 1- x Ca x) 2 Ce 2 O 7-δ (x = 0–0.3), (La 1- x Sr x) 2 Ce 2 O 7-δ (x = 0–0.2), and (La 1- x Ba x) 2 Ce 2 O 7-δ (x = 0–0.05). Substitution by A 2+ enhanced the degree of hydration, which may be attributed to increased concentration of hydratable oxygen vacancies and coordination of oxide ions by A 2+ in addition to La3+, while the basicity of the different A 2+ 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. [ABSTRACT FROM AUTHOR]