Proton solubility for La2Zr2O7 with a pyrochlore structure doped with a series of alkaline-earth ions

Alkaline-earth ion doped La2Zr2O7 samples with a pyrochlore structure, i.e., (La2−xMx)Zr2O7−δ and La2(Zr2−xMx)O7−δ (M=Mg, Ca, Sr and Ba), were prepared by conventional ceramic methods. The quantity of protons dissolved in the crystal was evaluated by evolved H2O gas, which was detected by a quadrupole mass spectrometer. The relationship between the proton solubility of the La2Zr2O7-based high temperature proton conductor and the basicity around the dopant, which is referred to as local basicity in the present paper, was studied. Proton solubility normalized by the positive charge introduced by doping was adopted to allow comparison of the proton solubility among the samples with different doping levels. Because the basicity of the alkaline-earth oxide is well-known to be ranked as MgO<CaO<SrO<BaO, it may be inferred that the local basicity of the heavier alkaline-earth dopants is higher than that of the lighter alkaline-earth dopants. The normalized proton solubility obtained was slightly dependent on the dopant alkaline-earth species in both the (La2−xMx)Zr2O7−δ and La2(Zr2−xMx)O7−δ systems, i.e., the normalized proton solubility was slightly large for the heavy alkaline-earth dopant. The proton solubility of the (La2−xMx)Zr2O7−δ system was approximately three times higher than that of the La2(Zr2−xMx)O7−δ system. This observation was attributed to existence of two different oxygen sites with different basicity in the pyrochlore crystal, i.e., the higher basicity of oxygen at 8b-site in Fd3m than oxygen at 48f-site.