The origin of triple conductivity and water uptake in layered double perovskites: A case study on lanthanum-substituted GdBaCo2O6−δ

Abstract

Some layered double perovskite cobaltites have been shown recently to absorb water and exhibit increasing electrical conductivity in humid atmospheres. However, the assumptions that their crystal lattice is capable of proton uptake, and that these oxides really possess triple (oxide ion, protonic and electronic) conductivity, have already been brought into question. We investigated in detail the crystal structure and phase composition of various lanthanum-substituted GdBaCo2O6−δ and came to several important conclusions. Firstly, in oxidative conditions (e.g. in air), the substitution of La for either only Gd or only Ba in GdBaCo2O6−δ results in formation of multiphase materials. For example, BaCo1–xGdxO3−δ exsolves from Gd1–xLaxBaCo2O6−δ due to the redistribution of La between Gd and Ba sites in Gd1–xLaxBaCo2O6−δ lattice. Secondly, a single-phase double perovskite can be synthesized in air only by simultaneously substituting, within certain limits, both Gd and Ba in GdBaCo2O6−δ with La. Finally, using Gd0.8La0.2Ba0.95La0.05Co2O6−δ and BaCo0.8Gd0.2O3−δ as examples, we demonstrated that while single-phase double perovskite does not hydrate, in the same conditions, the cubic perovskite BaCo0.8Gd0.2O3−δ (BaCo1–xGdxO3−δ is encountered in Gd1–xLaxBaCo2O6−δ as an impurity) absorbs significant amount of water. Thus, the water uptake by lanthanum-substituted GdBaCo2O6−δ is most likely to occur due to the impurities, and not the main double perovskite phase.