Electrical conductivity of Ln6–xZrxMoO12 + δ (Ln = La, Nd, Sm; x = 0.2, 0.6) ceramics during thermal cycling

Abstract

In this paper, we analyze the relationship between the microstructure of new polycrystalline electron–proton conductors, Ln_6–xZr_xMoO_{12 + δ} (Ln = La, Nd, Sm; x = 0.2, 0.6), and the reduction and hydration processes in these materials in humid atmospheres (air and argon). The La_5.8Zr_0.2MoO_12.1 solid solution with a rhombohedral structure possesses not only the highest electrical conductivity among the materials studied here but also high stability in various dry and humid, oxidizing (air) and reducing atmospheres. La_5.8Zr_0.2MoO_12.1 ceramic grains have a twin microstructure, and the conductivity of this material along the grain boundaries, consisting of ordered domains, differs little from its bulk conductivity. It seems likely that we observe a “domain wall” effect, typical of La_0.95Sr_0.05Ga_0.9Mg_0.1O_3–δ (LSGM) oxygen ion conductors [1]. In studies of Ln_6–xZr_xMoO_{12 + δ} (Ln = Nd, Sm; x = 0.2, 0.6) ceramics in humid atmospheres, we detected a grain-boundary contribution, which limited the total conductivity, like in perovskite BaZr_0.8Y_0.2O_3–δ. We believe that such conditions lead to a reduction process in these materials and that Mo⁶⁺ is reduced before Nd³⁺ and Sm³⁺. The process first occurs on grain boundaries.