Electrical conductivity of Ln6-x Zr x MoO12+δ (Ln = La, Nd, Sm; x=0.2, 0.6) ceramics during thermal cycling

In this paper, we analyze the relationship between the microstructure of new polycrystalline electron-proton conductors, Ln(6-x) Zr (x) MoO12 + delta (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 La5.8Zr0.2MoO12.1 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. La5.8Zr0.2MoO12.1 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 La0.95Sr0.05Ga0.9Mg0.1O3-delta (LSGM) oxygen ion conductors [1]. In studies of Ln(6-x) Zr (x) MoO12 + delta (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 BaZr0.8Y0.2O3-delta. 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.