Defect Structure, Oxygen Ion Conduction, and Conducting Mechanism in Ruddlesden-Popper Sr₃Zr_2-MO_7-0.5 (M = Ga, Y, In)

Ruddlesden-Popper (RP)-structured materials based on transition metals with a variable valence, such as Fe, Mn, Ni, and so on, have been well documented for their potential of being used as electrodes in solid-oxide fuel cells. However, RP materials with pure or dominant ionic conduction are rare. Here, a series of Zr-based RP materials Sr₃Zr_2-MO_7-0.5 (M = Ga, Y, In) with electrical conductivity as high as 3.25 × 10^-3 S cm^-1 at 900 °C in air was reported, which represents the highest conductivity for the Zr-based RP materials and is comparable to that of the recently reported In-based RP oxide-ion conductors, such as NdBaInO₄-based and La₂BaIn₂O₇-based materials. Under low oxygen partial pressure (pO₂), the doped samples show pure ionic conducting behaviors without n-type electronic conductivity. The defect formation energies, local structure around the oxygen vacancies, and oxide-ion-conducting mechanism of the acceptor-doped Sr₃Zr₂O₇-based materials were studied for the first time. The results revealed a two-dimensional oxide ion migration characteristic within the perovskite slabs. This work therefore provides a good reference for developing new oxide-ion conductors in the Zr-based RP-structured materials.