Defect Structure, Oxygen Ion Conduction, and Conducting\nMechanism in Ruddlesden–Popper Sr<sub>3</sub>Zr<sub>2–<i>x</i></sub>M<sub><i>x</i></sub>O<sub>7–0.5<i>x</i></sub

Ruddlesden–Popper\n(RP)-structured materials based on transition\nmetals with a variable valence, such as Fe, Mn, Ni, and so on, have\nbeen well documented for their potential of being used as electrodes\nin solid-oxide fuel cells. However, RP materials with pure or dominant\nionic conduction are rare. Here, a series of Zr-based RP materials\nSr<sub>3</sub>Zr<sub>2–<i>x</i></sub>M<sub><i>x</i></sub>O<sub>7–0.5<i>x</i></sub> (M = Ga,\nY, In) with electrical conductivity as high as 3.25 × 10<sup>–3</sup> S cm<sup>–1</sup> at 900 °C in air was\nreported, which represents the highest conductivity for the Zr-based\nRP materials and is comparable to that of the recently reported In-based\nRP oxide-ion conductors, such as NdBaInO<sub>4</sub>-based and La<sub>2</sub>BaIn<sub>2</sub>O<sub>7</sub>-based materials. Under low oxygen\npartial pressure (<i>p</i>O<sub>2</sub>), the doped samples\nshow pure ionic conducting behaviors without n-type electronic conductivity.\nThe defect formation energies, local structure around the oxygen vacancies,\nand oxide-ion-conducting mechanism of the acceptor-doped Sr<sub>3</sub>Zr<sub>2</sub>O<sub>7</sub>-based materials were studied for the\nfirst time. The results revealed a two-dimensional oxide ion migration\ncharacteristic within the perovskite slabs. This work therefore provides\na good reference for developing new oxide-ion conductors in the Zr-based\nRP-structured materials.