2. New Oxide-Ion Conductors of Dion–Jacobson-Type Layered Perovskites CsBi₂Ti₂NbO_10-δ

Oxide-ion conductors exhibit many important applications in many electrochemical devices, for example, solid-oxide fuel cells (SOFCs), solid-oxide electrolysis cell (SOECs), oxygen gas sensors, and oxygen separation membranes. Dion–Jacobson phases have attracted attention because of their wide chemical and physical performances, however, up to now, there are no reports of oxide-ion conductors with the Dion–Jacobson structure. Here, we have reported the new oxide-ion conductors of Dion–Jacobson-type layered perovskites for the first time. The oxide-ion migration energy barriers, E_b, of 69 Dion–Jacobson phases were calculated using 83 Dion–Jacobson-type crystal data, in order to find new oxide-ion conductors in the Dion–Jacobson phases. CsBi₂Ti₂NbO_10−δ was chosen because of its relatively low E_b. The bulk conductivity of CsBi₂Ti₂NbO_10−δ (δ is the oxygen vacancy content) was 8.9 × 10⁻² S cm⁻¹ at 1073 K, which was higher than that of the conventional yttria-stabilized zirconia (YSZ). There is a jump in conductivity between 823 and 873 K, which may can be attributed to the high level of oxygen vacancy concentration and the orthorhombic-to-tetragonal phase transition. In order to improve its conductivity, doping elements in the A (Bi site) and B sites (Ti/Nb site) were investigated. The CsBi_2−xM_xTi_2+yNb_1–yO_{10−x/2–y/2-δ} (M = alkaline earth metal; x = 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6; y = 0.2, 0.1, −0.1, and −0.2) compounds were synthesized and the electrical properties were investigated. The high conductivity of Dion–Jacobson-type layered perovskites CsBi₂Ti₂NbO_10-δ can be attributable to the large anisotropic thermal motions of oxygen atoms (especially O1 and O2), the existence of carrier (oxygen vacancy in high-temperature ranges) and the formation of an oxide-ion conducing layer with large bottleneck sizes (inner perovskite layers). The present finding of high oxide-ion conductivities in the Dion–Jacobson-type layered perovskites CsBi₂Ti₂NbO_10−δ would open new avenues in the design and discovery of other novel oxide-ion conductors. Parts of this paper have been published in the following journal articles. Zhang, W., Fujii, K., Niwa, E., Hagihala, M., Kamiyama, T., & Yashima, M. (2020). Oxide-ion conduction in the Dion–Jacobson phase CsBi₂Ti₂NbO_10−δ.Nature communications, 11(1), 1–8.