Oxide-Ion Conduction in the Dion–Jacobson-Type Layered Perovskites

Global environmental issues, energy issues, and resources are the biggest challenges for our society. Durable, clean, and renewable energy devices are essential to global economic growth and human development, which presents a huge challenge. This challenge is made even more difficult due to the environmental threat of global warming, which is mostly caused by the carbon dioxide (CO₂) emissions produced from the primary energy sources, such as fossil fuels.

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.

As introduced in the Chap. 2, Dion–Jacobson-type layered perovskites have attracted much attention because of their novel physical and chemical properties such as ferroelectricity, photocatalyst, magnetism, and electrical conduction. The research in the caption presents a series of Dion–Jacobson phases of oxide-ion conductors. Dion–Jacobson phase CsLa₂Ti₂NbO₁₀₋δ and its related compounds of CsLn₂Ti₂NbO₁₀ (Ln = La, Pr, Nd, Sm, Gd, Ho, and Yb) and CsLa_2–xM_xTi_2+yNb_1–yO_{10–x/2–y/2} (M = Ca, Sr, and Ba; x = 0, 0.1; y = 0.1, 0, and –0.1) were synthesized and some of the compositions’ electrical conductivity and crystal structure were investigated from room temperature to high temperature. Unlike the CsBi₂Ti₂NbO_10–δ discussed in the Chap. 2, Rietveld's analysis of high-temperature neutron and synchrotron X-ray diffraction data revealed no phase transition from room temperature to 800 °C in the mother phase of CsLa₂Ti₂NbO_10−δ, exhibiting good practical realizations and applications. Bond-valence-based energy landscapes of a test oxide ion show two-dimensional oxide-ion diffusion along the edges of an octahedron in the perovskite layers. The larger-sized Cs in CsLa₂Ti₂NbO_10–δ make the larger bottlenecks for oxide-ion migration, which leads to the lower activation energy for the oxide-ion conduction. The present finding of oxide-ion conductivities in the Dion–Jacobson phase CsLa₂Ti₂NbO_10–δ indicates that the wide possibility of Dion–Jacobson phase as oxide-ion conductors would open up new horizons in the discovery of other novel oxide-ion conductors, which based on the reported Dion–Jacobson phases.

Parts of this paper have been published in the following journal articles.

Zhang, W., Fujii, K., Ishiyama, T., Kandabashi, H., & Yashima, M. (2020). Dion–Jacobson-type oxide-ion conductor CsLa 2 Ti 2 NbO 10 − δ without phase transitions. Journal of Materials Chemistry A, 8(47), 25,085–25,093.

Oxide-ion conductors exhibit many applications in various electrochemical devices such as solid-oxide fuel cells, gas sensors, and separation membranes. And the oxide-ion conductivity of a material is strongly dependent on its crystal structure. Therefore, it is important to discover new structural families of oxide-ion conductors.