Dion–Jacobson-type oxide-ion conductor CsLa2Ti2NbO10−δ without phase transitions

Abstract

Oxide-ion conductors have received considerable attention owing to their wide applicability in many electrochemical devices such as solid-oxide fuel cells, oxygen gas sensors, and oxygen separation membranes. CsBi₂Ti₂NbO_10−δ (δ represents oxygen-vacancy content), a Dion–Jacobson-type layered perovskite, exhibits higher oxide-ion conductivity than the conventional yttria-stabilized zirconia. However, CsBi₂Ti₂NbO_10−δ undergoes a phase transition around 800 K, which may make it unsuitable for practical applications. In this study, we present a Dion–Jacobson-type oxide-ion conductor CsLa₂Ti₂NbO_10−δ that does not show phase transitions in the temperature range from 297 K to 1073 K. CsR₂Ti₂NbO_10−δ (R = La, Pr, Nd and Sm) oxides were synthesized. CsLa₂Ti₂NbO_10−δ shows the highest total electrical conductivity (σ_tot) among them. σ_tot of CsLa₂Ti₂NbO_10−δ is almost constant from 10⁻²² to 1 atm oxygen partial pressure at 873 K, supporting oxide-ion conduction. Proton conductivity was not observed between 673 K and 1273 K because the σ_tot measured in dry air agreed well with that measured in wet air at the same temperature. Electromotive force measurements by an oxygen concentration cell show an oxide-ion transport number of 0.99 in CsLa₂Ti₂NbO_10−δ at 873 K, which indicates that the major charge carrier in CsLa₂Ti₂NbO_10−δ is the oxide ion. The electrical properties were examined by an AC impedance method, showing a bulk ionic conductivity of 1.01 × 10⁻³ S cm⁻¹ at 1276 K with an activation energy of 0.932(2) eV. Neutron-diffraction and synchrotron X-ray diffraction data were processed using Rietveld refinements and revealed no phase transitions from 297 K to 1073 K in CsLa₂Ti₂NbO_10−δ. Bond-valence-based energy landscapes of a test oxide ion in CsLa₂Ti₂NbO_10−δ strongly suggest that oxide ions diffuse along the edges of the octahedra in perovskite-like slabs, leading to two-dimensional (2D) oxide-ion diffusion. We attribute the oxide-ion conduction in CsLa₂Ti₂NbO_10−δ to the existence of oxygen vacancies, anisotropic thermal motions of oxygen atoms, and the 2D oxide-ion conducting slab.