Mixed Conduction in A-Site Double-Perovskite Na1+xLa1-xZr2O6-δ Proton Conductors

Perovskite-type proton conductors exist in two structural forms, ABO₃ and A2B'B″O6. In this study, novel A-site double-perovskite proton conductors (A'A″B2O6) were proposed. Na_1+xLa_1-xZr₂O_6-δ (x = 0, 0.1, 0.2) perovskites were prepared by a solid-state reaction at 1200 °C. However, raising the sintering temperature to 1300 °C resulted in the Na to volatilize, converting the Na_1.1La_0.9Zr₂O_6-δ into La_0.9Zr₂O_6-δ. The conductivities of these materials in a humid atmosphere were tested using electrochemical impedance spectroscopy, and their carrier transport numbers were measured using the defect equilibria model and concentration cell method. Na_1.1La_0.9Zr₂O_6-δ and Na_1.2La_0.8Zr₂O_6-δ are predominantly proton conductors, with Na_1.1La_0.9Zr₂O_6-δ exhibiting the highest proton transport number of 0.52 at 800 °C. In contrast, NaLaZr₂O₆ is predominantly an electronic conductor, while La_0.9Zr₂O_6-δ functions as an oxide ion conductor. Due to their high protonic transport numbers, these Na_1+xLa_1-xZr₂O_6-δ A-site double-perovskite oxides present a promising avenue for the development of proton conductors.