Thermodynamic Equilibrium between Non-Stoichiometric Na-β-Alumina and α-Alumina

A solid-state electrochemical technique based on a potentiometric oxygen concentration cell has been used to characterize the thermodynamic stability of the phase mixture alpha-Al2O3 + Na-beta-Al2O3 by determining its Na2O activity. In combination with phase analysis based on Rietveld refinement of X-ray diffraction patterns the magnitude and variability of the stoichiometric composition of the beta-phase have been quantified. Upon variation of the Na2O content of the phase mixture, the Na2O activity resulting from the alpha/beta-equilibrium has proved to be stable because both phases are readily inter-convertible. This behaviour guarantees that the material is eminently suitable as a stable sodium electrode, while simultaneously functioning as a sodium-ion conducting solid electrolyte. The temperature dependence of the logarithm of the Na2O activity has been found to be two-part. This is in line with the trend of the majority of critically assessed literature data thus demonstrating that former apprehension about the prevalence of a possible methodical flaw due to electronic conduction was unsubstantiated. As a conclusion, the knowledge about electronic conduction through Na-beta-Al2O3 is advanced. The findings are the prerequisite for putting a straightforward solid-state CO2 sensor and sodium ion battery into practice. (c) 2024 The Electrochemical Society ("ECS"). Published on behalf of ECS by IOP Publishing Limited. All rights, including fortext and data mining, AI training, and similar technologies, are reserved.