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 α-Al2O3 + Na-β-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 β-phase have been quantified. Upon variation of the Na2O content of the phase mixture, the Na2O activity resulting from the α/β-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-β-Al2O3 is advanced. The findings are the prerequisite for putting a straightforward solid-state CO2 sensor and sodium ion battery into practice.