Ga-doped Ca 12 Al 14 O 33 mayenite oxide ion conductors: synthesis, defects, and electrical properties

Abstract

Although mayenite Ca 12 Al 14 O 33 has been known as an oxygen ion conductor for several decades, its relatively low oxide ion conductivity limits its applications in electrochemical devices. Thus, many efforts have been made by researchers, employing a doping strategy, in order to further improve its ionic conductivity, but with little success. In this work, a series of pure phase Ca 12 Al 14−x Ga x O 33+δ (0 ≤ x ≤ 1.2) materials were synthesized by a traditional solid state reaction method. Scanning electron microscopy (SEM) combined with energy dispersion spectrum (EDS) analyses disclosed well-sintered ceramics with uniform Ga distributions. The defect formation energies for Ga replacing the two distinguishable Al1 and Al2 sites in Ca 12 Al 14 O 33 calculated by static lattice atomistic simulation are nearly identical, ∼3.03 and ∼3.04 eV, respectively, consistent with the results of Rietveld refinements based on the XRD data, from which no preferred distribution of Ga on Al1 or Al2 site was observed. The electrical properties investigated by alternating current (AC) impedance spectroscopy show increased bulk conductivities for 0 ≤ x ≤ 0.4. Thus, here we present the first work that successfully improves the bulk oxide ion conductivity of Ca 12 Al 14 O 33 by Ga-doping.