Cobalt-doped Ca12Al14O33 mayenite oxide ion conductors: phases, defects, and electrical properties

Mayenite Ca12Al14O33, as a good oxygen ion conductor with conductivity slightly lower than stabilized ZrO2, has been investigated through doping strategy over the last few decades, but with little success in further improving its oxide ionic conductivity. Here, cobalt-doped Ca12Al14-xCoxO33+δ (0 ≤ x ≤ 1.6) materials were prepared by traditional solid-state reaction method, and then studied by complementary techniques, including X-ray diffraction (XRD), scanning electron microscope coupled with energy dispersion spectrum (EDS) analysis, X-ray photoelectron spectroscopy, and static lattice atomistic simulations. The results showed that these doped materials had much lower Co contents in the crystal structure than their nominal compositions, which was consistent with the high calculated defect formation energy (~ 6.25 eV). The minor divalent Co ions in the crystal structure would reduce the amount of mobile oxide ions and accordingly slightly decreased the bulk conductivities, while most of the Co ions existed in the form of Co2O3 and segregated along grain boundaries in the ceramic samples, which could apparently increase the grain boundary conductions of Ca12Al14O33.