Thermally Induced Oxygen Vacancies and High Oxide Ion Conduction in K₂ZnV₂O₇ with a Melilite-Related Structure

Donor-doped melilite materials with interstitial oxygen defects in the structure are good oxide ion conductors with negligible electronic conduction and show great potential in the ceramic electrolyte of intermediate-temperature solid oxide fuel cells (IT-SOFC). However, the parent melilite-structured materials with stoichiometric oxygen are usually insulators. Herein, we reported high and pure oxide ion conduction in the parent K₂ZnV₂O₇ material with a melilite-related structure, e.g., ∼1.14 × 10^-3 S/cm at 600 °C, which is comparable to that of the state-of-the-art yttrial-stabilized ZrO₂ applied in practical fuel cells. Neutron diffraction data revealed the interesting thermally induced formation of oxygen vacancies at elevated temperatures, which triggered the transformation of the material from electronically conducting to purely and highly oxide ion-conducting. The VO₄ tetrahedron with non-bridging terminal oxygen in K₂ZnV₂O₇ was proved to be the key structural factor for transporting oxygen vacancies. The molecular dynamic simulation based on the interatomic potential approach revealed that long-range oxide ion diffusion was achieved by breaking and re-forming the 5-fold MO₄ (M = Zn and V) tetrahedral rings. These findings enriched our knowledge of melilite and melilite-related materials, and creating oxygen vacancies in a melilite-related material may be a new strategy for developing novel oxide ion conductors.