Interstitial Oxygen Induced K+ Ions’ Order-to-Disorder Transition and High Oxide Ion Conduction in K2ZnV2O7

The undoped K2ZnV2O7 material has a melilite-like structure, with the K+ ions distributed orderly on two distinguishable crystallographic sites, which is the main difference with the case in a typical A2B3O7 melilite material where the A site cations occupy only one crystallographic position. Herein, we report that substitution of Ga for Zn in K2ZnV2O7 would induce the K+ ions' order-to-disorder transition and transform the K2ZnV2O7 from the melilite-like structure to a typical melilite phase, coupled with significantly enhanced oxide ion conductivity, e.g. ∼1.01 × 10–1 S/cm at 600 °C for the K2Zn0.7Ga0.3V2O7.15 sample, which is about 2 orders of magnitude higher than the parent material. The powder neutron diffraction data analysis results revealed two sites for the interstitial oxygen defects that are introduced by the donor-doping. The introduction of interstitial oxygen into the K layer strongly correlated with the K ions' order-to-disorder transition, while the interstitial oxygens within the 5-fold (Zn/Ga/V)O4 tetrahedral rings are responsible for the enhanced oxide ion conductivity. These findings provided a comprehensive understanding of the melilite-like and typical melilite materials for the mechanism of the stabilization and migration of interstitial oxygen defects.