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 K₂Zn_0.7Ga_0.3V₂O_7.15 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.