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.
The K 2 ZnV 2 O 7 material, as reported in our previous work, undergoes a phase transition from melilite-related structure to typical melilite structure upon Ga doping on the Zn site, with interstitial oxygen introduced and accompanied by an approximately two orders of magnitude ionic conductivity improvement (≈1.01 × 10 −1 S cm −1 at 600 °C for K 2 Zn 0.7 Ga 0.3 V 2 O 7.15 ). Herein, in order to further enhance the oxide-ion conduction by introducing more interstitial oxygen into the structure, a series of Ga and Mo codoped K 2 Zn 1- x Ga x V 2- y Mo y O 7+0.5 x +0.5 y (0 ≤ x ≤ 0.2, y = 0.1 and 0.2) materials are designed and synthesized via the traditional solid-state reaction method, with their phases and electrical properties being systematically studied. The results reveal that the Ga and Mo codoped materials retain the melilite-related structure and show a much lower conductivity, e.g., only ≈1.49 × 10 −2 S cm −1 at 600 °C for the composition of K 2 Zn 0.8 Ga 0.2 V 1.8 Mo 0.2 O 7.2 , despite its higher interstitial oxygen concentration compared to the single Ga-doped K 2 ZnV 2 O 7 (e.g., K 2 Zn 0.7 Ga 0.3 V 2 O 7.15 ). Therefore, the introduction of Mo into the structure appears to suppress the phase transition and decrease the mobility of interstitial oxygen for the K 2 ZnV 2 O 7 -based materials. These findings provide a more comprehensive perspective on the oxide-ion conduction optimization of melilite or melilite-related materials.
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