Local Structure and Conductivity in the BIGAVOX System

While BIMEVOX systems have attracted attention for their fast oxide-ion conductivity at intermediate temperatures, there are only a limited number of reports concerning their local structure. In this work, both long-range and local structures in the Bi2V1–x Ga x O5.5–x–δ (BIGAVOX, 0.025 ≤ x ≤ 0.40) system are investigated using X-ray powder diffraction (XRD) and a combination of X-ray total scattering, 51V and 71Ga solid-state nuclear magnetic resonance (NMR), and Raman spectroscopy, supported by electrical measurements using a.c. impedance spectroscopy. The three main BIMEVOX polymorphs, α, β, and γ, are observed at room temperature over the compositional ranges 0.025 ≤ x < 0.10, 0.10 ≤ x < 0.20, and 0.20 ≤ x < 0.40, respectively. Above x = 0.10, as more Ga is introduced into the lattice, a general growth of the distance between bismuthate and vanadate layers is observed, indicating increasing ionicity in the interaction between these layers. Ga is found to adopt octahedral and tetrahedral geometries, while V polyhedra include tetrahedral, pentacoordinate, and octahedral geometries. With increasing x-value, as the vacancy concentration increases, more octahedral V polyhedra transform to lower coordinate geometries, resulting in a decrease in the average V–O bond length. Reversible α ↔ β and β ↔ γ phase transitions are observed on heating the x = 0.05 composition, while the β ↔ γ and γ′ ↔ γ phase transitions are observed on heating the x = 0.15 and 0.20 compositions, respectively. The γ-BIGAVOX compositions (x = 0.20 and 0.25) generally show a high conductivity of ∼10–2 S cm–1 at 600 °C.