Crystal structure and conductivity of non-stoichiometric intermedium-temperature oxide electrolyte Bi₄(V_0.9Cu_0.1)_xO_6+2.35x (1.80 ≤ x ≤ 2.05)

The non-stoichiometric Bi₄(V_0.9Cu_0.1)_xO_6+2.35x (nx-BICUVOX) samples were designed and synthesized by solid-state reactions. The tetragonal γ-Bi₄(V_0.9Cu_0.1)_xO_6+2.35x solid solution is identified as 1.80 ≤ x ≤ 2.05 by X-ray diffraction (XRD) analysis. The unit cell of nx-BICUVOX shows an anisotropic in-plane layer shrinkage and the cell volume V decreases from 240.18(2) ų to 237.99(1) ų as x increases from 1.80 to 2.05. The nx-BICUVOX shows a large positive thermal expansion with the volume thermal expansion coefficient (TEC) estimated to be 5.20 × 10⁻⁵ K⁻¹, 5.48 × 10⁻⁵ K⁻¹, 5.90 × 10⁻⁵ K⁻¹ from RT to 700 °C, for Bi₄(V_0.9Cu_0.1)_1.80O_10.23, Bi₄(V_0.9Cu_0.1)_1.90O_10.465, Bi₄(V_0.9Cu_0.1)_2.00O_10.7, respectively. The nx-BICUVOX shows an obvious electronic insulating manner. Three peaks of V-2p, three peaks of O-1s and two peaks of Bi-4f are observed in X-ray photo-electron spectroscopy (XPS) of nx-BICUVOX, which agrees with the crystal structure of the nx-BICUVOX. The grain conductivity of nx-BICUVOX (x ≠ 2.00) is significantly higher than that of stoichiometric Bi₄(V_0.9Cu_0.1)_2.00O_10.7. The Bi₄(V_0.9Cu_0.1)_1.95O_10.5825 and Bi₄(V_0.9Cu_0.1)_2.05O_10.8175 show the highest grain conductivity, in which accompanying with the smallest activation energy. Therefore, the composition control of nx-BIMEVOX is an effective way in the research and development of new BIMEVOX electrolytes.