Insights into the relationship between crystal structure and electrical conductivity of Bi₄(V_0.9Co_0.1)_xO_6+2.35x electrolyte

The non-stoichiometric γ-phase Bi₄(V_0.9Co_0.1)_xO_6+2.35x (1.80 ≤ x ≤ 2.00) solid solutions were synthesized by the solid-state reaction method. The crystal structures of Bi₄(V_0.9Co_0.1)_xO_6+2.35x were obtained using powder X-ray diffraction (XRD). The Bi₄(V_0.9Co_0.1)_xO_6+2.35x is in space group I4/mmm and the cell parameters were obtained from Rietveld refinements. The cell parameter a decreases from 3.9426(1) to 3.9314(1) Å and the V decreases from 240.728(2) to 239.296(2) ų as x increases from 1.80 to 2.00. The Bi₄(V_0.9Co_0.1)_xO_6+2.35x exhibits an electronic insulator with a direct band gap ranging from 1.67 to 1.69 eV. When x = 1.85, the occupancy of O3 is higher than that of O2. With the presence of more oxygen vacancies located at the bridging equatorial positions, the conductivity of Bi₄(V_0.9Co_0.1)_1.85O_10.3475 reaches the maximum value of 2.3 × 10⁻³ S·cm⁻¹ at 350 °C. The total conductivity of Bi₄(V_0.9Co_0.1)_xO_6+2.35x is obviously higher than 1 × 10⁻³ S·cm⁻¹ at 400 °C. The achievement in this work greatly confirms that the design of non-stoichiometric ratios can effectively regulate the conductivity of BICOVOX system.