Effect of divalent dopants on defect structure and electrical properties of Bi2WO6

High-temperature tetragonal Bi4V2O11 has been reported as a good oxygen ionic conductor. The structure can be stabilized to ambient temperature by doping Cu, Ti or Nb to substitute V. In this work, Bi2WO6, which is almost isostructural with Bi4V2O11 but has no oxygen vacancy, was doped with Cu and Mg to induce extrinsic oxygen vacancies to enhance its ionic conductivity. Both solid solutions were synthesized by solid-state reaction method using oxides as the raw materials. Such solid solution may be represented by the formula of Bi2W1−xMxO6−2x, where M=Cu and Mg. The sintered samples were examined by XRD and SEM. The conductivity was measured at 300, 400, 500, 600, and 700 °C by the two-probe technique, using HP-34970 multi-meter. Both Bi2W0.9Cu0.1O5.8 and Bi2W0.9Mg0.1O5.8 exhibited a mixture of tetragonal and orthorhombic lattices from XRD patterns. However, the SEM micrograph of Bi2W0.9Cu0.1O5.8 showed CuO precipitation while no MgO precipitation in Bi2W0.9Mg0.1O5.8. Although Mg ions may incorporate into the structure of Bi2WO6, a new Mg-rich tetragonal phase may still appear when Mg is no longer to be surrounded by six oxygen atoms. The Mg-rich tetragonal phase is highly conductive because of its high oxygen vacancy concentration. The conductivity of Bi2W0.8Mg0.2O5.6 is 1.12×10−1 Ω−1 cm−1 at 700 °C and that is higher than 7.97×10−2 Ω−1 cm−1 of Bi2W0.8Cu0.2O5.6. Thus, the dissolution of Cu into the W cation-sublattice is limited.