High oxide-ion conductivity of monovalent-metal-doped bismuth vanadate at intermediate temperatures

Li- and Ag-doped Bi₂VO_5.5 were synthesized by solid-state reactions, and their high oxide-ion conductivities at intermediate temperatures between 400 and 600 °C were demonstrated. Li- and Ag-doped Bi₂VO_5.5 show a higher symmetry than Bi₂VO_5.5 at room temperature. In Bi₂(V_0.9Li_0.1)O_5.3 and Bi₂(V_0.9Ag_0.1)O_5.3, the tetragonal γ-phase, i.e., the high-temperature and disordered phase of Bi₂VO_5.5, is stabilized at room temperature. AC impedance spectroscopy reveals that Li- and Ag-doping markedly improve the electrical conductivity of Bi₂VO_5.5 below 570 °C. The dominant charge carrier is confirmed as oxide ions from impedance spectra, DC polarization measurements, and conductivity change with oxygen partial pressure. At around 500 °C, the oxide-ion conductivities of Li- and Ag-doped Bi₂VO_5.5 are 2-5 times higher than that of La_0.8Sr_0.2Ga_0.8Mg_0.115Co_0.085O₃, which is one of the best oxide-ion conductors.