Phase classification, electrical conductivity, and thermal stability of Bi₂(V_0.95TM_0.05)O_{5.5 + δ} (TM: Transition metal)

The electrical conductivity and phase transition temperatures of Bi ₂(V_0.95TM_0.05)O_{5.5 + δ} (TM (transition metal) = Ti, Zr, Hf, Nb, Ta, Cr, Mo, and W) were studied by X-ray diffraction analysis, AC impedance spectroscopy, and DC polarization measurements. To understand the changes in electrical conductivity, the structural phases were classified into three groups of α_f, β_f, and γ_f in terms of the network structures in the layer containing vanadium and oxygen. The phase transition behavior was substantially affected by TM doping. With the exception of Cr, TM doping decreased the β_f/γ_f transition temperature with respect to the undoped material. This was most significant in the case of Ta doping. The γ_f phase maintained a high oxide-ion conductivity of above 3 × 10^{- 1}S cm^{- 1} at 700 °C. The conductivity of α_f and the transition temperature between α_f and β_f exhibited a notable hysteresis upon heating and cooling. This phenomenon was prominent in Ti- and Nb-doped Bi ₂VO_5.5. TM doping also improved the long-term phase stability of γ_f against thermal decomposition into the BiVO₄ and Bi_3.5V_1.2O_8.25 phases at intermediate temperatures. Ti doping was the most effective method for suppressing thermal decomposition.