Phase classification, electrical conductivity, and thermal stability of Bi2(V0.95TM0.05)O5.5 + δ (TM: transition metal)

The electrical conductivity and phase transition temperatures of Bi2(V0.95TM0.05)O5.5 + 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 αf, βf, and γf 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 βf/γf transition temperature with respect to the undoped material. This was most significant in the case of Ta doping. The γf phase maintained a high oxide-ion conductivity of above 3 × 10− 1S cm− 1 at 700 °C. The conductivity of αf and the transition temperature between αf and βf exhibited a notable hysteresis upon heating and cooling. This phenomenon was prominent in Ti- and Nb-doped Bi2VO5.5. TM doping also improved the long-term phase stability of γf against thermal decomposition into the BiVO4 and Bi3.5V1.2O8.25 phases at intermediate temperatures. Ti doping was the most effective method for suppressing thermal decomposition.