Synthesis and ionic conductivity of PbBi6M2O15 (M=V, P, As) and SrBi6V2O15 solid solutions

PbBi6V2O15, PbBi6P2O15, PbBi6As2O15 and their solid solutions have been prepared by the mixed oxide route at 800–850 °C. Complete solid solutions were formed in the systems PbBi6V2O15–PbBi6P2O15, PbBi6V2O15–PbBi6As2O15 and PbBi6P2O15–PbBi6As2O15. In addition, substitution of Pb by Sr in PbBi6V2O15 resulted in a complete solid solution series. The composition and phase purity of PbBi6V2O15 and the hitherto unreported SrBi6V2O15 were confirmed by electron probe microanalysis (EPMA). Although their crystal structures remain unknown, all compositions appeared to be isostructural from X-ray diffraction (XRD) results and the data could be indexed on an orthorhombic cell derived from the cubic δ-Bi2O3 structure, with a∼23.9, b∼17.1 and c∼11.1 Å. Impedance spectroscopy and transference number measurements show these materials to be moderate oxide ion conductors with bulk conductivities of 5×10−5–10–7 S cm−1 at 500 °C with transference numbers ∼0.8 at 600–700 °C. Conductivity decreased in the order SrBi6V2O15≫PbBi6V2O15>PbBi6As2O15>PbBi6P2O15 indicating that lone pair electron effects associated with Pb2+ and cation size effects associated with V, As and P play an important role in controlling the anion transport in these materials.