Synthesis and ionic conductivity of PbBi 6M 2O 15 (M=V, P, As) and SrBi 6V 2O 15 solid solutions

Abstract

PbBi 6V 2O 15, PbBi 6P 2O 15, PbBi 6As 2O 15 and their solid solutions have been prepared by the mixed oxide route at 800–850 °C. Complete solid solutions were formed in the systems PbBi 6V 2O 15–PbBi 6P 2O 15, PbBi 6V 2O 15–PbBi 6As 2O 15 and PbBi 6P 2O 15–PbBi 6As 2O 15. In addition, substitution of Pb by Sr in PbBi 6V 2O 15 resulted in a complete solid solution series. The composition and phase purity of PbBi 6V 2O 15 and the hitherto unreported SrBi 6V 2O 15 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-Bi 2O 3 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 SrBi 6V 2O 15≫PbBi 6V 2O 15>PbBi 6As 2O 15>PbBi 6P 2O 15 indicating that lone pair electron effects associated with Pb 2+ and cation size effects associated with V, As and P play an important role in controlling the anion transport in these materials.