Synthesis, X-ray diffraction analysis, and transport properties of Bi5−xLnxNbO10(Ln = La, Gd, and Y) oxide-ion conductors

The effects of partial substitutions of Bi by La, Gd, and Y on the structure, thermal expansion and transport properties of modulated fluorite-type Bi5NbO10were investigated using X-ray thermodiffraction, alternative current impedance spectroscopy and the Wagner polarization method. Through conventional solid-state syntheses, the solid solutions Bi5−xLnxNbO10exist up to about x∼ 0.25 for La, and to x∼ 0.5 for Gd and Y. A doubling of their thermal expansion coefficient is observed above 540 °C. The type-II incommensurate modulation of these phases is correlated to the nature and amount of substitute, and disappears above 800 °C. Except for Bi5NbO10, which appears to be a pure ionic conductor, most of the solid solutions exhibit mixed conductivity with predominant ionic transport. Partial Gd and Y substitutions favor electron conductivity in the low temperature range, probably due to bismuth partial oxidation.