LAUSR

Abstract Undoped LnVO 4 and magnesium-doped Ln 0.95 V 0.95 Mg 0.10 O 4-δ (Ln = Pr, Sm, Gd, Dy and Er) orthovanadates were synthesized by solid state reaction method and characterized by XRD, SEM/EDS, electrical conductivity measurements in controlled atmospheres, and modified e.m.f. technique for determination of oxygen-ion transference numbers. XRD analysis showed the formation of phase-pure materials with tetragonal zircon-type structure and a decrease in lattice parameters with a decrease of ionic radius of rare-earth cations. Trace amounts of MgO and Mg-V-O phases revealed by SEM/EDS suggest that the solid solubility limit of magnesium cations in LnVO 4 lattice is somewhat lower than the nominal doping level, and that magnesium substitutes preferentially into the vanadium sublattice. LnVO 4 and Ln 0.95 V 0.95 Mg 0.10 O 4-δ orthovanadates show semiconducting behavior under oxidizing conditions at 450–950 °C and are predominantly oxygen-ionic conductors, except PrVO 4 that shows mixed conductivity. In the LnVO 4 series, electrical conductivity is the highest for PrVO 4 and SmVO 4 (~4 × 10 −4  S/cm at 800 °C) and decreases with increasing atomic number of rare-earth cation for the other compositions. Additions of magnesium results in a drop of electrical conductivity, by 1.5–2 times for most of compositions. Interstitial oxygen diffusion is discussed as a prevailing mechanism of ionic transport in undoped LnVO 4 , whilst acceptor-type magnesium doping suppresses the formation of interstitial oxygen ions. Humidity has a rather negligible impact on the electrical properties of substituted ceramics, indicating only minor (if any) protonic contribution to the total electrical transport of the studied orthovanadates.