Structural and Conducting Features of Niobium-Doped Lanthanum Tungstate, La27(W1-xNbx)5O55.55-δ

The most studied ceramic proton conductors are those based on the perovskite structure. However, these materials have some practical drawbacks including their poor tolerance to carbonation. Hence, proton-conducting ceramic materials with fluorite structure are currently under investigation. One of the most studied materials is the lanthanum tungstate, Ln(6)WO(12). Here, we report a new series of compounds La27W5-5xNb5xO55.5-5x/2 square(8.5+5x/2) obtained by niobium doping to optimize/increase the amount of oxygen vacancies. The limiting composition has been established as La27NbW4O55.0 square(9.0) with an astonishing 14% of oxygen vacancies. The materials have been studied by Rietveld analysis of high-resolution laboratory X-ray powder diffraction data and electron microscopy. Thermal analysis measurements in a wet atmosphere indirectly confirm the increase of oxygen vacancies as the amount of incorporated protons increases with the niobium content. A thorough electrical characterization has been carried out including overall conductivity measurements in wet and dried atmospheres, conductivity as a function of the oxygen partial pressure, and electronic contribution by the Hebb-Wagner polarization method. The data collected suggest that the proton conductivity is dominant below 600 degrees C. However, above 800 degrees C the conductivity values are almost independent of the water partial pressure which indicates that the oxide ion is the main charge carrier. The highest conductivity value was measured for La27NbW4O55, i.e., 0.01 S.cm(-1) at 800 degrees C compared to 0.004 S.cm(-1) for the nonsubstituted material La22W5O55.5. At temperatures below 800 degrees C, these materials are nearly pure ionic conductors with transport numbers higher than 0.98, while at higher temperatures these compounds are mixed ionic-electronic conductors displaying both n- and p-type electronic contributions.