Synthesis and Characterization of Nonsubstituted and Substituted Proton-Conducting La_6–<i>x</i>WO_{12–<i>y</i>}

Mixed proton–electron conductors (MPEC) can be used as gas separation membranes to extract hydrogen from a gas stream, for example, in a power plant. From the different MPEC, the ceramic material lanthanum tungstate presents an important mixed protonic–electronic conductivity. Lanthanum tungstate La_6–<i>x</i>WO_{12–<i>y</i>} (with <i>y</i> = 1.5<i>x</i> + δ and <i>x</i> = 0.5–0.8) compounds were prepared with La/W ratios between 4.8 and 6.0 and sintered at temperatures between 1300 and 1500 °C in order to study the dependence of the single-phase formation region on the La/W ratio and temperature. Furthermore, compounds substituted in the La or W position were prepared. Ce, Nd, Tb, and Y were used for partial substitution at the La site, while Ir, Re, and Mo were applied for W substitution. All substituents were applied in different concentrations. The electrical conductivity of nonsubstituted La_6–<i>x</i>WO_{12–<i>y</i>} and for all substituted La_6–<i>x</i>WO_{12–<i>y</i>} compounds was measured in the temperature range of 400–900 °C in wet (2.5% H₂O) and dry mixtures of 4% H₂ in Ar. The greatest improvement in the electrical characteristics was found in the case of 20 mol % substitution with both Re and Mo. After treatment in 100% H₂ at 800 °C, the compounds remained unchanged as confirmed with XRD, Raman, and SEM