Effects of protons and acceptor substitution on the electrical conductivity of La6WO12

Transport properties and non-stoichiometry of La1−xCaxW1/6O2 and La1−yW1/6O2 (x=0, 0.005, 0.05; y=0.05, 0.1) have been characterized by means of impedance spectroscopy, the EMF-technique, H+/D+ isotope exchange, and thermogravimetry in the temperature range 300–1200 °C as a function of oxygen partial pressure and water vapor partial pressure. The materials exhibit mixed ionic and electronic conductivities; n- and p-type electronic conduction predominate at high temperatures under reducing and oxidizing conditions, respectively. Protons are the major ionic charge carrier under wet conditions and predominates the conductivity below ∼750 °C. The maximum in proton conductivity is observed for LaW1/6O2 with values reaching 3×10−3 S/cm at approximately 800 °C. The high proton conductivity for the undoped material is explained by assuming interaction between water vapor and intrinsic (anti-Frenkel) oxygen vacancies.