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 degrees 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 similar to 750 degrees C. The maximum in proton conductivity is observed for LaW1/6O2 with values reaching 3 x 10(-3) S/cm at approximately 800 degrees C. The high proton conductivity for the undoped material is explained by assuming interaction between water vapor and intrinsic (anti-Frenkel) oxygen vacancies. (C) 2008 Elsevier Ltd. All rights reserved.