The electrical properties of 1mol%Ca-doped samples were characterized by means of alternating-current impedance spectroscopy and EMF transport-number techniques, as a function of O-pressure and the water vapor pressure, at 300 to 1200C. Protons, oxide ions, electron holes and electrons were found to contribute to the total conductivity; depending upon the conditions. Ionic conductivity predominated under reducing and weakly reducing conditions; with protons prevailing in wet atmospheres up to about 1000C. A p-type electronic conductivity prevailed at high temperatures under oxidizing conditions. The conductivity behavior was explained in terms of a point-defect model in which protons and O vacancies charge-compensated the acceptor doping. Thermodynamic and transport parameters were deduced from changes in the total conductivity following variations in the conditions. It was found that the hydration enthalpy became increasingly negative, with increasing oxide-ion density, in going from LaTaO4 to ErTaO4.

High-Temperature Proton Conductivity in Acceptor-Substituted Rare-Earth Ortho-Tantalates, LnTaO4. R.Haugsrud, T.Norby: Journal of the American Ceramic Society, 2007, 90[4], 1116-21