High-temperature proton conductivity in acceptor-substituted rare-earth ortho-tantalates, LnTaO4

The electrical properties of 1 mol% Ca-doped LnTaO(4) (Ln=La, Nd, Gd, Er) have been characterized by AC impedance spectroscopy and by the EMF transport number technique as a function of the oxygen pressure and the water vapor pressure from 300 degrees to 1200 degrees C. Protons, oxide ions, electron holes, and electrons contribute to the total conductivity, depending on the conditions. Ionic conductivity predominates under reducing and weakly reducing conditions, with protons prevailing in wet atmospheres up to temperatures of around 1000 degrees C. p-type electronic conductivity prevails at high temperatures under oxidizing conditions. The conductivity behavior has been interpreted according to a point-defect model where protons and oxygen vacancies charge compensate the acceptor doping. Thermodynamic and transport parameters have been extracted based on the changes in total conductivity upon variations in the conditions. The hydration enthalpy becomes increasingly negative with increasing oxide ion density from LaTaO4 to ErTaO4.