Protons in Acceptor-Doped La3NbO7 and La3TaO7

The conductivity of A- (Ca and Sr) and B-site (Ti) acceptor doped La3NbO7 and La3TaO7 has been characterized by means of constant frequency measurements and ac impedance spectroscopy in the temperature range of 300-1200 degrees C, and as a function of the partial pressures of oxygen and water vapor. Effects of H-D isotope shifts on the total conductivity and concentration cell measurements were applied to determine the predominating charge carriers in the system. The conductivity is mainly protonic under wet conditions at low temperatures, whereas oxide ion and electronic conduction gradually take over at higher temperatures. Electron holes (p-type conductivity) predominate under oxidizing conditions for the Ca- and Sr-doped materials. The Ti-doped material is an n-type conductor under reducing conditions. The conductivity behavior is explained with bases in point defect chemistry under the assumption that oxygen vacancies and protons charge compensate the acceptors. Thermodynamic and kinetic parameters for the different systems are derived.