Hydrogen ion conduction in iron-substituted strontium titanate, SrTi1−xFexO3−x/2 (0≤x≤0.8)

Conduction of hydrogen species in selected compositions of SrTi1−xFexO3−x/2 (0≤x≤0.8) has been investigated by ac conductivity and concentration cell emf transport number measurements, as a function of pO2, pH2O and temperature (500–1100°C). The main contributions to the total conductivity are n-type electronic, dominating at low pO2, ionic (oxygen vacancies) at intermediate pO2, and p-type electronic at high pO2. Hydrogen ion conduction is measurable, but the hydrogen containing species are minority defects in this system at the temperatures investigated, both in terms of concentration and conductivity. When the conditions are not too reducing, hydrogen ions are positive, assumed to be protons, H+. At lower pO2, however, there is a change in the sign of the emf of the water vapour concentration cell, and thus apparently of the charge of the hydrogen-containing charge carrying species. This can be rationalised by the presence and migration of substitutionally dissolved hydride ions, H−, but it is questioned whether these can be stable under the conditions used.