The bulk conductivity of polycrystalline 1 mol-% Fe-doped rutile TiO2 has been measured as a function of p H2O and temperature under oxidizing and reducing conditions. From the p H2O-dependency of the conductivity, it is concluded that protons are significant positive defects and, furthermore, that mixed p-type electronic and protonic, and n-type electronic conduction dominate under oxidizing and reducing conditions, respectively. H2O/D2O isotope exchange confirmed that protons are significant charge carriers under wet oxidizing conditions below approximately 600 °C. Thermodynamic parameters for the hydration reaction were obtained by modeling the experimental p H2O and temperature dependencies, assuming that the acceptor dopant (Fe3+) is charge compensated by protons and oxygen vacancies, and from ab initio density functional theory (DFT) calculations. The experimental data yield standard enthalpy changes of hydration of −130 ± 16 kJ/mol, whereas the calculated values are somewhat more negative; −155 to −162 kJ/mol. Based on such favorable thermodynamics of hydration, it is concluded that protons will be the dominating positive defect in TiO2 under most conditions of practical interest.

Mayenite, Ca 12 Al 14 O 33 , has been synthesized by a citric acid route, with final sintering at 1200-1350 °C. Phase purity has been characterized by X-ray diffraction and scanning electron microscopy. The hydration has been investigated under oxidizing and reducing conditions using thermogravimetry versus T and pH 2 O, as well as in situ H 2 O/D 2 O isotope exchange. The two data sets are in general correspondence and yield standard entropy and enthalpy changes of hydration (for 1 mol of H 2 O) of -123 ( 10 J/mol K and -240 ( 16 kJ/mol, respectively, in overall agreement with previous literature. The ac conductivity of sintered disks has been measured by the 4-point van der Pauw method versus pO 2 at 900-1200 °C. It reflects mainly n-type electronic conductivity under reducing conditions and ionic (mainly oxide ion) conductivity under oxidizing conditions. The conductivity contributions are modeled and discussed in terms of a defect model involving partial occupancy by oxide ions and hydroxide ions of the structural subnanocages and with electrons as minority defects. A new Kro ¨ger-Vink-compatible nomenclature for the partial occupancy, including electronic defects, is introduced and used to model hydration and the unusual pH 2 O dependency of the n-type electronic conductivity.

No abstract