An unusual technique for the study of nonstoichiometry: The thermal emission of electrons. Results for Y2O3 and TiO2

The thermal emission of electrons is presented as a useful technique for the study of nonstoichiometric oxides at high temperature. Results are reported for yttria and titanium dioxide, very different in their respective properties. For these compounds the density of emitted current follows a simple law, J ∝ P^x_O2, where P_O2 is the oxygen partial pressure and x is a constant that is not dependent on temperature. The electrical conductivity, when measured under the same conditions, follows a similar law. Therefore there is some evidence that at high temperature the chemisorption is not an important process, and the emission characteristics are then discussed in terms of a bulk nonstoichiometry. Data are obtained for yttrium oxide, as the width of the band gap E_g = 5.5 eV, the electron affinity χ = 2 eV. A reasonable defect for this oxide consists of oxygen vacancies V^2·_O and oxygen interstitials O^2′_i. The situation in the case of rutile is much more complicated as this oxide has a wide nonstoichiometric field with several suboxides and a nonisotropic structure. When the deviation to the stoichiometry is low the oxygen sublattice is stable and the main defects are titanium interstitials Ti^4·_i. When the compound is more reduced a surface reorganization then occurs which seems related to a crystallographic transformation leading to the Ti_nO_2n?1 suboxides. This technique give a lot of data on the properties of nonstoichiometric compounds in the vicinity of the surface at high temperature.