Low-temperature defect chemistry of oxides. I. General aspects and numerical calculations

Abstract

Using oxides as examples, the defect chemistry is systematically analyzed for a low-temperature regime, at which the oxygen exchange equilibrium reaction is no longer reversible, while the internal defect equilibrium reactions (in particular, the electronic transfer processes) may still be reversible. For the partially frozen-in states as well as for the complete equilibrium cases, defect concentrations are numerically calculated for idealized model oxides including pure, acceptor-doped, and donor-doped oxides. Foreign ions (major/minor, shallow/deep, acceptor/donor), oxygen vacancies, and oxygen interstitials are taken into account as redox-active defects. The deep-level (redox-active) defects often dominate defect concentrations in the partially frozen-in states, while the major dopants fix the concentrations in complete equilibrium. The temperature and oxygen partial pressure dependencies of defect concentrations in the partially frozen-in states are discussed. The description does not only allow one to extend the defect chemistry to lower temperatures, such as room temperature, but also offers a quantitative basis for manipulation and prediction of defect concentrations in ionic crystals. Thereby, the physical and chemical performance of such materials may be controlled at temperatures lower than those at which the oxygen nonstoichiometry is established. The results are equally relevant for applications in solid state physics (e.g., compound semiconductors) and in solid state chemistry (e.g., solid electrolytes, mixed conductors).