The influence of high temperature oxygen annealing on (100) oriented donor-doped SrTiO{sub}3 single crystals was studied. Crystalline precipitates were found on the optical scale on surfaces of lanthanum-doped as well as niobium-doped specimens with donor concentrations above 0.5 at.. The amount of the secondary phase increases with the doping level, oxidation temperature and oxidation time. EDX analyses of the crystallites reveal a SrO{sub}X composition. The formation of the observed secondary phase is discussed by means of the defect re-equilibration of the cation sub-lattice. In view of the point defect model for donor-doped perovskites, n-conducting SrTiO3 changes its compensation mechanism during an oxidation treatment from "electronic compensation" (N{sub}D = n) to "self-compensation" (N{sub}D = 2V{sub}(sr){sup}") by forming cation vacancies. Due to the favored Schottky-type disorder in perovskites, the formation of strontium vacancies is accompanied by a release of strontium from the regular lattice. Since the excess strontium is found to be situated at the surface in form of SrO-rich precipitates only, we propose the formation of strontium vacancies via a surface defect reaction and the chemical diffusion of strontium vacancies from the surface into the crystal as the most probable re-equilibration mechanism for the oxidation treatment of single crystals. The introduced mechanism is in contrast to an established model which proposes the formation of Ruddlesden-Popper intergrowth phases SrO·(SrTiO{sub}3){sub}n in the interior of the crystal.
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