Defect association in acceptor-doped SrTiO3: case study for Fe ' V-Ti(O)center dot center dot and Mn '' V-Ti(O)center dot center dot

The association of acceptor cations (Fe-Ti' and Mn-Ti) with oxygen vacancies in Fe- and Mn-doped SrTiO3 single crystals is investigated using in-situ EPR spectroscopy. Effective association enthalpies Delta(ass)H(eff)(0) and entropies Delta(ass)S(eff)(0) are determined, and Delta(ass)H(eff)(0) is found to depend strongly on the dopant concentration. This dependence can be roughly described by a correction term in the chemical potential that is proportional to the cube root of the defect concentration. Extrapolation to infinite dilution yields an association enthalpy of -26 kJ mol(-1). The association of oxygen vacancies can significantly reduce the ionic conductivity of these materials at temperatures up to ca. 200degreesC (Fe-Ti') or even higher (Mn-Ti), and must therefore be taken into account in the respective defect chemical models at moderate or low temperatures.