Grain growth in donor-doped SrTiO₃

The Nb donor concentration and cation stoichiometry dependence of grain growth in SrTiO₃ has been studied. The so-called donor anomaly, where grain growth is enhanced at low donor but suppressed at high donor concentrations, is observed only in nonstoichiometric compositions with an excess of B-site cations. Scanning transmission electron microscopy (STEM) observations indicate that this phenomenon is entirely a result of changes in the distribution of residual silicate phases. Exaggerated grain growth at low donor concentrations results from the presence of a continuously wetting grain boundary silicate, whereas inhibited growth at higher donor concentrations occurs in microstructures where the silicate phase is nonwetting. In stoichiometric compositions, however, grain growth rates are both slower and independent of donor concentration. In this composition regime grain growth appears to be limited by solid solution drag. The presence of residual silicates only in nonstoichiometric compositions implies a strong stoichiometry dependence of the silica solubility.