Core‐Shell Structure Formation in Nb ₂ O ₅ ‐Doped SrTiO ₃ by Oxygen Partial Pressure Change

A core‐shell structure was observed in SrTiO ₃ doped with 1.2 mol% of Nb ₂ O ₅ , after sintering in a reducing atmosphere (5H ₂ ‐95N ₂ ) and then in an oxidizing atmosphere (air). In undoped and Al ₂ O ₃ ‐doped SrTiO ₃ specimens, no core‐shell structure formed after the same sintering treatments as those for SrTiO ₃ doped with 1.2 mol% of Nb ₂ O ₅ . The measured chemical compositions of the core and shell regions of 1.2‐mol%‐Nb ₂ O ₅ ‐doped SrTiO ₃ grains showed that the Sr/(Ti + Nb) ratio of the shell regions grown in air was ~1% less than that of core regions grown in 5H ₂ ‐95N ₂ , which was in good agreement with a value predicted by available defect equations. Therefore, the observed core‐shell structure is thought to result from the formation of strontium vacancies in an oxidizing atmosphere.