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.