The Drastic Influence of the TiO ₂ Polymorph on the Formation of Core–Shell Structures in Na _0.5 Bi _0.5 TiO ₃ –25SrTiO ₃

ABSTRACT The formation of Na 0.5 Bi 0.5 TiO 3 –25SrTiO 3 solid solutions is highly sensitive to variations in the oxygen vacancy concentration, which strongly influence the Sr 2+ and O 2− interdiffusion during calcination. In addition to conventional approaches such as adjusting the Bi 2 O 3 content or doping, selecting the TiO 2 polymorph provides an additional strategy to control oxygen vacancies and with that microstructure and functional properties of ceramics. XRD revealed that rutile TiO 2 promotes the formation of intermediate Bi 12 TiO 20 , thereby suppressing Bi 2 O 3 sublimation, and reducing the final oxygen vacancy concentration. This yields pure Na 0.5 Bi 0.5 TiO 3 core fractions of up to 16.9%, surrounded by SrTiO 3 ‐rich shells. The core–shell formation correlates with an increased positive strain from 0.21% to 0.27%. Electrical properties, however, are more dependent on the oxygen vacancy content than core–shell presence. Thus, anatase‐based samples, benefiting from faster oxygen diffusion, exhibit more homogeneous microstructures with fewer core–shell structures and up to three orders of magnitude higher oxygen ion conductivity than rutile‐based ceramics. Interestingly, the opposite effect was found for pure NBT, highlighting the dependence on the solid solution. Overall, the results demonstrate that the TiO 2 polymorph selection is a powerful strategy for effectively controlling reaction pathways, defect chemistry, and the resulting functional properties.