Core–Shell Formation by Diffusion Along Grain Boundaries in Lead-Free Ferroelectric Ceramics

The search for lead-free ferroelectrics, driven by environmental concerns, has continued for decades, yet no material has fully replaced PbZrTiO₃-based compositions. Introducing core–shell architectures offers a promising route to enhance functional properties; however, their realization in lead-free systems remains limited. In this work, we present a novel method for obtaining core–shell structures in fully sintered lead-free ferroelectric ceramics by thermally treating two ceramic plates in direct contact—hereafter referred to as the contact method. This approach enabled core–shell formation in BaTiO₃, SrTiO₃, and Na₀.₅Bi₀.₅TiO₃ systems through interfacial diffusion. Bi over-stoichiometry in NBT strongly promoted shell formation and enhanced interdiffusion, yielding high chemical contrast with diffusion depths exceeding 300 µm. Furthermore, a fully developed core–shell architecture in BaTiO₃ was obtained, resulting in a significant improvement in resistivity. These findings demonstrate the potential of the contact method as a simple and versatile route for tailoring the microstructure and functional properties of lead-free ferroelectric ceramics.