Retaining field-dependent properties of NBT-6BT ceramics sintered under reducing atmospheres via defect-level engineering

The development of lead-free multilayer ceramic actuators (MLCAs) compatible with cost-effective Cu base-metal electrodes (BMEs) remains challenging due to the degradation of piezoceramic properties under the low oxygen partial pressure (PO2 ), required for co-sintering. In particular, Na0.5Bi0.5TiO3-based systems suffer from severe conductivity and loss of functional response under reducing conditions. In this work, donor-doped Na0.5Bi0.5TiO3–6BaTiO3 (NBT–6BT) ceramics are designed to retain their electromechanical performance under such atmospheres. Nb and Ta doping effectively suppress defectrelated conductivity, stabilize the defect chemistry, and preserve the ferroelectric response. As a result, Nb- and Ta-modified NBT–6BT exhibits a large strain of up to 0.36% even after sintering in reducing conditions, whereas undoped NBT–6BT fails completely under identical processing. These results highlight an effective strategy for defect engineering in NBT-based ceramics and demonstrate a viable pathway toward high-performance, Cu-compatible, lead-free MLCAs.