Defect and microstructure engineering via Ta/Zn Co-doping for wide temperature range BaTiO₃-Bi_0.5Na_0.5NbO₃ dielectric

The development of lead-free dielectric ceramics allowing the reliable operation above 150 °C is critical for advanced electronics. The BaTiO₃-Bi_0.5Na_0.5TiO₃ (BT-BNT) system is a promising candidate, nonetheless its temperature stability requires further enhancement. This work demonstrates that the synergistic co-doping of Ta^{5 +} and Zn²⁺ is an effective strategy to achieve this goal. The 0.90 BaTiO₃-0.10 Bi_0.5Na_0.5TiO₃-1.5 mol% Ta₂O₅ ceramic system was fabricated via solid-state reaction, and the effects of ZnO content (0–4.0 wt%) and sintering temperature (1175–1225 °C) were systematically investigated. The optimal composition with 2.0 wt% ZnO sintered at 1200 °C exhibits a high dielectric constant of ∼4415 and an excellent temperature stability, with a capacitance variation (ΔC/C_25°C) within ±15% over a broad temperature range of −45 °C to 240 °C. Comprehensive microstructural and chemical analysis were conducted, including XRD, SEM TEM, EDS, XPS, Raman spectra, and high-temperature impedance spectroscopy. The results reveal that ZnO doping plays a multifunctional role. It promotes grain growth and enhances relaxor behavior through the formation of polar nanoregions. Crucially, Zn²⁺ incorporation tailors the defect chemistry, leading to the formation of a resistive grain boundary layer enriched with A-site vacancies, which is identified as the key mechanism for suppressing conduction loss, enhancing dielectric constant and achieving high thermal stability. However, exceeding the solid solubility limit of ZnO (>2.0 wt%) causes the precipitation of secondary phases, degrading performance. This study highlights the precise defect engineering via co-doping as a vital pathway for developing high-performance, temperature-stable multi-layer ceramic capacitors.