Synergistically enhancing the piezoelectric activity and Curie temperature of CaBi₄Ti₄O₁₅ ceramics via co-doping Gd/Mn at the A/B-site

As a typical Aurivillius-type compound, CaBi₄Ti₄O₁₅ (CBT) is considered a strong competitor among high-temperature piezoelectric materials, but it is difficult to achieve both high piezoelectric activity and a high Curie temperature for CBT. In this work, the method of double-ion co-substituting at different crystalline sites was used to modify the electrical properties of CBT. The Gd/Mn co-doped CBT ceramics with the chemical formula of Ca_1−xGd_xBi₄Ti₄O₁₅+0.2 wt% MnO₂ (CBT–100xGM, x = 0–0.11) were prepared via the conventional sintering process. The phase and valence band structures, chemical compositions and microstructures, dielectric and ferroelectric properties, electrical conduction behaviors, and electroelastic and piezoelectric properties of the ceramics were characterized. The doping concentration effects of Gd³⁺ were analyzed according to the composition-dependent structures and properties of CBT–100xGM. The donor substitution of Gd³⁺ for Ca²⁺ at the A-site reduced the tolerance factor of the perovskite-like structure and decreased the concentration of intrinsic oxygen vacancies. While Mn³⁺ tended to substitute for Ti⁴⁺ at the B-site, the extrinsic oxygen vacancies are limited near the defect center of Ti(Mn) because of the formation of ( ${\mathrm{V}}_{\mathrm{O}}^{\cdot \cdot }$‒Mn_Ti^')^• as defect dipoles. The thermal depoling behavior of the CBT–100xGM ceramics between 300 and 700 °C was explained by the thermodynamic characteristics of the defect dipoles. The optimized composition with x = 0.08 (CBT–8GM) had a high T_C ≈ 809 °C and a high piezoelectric coefficient (d₃₃) ≈ 23 pC/N, as well as a piezoelectric voltage constant (g₃₃) value of up to 21.5×10⁻³ (V·m)/N. Moreover, it can maintain a residual d₃₃ ≈ 80% after being annealed at 700 °C. This good anti-thermal depoling ability endows this material with great application potential in high-temperature piezoelectric devices with operating temperatures exceeding 500 °C. The synergistic enhancement in the piezoelectric activity and Curie temperature of CBT can be attributed mainly to the donor-substituting effect of Gd³⁺ at the A-site, as well as the decreased elastic compliance contributed by MnO₂ as the B-site dopant.