Piezoelectric ceramics are widely used in the fields such as electronics, telecommunications, sensing, and acoustic due to their capability for bidirectional conversion between electrical and mechanical energy. The existing lead zirconate titanate ceramics dominate a wide range of practical applications due to their excellent piezoelectric properties and good manufacturability. However, lead is toxic to human-being health and environment during the processing, application, and disposal. Furthermore, their low Curie temperature (T_C<400 ℃) limits their application potential in the field of high temperature environment. CaBi₄Ti₄O₁₅ (CBT) is one of the piezoceramics with the T_C of < 790 ℃ and high thermal stability, which makes it a leading candidate for high-temperature applications. The potential ultrahigh temperature employment of CBT-based ceramics is, nevertheless, constrained by a low piezoelectric coefficient (d₃₃ < 10 pC/N), small dielectric constant, and poor poling capability. It is difficult to fully pole these ceramics in high electric fields because the volatilization of Bi during high-temperature sintering results in a stoichiometric deviation, a higher density of lattice defects, and reduced electrical resistivity. Recently, numerous researchers use chemical doping strategy to tailor the crystal structure and modify microdefects, yielding an initial success. The A-site doping of rare-earth ions (e.g., Er³⁺, Nd³⁺, and Ce³⁺) or alkali metal ions (e.g., Na⁺, K⁺, and Li⁺) is a practical way of improving the piezoelectric property via suppressing Bi volatilization, inducing lattice distortion and tailoring densification. According to some previous reports, rare-earth element doping promotes the d₃₃ of CBT-based ceramics to 20 pC/N. It is reported that Nd/Ce co-doping substantially elevates the d₃₃ to 28 pC/N in Na_0.5Bi_4.5Ti₄O₁₅, demonstrating that synergistic rare-earth element co-doping plays a critical role in enhancing crystal structure stability and defect control. Despite the promising progress of rare-earth element doping in bismuth layer-structured ferroelectrics, there is still little systematic research regarding the synergistic mechanisms of La/Ce co-doping in regulating the microstructure and piezoelectric properties of CBT ceramics. La³⁺ can incorporate into the A-site in CBT-based ceramics due to the similar ionic radius of La³⁺ and Bi³⁺, enhancing the lattice stability. Meanwhile, Ce can reduce oxygen vacancy concentration by charge compensation mechanism and improve densification. The synergistic interaction between them can boost polarization and piezoelectric property. In this work, CBT-based ceramics with La/Ce co-doping were prepared via conventional solid-state reaction. Moreover, the effect of doping concentration on the crystal structure, microstructure, and various electrical properties, including dielectric, ferroelectric, and piezoelectric characteristics was systematically investigated.

Ca(La_1/2Ce_1/2)_xBi_4-xTi₄O₁₅(CLaCe-x, x = 0.000, 0.025, 0.050, 0.075, 0.100) ceramics were synthesized via solid-state reaction. The raw materials were CaCO₃(99.99%)、Bi₂O₃(99.99%)、TiO₂(99.8%)、CeO₂(99.99%) and La₂O₃(99.99%). First, all the starting materials were weighed according to different stoichiometric ratios and ground in a ball mill with alcohol for 12 h. Then, the wet slurries were dried at 80 ℃ and calcined at 82 ℃ for 4 h. Afterwards, the powder was further ground. Consecutively, the dried powder with 0.5% polyvinyl alcohol was pressed into discs with a thickness of 0.7 mm and a diameter of 12 mm. The pellets were sintered at 1025-1075 ℃ for 1 h.

An improved piezoelectric coefficient of 19 pC/N is obtained, along with an outstanding Curie temperature of 791 ℃ can be achieved in the ceramic. The XRD patterns and SEM images confirm that all the ceramics show a pure single phase, indicating that La/Ce ions are dissolved into the original crystal lattice. Furthermore, the influence of doping ions on the grain size is negligible. The co-doping bigger radius ions at the A-site induce the expansion of lattice, which can improve the crystal structure. The dielectric loss of the ceramic is reduced, and its impedance is increased by over one order of magnitude at a high temperature due to this structure modification. The activation energy is calculated with the Arrhenius relation, manifesting that the oxygen vacancy concentration can reduce. Consequently, the ceramic resistivity is a boosted result from a lower concentration of hole carriers. This improved resistivity facilitates the poling process in high electric fields, leading to a sharp increase in the piezoelectric coefficient. Subsequently, P-E hysteresis loops are measured at 100 ℃. The CLaCe-0.075 ceramic exhibits a larger remanent polarization (P_r) than pure CBT, which is consistent with the enhancement in piezoelectric property. In summary, the La/Ce co-doping ceramics demonstrate the superior d₃₃, T_C, resistivity, and thermal stability, compared to most bismuth-layered piezoelectric ceramics. This work confirms that the rare-earth element co-doping is an efficient strategy for elevating the electrical performance of CBT-based ferroelectrics.

La/Ce co-doping CBT ceramics were fabricated via a solid-state reaction. The effect of rare-earth element doping on the structure and properties of bismuth-layered ceramics was systematically investigated. The results revealed that the ceramics remained phase-pure Aurivillius structures with a high relative density and a negligible variation in grain size. The high d₃₃ of 19 pC/N and T_C of 791 ℃ of the CLaCe-0.075 ceramic were achieved. In addition, the dielectric loss reduced to 1.62%, and the resistivity increased to 2.4 × 10⁸ Ω·cm at 400 ℃. The impedance spectroscopy analysis revealed that La/Ce co-doping effectively inhibited Bi volatilization, leading to a reduced oxygen vacancy concentration, which was responsible for the enhanced piezoelectric property. This work demonstrated that La/Ce co-doping could be an effective strategy for enhancing piezoelectric property of bismuth layer-structured ferroelectrics, providing valuable insights for the development of novel high-temperature piezoelectric materials.