Piezoelectric ceramics are critical components in sensors, actuators, energy converters, and micro-electromechanical systems. The lead-based compositions dominate in the existing market. However, the release of lead during their lifecycle poses severe environmental and human-being health risks, which contradicts global sustainable development goals. Therefore, developing high-performance lead-free alternatives becomes a paramount research priority. Among lead-free alternatives, potassium sodium niobate ((K,Na)NbO₃, KNN)-based ceramics stand out due to their low driving voltage, large electrostrain, and minimal hysteresis, making them performance-competitive with conventional PZT for precision actuators. Their performance is typically enhanced by constructing polymorphic phase boundaries and regulating microstructure. However, the existing doping strategies inadvertently degrade thermal stability, which remains a key bottleneck for practical applications. Rare-earth ions act as effective modifiers in KNN-based piezoceramics. They enhance a relaxor behavior, and improve thermal stability by shifting rhombohedral-tetragonal (R-T) phase transition below room temperature, and introduce photoluminescence due to their abundant energy levels. The integration of electrostrain and luminescence in KNN-based systems is thus significant, paving a way for developing novel bimodal functional materials for next-generation optoelectronic devices. In this work, the 0.96(K_0.48Na_0.52)(Nb_0.95Sb_0.05)O₃-0.04Bi_0.5(Na_0.82K_0.18)_0.5ZrO₃(0.96KNNS-0.04BNKZ) composition with a polymorphic phase transition characteristic was chosen as a matrix, and rare-earth oxides (Dy₂O₃ and Eu₂O₃) served as modifiers. The effect of rare-earth oxides doping on the microstructure, ferroelectric/electrostrain properties, and photoluminescence performance of the 0.96KNNS-0.04BNKZ ceramics was investigated.

0.96KNNS-0.04BNKZ-x%Dy/Eu (x = 0, 0.05, 0.10, 0.20, 0.40) lead-free ferroelectric ceramics were synthesized by a solid-state reaction method. The stoichiometric raw materials were weighed according to the formula. They were then ground in ethanol in a ball mill with zirconia balls for 24 h. The powders were calcined at 850 ℃ for 4 h, then ground again for 24 h. After drying, the powders were mixed with 10% polyvinyl alcohol (PVA) as a binder and then granulated. Subsequently, disc-shaped green compacts (with the sizes of 10 mm in diameter and 0.5 mm in thickness) were pressed and subjected to a debinding process at 550 ℃. The ceramics were obtained after sintering at 1130-1200 ℃ for 2 h. After polishing, silver electrodes were fired at 650 ℃ for electrical testing. The ceramics were sintered at at 1130 ℃ to 1200 ℃ for 2 h, depending on the doping levels. After polishing, silver electrodes were fired on the surfaces at 650 ℃ for electrical characterization. The phase structure was analyzed by an X-ray diffractometer (D8 advance). The surface morphology was determined by a field emission scanning electron microscope (Merlin compact, Carl Zeiss Co., Germany). The temperature-dependent dielectric constant was measured by a broadband dielectric spectrometer (Novocontrol concept 40). The ferroelectric and electric-field-induced strain were characterized by a ferroelectric analyzer (TF 2000) coupled with a laser interferometer (SP-S 120/500). The photoluminescence properties were evaluated by a fluorescence spectrometer (FLS920).

Dy/Eu-doped 0.96KNNS-0.04BNKZ ceramics exhibit a single perovskite structure. At low doping levels (i.e., x ≤ 0.10%), the rhombohedral-tetragonal (R-T) coexisting phase maintains, while a higher doping (i.e., x ≥ 0.20%) induces a transition to the pseudocubic (P_c) phase, with no secondary phases appears in all the samples. All the ceramics present dense and uniform microstructures. The average grain size gradually decreases with increasing Dy/Eu doping, indicating that rare-earth ions effectively inhibit grain growth. In terms of electrical properties, a moderate Dy/Eu doping (i.e., Dy: x=0.10%; Eu: x=0.05%) enhances ferroelectricity, as reflected by improved P-E loop squareness and increased saturated polarization (P_max). The 0.10%Dy and 0.05%Eu samples achieve an optimal electrostrain performance, delivering unipolar strains (S_uni) of 0.257% and 0.241% at 70 kV/cm, which are accompanied by large piezoelectric strain coefficients (d₃₃^*) of 367 pm/V and 344 pm/V, respectively. An appropriate doping optimizes the domain structure and facilitates domain switching in an external electric field, thereby improving both ferroelectric and electrostrain properties. In contrast, excessive doping leads to a severe lattice distortion, notably octahedral tilting, which pins domain walls and hinders their reorientation in response to the electric field, ultimately compromising ferroelectric and electrostrain performance. Furthermore, the 0.05% Dy- and 0.05% Eu-doped ceramics exhibit excellent thermal stability, showing less than 10% variation in electrostrain at 25-100 ℃, which demonstrates their great potential for use in piezoelectric actuators. In addition, Dy/Eu doping introduces a pronounced photoluminescence to the ceramics. Under blue-light excitation, Dy³⁺-doped samples yield an intense yellow emission, whereas Eu³⁺-doped samples produce a bright orange-red emission.

The introduction of Dy/Eu could promote a phase transition from a mixed R-T phase to a P_c phase. An appropriate doping effectively enhanced the ferroelectric and electrostrain properties of the materials. Specifically, the 0.10% Dy- and 0.05% Eu-doped ceramics achieved unipolar strains (S_uni) of 0.257% and 0.241% at an electric field of 70 kV/cm, with corresponding piezoelectric strain coefficients (d₃₃^*) of 367 pm/V and 344 pm/V, respectively. Furthermore, the 0.05% Dy- and 0.05% Eu-doped samples exhibited excellent thermal stability, with a variation in electrostrain of less than 10% at 25-100 ℃, demonstrating a great potential for piezoelectric actuator applications. In addition, Dy/Eu doping imparted outstanding photoluminescence properties to the ceramics. Under blue-light excitation, Dy³⁺-doped samples exhibited an intense yellow emission, while Eu³⁺-doped samples displayed a bright orange-red emission. The combination of electrical and luminescent characteristics could make this material a promising candidate for optoelectronic coupling devices.