Er-doped 0.96(K_0.48Na_0.52)(Nb_0.965Sb_0.035)O₃-0.04(Bi_0.5Na_0.5)ZrO₃ Multifunctional Ceramics with Good Temperature Stability

Lead-free 0.96(K_0.48Na_0.52)(Nb_0.965Sb_0.035−xEr_x)O₃-0.04(Bi_0.5Na_0.5)ZrO₃ (KNNS_0.035−xE_x-BNZ) multifunctional ceramics were prepared by the conventional solid-state reaction method combined with the two-step sintering method. The results show that the ceramics doped with a moderate amount of Er³⁺ show good piezoelectric properties and excellent temperature stability, while displaying bright up-conversion photoluminescence. Er³⁺-doping shifts the orthorhombic–tetragonal phase transition temperature (T_O-T) down to below room temperature and enhances the temperature stability of the ceramics. For a 0.2 mol%Er³⁺-doped sample, the room-temperature piezoelectric constant d₃₃ reached 343 pC/N. The sample shows good resistance to thermal depoling as noted that the d₃₃ value remaining at more than 95% of that at room temperature when the temperature was increased to 230 °C. Relying on the luminescence behavior of rare earth Er³⁺ ions, Er³⁺-doped samples exhibit a strong photoluminescence with a bright green up-conversion emission under excitation at 980 nm. Two green emissions, at 525 nm and 550 nm, corresponded to the transitions from ²H_11/2 → ⁴I_15/2 to ⁴S_3/2 → ⁴I_15/2, and a weak red emission at 660 nm was assigned to the ⁴F_9/2 → ⁴I_15/2 transition obtained under the excitation of a 980-nm laser. The multifunctional properties in the Er-doped samples make them promising for multifunctional device applications.