High piezoelectricity and optical transmittance are ordinarily difficult to simultaneously achieve in randomly oriented ceramics or textured ones, affecting their further applications in optoelectronic field. How to break this trade-off relationship becomes a challenge for realizing electrical-optical coupling. In this paper, Er³⁺-doped randomly oriented (K_0.5Na_0.5)NbO₃ (abbreviated as xEr-KNN-R) and textured (K_0.5Na_0.5)NbO₃ (abbreviated as xEr-KNN-T) ceramics were prepared via tape-casting. the results showed that the xEr-KNN-T ceramics could have a high degree of orientation and a high relative density (up to 98.8%). The piezoelectric coefficient d₃₃ of the textured ceramics substantially boosted to 121 pC/N compared to randomly-oriented ones (30 pC/N), without significant decrease of the Curie temperature. Also, refinement of grains with tiny pores and relaxor-like behavior could endow the textured ceramics with moderate optical transparency (about 30% in the visible region). Meanwhile, the xEr-KNN-T ceramics exhibited enhanced up-conversion green and red luminescence intensity. The translucent-piezoelectric ceramics could provide insight for further development of high-performance optoelectronic devices.

x% (x = 0, 0.50, 0.75, 1.00, 1.25) Er³⁺ doped KNN with a nominal composition of (K_0.5Na_0.5)_1-xEr_xNbO₃ (abbreviated as xEr-KNN) matrix powders were synthesized via conventional solid-state reaction with K₂CO₃ (99.99%), Na₂CO₃ (99.99%), Nb₂O₅ (99.99%) and Er₂O₃ (99.9%) as raw materials. Anisotropic plate-like NN templates (with about 20 µm in length and width and about 5µm in thickness) were synthesized by a two-step molten salt method. The textured ceramics (abbreviated as xEr-KNN-T) were prepared via reaction template grain growth. Meanwhile, the non-textured randomly oriented ceramics (abbreviated as xEr-KNN-R) prepared without NN templates were used for comparison purpose. The microstructures of samples were determined by scanning electron microscopy (SEM, SUPRA 55 SAPPHIRE, Carl Zeiss Co., Germany) on the surface parallel to the casting direction and the section perpendicular to the casting direction using the accelerating voltage in a range of 0.02-30 kV. To measure the electrical properties, silver paste was coated on both sides of the sintered samples, and then poled in a silicone oil under 3 kV/mm at 150 ℃ for 30 min. The temperature dependences of the relative dielectric permittivity (ε_r) and dielectric loss (tanδ) of the ceramics were recorded by an Impedance Analyzer (Agilent 4294, Agilent Technologies Inc., USA), with an oscillation voltage of 1 V, at a frequency range of 1 kHz-1 MHz and a temperature range of 25-525℃ at a heating rate of 2℃/min. The piezoelectric coefficients (d₃₃) were characterized by a quasi-static piezoelectric constant testing meter (ZJ-6A, Institute of Acoustics, Chinese Academy of Sciences, China). The polarization-electric field (P-E) loops of the ceramics were determined by a ferroelectric test system (TF Analyzer 2000, aixACCT Co., Germany) with the frequency of 200 Hz. The optical transmittance was measured by ultraviolet-visible-near infrared spectroscopy (Lambda 950, PerkinElmer Co., USA). The PL emission spectra were recorded by a spectro-fluorometer (USB4000, Ocean Optics Co., USA) equipped with a 980-nm diode laser (MDL-III, CNI Optoelectronics Tech. Co., China).

Based on the SEM image of NN template, the NN grains synthesized by the topochemical microcrystal conversion (TMC) well inherit the anisotropic morphology of the precursor BNN5, showing a typical flake morphology. The length and width of NN grains are approximately 10-20 µm and the thickness is approximately 0.5-1.0 µm, meeting the size requirement of appropriate NN template used for tape-casting. From the optical transmittance curve, the addition of large size NN template with anisotropy is unfavorable to the transmittance of higher isotropy, resulting in the reduced transmittance of xEr-KNN-T. The XRD patterns of xEr-KNN-T show sharp diffraction peaks with a good crystallinity, presenting a typical ABO₃-type perovskite structure. Er³⁺ diffuses into the KNN lattice, and the NN template has a solid solution with the KNN matrix. Based on the SEM images of all the ceramics, the conventional solid-state reaction, the tape-casting process alone cannot improve the microstructure of KNN, but leads to deterioration in density. The existence of NN template leads to relatively small-sized grains of textured KNN-T ceramics. From the up-conversion PL spectra, the introduced flake-shaped NN template can reduce the symmetry of local environment around the KNN matrix and then cause local heterogeneity and change of crystal field round Er³⁺, thus increasing the up-conversion PL intensity of the xEr-KNN-T ceramics. In electrical tests (i.e., dielectric temperature spectrum and P-E loops), compared with xEr-KNN-R, the xEr-KNN-T ceramics have an improved piezoelectric coefficient (i.e., d₃₃ = 121 pC/N), while maintaining moderate ferroelectricity and a high Curie temperature (i.e., 371℃).

Er³⁺-doped KNN ceramics with random orientation and texture were prepared by a tape-casting method. The xEr-KNN-T ceramics exhibited a high relative density (i.e., up to 98.8%) and a uniform microscopic morphology (with the grain size of 0.33 µm). The piezoelectric performance of xEr-KNN-T ceramics was optimized to a large extent (i.e., d₃₃ = 121 pC/N), which was higher than that of the xEr-KNN-R ceramics, without lowering T_C. Meanwhile, xEr-KNN-T ceramics had a moderate optical transmittance (approximately 30% in the visible region), realizing the coexistence of piezoelectricity and transparency. Enhanced up-conversion luminescence characteristics were displayed in xEr-KNN-T. The translucent-piezoelectric KNN textured ceramics could provide a possibility and an enlightenment for the research of other high-performance lead-free optoelectronic materials.