Nd 3+ ‐Modified KNN‐Based Ceramics With Coupled Optical and Piezoelectric Properties

Transparent lead‐free piezoelectric ceramics are promising for multifunctional optoelectronic devices. However, the simultaneous realization of high optical transparency, piezoelectricity, and light‐tunable luminescence remains challenging. In this study, Nd 3+ ‐modified 0.94(K 0.5 Na 0.5 ) 1−3 x Nd x NbO 3 –0.06Sr(Zn 1/3 Nb 2/3 )O 3 ceramics were designed to regulate phase structure, microstructure, defect chemistry, and optical−electrical coupling. Nd 3+ doping has shown to drive the structural evolution from a tetragonal‐dominated phase toward a tetragonal and cubic coexist phase, while regulating grain growth and enhancing densification. The optimized composition with x = 0.01 exhibits a high relative density of 99.7%, refined grains, an optical transmittance of 67.26% at 780 nm, and an enlarged optical bandgap of 3.02 eV. Meanwhile, characteristic Nd 3+ upconversion emissions are observed under 808 nm excitation, and both transmittance and upconversion photoluminescence can be modulated by 365 nm light irradiation. This photochromic‐induced luminescence quenching is attributed to the presence of light‐triggered color centers, which are often associated with various types of defects including alkali vacancies, oxygen vacancies, and dopant‐related defects. Benefiting from the balanced pseudo‐cubic structure, dense microstructure, enhanced dielectric response, and retained ferroelectric polarization, the x = 0.01 ceramic achieves a maximum piezoelectric coefficient of 136 pC/N. This work provides an effective strategy for integrating transparency, piezoelectricity, upconversion photoluminescence, and photochromic modulation in lead‐free KNN‐based ceramics.