Unconventional Photo‐Dielectric Effect of K 0.5 Na 0.5 Nb 0.992 Cu 0.008 O 3‐δ Lead‐Free Ceramics

Conventional photo‐dielectric effect (PDE), that is, the phenomenon in which light illumination causes an increase in dielectric constant (ε r ) accompanied by a significant increase in dielectric loss (tanδ), is generally observed only at low frequencies (typically below 10 4 Hz). In this work, effect of light irradiation on dielectric properties of K 0.5 Na 0.5 Nb 0.992 Cu 0.008 O 3‐δ (KNNC) lead‐free ceramics was studied. A distinct unconventional PDE behavior was observed in KNNC, deviating from the conventional one. Under light irradiation, ε r of KNNC increases significantly even at frequencies above 10 4 Hz, while tanδ is almost unaffected. With switching the light on and off, the change in ε r exhibits a pronounced switching effect. The photo‐induced increase in ε r depends on both the light wavelength and light intensity. The conductivity of KNNC under light irradiation remains on the order of 10 −13 S/cm, indicating excellent electrical insulation. Density functional theory was used to analyze the band structure of KNNC, confirming that Cu 2+ ‐doping introduces in‐gap states. The contributions of the in‐gap states and thermal effects to the unconventional PDE of KNNC were discussed.