Study on the structural, ferroelectric, and optical properties of KNN ferroelectric ceramics doped with Cu

In this work, the influence of Cu doping (0, 0.5, and 1.0 mol% Cu) on the structural, ferroelectric, dielectric, and optical properties of the K0.5Na0.5NbO3 (KNN) electroceramic system was investigated. Bulk samples were synthesized via the conventional ceramic method, calcined at 900 °C for 4 h, pressed uniaxially at 385 MPa, and sintered at 1100 °C for two different time intervals:1 and 3 h. The XRD patterns of the calcined powders confirmed the formation of the perovskite structure, and the SEM micrographs of the sintered samples revealed grain sizes between 1 and 2 μm. Hysteresis loops (polarization vs. applied electric field) revealed ferroelectric behavior in the pure KNN and KNN1%Cu samples; in contrast, KNN0.5%Cu showed a pinched hysteresis loop, characteristic of antiferroelectric or aging behavior. Dielectric function as a function of temperature allows identifying the reported temperatures of the crystalline phase transitions of KNN, orthorhombic–tetragonal, and tetragonal–cubic, which occur at approximately 200 °C and 420 °C respectively; the highest temperature corresponds to the Curie temperature, where a transition to the paraelectric phase occurs. First-principles calculations were realized to determine the band structure and density of states for pristine and Cu-doped KNN. The calculated energy gap, E g , was compared with that measured experimentally, where we found an appreciable reduction in E g , with Cu doping from 3.13 eV for pure KNN to 1.75 eV for KNN with 1% Cu.