Study on the structural, ferroelectric, and optical properties of KNN ferroelectric ceramics doped with Cu
摘要
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, Eg, was compared with that measured experimentally, where we found an appreciable reduction in Eg, with Cu doping from 3.13 eV for pure KNN to 1.75 eV for KNN with 1% Cu.