Bandgap narrowing and ferroic modulation in KNbO3 ceramics composited by spinel AFe2O4 (A = Mg, Zn, Ni, Co)

Lead-free multiferroic composite ceramics, KNbO3-AFe2O4 (KNO-AFO, A = Mg, Zn, Ni, Co), are synthesized. The microstructure and property characterizations reveal that the introduced AFO can intrinsically modulate the optical/ferroelectric/ferromagnetic properties due to the independent existence of the two phases in the composite ceramics, including the significantly narrowing of the optical bandgap from the intrinsic ~3.2 eV of KNO down to ~1.2 eV, of which the modulation behavior is in line with the prediction of the firstprinciples calculations, and more importantly, the maintaining of the excellent ferromagneticity (with a remnant magnetization value of 0.8 - 36.5 emu/g) and ferroelectricity (~1 - 4 mu C/cm2) but for slightly deteriorating of the leakage as compared to the anti-magnetic KNO. In addition, different spinel ferrites composited to KNO present different functions for the ferroelectric/ferromagnetic properties, i.e., the orthospinel phase mainly retaining the ferroelectricity of the composite ceramics while the anti-spinel phase responsible for the ferromagnetism, especially in the KNO-CFO ceramics that not only have the strongest narrowing effect of the optical bandgap but also demonstrate the most excellent magnetic properties. This makes the composite ceramics promising for a wide range of applications in ferric-based storage and photovoltaics, and should offer more options for the research of multifunctional materials and devices. (c) 2022 Published by Elsevier B.V.