Structural phase transition, narrow band gap, and room-temperature ferromagnetism in [KNbO₃]_1-x[BaNi_1/2Nb_1/2O_3-δ]_xferroelectrics

Structural phase transition, narrow band gap (E_g), and room-temperature ferromagnetism (RTFM) have been observed in the [KNbO₃]_1-x[BaNi_1/2Nb_1/2O_3-δ]_x(KBNNO) ceramics. All the samples have single phase perovskite structure, but exhibit a gradual transition behaviour from the orthorhombic to a cubic structure with the increase of x. Raman spectroscopy analysis not only corroborates this doping-induced change in normal structure but also shows the local crystal symmetry for x 0.1 compositions to deviate from the idealized cubic perovskite structure. A possible mechanism for the observed specific changes in lattice structure is discussed. Moreover, it is noted that KBNNO with compositions x = 0.1-0.3 have quite narrow E_gof below 1.5 eV, much smaller than the 3.2 eV band gap of parent KNbO₃(KNO), which is due to the increasing Ni 3d electronic states within the gap of KNO. Furthermore, the KBNNO materials present RTFM near a tetragonal to cubic phase boundary. With increasing x from 0 to 0.3, the magnetism of the samples develops from diamagnetism to ferromagnetism and paramagnetism, originating from the ferromagnetic-antiferromagnetic competition. These results are helpful in the deeper understanding of phase transitions, band gap tunability, and magnetism variations in perovskite oxides and show the potential role, such materials can play, in perovskite solar cells and multiferroic applications.