Modified structural, optical, magnetic and ferroelectric properties in (1-x)BaTiO₃-xBaCo_0.5Nb_0.5O_3-δ ceramics

Lead-free perovskite ceramics with the formula of (1-x)BaTiO₃-xBaCo_0.5Nb_0.5O_3-δ ((1-x)BT-xBCN), exhibit structural phase transition, modified band-gap (E_g), ferromagnetic and ferroelectric orders at room-temperature (RT). All ceramics have no impurity phase but the structural transition from tetragonal to cubic phase is observed with the increase of x. A possible mechanism is proposed to explain the variation of crystal structure. Raman scattering analysis not only proves the existence of doping-induced phase transition but also indicates the changed bonding vibration modes between adjacent atoms. The appearance of two band-gaps in doping samples, the low E_g is about 1.3 eV, obviously smaller than that of 3.2 eV for pure BaTiO₃ (BT), which is attributed to the electronic orbital of Co 3d within the gap of BT. Moreover, the RT ferromagnetic behavior is obtained at x = 0.05, which can be interpreted via the F center exchange model. These results show the potential applications for perovskite oxides on solar energy conversion apparatus and multiferroic devices.