Structural phase transition, optical bandgap, interband electronic transition, and improved magnetism in bivalent Ca-, Sr-, Pb-, and Ba-doped BiFeO₃ ceramics

Perovskite compounds Bi_0.7A_0.3FeO_3−δ (BAFO) (A = Ca, Sr, Pb, and Ba) with improved optical and magnetic properties have been synthesized by solid-state method. The effect of bivalent ions substitution on the phase transition, microstructure, optical, and magnetic properties are investigated in detail. X-ray diffraction and Raman scattering spectra analyses indicate the phase transition from rhombohedral to cubic in doped samples and the coexistence of the two phases in BBFO, inducing strong lattice distortions. A blue shift observed in d–d transitions energies can reveal the decreased internal chemical pressure, thus leading to the slight increase in bandgaps of BSFO, BPFO, and BBFO. While, narrowed bandgap of 1.96 eV can be found in BCFO for the increased tailing of the conduction band edge into the gap. Besides, room-temperature ferromagnetism has been observed in Sr-, Pb-, and Ba-doped samples which can be resulted from the suppression of spiral structure of BFO. These results lay a solid foundation on further exploring the potential abilities of BFO ceramics as multiferroic photovoltaic materials.