Effects of alkaline-earth dopants on structural, optical and magnetic properties of Bi₂Fe₄O₉ powders

The pure Bi₂Fe₄O₉ and Bi_{2 (1−x)} A_2xFe₄O₉ (A = Ca, Ba, x = 0.03) powders are synthesized via a modified solid-state reaction method to study the effects of alkaline-earth metal ions doping on crystal structural, optical and magnetic properties. Both X-ray diffraction and Raman spectroscopy data reveal that all the powders are Mullite-type Bi₂Fe₄O₉ orthorhombic single phase without any impurities. Much greater structural distortion in Bi_1.94Ba_0.06Fe₄O₉ than that of Bi_1.94Ca_0.06Fe₄O₉ is observed. The chemical compositions of Ba²⁺ and Ca²⁺ doped powders have been investigated with energy dispersive X-ray spectroscopy (EDS). X-ray photoelectron spectroscopy results indicate that oxygen vacancies could be found in all doped powders. The ratio of Fe²⁺ in the total Fe ions is almost unchanged by Ca doping and increases a little with Ba substitution. Compared with that of pure Bi₂Fe₄O_9, the band gap values decrease slightly in Bi_1.94Ca_0.06Fe₄O₉ but drop dramatically in Bi_1.94Ba_0.06Fe₄O₉. A clear and obvious ferromagnetic behavior is found in Bi_1.94Ba_0.06Fe₄O₉ at 10 K. However, Bi_1.94Ca_0.06Fe₄O₉ shows a weak ferromagnetism with enhanced magnetization and Bi₂Fe₄O₉ exhibits antiferromagnetism with a linear M–H relationship. The varied bandgap and magnetization resulting from the alkaline-earth metal ionic species are discussed in terms of structural distortion due to the ionic radius size effect.