Tunability of magnetization and bandgap in mullite-type Bi2Fe4O9 ceramics through non-magnetic ions

Polycrystalline Bi2Fe4O9 and Bi2Fe3.6Me0.4O9 (Me=Al and Ti) ceramics were prepared using a modified Pechini method. It is observed that the introduction of Ti apparently increases low-temperature magnetization of Bi2Fe4O9, while the Al substitution has weakly effect. Meanwhile, the optical bandgap reduces from 2.06eV for Bi2Fe4O9 to1.86eV for Ti substitution, and 2.01eV for Al substitution, respectively. All results may be ascribed to the random replacement of both octahedral and tetrahedral Fe ions for Al substitution while preferly replacement of octahedral sites for Ti substitution, as confirmed by the Rietveld refinement and Raman spectra.