Processing optimization and sintering time dependent magnetic and optical behaviors of Aurivillius Bi₅Ti₃FeO₁₅ ceramics

Aurivillius Bi₅Ti₃FeO₁₅ (BTF) ceramics were synthesized using the conventional solid state reaction method by optimizing excess of Bi₂O₃ and sintering time. Their structures, magnetic, and optical properties were investigated in detail. The optimum process to sinter pure Aurivillius BTF ceramics was confirmed to be 3 wt. % excess Bi₂O₃ to compensate the Bi volatilization at 1050 °C for 240 min (BTF-240M). The microstructure and crystalline structure of the BTF ceramics had little dependence on the sintering time from the x-ray diffraction (XRD) and scanning electron microscopic data. Nevertheless, the magnetic and optical properties were closely related with the sintering time. The overall magnetic behavior of these BTF ceramics was superparamagnetic (SPM), whereas there were unambiguous clues for the existence of antiferromagnetic (AFM) interactions. However, whether the SPM behavior was intrinsic or arised from a tiny amount of spinel Fe₃O₄ impurity phase cannot be thoroughly ruled out in the XRD detection limit in the present stage. The AFM interactions were weakened upon extending the sintering time. The effective magnetic moment (μ_eff), however, demonstrated different dependency on the sintering time. It increased with the sintering time from 80 min to 240 min, and then dropped with further extending the sintering time. Compared with other BTF ceramics, the BTF-240M ceramic showed the highest values of the refractive index n and real part ε₁, as well as the lowest ones of the extinction coefficient k and imagine part ε₂ in whole photon energy range. Finally, a direct inter-band transition was confirmed for these BTF ceramics and optical energy band gaps were determined to be about 3.08, 3.18, and 3.39 eV for 80 min, 150 min, and 240 min sintered BTF ceramics, respectively, yet abnormal optical behavior was observed in BTF-360M ceramic.