Nanoparticle Precursor into Polycrystalline Bi₂Fe₄O₉: An Evolutionary Investigation of Structural, Morphological, Optical, and Vibrational Properties

Mullite-type Bi₂Fe₄O₉ was synthesized using a polyol-mediated method. X-ray powder diffraction (XRD) revealed that the as-synthesized sample is nanocrystalline. It transformed into a rhombohedral perovskite-type BiFeO₃ followed by a second transformation into mullite-type Bi₂Fe₄O₉ during heating. Each structural feature, from as-synthesized into crystalline phase, was monitored through temperature-dependent XRD in situ. The locally resolved high resolution transmission electron micrographs revealed that the surface of some heated samples is covered by 4–13 nm sized particles which were identified from the lattice fringes as crystalline Bi₂Fe₄O₉. XRD and Raman spectra demonstrate that the nucleation of both BiFeO₃ and Bi₂Fe₄O₉ might simultaneously commence; however, their growth and ratios are dependent on temperature. The diffuse UV/vis reflectance spectra showed fundamental absorption edges between 1.80(1) and 2.75(3) eV. A comparative study between the “derivation of absorption spectrum fitting method” (DASF) and the Tauc method suggests Bi₂Fe₄O₉ to be a direct band gap semiconductor