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 um 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 their growth and ratios are dependent on temperature. The diffuse UV/vis is 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.