Structural, Spectroscopic, and Thermoanalytic Studies on Bi₂Fe₄O₉: Tunable Properties Driven by Nano- and Poly-crystalline States

We report on average crystallite size-dependent structural, spectroscopic and thermoanalytic studies of Bi 2 Fe 4 O 9 synthesized by a sol-gel method. In-situ heating X-ray diffraction revealed transformation of an X-ray amorphous precursor into a rhombohedral perovskite-type BiFeO 3 followed by a second transformation into orthorhombic mullite-type Bi 2 Fe 4 O 9 phase. Twelve samples with average crystallite sizes between 35.3(4) and 401(17) nm were produced using calcination of the precursor for 2 h at temperatures between 900 and 1073 K. The average crystallite sizes calculated from X-ray diffraction and transmission electron microscopy for selective samples are in excellent agreement. X-ray powder diffraction data Rietveld refinements demonstrate each structural feature as a function of the average crystallite size. Both in the in-situ and ex-situ experiments the metric parameters evolve through expansion of a - and contractions of b - and c -cell parameters, leading to a parabolic expansion of the cell volume. The associated nanoproperties significantly differ across a critical average crystallite size of 122(2) nm, such as metric parameters, polyhedral distortions, vibrational mode frequencies, electronic bandgaps and hyperfine parameters. The antiferromagnetic transition temperature was found to be 249(1) K for an average crystallite size of 86(1) nm, and its transition enthalpy significantly decreases below the critical size as observed by differential scanning calorimetry. The second absorption feature in the UV/vis, which is typical for bulk Bi 2 Fe 4 O 9, was found to be mainly caused by the d-d transition of the Fe 3+ cations in the FeO 4 tetrahedra. The nanosize dependent Raman line-shape has been modeled using an analytical expression. The temperature-dependent Raman spectra helped us to understand the crystallite size-dependent vibrational features of Bi 2 Fe 4 O 9 .