Tartrate-chelated aqueous gel-route synthesis of multiferroic BiFeO3 nanoparticles: Calcination-dependent thermal, structural, and morphological evolution

Bismuth ferrite (BiFeO 3 , BFO) is the most extensively investigated single-phase, room-temperature multiferroic oxide, but obtaining phase-pure nanoparticles remains difficult because of the narrow thermodynamic stability window of the perovskite phase and the persistent formation of Bi 2 O 3 and Fe 2 O 3 secondary phases. In this work, BiFeO 3 nanoparticles were synthesized through an aqueous organic gel route in which tartaric acid (C 4 H 6 O 6 ) served as the chelating and gel-forming agent for Bi 3 + and Fe 3+ nitrate precursors, in place of the more commonly reported citrate- or ethylenediaminetetraacetic acid (EDTA)-based routes — the central methodological novelty of this study, motivated by tartaric acid's lower cost, simpler two-step acid dissociation relative to citric acid, and substantially better biodegradability relative to EDTA. The Bi–Fe–tartrate precursor gel was dried and calcined for 2 h at six temperatures between 400 and 550 °C, and the resulting powders were characterized by Fourier-transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), and thermogravimetric/differential thermal analysis (TG/DTA). TG/DTA of the precursor gel showed a three-stage mass loss (≈91% of the initial mass) up to ≈ 450 °C, associated with dehydration, tartrate-complex decomposition, and residual-carbon burnout, with an exothermic feature near 280 °C marking the onset of perovskite crystallization. FT-IR spectra confirmed progressive sharpening of the diagnostic Fe–O and Bi–O octahedral stretching bands (≈440–555 cm −1 ) with increasing calcination temperature. XRD revealed a mixed-phase product containing Bi 2 O 3 and Fe 2 O 3 impurities at 400–475 °C, converging to a predominantly single-phase rhombohedral BiFeO 3 (space group R3c) from 500 °C onward, in reasonable agreement with JCPDS card no. 01–076–6047. The average crystallite size, calculated from the Scherrer equation, increased monotonically from 28 ± 3 nm at 450 °C to 39 ± 4 nm at 550 °C, remaining below the 62–64 nm periodicity of the BFO spin cycloid throughout, while FESEM confirmed quasi-spherical to quasi-rhombic particles in the 20–60 nm range. These results identify tartaric acid as an effective, low-cost chelating agent for producing sub-cycloidal, phase-pure BiFeO 3 nanoparticles of interest for magnetoelectric applications.