Effects of precursor chemistry and thermal treatment conditions on obtaining phase pure bismuth ferrite from aqueous gel precursors

Phase pure BiFeO3 powders are synthesized by an entirely aqueous solution–gel route, starting from water soluble Fe(III) nitrate or citrate, and Bi(III) citrate as precursors. In order to obtain stable solutions, which transform to homogeneous gels upon drying, the pH is adjusted to 7 and a citric acid content equimolar to the metal ions is selected.The presence of nitrate strongly accelerates the thermo-oxidative decomposition step of the precursor gel around 200 °C, and the decomposition is finished at a lower temperature for the nitrate containing precursor (460 °C) than without nitrates (500 °C) in dynamic dry air. An oxidative ambient is required to fully decompose the precursor.The presented synthesis allows very low temperature (400 °C) crystallization of BiFeO3 together with a secondary phase, as shown by high temperature XRD. This parasitic phase remains up to high temperatures, where decomposition of BiFeO3 is observed from 750 °C onwards, and Bi2Fe4O9 is formed. However, optimization of the furnace treatment, considering anneal temperatures and heating rates showed that phase pure BiFeO3 can be obtained, with the heating rate being the crucial factor (5 °C/min). The chemical purity of the powders is confirmed by FTIR, and the antiferromagnetic to paramagnetic phase transition is demonstrated by DSC measurements.