Ambient-temperature synthesis of metal-bearing ferrites: How and why?

Various metal-bearing ferrites were produced directly from aqueous solutions at 25°C by simultaneous control of the oxidizing conditions and pH. In order to make clear the involved mechanisms, this work investigated the correspondence between the reaction conditions of formation of ferrite-type compounds and their crystallinity, nature of incorporated water and developed magnetic characteristics. The formation of a Zn-bearing ferrite was considered as a first-case study. X-ray diffractometry (XRD), Fourier transform infrared (FT-IR) and extended X-ray absorption fine structure spectroscopy (EXAFS) measurements were undertaken. It was found that the crystallinity, the dehydration of the intermediate and the diminution of the sulfate contents in the ferrite-type precipitates could be promoted by: (i) increasing the Fe/Zn mole ratios; (ii) a suitable duration of the aeration of the suspensions at pH 11.0 (contact stage) or, (iii) by aging of the precipitates in their mother liquors at 25°C. These effects were attributed to the suitable progress of the oxidation-hydrolysis reactions of Fe(II) entities and the loss of molecular water from the intermediate compound, which also explained the observed enhancement in the saturation magnetization (M_s) of the precipitates. Furthermore, the analysis of the local structure of Fe and Zn atoms by EXAFS evidenced that the ambient temperature ferrite exhibited a similar structure than the one produced by the ceramic method (above 1000°C) and Zn atoms were fully incorporated into the ferrite frameworks occupying the most stable position; i.e. the tetrahedral sites.