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

Various metal-bearing ferrites were produced directly from aqueous solutions at 25 degrees 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 degrees 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 degrees C) and Zn atoms were fully incorporated into the ferrite frameworks occupying the most stable position; i.e. the tetrahedral sites. (C) 1999 Elsevier Science S.A. All rights reserved.