Microstructural and magnetic properties of Fe/Cr-substituted ferrite composites

Composites made of iron and Cr doped magnetite are obtained by precipitatio n from the metallic chlorides salts into a KOH solution heated under reflux . A KOH concentration of 14 mol l(-1) is required. The composition of the s amples is determined by X-ray diffraction, TG measurements, chemical analys is and Mossbauer spectrometry. The higher the chromium content, the smaller the lattice parameter and the lower the metal ratio. For Cr/Fe less than o r equal to 0.1, the chemical formula of the composites can be written Fe-A/ (Fe(x)(nu+)Cr(y)(3+)square O-3-(x+y)(4))(1-A), 0 less than or equal to y le ss than or equal to 0.23. We show that only in-field Mossbauer spectrometry at low temperature is suitable to estimate unambiguously both the proporti ons of octahedral and tetrahedral iron sites in spinel and the valence of d elocalised iron state, if any. A peculiar A-site occupation by either vacan cies or Cr3+ ions is observed. This is probably due to the low temperature synthesis. The two phases, metal and spinel are found to be intimately mixe d within the nanometer sized particles. For Cr/Fe ratios higher than 0.1, h ydroxide phase is also present.