Influence of Co/Fe Ratio on the Synthesis of Cobalt Containing Metal–Spinel Composites by Using Iron Disproportionation

Metal–spinel composites made up of an iron–cobalt alloy and a cobalt-doped magnetite are synthesized by using the disproportionation of Fe(OH)2and the reduction of Co(II) by Fe0in a concentrated and hot KOH solution. The effect of the Co/Fe ratio (0 ≤ Co/Fe ≤ 1) on the composition and the structure is described. Two KOH concentrations, 9.0 and 11.5 mol/liter, have been considered. A partial dissolution of cobalt hydroxide in KOH occurs. The metallic part always contains an iron–cobalt alloy of bcc structure. Additional diffraction lines occur, favored by high Co/Fe ratios. They are assigned to a metallic phase, either a fcc structure and an unknown structure close to the hexagonal cobalt, or a cobalt–iron alloy of hexagonal structure witha= 7.136 (2) Å andc= 2.186 (2) Å. In vacuum, the composites undergo a transformation into the wüstite phase. The temperature range of the transformation increases with a further increase of Co/Fe.