Spinel ferrites of the quaternary system Cu-Ni-Fe-O: Synthesis and characterization

The decomposition of the freeze dried Cu(II)-Ni(II)-Fe(III) formate precursors at 1000°C in air yields complex oxides CuxNi1−xFe2O4±δ (0 ≤ x ≤ 1) with a cubic spinel structure. For x < 0.7, single phase spinels are formed at 1000°C. However, for 0.7 ≤ x ≤ 1, Copper oxide (CuO) is identified as a second phase and the formation of a pure spinel phase requires an increase of the iron content in the mixture. For example, Cu0.81Ni0.1Fe2.09O4 is a single phase at 1000°C/air. Other single spinel phases Cu0.5+yNi0.5−y−zFe2+zO4±δ, 0 ≤ (y + z) ≤ 0.5, in the phase triangle Cu0.5Ni0.5Fe2O4–CuFe2O4–Cu0.5Fe2.5O4 have been synthesized under special p(O2)/T—synthesis conditions. The increase of the iron content requires an increase of the reaction temperature and/or a decrease of the p(O2) in the reaction gas stream. The oxygen exchange between Cu0.9Fe2.1O4.02 and the reducing gaseous phases shows that the non stoichiometry δ of copper ferrite is only about ±0.03. Significant changes in the oxygen content lead to the separation in different phases. The electrical and magnetic properties of copper ferrite samples depend on their chemical composition and preparation conditions.