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

Abstract

The decomposition of the freeze dried Cu(II)-Ni(II)-Fe(III) formate precursors at 1000°C in air yields complex oxides Cu_xNi_1−xFe₂O_4±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, Cu_0.81Ni_0.1Fe_2.09O₄ is a single phase at 1000°C/air. Other single spinel phases Cu_0.5+yNi_0.5−y−zFe_2+zO_4±δ, 0 ≤ (y + z) ≤ 0.5, in the phase triangle Cu_0.5Ni_0.5Fe₂O₄–CuFe₂O₄–Cu_0.5Fe_2.5O₄ 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 Cu_0.9Fe_2.1O_4.02 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.