Preparation and formation mechanism of Cr-free spinel-structured high entropy oxide (MnFeCoNiCu)3O4

In this work, two Cr-free high entropy oxides (HEOs), an equimolar (MnFeCoNiCu)3O4 and a non-equimolar (Mn0.272Fe0.272Co0.272Ni0.092Cu0.092)3O4 have been synthesized by a solid-state reaction method. The reaction sequence and electrical conductivity were also studied for these two HEOs. It is demonstrated that a rock-salt phase containing a solid solution of NiO and CuO appears in the synthesizing process of (MnFeCoNiCu)3O4, which is ascribed to the incomplete solubilization of rock-salt phase in the spinel phase. For (Mn0.272Fe0.272Co0.272Ni0.092Cu0.092)3O4, a single spinel phase (Fd-3m) is obtained at 750 °C, which is much lower than that of the (MnFeCoNiCu)3O4 sample. Furthermore, Mn, Fe, Co, Ni elements exist in the chemical states of +2 and + 3, and Cu exists in Cu2+ state. The electrical conductivity of (Mn0.272Fe0.272Co0.272Ni0.092Cu0.092)3O4 is approximately 15.77 S cm-1 at 800 °C, which is nearly three times higher than that of the (MnFeCoNiCu)3O4 sample.