Accessing metastable rocksalt phases in ferrimagnetic (CrMnFeCoNi)O and (CrMnFeCoZn)O high-entropy oxides through a mechanical alloying pathway

ABSTRACT Phase evolution, microstructure, and properties of mechanically alloyed (CrMnFeCoNi)O (Ni-HEO) and (CrMnFeCoZn)O (Zn-HEO) high-entropy oxide (HEO) powders are systematically investigated. Combined X-ray diffraction, transmission electron microscopy, and Raman spectroscopy demonstrates that both systems form a predominant face-centered cubic (FCC) phase with minor spinel and amorphous phases after 100 h of milling. Nevertheless, their formation mechanisms differ fundamentally. Ni-HEO develops by solid solution into a rocksalt structure based on the NiO matrix. While, Zn-HEO undergoes a complete reconstruction from wurtzite ZnO to a novel rocksalt FCC phase. Zn-HEO exhibits lattice contraction, unlike the lattice expansion in Ni-HEO, reflecting the rocksalt structure's cation accommodation capability. Microstructural characterization confirms that both HEO powders exhibit agglomerated, spherical-like particles with a bimodal size distribution. Chemically, Ni-HEO possesses abundant oxygen vacancies and a pronounced reduction of high-valence metal cations to their +2 oxidation state. In contrast, Zn-HEO displays a nearly defect-free structure, and negative chemical shifts. These distinctions directly govern functional properties. Ni-HEO exhibits higher saturation magnetization (∼6.0 emu/g) and lower coercivity (∼98 Oe), while Zn-HEO shows lower room-temperature resistivity (0.896 MΩ∙cm). The zero-field-cooled and field-cooled magnetization curves indicate the presence of ferromagnetic interactions in both HEOs, resembling those of spinel-structured rather than rocksalt-structured HEOs. Both HEOs exhibit a typical negative temperature coefficient of resistance with similar activation energies, despite demonstrating distinctly different conduction mechanisms. The successful synthesis of these two distinct HEOs via mechanical alloying underscores the critical role of the principal oxide components in tailoring phase evolution and final material characteristics.