Thermally driven rock-salt to spinel transition in high-entropy (CoFeMnNiZn)xOy oxides: Effects on magnetic and mechanical properties

High-entropy (CoFeMnNiZn) x O y oxides with rock-salt and spinel structures were synthesized by a one-step solution combustion synthesis method employing metal nitrates as oxidizers and glycine as fuel. Two distinct combustion regimes were established by adjusting the fuel-to-oxidizer ratio. Fuel-lean conditions favored the formation of a single-phase rock-salt (CoFeMnNiZn)O, whereas fuel-rich conditions led to the formation of spinel (CoFeMnNiZn) 3 O 4 through the re-oxidation of reduced intermediate phases. Thermal analysis and spark plasma sintering (SPS) revealed a reversible transformation between the rock-salt and spinel structures driven by the redox environment, i.e. oxygen uptake and release processes. SPS enabled densification up to ~97 % of the theoretical density and promoted grain growth while preserving nanoscale crystallinity, which is important for structural reversibility. The structural evolution was tracked using X-ray diffraction, Raman spectroscopy, and electron microscopy. Entropy calculations confirmed the high-entropy character of both phases. The sample retaining partial spinel content showed higher hardness and magnetization, while the fully transformed counterpart exhibited higher coercivity. These insights offer a tunable platform for designing functional high-entropy oxides with broad technological relevance. • Thermally driven phase change observed between rock-salt and spinel in (CoFeMnNiZn)xOy oxides. • Fuel-oxidizer ratio controls phase: lean yields rock-salt, rich leads to spinel formation. • Spinel-rich samples show higher hardness and magnetization than rock-salt one. • Nanoscale crystallinity enables reversible structure and tunable properties of the high-entropy oxides.