Thermal stability and electrical properties of XFe2O4 (X = Co,Cu,Fe,Mg,Mn,Ni) high-entropy spinel ferrites prepared by reactive flash sintering

This study investigates the high-temperature stability and phase composition of two high-entropy oxides (HEOs), (Mn 0.2 Co 0.2 Ni 0.2 Cu 0.2 Fe 0.2 )Fe 2 O 4 and (Mn 0.2 Co 0.2 Ni 0.2 Cu 0.2 Mg 0.2 )Fe 2 O 4 , prepared as single-phase samples using the reactive flash sintering technique. Results show that the annealing temperature in a nitrogen atmosphere has a significant impact on the stability of the compounds. The destabilization of the spinel structure occurs in a two-step process: spinel HEO → spinel HEO + Fe 2 O 3 → spinel HEO + Cu based-oxide. This sequence is inferred from in-situ XRD experiments and calorimetric analysis, and confirmed by TEM observations. Impedance spectroscopy analysis revealed a complex, thermally activated electrical response comprising bulk and grain boundary contributions. AC conductivity follows Jonscher’s universal power law, with a temperature dependence of the S parameter consistent with overlapping large polaron tunneling. These findings provide insight into charge transport and relaxation processes in the prepared HEOs, improving their understanding for potential electrical applications. • Synthesis and sintering in a single step of XFe 2 O 4 (X = Co,Cu,Fe,Mg,Mn,Ni) HEOs by reactive flash sintering, RFS. • Study of the thermal stability by in-situ and ex-situ measurements. • Destabilization: spinel → spinel + Fe 2 O 3 → spinel + Cu-based oxide. • Study of the electrical properties of HEOs prepared by RFS.