The selective site occupation, structural and thermal stability of high entropy (CoCrFeMnNi)3O4 spinel

Abstract

The selective site occupation may occur in high entropy spinel oxide (HESO), which is essential for the structural and thermal stability. A single-phase (Co0.2Cr0.2Fe0.2Mn0.2Ni0.2)3 O4 was therefore investigated to reveal its high stability in a wide temperature from room temperature to 1400°C in air/nitrogen. Characteristic features and DFT calculations proved the derived structural model, [Fe0.5Co0.5]Td [Co0.1Fe0.1Cr0.6Mn0.6Ni0.6]Oh O4, which highlighted the selective site occupation in HESO. The real structure presented here provides an alternate viewpoint on the cationic arrangement in HESO, which presented low crystal field stable energy value (−25.74 Dq) and low formation energy (−5.884 eV/atom). The HESO exhibited a low weight loss of 1.68 wt% at 1400 °C in nitrogen, which was 42.86 % lower than nickel ferrite spinel. No precipitation or decomposition was observed in the single-phase HESO in air. However, since the Gibbs free energy change of possible precipitation reactions of NiO and CoO oxides were negative below 1594 K, the precipitation of rock-salt phase close to (NiCo)1-δ could be observed in nitrogen due to oxygen deficiency.