A structural and thermal investigation of Li-doped high entropy (Mg, Co, Ni, Cu, Zn)O obtained by co-precipitation

This work investigates the lithium doping effect on the structural and thermal behavior of (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O high entropy oxide (HEO) obtained by co-precipitation. The powders are characterized by differential thermal and thermogravimetric analyses (DTA-TG), mass spectroscopy, dilatometry, X-ray diffraction (XRD), magic angle spinning nuclear magnetic resonance (MAS NMR), Raman spectroscopy, X-ray photoelectron spectra (XPS) and electron spin resonance (ESR). The results point out that Co3+ ions (within a spinel phase) are reduced to Co2+ before the formation of the high entropy oxide in the absence of Li. Conversely, no reduction is observed in the case of Li-doping, thus indicating the presence of Co3+ within the high entropy rock salt lattice. At high temperature (>1050–1150 °C), the HEO phase loses oxygen changing the charge compensation mechanism for Li incorporation (mostly based on the presence of 3 + cations and oxygen vacancies at low and high temperatures, respectively). Moreover, it is found that lithium lies in two well-distinct chemical environments in HEO, which cannot be completely explained by assuming a random organization of the high entropy phase. This suggests the existence of some short-range order and possible M3+-Li+ pairs.