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 (Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2)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. • Li dissolves into the rock salt HEO phase in substitutional positions. • Single-phase HEO is formed without the Co3+ reduction in the case of Li-doping. • Li sits in two well-distinct chemical environments within the HEO structure. • The presence of some kind of short-range order within the Li-doped HEO is hypothesized. [ABSTRACT FROM AUTHOR]