Comparative study of a high-entropy metal disulfide and its parent compounds using x-ray absorption spectroscopy

We study the local electronic structure of an equimolar-multimetal solid solution of high-entropy metal disulfide (Fe,0.16em0exCo,0.16em0exNi,0.16em0exCu)S₂ and its parent compounds MS₂ (M=Fe, Co, Ni, and Cu) using x-ray absorption spectroscopy (XAS). The Fe, Co, and Ni L2,3-edge absorption spectra indicate a divalent metal state both in (Fe, Co, Ni, Cu)S₂ and its parent compounds, except for the Cu L2,3-edge absorption spectra. The Cu L2,3-edge spectra of CuS₂ and (Fe,0.16em0exCo,0.16em0exNi,0.16em0exCu)S₂ show satellites, which rule out the divalent Cu but can be analyzed as a combination of monovalent and trivalent copper states. The L2,3-edge XAS spectral analysis with charge-transfer multiplet cluster model calculations was carried out for (Fe,0.16em0exCo,0.16em0exNi,0.16em0exCu)S₂ and its parent compounds. The estimated electronic parameters indicate a negative charge-transfer energy for the parent compounds and high-entropy compound (with the Ni L edge in the high-entropy compound being an exception), which corresponds to a p-p type lowest energy excitation in the extended Zaanen-Sawatzky-Allen phase diagram. The analysis shows that the charge-transfer energy Δ decreases and the on-site Coulomb energy Udd increases systematically from Fe to Cu. The results suggest that in the high-entropy compound compared to the parent compounds, the hybridization strengths are weaker for Fe 3d-S $3p$ and Co 3d-S $3p$ and stronger for Ni 3d-S $3p$ and Cu 3d-S $3p$ bonds. This behavior is consistent with the longer Fe-S and Co-S bond distances and shorter Ni-S and Cu-S bond distances in the high-entropy compound compared to the parent compounds. The results indicate modifications in the structural lattice parameters of the high-entropy compound are reflected in the electronic structure and provides evidence for the so-called cocktail effect in the high-entropy compound.