Electronic structure change of NiS2−xSex in the metal-insulator transition probed by X-ray absorption spectroscopy

The electronic structure change of NiS2−xSex as a function of Se concentration x has been studied by Ni L-edge X-ray absorption spectroscopy (XAS). The XAS spectra show distinct features in Ni L3 edge, indicating whether the system is insulating or metallic. These features can be semi-quantitatively explained within the framework of the configurational interaction cluster model (CICM). In the S-rich region, relatively large charge-transfer energy (Δ ~ 5 eV) from ligand p to Ni 3d states and a little small p-d hybridization strength (Vpdσ ~ 1.1 eV) can reproduce the experimental spectra in the CICM calculation, and vice versa in the Se-rich region. Our analysis result is consistent with the Zaanen-Sawatzky-Allen scheme that the systems in S-rich side (x ≤ 0.5) are a charge transfer insulator. However, it also requires that the Δ value must change abruptly in spite of the small change of x near x = 0.5. As a possible microscopic origin, we propose a percolation scenario where a long range connection of Ni[(S,Se)2]6 octahedra with Se-Se dimers plays a key role to gap closure.