Electronic Structure Change of NiS$_{2-x}$Se$_x$ in the Metal-Insulator Transition Probed by X-ray Absorption Spectroscopy

TL;DR

This study investigates how the electronic structure of NiS$_{2-x}$Se$_x$ changes with Se concentration using Ni L-edge X-ray absorption spectroscopy, revealing a transition from insulating to metallic states linked to local structural changes.

Contribution

It introduces a detailed analysis of the electronic structure evolution in NiS$_{2-x}$Se$_x$ across the metal-insulator transition using XAS and CICM, highlighting the role of local bonding and a percolation scenario.

Findings

Ni L-edge XAS spectra distinguish insulating and metallic states.

Charge-transfer energy $$ changes abruptly near $x=0.5$.

Percolation of Ni(S,Se)$_6$ octahedra influences gap closure.

Abstract

The electronic structure change of NiS$_{2-x}$Se$_x$ 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 $L_3$ 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 ($\Delta \sim 5$ eV) from ligand $p$ to Ni 3$d$ states and a little small $p$-$d$ hybridization strength ($V_{pd\sigma} \sim 1.1$ 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 ($x \leq$ 0.5) are a charge transfer insulator. However, it also requires that the $\Delta$ 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.