Electronic and magnetic properties of NiS_{2-x}Se_x: a comparative study

TL;DR

This study investigates the electronic and magnetic properties of NiS_{2-x}Se_x using various computational methods, revealing the importance of Fock exchange in accurately describing the insulating state and magnetic order.

Contribution

It demonstrates that mixing Fock exchange with local approximations accurately reproduces the insulating ground state and metallic behavior in doped NiS_{2-x}Se_x, clarifying the nature of the gap.

Findings

Fock exchange improves insulating state description

GGA+U fails to capture non-magnetic doped states

GW underestimates the gap and cannot correct metallic character

Abstract

The metal-insulator transition and the problem of d-electron delocalization are investigated in the pyrite system NiS_{2-x}Se_x under pressure using density functional theory (DFT). We test several approximations, including the generalized gradient approximation (GGA), the GGA+U approach, and hybrid functionals. In addition we apply the GW approximation and perform Hartree-Fock calculations. The important role of the chalcogen dimers in the electronic structure is discussed within GGA, which sufficiently describes the role of the lattice in the metal-insulator transition. In addition, the magnetic phase diagram is determined. However, the electronic properties are inadequately described -- the insulating ground state of NiS_2 cannot be obtained -- and the magnetic order is slightly overestimated. If correlations are taken into account within GGA+U, the insulator is found, but the non-magnetic ground state of the doped samples is not accessible. Mixing Fock exchange with local approximations as GGA correctly reproduce the insulating ground state in NiS_2, and metallic ground states with doping and pressure. The insulator is -- in contrast to earlier suggestions -- of Mott type, i. e. the gap opens between Ni 3d {eg} states. Due to the Fock term, the magnetic order is strongly overestimated. The applied GW calculations are not able to correct the metallic character of NiS_2 and underestimate the gap by one order of magnitude.