First-Principles Study of X-ray Absorption Spectra of FeS2

Abstract

It is well known for a long time that FeS2 has two stable crystal phases, cubic pyrite (space group Pa 3) and orthorhombic marcasite (Pnnm) structures. Surprisingly, local coordinations around Fe and S atoms are quite similar between the two phases. For example, Fe is almost octahedrally coordinated by six S atoms at a distance of 2.26 A in pyrite and at 2.23–2.25 A in marcasite (see Table I). Dumbbell structure of the nearest neighbor S atoms with a bond distance of about 2.2 A is commonly seen in the both phases. X-ray absorption spectroscopy (XAS) is a useful tool to investigate the element-specific electronic structure of condensed matter systems. In particular, local geometry and symmetry around a given site are also analyzed by combining electronic structure calculations with experiments. XAS spectra at S and Fe K-edges have been measured for NiAs-type FeS and pyrite FeS2 and the observed first peak at the S K-edge and the pre-edge structure at the Fe K-edge are attributed to a transition to the empty anti-bonding states formed by hybridization of Fe 3d and S 3p orbitals. Although several first-principles density-functional-theory (DFT) calculations have been reported for Fe sulfides including pyrite and marcasite FeS2 since then, no theoretical analysis on XAS spectra has been performed for Fe–S systems. Discharge and charge reactions in Li/FeS2 and Na/FeS2 were experimentally studied for ion-battery applications. Among them, Kitajou et al. have measured XAS spectra at Fe K-edge for pyrite FeS2 and Na discharged FeS2 and found some variations in the main peak region while the pre-edge structure retains its shape by discharge. In this note, we present some theoretical results of XAS spectra of FeS2 in the pyrite and marcasite phases by performing firstprinciples DFT calculations. Electronic-structure calculations are carried out within the generalized gradient approximation (GGA) by Perdew, Burke, and Ernzerhof (PBE). One-electron Kohn–Sham equations are self-consistently solved by the all-electron full-potential linearized augmented plane wave method (FLAPW) in a scalar relativistic scheme. Experimental crystal structure including lattice constants and internal parameters is assumed throughout the calculation. XAS spectrum can be computed by using Fermi’s golden rule for the transition probability from an initial state jii to a final state j f i by x-ray with the photon energy h ! as Ið!Þ 1⁄4 2 h X