Electronic structure of FeS2:: The crucial role of electron-lattice interaction

Using the results of fully self-consistent all-electron first-principles calculations for semiconducting iron pyrite we discuss the major factors governing the semiconducting properties as well as the chemical bonding of this material. The calculations are based on density functional theory within the local density approximation and employ the augmented spherical wave method in its scalar-relativistic implementation. The electronic properties are dominated by strongly hybridized Fe 3d and S 3p states. The chemical bonding is analyzed using an ab initio implementation of the crystal orbital overlap population. Chemical stability is shown to result mainly from the Fe-S bonding. While the upper part of the valence band is formed mainly from Fe 3dt(2g)-derived states the conduction band comprises the e(g)-derived levels. Thr conduction band minimum, in contrast, is exclusively due to S 3p states, this fact explaining the observed high optical absorption. For the same reason the optical properties are strongly influenced by the short sulfur-sulfur bonds. We demonstrate that only small deviations in the sulfur pair bond lengths involve rather drastic changes of the near-gap electronic states which might even turn the indirect band gap into a direct one. These findings allow us to understand the rather high sensitivity of the optical band gap to the incorporation of defects. Finally, our results open perspectives for photovoltaic applications of FeS2.