Localized and Bandlike Valence-Electron States in FeS2and NiS2

Photoemission energy distributions for $20<~h\nu <~50$eV for semiconducting Fe${\mathrm{S}}_2$ (fool's gold) show a narrow, ∼1 eV wide, localized uppermost valence level, whereas the corresponding levels of semiconducting Ni${\mathrm{S}}_2$ are broad (∼3.5 eV) and bandlike. Self-consistent-field $X\alpha$ cluster calculations indicate that this level in Fe${\mathrm{S}}_2$ is the $3d$-like $t_{2g}$ state, while in Ni${\mathrm{S}}_2$ the uppermost level is the more bandlike $e_g$ state, with $t_{2g}$ states about 2 eV lower that overlap the broad S $3p$-derived valence bands.