CRYSTAL ORBITAL CONTRIBUTIONS TO THE PYRRHOTITE VALENCE BAND WITH XPS EVIDENCE FOR WEAK FE-FE π BOND FORMATION

Abstract

Synchrotron excited X-ray photoelectron spectra (SXPS) of hexagonal pyrrhotite reveal three distinct Fe 3d-derived photopeaks within its outer valence band. The t2gα band (majority spin) is centered at about 2.5 eV, the eg5>α band at about 1.0 eV and the t2gβ (minority spin) contribution at about 0.25 eV. From these data the ligand field splitting energy is 1.5 (±0.2) eV and the majority spin pairing energy is 2.25 (±0.2) eV. These are the first such XPS measurements for this mineral. S 3p-derived bonding and non-bonding bands are identified, with the former centred at about 6.5 eV and the latter near 4.5 eV. The XPS results are remarkably consistent with SCF-Xα scattered wave molecular orbital calculations. Although the calculations and the collected spectra are consistent, they differ from a recent interpretation of the pyrrhotite valence band. An explanation for the discrepant results is provided. Auger resonant enhancement of Fe 3d photopeaks at 60 eV photon energy results in the t2gα emission (at 2.5 eV) being strongly enhanced and broader than the t2gβ emission (0.25 eV). The explanation of these observations requires the presence of weak Fe-Fe π and π*ystal (molecular) orbitals located near 2.5 eV, and separated by no more than about 0.5 eV. The π-bonded crystal orbitals are derived from weak mixing of adjacent Fe t2g atomic orbitals along the c crystallographic axis.