Crystal orbital contributions to the pyrrhotite valence band with XPS evidence for weak Fe-Fe π bond formation

Synchrotron excited X-ray photoelectron spectra (SXPS) of hexagonal pyrrhotite reveal three distinct Fe 3d-derived photopeaks within its outer valence band. The t(2g)alpha band (majority spin) is centered at about 2.5 eV, the e(g)alpha band at about 1.0 eV and the t(2g)beta (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-Xalpha 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 t(2g)alpha emission (at 2.5 eV) being strongly enhanced and broader than the t(2g)beta emission (0.25 eV). The explanation of these observations requires the presence of weak Fe-Fe pi and pi* crystal (molecular) orbitals located near 2.5 eV, and separated by no more than about 0.5 eV. The pi-bonded crystal orbitals are derived from weak mixing of adjacent Fe t(2g) atomic orbitals along the c crystallographic axis.