New Interpretation and Approach to Curve Fitting Synchrotron X-ray Photoelectron Spectra of (Fe,Ni)9S8 Fracture Surfaces

Abstract The surface electronic structure of (Fe,Ni)9S8 was investigated using synchrotron X-ray photoelectron spectroscopy (SXPS). SXPS S 2p and valence band (VB) spectra, collected as a function of photon energy, have resolved two distinct bulk states and two surface chemical states present at (Fe,Ni)9S8 fracture surfaces. The SXPS S 2p spectra show two distinct sulfur sites within the lattice that have been attributed to 4-coordinate sulfur at 161.7 eV, and 5-coordinate sulfur at 162.2 eV. Upon fracture, sulfur sites are exposed at multiple fracture planes and lose at least one bonding partner, resulting in surface 3-coordinate sulfur at 161.1 eV, and a slightly broader peak at 161.7 eV attributed to 4-coordinate sulfur surface monomer species overlapping the bulk 4-coordinate species. Significant asymmetric tail shapes were observed in each of the S 2p spectra and have been identified as ligand-to-metal charge transfer satellites (LMCT). A practical approach for curve fitting the (Fe,Ni)9S8 S 2p spectrum has been proposed.