Synchrotron XPS evidence for Fe²⁺-S and Fe³⁺-S surface species on pyrite fracture-surfaces, and their 3D electronic states

Abstract

A X-ray photoelectron Fe 2p_3/2 spectrum of a pristine pyrite fracture surface was collected using synchrotron radiation with the source tuned to 800 eV. Comparison of this highly surface sensitive Fe 2p spectrum with Fe 2p spectra collected by conventional means (1487 eV AlKa source) reveals that the high binding energy tail of the pyrite Fe 2p_3/2 line results primarily from Fe surface state contributions. The three major contributions to the spectrum are interpreted to be: (1) Fe²⁺ resident on bulk sites; (2) Fe²⁺ resident on surfaces, edges and corners; (3) Fe³⁺ surface states produced during fracture by an auto-redox reaction involving Fe and S. The intense main peak is ascribed to the bulk state, whereas the high binding energy tail of the spectrum is composed primarily of Fe²⁺ and Fe³⁺ surface state contributions.

Fe²⁺ on bulk sites is octahedrally coordinated (O_h symmetry). All valence electrons of Fe on bulk sites are paired (diamagnetic) and a singlet photopeak at 707 eV is consequently produced. Fracture produces Fe²⁺ surface states with lower coordination than bulk sites. Fe²⁺ located at surfaces, edges and corners experiences modified Ligand Field Stabilization Energies (LFSE) which results in stabilization of the d_z2 orbital and destabilization of the dxy orbital. Promotion of a dxy electron to the d_z2 orbital makes surface Fe²⁺ surface states paramagnetic resulting in multiplet splitting of their associated photopeaks. The Fe³⁺ surface state is necessarily paramagnetic and its photoemissions are consequently multiply split.

Analysis of photopeak structures and binding energy splittings of Fe²⁺ and Fe³⁺ surface states demonstrates that they are located at the appropriate binding energies, and span the appropriate energy range, to satisfactorily explain the high binding energy tail on of the Fe 2p_3/2 spectrum.