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XPS measurement of fivefold and sixfold coordinated sulfur in pyrrhotites and evidence for millerite and pyrrhotite surface species

¹ Department of Earth Sciences, University of Western Ontario, London, Ontario, N6A 5B7, Canada
² Wilhelm-Ostwald-Institut, Fakultät für Chemie und Mineralogie, Universität Leipzig, Linnéstrasse 2, D-O4103, Germany
³ Deptartment of Chemistry, University of Western Ontario, London, Ontario, N6A 5B7, Canada

Correspondence: ^* E-mail address: hwn{at}julian.uwo.ca

Collection of S 2p XPS spectra of millerite (NiS), using a conventional (AlK) and a synchrotron photon source, demonstrates the presence of one surface species on millerite (NiS) and spectral deconvolution indicates a second surface contribution. The observed surface core-level shift (binding energy = 161.1 eV) is attributed to a surface monomeric species (S^2–) whereas the second contribution probably is a surface dimeric species (S₂^2–, binding energy = 162.3 eV). Surface dimers, if present, indicate surface reconstruction of millerite surfaces upon cleavage. Monoclinic (Fe₇S₈) and hexagonal (Fe₁₀S₁₁) pyrrhotite are non-stoichiometric due to vacancies on metal sites. The conventional S 2p XPS spectra of these phases, which sample primarily bulk sulfur states, reveal contributions from fivefold and sixfold coordinated S atoms, the proportion of which is consistent with structural refinement studies. The more intense signal is derived from S in fivefold coordination (80–85%) and the remainder represents the sixfold contribution. Comparison of a highly surface sensitive S 2p XPS spectrum of pyrrhotite (photon energy tuned to 210 eV) with a conventional S 2p XPS signal (AlK source) indicates the presence of a monomeric surface species (S^2–). Spectral deconvolution of the surface sensitive spectrum indicates another contribution near 162 eV, the origin of which is uncertain. It may represent S atoms in sixfold coordination, surface dimers (S₂^2–) or both.

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