Electronic environments in Ni3Pb2S2 (shandite) and its initial oxidation in air

Polycrystalline Ni₃Pb₂S₂ (shandite) was synthesised as a non-porous ingot to enable representative surfaces prepared by fracture under UHV to be characterised by X-ray photoelectron spectroscopy (XPS) before and after exposure to air. For an unoxidised surface, the S 2p _3/2 binding energy was found to be significantly lower than those reported previously for shandite itself and other sulfides having shandite structure, and consistent with the physical and chemical properties of the shandites. The core electron binding energies for the three constituent elements were in agreement with the formal oxidation state representation Ni ⁰ ₃Pb^II ₂S^-II ₂, analogous to that deduced previously for Ni₃Sn₂S ₂. Shandite surfaces were found to oxidise rapidly when initially exposed to air under ambient conditions, and concomitant with the formation of the Ni-O and Pb-O species, to restructure to NiS- and PbS-like surface phases having S core electron binding energies no higher than those for shandite.