Transferability of phase shifts for extended x-ray-absorption fine-structure studies of metal sulfides and sulfur on nickel surfaces

Abstract

Extended x-ray-absorption fine-structure (EXAFS) spectra of \ensuremath{\alpha}-MnS, CoS, \ensuremath{\alpha}-NiS, \ensuremath{\beta}-NiS, ${\mathrm{Ni}}_{3}$${\mathrm{S}}_{2}$, wurtzite ZnS, and GaS have been recorded at the S K edge. These temperature-dependent data together with Ni K-edge EXAFS from \ensuremath{\beta}-NiS are analyzed to investigate a breakdown of transferability in S-Ni EXAFS scattering phase shifts. A comparison of our data with theoretical calculations of the EXAFS indicates that the S-metal bond distance for CoS and \ensuremath{\beta}-NiS is underestimated by 0.07 \AA{}, while all the remaining sulfides produce values within 0.02 \AA{} of the bond lengths expected from x-ray diffraction. This explains discrepancies in the literature for EXAFS-derived surface S-Ni bond distances, some of which have employed S-Ni phase shifts transferred from \ensuremath{\beta}-NiS while others have used ${\mathrm{Ni}}_{3}$${\mathrm{S}}_{2}$. The latter are found to give results agreeing with those of other surface structural techniques. We suggest a revision of published S-Ni distances for surface EXAFS of sulfur on Ni(100), Ni(110), and Ni(111). We point out that all determinations of surface structure using electron-scattering techniques will suffer from the same problems of inaccuracies of calculated phase shifts.