Evolution of local electronic structure in cubic Mg1−xFexS by S K-edge XANES spectroscopy

Synchrotron radiation S K-edge XANES spectra are reported for rocksalt (B1) structure Mg1−xFexS, with x=0–0.68. The edge peak for pure MgS is at 2474.5 eV, consistent with that of a large (3.7 eV) band gap monosulfide semiconductor. XANES spectra of all Fe-bearing compositions contain a pre-edge shoulder or peak which progressively evolves with increase in Fe content into a feature analogous to the edge peak of the S K-edge XANES spectrum of FeS (NiAs-type; B8 structure). The pre-peak is, therefore, assigned to transition of S 1s electrons to unoccupied hybridized S 3p σ∗ antibonding and Fe 3d(eg) states. Evolution of S K-edge XANES spectra in MgS–FeS binary compositions with increase in Fe content shows: (1) increasing probability of a transition from the S 1s core level to unoccupied S 3p σ∗ antibonding states hybridized with Fe 3d(egβ) states, and (2) a distinguishable phase boundary at ∼68 mol% FeS, for B1 phase coexisting with B8, consistent with previous P-XRD studies. The S K-edge XANES spectrum of the hypothetical cubic (B1) FeS phase has been derived from the spectrum of Mg0.375Fe0.625S.