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Evolution of local electronic structure in alabandite and niningerite solid solutions [(Mn,Fe)S, (Mg,Mn)S, (Mg,Fe)S] using sulfur K- and L-edge XANES spectroscopy

¹ Department of Earth Sciences,University of Western Ontario,London, Ontario N6A 5B7, Canada
² Faculty of Physics, Rostov State University,344090, Rostov-on-Don, Russia

Correspondence: ^* E-mail: Shannon.Farrell{at}drdc-rddc.gc.ca

Synchrotron radiation S K- and L-edge X-ray absorption near-edge structure (XANES) spectra are reported for the cubic, rocksalt (B1) structure sulfides niningerite (MgS), alabandite (MnS), and oldhamite (CaS), and for their solid solutions (Mn,Fe)S and (Mg,Mn)S, and S L-edge XANES spectra are reported also for (Mg,Fe)S solid solutions. Pre-edge features at the S K-edge are attributed to transition of S 1s electrons to the lowest available unoccupied S 3p * antibonding states hybridized with metal 3d(e_g) states, and at the S L-edge to transition of S 2p electrons to unoccupied S 3s *, 4s *, and 3d antibonding states hybridized with metal 3d(e_g) states, and to a lesser extent 3d(t_2g) states.

The S K-edge XANES spectra for the solid solutions show a progressive participation of 3d orbitals in metal-S bonding with increase in substitution by Fe in (Mn,Fe)S and (Mg,Fe)S and Mn in (Mg,Mn)S through progressive increase in the area of the pre-edge feature. However, the pre-peak area does not increase linearly in each solid solution series showing that a real change in bulk electronic properties has occurred. Increase in pre-peak area reflects an increase in overall attainability of metal 3d states for hybridization with S 3p * antibonding states as proportionally more metal 3d orbitals become available. The S L-edge XANES spectra show progressive evolution of pre-edge features at the L₃- and L₂-edges (a₁ and a₂, respectively). Only a₂ is present in the S L-edge XANES spectrum of FeS (troilite), and with progressive decrease in Fe content in (Mn,Fe)S and (Mg,Fe)S solid solutions, a₁ first appears, then becomes dominant. Since a₁ is attributed to transition of S 2p_3/2 electrons to S 3s * states hybridized with metal 3d(e_g) and 3d(t_2g) states, this appears to represent an increased contribution from metal-S -bonding. The results show that the size and position of the pre-edge features to the S K- and L-edges are controlled more by the DOS of hybridized 3d(e_g^ß) and 3d(t_2g ^ß) states and nearest-neighbor coordination of the metal atoms than by the precise coordination of S and the extended structure of the sulfide.

The full multiple scattering approach has been applied to the calculation of the S K-edge XANES spectra of MgS, MnS, and CaS. Results are consistent with experimental XANES spectra, especially for the pre-edge features, which are often neglected in such calculations.

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