Sulfur K- and L-edge XANES spectroscopy of monosulfide solid solutions

Synchrotron radiation S K- and L-edge XANES spectra and unit-cell parameters are reported for a variety of monosulfides. These include; (1) the cubic, rocksalt (B1) structure sulfides niningerite (MgS), alabandite (MnS) and oldhamite (CaS), and their solid solutions (Mg,Fe)S, (Mn,Fe)S and (Mg,Mn)S, and (2) the non-stoichiometric binary and ternary Fe-Co-Ni monosulfide solid solution (mss; M_0.923S, M = Fe, Co, Ni), with the NiAs-type structure.; The absorption edge feature of the S K- and L-edge XANES spectra represents transition of S 1s and 2p electrons to unoccupied S 3p σ* and S 3s σ*, 4s σ* and 3 d antibonding states, respectively. Pre-edge (edge feature for the Fe-Co-Ni mss) features at the S K and L-edge are attributed to transitions to these same states hybridized with metal 3d(e_g) states and to a lesser extent 3d(t_2g) states.; Increase in pre-peak area reflects an increase in overall attainability of metal 3d states for hybridization with S 3p σ* (K-edge) or S 3s σ* (L-edge) antibonding states as proportionally more metal 3d orbitals become available. Results indicate that the absorption of synchrotron radiation reflects the local electronic structure of individual absorber atoms (i.e., the SM₆ cluster), and is not a group (crystal energy band) effect. In particular, 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 β) and (t₂gβ) states and nearest-neighbour coordination of the metal atoms than by the precise coordination of S and the extended structure of the sulfide.; Low-temperature phase relations for (Fe,Co)_1−xS, coexisting with Co-pentlandite, with the NiAs-type structure (Co-mss) unmixes abruptly below 425°C, giving co-existing Fe-rich (Co- mss1; 1C structure) and Co-rich (Co-mss2; 3C structure) phases. There is evidence of a metastable solvus within the equilibrium miscibility gap and a narrow field of spontaneous phase separation as observed by the appearance of satellite reflections to h0ℓ NiAs-type subcell reflections in products quenched from high temperature.