Interatomic distances in pyrite-structure disulfides – a case for ellipsoidal modeling of sulfur ions

The geometry of crystalline bonding in pyrite-structure disulfides MS₂ is investigated for the M = Mn, Fe, Co, Ni, Cu series. The decomposition of interatomic distances by the ionic radii approach is first shown to yield metal ion values r_M inconsistent with state-of-the-art data. The bonding geometry is subsequently analyzed by assuming sulfur ions to become ellipsoidally deformed in accordance with their crystallographic site symmetry. It is derived, how the S bonding coordination can be modeled by a polar radius r_‖ in direction of the trigonal axis and an equatorial radius r_⊥ perpendicular to it. Sulfur ions are found to compress along the symmetry axis yielding r_⊥/r_‖ ratios between 1.55 and 1.73 in the MnS₂–CuS₂ series and the additivity of the interatomic M–S distance from ion-specific parameters is re-established. A constant volume of V_S = 0.0133 nm³ is consistently attained by sulfur ions in all MS₂ investigated. Finally, the crystal electric field acting at sulfur sites is uncovered to determine the ion deformation ratio (r_⊥–r_‖/) with unexpected precision. These results imply that polarizable ions at polar lattice sites should appropriately be modeled by ellipsoids rather than by spheres. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)