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 rM 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 VS = 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‖/$ \root 3 \of {3V_{\rm S}/(4\pi)} $) 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)