LOW-TEMPERATURE CRYSTAL STRUCTURES OF STIBNITE (SB2S3) IMPLYING ORBITAL OVERLAP OF SB 5S2 INERT PAIR ELECTRONS

The crystal structure of stibnite (Sb₂S₃, Pnma, a=11.314(2), b=3.837(2), c=11.234(3) A, V=487.7(3) A³ at 293 K) was refined in situ at, 230, 173, and 128 K. It is a major characteristic of the structure that the Sb-S secondary bonds enclosing Sb 5s² inert lone pair electrons at 293 K are significantly shorter than the corresponding sum of the Sb and S van der Waals radii. The thermal contraction behavior of two kinds of the SbS₇ coordinated polyhedra building the structure showed that the isotropic shrinkage is dramatically induced in the unit-cell volume. Although contraction in both the polyhedral volume and the mean distance from centroid to ligands leads to the reduction in distance between Sb and S atoms, the sphericity values indicate isotropic contractions for two SbS₇ coordinated polyhedra. Moreover, concerning the temperature dependence of the polyhedral volume and the centroid-central atom distances there are continuous contractions with decreasing temperature. Consequently these geometries of nonspherical charge distribution around Sb³⁺ cation remain unchanged at low temperature. This is because that the crystal structure of stibnite at low temperature induces contraction with attractive interactions, which is called the "orbital overlap" between Sb 5s² inert lone pair electrons and ligand orbitals to maintain the coordination environment. In this case, Sb 5s² lone pair electrons are not inert, but active. Such orbital overlaps of inert lone electron pairs can provide an reasonable explanation for lower band gap energy of the binary compounds containing heavy elements such as Sb, Te, Pb, and Bi, which is directly related to color, luster and semiconductivity of their minerals or compounds.