THEORETICAL-STUDIES ON THE STRUCTURAL AND SPECTRAL PROPERTIES OF SILICON SULFIDE GLASSES

Local structural units existing in silicon sulfide and alkali-metal silicon sulfide glasses have recently been identified by comparison of their Si-29 NMR spectra with those of model compounds. In constrast to silicon oxides, where the Si is shielded by corner sharing of the silicate tetrahedra and deshielded by edge sharing, the Si sulfides show only a small shielding arising from corner sharing but a substantial shielding resulting from edge sharing. To help in understanding this and other differences between the silicon oxides and sulfides, we have used ab initio quantum mechanical methods to calculate the equilibrium structures, vibrational spectra and NMR spectra for some simple molecular models of the Si sulfide species occurring in condensed phases. The compounds studied include the series SiH4-Si(SH)4, the isoelectronic compounds SiCl4, SiS44-, and Si(SH)4, the tetrahedral corner-sharing molecules (SiH3)2S and (H2SiS)3, the edge-sharing molecules (H2SiS)2, Si2S4H4, and Si3S4H4, the face-sharing dimer Si2S3H2, and the edge-sharing, corner-sharing combination Si4S5H6. Accurate structures, energetics, and vibrational spectra can be obtained for these molecules at the polarized split-valence ab initio SCF level. The calculated vibrational spectrum for Si3S4H4 matchs well against that of solid SiS2. However, accurate NMR shieldings require at least a doubly polarized basis on the Si and S and single polarization functions on the H. At the highest basis set level employed we find the Si in the edge-sharing dimer (H2SiS)2 to be shielded with respect to that in the corner-sharing trimer (H2SiS)3 by a few ppm, in agreement with experiment for their methylated gas-phase analogues. However, this difference between Si chemical shifts for these corner- and edge-sharing gas-phase compounds is substantially smaller than that observed in the analogous solids. We offer a qualitative explanation for this result based on the relative energies and hardnesses of edge-sharing and corner-sharing oligomers. Our calculations also reveal differences in S nuclear quadrupole coupling constants and NMR shieldings between the different species which may be useful for their characterization.