First-principles study on the energetics and vibrational properties of the S2 - impurity in alkali-halide crystals

We have performed first-principles electronic structure calculations for the S2 and S2- molecules in the vacuum and found that the calculated equilibrium bond lengths and vibrational spectroscopic constants are in good agreement with the experiments as well as the previous calculations. Based on this conformation we extended these calculations to the substitutional S2- molecule in the alkali halides, such as NaCl, NaBr, NaI, KCI, KBr, KI, RbCl, RbBr, and RbI. It is found from these calculations that the S2- center aligned to [110] direction is generally favorable over those with the [100] and [111] orientations, in agreement with the experiments. We also found that the vibrational frequencies of the S2- anion in the alkali halides shift to higher energies with decreasing lattice parameters. These results suggest that the change in the bond length of the S2- anion in the matrices depends on the species of halogen atom, while the frequencies are influenced by the species of alkali atom.