Density functional theory investigation of S2 in KCl: evidence for the existence of a di-vacancy site

Electron paramagnetic resonance experiments have shown that, depending on the doping procedure, two different S₂⁻ centers may coexist in KCl. These centers have the ²B_2g and the ²B_3g ground state, respectively. As no experimental ligand hyperfine data are available, it could not be determined whether the S₂⁻ molecular ion replaces a single halide ion (mono-vacancy site) or two nearest neighbor halide ions (di-vacancy site). Also, other defect models could a priory be considered. In this work, cluster in vacuo density functional theory calculations of the g and ³³S hyperfine tensors show that the S₂⁻ ion at a mono-vacancy site has the ²B_2g ground state, whereas S₂⁻ in a di-vacancy exhibits a ²B_3g ground state. For the latter center, the possibility of charge compensation by a cation vacancy is also considered. The calculations indicate that a possible vacancy is not in the direct vicinity (nearest or next-nearest neighbor) of the S₂⁻ ion.