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 2B2g ( g x [ 1 ¯ 10 ] < g y [ 001 ] ) and the 2B3g ( g x [ 1 ¯ 10 ] > g y [ 001 ] ) 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 2B2g ground state, whereas S2− in a di-vacancy exhibits a 2B3g 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.