Identification of the EPR signal of S-2(-) in green ultramarine pigments

Green and blue ultramarine pigments are characterized by the sodalite structure Na-6(Al6Si6O24) and colored inserted species. These chromophores are sulfur species: S-3(-) (blue) and S-2(-) (yellow). Both radicals are encapsulated inside the beta-cages. They contribute to the EPR spectrum of ultramarine pigments. The well-known strong EPR signal observed in all ultramarine pigments Continuous-Wave (CW) spectra has long been assigned to S-3(-) (g = 2.029). In contrast, the S-2(-) contribution is still subject to controversy because its signal in ultramarine pigments was not resolved even at low temperature in CW-EPR experiments. In this study, we identify unambiguously for the first time by CW-EPR and field sweep-echo detected (FS-ED) EPR the signal of S-2(-) in ultramarine pigments and we determine its tensor components: g(1) = 2.69(6), g(2) = 2.03(4) and g(3) = 1.86(4).