Identification of the EPR signal of S2− in green ultramarine pigments

Abstract

Green and blue ultramarine pigments are characterized by the sodalite structure Na₆(Al₆Si₆O₂₄) and colored inserted species. These chromophores are sulfur species: S₃⁻ (blue) and S₂⁻ (yellow). Both radicals are encapsulated inside the β-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 S₃⁻ (g = 2.029). In contrast, the S₂⁻ 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 S₂⁻ in ultramarine pigments and we determine its tensor components: g₁ = 2.69(6), g₂ = 2.03(4) and g₃ = 1.86(4).