title: Identification of the EPR signal of S₂⁻ in green ultramarine pigments.
abstract: Green and blue ultramarine pigments are characterized by the sodalite structure Na(6)(Al(6)Si(6)O(24)) and colored inserted species. These chromophores are sulfur species: S(3)(-) (blue) and S(2)(-) (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 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).
authors.affiliation: Laboratoire de Spectrochimie Infrarouge et Raman (LASIR), Bât. C5 USTL, 59655 Villeneuve d'Ascq, France.
authors.forename: Katarzyna
authors.initials: K
authors.lastname: Raulin
authors.forename: Noëlle
authors.initials: N
authors.lastname: Gobeltz
authors.forename: Hervé
authors.initials: H
authors.lastname: Vezin
authors.forename: Nadia
authors.initials: N
authors.lastname: Touati
authors.forename: Bertrand
authors.initials: B
authors.lastname: Ledé
authors.forename: Alain
authors.initials: A
authors.lastname: Moissette