Electron spin relaxation of S₃ ^- in ultramarine blue and lapis lazuli

The blue color that has made lazurite (lapis lazuli) a prized mineral is due to the same S₃ ^- radical that is in synthetic ultramarine blue (UMB), which has been proposed as a CW EPR standard. Continuous wave and pulsed EPR spectra and relaxation times of S₃ ^- are compared for three commercial sources of synthetic UMB, for samples of lapis lazuli from Afghanistan, Chile, Colorado USA, and Pakistan, and a solution in DMSO:dioxane. The spin concentrations in the UMB samples were high, in the range of 3×10²⁰ to 5×10²⁰ spins/g. The field-swept echo detected spectra of UMB samples have lineshapes at 4.2 K that depend on the field at which phase adjustment is performed, indicating strong spin-spin interaction. The spectra of the minerals included large spectral contributions from Mn²⁺, in addition to S₃ ^- for which the concentrations were 6×10¹⁸ to 2.9×10¹⁹ spins/g. Features in the spin-echo detected spectra attributed to forbidden Mn²⁺ transitions were confirmed by comparison with Mn²⁺ spectra in CaO powder. Large distributions in relaxation times caused derived results to depend strongly on the experimental acquisition windows for echo decay and inversion recovery curves. Short phase memory times are attributed to spin-spin interactions and to motion of the S₃ ^- in the lattices. Relatively weak temperature dependence of spin lattice relaxation rates below about 25 K is attributed to substantial spin-spin interaction and cross relaxation. The strong spin-spin interaction is not present for 0.4 mM S₃ ^- in DMSO:dioxane. The shorter T₁ for S₃ ^- than for SO₂ ^- or SO₃ ^- is attributed to stronger spin orbit coupling.