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 or around 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.