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.