S-2 and S-3 radicals and the S2–4 polysulfide ion in lazurite, haüyne, and synthetic ultramarine blue revealed by resonance Raman spectroscopy

Abstract Taking advantage of the Raman resonance effect, we employed 405 and 532 nm excitations to (1) identify sulfur species present in lazurite, haüyne, and synthetic ultramarine blue pigments and (2) investigate the enigmatic ~485 cm–1 band found previously in Raman spectra of lazurite and haüyne collected with 458 nm excitation. In spectra of lazurite and haüyne, bands of the sulfate ion and S2− and S3− radicals can be seen. Spectra collected using 405 nm excitation show the enhancement of the intensity of ν1(S2−) band and its nν1 (n ≤ 7) progression. Spectra collected using 532 nm incident light show the enhancement of intensity of ν1(S3−), ν2(S3−), and ν3(S3−) bands and the nν1 (n ≤ 9) and ν2 + nν1 progressions of the ν1(S3−) band. In spectra collected with 405 nm excitation, we also found features that we ascribe to the S42− polysulfide ion. These include the ν1 symmetric S-S stretching band at ~481 cm–1, the ν2 symmetric S-S stretching band at ~443 cm–1 (only present in spectra of some lazurite samples), the ν3 symmetric S-S bending at 223 cm–1 and the nν1 (n ≤ 5) and nν1+ν3 progressions of the ν1(S42−) band. We observed that under laser illumination, the S42− polysulfide ion rapidly decomposes into two S2− radicals in lazurite while it remains stable in haüyne. In spectra of synthetic ultramarine blue pigments, only features of S2− and S3− radicals were observed. Finally, we verified the identity of the radical and polysulfide ions with ab initio molecular dynamics calculations. We conclude that Raman resonance spectroscopy is a powerful qualitative method to detect polysulfide and sulfur radical species with concentrations below the detection limit of conventional analytical techniques. Owing to the high stability of S42− in haüyne, this mineral structure appears promising as a host material for S42− entrapment, making it potentially useful for applications in optoelectronics.