XRD, XPS and 119Sn NMR study of tin sulfides obtained by using chemical vapor transport methods

Application of the chemical vapor transport method to the Sn-S system allowed three different phases (viz. SnS, SnS2 and Sn2S3) to be synthesized. No evidence of the formation of other, previously reported mixed valence compounds such as Sn3S4 or Sn4S5 was found, whichever the Sn:S atomic ratio and temperature gradient used. Except for SnS2, which was always obtained as a pure phase as a result of starting from the required stoichiometry, a mixed phase was invariably obtained. The XPS spectrum for Sn2S3 was only slightly different from those for SnS and SnS2 in spite of the presence of Sn(II) and Sn(IV) in the former compound, which hindered the identification of mixed valence compounds in the Sn-S system by the use of this spectroscopic technique. By contrast, the chemical shifts, anisotropy and skew parameters for Sn2S3 as obtained by 119Sn NMR were markedly different from those for SnS and SnS2, and reflected the severe distortion of Sn(IV) and, especially, Sn(II) in the former compound relative to the latter two. The time scale for this resonance technique is shorter than the lifetime of the valence states, which allows one to unambiguously distinguish the two oxidation states of Sn