Are Selenides the Same as Sulfides? Structure, Spectroscopy, and Properties of Narrow-Gap Rare-Earth Semiconductors Sn(SSe) ( = La, Ce; = 0-0.8).

The ternary rare-earth sulfides SnS ( = La-Nd) and the partial solid solutions Sn(SSe) ( = La, Ce; = 0-0.8) were prepared in the form of polycrystalline samples by reaction of the elements at 900 °C and as single crystals in the presence of KBr flux. They adopt the LaSnS-type structure (orthorhombic, space group , = 2) consisting of chains of edge-sharing Sn octahedra separated by atoms. Although the cell parameters evolve smoothly in Sn(SSe), detailed structural analysis by single-crystal X-ray diffraction revealed a pronounced preference for the Se atoms to occupy two out of the three chalcogen sites, which offers a rationalization for why the all-selenide end-members SnSe do not form. Solid-state Sn NMR spectra confirmed the nonrandom distribution of SnSSe local environments, which could be resolved into individual resonances. The Raman spectra of SnS compounds show an intense peak at 307-320 cm assigned to a symmetric A mode, which is dominated by Sn-S bonds; the Raman peak intensities varied with Se substitution in LaSn(SSe). Optical diffuse reflectance spectra, band structure calculations, and electrochemical impedance spectra indicated that these compounds are narrow band gap semiconductors; the optical band gaps are insensitive to substitution in SnS (0.7 eV) but they gradually decrease with greater Se substitution in Sn(SSe) (0.7-0.4 eV).