Are Selenides the Same as Sulfides? Structure, Spectroscopy, and Properties of Narrow-Gap Rare-Earth Semiconductors RE 2 Sn(S 1- x Se x ) 5 ( RE = La, Ce; x = 0-0.8).

The ternary rare-earth sulfides RE 2 SnS 5 ( RE = La-Nd) and the partial solid solutions RE 2 Sn(S 1- x Se x ) 5 ( RE = La, Ce; x = 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 La 2 SnS 5 -type structure (orthorhombic, space group Pbam , Z = 2) consisting of chains of edge-sharing Sn Ch 6 octahedra separated by RE atoms. Although the cell parameters evolve smoothly in RE 2 Sn(S 1- x Se x ) 5 , 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 RE 2 SnSe 5 do not form. Solid-state 119 Sn NMR spectra confirmed the nonrandom distribution of SnS 6- n Se n local environments, which could be resolved into individual resonances. The Raman spectra of RE 2 SnS 5 compounds show an intense peak at 307-320 cm -1 assigned to a symmetric A 1g mode, which is dominated by Sn-S bonds; the Raman peak intensities varied with Se substitution in La 2 Sn(S 1- x Se x ) 5 . 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 RE substitution in RE 2 SnS 5 (0.7 eV) but they gradually decrease with greater Se substitution in RE 2 Sn(S 1- x Se x ) 5 (0.7-0.4 eV).