A Series of Noncentrosymmetric Antimony Sulfides Ln8Sb2S15 (Ln = La, Pr, Nd) – Syntheses, Crystal and Electronic Structures, and NLO Properties

Abstract A series of noncentrosymmetric sulfides Ln 8 Sb 2 S 15 (Ln = La, Pr, Nd) were prepared from stoichiometric mixtures of the elements at 1223 K in an evacuated silica tube. The compounds Ln 8 Sb 2 S 15 with Ln = La and Nd are isostructural to Pr 8 Sb 2 S 15 and crystallize in the tetragonal noncentrosymmetric space group I 4 1 cd. Their structure contains discrete [SbS 3 ] 3– trigonal pyramids separated by Ln 3+ cations and S 2– anions. La 8 Sb 2 S 15 shows second harmonic generation with intensities 1.2 times that of the commercially used IR NLO (nonlinear optics) material AgGaS 2 (at 2.05 μm laser). It exhibits excellent thermal stability up to 663 °C. Studies with UV/Vis–NIR diffuse reflectance spectroscopy show that La 8 Sb 2 S 15 has an optical gap of around 2.3 eV, and a DFT study indicates a direct band gap with an electronic transfer excitation of S 3p electrons to a La 5d orbital.