Ba$_6$(Cu$_x$Z$_y$)Sn$_4$S$_{16}$(Z = Mg, Mn, Zn, Cd, In, Bi, Sn): High chemical flexibility resulting in good nonlinear-optical properties

Abstract

Seven acentric sulfides Ba$_6$(Cu$_x$Z$_y$)Sn$_4$S$_{16}$ (Z = Mg, Mn, Zn, Cd, In, Bi, Sn) were grown by a high-temperature salt flux method. The crystal structures of the Ba$_6$(Cu$_x$Z$_y$)Sn$_4$S$_{16}$ (Z = Mg, Mn, Zn, Cd, In, Bi, Sn) compounds were determined by single crystal X-ray diffraction with the aid of solid-state NMR spectroscopy. The Ba$_6$(Cu$_x$Z$_y$)Sn$_4$S$_{16}$ (Z = Mg, Mn, Zn, Cd, In, Bi) compounds are isostructural and crystallize in the Ba$_6$Ag$_4$Sn$_4$S$_{16}$ structure type. The Sn-containing compound exhibits high structural similarity to Ba$_6$(Cu$_x$Z$_y$)Sn$_4$S$_{16}$ (Z = Mg, Mn, Zn, Cd, In, Bi) with the presence of an interstitial atomic position partially occupied by Sn atoms. The chemical bonding characteristics of Ba$_6$(Cu$_{2.9}$Sn$_{0.4}$)Sn$_4$S$_{16}$ were understood with electron localization function calculations coupled with crystal orbital Hamilton population calculations. The Ba−S and Cu−S interactions are dominantly ionic, but the Sn−S interactions consist of strong covalent bonding characteristics in Ba$_6$(Cu$_{2.9}$Sn$_{0.4}$)Sn$_4$S$_{16}$. The monovalent Cu atoms, mixed with certain metals with various oxidation states, significantly shift the optical properties of the Ba$_6$(Cu$_x$Z$_y$)Sn$_4$S$_{16}$ (Z = Mg, Mn, Zn, Cd, In, Bi) compounds. This results in a good balance between the second harmonic generation (SHG) response and laser damage threshold (LDT). Ba$_6$(Cu$_{1.9}$Zn$_{1.1}$)Sn$_4$S$_{16}$ possesses a high SHG response and a high LDT of 2.8 × AGS and 3 × AGS, respectively. A density functional theory calculation revealed that CuS$_4$ and SnS$_4$ tetrahedra significantly contribute to the SHG response in Ba$_6$(Cu$_2$Mg)Sn$_4$S$_{16}$, which also confirmed that CuS$_4$ tetrahedra are crucial for the stability and optical properties of the Ba$_6$(Cu$_x$Z$_y$)Sn$_4$S$_{16}$ (Z = Mg, Mn, Zn, Cd, In, Bi, Sn) compounds revealed by electronic structure analysis.