Syntheses, structures, optical properties, and electronic structures of Ba6Cu2GSn4S16 (G = Fe, Ni) and Sr6D2FeSn4S16 (D = Cu, Ag)

Ba6(CuxZy)Sn4S16 (Z = Mg, Mn, Zn, Cd, In, Bi, Sn): High Chemical Flexibility Resulting in Good Nonlinear-Optical Properties

Seven acentric sulfides Ba₆(Cu_xZ_y)Sn₄S₁₆ (Z = Mg, Mn, Zn, Cd, In, Bi, Sn) were grown by a high-temperature salt flux method. The crystal structures of the Ba₆(Cu_xZ_y)Sn₄S₁₆ (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 Ba₆(Cu_xZ_y)Sn₄S₁₆ (Z = Mg, Mn, Zn, Cd, In, Bi) compounds are isostructural and crystallize in the Ba₆Ag₄Sn₄S₁₆ structure type. The Sn-containing compound exhibits high structural similarity to Ba₆(Cu_xZ_y)Sn₄S₁₆ (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 Ba₆(Cu_2.9Sn_0.4)Sn₄S₁₆ 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 Ba₆(Cu_2.9Sn_0.4)Sn₄S₁₆. The monovalent Cu atoms, mixed with certain metals with various oxidation states, significantly shift the optical properties of the Ba₆(Cu_xZ_y)Sn₄S₁₆ (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). Ba₆(Cu_1.9Zn_1.1)Sn₄S₁₆ possesses a high SHG response and a high LDT of 2.8 × AGS and 3 × AGS, respectively. A density functional theory calculation revealed that CuS₄ and SnS₄ tetrahedra significantly contribute to the SHG response in Ba₆(Cu₂Mg)Sn₄S₁₆, which also confirmed that CuS₄ tetrahedra are crucial for the stability and optical properties of the Ba₆(Cu_xZ_y)Sn₄S₁₆ (Z = Mg, Mn, Zn, Cd, In, Bi, Sn) compounds revealed by electronic structure analysis.