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

Highlights: • Ba₆Cu₂MSn₄S₁₆ (M = Fe, Ni) and Sr₆N₂FeSn₄S₁₆ (N = Cu, Ag) were synthesized. • Four isostructural compounds crystallize in space group $I\bar{4}3d$ of the non-centrosymmetric three-dimensional cubic structure. • Single crystals of target compounds were grown with flux KI. -- Abstract: Ba₆Cu₂GSn₄S₁₆ (G = Fe, Ni), and Sr₆D₂FeSn₄S₁₆ (D = Cu, Ag) were synthesized in an A-B-P-Q (A = Sr, Ba; B = Cu, Ag, Li; P = Sn, Ge; Q = S, Se, Te) system, through the introduction of the Fe and Ni atoms. The four compounds were found to be isostructural. They crystallize in space group $I\bar{4}3d$ (no. 220) of the three-dimensional cubic structure. The structure is composed of SnS₄ and (Cu|G)S₄ by sharing corners with Ba²⁺ cations distributed in the channels of the Ba₆Cu₂GSn₄S₁₆ (G = Fe, Ni). The structure is composed of SnS₄ and (D|Fe)S₄ by sharing corners with Sr²⁺ cations distributed in the channels of the Sr₆D₂FeSn₄S₁₆ (D = Cu_, Ag). The UV–vis diffuse reflectance spectra showed the Ba₆Cu₂FeSn₄S₁₆, Ba₆Cu₂NiSn₄S₁₆, Sr₆Cu₂FeSn₄S₁₆, and Sr₆Ag₂FeSn₄S₁₆ energy band gaps to be 1.20 eV, 0.82 eV, 1.53 eV and 1.87 eV, respectively.