Sr₅Ga₈O₃S₁₄: A Nonlinear Optical Oxysulfide with Melilite-Derived Structure and Wide Band Gap

Sr₅Ga₈O₃S₁₄, which is a novel nonlinear optical oxysulfide, was discovered via solid-state reaction. The compound crystallizes in noncentrosymmetric space group P2₁2₁2 (No. 18) of the orthorhombic system. The crystal structure is derived from the melilite structure and composed of anionic [Ga8O3S14]∞210− layers intercalated by Sr²⁺ cations. The anionic layers are constructed by [GaOS₃]^5– and [GaS₄]^5– tetrahedra via either corner sharing or edge sharing, while typical melilite only features edge-sharing mode. Because of the existence of electronegative O atoms, it possesses a large band gap (3.9 eV) with a wide transparent region of 0.3–13.4 μm. It exhibits second-harmonic-generation response, which is 0.8 times that of the reference AgGaS₂ (AGS) in the particle size range of 20–41 μm under laser irradiation at a wavelength of 2.09 μm laser. Density functional theory calculations indicate that the band edges are mainly composed of the Ga–S bond orbitals, with little participation of O 2p near the valence band maximum.