Syntheses, Characterization, and Optical Properties of Ternary Ba–Sn–S System Compounds: Acentric Ba7Sn5S15, Centric BaSn2S5, and Centric Ba6Sn7S20

Three new ternary Ba–Sn–S system compounds, acentric Ba7Sn5S15, centric BaSn2S5, and centric Ba6Sn7S20 have been designed and synthesized by a conventional high-temperature solid-state reaction method using the evacuated silica tubes. The crystal structure of Ba7Sn5S15 shows the coexistence of a SnS4 tetrahedral and a Sn2S3 trigonal bipyramid. Importantly, the larger dipole moment of the [Sn2S3]2– trigonal bipyramid group and the polarity enhancement of the bipyramidal arrangements result in a strong SHG effect at 2.05 μm, which is 10 times of the SHG intensity of the benchmark AgGaS2 with the particle size of 30–46 μm and twice as much as that with the particle size of 150–212 μm. Evidently, the acentric Ba7Sn5S15 is a novel IR NLO crystal material with a wide mid-IR window and a strong SHG effect, which is the first reported among the Ba–Sn–S ternary system. Moreover, Ba7Sn5S15 can achieve type-I phase-matching that can be used for practical applications. In the centric BaSn2S5, all Sn atoms are coordinated by five S atoms to form novel SnS5 trigonal bipyramid polyhedrons. In the other centric Ba6Sn7S20, there is the coexistence of the two coordination patterns with a SnS5 trigonal bipyramid and SnS4 tetrahedral polyhedrons, featuring a special crystal structure in the Ba–Sn–S system.