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 Ba(7)Sn(5)S(15), centric BaSn(2)S(5), and centric Ba(6)Sn(7)S(20) have been designed and synthesized by a conventional high-temperature solid-state reaction method using the evacuated silica tubes. The crystal structure of Ba(7)Sn(5)S(15) shows the coexistence of a SnS(4) tetrahedral and a Sn(2)S(3) trigonal bipyramid. Importantly, the larger dipole moment of the [Sn(2)S(3)](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 AgGaS(2) 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 Ba(7)Sn(5)S(15) 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, Ba(7)Sn(5)S(15) can achieve type-I phase-matching that can be used for practical applications. In the centric BaSn(2)S(5,) all Sn atoms are coordinated by five S atoms to form novel SnS(5) trigonal bipyramid polyhedrons. In the other centric Ba(6)Sn(7)S(20), there is the coexistence of the two coordination patterns with a SnS(5) trigonal bipyramid and SnS(4) tetrahedral polyhedrons, featuring a special crystal structure in the Ba-Sn-S system.