Ba8Sn4S15: A Strong Second Harmonic Generation Sulfide with Zero-Dimensional Crystal Structure

A new noncentrosymmetric (NCS) ternary sulfide, Ba8Sn4S15, was synthesized by a conventional solid-state reaction. The compound crystallizes in the orthorhombic space group Pca21 (29), with unit cell parameters a = 28.727(5) Å, b = 8.5220(14) Å, c = 25.438(4) Å, Z = 8, and V = 6227.5(18) Å3. The major structure of Ba8Sn4S15 formed by isolated SnS4 tetrahedra and SnS3 pyramids features the zero-dimensional (0D) characteristic and shows the coexistence of divalent Sn­(II) and tetravalent Sn­(IV). The optical band gap of Ba8Sn4S15 is 2.31 eV with a wide transparent region of 0.73–25 μm. Importantly, the large dipole moment and the polarity enhancement of the SnS3 pyramid unit make Ba8Sn4S15 exhibit strong second harmonic generation (SHG) nonlinear optical response, which is about 10 times larger than that of the benchmark AgGaS2 in the particle size of 25–45 μm at the laser radiation wavelength of 2.05 μm with a non phase-matchable behavior. The density functional theory (DFT) studies confirm the experimental results and show the strong SHG nonlinear optical response is ascribed to the electronic transitions from the S­(SnS3)-3p, Sn­(SnS3)-5s and 5p states to the S­(SnS4)-3p, Sn­(SnS4)-5s and 5p states.