Synthesis and characterization of a new mid-infrared transparent compound: acentric Ba5In4Te4S7.

A new noncentrosymmetric (NCS) sulfide, Ba5In4Te4S7, was synthesized by a conventional solid-state reaction in evacuated closed silica tubes. The compound crystallizes in the orthorhombic space group Imm2 (44), with unit cell parameters a = 39.110(3) Å, b = 4.3763(3) Å, c = 7.3452(6) Å, Z = 2, and V = 1257.18(17) Å(3). The 3D framework of Ba5In4Te4S7 is composed of infinite 1(∞)[InS2Te2](5-) and 1(∞)[In2S3Te2](4-) anionic chains. The optical band gap of Ba5In4Te4S7 is 2.13 eV and it shows the UV-visible cutoff at 0.57 µm and the infrared transparency extends to 25 µm. The compound exhibits a powder second harmonic generation (SHG) signal at 2.05 µm with about half of the AgGaS2 at a particle size of 74-106 µm. According to first-principles calculation, the calculated major SHG tensor element is d32 = 18.8 pm V(-1). The SHG process of nonlinear optical response of Ba5In4Te4S7 originates from the electronic transitions from occupied S-3p and Te-5p states to unoccupied In-5s and In-5p states.