Syntheses and characterization of new mid-infrared transparency compounds: centric Ba2BiGaS5 and acentric Ba2BiInS5.

Two new mid-infrared transparency compounds, centric Ba(2)BiGaS(5) (1) and acentric Ba(2)BiInS(5) (2), were synthesized from a high-temperature solid-state reaction in evacuated closed silica tubes. Their crystal structures were determined by a single crystal X-ray diffraction method at 293 K. The results of crystal structure solution indicate that compound 1 crystallizes in the centrosymmetric space group Pnma with trans- (1)(∞)[BiGaS(5)](4-) chain structure, while compound 2 crystallizes in the noncentrosymmetric polar space group Cmc2(1) with cis- (1)(∞)[BiInS(5)](4-) chain structure. Two types of lone-pair electrons alignment fashions within (1)(∞)[BiMS(5)](4-) chains result in destructive (for 1) or constructive (for 2) dipole moments, as illustrated in the crystal structures and the partial electron density maps based on the first-principles electronic structure computations. Powder second-harmonic generation (SHG) experiments with a 2.05 µm pumping laser show that the SHG efficiency of the polar compound 2 is approximately 0.8 times that of KTiOPO(4) (KTP) reference. Furthermore, SHG signal intensity measurements using different size particles of powder samples indicate that compound 2 can also achieve type I phase-matching, which makes the compound promising for practical applications.