Syntheses and Characterization of New Mid-Infrared Transparency Compounds: Centric Ba₂BiGaS₅ and Acentric Ba₂BiInS₅

Two new mid-infrared transparency compounds, centric Ba2BiGaS5 (1) and acentric Ba2BiInS5 (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∞[BiGaS5]4– chain structure, while compound 2 crystallizes in the noncentrosymmetric polar space group Cmc21 with cis- 1∞[BiInS5]4– chain structure. Two types of lone-pair electrons alignment fashions within 1∞[BiMS5]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 KTiOPO4 (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.