Infrared Nonlinear optical (NLO) materials, as important frequency-shifting crystals, have been extensively applied in many fields, such as national defense, communication, medical treatment, and security. Compared with oxide materials, the outstanding infrared (IR) NLO materials have been rarely discovered. Therefore, it is a challenge to find excellent NLO materials with broad IR transparency range, large NLO response, and high damage resistance.

A research group led by Prof. PAN Shilie at Xinjiang Technical Institute of Physics & Chemistry (XTIPC), Chinese Academy of  Sciences, designed and synthesized new nonlinear optical materials in the Ba-Hg-S/Se systems, BaHgS₂ and Ba₈Hg₄S₅Se₇, by introducing the Hg element into metal chalcogenides. 

Recently, the Hg-containing compounds have attracted researchers’ attention as Hg is a largely polarizable metal with variable coordination environments and has a great potential to form new special structures.This provides Prof. PAN Shilie and his colleagues a new way to design new compounds by introducing the Hg element into chalcogenides. Besides, a new family of metal chalcogenides containing the Hg element with outstanding properties was discovered, such as HgGa₂S₄ and A₂Hg₃M₂S₈ (A = K, Rb; M =Ge, Sn). Based on theoretical studies, introducing alkaline-earth metal into structure could effectively broaden the band gap and improve its laser-damaged threshold.  

Researchers obtained two metal chalcogenides  in the Ba-Hg-S/Se systems, BaHgS₂ and Ba₈Hg₄S₅Se₇ through choosing the proper research system-metal chalcogenides and introducing the largely polarizable metal-Hg ions. They found BaHgS₂ crystallizes in the orthorhombic space group Pmc2₁, and exhibits a good second harmonic generation (SHG) response of ∼6.5 times compared with that of reference AgGaS₂ at a fundamental wavelength (2.09 μm) .

In order to achieve the transmission range of two crystals, IR, Raman and diffuse reflection spectra were systemically studied. It is  proven that two compounds have good transmission region up to 0.6-20 μm. With the help of theoretical calculation method, the electronic structures and nonlinear coefficient were also calculated to verify the accuracy of experimental results. Based on advanced calculating software, the detail data were obtained and in agreement with experimental values. Therefore, both of experimental and theoretical results verify the accuracy of the data.  

Moreover, Prof. PAN Shilie and his colleagues obtained through introducing the Se atoms into the structure of Ba₂HgS₃, a disordered metal chalcogenide, Ba₈Hg₄S₅Se₇ (BHSSe), which crystallizes in the orthorhombic space group Pnma and has a similar structure with Ba₂HgS₃. Meanwhile, researchers also showed the crystal structure and important physical properties of centric BHSSe compound. 

The result, published in Inorg. Chem., gives a potential IR NLO crystal for the development of infrared field.  

This work was supported by the Western Light of CAS, the National Natural Science Foundation of China and Main Direction Program of Knowledge Innovation of CAS, etc. 

Figure:The crystal structure of BaHgS₂ and its relative intensity of SHG signals（Image by XTIPC)