The discovery of new nonlinear optical (NLO) materials for infrared (IR) light sources generation is essential for the development of critical technologies on tunable lasers. As for diamond-like semiconductors (DLSs), their inherently noncentrosymmetric (NCS) structures can be exploited in the pursuit of excellent properties and satisfy the prerequisites for frequency conversion application.

Current benchmark IR NLO materials including the chalcopyrite-type AgGaS₂, AgGaSe₂, and ZnGeP₂ are the members of DLSs. Therefore, the exploration of new IR NLO materials with optimal key performances (concurrently large NLO coefficient and impressive laser damage threshold (LDT)) has become imperative.  

A research team led by Prof. PAN Shilie at the Xinjiang Technical Institute of Physics & Chemistry of Chinese Academy of Sciences (CAS) chose the d¹⁰ elements-containing metal sulfides as research system and synthesized a new mercury-based quaternary DLS, Li₄HgGe₂S₇ via solid-state reaction in vacuum-sealed silica tubes. This study was published as back cover in Chem. Commun.

Single-crystal X-ray diffraction (XRD) analysis showed that Li₄HgGe₂S₇ crystallizes in the monoclinic Cc space group. Based on the analysis of its structure, researchers found that Li₄HgGe₂S₇ could be viewed as the first example of exhibiting the unusual 10−membered LiS₄ rings for known quaternary DLSs.  

Properties investigations showed that Li₄HgGe₂S₇ satisfies the essential requirements as excellent IR NLO material and achieves a delicate balance of concurrently large NLO coefficient and impressive LDT, which are comparable to those of famous IR NLO materials (HgGa₂S₄ and BaGa₄Se₇). Moreover, theoretical calculation results verified well with the experimental values and revealed the origin mechanism of the second harmonic generation (SHG) effect. 

The study provided a good way to design new promising IR NLO materials in the research system of d¹⁰ elements-containing DLSs. It was supported by the Western Light Foundation of CAS and the National Natural Science Foundation of China.