It is known to all that an outstanding Infrared nonlinear optical (IR NLO) material must have the desired characters such as broad spectral transmission region, good chemical stability, a large nonlinear coefficient, and high laser-damage threshold. However, commercial infrared nonlinear crystals (such as AgGaS₂ and ZnGeP₂) have some drawbacks, i.e. low laser damage threshold (LDT) and two-photon absorption (TPA), which seriously hinder their application.

A research group led by Prof. PAN Shilie at Xinjiang Technical Institute of Physics & Chemistry (XTIPC) of Chinese Academy of Sciences designed and synthesized a new family of Non-centrosymmetric (NCS) quaternary metal sulfides, Na₂Hg₃M₂S₈ (I, M = Si; II, M = Ge; III, M = Sn), by combining the alkali metal (Na), d¹⁰ element (Hg) and asymmetric tetrahedral units MS₄ (M= Si, Ge, Sn) into crystal structure.

Hg-containing compounds have been developed as the promising IR NLO materials in the frequency-conversion region in recent years, such as HgGaS₄, BaHgS₂, etc. However, how to achieve a suitable balance of the LDT and NLO coefficient for IR NLO material is a big challenge. Researchers designed and synthesized three IR NLO materials in the Na-Hg-M-S systems, including the Na₂Hg₃Si₂S₈, Na₂Hg₃Ge₂S₈, and Na₂Hg₃Sn₂S₈.

They adopted the high-temperature solid-state method to complete the reaction in the vacuum sealed silica tubes under vacuum degree of 10-3 Pa. The obtained products were carefully washed with N,N-dimethylformamide (DMF) to remove the excess S and other byproducts. Many crystals were obtained and existed stably in air for half a year. Single-crystal X-ray diffraction (XRD) analysis shows that all of them crystallize in same NCS space group P-4c2. 

Overall properties investigation on Na₂Hg₃M₂S₈ show that they satisfy the key requirements as potential IR NLO crystals, including wide transparent region (~ 20 μm), good chemical stability (non–deliquescent), strong SHG responses with a phase-matching behavior at 2.09 μm, and high laser-damage thresholds (LDT). Remarkably, Na₂Hg₃Si₂S₈ and Na₂Hg₃Ge₂S₈ achieve the suitable balance of large SHG coefficients and high LDTs, respectively.

Researchers also found that three compounds exhibited the novel multiple types of tunnel structures. A special phenomenon that structural symmetries show a gradually rising tendency from Cs to Na analogues was rarely observed.

The study provides a good way to design new promising IR NLO materials by introducing the alkali metal and d¹⁰ elements and has been published in Chem. Mater. 

This work was supported by the Xinjiang Program of Cultivation of Young Innovative Technical Talents, the Western Light Foundation of CAS, and the National Natural Science Foundation of China.

The crystal structure of Na₂Hg₃Ge₂S₈ and good balance of LDT and SHG coefficient for Na₂Hg₃Si₂S₈ and Na₂Hg₃Ge₂S₈ compounds. (Image by XTIPC)