Chalcopyrite-type AgGaQ₂ (Q = S, Se) and ZnGeP₂ are the main commercial infrared nonlinear optical (IR NLO) materials. However, some inherent drawbacks still seriously hinder their application range. Note that excellent IR NLO materials should satisfy the following conditions: wide bandgap (E_g >3.0 eV), large NLO coefficient (d_ij ≥ 0.5 × AgGaS₂), and suitable Δn (0.03−0.10).  

Considering the performance features of chalcopyrites, a research group led by Prof. PAN Shilie at Xinjiang Technical Institute of Physics & Chemistry of Chinese Academy of Sciences  (XTIPC), designed and synthesized four new IR NLO materials Li₂BaM^IVQ₄ (M^IV = Ge, Sn; Q = S, Se) system with the classical AgGaQ₂ as the templates. In this design strategy, M^IVQ₄ tetrahedra as the critical “NLO active units” are used to substitute the GaQ₄ units to maintain the good d_ij of AgGaQ₂ and substitution of highly electropositive elements (alkali/alkaline-earth metal) for the Ag atoms can also increase E_g.  The finding was published in J. Am. Chem. Soc..

Structural analysis showed that all of them crystallize in the 4̅2m point group, which is similar with those of chalcopyrite-type AgGaQ₂. Note that all of them exhibit the obvious tetragonal compression along the c axis compared with the structures of classical chalcopyrites, which can be explained that the larger cations prefer a coordination number 8 than only 4 in the structures of chalcopyrites. Then their structure further compress themselves to meet the demand for higher coordination spheres.

More importantly, these new compounds could be also viewed as the first examples that isostructural structures are found in the I₂−Ba−M^IV−Q₄ system with different M^IV (M^IV = Ge, Sn) or Q (Q = S, Se) atoms. 

Overall properties investigation shows that title sulfides exhibit the good balance of wide E_g, high LDTs, good d_ij, and suitable Δn, which indicates that they are promising candidates for IR NLO application and eliminate the inherent defects (low LDT and strong TPA) of commercial chalcopyrite-type materials. This work also demonstrates that designing the new materials with chalcopyrite-like structures provides a predictive guide to explore new promising IR NLO candidates. 

The work was supported by the National Natural Science Foundation of China, Ten Thousand People Plan Backup Project, and National Key Research Project.    

Figure:Materials design and performance comparison (Image by XTIPC)