In recent years, the infrared non-linear optical crystal has been developed as a key frequency conversion device for infrared laser output. However, the commercialized infrared nonlinear optical crystals affect their applications due to problems such as low laser damage threshold and strong two-photon absorption. Based on this, to explore a new type of excellent infrared nonlinear optical crystal material has become a research focus, but also requires it to have a wide range of infrared transmission, a large nonlinear coefficient, band gap and chemical stability and so on. Since there is an inverse relationship between the large bandgap and the nonlinear optical coefficient, how to achieve the balance between the large bandgap and the high nonlinearity coefficient (requiring bandgap Eg> 3.0 eV and nonlinear effect dij> 10 × KDP) Explore the important research direction of new infrared nonlinear optical materials. In recent years, the team of optoelectronic functional materials of Xinjiang Institute of Physics and Chemistry, Chinese Academy of Sciences has been devoted to the study of infrared nonlinear optical materials. In previous literature research, it was found that there are not many infrared nonlinear materials that can achieve the above balance of performance. Most of these materials The structure has tetrahedral elements, alkali metals or alkaline earth metals. Therefore, the researchers prefer Na-Ba-MQ (M = Ge, Sn; Q = S, Se) as the research object, through the high temperature solid-state reaction in a closed vacuum quartz tube, finally obtained four excellent performance infrared non-linear Optical materials, namely Na2BaSnS4, Na2BaSnSe4, Na2BaGeS4 and Na2BaGeSe4. Structural analysis revealed that Na2BaSnS4 crystallizes in tetragonal I-42d space group, whereas the other three crystals crystallize in the tripartite R3c space group. It is noteworthy that in the quaternary chalcogenides, the crystal structure has been changed by simple elemental substitution such as Se substitution or Sn substitution for Ge, etc., but the transformation between biaxial crystals (such as single Skew to orthogonally), or biaxial to uniaxial (such as orthorhombic to tetragonal, orthogonally crossed), the discovery of this work has resulted in a structural shift between uniaxial grains Tripartite), which is the first time in chalcogenides found. In addition, two materials, Na2BaSnS4 and Na2BaGeS4, have been found to have good non-linear potential through performance testing, including large nonlinear coefficients, high laser damage threshold, and suitable birefringence to meet the requirements of excellent IR nonlinear optical materials. More importantly, both of them well balance the large band gap with nonlinear coefficients such as Na2BaSnS4 (Eg = 3.27 eV, dij = 17 × KDP) and Na2BaGeS4 (Eg = 3.70 eV, dij = 10 × KDP ), While its performance can be comparable to excellent infrared nonlinear materials LiGaS2 and LiInS2. The results of this study provide an effective strategy for designing a new infrared nonlinear optical material that balances the band gap with the nonlinear coefficient. The research was published in Angew. Chem. Int. Ed. Related research has been funded by the National Natural Science Foundation of China. Papers link The structural transformation of Na2BaMQ4 (M = Ge, Sn; Q = S, Se) crystals and the performance indexes of Na2BaSnS4 and Na2BaGeS4.
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