Na2BaMQ4 (M = Ge, Sn; Q = S, Se): Infrared Nonlinear Optical Materials with Excellent Performances and that Undergo Structural Transformations

Infrared nonlinear optical (IR NLO) materials with excellent performances are particularly important in laser technology. However; to design and synthesize an efficient IR NLO material with a balance between the optical band gap and the NLO coefficient is still a huge challenge. With this in mind, four new IR NLO materials Na2BaSnS4, Na2BaSnSe4, Na2BaGeS4,, and Na,BaGeSe4 were successfully designed and synthesized. The compounds exhibit excellent properties with a suitable balance of band gap and NLO coefficient measured for Na2BaSnS4 (3.27 eV and about 17 x KDP, that is, about 17 times that of KH2PO4(KDP)) and Na2BaGeS4 (3.7 eV and about 10 x KDP), demonstrating that the systems satisfy the key requirements as promising IR NLO candidates. Remarkably, the new compounds also undergo a novel structural transformation from tetragonal to trigonal systems, the first time that this has been reported fur quaternary metal chalcogen ides.