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 Na₂BaGeSe₄ 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×KDP, that is, about 17 times that of KH₂PO₄ (KDP)) and Na₂BaGeS₄ (3.7 eV and about 10×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 for quaternary metal chalcogenides.