The development of frequency-conversion technology in the infrared region is in urgent need of new excellent infrared nonlinear optical (IR NLO) materials. How to achieve a good balance between laser damage threshold (LDT) and NLO coefficient (dij) for new IR NLO candidates is still a challenge. The combination of the highly electropositive alkali metal (Na) and Zn with d(10) electronic configuration into crystal structure affords one new IR NLO material, Na2ZnGe2S6. It exhibits excellent properties including a wide transparent region (0.38-22 μm), large band gap (3.25 eV), and especially a balance between a strong NLO coefficient (30-fold that of KDP) and a high LDT (6-fold that of AgGaS2), indicating a promising application in the IR region. Moreover, novel common-vertex-linked wavelike ∞[GeS3]n chains are interestingly discovered in Na2ZnGe2S6, which rarely exist in the reported thiogermanides containing alkali metals. In addition, calculated SHG density and dipole moment demonstrate that the large NLO response is mainly attributed to the cooperative effects of the [GeS4] and [ZnS4] units.

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N onlinear optical (NLO) crystals are of great importance in producing coherent ultraviolet−visible (UV−vis), deep-UV (DUV), or infrared (IR) light sources for civil and military applications by frequency-conversion technology. 1−23 Borates are widely applied as short-wave NLO materials because of their large bandgaps and high laser damage threshold that originate from strongly covalent B−O bonds in [BO 3 ] 3− and [BO 4 ] 5− anionic groups, such as β-BaB 2 O 4 , 1 LiB 3 O 5 , 2 KBe 2 BO 3 F 2 , 3 etc.; unfortunately, they cannot be used in the IR region because of absorption resulting from their B− O vibration modes which limits the infrared transmission. Meanwhile, chalcogenides, 6 owing to its wide optical transmission range and strong NLO response, are wildly researched for IR NLO materials. Recently, in view of the performance drawbacks (low laser damage threshold (LDT) and harmful two photon absorption) of commercial IR NLO materials, 24−26 it is urgent to explore new excellent IR NLO materials 27−33 with suitable balance between wide bandgap (E g ≥ 3.0 eV) and large second harmonic generation (SHG) effect (NLO coefficient d ij ≥ 0.5 × benchmark AgGaS 2 ). 34