A new family of noncentrosymmetric isostructural compounds,Na2Hg3M2S8(M = Si,Ge,and Sn),was successfully synthesized.They crystallize in the tetragonal space group P4-c2 with Z = 2.Their major structures are composed of infinite cross-connected ∞(HgS3)n chains and isolated [MS4] ligands and show the interesting tunnel features.Interestingly,compared with the structures of A2Hg3M2S8(A = alkali metal,Na-Cs),it can be found that the structural symmetries show a gradually rising tendency from Cs to Na analogues as a result of cation size effect,which rarely exists in quaternary alkali metal chalcogenides.Property measurements show that title compounds exhibit strong second harmonic generation(SHG)effects with a phase-matching behavior at 2.09 μm,wide transparency range in the infrared(IR)region,and large laser-damage thresholds(LDTs).Remarkably,Na2Hg3Si2S8 and Na2Hg3Ge2S8 achieve the suitable balance between large SHG effects(1.3 and 2.2 × benchmark AgGaS2)and high LDTs(4.5 and 3 × AgGaS2),respectively,and can be expected to be potential nonlinear optical(NLO)candidates in the IR region.Moreover,band structures and NLO properties of title compounds are also theoretically studied,and the calculated NLO coefficients are consistent with the experimental observations.