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A new family of noncentrosymmetric isostructural compounds, Na 2 Hg 3 M 2 S 8 (M = Si, Ge, and Sn), was successfully synthesized. They crystallize in the tetragonal space group P 4̅ c 2 with Z = 2. Their major structures are composed of infinite cross-connected ∞ (HgS 3 ) n chains and isolated MS 4 ligands and show the interesting tunnel features. Interestingly, compared with the structures of A 2 Hg 3 M 2 S 8 (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, Na 2 Hg 3 Si 2 S 8 and Na 2 Hg 3 Ge 2 S 8 achieve the suitable balance between large SHG effects (1.3 and 2.2 × benchmark AgGaS 2 ) and high LDTs (4.5 and 3 × AgGaS 2 ), 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.
Wu et al. (Tue,) studied this question.