AgGaS2 (AGS) is the most commonly used commercial infrared nonlinear optical material. However, AGS has a narrow band gap (Eg = 2.58 eV) and a low laser‐induced damage threshold (LIDT), primarily attributed to its mobile liquid‐like Ag+ constituent and the unstable Ag–S chemical bond. Herein, we propose a “band reformation of AGS” strategy, which leads to the success syntheses of four lanthanide sulfides, LiLnGeS4, crystalizing in an asymmetric Ama2 structure. LiLaGeS4 demonstrates that eliminating the presence of Ag‐4d band increases the Eg to 3.32 eV and enhances the LIDT (14–29 × AGS, measured by both powder and single crystal); while increasing the nonbonding density of states of the S‐3p band enhances the 2nd‐nonlinear optical coefficient (1.06 × AGS). Besides, the bond length discrepancy between [LiS4], [GeS4] and [LaS8] units leads to a moderate birefringence (Δn = 0.052). Such a unique structure further results in extremely small thermal expansion with αL = 0.41–1.74 × 10‐5 K‐1, along different crystallographic axes. Our theoretical studies indicate that the synergy of the structure building units contribute to the second harmonic generation performance. These results suggest that the “band reformation of AGS” strategy provides effective guidance to discover new NLO crystals with optimized performance.

中文翻译：

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LiLnGeS4 (Ln = La–Nd)：通过“AGS 的能带重组”设计高性能红外非线性光学硫化物


AgGaS2 (AGS) 是最常用的商用红外非线性光学材料。然而，AGS 具有窄带隙 (Eg = 2.58 eV) 和低激光诱导损伤阈值 (LIDT)，这主要归因于其可移动的液体状 Ag+ 成分和不稳定的 Ag-S 化学键。在此，我们提出了“AGS能带重组”策略，成功合成了四种镧系元素硫化物LiLnGeS4，并以不对称Ama2结构结晶。 LiLaGeS4 表明，消除 Ag-4d 带的存在可将 Eg 增加至 3.32 eV，并增强 LIDT（14-29 × AGS，通过粉末和单晶测量）；同时增加S-3p带的非键态密度会增强第二非线性光学系数（1.06×AGS）。此外，[LiS4]、[GeS4]和[LaS8]单元之间的键长差异导致中等双折射（Δn = 0.052）。这种独特的结构进一步导致沿不同晶轴的热膨胀极小，αL = 0.41–1.74 × 10-5 K-1。我们的理论研究表明，结构构建单元的协同作用有助于二次谐波的产生性能。这些结果表明，“AGS能带改造”策略为发现具有优化性能的新型非线性光学晶体提供了有效指导。