LiLnGeS₄ (Ln=La−Nd): Designing High Performance Infrared Nonlinear Optical Sulfides through “Band Reformation of AGS”

AgGaS₂ (AGS) is the most commonly used commercial infrared nonlinear optical (IR NLO) material. However, AGS has a narrow band gap (E_g=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 successful syntheses of four lanthanide sulfides, LiLnGeS₄ (Ln=La-Nd), crystalizing in an asymmetric Ama2 structure. LiLaGeS₄ demonstrates that eliminating the presence of Ag-4d band increases the E_g 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 2^nd-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 α_L=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.