Design of Pentanary Mixed-Chalcogenides Ag2In2SiS6–xSex (x = 1, 2) Based on the Bucket Effect: Local Structural Difference and High-Performance Nonlinear-Optical Properties Realized by Partial Congener Substitution

Noncentrosymmetric chalcogenides are promising candidates for infrared nonlinear-optical (NLO) crystals, and exploring high-performance ones is a hot topic and challengeable. Herein, the combination of AgQ 4, InQ 4, and SiQ 4 (Q = S, Se) units with different S/Se ratios resulted in the discovery of the tetrahedral chalcogenides Ag 2 In 2 SiS 4 Se 2 ( 1 ) and Ag 2 In 2 SiS 5 Se ( 2 ). They both crystallize in the monoclinic Cc space group with different local structures. Co-occupied S/Se sites only exist in 2, and the arrangement of [In 2 SiQ 3 ] six-membered rings builds different helical chains and 3D [(In 2 SiQ 6 ) 2– ] n polyanionic frameworks in 1 and 2 . They show balanced NLO performances, including phase-matchable moderate NLO responses (0.7 and 0.5 × AGS) and enhanced laser-induced damage thresholds (4.5 and 5.1 × AGS). Theoretical calculations reveal that their NLO responses are predominantly contributed by the AgQ 4 and InQ 4 units.