Strong Second‐Harmonic Generation Responses in Salt‐Inclusion Oxychalcogenides Enabled by the Highly Distorted Polyhedra with Mixed‐Anions

Abstract Nonlinear optical (NLO) crystals demonstrate notable NLO effects, which are predominantly attributed to their anionic groups. However, the role of cations has long been overlooked. For the first time, S 2− , Cl − , and O 2− are successfully incorporated into two new salt‐inclusion oxychalcogenides, namely, [La 4 Cl 3 S][GaGeS 6 O] ( 1 ) and [La 4 Cl 2 S 2 ][Ge 2 S 6 O] ( 2 ), which are characterized by highly distorted and polarizable polycations and anionic groups. Notably, they exhibit substantial second‐harmonic generation (SHG) intensities of 2.7 and 3.8 × AgGaS 2 @1700 nm, respectively. To the best of knowledge, the latter represents the highest value recorded for all oxychalcogenides constructed from tetrahedral structural units. Crystal structure analysis and theoretical calculations reveal that the distorted tetrahedral units in the host lattice and the polyhedral units in the guest lattice considerably contributed to the SHG coefficients. Furthermore, compounds 1 and 2 display favorable laser‐induced damage thresholds (4.4 and 7.2 × AgGaS 2 @1064 nm) along with expanded transparency windows. This finding underscores the potential of host–guest architectures with multiple anions for markedly enhancing SHG coefficients, which paves the way for the development of new NLO materials.