La₃(Ga₃S₃O₃)(Si₂O₇): An Oxychalcogenide Demonstrating Ultrawide Optical Bandgap and Favorable Birefringence Propelled by Divergent Anionic Groups

The exceptional structural plasticity and well-balanced properties of oxychalcogenides make them highly desirable as infrared nonlinear optical (IR NLO) materials. A breakthrough in the design of high-performance oxychalcogenides involves integrating and assembling multiple anionic units to maximize their functions. Following this approach, we developed La 3 (Ga 3 S 3 O 3 )(Si 2 O 7 ) (LGSSO) by simultaneously incorporating [Ga 3 O 3 S 6 ] and [Si 2 O 7 ] groups. The potential of LGSSO as an IR NLO material is evident from its wide bandgap (4.82 eV, runner-up in NLO oxychalcogenides), high laser-induced damage threshold (8.7 × AgGaS 2 at 1064 nm), attractive birefringence (0.122 at 546 nm), and moderate phase-matching second-harmonic generation response (1.7 × KH 2 PO 4 at 1064 nm, 0.3 × AgGaS 2 at 1910 nm). Theoretical studies indicate that the [LaS 2 O 6 ] and [GaO 2 S 2 ] contribute significantly to the NLO coefficient, while the [Ga 3 O 3 S 6 ] trimers with pronounced polarizability anisotropy play a pivotal role in providing a substantial birefringence. This work offers a tangible paradigm for exploring well-performed oxychalcogenide NLO material.