A new family of quaternary thiosilicates SrA2SiS4 (A = Li, Na, Cu) as promising infrared nonlinear optical crystals

Infrared nonlinear optical (IR NLO) crystals are the key materials for the evolution of solid-state IR lasers. Commercial IR NLO crystals are limited in practical applications due to their low laser damage thresholds (LDTs). From the above considerations, we have proposed the introduction of [SiS4] ligands into structures to simultaneously enhance the LDT and maintain an impressive second harmonic generation (SHG) response for designing new NLO materials. In this work, a new family of quaternary thiosilicates, SrA2SiS4 (A = Li, Na, Cu), were successfully synthesized by regulating the optimal nonstoichiometric ratio. Note that the title compounds are the first discovered thiosilicates in the 30 known I2–AE–IV–VI4 (I = Li, Na, Cu, Ag; AE = Ba, Sr; IV = Si, Ge, Sn; VI = S, Se) research systems and they also undergo interesting structural changes with monovalent cation substitution in the structures because of the cationic size effect. Systematic performance tests show that all of them possess the essential phase-matching behavior and satisfy the balance demand between a large bandgap and good SHG response as promising IR NLO candidates. Their inherent structure–performance relationship was also investigated based on first-principles calculations and the calculated SHG coefficients and birefringences are in agreement with the experimental results. This research indicates that thiosilicates can be expected to be a promising research system for the future exploration of new IR NLO materials.