Mixed-Anion-Oriented Design of LnMGa3S6O (Ln = La, Pr, and Nd; M = Ca and Sr) Nonlinear Optical Oxysulfides with Targeted Property Balance

Nonlinear optical (NLO) crystals are of importance on extending infrared (IR) laser wavelengths. Considering their performance drawbacks in commercial IR NLO crystals, a recent challenge in exploring new excellent IR NLO crystals is how to break the inherent conflict between a wide bandgap (E g ≥ 3.0 eV) and large NLO effect (d ij ≥ 0.5 × AgGaS2) and simultaneously enlarge the birefringence (Δn) for a requisite phase-matching (PM) behavior. For that reason, rational combination of mixed-anion functional groups into a crystal structure affords the successful design and synthesis of six LnMGa3S6O (Ln = La, Pr, and Nd; M = Ca and Sr) NLO oxysulfides. Among them, LaMGa3S6O satisfy the property-balance demand (E g: 3.21–3.27 eV and d ij: 0.9–1.0 × AgGaS2) as promising PM NLO crystals through gathering their property advantages between LaMGa3O7 and LaMGa3S7 by mixed-anion-oriented performance engineering. A study on the structure–property relationship indicates that heteroleptic (Ln/M)­S7O and GaS3O anionic groups are proven as promising NLO-active units and offer a great synergistic effect on the NLO origin. This work as a visualized model not only provides a first clear cognition on varying properties from oxide to sulfide to oxysulfide but also highlights the feasibility of mixed-anion-oriented design of new NLO candidates with balanced performances.