Heteroanionic Melilite Oxysulfide: A Promising Infrared Nonlinear Optical Candidate with a Strong Second-Harmonic Generation Response, Sufficient Birefringence, and Wide Bandgap

The achievement of balanced performance with a strong second-harmonic generation (SHG) response, proper birefringence, and wide band gap concurrently is a crucial but challenging task in infrared nonlinear optical (IR-NLO) crystals. Here, we theoretically confirmed that the heteroanionic oxysulfide tetrahedron would produce improved polarizability anisotropy and quadratic hyperpolarizability compared with the monoanionic oxide or sulfide tetrahedra. When this anion-mixing strategy was applied, melilite oxysulfide with the four representative members A2GeGa2OS6 (A = Ca, Sr) and Sr2MGe2OS6 (M = Zn, Cd) was successfully discovered as a promising IR-NLO material system. Remarkably, compared with the monoanionic melilite oxides, these compounds exhibited unbiased performances of proper birefringences (0.106–0.143), strong SHG responses (>10× melilite oxide Ba2CdGe2O7, 1.3–2.1× AgGaS2 (AGS) @1570 nm) with phase-matchable ability, wide band gaps (2.95–3.15 eV), and large laser-induced damage thresholds (LIDTs, 5.6–13.4× AGS). The high structure tolerance of melilite offers a great possibility to improve the SHG response via tuning the orbital composition and hybridization near band edges. This work provides an effective approach for the design of high-performance IR-NLO materials.