Double alkaline earth metals sulfide SrMgGeS4 with high laser-induced damage threshold and strong second-harmonic generation

The exploration of anti-laser damage wide band gap infrared (IR) nonlinear optical (NLO) materials is urgently needed but challenging due to the contradiction between wide band gap and large NLO coefficient. Herein, by introducing [MQ4] (Q = S, Se) tetrahedral units into the AIIBIICIVD4VI family, three double alkaline earth metals chalcogenides SrMgGeS4, BaMgSiSe4 and SrMgSiSe4 with unprecedented [MgGeS6] or [MgSiSe6] dimers are rationally designed and fabricated. The compounds display an interesting structural change from centrosymmetric (CS) I41/acd (BaMgSiSe4, the first CS compound in this family) to non-centrosymmetric Ama2 (SrMgSiSe4) and Fdd2 (SrMgGeS4). Importantly, SrMgGeS4 shows a wide band gap (3.58 eV), a high laser-induced damage threshold (∼6 × AGS), a moderate birefringence (∼0.11@1064 nm), and a strong second-harmonic generation (SHG) response (∼1.1 × AGS), achieving a good balance between wide band gap (Eg ≥ 3.5 eV) and large SHG response (≥1.0 × AGS) for an excellent IR NLO material. The results enrich the chemical and structural diversity of chalcogenides and give insights into the design of high-performance IR NLO materials.