AlGaSe₃: Tuning Stability via Partial Cation Substitution and Achieving a Nonlinear Optical Property Leap

Simultaneously achieving a large second-harmonic generation (SHG) response and high laser-induced damage threshold (LIDT) remains a critical scientific challenge in mid-infrared nonlinear optical (NLO) materials. Computational studies demonstrate that Al₂Se₃ exhibits a large NLO coefficient and an ultrawide bandgap among selenides. However, its instability excludes the application potential, which attracts us to design its derivatives. Novel AlGaSe₃ can be obtained via a chemical substitution strategy when taking Al₂Se₃ as the template. AlGaSe₃ crystallizes in the noncentrosymmetric space group <i>Cc</i>, isostructural with Al₂Se₃, and its three-dimensional diamond-like structure consists of two distinct MSe₄ (M = Al/Ga) tetrahedral units. AlGaSe₃ exhibits an NLO effect of 1.7 × Al₂Se₃ and 0.7 × AgGaS₂ (AGS) at 2.1 μm and an LIDT of 4.3 × AGS while maintaining a relatively wide bandgap of 2.46 eV among selenides. Theoretical calculations and structural analyses reveal that the NLO performances of Al₂Se₃ and AlGaSe₃ primarily originate from the synergistic interaction between two types of tetrahedral units. The enhanced SHG effect in AlGaSe₃ is attributed to the more distorted tetrahedra, distinct orientation differences among them, and closer packing of them. This work provides an effective strategy for designing high-performance NLO materials.