Ba4Al4SnS12: balancing bandgap and nonlinear optical performance in thioaluminate via multifunctional group engineering.

The exploration of infrared nonlinear optical (IR NLO) crystals with excellent performance has attracted increasing attention in view of the inherent drawbacks (low laser damage threshold (LDT) or harmful two-photon absorption (TPA)) of commercial NLO crystals. However, the critical performances (wide bandgap (>3.0 eV) and good nonlinearity (>0.5 × AgGaS2)) show an inverse relationship in IR NLO crystals; thus, the judicious selection and combination of functional motifs are critical, as performance is fundamentally governed by structure. For that reason, we propose a multifunctional group engineering strategy incorporating [AlS4] for a wide bandgap and [SnS4] for large nonlinearity to design a new NLO chalcogenide that affords the successful synthesis of thioaluminate, Ba4Al4SnS12. Experimental study shows that Ba4Al4SnS12 achieves an optimal performance balance between a wide bandgap (3.06 eV) and a moderate second-harmonic generation (SHG) response (0.56 × AgGaS2) with requisite phase-matching behavior (Δn = 0.042), further indicating that Ba4Al4SnS12 could be regarded as a potential IR NLO candidate. Theoretical analysis demonstrates the respective contributions of distinct tetrahedral groups to the bandgap and NLO coefficient and validates the feasibility of multifunctional group engineering. Therefore, this study provides a principle for selecting excellent functional groups and research systems, illuminating a feasible pathway toward future NLO crystals with superior performance.