Realizing Excellent Infrared Nonlinear Optical Performance in Eu-Based Chalcogenides via Rational Cross Substitution Strategy

In recent years, rare-earth-based chalcogenides have gained attention promising materials in the field of infrared nonlinear optical (IR-NLO) applications owing to their exceptional physicochemical properties. However, they frequently encounter challenges such as adverse two-photon absorption and low laser-induced damage thresholds (LIDTs) caused by narrow optical band gaps ( E g ), which limit their practical utility. In this study, we started with the centrosymmetric (CS) parent compound EuGa 2 S 4 to develop two new noncentrosymmetric (NCS) Eu-based chalcogenides, namely, EuZnSiS 4 and EuCdSiS 4, employing a rational cross-substitution strategy. Despite having identical stoichiometry, both compounds crystallize in distinct NCS orthorhombic space groups [ Fdd 2 (no. 43) vs Ama 2 (no. 40)], as confirmed by single-crystal structure analysis. Their crystal structures feature highly distorted tetrahedral motifs interconnected via corner-sharing, forming unique two-dimensional layers that host Eu 2+ cations. Furthermore, both compounds exhibit robust phase-matching second-harmonic generation (SHG) intensities of 1.5 × AgGaS 2 for EuZnSiS 4 and 2.8 × AgGaS 2 for EuCdSiS 4 under 2050 nm excitation. They also demonstrate high LIDTs (approximately 14–17 × AgGaS 2 ), wide E g (>2.5 eV), and transparency windows extending up to 18.2 μm. Particularly noteworthy, EuCdSiS 4 stands out as a pioneering example in the Eu-based IR-NLO system for successfully combining a broad E g (>2.56 eV, equivalent to that of AgGaS 2 ) with a significant SHG effect (>1.0 × AgGaS 2 ) simultaneously. Structural analyses and theoretical insights underscore that the reasonable combination of asymmetric functional units plays a pivotal role in driving the CS-to-NCS structural transformation and enhancing the NLO and linear optical properties of these Eu-based chalcogenides. This study presents a promising chemical pathway for advancing rare-earth-based functional materials and suggests exciting opportunities for their future applications in IR-NLO technologies.