CdY ₂ Si ₂ S ₈ : A Wide Bandgap IR NLO Material with d ¹⁰ Metal-Induced Structure Transformation from CS to NCS

Infrared (IR) nonlinear optical (NLO) materials are crucial for high-tech industries and military fields. Research on searching novel IR NLO crystals has appealed international interests. In this work, three rare-earth (RE) chalcogenide compounds─SrY₂Si₂S₈ (SYSS), BaY₂Ge₂S₈ (BYGS), and CdY₂Si₂S₈ (CYSS)─were obtained and systematically characterized. Structural transformations from centrosymmetric (CS) to noncentrosymmetric (NCS) space group (C2/c to Cc) were discovered, caused by the smaller ionic radii of Cd²⁺ and different d¹⁰ orbital configuration compared with Ba²⁺ and Sr²⁺, resulting in the vanishing of the 2-fold rotation axis symmetry and the nonlinear activity in CYSS. Specially, CYSS is not only the first d¹⁰ metal chalcogenide in the AII-RE₂-M₂-Q₈ family (including 66 members), but also the first NCS compound. Moreover, CYSS realized moderate-satisfactory properties, namely, broad IR transparency (to 16.3 μm), wide bandgap (3.30 eV), high laser-induced damage threshold (3 × AGS), moderate birefringence (0.09 @1064 nm), and an SHG response of 0.4 × AGS. This work directly proved that substituting alkaline-earth metals with d¹⁰ metals was an efficient way to harvest NCS structures, which is beneficial to developing new IR NLO materials.