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.