Large Mid-Infrared SHG Activities in Eu(II)-Based Quaternary Chalcogenides

Complex metal-chalcogenides have received growing attention for second-harmonic generation (SHG) activity arising from their noncentrosymmetric structures. In this area, the impact of the Eu(II)-cation on their structures and optical properties has not been well explored. Synthetic investigations of the Eu-Ag-IV-Ch (IV = Sn or Ge; Ch = S or Se) systems have unveiled four rare examples of Eu(II)-based quaternary chalcogenides exhibiting very large mid-IR SHG responses within the chemically-rich systems II-I2-IV-Ch4 and II3-I2-IV2-Ch8 (Ch = S or Se; I, II, and IV = monovalent, divalent and tetravalent cations, respectively). Their structures were characterized by single-crystal X-ray diffraction (SCXRD) methods to crystallize in noncentrosymmetric space groups, I\bar{4}2m for EuAg2GeS4 (I) and I\bar{4}3d for Eu3Ag2Ge2Se8 (II) Eu3Ag2Sn2Se8 (III) and Eu3Ag2Sn2S8 (IV). The structures consist of body-centered arrangements of (Ge/Sn)Ch4 tetrahedra that are fully oriented and bridged by flattened AgCh4 tetrahedra into 3D networks and charge-balanced by Eu(II) cations. Their crystalline powders exhibit mid-IR (2.09 µm) SHG responses among the largest reported to date, ranging from a large ~1.9 × AGS (AgGaS2) for I, to remarkably high activities of ~30 for III, ~70 for II, and ~100 for IV × AGS. Spin-polarized band structure calculations showed the valence and conduction band edge states stem from interactions of the Ag-to-S/Se and Ge/Sn-to-S/Se based states, respectively, with increasing contributions of the Eu(II) 4f7-based in the order of I < III < II < IV. Most interestingly, this trend strongly correlates with the SHG activity, suggesting a potential new strategy for understanding and attaining cutting-edge SHG properties within Eu(II)-based chalcogenides. Thus, these results unveil a deeper understanding of structure-optoelectronic/SHG property relationships.