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 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, I4 ̅2m for EuAg2GeS4 (1) and I4 ̅3d for Eu3Ag2Ge2Se8 (2) Eu3Ag2Sn2Se8 (3) and Eu3Ag2Sn2S8 (4). 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) for1, to remarkably high activities of ~1.9 for 3, ~4.7 for 2, and ~7.0 for 4 × 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 1 < 3 < 2 < 4. Interestingly, this trend correlates with the SHG activity, suggesting a potential new relationship 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.