Large Mid-Infrared\nSecond-Harmonic Generation in Eu(II)-Based\nQuaternary Chalcogenides

Complex metal chalcogenides have received growing attention\nfor\nsecond-harmonic generation (SHG) activity arising from their noncentrosymmetric\nstructures. In this area, the impact of the Eu(II)-cation on their\nstructures and optical properties has not been well explored. Synthetic\ninvestigations of the Eu–Ag–IV-Ch (IV = Sn or Ge; Ch\n= S or Se) systems have unveiled four Eu(II)-based quaternary chalcogenides\nexhibiting very large mid-IR SHG responses within the chemically rich\nsystems II–I<sub>2</sub>-IV-Ch<sub>4</sub> and II<sub>3</sub>–I<sub>2</sub>-IV<sub>2</sub>–Ch<sub>8</sub> (Ch =\nS or Se; I, II, and IV = monovalent, divalent, and tetravalent cations,\nrespectively). Their structures were characterized by single-crystal\nX-ray diffraction methods to crystallize in noncentrosymmetric space\ngroups, <i>I</i>4̅2<i>m</i> for EuAg<sub>2</sub>GeS<sub>4</sub> (<b>1</b>) and <i>I</i>4̅3<i>d</i> for Eu<sub>3</sub>Ag<sub>2</sub>Ge<sub><b>2</b></sub>Se<sub>8</sub> (<b>2</b>) Eu<sub>3</sub>Ag<sub>2</sub>Sn<sub>2</sub>Se<sub>8</sub> (<b>3</b>), and Eu<sub>3</sub>Ag<sub>2</sub>Sn<sub>2</sub>S<sub>8</sub> (<b>4</b>). The structures\nconsist of body-centered arrangements of (Ge/Sn)Ch<sub>4</sub> tetrahedra\nthat are fully oriented and bridged by flattened AgCh<sub>4</sub> tetrahedra\ninto three-dimensional (3D) networks and charge-balanced by Eu(II)\ncations. Their crystalline powders exhibit mid-IR (2.09 μm)\nSHG responses among the largest reported to date, ranging from ∼1.9\n× AGS (AgGaS<sub>2</sub>) for <b>1</b>, to increasing activities\nof ∼1.9 for <b>3</b>, ∼4.7 for <b>2</b>,\nand ∼7.0 for <b>4</b> × AGS. Spin-polarized band\nstructure calculations showed that the valence and conduction band\nedge states stem from interactions of the Ag-to-S/Se-based and Ge/Sn-to-S/Se-based\nstates, respectively, with increasing contributions of the Eu(II)\n4f<sup>7</sup>-based states in the order of <b>1</b> < <b>3</b> < <b>2</b> < <b>4</b>. Interestingly,\nthis trend correlates with the SHG activity, suggesting a potential\nnew relationship for understanding and attaining cutting-edge SHG\nproperties within Eu(II)-based chalcogenides. Thus, this family helps\ntounveil a deeper understanding of structure–optoelectronic/SHG\nproperty relationships.