Two isostructural ternary acentric sulfides, Y 6 Si 2.5 S 14 (1) and Y 6 Ge 2.5 S 14 (2) were re-investigated to understand the origin of the chemical flexibility of RE 6 B x C y Ch 14 (RE=Y, LaÀ Lu; B=Si, Ge, Sn, Al, Ga; C = monovalent M + (Ag, Na, Li, etc.), divalent M 2 + (Mg, Cr, Ni, Zn, etc.), trivalent M 3 + (Al, In, Ga, etc.), tetravalent M 4 + (Si, Ge, Sn) and pentavalent M 5 + (Sb), Ch=S, Se), which consists of ~444 isostructural compounds. Y 6 IV 2.5 S 14 (IV=Si, Ge) were synthesized by a high-temperature salt flux method. The crystal structures of Y 6 IV 2.5 S 14 (IV=Si, Ge) are constructed by [YS 8 ] polyhedra, [Si1S 6 ] octahedra, and [Si2S 4 ] tetrahedra. The Si1 atom displaces from the center of [Si1S 6 ] octahedra with partial occupancy, which can be replaced by various metals, and mainly accounts for the chemical flexibility of the RE 6 B x C y Ch 14 family. The bonding pictures of Y 6 Si 2.5 S 14 were studied by electron localization function (ELF) and crystal orbital Hamilton population (COHP) calculations. Y 6 Si 2.5 S 14 is evaluated as an indirect semiconductor with a bandgap of 2.4(1) eV measured by UV-Vis. The indirect bandgap of Y 6 Ge 2.5 S 14 is 1.7(1) eV. Y 6 IV 2.5 S 14 (IV=Si, Ge) are not type-I phase-matchable materials. For samples of 47 μm particle size, Y 6 Si 2.5 S 14 and Y 6 Ge 2.5 S 14 own good second harmonic generation (SHG) responses of ~3.0 × AGS and ~2.8 × AGS respectively. Y 6 Si 2.5 S 14 and Y 6 Ge 2.5 S 14 possess high laser damage threshold (LDT) of ~5.5 × AGS and ~5.2 × AGS respectively.
A new acentric sulfide Y 4 Si 3 S 12 was grown by a high temperature vapor transport reaction. The crystal structure of Y 4 Si 3 S 12 was determined by single crystal X-ray diffraction. Y 4 Si 3 S 12 is isostructural to La 4 Ge 3 S 12 . The three dimensional structure of Y 4 Si 3 S 12 is constructed by [Y1S 7 ] augmented triangular prisms, [Y2S 6 ] triangular prisms and [SiS 4 ] tetrahedra through sharing vertices and edges. Y 4 Si 3 S 12 is revealed as an indirect bandgap semiconductor with a calculated bandgap of 2.1 eV, which is close to 2.5(1) eV experimentally measured by UV-Vis. The YÀ S interactions and SiÀ S interactions are predicated to be strong ionic bonds and covalent bonds, respectively, by electron localization function coupled with crystal orbital Hamilton population calculations. Y 4 Si 3 S 12 is verified by DFT calculations to have moderate birefringence, with incident 1900 nm laser, Δn = 0.09. DFT calculations also predicted that Y 4 Si 3 S 12 possesses moderate second harmonic generation response with χ 111 = 15.03 pm/V. The nonlinear optical properties of Y 4 Si 3 S 12 are mainly contributed by YÀ S interactions revealed by DFT calculations.
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