K2 ZnSn3 Se8 : A Non-Centrosymmetric Zinc Selenidostannate(IV) Featuring Interesting Covalently Bonded [ZnSn3 Se8 ]2− Layer and Exhibiting Intriguing Second Harmonic Generation Activity

d6versus d10, Which Is Better for Second Harmonic Generation Susceptibility? A Case Study of K2TGe3Ch8 (T = Fe, Cd; Ch = S, Se)

Two acentric chalcogenide compounds, K₂CdGe₃S₈ and K₂CdGe₃Se₈, were synthesized via conventional high-temperature solid-state reactions. The crystal structures of K₂CdGe₃S₈ and K₂CdGe₃Se₈ were accurately determined by single-crystal X-ray diffraction and crystallize in the K₂FeGe₃S₈ structure type. K₂CdGe₃S₈ is isostructural to K₂FeGe₃S₈ with superior nonlinear optical properties. For the second harmonic generation (SHG) response, K₂CdGe₃S₈ is 18× K₂FeGe₃S₈ for samples of particle size of 38-55 μm. The superior nonlinear optical properties of K₂CdGe₃S₈ over K₂FeGe₃S₈ are mainly contributed by the chemical characteristics of Cd compared with Fe, which are elucidated by nonlinear optical property measurements, electronic structure calculations, and density functional theory calculations. The [CdS₄] tetrahedra within K₂CdGe₃S₈ exhibit a higher degree of distortion and larger volume compared to the [FeS₄] tetrahedra in K₂FeGe₃S₈. This study possesses a good platform to investigate how d-block elements contribute to the SHG response. The fully occupied d¹⁰-elements are better for SHG susceptibility than d⁶-elements in this study. K₂CdGe₃S₈ is a good candidate as an infrared nonlinear optical material of high SHG response (2.1× AgGaS₂, samples of particle size of 200-250 μm), type-I phase-matching capability, high laser damage threshold (6.2× AgGaS₂), and good stability.

Triclinic Layered A2ZnSi3S8 (A = Rb and Cs) with Large Optical Anisotropy and Systematic Research on the Inherent Structure-Performance Relationship in the A2MIIBMIV3Q8 Family

Two triclinic A₂ZnSi₃S₈ (A = Rb and Cs) with layered structures were successfully synthesized, and their physicochemical performances including optical bandgap, thermal behavior, and optical anisotropy were investigated. A₂ZnSi₃S₈ could be viewed as the first discovered Si-based examples in the known A₂M^IIM^IV₃Q₈ family (2-1-3-8 system; A = monovalent alkali metal; M^II = divalent transition metal; M^IV = group 14 metal; Q = chalcogen). The A₂M^IIM^IV₃Q₈ family members crystallize in five different space groups (P1̅, P2₁, P2₁/n, P2₁2₁2₁, and Pa3̅), and their structural transformation and optical performances (bandgap, NLO coefficient, and birefringence) were systematically studied based on the first-principles calculation among 13 A₂M^IIBM^IV₃Q₈ (M^IIB = Zn, Cd, and Hg) compounds without cubic β-K₂ZnSn₃S₈. Research result shows that the above 13 compounds exhibit the layered structures, but diverse wavelike layers and their optical anisotropy (Δn) undergo an increasing trend range from the triclinic to orthorhombic systems. Moreover, P2₁2₁2₁ compounds have very weak NLO effects compared with those of the P2₁ compounds since the polarization directions of anionic groups (M^IIBQ₄ and M^IVQ₄) in P2₁2₁2₁ compounds are directing oppositely and almost completely canceled out by the dipole moment calculation, which further indicates that P2₁ compounds exhibiting the relatively strong NLO effect and large optical anisotropy could be expected as potential IR NLO candidates.

Centrosymmetric or Noncentrosymmetric? Transition Metals Talking in K2TGe3S8(T = Co, Fe)

Two new quaternary sulfides K₂TGe₃S₈(T = Co, Fe) have been synthesized by a high-temperature solid-state routine and flux growth method. The crystal growth process of K₂TGe₃S₈(T = Co, Fe) was elucidated by in situ powder X-ray diffraction and DSC thermal analysis. The millimeter-sized crystals of K₂TGe₃S₈(T = Co, Fe) were grown. K₂CoGe₃S₈ crystallizes in a new structure type in centrosymmetric space group P1 (no. 2) with unit cell parameters of a = 7.016(1) Å, b= 7.770(1) Å, c = 14.342(1) Å, α = 93.80(1)°, β = 92.65(1)°, γ = 114.04(1)°. K₂FeGe₃S₈ crystallizes in the K₂FeGe₃Se₈ structure type and the noncentrosymmetric space group P2₁ (no. 4) with unit cell parameters of a = 7.1089(5)Å, b = 11.8823(8) Å, c = 16.7588(11) Å, β = 96.604(2)°. There is a high structural similarity between K₂CoGe₃S₈ and K₂FeGe₃S₈. The larger volume coupled with higher degrees of distortion of the [FeS₄] tetrahedra compared to the [CoS₄] tetrahedra accounts for the structure's shift from centrosymmetric to noncentrosymmetric. The theory simulation confirms that [TS₄]_{T= Co or Fe} tetrahedra play a crucial role in controlling the structure and properties of K₂TGe₃S₈(T = Co, Fe). The measured optical bandgaps of K₂CoGe₃S₈ and K₂FeGe₃S₈ are 2.1(1) eV and 2.6(1) eV, respectively. K₂FeGe₃S₈ shows antiferromagnetic ordering at 24 K while no magnetic ordering was detected in K₂CoGe₃S₈. The magnetic measurements also demonstrate the divalent nature of transition metals in K₂TGe₃S₈(T = Co, Fe).

Li7 Cd4.5 Ge4 Se16 and Li6.4 Cd4.8 Sn4 Se16 : Strong Nonlinear Optical Response in Quaternary Diamond-Like Selenide Networks

Two new selenides with diamond-like structures, Li₇ Cd_4.5 Ge₄ Se₁₆ and Li_6.4 Cd_4.8 Sn₄ Se₁₆ , were synthesized by using a conventional high-temperature solid-state reaction method. They crystallize in the space group Pna2₁ (no. 33) of the orthorhombic system. Their three-dimensional frameworks consist of corner-sharing LiSe₄ , CdSe₄ , and MSe₄ (M=Ge, Sn) tetrahedra. These two compounds exhibit strong powder second-harmonic generation responses that are about 1.2 and 2.5 times that of the benchmark AgGaS₂ at a laser wavelength of λ=2.09 μm, and also demonstrate type I phase-matchable behavior. The optical bandgaps were determined to be 2.18 and 1.95 eV for Li₇ Cd_4.5 Ge₄ Se₁₆ and Li_6.4 Cd_4.8 Sn₄ Se₁₆ , respectively. Furthermore, these two materials exhibit congruent melting behavior at rather low temperatures of 985 and 1060 K, respectively, which makes bulk single crystal growth by using the Bridgman-Stockbarger method possible. Our study indicates that these two materials show advantages over the traditional IR NLO material CdSe and are promising for practical applications.