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Interlayer sliding modulation in van der Waals (vdW) layered materials opens opportunities to seek tunable applications for photonics and optoelectronics at the nanoscale. In this work, focusing on ternary chalcogenides, dozens of typical vdW layered materials with the formula of A2MZ4 (A, M = metal cation; Z = S/Se) were systematically classified and studied to illustrate the interlayer sliding modulation performance on nonlinear optical (NLO) effect and underlying mechanism. Based on first-principles analysis, the screened Cd2GeSe4 exhibits large second harmonic generation (SHG) deviation (>36 times between high- and low-SHG states) under sliding among all studied materials. This phenomenon arises from the polarization modulation induced by the vdW interlayer charge redistribution of two tetrahedral primitives in the ternary edge-sharing tetrahedral structures. Our findings present a strategy to substantially modulate NLO properties by altering the interlayer charge coupling which is commonly thought to be very weak in vdW layered materials. [ABSTRACT FROM AUTHOR]

A new sulfide, SnGa2GeS6, has been synthesized, which represents the first member in the quaternary Sn/M/M′/Q (M = Ga, In; M′ = Si, Ge; Q = S, Se, Te) system. It adopts a new structure type in the non-centrosymmetric space group Fdd2. In the structure, Sn2+ is coordinated to a distorted square-pyramid of five S atoms, demonstrating the stereochemical activity of the lone electron pair, while the Ge atom and Ga atom are both tetrahedrally coordinated to four S atoms. The SnS5 square-pyramids and the MS4 (M = Ga, Ge) tetrahedra are connected to each other via corner and edge-sharing to generate a three-dimensional framework. The compound exhibits a powder second harmonic generation signal at 2 μm whose strength is about one-fourth that of the benchmark material AgGaS2, which may be explained in view of the macroscopic arrangement of the SnS5 square-pyramids and the MS4 tetrahedra. Moreover, based on UV-vis-NIR spectroscopy measurements and the electronic structure calculations, SnGa2GeS6 has two optical transitions at about 1.12 eV and 2.04 eV respectively. [ABSTRACT FROM AUTHOR]

Two new quaternary selenides, namely Ba4Ga4SnSe12 and Ba6Ga2SnSe11, have been synthesized for the first time, representing the first two members in the A/M/Sn/Q (A = alkaline-earth metal; M = Al, Ga, In; Q = S, Se, Te) system. Ba4Ga4SnSe12 crystallizes in the non-centrosymmetric space group P4̅21/c of the tetragonal system and has a three-dimensional structure. Its three-dimensional framework is built up from corner-sharing GaSe4 and SnSe4 tetrahedra with eight-coordinated Ba2+ cations residing in the cavities. Ba6Ga2SnSe11 crystallizes in a new structure type in the monoclinic centrosymmetric space group P21/c. The structure of Ba6Ga2SnSe11 features a zero-dimensional structure containing totally isolated distorted SnSe4 tetrahedra and a discrete Ga2Se7 unit with Ba2+ cations located between them. On the basis of the diffuse-reflectance spectra, the band gaps are 2.16 (2) eV and 1.99 (2) eV for Ba4Ga4SnSe12 and Ba6Ga2SnSe11 respectively. In addition, the electronic structure calculation of Ba4Ga4SnSe12 indicates that it is a direct-gap semiconductor with the band gap mainly determined by the [Ga4SnSe12]8− anionic framework. [ABSTRACT FROM AUTHOR]

Two new quaternary selenides, namely Ba4Ga4SnSe12 and Ba6Ga2SnSe11, have been synthesized for the first time, representing the first two members in the A/M/Sn/Q (A = alkaline-earth metal; M = Al, Ga, In; Q = S, Se, Te) system. Ba4Ga4SnSe12 crystallizes in the non-centrosymmetric space group P4̅21/c of the tetragonal system and has a three-dimensional structure. Its three-dimensional framework is built up from corner-sharing GaSe4 and SnSe4 tetrahedra with eight-coordinated Ba2+ cations residing in the cavities. Ba6Ga2SnSe11 crystallizes in a new structure type in the monoclinic centrosymmetric space group P21/c. The structure of Ba6Ga2SnSe11 features a zero-dimensional structure containing totally isolated distorted SnSe4 tetrahedra and a discrete Ga2Se7 unit with Ba2+ cations located between them. On the basis of the diffuse-reflectance spectra, the band gaps are 2.16 (2) eV and 1.99 (2) eV for Ba4Ga4SnSe12 and Ba6Ga2SnSe11 respectively. In addition, the electronic structure calculation of Ba4Ga4SnSe12 indicates that it is a direct-gap semiconductor with the band gap mainly determined by the [Ga4SnSe12]8− anionic framework. [ABSTRACT FROM AUTHOR]

The first two members in the quaternary A/A′/M/Q (A = alkali metal; A′ = alkaline-earth metal; M = As, Sb, Bi; Q = S, Se, Te) system, namely the KBaMSe 3 (M = As, Sb) selenides, have been synthesized by solid state reactions. They are isostructural and crystallize in the centrosymmetric space group P 2 1 / c of the monoclinic system. In the structure, the trivalent M atom (M = As, Sb) is coordinated to three Se atoms forming a trigonal pyramid with the Se atoms serving as the triangle base, showing the stereochemical activity of the ns 2 lone pair electron. The MSe 3 (M = As, Sb) trigonal pyramids are totally isolated from each other with K + and Ba 2+ cations located between them. The optical band gaps of 2.26 (2) eV for KBaAsSe 3 and 2.04 (2) eV for KBaSbSe 3 , were deduced from the diffuse reflectance spectra. The first principles calculations were performed to study the electronic structures of KBaMSe 3 (M = As, Sb) compounds and the results indicated that these two compounds are indirect band gap semiconductors. [ABSTRACT FROM AUTHOR]

[Display omitted] • Recent developments in mid-IR NLO materials utilizing f 0 rare earth chalcogenides (RE = Sc, Y, La) are reviewed. • The advantages of utilizing f 0 rare earth chalcogenides as mid-IR NLO materials are thoroughly analyzed. • Several useful conclusions and considerations for future research are presented. Mid-infrared (mid-IR) nonlinear optical (NLO) materials are extensively researched as a vital optoelectronic material family with widespread applications ranging from civil to military fields. Metal chalcogenides possess a wide range of IR transparency and vast structural diversity, making them ideal system for exploration of novel mid-IR NLO materials. Incorporating rare earth elements with 4 f 0 and 5 f 0 electrons, including Sc, Y, and La, into chalcogenide structures in mid-IR NLO materials has been actively studied in the recent years. In this review, we categorize all typical f 0 rare earth (RE = Sc, Y, La) chalcogenides in the inorganic crystal structure database based on their NLO functional groups. We then assess their energy band gaps, NLO coefficients, birefringence, IR cut-offs, and mechanical properties using available experimental data and first-principles results and investigate the correlation between structural features and mid-IR NLO performance. It is showed that the f 0 rare earth chalcogenides can possess suitable birefringence and NLO response for mid-IR NLO applications, while not compromising energy band gap, thus may have a well-balanced mid-IR NLO performance. The prospects of novel f 0 rare earth chalcogenides with favorable mid-IR NLO performance are also discussed. [ABSTRACT FROM AUTHOR]

Abstract: A new series of transition-metal rare-earth chalcogenides, Ln 3FeGaQ7 (Ln=Nd, Sm, Gd, Dy, Q=S; Ln=Nd, Gd, Dy, Q=Se), have been synthesized by solid state reactions. They are isostructural and crystallize in the space group P63. They adopt a three-dimensional framework composed of LnQ7 monocapped trigonal prisms with the interesting [FeS3]4− chains and isolated GaQ4 tetrahedra lying in two sets of channels in the framework. Magnetic susceptibility measurements on Ln 3FeGaQ7 (Ln=Gd, Dy; Q=S, Se) indicate that they are paramagnetic and obey the Curie–Weiss law. Based on the diffuse reflectance spectra, Ln 3FeGaQ7 (Ln=Gd, Dy; Q=S, Se) should have band gaps smaller than 0.5eV. Electronic conductivity measurement on Dy3FeGaSe7 demonstrates semiconducting behavior with σ 300=0.124S/cm. The first-principles calculations were also performed to study the electronic structures of these compounds. [Copyright &y& Elsevier]