Synthesis, Crystal Structure, and Optical Properties of Noncentrosymmetric Na2ZnSnS4

Rational Design of a Promising Oxychalcogenide Infrared Nonlinear Optical Crystal

Oxychalcogenides with the performance-advantages of both chalcogenides and oxides are emerging materials class for infrared (IR) nonlinear optical (NLO) crystals that can expand the wavelength of the solid-state laser to...

Rational Crystal Structure Design and Nonlinear-Optical Properties of Noncentrosymmetric RbCu2NbS4

Crystal structure design based on known materials is an efficient strategy for exploring new compounds with evident second-harmonic-generation (SHG) effects. By the introduction of Rb⁺ ions into the 3D framework of Cu₃NbS₄, the new compound RbCu₂NbS₄ (space group Ama2) composed of [Cu22∞NbS4]- layers is obtained. The band gap of RbCu₂NbS₄ is 2.1 eV, indicating that this compound is a semiconductor. Band-structure calculations indicate that the electronic transition mainly occurs from the S 3p/Cu 3d to Nb 4d states. The intercalation of Rb⁺ ions induces a high degree of local distortion and symmetry reduction, which results in a dipole moment of 11.7 Debye for RbCu₂NbS₄. RbCu₂NbS₄ shows a moderate SHG response of 0.33 × AgGaS₂ (particle size of 20-41 μm).

Na2MgSnS4 – a new member of the A 2 I B II C IV X 4 family of compounds

A new member of the A 2 I B II C IV X 4 compound family, Na2MgSnS4, has been synthesized by ball milling of the binary sulfides SnS, Na2S, and MgS with elemental sulfur in a high-energy planetary mill, followed by annealing in an atmosphere of H2S (T = 600 °C/3 h). Na2MgSnS4 adopts the NaCrS2-type structure (rhombohedral, space group R 3 ¯ $\overline{3}$ m) with a = 3.7496(11) and c = 19.9130(6) Å. The Na atoms occupy Wyckoff position 3b, whereas the Mg and Sn atoms are statistically distributed on the cation sites 3a; all cations are surrounded by six sulfur atoms.

First-Principles Design and Simulations Promote the Development of Nonlinear Optical Crystals

A hot topic in materials science is to search for nonlinear optical (NLO) crystals, which are indispensable in current laser technology, future optical information, and precision measurements. In the period of the 1980s and 1990s, the anionic group theory proposed by Prof. Chuangtian Chen has greatly promoted the inventions of BaB₂O₄ (BBO), LiB₃O₅ (LBO), and KBe₂BO₃F₂ (KBBF) which are widely applied in the ultraviolet (UV) spectral region today. From the beginning of this century, the rapid development of laser science and technology urgently demands new NLO crystals for wider application ranges. However, commercial NLO crystals in deep-UV and mid-infrared (mid-IR) regions are scarce. The challenge arises from the stringent criteria at various wavelengths and inefficient exploration strategy. As such, more comprehensive and quantitative theoretical guidance is necessary to improve and supplement the NLO structure-property understandings. Benefiting from high-performance computing resources, first-principles design and simulations came into being, which is more applicable to the understanding of mid-IR NLO mechanism and suitable for the efficient design of new NLO structures for current needs. In the past decade, a complete set of computational research programs based on first-principles simulations have been developed, which have promoted the development of NLO crystals in the deep-UV and mid-IR regions, and guided the subsequent and further experimental explorations. Based on our developed first-principles materials design system, the discoveries of NLO materials have ranged from basic theoretical design to rapid-prototyping and final experimental synthesis. In this Account, we will concisely summarize our ab initio guided and forward-looking studies on NLO crystals, which are our original contributions to this field and can be consulted by other material fields. First, we will review the development of NLO crystals and the important features of NLO materials. Second, we will summarize the important role of computer-aided design in advancing the NLO material field and our developed NLO material design system based on the first-principles simulations. Third, we will introduce the first-principles design for new deep-UV NLO crystals using two novel design proposals, i.e., interlayer cationic replacement and intralayer anionic substitution. Meanwhile, we will illustrate the hierarchical molecular engineering optimizations for mid-IR NLO crystals by illustrating an extended mid-IR NLO family pedigree, from which many promising mid-IR NLO systems were predicted theoretically and confirmed experimentally. Finally, we will give an outlook to explore new functional NLO crystals guided by our first-principles design and simulations. We believe that the computer-assisted exploration for new functional NLO materials is useful for understanding structure-property relationships and can provide researchers with a new approach to cost-effective and data-driven materials design.

A highly distorted HgS4 tetrahedron-induced moderate second-harmonic generation response of EuHgGeS4

The first Eu,Hg-based chalcogenide EuHgGeS₄ exhibits phase-matchable SHG activity with an intensity ∼0.9 times that of AgGaS₂.

Ionothermal Synthesis of Metal Chalcogenides M2Ag3Sb3S7 (M = Rb, Cs) Displaying Nonlinear Optical Activity in the Infrared Region

Two interesting non-centrosymmetric metal chalcogenides, Rb₂Ag₃Sb₃S₇ and Cs₂Ag₃Sb₃S₇, were synthesized by an ionothermal approach. Crystals of compounds Rb₂Ag₃Sb₃S₇ and Cs₂Ag₃Sb₃S₇ possess isomorphic configuration, consisting of two-dimensional (2D) anionic networks _∞[Ag₃Sb₃S₇]^2-, which are split by alkali-metal M⁺ cations. The band gaps are 2.11 and 2.02 eV for Rb₂Ag₃Sb₃S₇ and Cs₂Ag₃Sb₃S₇, respectively. Second-harmonic generation (SHG) studies revealed that Rb₂Ag₃Sb₃S₇ affords a powder SHG performance of ∼0.5 × AgGaS₂ with type-I phase matching, while Cs₂Ag₃Sb₃S₇ shows a slightly stronger SHG performance of ∼0.6 × AgGaS₂ with type-I phase matching. Both compounds possess broad transparency ranges (∼0.6-20 μm), suggesting their potential as infrared (IR) nonlinear optical (NLO) materials. The laser damage thresholds (LDTs) of both Rb₂Ag₃Sb₃S₇ and Cs₂Ag₃Sb₃S₇ are about 2.3 × AgGaS₂. The calculated birefringence indexes Δn are 0.1885 and 0.1944 at 1.064 μm, respectively, which are sufficiently large enough to achieve phase matching. Theoretical studies using density functional theory have been implemented to further understand the relationship between their NLO properties and band structures.