Li2HgMS4 (M = Si, Ge, Sn): New Quaternary Diamond-Like Semiconductors for Infrared Laser Frequency Conversion

Recent advances in rational structure design for nonlinear optical crystals: leveraging advantageous templates

Nonlinear optical (NLO) crystals that can expand the spectral range of laser outputs have attracted significant attention for their optoelectronic applications. The research progress from the discovery of new single crystal structures to the realization of final device applications involves many key steps and is very time consuming and challenging. Consequently, exploring efficient design strategies to shorten the research period and accelerate the rational design of novel NLO materials has become imperative to address the pressing demand for advanced materials. The recent shift in paradigm toward exploring new NLO crystals involves significant progress from extensive "trial and error" methodologies to strategic approaches. This review proposes the concept of rational structure design for nonlinear optical crystals leveraging advantageous templates. It further discusses their optical characteristics, promising applications as second-order NLO materials, and the relationship between their structure and performance, and highlights urgent issues that need to be addressed in the field of NLO crystals in the future. The review aims to provide ideas and driving impetus to encourage researchers to achieve new breakthroughs in the next generation of NLO materials.

Flexible Anionic Groups-Activated Structure Dissymmetry for Strong Nonlinearity in Ln2 Ae3 MIV 3 S12 Family

Structural dissymmetry and strong second-harmonic generation (SHG) responses are key conditions for nonlinear optical (NLO) crystals, and targeted combinatorial screening of suitable anionic groups has become extremely effective. Herein, optimal combination of flexible SnSn (n = 5, 6) groups and highly electropositive cations (lanthanides (Ln3+ ) and alkaline earth (Ae2+ : Sr, Ca) metals) affords the successful synthesis of 12 NLO thiostannates including Ln2 Sr3 Sn3 S12 (Pmc21 ) and Ln2 Ca3 Sn3 S12 (P-62m); whereas 17 rigid GeS4 or SiS4 tetrahedra-constructed Ln2 Ae3 Ge3 S12 and Ln2 Ae3 Si3 S12 crystallize in the centrosymmetric (CS) Pnma. This unprecedented CS to noncentrosymmetric (NCS) structural transformation (Pnma to P-62m to Pmc21 ) in the Ln2 Ae3 MIV 3 S12 family indicates that chemical substitution of the tetrahedral GeS4 /SiS4 units with SnSn breaks the original symmetry to form the requisite NCS structures. Remarkably, strong polarization anisotropy and hyperpolarizability of the Sn(4+) S5 unit afford huge performance improvement from the nonphase-matching (NPM) SHG response (1.4 × AgGaS2 and Δn = 0.008) of La2 Ca3 Sn3 S12 to the strong phase-matching (PM) SHG effect (3.0 × AgGaS2 and Δn = 0.086) of La2 Sr3 Sn3 S12 . Therefore, Sn(4+) S5 is proven to be a promising "NLO-active unit." This study verifies that the coupling of flexible SnSn building blocks into structures opens a feasible path for designing targeted NCS crystals with strong nonlinearity and optical anisotropy.

KHg4Ga3S9: A Hg-Based Sulfide with Nonlinear-Optical Activity in the A-MII-MIII-Q (A = Alkali Metal; MII = d10 Metal; MIII = Ga, In; Q = S, Se) System

The search for new high-performance infrared (IR) nonlinear-optical (NLO) materials is a hot topic in the fields of laser chemistry and inorganic solid-state chemistry. Here, a new Hg-based sulfide KHg4Ga3S9 in the family of A-MII-MIII-Q (A = alkali metal; MII = d10 metal; MIII = Ga, In; Q = S, Se) was synthesized. It crystallizes in the orthogonal system of the C2221 structure, which is rare for IR NLO chalcogenides. Its anionic framework {[Hg4Ga3S9]-}∞ is constructed by two types of interconnected helical chains, viz., the inner layer ({[Hg6Ga2S29/3]4/3-}∞) and the outer layer ({[Hg2Ga4S25/3]2/3-}∞). It exhibits a moderate NLO response and a high laser-induced damage threshold. Theoretical calculations indicate that the HgS4 unit accounts for its much larger NLO response compared to RbCd4Ga3S9. The influence of alkali metals and d10 metals on the initial phase-matching wavelength is also discussed. This work provides inspiration for improving the properties of NLO materials' properties.

Synthesis, Crystal Structure, and Nonlinear-Optical Properties of a Diamond-Like Chalcogenide Cu2GeS3

Metal sulfides with diamond-like (DL) structures generally exhibit excellent mid-IR nonlinear-optical (NLO) properties. Here, Cu₂GeS₃ (CGS) as a member of the DL chalcogenides was synthesized by a high-temperature solid-state method, and the optical properties were carefully studied experimentally and theoretically. The results revealed that CGS has a large second harmonic generation (0.8 × AgGaSe₂) and a moderate birefringence of 0.067 at 1064 nm. In addition, the linear and NLO properties of the A₂MS₃ (A = Cu, Li; M = Ge, Si) series of compounds were evaluated and compared with the help of first-principles calculations.

Ba3HgGa2S7: A Zero-Dimensional Quaternary Sulfide Featuring a Unique [Hg2Ga4S14]12- String and Exhibiting a High Photocurrent Response

The discovery of new types of metal sulfides is attractive because of their rich structures and diverse physical properties. Here, a novel quaternary sulfide, Ba₃HgGa₂S₇ (BHGS), is obtained by a solid-state reaction at 1123 K. It crystallizes in the monoclinic space group P2₁/c, and its zero-dimensional structure features two seesaw-like HgS₂ units and four GaS₄ tetrahedra, constructing a unique [Hg₂Ga₄S₁₄]^12- string. BHGS has a wide band gap of 3.64 eV and a large birefringence of 0.09 at 2100 nm. Specifically, BHGS exhibits a remarkable photocurrent response. This work may be extended to a new family of AE₃M^IIM^III₂Q₇ (AE = Mg, Ca, Sr, Ba; M^II = Zn, Cd, Hg; M^III = Al, Ga, In; Q = S, Se) chalcogenides.

Ba2HgTe5: a Hg-based telluride with giant birefringence induced by linear [HgTe2] units

Ba2HgTe5, the first Hg-based telluride birefringent material, was successfully synthesized. The analysis of the response electron distribution anisotropy illustrates that the large birefringence of Ba2HgTe5 originates from the linear [HgTe2] unit.

Cu4MnGe2S7 and Cu2MnGeS4: two polar thiogermanates exhibiting second harmonic generation in the infrared and structures derived from hexagonal diamond

The new, quaternary diamond-like semiconductor (DLS) Cu₄MnGe₂S₇ was prepared at high-temperature from a stoichiometric reaction of the elements under vacuum. Single crystal X-ray diffraction data were used to solve and refine the structure in the polar space group Cc. Cu₄MnGe₂S₇ features [Ge₂S₇]^6- units and adopts the Cu₅Si₂S₇ structure type that can be considered a derivative of the hexagonal diamond structure. The DLS Cu₂MnGeS₄ with the wurtz-stannite structure was similarly prepared at a lower temperature. The achievement of relatively phase-pure samples, confirmed by X-ray powder diffraction data, was nontrival as differential thermal analysis shows an incongruent melting behaviour for both compounds at relatively high temperature. The dark red Cu₂MnGeS₄ and Cu₄MnGe₂S₇ compounds exhibit direct optical bandgaps of 2.21 and 1.98 eV, respectively. The infrared (IR) spectra indicate potentially wide windows of optical transparency up to 25 μm for both materials. Using the Kurtz-Perry powder method, the second-order nonlinear optical susceptibility, χ⁽²⁾, values for Cu₂MnGeS₄ and Cu₄MnGe₂S₇ were estimated to be 16.9 ± 2.0 pm V^-1 and 2.33 ± 0.86 pm V^-1, respectively, by comparing with an optical-quality standard reference material, AgGaSe₂ (AGSe). Cu₂MnGeS₄ was found to be phase matchable at λ = 3100 nm, whereas Cu₄MnGe₂S₇ was determined to be non-phase matchable at λ = 1600 nm. The weak SHG response of Cu₄MnGe₂S₇ precluded phase-matching studies at longer wavelengths. The laser-induced damage threshold (LIDT) for Cu₂MnGeS₄ was estimated to be ∼0.1 GW cm^-2 at λ = 1064 nm (pulse width: τ = 30 ps), while the LIDT for Cu₄MnGe₂S₇ could not be ascertained due to its weak response. The significant variance in NLO properties can be reasoned using the results from electronic structure calculations.

Helical {[HgS]}n Chain-Induced Balanced Nonlinear-Optical Performance for Trigonal Mercury Sulfide

Large second-harmonic-generation (SHG) response and enhanced laser-induced damage threshold (LIDT) compared with benchmark AgGaS₂ (AGS), together with a simple chemical composition and exclusion of the light-sensitive Ag element, are vital for exploration of superior IR nonlinear-optical (NLO) materials beyond AGS. Herein, we report trigonal HgS as a highly promising IR NLO material. It was obtained by a facile one-pot hydrothermal reaction, and its noncentrosymmetric P3₂21 structure features a unique dumbbell-shaped HgS₂ group and a neutral helical {[HgS]}_n chain, which first served as a SHG functional motif. Its polycrystalline powder sample exhibits a phase-matchable SHG response comparable to that of AGS and a LIDT value of 1.2 × AGS. Structural analysis and theoretical calculation suggest that the {[HgS]}_n chain plays an important role for excellent NLO performance. This work not only provides a strong candidate for IR NLO application but also introduces a new SHG functional motif for the design of future NLO materials.

Highly Distorted [HgS4] Motif-Driven Structural Symmetry Degradation and Strengthened Second-Harmonic Generation Response in the Defect Diamond-Like Chalcogenide Hg3P2S8

Chalcogenides with diamond-like (DL) structures are a treasury of infrared nonlinear optical (NLO) materials. Here, a ternary Hg-based chalcogenide with a defect DL structure, Hg₃P₂S₈, is synthesized by solid-state reaction. Driven by the highly distorted [HgS₄] tetrahedra, this compound displays an interesting structural symmetry degradation from tetragonal to orthorhombic compared with its analogue Zn₃P₂S₈. Meanwhile, the overall performances of Hg₃P₂S₈ are quite remarkable, including a very strong phase-matchable second-harmonic generation (SHG) response (4.2 × AgGaS₂), large band gap (2.77 eV), wide IR transparent range (0.45-16.7 μm), and high laser-induced damage threshold (4 × AGS). Furthermore, the theoretical analysis and local dipole moment calculations elucidate that the highly distorted [HgS₄] tetrahedra contribute a lot to the enhancement of the SHG effect. This discovery will motivate the exploration of other DL Hg-based chalcogenides serving as high-performing mid-IR NLO materials.

Syntheses and crystal structures of the quaternary thio-germanates Cu4FeGe2S7 and Cu4CoGe2S7

The quaternary thio-germanates Cu4FeGe2S7 (tetra-copper iron digermanium hepta-sulfide) and Cu4CoGe2S7 (tetra-copper cobalt digermanium hepta-sulfide) were prepared in evacuated fused-silica ampoules via high-temperature, solid-state synthesis using stoichiometric amounts of the elements at 1273 K. These isostructural compounds crystallize in the Cu4NiSi2S7 structure type, which can be considered as a superstructure of cubic diamond or sphalerite. The monovalent (Cu+), divalent (Fe2+ or Co2+) and tetra-valent (Ge4+) cations adopt tetra-hedral geometries, each being surrounded by four S2- anions. The divalent cation and one of the sulfide ions lie on crystallographic twofold axes. These tetra-hedra share corners to create a three-dimensional framework structure. All of the tetra-hedra align along the same crystallographic direction, rendering the structure non-centrosymmetric and polar (space group C2). Analysis of X-ray powder diffraction data revealed that the structures are the major phase of the reaction products. Thermal analysis indicated relatively high melting temperatures, near 1273 K.

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.

A review of the AI2BIICIVDVI4 family as infrared nonlinear optical materials: the effect of each site on the structure and optical properties

The AI2B^IIC^IVDVI4 family as promising infrared NLO materials is summarized. The influence of each site substitutions on the structures and properties is systematically analyzed.

Adjustable optical nonlinearity in d10 cations containing chalcogenides via dp hybridization interaction

The role of d¹⁰-cations on band gap and nonlinearity in Cd/Hg-containing metal chalcogenides was investigated using DFT.

Structural insights into T2-cluster-containing chalcogenides with vertex-, edge- and face-sharing connection modes of NaQ6 ligands: Na3ZnMIIIQ4 (MIII = In, Ga; Q = S, Se)

New series of T₂-cluster-containing chalcogenides exhibiting the novel connection mode of NaS₆ ligands and torsional adjacent T₂-clusters were reported.