Rational design via dual-site aliovalent substitution leads to an outstanding IR nonlinear optical material with well-balanced comprehensive properties

Realizing Excellent Infrared Nonlinear Optical Performance in Eu-Based Chalcogenides via Rational Cross Substitution Strategy

In recent years, rare-earth-based chalcogenides have gained attention promising materials in the field of infrared nonlinear optical (IR-NLO) applications owing to their exceptional physicochemical properties. However, they frequently encounter challenges such as adverse two-photon absorption and low laser-induced damage thresholds (LIDTs) caused by narrow optical band gaps (Eg), which limit their practical utility. In this study, we started with the centrosymmetric (CS) parent compound EuGa2S4 to develop two new noncentrosymmetric (NCS) Eu-based chalcogenides, namely, EuZnSiS4 and EuCdSiS4, employing a rational cross-substitution strategy. Despite having identical stoichiometry, both compounds crystallize in distinct NCS orthorhombic space groups [Fdd2 (no. 43) vs Ama2 (no. 40)], as confirmed by single-crystal structure analysis. Their crystal structures feature highly distorted tetrahedral motifs interconnected via corner-sharing, forming unique two-dimensional layers that host Eu2+ cations. Furthermore, both compounds exhibit robust phase-matching second-harmonic generation (SHG) intensities of 1.5 × AgGaS2 for EuZnSiS4 and 2.8 × AgGaS2 for EuCdSiS4 under 2050 nm excitation. They also demonstrate high LIDTs (approximately 14-17 × AgGaS2), wide Eg (>2.5 eV), and transparency windows extending up to 18.2 μm. Particularly noteworthy, EuCdSiS4 stands out as a pioneering example in the Eu-based IR-NLO system for successfully combining a broad Eg (>2.56 eV, equivalent to that of AgGaS2) with a significant SHG effect (>1.0 × AgGaS2) simultaneously. Structural analyses and theoretical insights underscore that the reasonable combination of asymmetric functional units plays a pivotal role in driving the CS-to-NCS structural transformation and enhancing the NLO and linear optical properties of these Eu-based chalcogenides. This study presents a promising chemical pathway for advancing rare-earth-based functional materials and suggests exciting opportunities for their future applications in IR-NLO technologies.

Structural strategy for advancing nonlinear optical effects in 1D-[MX2]∞ chains: internal distortion and atomic types

This work aimed to alleviate the limitations of existing mid-infrared nonlinear-optics (MIR-NLO) crystals by conducting theoretical research on 1D-[MX2]∞ (1D = one-dimensional; M = metallic element; X = anionic element) structures in relation to NLO. An analysis was conducted on the electronic structure and optical properties of six selenides (BaZnGeSe4, KxBa1-x/2Ga2Se4, KxBa1-x/2GayIn2-ySe4, KxBa1-x/2In2Se4, BaZnSiSe4, SrZnSiSe4) and three arsenides (Cs2SnAs2, Rb2SnAs2, K2SnAs2) using first principles. Afterwards, the inherent characteristics of these 1D-[MX2]∞ chains were further examined. The calculation results suggest that the presence of internal distortion (non-centrosymmetric) and high electronegativity in M elements can greatly enhance the NLO capability of 1D-[MX2]∞ structures. Furthermore, the first-ever prediction of K2SnAs2's NLO ability has been made. Ultimately, the establishment of a theoretical structure (K2BaSn2As4) provides guidance for the subsequent creation of high-performance MIR-NLO crystals.

Rare-earth-based chalcogenides and their derivatives: an encouraging IR nonlinear optical material candidate

With the continuous development of laser technology and the increasing demand for lasers of different frequencies in the infrared (IR) spectrum, research on infrared nonlinear optical (NLO) crystals has garnered growing attention. Currently, the three main commercially available types of borate materials each have their drawbacks, which limit their applications in various areas. Rare-earth (RE)-based chalcogenide compounds, characterized by the unique f-electron configuration, strong positive charges, and high coordination numbers of RE cations, often exhibit distinctive optical responses. In the field of IR-NLO crystals, they have a research history spanning several decades, with increasing interest. However, there is currently no comprehensive review summarizing and analyzing these promising compounds. In this review, we categorize 85 representative examples out of more than 400 non-centrosymmetric (NCS) compounds into four classes based on the connection of different asymmetric building motifs: (1) RE-based chalcogenides containing tetrahedral motifs; (2) RE-based chalcogenides containing lone-pair-electron motifs; (3) RE-based chalcogenides containing [BS3] and [P2Q6] motifs; and (4) RE-based chalcohalides and oxychalcogenides. We provide detailed discussions on their synthesis methods, structures, optical properties, and structure-performance relationships. Finally, we present several favorable suggestions to further explore RE-based chalcogenide compounds. These suggestions aim to approach these compounds from a new perspective in the field of structural chemistry and potentially uncover hidden treasures within the extensive accumulation of previous research.

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.

Ca2 Ln(BS3 )(SiS4 ) (Ln = La, Ce, and Gd): Mixed Metal Thioborate-Thiosilicates as Well-Performed Infrared Nonlinear Optical Materials

The first examples of thioborate-thiosilicates, namely Ca2 Ln(BS3 )(SiS4 ) (Ln = La, Ce, and Gd), are synthesized by rationally designed high-temperature solid-state reactions. They crystalize in the polar space group P63 mc and feature a novel three-dimensional crystal structure in which the discrete [BS3 ]3- and [SiS4 ]4- anionic groups are linked by Ca2+ and Ln3+ cations occupying the same atomic site. Remarkably, all three compounds show comprehensive properties required as promising infrared nonlinear optical materials, including phase-matchable strong second harmonic generation (SHG) responses at 2.05 µm (1.1-1.2 times that of AgGaS2 ), high laser-induced damage thresholds (7-10 times that of AgGaS2 ), wide light transmission range (0.45-11 µm), high thermal stabilities (>800 °C), and large calculated birefringence (0.126-0.149 @1064 nm), which justify the material design strategy of combining [BS3 ]3- and [SiS4 ]4- active units. Theoretical calculations suggest that their large SHG effects originate mainly from the synergy effects of the LnS6 , BS3 , and SiS4 groups. This work not only broadens the scope of research on metal chalcogenides but also provides a new synthetic route for mixed anionic thioborates.

Chiral and Polar Duality Design of Heteroanionic Compounds: Sr18 Ge9 O5 S31 Based on [Sr3 OGeS3 ]2+ and [Sr3 SGeS3 ]2+ Groups

Chirality and polarity are the two most important and representative symmetry-dependent properties. For polar structures, all the twofold axes perpendicular to the principal axis of symmetry should be removed. For chiral structures, all the mirror-related symmetries and inversion axes should be removed. Especially for duality (polarity and chirality), all of the above symmetries should be broken and that also represents the highest-level challenge. Herein, a new symmetry-breaking strategy that employs heteroanionic groups to construct hourglass-like [Sr3 OGeS3 ]2+ and [Sr3 SGeS3 ]2+ groups to design and synthesize a new oxychalcogenide Sr18 Ge9 O5 S31 with chiral-polar duality is proposed. The presence of two enantiomers of Sr18 Ge9 O5 S31 is confirmed by the single-crystal X-ray diffraction. Its optical activity and ferroelectricity are also studied by solid-state circular dichroism spectroscopy and piezoresponse force microscopy, respectively. Further property measurements show that Sr18 Ge9 O5 S31 possesses excellent nonlinear optical properties, including the strong second harmonic generation efficiency (≈2.5 × AGS), large bandgap (3.61 eV), and wide mid-infrared transparent region (≈15.3 µm). These indicate that the unique microstructure groups of heteroanionic materials are conducive to realizing symmetry-breaking and are able to provide some inspiration for exploring the chiral-polar duality materials.

Breaking Through the Trade-Off Between Wide Band Gap and Large SHG Coefficient in Mercury-Based Chalcogenides for IR Nonlinear Optical Application

It is substantially challenging for non-centrosymmetric (NCS) Hg-based chalcogenides for infrared nonlinear optical (IR-NLO) applications to realize wide band gap (Eg > 3.0 eV) and sufficient phase-matching (PM) second-harmonic-generation intensity (deff > 1.0 × benchmark AgGaS2 ) simultaneously due to the inherent incompatibility. To address this issue, this work presents a diagonal synergetic substitution strategy for creating two new NCS quaternary Hg-based chalcogenides, AEHgGeS4 (AE = Sr and Ba), based on the centrosymmetric (CS) AEIn2 S4 . The derived AEHgGeS4 displays excellent NLO properties such as a wide Eg (≈3.04-3.07 eV), large PM deff (≈2.2-3.0 × AgGaS2 ), ultra-high laser-induced damage threshold (≈14.8-15 × AgGaS2 ), and suitable Δn (≈0.19-0.24@2050 nm), making them highly promising candidates for IR-NLO applications. Importantly, such excellent second-order NLO properties are primarily attributed to the synergistic combination of tetrahedral [HgS4 ] and [GeS4 ] functional primitives, as supported by detailed theoretical calculations. This study reports the first two NCS Hg-based materials with well-balanced comprehensive properties (i.e., Eg > 3.0 eV and deff > 1.0 × benchmark AgGaS2 ) and puts forward a new design avenue for the construction of more efficient IR-NLO candidates.

The first polyanion-substitution-driven centrosymmetric-to-noncentrosymmetric structural transformation realizing an excellent nonlinear optical supramolecule [Cd4P2][CdBr4]

Crystallographically, noncentrosymmetricity (NCS) is an essential precondition and foundation of achieving nonlinear optical (NLO), pyroelectric, ferroelectric, and piezoelectric materials. Herein, structurally, octahedral [SmCl6]3- is substituted by the acentric tetrahedral polyanion [CdBr4]2-, which is employed as a templating agent to induce centrosymmetric (CS)-to-NCS transformation based on the new CS supramolecule [Cd5P2][SmCl6]Cl (1), thereby providing the NCS supramolecule [Cd4P2][CdBr4] (2). Meanwhile, this replacement further results in the host 2D ∞2[Cd5P2]4+ layers converting to yield the twisted 3D ∞3[Cd4P2]2+ framework, which promotes the growth of bulk crystals. Additionally, phase 2 possesses well-balanced NLO properties, enabling considerable second-harmonic generation (SHG) responses (0.8-2.7 × AgGaS2) in broadband spectra, the thermal expansion anisotropy (2.30) together with suitable band gap (2.37 eV) primarily leading to the favorable laser-induced damage threshold (3.33 × AgGaS2), broad transparent window, and sufficient calculated birefringence (0.0433) for phase-matching ability. Furthermore, the first polyanion substitution of the supramolecule plays the role of templating agent to realize the CS-to-NCS transformation, which offers an effective method to rationally design promising NCS-based functional materials.

Cs4 Zn5 P6 S18 I2 : the Largest Birefringence in Chalcohalide Achieved by Highly Polarizable Nonlinear Optical Functional Motifs

Chalcohalides not only keep the balance between the nonlinear optical (NLO) coefficient and wide band gap, but also provide a promising solution to achieve sufficient birefringence for phase-matching ability in NLO crystals. In this study, a novel chalcohalide, Cs4 Zn5 P6 S18 I2 (1) is successfully synthesized, by incorporating the highly electropositive Cs and the large electronegative I element into the zinc thiophosphate. Its 3D open framework features an edge-shared by distorted [ZnS4 ], ethanol-like [P2 S6 ], and unusual [ZnS2 I2 ] polyhedrons, which is inconsistent with the soft-hard-acids-bases theory. Remarkably, compound 1 simultaneously exhibits the large second-harmonic generation (SHG, 1.1×AgGaS2 , @1.3 µm) and a wide band gap (3.75 eV) toward a high laser-induced damage threshold (16.7×AgGaS2 , @1.06 µm), satisfying the rigorous requirements for a prominent infrared NLO material with concurrent SHG intensity (≥0.5×AGS) and band gap (≥3.5 eV). Moreover, to the best of the knowledge, the experimental result shows that phase 1 has the largest birefringence (0.108, @546 nm) in chalcohalide and meets phase-matching behavior demand originating from the polarizable anisotropy of NLO-functional motifs. This finding may provide great opportunities for designing birefringent chalcohalides.

Nonlinear Optical Sulfides LiMGa8 S14 (M = Rb/Ba, Cs/Ba) Created by Li+ Driven 2D Centrosymmetric to 3D Noncentrosymmetric Transformation

Cations that can regulate the configuration of anion group are greatly important but regularly unheeded. Herein, the structural transformation from 2D CS to 3D noncentrosymmetric (NCS, which is the prerequisite for second-order NLO effect) is rationally designed to newly afford two sulfides LiMGa8 S14 (M = Rb/Ba, 1; Cs/Ba, 2) by introducing the smallest alkali metal Li+ cation into the interlamination of 2D centrosymmetric (CS) RbGaS2 . The unusual frameworks of 1 and 2 are constructed from C2 -type [Ga4 S11 ] supertetrahedrons in a highly parallel arrangement. 1 and 2 display distinguished NLO performances, including strong phase-matchable second-harmonic generation (SHG) intensities (0.8 and 0.9 × AgGaS2 at 1910 nm), wide optical band gaps (3.24 and 3.32 eV), and low coefficient of thermal expansion for favorable laser-induced damage thresholds (LIDTs, 4.7, and 7.6 × AgGaS2 at 1064 nm), which fulfill the criteria of superior NLO candidates (SHG intensity >0.5 × AGS and band gap >3.0 eV). Remarkably, 1 and 2 melt congruently at 873.8 and 870.5 °C, respectively, which endows them with the potential of growing bulk crystals by the Bridgeman-Stockbarge method. This investigated system provides a new avenue for the structural evolution from layered CS to 3D NCS of NLO materials.

Three-in-One Tetrahedral Functional Units Constructing Diamondlike Ag2In2SiS3.06Se2.94 with High-Performance Nonlinear-Optical Activity

Exploration of new functional materials with enhanced performance from known ones is always an attractive strategy. A new infrared (IR) nonlinear-optical (NLO) mixed chalcogenide Ag2In2SiS3.06Se2.94 (1), was obtained through partial congener substitution originated from Ag2In2SiS6 (0). 1 crystallizes in the monoclinic space group Cc, and its three-dimensional (3D) polyanionic network is composed of {[In4Si2Se5(S/Se)11]12-}∞ helical chains sharing S/Se(5) corner atoms with cavities embedded with counterion Ag+ ions. It exhibits a much enhanced NLO response compared to that of 0, reaching 1.1 × AgGaS2. Further theoretical analysis results indicate that the large NLO response can be attributed to the synergistic effect of AgQ4 and InQ4 tetrahedral functional motifs. This work not only reports a new high-performance IR NLO material but also enriches the partial ion substitution strategy to obtain new functional materials.

Rational Design of a Rare-Earth Oxychalcogenide Nd3 [Ga3 O3 S3 ][Ge2 O7 ] with Superior Infrared Nonlinear Optical Performance

Inorganic chalcogenides have been studied as the most promising infrared (IR) nonlinear optical (NLO) candidates for the past decades. However, it is proven difficult to discover high-performance materials that combine the often-incompatible properties of large energy gap (E_g ) and strong second harmonic generation (SHG) response (d_eff ), especially for rare-earth chalcogenides. Herein, centrosymmetric Cs₃ [Sb₃ O₆ ][Ge₂ O₇ ] is selected as a maternal structure and a new noncentrosymmetric rare-earth oxychalcogenide, namely, Nd₃ [Ga₃ O₃ S₃ ][Ge₂ O₇ ], is successfully designed and obtained by the module substitution strategy for the first time. Especially, Nd₃ [Ga₃ O₃ S₃ ][Ge₂ O₇ ] is the first case of breaking the trade-off relationship between wide E_g (>3.5 eV) and large d_eff (>0.5 × AgGaS₂ ) in rare-earth chalcogenide system, and thus displays an outstanding IR-NLO comprehensive performance. Detailed structure analyses and theoretical studies reveal that the NLO effect originates mainly from the cooperation of heteroanionic [GaO₂ S₂ ] and [NdO₂ S₆ ] asymmetric building blocks. This work not only presents an excellent rare-earth IR-NLO candidate, but also plays a crucial role in the rational structure design of other NLO materials in which both large E_g and strong d_eff are pursued.

Polarization-dependent second harmonic generation in peptide crystals: effects of molecular packing

A series of chiral peptide luminophores containing the coumarin moiety was synthesized via a simple and efficient solution-based procedure. The peptides, containing either L-Phe, or L-Ala, or L-Leu (designated, respectively, as p1, p2, and p3), self-aggregate to form anti-parallel sheet-like structures. The self-assembly of the peptide luminophores leads to non-centrosymmetric crystals which display significant second harmonic generation (SHG). The dependence of the SHG intensity on the input laser polarization revealed a strong correlation between the SHG and the crystal packing. In the polar plots, the SHG intensity as a function of the linear polarization orientation of the input laser beam gave a four-petal pattern for p1, a predominantly two-petal pattern for p2, and a dumbbell-shaped pattern for p3. This reflects the dependence of the second order optical susceptibility tensor on the crystal symmetry. The polar plots can be fitted very well with the theoretical expressions derived from the second order polarization equation after incorporating crystal symmetry in the second order optical susceptibility tensor. The strong polarization-dependent SHG from organic crystals may be interesting for polarization controlled nonlinear optical switches, sensors, and actuators.

Ba5Ga2SiO4S6: a Phase-Matching Nonlinear Optical Oxychalcogenide Design via Structural Regulation Originated from Heteroanion Introduction

Tailored structural regulation to obtain a new non-centrosymmetric (NCS) compound with excellent optical properties is highly desirable but remains a challenge for nonlinear optical (NLO) material design. In this work, centrosymmetric celsian-type BaGa₂Si₂O₈ was selected as a template structure, and a novel NCS oxychalcogenide, namely, Ba₅Ga₂SiO₄S₆, was successfully designed via the introduction of heteroanions under high-temperature solid-state conditions. Ba₅Ga₂SiO₄S₆ adopts the monoclinic space group of Cc (no. 9) and is formed by charges balancing Ba²⁺ cations and discrete [Ga₂SiO₄S₆] clusters made of corner-sharing [SiO₄] and [GaOS₃] tetrahedra. Notably, Ba₅Ga₂SiO₄S₆ exhibits the critical requirements as a potential UV NLO candidate, including a phase-matching second-harmonic generation intensity (∼1.0 × KDP), a beneficial laser-induced damage threshold (1.2 × KDP), a large birefringence (Δn = 0.10@546 nm), and a short UV absorption cutoff edge (ca. 0.26 μm). Furthermore, the theoretical calculation is implemented to provide a deeper analysis of the structure-activity relationship. The investigated example of structural regulation originated from heteroanion introduction in this study may offer a feasible strategy for high-performance NLO candidate design.

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.

Oxychalcogenides as Promising Ultraviolet Nonlinear Optical Candidates: Experimental and Theoretical Studies of AEGeOS2 (AE = Sr and Ba)

Oxychalcogenides have gained widespread attention as promising infrared nonlinear optical (IR-NLO) candidates. However, high-performance oxychalcogenides have rarely been reported in the ultraviolet (UV) region owing to the low energy gaps (E_g < 4.0 eV). Herein, two non-centrosymmetric (NCS) oxychalcogenides with one-dimensional (1D) chain structures and wide E_g (>4.3 eV), namely, AEGeOS₂ (AE = Sr and Ba), have been discovered by combined experiments and theory calculations as a new source of UV-NLO materials. Significantly, they exhibit excellent comprehensive performance comparable to the commercial UV-NLO material KH₂PO₄ (KDP), including large phase-matching ranges (>380 nm), sufficient second harmonic generation intensities (0.7-1.1 × KDP), high laser-induced damage thresholds (1.2 × KDP), wide transparent regions (0.26-12.2 μm), and good thermal stability (up to 1100 K). Moreover, systematic structure-activity relationship analysis illustrates that the 1D homochiral helical [GeOS₂]^2- chains composed of heteroanionic [GeS₂O₂] units make major contribution to the desirable UV-NLO performance. This work makes the two compounds shine out as new energy in the UV-NLO field and offers a new perspective for the exploration of structure-driven functional oxychalcogenides.