Analysis and prediction of mid-IR nonlinear optical metal sulfides with diamond-like structures

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.

Layered Chalcogenide Scintillators Enabled by Reversible Hydrous-Induced Phase Transformation for High-Resolution X-ray Imaging

Scintillation materials have been widely used in various fields, such as medical diagnosis and industrial detection. Chalcogenides have the potential to become a new generation of high-performance scintillation materials due to their high effective atomic number and good resistance to radiation damage. However, research on their application in radiation detection is currently very scarce. Herein, single crystals of rare earth ion-doped ternary chalcogenides NaGaS2/Eu were grown by a high-temperature solid-phase method. It exhibits unique characteristics of structure transformation by absorbing water molecules from the air. To maintain the anhydrous phase of the material, we have used a strategy of organic-inorganic composites of epoxy resin and NaGaS2/Eu to prepare devices for radiation detection and discuss the irradiation luminescence properties of the two phases. The anhydrous phase of NaGaS2/Eu demonstrates excellent sensitivity to X-rays, with a low detection limit of 250 nGy s-1, which is approximately 1/22 of the medical imaging dose. Additionally, composite flexible films were prepared, which exhibited excellent performance in X-ray imaging. These films enable clear observation of a wide range of objects with a high spatial resolution of up to 13.2 line pairs per millimeter (lp mm-1), indicating that chalcogenide holds promising prospects in the realm of X-ray imaging applications.

A Machine Learning Study on High Thermal Conductivity Assisted to Discover Chalcogenides with Balanced Infrared Nonlinear Optical Performance

Exploration of novel nonlinear optical (NLO) chalcogenides with high laser-induced damage thresholds (LIDT) is critical for mid-infrared (mid-IR) solid-state laser applications. High lattice thermal conductivity (κL ) is crucial to increasing LIDT yet often neglected in the search for NLO crystals due to lack of accurate κL data. A machine learning (ML) approach to predict κL for over 6000 chalcogenides is hereby proposed. Combining ML-generated κL data and first-principles calculation, a high-throughput screening route is initiated, and ten new potential mid-IR NLO chalcogenides with optimal bandgap, NLO coefficients, and thermal conductivity are discovered, in which Li2 SiS3 and AlZnGaS4 are highlighted. Big-data analysis on structural chemistry proves that the chalcogenides having dense and simple lattice structures with low anisotropy, light atoms, and strong covalent bonds are likely to possess higher κL . The four-coordinated motifs in which central cations show the bond valence sum of +2 to +3 and are from IIIA, IVA, VA, and IIB groups, such as those in diamond-like defect-chalcopyrite chalcogenides, are preferred to fulfill the desired structural chemistry conditions for balanced NLO and thermal properties. This work provides not only an efficient strategy but also interpretable research directions in the search for NLO crystals with high thermal conductivity.

NaMoO3(IO3)(H2O): water molecule introduction induces strong second harmonic generation response, widened band gap and large anisotropy

Exploring feasible tactics to induce the formation of non-centrosymmetric (NCS) structures, especially from centrosymmetric (CS) structures, is essential for the development of nonlinear optical crystals with more potential. An NCS alkali metal-containing molybdenum iodate hydrate, namely, NaMoO3(IO3)(H2O), was designed based on the CS matrix NaMoO3(IO3) via introducing a water molecule into the structure. The introduction of one crystalline water molecule results in the rearrangement of Λ-shaped cis-[MoO4(IO3)2] units, and the proper array of the cis-[MoO4(IO3)2] units in NaMoO3(IO3)(H2O) results in its strong SHG response of 4.6 × KH2PO4. In addition, NaMoO3(IO3)(H2O) exhibits a wider optical bandgap of 3.44 eV and a larger birefringence of 0.231 than its matrix. Furthermore, the framework of NaMoO3(IO3)(H2O) is highly similar to that of α-KMoO3(IO3), with water molecules assisting Na+ cations in occupying the position of K+. However, due to the extra hydrogen bond of water molecules, the [MoO3(IO3)]∞ layers in NaMoO3(IO3)(H2O) retain a parallel-stacking arrangement, different from the antiparallel arrangement of layers in α-KMoO3(IO3) with a centric structure. This study confirms the feasibility of applying a water molecule to adjust the orientation of basic building block units to assemble an NCS structure based on CS crystals.

Nonlinear Optical Phosphide CuInSi2P4: The Inaugural Member of Diamond-Like Family I-III-IV2-V4 Inspired by ZnGeP2

Noncentrosymmetric phosphides have garnered significant attention as promising systems of infrared (IR) nonlinear optical (NLO) materials. Herein, a new quaternary diamond-like phosphide family I-III-IV2-V4 and its inaugural member, namely, CuInSi2P4 (CISP), were successfully fabricated by isovalent and aliovalent substitution based on ZnGeP2. First-principles calculations revealed that CISP has a large NLO coefficient (d14 = 110.8 pm/V), which can be attributed to the well-aligned tetrahedral [CuP4], [InP4], and [SiP4] units. Remarkably, the extremely small thermal expansion anisotropy (0.09) of CISP enables it to exhibit a considerable laser-induced damage threshold (LIDT, 5.0 × AgGaS2@1.06 μm) despite the relatively narrow band gap (0.81 eV). This work improves the chemical diversity of inorganic phosphide and promotes the development of phosphide systems, which may provide valuable perspectives for future exploration of IR NLO materials.

Sr(NH2SO3)(NO3)·H2O: An Ultraviolet Nonlinear Optical Material Exhibiting Strong Second-Harmonic Generation Response and Sufficient Birefringence

Second-harmonic generation (SHG) response and birefringence stand as fundamental optical properties for nonlinear optical (NLO) materials. Up to now, engineering a strong SHG response and substantial birefringence in the sulfamate system has proven to be exceedingly challenging. A novel noncentrosymmetric compound, Sr(NH2SO3)(NO3)·H2O, has been meticulously designed by introducing the (NO3)- group characterized by significant polarizability anisotropy and hyperpolarizability into the polar non-π-conjugated (NH2SO3)- system. This compound exhibits a robust SHG response (5.2 × KDP), ample birefringence (0.077 at 546.1 nm), and a short ultraviolet (UV) absorption edge (290 nm). The linear and nonlinear optical properties can be attributed to the (NO3)- group and the (NH2SO3)- group. This study presents an effective approach that contributes to the design of more composite anionic UV NLO materials with excellent and balanced optical properties.

Rational Design of a Niobium Tellurite Crystal Nb2Te3O11 Exhibiting Good Overall Infrared NLO Performance by Structural Genetic Engineering

Nonlinear optical (NLO) materials have aroused increasing interest owing to their promising applications in optoelectronic technologies. Herein, we present the synthesis of an acentric niobium tellurite crystal, Nb2Te3O11, extracted via a spontaneous crystallization approach. It adopts a unique three-dimensional (3D) structure constructed by the distorted [TeO3], [TeO4], and [NbO6] fundamental building units. The title compound undergoes incongruent melting at approximately 807 °C. Optical characterizations demonstrate that Nb2Te3O11 possesses an extended transparency window beyond 5 μm, along with a large band gap value of 3.1 eV. Moreover, the as-synthesized Nb2Te3O11 displays an appreciable second-harmonic generation (SHG) response of 2 × KDP and a notable birefringence of 0.11 under 1064 nm for achieving phase-matching. In addition, theoretical calculation investigations suggest that the intriguing optical properties are ascribed to the cooperative effect of three types of NLO-active motifs: [TeO3] pyramids, [TeO4] seesaws, and [NbO6] octahedra. These attributes provide new functional insights into Nb2Te3O11 and enrich the family of NLO crystals in the mid-infrared region.

Eu2MGe2OS6 (M = Mn, Fe, Co): Three Melilite-Type Rare-Earth Oxythiogermanates Exhibiting Balanced Nonlinear-Optical Behaviors

Metal oxychalcogenides as candidates for novel mid-infrared nonlinear-optical materials have attracted great interest due to the distinctive advantages of oxides and chalcogenides in this field. Herein, the first melilite-type rare-earth (RE) oxythiogermanates Eu2MGe2OS6 [M = Mn (1), Fe (2), Co (3)] are obtained by combining RE metals with localized f electrons, magnetic transition metals with delocalized d electrons, and the highly distorted mixed anionic group [GeOS3] into one structure. They belong to the tetragonal P4̅21m space group, and highly distorted [EuOS7] bicapped trigonal prisms bridge adjacent {[MGe2OS6]4-}∞ layers to build the three-dimensional network. Their optical band gaps are determined as 2.40, 2.11 and 2.14 eV, and they show moderate second-harmonic-generation (SHG) responses (0.3, 0.3 and 0.5 × AGS) and large laser-induced damage thresholds (2.77-8.31 × AGS). Theoretical calculation results indicate that the synergistic effect of [EuOS7] and [MS4] units acts on the SHG effect. This work enriches the crystal chemistry of melilite-structure materials.

[Ba4 (S2 )][ZnGa4 S10 ]: Design of an Unprecedented Infrared Nonlinear Salt-Inclusion Chalcogenide with Disulfide-Bonds

Salt-inclusion chalcogenides (SICs) have been receiving widespread attention due to their large second harmonic generation (SHG) responses and wide bandgaps, however most of them suffer from small birefringence limiting their technical application. Herein, by introducing the π-conjugated (S2 )2- units in the ionic guest of salt-inclusion structure, the first disulfide-bond-containing SIC, [Ba4 (S2 )][ZnGa4 S10 ] has been synthesized. It exhibits the widest bandgap up to 3.39 eV among polychalcogenides and strong SHG response as large as that of AgGaS2 (AGS). Importantly, its birefringence reaches a max value of 0.053@1064 nm among AGS-like SICs, indicating it is a promising IR nonlinear optical (NLO) material. Theoretical calculations reveal that the π-conjugated (S2 )2- units and covalent GaS layers favor the enhanced birefringence and large SHG response. This work provides not only a new type of SIC for the first time, but also new lights on the design of IR NLO materials.

Three-in-One Strategy Constructing the First High-Performance Nonlinear Optical Sulfide Crystallizing with the P43 Space Group

The balance between large nonlinear optical (NLO) effect and wide bandgap is the key scientific issue for the exploration of infrared NLO materials. Targeting this issue, two new pentanary chalcogenides KGaGe1.37 Sn0.63 S6 (1) and KGaGe1.37 Sn0.63 Se6 (2) are obtained by the three-in-one strategy, viz. three types of fourfold-coordinated metal elements co-occupying the same site. They crystallize in the tetragonal P43 (1) and monoclinic Cc (2) space group. Their structures can be evolved from benchmark AgGaS2 (AGS) by suitable substitution. Remarkably, 1 is the first NLO sulfide crystallizing with the P43 space group, representing a new structure-type NLO material. The structural relationship between 1 and 2 and the evolution from 1, 2 to AGS are also analyzed. Both 1 and 2 show balanced NLO properties. Specifically, 1 exhibits phase-matchable SHG response of 0.6 × AGS, a wide bandgap of 3.50 eV, and a high laser damage threshold of 6.24 × AGS. Theoretical calculation results suggest that the Ga/Ge/Sn element ratios of the co-occupied sites of 1 and 2 are the most appropriate for stabilizing the structures. The strategy adopted here will provide some inspiration for exploring new high-performance NLO materials.

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.

Structure and Optical Properties of LixAg1-xGaSe2 and LixAg1-xInSe2

Complete substitution of Li atoms for Ag atoms in AgGaSe₂ and AgInSe₂ was achieved, resulting in the solid solutions Li_xAg_1-xGaSe₂ and Li_xAg_1-xInSe₂. The detailed crystal structures were determined by single-crystal X-ray diffraction and solid-state ⁷Li nuclear magnetic resonance spectroscopy, which confirm that Li atoms occupy unique sites and disorder only with Ag atoms. The tetragonal CuFeS₂-type structure (space group I4̅2d) was retained within the entirety of the Ga-containing solid solution Li_xAg_1-xGaSe₂, which is noteworthy because the end-member LiGaSe₂ normally adopts the orthorhombic β-NaFeO₂-type structure (space group Pna2₁). These structures are closely related, being superstructures of the cubic sphalerite and hexagonal wurtzite prototypes adopted by diamond-like semiconductors. For the In-containing solid solution Li_xAg_1-xInSe₂, the structure transforms from the tetragonal to orthorhombic forms as the Li content increases past x = 0.50. The optical band gaps increase gradually with higher Li content, from 1.8 to 3.4 eV in Li_xAg_1-xGaSe₂ and from 1.2 to 2.5 eV in Li_xAg_1-xInSe₂, enabling control to desired values, while the second harmonic generation responses become stronger or are similar to those of benchmark infrared nonlinear optical materials such as AgGaS₂. All members of these solid solutions remain congruently melting at accessible temperatures between 800 and 900 °C. Electronic structure calculations support the linear trends seen in the optical band gaps and confirm the mostly ionic character present in Li-Se bonds, in contrast to the more covalent character in Ga-Se or In-Se bonds.

Polar Phosphorus Chalcogenide Cage Molecules: Enhancement of Nonlinear Optical Properties in Adducts

Inorganic adducts are an emerging class of infrared nonlinear optical (NLO) materials. However, although the reported NLO adducts have sufficient birefringences and significant laser-induced damage thresholds (LIDTs), they commonly suffer from weak second harmonic generation (SHG) responses. In this work, a series of polar phosphorus chalcogenide cage molecules with strong hyperpolarizabilities were theoretically screened out to enhance the SHG responses of adducts. Accordingly, (CuI)₃ (P₄ S₄ ), (CuI)₃ (P₄ Se₄ ), (CuBr)₇ (P₄ S₃ )₃ and (CuBr)₇ (P₄ Se₃ )₃ with target polar cage molecules were successfully synthesized. As expected, they exhibit enhanced SHG responses while keeping moderate birefringences and high LIDTs. Notably, (CuBr)₇ (P₄ Se₃ )₃ possesses the largest SHG response (3.5×AGS@2.05 μm) among the known inorganic NLO adducts. Our study confirms that introducing NLO-active cage molecules into adducts is an efficient strategy for high-performance NLO materials.

KREP2 Se6 (RE = Sm, Gd, Tb): The First Rare-Earth Selenophosphates with Remarkable Nonlinear Optical Activities Realized by Synergistic Effect of RE- and P-Based Motifs

Rare-earth (RE) chalcogenides have been extensively studied as infrared nonlinear optical (NLO) materials because of their nice integrated performances; however, very few RE chalcophosphates are involved for this topic. Here, three quaternary RE selenophosphates, KSmP₂ Se₆ (1), KGdP₂ Se₆ (2), and KTbP₂ Se₆ (3), are profoundly studied for their NLO potentials. Their noncentrosymmetric P21 structures feature RESe8-bicapped trigonal prisms and ethane-like [P₂ Se₆ ]₄ - dimers built {[REP₂ Se₆ ]-}∞ layers. As the first studied NLO-active RE selenophosphates, 1-3 exhibit second harmonic generation (SHG)responses ≈0.34-1.08 × AgGaS₂ at 2.10 µm and laser-induced damage thresholds (LIDTs) ≈1.43-4.33 × AgGaS₂ , and they all show phase-matchable behaviors, indicating their wonderful balanced NLO properties. Theoretical calculations demonstrate that the synergistic effect between RESe₈ and P₂ Se₆ units makes the major contribution to the SHG responses.

Characterizations and Nonlinear-Optical Properties of Pentanary Transition-Metal Oxysulfide Sr2CoGe2OS6

As one type of material containing multiple anions, oxysulfides can combine the advantages of oxides and sulfides and are deeply studied as nonlinear-optical (NLO) materials. Herein, a new melilite-type pentanary oxysulfide Sr₂CoGe₂OS₆ is studied. It crystallizes in the noncentrosymmetric tetragonal space group P4̅2₁m, and its structure features GeOS₃ and CoS₄ tetrahedra-built {[CoGe₂OS₆]^4-}_∞ layers. Its powder sample exhibits a moderate phase-matchable NLO response and a high laser-induced damage threshold. The NLO response is mainly determined by CoS₄ tetrahedra according to the theoretical calculation results. This work indicates that transition-metal oxysulfides can also be considered as potential infrared NLO materials.

Wide band gap thiophosphates ASrPS4 (A = Li, Na, K, Rb, Cs): cation size effect induced successive structural transformation

Metal thiophosphates have aroused much research interest due to their structural chemistry and possible applications as infrared functional materials. In this study, five quaternary Sr-based alkali metal thiophosphates ASrPS₄ (A = Li, Na, K, Rb, Cs) were obtained. Their structural comparison shows that their symmetry undergoes transformation from tetragonal (I4₁/acd) to monoclinic (P2₁/c) to orthorhombic (Pnma) system, which is induced by the cation size effects and coordination features of different alkali metal cations. The experimental and theoretical results demonstrate that the band gaps of all title compounds are large, namely 3.6-3.9 eV (experimental results) and 3.78-4.12 eV (HSE06). Theoretical analyses indicate that the [PS₄] group could be regarded as a good unit for designing wide band gap compounds, and the birefringence of NaSrPS₄ is 0.08 at the fundemental 1064 nm wavelength, which shows that it may be a potential infrared birefringent material.

Ag4SnGe2S7: A Noncentrosymmetric Chalcogenide in I4-II-IV2-VI7 System with Non-Diamond-Like Structure Featuring 1D ∞[SnGe2S8]6- Infinite Chain

The I₄-II-IV₂-VI₇ metal chalcogenide system has become an attractive research system because of its excellent physical and chemical properties. Here, we report the discovery of a new Sn^II-based quaternary chalcogenide in the I₄-II-IV₂-VI₇ system, Ag₄SnGe₂S₇, with a non-diamond-like structure and crystallizing in the Cc space group. The compound is characterized by isolated pyramid-like [SnS₃] units and one-dimensional _∞[SnGe₂S₈]^6- infinite chains with two orientations formed by the corner-sharing connected [SnGe₂S₈]^6- units. It has a band gap of 2.40 eV and is insensitive to air and moisture.

LiSrSbS3: parallel configurations of lone pair electrons inducing a large birefringence

Enhancement of birefringence is significant since the birefringent materials can create and control polarized light and be used extensively in various advanced optical systems. By optimizing the arrangement of [SbS₃] units with stereo-chemical active lone pair electrons, a new quaternary thioantimonate LiSrSbS₃ with a large birefringence has been successfully synthesized by a high temperature solid-state reaction method. LiSrSbS₃ crystallizes in the monoclinic space group of P2₁/c. In the structure, the isolated infinite [LiS₄] chains and zigzag [SrS₆] chains are alternately connected with each other to compose a three-dimensional (3D) framework, and the isolated pyramid [SbS₃] units are located between them. To analyze the source of large birefringence, the electronic structure and optical properties of LiSrSbS₃ were further investigated by the first-principles method, and the results show that the optimized arrangement [SbS₃] trigonal pyramid induces a large birefringence.

Mixed-Anion-Oriented Design of LnMGa3S6O (Ln = La, Pr, and Nd; M = Ca and Sr) Nonlinear Optical Oxysulfides with Targeted Property Balance

Nonlinear optical (NLO) crystals are of importance on extending infrared (IR) laser wavelengths. Considering their performance drawbacks in commercial IR NLO crystals, a recent challenge in exploring new excellent IR NLO crystals is how to break the inherent conflict between a wide bandgap (E_g ≥ 3.0 eV) and large NLO effect (d_ij ≥ 0.5 × AgGaS₂) and simultaneously enlarge the birefringence (Δn) for a requisite phase-matching (PM) behavior. For that reason, rational combination of mixed-anion functional groups into a crystal structure affords the successful design and synthesis of six LnMGa₃S₆O (Ln = La, Pr, and Nd; M = Ca and Sr) NLO oxysulfides. Among them, LaMGa₃S₆O satisfy the property-balance demand (E_g: 3.21-3.27 eV and d_ij: 0.9-1.0 × AgGaS₂) as promising PM NLO crystals through gathering their property advantages between LaMGa₃O₇ and LaMGa₃S₇ by mixed-anion-oriented performance engineering. A study on the structure-property relationship indicates that heteroleptic (Ln/M)S₇O and GaS₃O anionic groups are proven as promising NLO-active units and offer a great synergistic effect on the NLO origin. This work as a visualized model not only provides a first clear cognition on varying properties from oxide to sulfide to oxysulfide but also highlights the feasibility of mixed-anion-oriented design of new NLO candidates with balanced performances.

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.

Heteroanionic Control of Exemplary Second-Harmonic Generation and Phase Matchability in 1D LiAsS2-xSex

The isostructural heteroanionic compounds β-LiAsS_2-xSe_x (x = 0, 0.25, 1, 1.75, 2) show a positive correlation between selenium content and second-harmonic response and greatly outperform the industry standard AgGaSe₂. These materials crystallize in the noncentrosymmetric space group Cc as one-dimensional ¹/_∞ [AsQ₂]^- (Q = S, Se, S/Se) chains consisting of corner-sharing AsQ₃ trigonal pyramids with charge-balancing Li⁺ atoms interspersed between the chains. LiAsS_2-xSe_x melts congruently for 0 ≤ x ≤ 1.75, but when the Se content exceeds x = 1.75, crystallization is complicated by a phase transition. This behavior is attributed to the β- to α-phase transition present in LiAsSe₂, which is observed in the Se-rich compositions. The band gap decreases with increasing Se content, starting at 1.63 eV (LiAsS₂) and reaching 1.06 eV (β-LiAsSe₂). Second-harmonic generation measurements as a function of wavelength on powder samples of β-LiAsS_2-xSe_x show that these materials exhibit significantly higher nonlinearity than AgGaSe₂ (d₃₆ = 33 pm/V), reaching a maximum of 61.2 pm/V for LiAsS₂. In comparison, single-crystal measurements for LiAsSSe yielded a d_eff = 410 pm/V. LiAsSSe, LiAsS_0.25Se_1.75, and β-LiAsSe₂ show phase-matching behavior for incident wavelengths exceeding 3 μm. The laser-induced damage thresholds from two-photon absorption processes are on the same order of magnitude as AgGaSe₂, with S-rich materials slightly outperforming AgGaSe₂ and Se-rich materials slightly underperforming AgGaSe₂.

Shedding Light on the Structure and Characterization of K2ZnGe2O6: A Phase-Matchable Nonlinear Optical Crystal

Nonlinear optical (NLO) materials have recently aroused great interest owing to their capability of frequency conversion in solid-state lasers. Herein, we report an acentric zinc germanate K₂ZnGe₂O₆ obtained successfully through spontaneous crystallization methods. It affords a novel three-dimensional (3D) framework comprised of [GeO₄] and [ZnO₄] motifs with K atoms located in the tunnels. K₂ZnGe₂O₆ displays a moderate second-harmonic-generation (SHG) intensity (0.73 × KDP) with phase-matchable behavior. Optical characterization demonstrated that it has a UV cutoff edge located at 368 nm with a large energy band of 3.23 eV, accompanied by a wide transmission window, covering a 3-5 μm atmospheric window. Moreover, thermal properties implied that it possesses intriguing thermal stability of 987 °C and a congruent melting nature. Additionally, first-principles calculations unveiled that the NLO performance was primarily attributed to the collective effect of [GeO₄] and [ZnO₄] building units. These findings indicate that K₂ZnGe₂O₆ is a potential NLO crystal.

Design of Infrared Nonlinear Optical Compounds with Diamond-like Structures and Balanced Optical Performance

Infrared (IR) nonlinear optical (NLO) crystals are the major materials to widen the output range of solid-state lasers to mid-infrared regions, but they are still inadequate for application due to the difficulties in balancing the large band gaps and strong NLO response. The diamond-like structure is a potential structural template to explore IR NLO materials. Herein, a computational workflow is proposed for exploring compounds with diamond-like structures, a series of LiMgGaSe₃ structures were predicted successfully through this workflow, and LiMgGaSe₃-I-III exhibited good optical performances in a large band gap (2.75-2.92 eV), strong SHG response (1.2-1.3 × AGS), and suitable birefringence (0.0470-0.0783 at 1064 nm). The in-depth mechanism explorations strongly demonstrate that the synergistic effect of alkaline earth metal tetrahedral [MgSe₄] and [GaSe₄] units is the main origin of large SHG response. The foregoing results suggest that our workflow can accelerate the discovery of new mid-IR NLO materials with diamond-like structures.

The Combination of Structure Prediction and Experiment for the Exploration of Alkali-Earth Metal-Contained Chalcopyrite-Like IR Nonlinear Optical Material

Design and fabrication of new infrared (IR) nonlinear optical (NLO) materials with balanced properties are urgently needed since commercial chalcopyrite-like (CL) NLO crystals are suffering from their intrinsic drawbacks. Herein, the first defect-CL (DCL) alkali-earth metal (AEM) selenide IR NLO material, DCL-MgGa2 Se4 , has been rationally designed and fabricated by a structure prediction and experiment combined strategy. The introduction of AEM tetrahedral unit MgSe4 effectively widens the band gap of DCL compounds. The title compound exhibits a wide band gap of 2.96 eV, resulting in a high laser induced damage threshold (LIDT) of ≈3.0 × AgGaS2 (AGS). Furthermore, the compound shows a suitable second harmonic generation (SHG) response (≈0.9 × AGS) with a type-I phase-matching (PM) behavior and a wide IR transparent range. The results indicate that DCL-MgGa2 Se4 is a promising mid-to-far IR NLO material and give some insights into the design of new CL compound with outstanding IR NLO properties based on the AEM tetrahedra and the structure predication and experiment combined strategy.

Metal oxyhalides: an emerging family of nonlinear optical materials

Second-order nonlinear optical (NLO) materials have drawn enormous academic and technological attention attributable to their indispensable role in laser frequency conversion and other greatly facilitated applications. The exploration of new NLO materials with high performances thus has long been an intriguing research field for chemists and material scientists. However, an ideal NLO material should simultaneously satisfy quite a few fundamental yet rigorous criteria including a noncentrosymmetric structure, large NLO coefficients, desired transparent range, large birefringence, high laser damage threshold, and availability of a large-size single crystal. Therefore, the identification of promising compound systems, targeted design, and experience-based syntheses are crucial to discover novel NLO materials working in the spectral region of interest. As an important family of mixed-anion compounds, versatile metal oxyhalides containing metal-centered oxyhalide functional units ([MO m X n ] (X = F, Cl, Br, and I)) are becoming a marvelous branch for interesting NLO materials. Especially, when the central metals are d0/d10 transition metals or heavy post-transition metals, a number of novel NLO materials with superior functionalities are expected. Our thorough review on the recent achievements of metal oxyhalides for NLO materials are divided into the fast-growing NLO metal oxyhalides with single type halogen anions and the newly identified NLO metal oxyhalides with mixed halogen anions. Here we mainly focus on the design strategy, structural chemistry, NLO-related properties, and structure-property correlation of the metal oxyhalides with relatively large NLO responses. We hope this review can provide an insight on the rational design and future development of emerging metal oxyhalides for NLO and other applications.

NaGa3Se5: An Infrared Nonlinear Optical Material with Balanced Performance Contributed by Complex {[Ga3Se5]-}∞ Anionic Network

Second-order nonlinear optical (NLO) materials are extensively applied in laser-related techniques. For developing IR NLO materials, chalcogenides are the main candidates. Here, NaGa₃Se₅ was explored as inspired by its unique anionic structure. It crystallizes with the orthorhombic chiral P2₁2₁2₁ structure, featuring 12 types of GaSe₄ tetrahedra built into a three-dimensional {[Ga₃Se₅]^-}_∞ anionic network, representing a new NLO-functional motif. NaGa₃Se₅ exhibits large and phase-matchable NLO response 1.37 × AgGaS₂. It has the largest band gap among the noncentrosymmetric A-M^III-Se (A = alkali metal; M = Ga, In) compounds. The NLO properties' origin is explored via theoretical analysis. The success of NaGa₃Se₅ contributes a practical case for exploring new NLO materials.

The Third-Order Nonlinear Optical Properties of Sb2S3/RGO Nanocomposites

Antimony sulfide/reduced graphene oxide (Sb2S3/RGO) nanocomposites were synthesized via a facile, one-step solvothermal method. XRD, SEM, FTIR, and Raman spectroscopy were used to characterize the uniform distribution of Sb2S3 nanoparticles on the surface of graphene through partial chemical bonds. The third-order nonlinear optical (NLO) properties of Sb2S3, RGO, and Sb2S3/RGO samples were investigated by using the Z-scan technique under Nd:YAG picosecond pulsed laser at 532 nm. The results showed that pure Sb2S3 particles exhibited two-photon absorption (TPA), while the Sb2S3/RGO composites switched to variable saturated absorption (SA) properties due to the addition of different concentrations of graphene. Moreover, the third-order nonlinear susceptibilities of the composites were also tunable with the concentration of the graphene. The third-order nonlinear susceptibility of the Sb2S3/RGO sample can achieve 8.63 × 10−12 esu. The mechanism for these properties can be attributed to the change of the band gap and the formation of chemical bonds supplying channels for photo-induced charge transfer between Sb2S3 nanoparticles and the graphene. These tunable NLO properties of Sb2S3/RGO composites can be applicable to photonic devices such as Q-switches, mode-locking devices, and optical switches.

Quaternary Noncentrosymmetric Rare-Earth Sulfides Ba4RE2Cd3S10 (RE = Sm, Gd, or Tb): A Joint Experimental and Theoretical Investigation

Multinary rare-earth chalcogenides with d-block transition metals have attracted considerable attention owing to their intriguing structural architectures and promising practical applications. In this work, three quaternary rare-earth sulfides, Ba₄RE₂Cd₃S₁₀ (RE = Sm, Gd, or Tb), have been obtained by the high-temperature solid-state method. These compounds are isostructural and belong to the noncentrosymmetric orthorhombic space group Cmc2₁ (No. 36). The basic structural unit contains unique two-dimensional anionic [RE₂Cd₃S₁₀]^8- layers, which are separated by Ba²⁺ cations. Remarkably, Ba₄Sm₂Cd₃S₁₀ exhibits a high second-harmonic-generation intensity (1.8 times that of AgGaS₂) and a significantly higher laser-induced damage threshold (14.3 times that of AgGaS₂), which is the first case possessing an infrared (IR) nonlinear optical (NLO) property in the quaternary AE/RE/TM/Q (AE = alkaline-earth metals; RE = rare-earth metals; TM = d-block transition metals; and Q = chalcogen) systems. Moreover, theoretical investigations of the structure-property relationship indicate that the combined action of various types of NLO-active units makes the main contribution to the SHG activity. This discovery may shed light on broadening the frontiers of IR-NLO materials.

Two New Tellurite Halides with Cationic Layers: Syntheses, Structures, and Characterizations of CdPb2Te3O8Cl2 and Cd13Pb8Te14O42Cl14

Two new tellurite halides, CdPb2Te3O8Cl2 and Cd13Pb8Te14O42Cl14 with mixed cationic layered structures, have been synthesized by high-temperature solution method. CdPb2Te3O8Cl2 crystallizes in the noncentrosymmetric Aba2 space group, built by [CdPb2Te3O8]...

Sn7Br10S2: The First Ternary Halogen-Rich Chalcohalide Exhibiting A Chiral Structure and Pronounced Nonlinear Optical Properties

Infrared nonliear optical (IR NLO) materials are significant in laser technology for civil and military uses. Here, we report the synthesis, structural chemistry and NLO properties of a halogen-rich chalcohalide Sn7Br10S2. Its noncentrosymmetric (NCS, P63) structure can be considered as partially aliovalent anion substitution of SnBr2 (P63/m) induced centrosymmetric (CS) to NCS structural transformation. The 3D ∞[Sn(1)6Sn(2)6Br6X6]6- (X = Br/S) channel framework is consisting of Sn(1)BrX2 and Sn(2)X3 trigonal pyramids. It exhibits excellent NLO performances, including strong phase-matchable NLO response 1.5 × AgGaS2 and high laser-induced damage threshold 6.3 × AgGaS2. Structure-NLO performance relationship investigation confirm that the effective arrangement of Sn(1)BrX2 and Sn(2)X3 units predominantly contribute to the large SHG response. These results indicate Sn7Br10S2 is a potential IR NLO candidate, and provide a new feasible system as promising NLO materials.

Structural Chemistry and Excellent Nonlinear Optical Properties of a Series of Ternary Selenides GaxIn2-xSe3

Novel nonlinear optical (NLO) materials possessing simple chemical compositions and facile syntheses are competitive when considering their practical application. Here, a series of ternary selenides Ga_xIn_2-xSe₃ (x = 0.07, 0.38, 0.45, and 0.81) that crystallize in a chiral P6₅ structure are obtained by melting Ga, In, and Se elements. Their three-dimensional structures are built by (Ga/In)Se₄ tetrahedra and InSe₅ trigonal bipyramids. The hexagonal modification's phase stability is analyzed by energy calculation, and their optical band gaps are determined to be 1.72-1.99 eV. They exhibit large NLO responses that are 1.41-1.64 times that of the benchmark AgGaS₂. The results of density functional theory calculations suggest that introduction of Ga onto the In site in (InSe₄)^5- units can form a deformed tetrahedron with more distortion in the structure, and the (InSe₅)^7- units contribute a large amount of birefringence to the structure. This work is the first to investigate the ternary chalcogenides M₂Q₃ (M = Ga or In; Q = S or Se) as new types of infrared NLO crystals with excellent performances, which will stimulate more interest in those possessing simple compositions and outstanding performances.

Noncentrosymmetric Ba6In2Q10 (Q = S, Se): Structural Chemistry and Nonlinear-Optical Activity

The indispensable condition for laser frequency-doubling crystal materials is that they crystallize in the noncentrosymmetric (NCS) structures. Here, ternary NCS Ba₆In₂S₁₀ (1) and Ba₆In₂Se₁₀ (2) (P6₃) were synthesized via conventional solid-state reactions. Their zero-dimensional structures feature isolated InQ₄ tetrahedra and Q₂^2- (Q = S, Se) dimers that are separated by Ba²⁺ counter cations. The structural relevance and differences with the centrosymmetric Ba₁₂In₄S₁₉ and Ba₁₂In₄Se₂₀ are analyzed in detail. Both 1 and 2 show obvious laser frequency-doubling activity, and their optical band gaps are 3.72 and 1.78 eV, respectively. This work provides a new type of IR nonlinear-optical materials.

Ca3(TeO3)2(MO4) (M = Mo, W): Mid-Infrared Nonlinear Optical Tellurates with Ultrawide Transparency Ranges and Superhigh Laser-Induced mage ThreDasholds

Intense interests in mid-infrared (MIR) nonlinear optical (NLO) crystals have erupted in recent years due to the development of optoelectronic applications ranging from remote monitoring to molecular spectroscopy. Here, two polar crystals Ca₃(TeO₃)₂(MO₄) (M = Mo, W) were grown from TeO₂-MO₃ flux by high-temperature solution methods. Ca₃(TeO₃)₂(MoO₄) and Ca₃(TeO₃)₂(WO₄) are isostructural, which feature novel structures consisting of asymmetric MO₄ tetrahedra and TeO₃ trigonal pyramids. Optical characterizations show that both crystals display ultrawide transparency ranges (279 nm to 5.78 μm and 290 nm to 5.62 μm), especially high optical transmittance over 80% in the important atmospheric transparent window of 3-5 μm, and superhigh laser damage thresholds (1.63 GW/cm² and 1.50 GW/cm²), 54.3 and 50 times larger than that of state-of-the-art MIR NLO AgGaS₂, respectively. Notably, they exhibit the widest band gaps and the loftiest laser-induced threshold damages among the reported tellurates so far. Moreover, Ca₃(TeO₃)₂(MO₄) exhibit type I phase matching at two working wavelengths owing to their large birefringence and strong second-harmonic generation responses from the distorted anions, as further elucidated by the first-principles calculations. The above characteristics indicate that Ca₃(TeO₃)₂(MO₄) crystals are high-performance MIR NLO materials, especially applying in high-power MIR laser operations.