Heteroanionic Melilite Oxysulfide: A Promising Infrared Nonlinear Optical Candidate with a Strong Second-Harmonic Generation Response, Sufficient Birefringence, and Wide Bandgap

Ae3[TO3][SnOQ3] (Ae = Sr, Ba; T = Si, Ge; Q = S, Se) and Ba3[CO3][MQ4] (M = Ge, Sn; Q = S, Se): Design and Syntheses of a Series of Heteroanionic Antiperovskite-Type Oxychalcogenides

The heteroanionic materials (HAMs) have attracted more and more attention because they can better balance the functional properties of materials. However, their rational structural design is still a great challenge. Here, by using the antiperovskite Ba3S[GeS4] as a template and calculating the tolerance factor (t) as a reference, eight heteroanionic oxychalcogenides with balanced properties were finally synthesized by a partially group-substitution method. Among them, Ba3[CO3][MQ4] (M = Ge, Sn; Q = S, Se) are centrosymmetric (CS) crystals and realize optimization of band gaps and birefringence. For Ae3[TO3][SnOQ3] (Ae = Sr, Ba; T = Si, Ge; Q = S, Se), thanks to the novel [TO4SnQ3] polyanionic groups for the regulation to the antiperovskite structures and the contributions to the nonlinear optical (NLO) properties, they achieve the structural transition from CS to noncentrosymmetry and accomplish an excellent balance among the critical performance parameters as the potential candidates for the infrared NLO materials, including phase-matchable behavior, wide band gaps (Eg = 3.26-3.95 eV), high laser damage threshold (LDT = 3.2-4.4 × AgGaS2), suitable birefringence (Δn = 0.065-0.098@2090 nm) and sufficiently strong second-harmonic generation responses (about 0.6-0.9 × AgGaS2). Moreover, benefiting from crystallization in the polar space groups, they exhibit ferroelectricity and piezoelectricity at room temperature. As far as we know, this is the first reported fully inorganic antiperovskite ferroelectric. These demonstrate that our strategy is desirable and can provide some unique insights into the development of HAMs or antiperovskite materials with specific functions or structures.

Synthesis and Properties of BaMTeS (M = Fe, Mn, Zn) and the Disordered Structural Analog BaGe0.5TeS

A series of tertiary sulfide-tellurides, BaMxTeS (M = Fe, Mn, Zn, Ge), has been synthesized by solid-state synthesis. The compounds assume an orthorhombic crystal structure, described by the Cmcm (No. 63) space group, and are structural analogs of the BaMSO (M = Co, Zn) phases. The properties of all four analogs are investigated by DFT analysis. As only the BaFeTeS analog was prepared as a relatively pure phase, this homologue was subject to further experimental investigations, including heat capacity, magnetometry, and Mössbauer spectroscopy. BaFeTeS exhibits no obvious phase transition between 2 and 300 K, has no paramagnetic behavior, and lacks long-range magnetic ordering. However, the Mössbauer spectra, as well as electrical resistance data, indicate a hidden transition near 200 K that is tentatively explained by a dynamic charge-density-wave mechanism, based on a resonating valence bond (RVB) model.

Homoatomic Polychalcogenide Nonlinear Optical Anionic Groups with Ultra-Large Optical Anisotropy

Functional assembly of nonlinear optical (NLO) motifs with a large optical anisotropy is vital to the development of advanced NLO and birefringent materials. In this work, we highlight that, in addition to heteroatomic NLO motifs, homoatomic anionic clusters formed by aggregated anions (S, Se, Te) exhibit diverse chain-, ring-, and cage-like chemical structures as well as one-, two-, and three-dimensional motif alignments. The rich structural chemistry enables homoatomic polychalcogenides (HAPCs) to exhibit asymmetric structural features and anisotropic optical properties, with great potential for NLO and birefringent performance. Focusing on totally 55 binary HAPCs A2Qn (n = 2, 3, 4, 5; A = Na, K, Rb, Cs; Q = S, Se, Te) and their ternary analogues, we employ the state-of-the-art first-principles approach to systematically investigate the modulation evolution of their NLO and birefringent properties. Remarkably, Rb2Te3 and Na2TeSe2 exhibit rarely colossal birefringence (>1.0@10 μm) and NLO effects (>20 × AgGaS2), much larger than conventional NLO chalcogenides. Na2Te3 presents the largest birefringence to date (∼3.48@1, 2.72@2, 2.34@10 μm), indicating the unique structural superiority of HAPC in terms of ultra-large birefringence. By mining the intrinsic mechanism, the HAPC anionic groups are identified as novel mid-infrared NLO "material genes", furnishing unique NLO and birefringent performance for the design of novel optoelectronic materials.

Sr2HgGe2OS6: A Hg-Based Oxychalcogenide Infrared Nonlinear Optical Material Exhibiting Favorable Balance between a Large Band Gap and Strong Second Harmonic Generation Response

Currently, oxychalcogenides with mixed-anion groups that integrate the property advantages of oxides (wide optical band gap) and chalcogenides [strong second harmonic generation (SHG) response] through chemical substitution engineering have attracted widespread interest and are considered to be important candidates for infrared (IR) nonlinear optical (NLO) materials. Herein, the first Hg-based oxychalcogenide Sr2HgGe2OS6 with mixed anion [GeOS3] units has been successfully synthesized through a spontaneous crystallization method, which exhibits a favorable balance between the strong SHG response (0.7 × AgGaS2) and large optical band gap (2.9 eV). In addition, Sr2HgGe2OS6 shows high laser-induced damage threshold (LIDT, 2.1 × AgGaS2) as well as phase-matching (PM) performance. Theoretical calculations indicate that the Sr2HgGe2OS6 encompasses large birefringence of 0.128@2090 nm (3.3 × AgGaS2) and its SHG density mainly comes from [HgS4] tetrahedra and [GeOS3] units. This work not only demonstrates that Sr2HgGe2OS6 is a promising IR NLO material but also provides new ideas for the exploration of Hg-based oxychalcogenide IR NLO materials.

Ba10In2Mn11Si3O12S18: First Hexanary Oxychalcogenide Containing an Infrequent Three-Dimensional Noncentrosysmmetric Framework

Noncentrosymmetric (NCS) oxychalcogenides have attracted great attention in recent years due to their immense potential as candidates for IR nonlinear-optical (NLO) applications. Despite notable advancements in this field, the discovery of oxychalcogenides with three-dimensional (3D) framework structures remains a formidable challenge. In this study, we report the discovery of the first hexanary oxychalcogenide, Ba10In2Mn11Si3O12S18, exhibiting second-order NLO activity, using a high-temperature solid-phase method. This compound showcases a novel structure type, featuring an uncommon NCS 3D [In2Mn11Si3O12S18]20- framework formed by vertex-sharing [(Mn/In)S6] octahedra, [(Mn/In)OS3] tetrahedra, and [SiO4] tetrahedra, with charge-balanced Ba2+ cations occupying the channels. Our study serves as a source of inspiration for researchers to further investigate the synthesis of novel NLO-active oxychalcogenides with 3D frameworks.

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.

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.

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.

Improved Birefringence Activated by Tetrahedra Decorated with a Single Linear Unit

Performance enhancement induced by structural modification has long been the target in materials science fields. Direct evidence to witness the effectivity of one strategy is challenging and necessary. In this work, a tetrahedra-decoration strategy was proposed to improve the birefringent performance sharply, namely decorating the tetrahedra with a single linear [S2 ] unit. The strategy was verified by comprehensive characterization of two thiogermanates K2 BaGeS4 and K2 BaGeS5 , which crystallize in the same space group, have similar unit cells and the same units arrangements. Theoretical characterization verified that the [GeS5 ] group has much larger polarization anisotropy than [GeS4 ], further demonstrated that the linear [S2 ] led to the sharp birefringence enlargement of K2 BaGeS5 (0.19 vs 0.03 of K2 BaGeS4 ). This work provides a new guiding thought to improve the birefringence performance.

Target-Oriented Synthesis of Borate Derivatives Featuring Isolated [B3O3] Six-Membered Rings as Structural Features

Borates provide an excellent platform for investigating the optical nonlinearity and linearity of crystals as photoelectric functional materials. In our work, borate derivatives with isolated [B₃O₃] six-membered rings as structural features are the preferred system due to their simple functional units and excellent properties. Herein, by utilizing the target-oriented synthesis, a series of borate derivatives, A₂[B₃O₃F₄(OH)] (A= NH₄, Rb, Cs) (ABOFH), K_2.3Cs_0.7B₃O₃F₆ (KCsBOF), and Cs₃[B₃O₃(OH)₃]Cl₃ (CsBOHCl), with novel heteroanionic groups containing [BO_xF_4-x] (x = 0-3) and/or [BO₂(OH)] units were obtained. ABOFH, KCsBOF, and CsBOHCl construct different two-dimensional pesudolayers featuring [B₃O₃F₄(OH)], [B₃O₃F₆], and [B₃O₃(OH)₃] units, respectively. Also, the optical properties and the arrangement information of these anionic groups were studied. Among the total five compounds, (NH₄)₂[B₃O₃F₄(OH)] and Cs₃[B₃O₃(OH)₃]Cl₃ with enlarged birefringence and sufficient band gaps were screened out as promising birefringent crystals due to the optimally aligned configuration of birefringence-active heteroanionic units. The successful results of target-oriented synthesis indicate a more profound conclusion that the borate system now has more diversified structural chemistry, and an effective strategy was proposed to modify the arrangement and species of anionic units to optimize the performance of optical crystals.

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.

Oxychalcogenides: A Promising Class of Materials for Nonlinear Optical Crystals with Mixed-Anion Groups

Infrared nonlinear optical (IR NLO) materials are of great significance in the development of IR laser technology. But rationally designing high-performance IR NLO materials remains a huge challenge due to the conflict between the necessary properties required for NLO materials. Notably, oxychalcogenides with mixed-anion groups have drawn extensive interest as a family of important IR NLO candidates because they integrate the property advantages of oxides and chalcogenides by chemical substitution engineering. In this review, we provide a survey of reported oxychalcogenides and aim to present the development of NLO oxychalcogenides from the perspective of rational design of their structural chemistry. Furthermore, we focus on the relationships between partial substitution and structural symmetry as well as optical properties. These provide some helpful guidance for the further exploration and design of novel oxychalcogenide materials with excellent NLO performance in the future.

A non-centrosymmetric chalcohalide synthesized through the combination of chemical tailoring with aliovalent substitution

A new non-centrosymmetric (NCS) chalcohalide, [Sr₄Cl₂][Ge₃S₉], was successfully designed and synthesized through combining chemical tailoring with aliovalent substitution strategies from the maternal [NaSr₄Cl][Ge₃S₁₀]. It can exhibit a large SHG effect (0.97 × AgGaS₂), a wide band gap of 3.71 eV, and a high LDT (∼16 × AgGaS₂). These results indicate that [Sr₄Cl₂][Ge₃S₉] may be a potential infrared nonlinear optical crystal.

Intense d-p Hybridization in Nb3 O15 Tripolymer Induced the Largest Second Harmonic Generation Response and Birefringence in Germanates

We herein report two asymmetric germanate crystals, KNbGe₃ O₉ and K₃ Nb₃ Ge₂ O₁₃ , with different structures and optical properties derived from divergent polymerized forms of GeO₄ and NbO₆ groups. Remarkably, K₃ Nb₃ Ge₂ O₁₃ achieved a rare combination of the strongest second harmonic generation (SHG) response of 17.5×KDP @ 1064 nm and the largest birefringence of 0.196 @ 546 nm in germanates. It features unique [Nb₃ O₁₂ ]_∞ tubular chains constructed by circular Nb₃ O₁₅ tripolymers. Theoretical calculations reveal that the d-p interactions in the Nb₃ O₁₅ group are responsible for outstanding optical properties. This work emphasizes the significance of the polymerizable functional units in obtaining high-performance nonlinear optical (NLO) crystals.

The Antiperovskite-Type Oxychalcogenides Ae3 Q[GeOQ3 ] (Ae = Ba, Sr; Q = S, Se) with Large Second Harmonic Generation Responses and Wide Band Gaps

Oxychalcogenides capable of exhibiting excellent balance among large second-harmonic generation (SHG) response, wide band gap (Eg ), and suitable birefringence (Δn) are ideal materials class for infrared nonlinear optical (IR NLO) crystals. However, rationally designing a new high-performance oxychalcogenide IR NLO crystal still faces a huge challenge because it requires the optimal orientations of the heteroanionic groups in oxychalcogenide. Herein, a series of antiperovskite-type oxychalcogenides, Ae3 Q[GeOQ3 ] (Ae = Ba, Sr; Q = S, Se), which were synthesized by employing the antiperovskite-type Ba3 S[GeS4 ] as the structure template. Their structures feature novel three-dimensinoal frameworks constructed by distorted [QAe6 ] octahedra, which are further filled by [GeOQ3 ] tetrahedra to form antiperovskite-type structures. Based on the unique antiperovskite-type structures, the favorable alignment of the polarizable [GeOQ3 ] tetrahedra and distorted [QAe6 ] octahedra have been achieved. These contribute the ideal combination of large SHG response (0.7-1.5 times that of AgGaS2 ), wide Eg (3.52-4.10 eV), and appropriate Δn (0.017-0.035) in Ae3 Q[GeOQ3 ]. Theoretical calculations and crystal structure analyses revealed that the strong SHG and wide Eg could be attributed to the polarizable [GeOQ3 ] tetrahedra and distorted [QAe6 ] octahedra. This research provides a new exemplification for the design of high-performance IR NLO materials.

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.

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.

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.

Synthesis, structural evolution and optical properties of a new family of oxychalcogenides [Sr3VO4][MQ3] (M = Ga, In, Q = S, Se)

New oxychalcogenides [Sr3VO4][MQ3] (M = Ga, In, Q = S, Se) were successfully synthesized. The Ga analogues feature a 0-D structure containing isolated [Ga2Q6]6− dimers, while the In analogues feature a 1-D structure containing ∞[InQ3]3− chains.