Toward the Rational Design of Novel Noncentrosymmetric Materials: Factors Influencing the Framework Structures

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.

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.

Ba2ScBSi2O9: A Mixed-Coordination Borosilicate with a Low B/Si Ratio Exhibiting Enhanced Second Harmonic Generation Response

Rational chemical substitution is an effective way to regulate structure and enrich property. Herein, a new noncentrosymmetric borosilicate, Ba2ScBSi2O9, was successfully synthesized by substituting CaO6 units in Ba2CaB2Si4O14 with ScO6 octahedra, with comparatively strong covalency. This substitution not only effectively prevents polymerization of the B-O groups, resulting in an intriguing structural transformation from tetrahedral-coordinated borosilicate of Ba2CaB2Si4O14 to mixed-coordinated borosilicate Ba2ScBSi2O9, but also enhances its second harmonic generation response (2 × KDP), that is nearly four times higher than its parent structure while keeping a short ultraviolet (UV) cutoff edge (λcutoff < 190 nm). In addition, the polar space group of Pca21 for Ba2ScBSi2O9 achieves its ferroelectric polarization reversal capability, which makes quasi-phase-matching technology possible to counteract the nonphase-matching caused by small birefringence of silicates. This work indicates the unique role of heterovalent substitution in regulating structure and performance, providing new insights for exploring borosilicate with versatile functionality.

Polycatenanes Formed of Self-Assembled Metal-Organic Cages

Poly-[n]-catenanes (PCs) self-assembled of three-dimensional (3D) metal organic cages (MOCs) (hereafter referred to as PCs-MOCs) are a relatively new class of mechanically interlocked molecules (MIMs) that combine the properties of MOCs and polymers. The synthesis of PCs-MOCs is challenging because of the difficulties associated with interlocking MOCs, the occurrence of multiple weak supramolecular electrostatic interactions between cages, and the importance of solvent templating effects. The high density of mechanical bonds interlocking the MOCs endows the MOCs with mechanical and physical properties such as enhanced stability, responsive dynamic behavior and low solubility, which can unlock new functional properties. In this Minireview, we highlight the benefit of interlocking MOCs in the formation of PCs-MOCs structures as well as the synthetic approaches exploited in their preparation, from thermodynamic to kinetic methods, both in the solution and solid-states. Examples of PCs-MOCs self-assembled from various types of nanosized cages (i.e., tetrahedral, trigonal prismatic, octahedral and icosahedral) are described in this article, providing an overview of the research carried out in this area. The focus is on the structure-property relationship with examples of functional applications such as electron conductivity, X-ray attenuation, gas adsorption and molecular sensing. We believe that the structural and functional aspects of the reviewed PCs-MOCs will attract chemists in this research field with great potential as new functional materials in nanotechnological disciplines such as gas adsorption, sensing and photophysical properties such as X-ray attenuation or electron conductivity.

A Noncentrosymmetric Metal-Free Borophosphate: Achieving a Large Birefringence and Excellent Stability by Covalent-Linkage

Organic-inorganic hybrid linear and nonlinear optical (NLO) materials have received increasingly wide spread attention in recent years. Herein, the first hybrid noncentrosymmetric (NCS) borophosphate, (C5H6N)2B2O(HPO4)2 (4PBP), is rationally designed and synthesized by a covalent-linkage strategy. 4-pyridyl-boronic acid (4 PB) is considered as a bifunctional unit, which may effectively improve the optical properties and stability of the resultant material. On the one hand, 4 PB units are covalently linked with PO3(OH) groups via strong B-O-P connections, which significantly enhances the thermal stability of 4PBP (decomposition at 321, vs lower 200 °C of most of hybrid materials). On the other hand, the planar π-conjugated C5H6N units and their uniform layered arrangements represent large structural anisotropy and hyperpolarizability, achieving the largest birefringence (0.156 @ 546 nm) in the reported borophosphates and a second-harmonic generation response (0.7 × KDP). 4PBP also exhibits a wide transparency range (0.27-1.50 µm). This work not only provides a promising birefringent material, but also offers a practical covalent-attachment strategy for the rational design of new high-performance optical materials.

Noncentrosymmetric Crystal [C10H8NO2]2SiF6·H2O with Large Birefringence

Crystals with noncentrosymmetric structures are applied in many fields, but reported compounds have a high probability of forming a centrosymmetric structure. Here, by hydrogen-bonding the π-conjugated [C10H8NO2]+ cation with the separated [SiF6]2- octahedron, a noncentrosymmetric isoquinoline hexafluorosilicate monohydrate optical crystal of [C10H8NO2]2SiF6·H2O was formed under the regulatory influence of hydrogen bonding. It not only possesses a moderate second harmonic generation response (1.0 × KDP) but also has a large birefringence (0.282 at 550 nm), which is greater than those of most commercial birefringent crystals. In addition, the UV-vis-NIR diffuse reflectance spectrum and thermal stability analysis are also reported. Our finding gives insight into how to design noncentrosymmetric structural compounds in the organic-inorganic system.

Assembly of π-Conjugated [B3O6] Units by Mer-Isomer [YO3F3] Octahedra to Design a UV Nonlinear Optical Material, Cs2YB3O6F2

Achieving the extreme balance of the key performance requirements is the crucial to breakthrough the application bottleneck for nonlinear optical (NLO) materials. Herein, by assembly of the π-conjugated [B3O6] functional species with the aid of structure-directing property of mer-isomer [YO3F3] octahedra, a new ultraviolet (UV) NLO material, Cs2YB3O6F2 with aligned arrangement of coplanar [B3O6] groups has been synthesized. The polar material exhibits the rare coexistence of the largest second harmonic generation response of 5.6×KDP, the largest birefringence of 0.091 at 532 nm, the shortest Type I phase-matching down to 200.5 nm and deep-ultraviolet transparency among reported acentric rare-earth borates with [B3O6] groups. Remarkably, benefiting from the enhanced bonding force among functional units [B3O6], a firm three-dimensional framework is constructed, which facilitates the growth of large crystals. This can be proved by a block shape crystal with dimensional of 6×5×4 mm3, indicating that it was a promising UV NLO crystal. This work provides a powerful strategy to design UV NLO materials with good performances.

An Unprecedented [BO2]-Based Deep-Ultraviolet Transparent Nonlinear Optical Crystal by Superhalogen Substitution

Solid-state structures with the superhalogen [BO2]- have thus far only been observed with a few compounds whose syntheses require high reaction temperatures and complicated procedures, while their optical properties remain almost completely unexplored. Herein, we report a facile, energy-efficient synthesis of the first [BO2]-based deep-ultraviolet (deep-UV) transparent oxide K9[B4O5(OH)4]3(CO3)(BO2) ⋅ 7H2O (KBCOB). Detailed structural characterization and analysis confirm that KBCOB possesses a rare four-in-one three-dimensional quasi-honeycomb framework, with three π-conjugated anions ([BO2]-, [BO3]3-, and [CO3]2-) and one non-π-conjugated anion ([BO4]5-) in the one crystal. The evolution from the traditional halogenated nonlinear optical (NLO) analogues to KBCOB by superhalogen [BO2]- substitution confers deep-UV transparency (<190 nm), a large second-harmonic generation response (1.0×KH2PO4 @ 1064 nm), and a 15-fold increase in birefringence. This study affords a new route to the facile synthesis of functional [BO2]-based oxides, paving the way for the development of next-generation high-performing deep-UV NLO materials.

Ba2Ga2F6(IO3)(PO4): the first fluoride-containing iodate-phosphate with a 1D [Ga2F6(IO3)(PO4)]4- helix chain

Herein, the first F-containing iodate-phosphate, namely Ba2Ga2F6(IO3)(PO4), was prepared via a hydrothermal reaction, in which HPF6 (70 wt% solution in water) was used as the source of both fluoride and phosphate anions for the first time. Ba2Ga2F6(IO3)(PO4) features an unprecedented 1D [Ga2F6(IO3)(PO4)]4- helix chain, composed of a 1D Ga(1)(IO3)O4F chain via the bridging of 0D Ga(2)(PO4)F5. The UV-Vis spectrum shows that Ba2Ga2F6(IO3)(PO4) has a wide bandgap with a short-UV absorption edge (4.35 eV; 253 nm). Birefringence measurement under a polarizing microscope shows that Ba2Ga2F6(IO3)(PO4) displays a moderate birefringence of 0.072@550 nm, which is consistent with the value (0.070@550 nm) obtained by DFT calculations, indicating that Ba2Ga2F6(IO3)(PO4) has potential applications as a short-UV birefringent material. This study highlights the crucial role played by the incorporation of specific functional groups into compounds, shedding light on their contribution to promising inorganic functional materials.

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.

Deep-Ultraviolet Bialkali-Rare Earth Metal Anhydrous Sulfate Birefringent Crystal

Birefringence is an important linear optical property of anisotropic crystals that plays a significant role in regulating light polarization. A new bialkali-rare earth metal sulfate, NaRbY2(SO4)4 compound, consisting of non-π-conjugated alkali metals and rare earth metal-centered dodecahedral YO8 has been synthesized. The structure analysis suggests that the three-dimensional (3D) structure of the compound is found to be attributable to the combination of dodecahedral YO8 and tetrahedral SO4 groups with Na+ and Rb+ located in the cavities. The ultraviolet, visible, and near-infrared (UV-vis-NIR) spectra reveal that the compound exhibits transparency at a wavelength of less than 200 nm. The observed birefringence of the compound is 0.045@550 nm, which is comparatively larger than that of most deep-ultraviolet (DUV) birefringent crystals. The birefringence mainly originated from the YO8 dodecahedron, which is suggested by first-principles calculations. This research work can provide a useful perspective to explore new DUV sulfates with excellent birefringence.

[(C5H6N2)2H](Sb4F13): a polyfluoroantimonite with a strong second harmonic generation effect

It is of great difficulty to create a new antimonite with second-harmonic-generation (SHG) intensity larger than 6 times that of KDP. In this study, a polyfluoroantimonite strategy has been proposed to explore fluoroantimonites with large nonlinear optical (NLO) coefficients. Under the cooperation of chemical (highly asymmetric π-conjugated organic amine) and physical (viscous reaction medium ethylene glycol) methods, two novel polyfluoroantimonites, namely, (3PC)2(Sb4F14) and (3AP)2(Sb4F13), have been achieved. Interestingly, these two structures contain two new polyfluoroantimonite groups respectively, an isolated (Sb4F14)2- four-member polyhedral ring and an infinite [Sb4F13]∞- helical chain. More importantly, the polar (3AP)2(Sb4F13) displays a strong SHG intensity of 8.1 × KDP, a large birefringence of 0.258@546 nm and a high laser-induced damage threshold (LIDT) value of 149.7 MW cm-2. Theoretical calculations indicated that its strong SHG effect stems from the synergistic effect of the helical [Sb4F13]∞- polyfluoroantimonite chain and π-conjugated 3AP+ cation, with a contribution ratio of 48.93% and 50.77% respectively. This work provides a new approach for the design and synthesis of high-performance fluoroantimonites.

Discovery of a new bimetallic borate with strong optical anisotropy activated by π-conjugated [B2O5] units

Birefringent materials with high optical anisotropy have been identified as a research hotspot owing to their significant scientific and technological significance in modern optoelectronics for manipulating light polarization. Researchers studying borate systems have discovered that adding π-conjugated units placed in parallel can significantly increase the birefringence of crystalline solids; some examples include [BO3] units, [B2O5] units, and [B3O6] units. However, there are not many borates with strictly parallel configurations of π-conjugated [B2O5] units. In this study, a new bimetallic borate Sr2Cd4(B2O5)3 with near-parallel arrangement of π-conjugated [B2O5] units was discovered. Sr2Cd4(B2O5)3 possesses the maximum number density of [B2O5] units, shortest dihedral angle of [B2O5] units (between the two [BO3]), and largest degree of [CdO6] octahedral distortion among all the currently known Sr-Cd-B-O tetragonal system borates, making it demonstrate a large birefringence of 0.102 at 532 nm. Theoretical analysis proves that π-conjugated [B2O5] anions are the primary source of the large birefringence of Sr2Cd4(B2O5)3.

PbBeB2O5: A High-Performance Ultraviolet Nonlinear-Optical Crystal with Functional [BeB2O8]8- Group

High-performance nonlinear-optical (NLO) crystals need to simultaneously meet multiple basic and conflicting performance requirements. Here, by using a partial chemical substitution strategy, the first noncentrosymmetric (NCS) PbBeB2O5 crystal with a BeB2O8 group was synthesized, exhibiting a two-dimensional [BeB2O5]∞ layer constructed by interconnecting BeB2O8 groups and bridged PbO4 with an active lone pair. The crystal shows a promising UV NLO functional feature, including a strong SHG effect of 3.5 × KDP (KH2PO4), large birefringence realizing phase matchability in the whole transparency region from 246 to 2500 nm, a short UV absorption edge of 246 nm, and single-crystal easy growth. Remarkably, theoretical studies reveal that the BeB2O8 group has high nonlinear activity, which could stimulate the discovery of a series of excellent NLO beryllium borates.

Superalkali nature of the Si9M5 (M = Li, Na, and K) Zintl clusters: a theoretical study on electronic structure and dynamic nonlinear optical properties

Zintl clusters have attracted widespread attention because of their intriguing bonding and unusual physical properties. We explore the Si9 and Si9M5 (where M = Li, Na, and K) Zintl clusters using the density functional theory combined with other methods. The exothermic nature of the Si9M5 cluster formation is disclosed, and the interactions of alkali metals with pristine Si9 are shown to be noncovalent. The reduced density gradient analysis is performed, in which increased van der Waals interactions are observed with the enlargement of the size of alkali metals. The influence of the implicit solvent model is considered, where the hyperpolarizability (βo) in the solvent is found to be about 83 times larger than that in the gas phase for Si9K5. The frequency-dependent nonlinear optical (NLO) response for the dc-Kerr effect is observed up to 1.3 × 1011 au, indicating an excellent change in refractive index by an externally applied electric field. In addition, natural bonding orbitals obtained from the second-order perturbation analysis show the charge transfer with the donor-acceptor orbitals. Electron localization function and localized orbital locator analyses are also performed to better understand the bonding electrons in designed clusters. The studied Zintl clusters demonstrate the superalkali character in addition to their remarkable optical and nonlinear optical properties.

Hydrogen bonding bolstered head-to-tail ligation of functional chromophores in a 0D SbF3·glycine adduct for a short-wave ultraviolet nonlinear optical material

The key properties of nonlinear optical (NLO) materials highly rely on the quality of functional chromophores (FCs) and their optimized interarrangement in the lattice. Despite the screening of various FCs, significant challenges persist in optimizing their arrangement within specific structures. Generally, FC alignment is achieved by designing negatively charged 2D layers or 3D frameworks, further regulated by templating cations. In this study, a novel 0D adduct NLO material, SbF3·glycine, is reported. Neutrally charged 0D [SbF3C2H5NO2] FCs, comprising [SbF3] pyramids and zwitterionic glycine, are well-aligned in the structure. The alignment is facilitated by the hydrogen bonding, reinforcing a 'head-to-tail' ligation of [SbF3C2H5NO2] FCs. Consequently, the title compound exhibits favorable NLO properties, including a large second-harmonic generation efficiency (3.6 × KDP) and suitable birefringence (cal. 0.057 @ 1064 nm). Additionally, its short absorption cut-off edge (231 nm) positions it as a promising short-wave ultraviolet NLO material. Importantly, the binary SbF3-amino acid system is expected to serve as a new resource for exploring ultraviolet NLO crystals, owing to the abundance of the amino acid family.

Na2CeF6: A Highly Laser Damage-Tolerant Double Perovskite Type Ce(IV) Fluoride Exhibiting Strong Second-Harmonic Generation Effect

Perovskite structure compounds are significant candidates for designing new optical function materials due to their structural variability. Here, an inorganic tetravalent cerium fluoride, Na2CeF6, is derived from the perovskite structure through double-site cation co-substitution. Na2CeF6 crystalizes in the non-centrosymmetric space groupP 6 ¯ 2 m ${P}^{\bar{6}}2m$ . Edge-sharing connected NaF9 and CeF9 polyhedra build the whole 3D structure of Na2CeF6. Importantly, it represents the first Ce(IV) fluoride nonlinear optical (NLO) crystal and can produce a strong and phase-matchable second-harmonic generation (SHG) effect of ≈2.1 times that of KH2PO4 (KDP), making it the strongest among non-lone pair electron metal fluoride system. Further, it exhibits a high laser-induced damage threshold (LIDT) of 74.65-76.25 MW cm-2, which is over 20 times that of AgGaS2. It also exhibits a wide transparent region (0.5-14.3 µm). This work provides a facile route for exploring high-performance halide NLO materials.

Centrosymmetric CaBaMF8 and Noncentrosymmetric Li2CaMF8 (M = Zr, Hf): Dimension Variation and Nonlinear Optical Activity Resulting from an Isovalent Cation Substitution-Oriented Design

Zirconium/hafnium fluorides have recently been recognized as potential nonlinear optical (NLO) materials with short ultraviolet (UV) cutoff edges, which is significant in laser science and industry. The synthesis of noncentrosymmetric (NCS) materials based on centrosymmetric (CS) compounds by an isovalent cation substitution-oriented design is an emerging strategy in the NLO territory. Here, two isostructural and novel fluorides, CaBaMF8 (M = Zr (1), Hf (2)), have been synthesized through the combination of alkaline earth metals, zirconium/hafnium, and fluorine elements. They feature zero-dimensional and CS structures composed by an isolated MF8 (M = Zr, Hf) dodecahedron and dissociative Ca2+ and Ba2+ cations, and they display short UV cutoff edges (<200 nm) as well. Two three-dimensional fluorides Li2CaMF8 (M = Zr (3), Hf (4)) are obtained by replacing Ba with alkali metal Li atom, which not only represent phase-matchable second-harmonic-generation activities (0.36, 0.30× KH2PO4 (KDP)) at 1064 nm but also maintain short UV cutoff edges with high reflectance. This work has largely enriched the family of NCS zirconium/hafnium fluorides reaching the short UV region.

Unearthing Superior Inorganic UV Second-Order Nonlinear Optical Materials: A Mineral-Inspired Method Integrating First-Principles High-Throughput Screening and Crystal Engineering

Natural minerals, with their adaptable framework structures exemplified by perovskite and lyonsite, have sparked substantial interest as potential templates for the design of advanced functional solid-state materials. Nonetheless, the quest for new materials with desired properties remains a substantial challenge, primarily due to the scarcity of effective and practical synthetic approaches. In this study, we have harnessed a synergistic approach that seamlessly integrates first-principles high-throughput screening and crystal engineering to reinvigorate the often-overlooked fresnoite mineral, Ba2 TiOSi2 O7 . This innovative strategy has culminated in the successful synthesis of two superior inorganic UV nonlinear optical materials, namely Rb2 TeOP2 O7 and Rb2 SbFP2 O7 . Notably, Rb2 SbFP2 O7 demonstrates a comprehensive enhancement in nonlinear optical performance, featuring a shortened UV absorption edge (260 nm) and a more robust second-harmonic generation response (5.1×KDP). Particularly striking is its significantly increased birefringence (0.15@546 nm), which is approximately 30 times higher than the prototype Ba2 TiOSi2 O7 (0.005@546 nm). Our research has not only revitalized the potential of the fresnoite mineral for the development of new high-performance UV nonlinear optical materials but has also provided a clearly defined roadmap for the efficient exploration of novel structure-driven functional materials with targeted properties.

Secondary-Bond-Driven Construction of a Polar Material Exhibiting Strong Broad-Spectrum Second-Harmonic Generation and Large Birefringence

Considerable effort has been invested in the development of non-centrosymmetric (NCS) inorganic solids for ferroelectricity-, piezoelectricity- and, particularly, optical nonlinearity-related applications. While great progress has been made, a persistent problem is the difficulty in constructing NCS materials, which probably stems from non-directionality and unsaturation of the ionic bonds between metal counter-cations and covalent anionic modules. We report herein a secondary-bond-driven approach that circumvents the cancellation of dipole moments between adjacent anionic modules that has plagued second-harmonic generation (SHG) material design, and which thereby affords a polar structure with strong SHG properties. The resultant first NCS counter-cation-free iodate, VO2 (H2 O)(IO3 ) (VIO), a new class of iodate, crystallizes in a polar lattice with∞ 1 [ ${{}_{{\rm { \infty }}}{}^{{\rm { 1}}}{\rm { [}}}$ VO2 (H2 O)(IO3 )] zigzag chains connected by weak hydrogen bonds and intermolecular forces. VIO exhibits very large SHG responses (18 × KH2 PO4 @ 1200 nm, 1.5 × KTiOPO4 @ 2100 nm) and sufficient birefringence (0.184 @ 546 nm). Calculations and crystal structure analysis attribute the large SHG responses to consistent polarization orientations of the∞ 1 [ ${{}_{{\rm { \infty }}}{}^{{\rm { 1}}}{\rm { [}}}$ VO2 (H2 O)(IO3 )] chains controlled by secondary bonds. This study highlights the advantages of manipulating the secondary bonds in inorganic solids to control NCS structure and optical nonlinearity, affording a new perspective in the development of high-performance NLO materials.

Rational Design of Novel Promising Infrared Nonlinear Optical Materials: Structural Chemistry and Balanced Performances

ConspectusSecond-order nonlinear optical (NLO) materials are currently a hot topic in modern solid-state chemistry and optics because they can produce coherent light by frequency conversion. Noncentrosymmetric (NCS) structure is not only the prerequisite for NLO materials but also a challengeable issue because materials tend to be centrosymmetric (CS) in terms of thermodynamical stability. Among NLO materials, an excellent infrared (IR) candidate should simultaneously meet several strict key conditions including a large NLO coefficient, high laser-induced damage threshold (LIDT), phase-matchable (PM) behavior, and so on. Achieving a balance between the large NLO effect and high LIDT is difficult, as they have contradictory requirements for chemical bonds. Considering the urgent need of the high-power IR laser market and the drawbacks of the available ones, exploring new high-performance IR NLO crystals is necessary while challenging. In this Account, we first briefly introduce the status and advancement of IR NLO crystals and emphasize the criteria of an excellent candidate. Then, we will introduce five simple methods developed by us to discover practical NLO candidates through understanding of the chemical composition-structure-NLO performance relationship. (1) A rarely investigated system with simple chemical compositions as new-type NLO crystals, namely, adducts containing S8 molecules, are developed. Combining a chairlike S8 unit with other units through van der Waals forces has successfully obtained several high-performance NLO adducts. (2) The main trend in exploring new NLO crystals is that the chemical composition is more and more diversified and the structure is more and more complex, and expensive and chemically active alkaline and alkaline earth metals are usually introduced as counter cations. In contrast, the research on systems with simple chemical compositions, simple structures, and low costs has been continuously ignored. The binary M2Q3 (M = Ga, In; Q = S, Se) family with rich acentric modifications has been systematically investigated, and they all exhibit strong SHG effects and high LIDTs. (3) We first proposed the concept of inducing CS structures transformed to NCS ones by partial cation substitution to design novel NLO crystals. Considering the huge number of CS structures in the database compared to the number of NCS structures, it is an attractive method to apply CS structures as the parents to obtain potential NLO materials via partial-substitution-induced symmetry breaking. A series of chalcogenides with high NLO performances have been successfully obtained by us in this way. (4) We investigated the first NLO-active rare earth (RE) chalcophosphates and developed this family systematically, and they demonstrate wonderful comprehensive NLO properties. (5) We created a novel mixed-anion system for NLO applications, namely, chalcogenide borates. Usually, mixed-anion compounds can engender a synergistic effect to obtain desired IR NLO properties. Our recent progress on this system suggests that chalcogenide borates are potential candidates for IR NLO applications, although the study is still in its infancy. Finally, we state the current problems of IR NLO materials and give some perspectives for their future development.

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.

Sb4O5I2: Enhancing Birefringence through Optimization of Sb/I Ratio for Alignment of Stereochemically Active Lone Pairs

Birefringent crystals are the key components of functional optics, contributing significantly to scientific and technological advancements. To enhance birefringence, the presence of stereochemically active lone pairs offers a unique opportunity. In fact, strengthening the stereochemical activity and aligning uniformly lone pairs face tough challenges. Herein, an anisotropic layered crystal, Sb4O5I2, is discovered to exhibit enhanced birefringence. The influence of crystal symmetry on the birefringence of Sb4O5X2 (X = Cl, Br, or I) is found to be minor. Instead, the asymmetric nature of ABUCBs (i.e., cis-X3[SbO3]6- and cis-X3[SbO4]8-) plays a crucial role in enhancing the optical anisotropy. And the orientation of these ABUCBs is equally important. We demonstrate that by adjusting the Sb/I ratio from 5:1 to 2:1, all of the intralayer Sb atoms in Sb5O7I-P63 are forced onto the surface position. This structural adjustment leads to strengthened ionic bonding interactions, enhanced activity of the lone pairs, and uniform alignments of the ABUCBs in Sb4O5I2. Consequently, this results in a 6-fold increase in birefringence.

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.

Na6Mg3P4S16 and RbMg2PS4Cl2: two Mg-based thiophosphates with ultrawide bandgaps resulting from [MgS6] and [MgSxCl6-x] octahedra

Designing wide-bandgap chalcogenides is one of the most important ways of obtaining high-performance infrared (IR) functional materials. In this work, two Mg-based metal thiophosphates, namely Na6Mg3P4S16 (NMPS) and RbMg2PS4Cl2 (RMPSC), were successfully obtained by introducing [MgS6] and [MgSxCl6-x] octahedra into thiophosphates. In addition, their crystal structures were determined, a first for Mg-containing [PS4]-based thiophosphates to the best of our knowledge. Their bandgaps were investigated in theoretical ways and verified by taking experimental measurements, and determined to be 3.80 eV for NMPS and 3.93 eV for RMPSC, values greater than those of the other investigated thiophosphate halides. The wide bandgaps of NMPS and RMPSC were attributed, based on theoretical calculations, to the [MgSxCl6-x] (x = 0-6) octahedron.

Rb3MgB5O10 and LiBaAl(BO3)2: covalent tetrahedra MO4-containing borates with deep-ultraviolet cutoff edges

Borates are favored by materials scientists and chemists because of the significant electronegativity difference between B and O atoms and their flexible assembly modes resulting in abundant structures and excellent properties. For the design of deep-ultraviolet (DUV) optical crystals with excellent macroscopic performance, it is crucial to choose appropriate cations and anionic groups and microscopically reasonable assembly patterns. Herein, by introducing covalent tetrahedra ([MO4], M = Mg, Al), two new mixed alkali metal and alkaline earth metal borates, Rb3MgB5O10 and LiBaAl(BO3)2, were synthesized using the melt method and high-temperature solution method. They contain M-B-O two-dimensional (2D) layers (2∞[MgB5O10] and 2∞[Al(BO3)2], respectively) composed of isolated B-O groups ([B5O10]5- and [BO3]3-, respectively) and metal-centered tetrahedral connectors ([MgO4]6- and [AlO4]5-, respectively). Combining experiments and theoretical calculations shows that the two compounds have short cutoff edges (<200 nm) and moderate birefringences. Further analysis manifests that the isolated [MO4] covalent tetrahedra can optimize the arrangement of anion groups, guarantee the balanced optical properties of materials, and point out the direction for further exploration of novel borate structures.

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.

Breaking the Inherent Interarrangement of [B3O6] Clusters for Nonlinear Optics with Orbital Hybridization Enhancement

The [B3O6] group as a prime functional unit provides borates with intrinsic properties that are modified by coordination to cations. Inherent [B3O6] cluster structures in borates exclusively made of them have a near-plane configuration, with more than 90% of them having a maximum dihedral angle of zero and the remaining ones being less than 13°. Although such an inherent configuration can produce considerable birefringence for good phase-matching ability, this is not conducive to obtaining high conversion efficiency and beam quality due to the walk-off effects in the nonlinear optical process. In this article, two new borate halides Ca2B3O6X (X = Cl and Br) were reported, in which the confinement effects of distorted halogen-centered secondary building blocks compress the existence space of [B3O6] primitives, resulting in the nonparallel arrangement between [B3O6] clusters in this series. Both compounds show large second harmonic generation effects, and more importantly, the broken inherent interarrangement of [B3O6] clusters makes them a moderate birefringence and small walk-off angle. Their moderate birefringence is due to the large angular alignment between [B3O6] clusters, resulting from the orbital hybridization between the Ca s and the O p orbitals of the terminal O atoms on [B3O6] clusters. Our model supports this viewpoint and offers guidelines for rearranging [B3O6] clusters' arrangements in borates.

Highly Birefringent d0 Transition Metal Fluoroantimonite in the Mid Infrared Band: Order-Disorder Regulation by Cationic Size

An unusual O/F ordered d0 transition metal fluoroantimonite, namely, K2SbMoO2F7, has been created by the cationic size effect of alkali metals. It features the largest birefringence of 0.220@550 nm among inorganic antimonites with a halogen element, which is an order of magnitude larger than the disordered A2SbMoO2F7 (A = Rb, Cs). These three new compounds exhibit two different structures, although all of the structures were made of [SbMoO2F7]2- chains formed by SbF5 square pyramids and MoO2F4 octahedrons. A transparent single crystal of K2SbMoO2F7 with dimensions of 7.0 × 5.0 × 1.0 mm3 has been successfully grown by the aqueous solution volatilization method. The UV-vis-MIR transmission spectrum showed that K2SbMoO2F7 can display excellent transmittance in the range of 0.5-5.0 μm and 6.0-9.8 μm, indicating its application potential as a birefringent material in the mid infrared band. This work offers a fresh approach to the design and synthesis of mid infrared birefringent materials.

New Functional Groups Design toward High Performance Ultraviolet Nonlinear Optical Materials

ConspectusThe invention of the laser is a pivotal milestone in the evolution of modern science and technology. Second-order nonlinear optical (NLO) crystals, which possess the ability to convert frequencies, have found widespread applications in laser science, information transmission, industrial Internet, and other cutting-edge fields within materials and optics. As modern science and technology continue to advance at a rapid pace, existing ultraviolet (UV) and deep ultraviolet (DUV) NLO crystals struggle to meet the ever-growing demands of various applications. Consequently, the development of novel UV and DUV NLO crystals has become an urgent necessity. For a UV NLO crystal to be considered outstanding in the UV/DUV range, it must exhibit three fundamental yet crucial properties: large second-order NLO coefficients, suitable birefringence, and short UV cutoff edge corresponding to a wide band gap. However, these key factors often conflict with one another, making it challenging to achieve a harmonious balance within a single crystal. It is widely believed that these mutually constrained optical properties are codetermined by microscopic NLO-active units and macroscopic structure features. Therefore, how to design high performance UV NLO-active groups to balance these three key properties is an essential scientifically question and serious challenge. In this Account, we present three strategies for designing high-performance UV NLO-active groups: (1) The "tetrahedron partial substitution" strategy by employing various substituents to replace one or more atoms in the traditional nonpolar tetrahedral groups, might achieve the aim of increasing the polarizability anisotropy and hyperpolarizability of the newly formed polar tetrahedral functional groups, such as from SO4 to SO3NH2 or SO3CH3 groups. (2) The "structure-analogue" strategy to develop a range of organic functional groups exhibiting more strong polarizability anisotropy and hyperpolarizability by using inorganic π-conjugated groups, such as BO3 and B3O6 groups, as templates. (3) The "two in one" strategy for integrating groups featuring planar triangle configurations and tetrahedrons to create NLO-active functional groups possessing large band gaps, strong hyperpolarizability, and moderate polarizability anisotropy. These three strategies successfully guide us to design and explore various kinds of organic-inorganic composite NLO crystal materials with excellent performances, like Ba(SO3CH3)2, M(SO3NH2)2 (M = Sr, Ba), C(NH2)3SO3F, KLi(HC3N3O3)·2H2O, KLi(C3H2O4)·H2O, and so on. Finally, we briefly conclude these strategies and propose some prospects for exploring new excellent UV/DUV NLO materials with practical applications. These findings could inspire novel thoughts for researchers designing new UV/DUV NLO materials and providing abundant materials used in UV/DUV regions.

Stereochemically Active Lone Pair Induced Polar Tri-layered Perovskite for Record-Performance Polarized Photodetection

Bulk photovoltaic effect, a promising optoelectronic phenomenon for generating polarized dependent steady-state photocurrent, has been widely applied in various photodetectors. However, incorporating stereochemically active lone pair to construct bulk photovoltage in organic-inorganic hybrid perovskite (OIHP) is still elusive and challenging. Herein, bulk photovoltage (1.2 V) has been successfully achieved by introducing the stereo-chemically active lone pair perovskitizer to construct a polar tri-layered hybrid perovskite, namely, (IBA)2 MHy2 Pb3 Br10 (1, IBA=iso-butylamine, MHy=methylhydrazine). Strikingly, owning to the promising bulk photovoltage, 1-based detectors exhibit an ultra-highly sensitive polarized photodetection (polarization ratio of up to 24.6) under self-powered mode. This ratio surpasses all the reported two-dimension OIHP single-crystal photodetectors. In addition, detectors exhibit outstanding responsivity (≈200 mA W-1 ) and detectivity (≈2.4×1013 Jones). More excitingly, further investigation confirms that lone pair electrons in MHy+ result in the separation of positive and negative charges to produce directional dipoles, which further directional alignment to generate bulk photovoltage, thereby resulting in polarization-dependent photocurrent. Our findings provide a new demonstration for polar multilayer materials' construction and may open opportunities for a host of high-sensitive polarized photodetection.

Hydroxyfluorooxoborate (NH4)[C(NH2)3][B3O3F4(OH)] for exploring the effects of cation substitution on structure and optical properties

Cation substitution is a straightforward but effective technique for improving the structure and properties; however, controlling directed substitution still poses significant difficulties. Herein, a metal-free hydroxyfluorooxoborate (NH4)[C(NH2)3][B3O3F4(OH)] has been synthesized using the strategy of heterologous substitution based on the template of A2[B3O3F4(OH)]. Tunable structure and optical properties have been achieved via varied A-site cation substitution. The intrinsic mechanism for this tunability was established by crystallography and theoretical research.

Protonated Organic Diamines as Templates for Layered and Microporous Structures: Synthesis, Crystal Chemistry, and Structural Trends among the Compounds Formed in Aqueous Systems Transition Metal Halide or Nitrate-Diamine-Selenious Acid

Systematic studies of crystalline compounds formed in aqueous systems containing aliphatic diamines, divalent transition metal halides, and selenious acid resulted in the discovery of a large family of new complex species corresponding to several new structure types. With ethylenediamine (en), layered (enH2)[M(HSeO3)2X2] compounds are the most commonly formed species which constitute a significant contribution to the family of layered hydrogen selenites containing neutral [M(HSeO3)2] (M = Mg, Mn, Co, Ni, Cu, Zn, Cd) 2D building blocks. In contrast to some previous suggestions, piperazine (pip), as well as its homologue N-methylpiperazine, mostly give rise to quite different, sometimes more complex, structures of varied dimensionality while the (pipH2)[M(HSeO3)2X2] compounds are formed only with M = Cu and Cd. In addition, metal-, halide-, or selenium-free by-product species are observed. The SeIV can be present in a multitude of forms, including H2SeO3, HSeO3-, SeO32-, and Se2O52-, reflecting amazing adaptability to the shape of the templating cations.

From H12C4N2CdI4 to H11C4N2CdI3: a highly polarizable CdNI3 tetrahedron induced a sharp enhancement of second harmonic generation response and birefringence

In this study, we identify a novel class of second-order nonlinear optical (NLO) crystals, non-π-conjugated piperazine (H10C4N2, PIP) metal halides, represented by two centimeter-sized, noncentrosymmetric organic-inorganic metal halides (OIMHs), namely H12C4N2CdI4 (P212121) and H11C4N2CdI3 (Cc). H12C4N2CdI4 is the first to be prepared, and its structure contains a CdI4 tetrahedron, which led to a poor NLO performance, including a weak and non-phase-matchable second harmonic generation (SHG) response of 0.5 × KH2PO4 (KDP), a small birefringence of 0.047 @1064 nm and a narrow bandgap of 3.86 eV. Moreover, H12C4N2CdI4 is regarded as the model compound, and we further obtain H11C4N2CdI3via the replacement of CdI4 with a highly polarizable CdNI3 tetrahedron, which results in a sharp enhancement of SHG response and birefringence. H11C4N2CdI3 exhibits a promising NLO performance including 6 × KDP, 4.10 eV, Δn = 0.074 @1064 nm and phase matchability, indicating that it is the first OIMH to simultaneously exhibit strong SHG response (>5 × KDP) and a wide bandgap (>4.0 eV). Our work presents a novel direction for designing high-performance NLO crystals based on organic-inorganic halides and provides important insights into the role of the hybridized tetrahedron in enhancing the SHG response and birefringence.

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.

α- and β-(C4H5N2O)(IO3)·HIO3: Two SHG Materials Based on Organic-Inorganic Hybrid Iodates

Organic-inorganic hybrid nonlinear optical (NLO) materials are highly anticipated because of the integration of both merits of the organic and inorganic moieties. Herein, the 2-pyrimidinone cation (C4H5N2O)+ has been incorporated into the iodate system to form two polymorphic organic-inorganic hybrid iodates, namely, α- and β-(C4H5N2O)(IO3)·HIO3. They crystallize in different polar space groups (Ia and Pca21), and their structures feature one-dimensional (1D) chain structures composed of (C4H5N2O)+ cations, IO3- anions, and HIO3 molecules interconnected via hydrogen bonds. α- and β-(C4H5N2O) (IO3)·HIO3 exhibit strong and moderate second-harmonic-generation (SHG) responses of 6.4 and 0.9 × KH2PO4 (KDP), respectively, the same band gaps of 3.65 eV, and high powder laser-induced damage threshold (LIDT) values [51 and 57 × AgGaS2 (AGS)]. The results of theoretical calculations revealed that the large SHG effect of α-(C4H5N2O)(IO3)·HIO3 originated from the IO3 and HIO3 groups. This work indicates that (C4H5N2O)+ is a potential group for designing new NLO materials with brilliant optical performances.

A Polar Tetragonal Tungsten Bronze with Colossal Second-Harmonic Generation

A polar tetragonal tungsten bronze, Pb1.91 K3.22 □0.85 Li2.96 Nb10 O30 (□: vacancies), has been successfully synthesized by a high temperature solid-state reaction. Single crystal and powder X-ray diffraction indicate that the structure of Pb1.91 K3.22 □0.85 Li2.96 Nb10 O30 crystallizing in the noncentrosymmetric (NCS) space group, P4bm, consists of 3D framework with highly distorted NbO6 , LiO9 , PbO12 , and (Pb/K)O15 polyhedra. While NCS Pb1.91 K3.22 □0.85 Li2.96 Nb10 O30 undergoes a reversible phase transition between polar (P4bm) and nonpolar (P4/mbm) structure at around 460 °C, the material decomposes to centrosymmetric Pb1.45 K3.56 Li3.54 Nb10 O30 (P4/mbm) once heated to 1200 °C. Powder second-harmonic generation (SHG) measurements with 1064 nm radiation indicate that Pb1.91 K3.22 □0.85 Li2.96 Nb10 O30 exhibits a giant phase-matchable SHG intensity of ≈71.5 times that of KH2 PO4 , which is the strongest intensity in the visible range among all nonlinear optical materials reported to date. The observed colossal SHG should be attributable to the synergistic effect of dipole moments from the well-aligned NbO6 octahedra, the constituting distortive channels with vacancies, and highly polarizable cations.

Three Polyborates with High-Symmetry [B12O24] Units Featuring Different Dimensions of Anion Groups

Three polyborates, namely, LiNa₁₁B₂₈O₄₈, Li_1.45Na_7.55B₂₁O₃₆, and Li₂Na₄Ca₇Sr₂B₁₃O₂₇F₉, were synthesized via the high-temperature solution method. All of them feature high-symmetry [B₁₂O₂₄] units, yet their anion groups exhibit distinct dimensions. LiNa₁₁B₂₈O₄₈ features a three-dimensional anionic structure of ³[B₂₈O₄₈]_∞ framework, which is composed of three units: [B₁₂O₂₄], [B₁₅O₃₀], and [BO₃]. Li_1.45Na_7.55B₂₁O₃₆ possesses a one-dimensional anionic structure of ¹[B₂₁O₃₆]_∞ chain consisting of [B₁₂O₂₄] and [B₉O₁₈] units. The anionic structure of Li₂Na₄Ca₇Sr₂B₁₃O₂₇F₉ is composed of two zero-dimensional isolated units, namely, [B₁₂O₂₄] and [BO₃]. The novel FBBs [B₁₅O₃₀] and [B₂₁O₃₉] are present in LiNa₁₁B₂₈O₄₈ and Li_1.45Na_7.55B₂₁O₃₆, respectively. The anionic groups in these compounds exhibit a high degree of polymerization, thereby augmenting the structural diversity of borates. And the crystal structure, synthesis, thermal stability, and optical properties were meticulously discussed to guide the synthesis and characterization of novel polyborates.

Cs2Mg(H2C3N3S3)4·8H2O: An Excellent Birefringent Material with Giant Optical Anisotropy in π-Conjugated Trithiocyanurate

The design of giant birefringence was performed by adjusting cations to make parallel and compact alignments of π-conjugated (H_xC₃N₃S₃)^x-3, where x = 1 and 2) groups with large polarizability anisotropy. Finally, the first mixed alkali/alkali-earth-metal trithiocyanurates, A₂B(H₂C₃N₃S₃)₄·nH₂O (A = K, Rb, Cs; B = Mg, Sr; n = 5-8, 12), were designed and synthesized successfully. Importantly, Cs₂Mg(H₂C₃N₃S₃)₄·8H₂O (III) and K₂Sr(H₂C₃N₃S₃)₄·5H₂O (IV) possess large birefringences of 0.580 and 0.194 at 800 nm, respectively, of III has the largest birefringence among all practical birefringent crystals, cyanurates, and hydroisocyanurates.

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.

Controlling the Nonlinear Optical Behavior and Structural Transformation with A-Site Cation in α-AZnPO4 (A = Li, K)

Regulating the crystal structure by A-site cation substitution is one of the effective methods to explore high-performance nonlinear optical (NLO) materials. Herein, two non-centrosymmetric (NCS) compounds, α-MZnPO₄ (M = Li, K) with short UV absorption edges 221 and 225 nm, are obtained by performing A-site cation substitution method. It is noteworthy that α-LiZnPO₄ (α-LZPO) achieves >10 times second harmonic generation (SHG) response (2.3 × KDP) enhancement compared with that of α-KZnPO₄ (α-KZPO) (0.2 × KDP), which is the only case among phosphates with different A-site cations. By structural comparison, it is found that the A-site cations play important roles for anion rearrangements, and further the structure features of the two compounds by designing two suppositional crystal models as well as performing other theoretical calculations are analyzed. The study confirms the feasibility to design promising NLO materials with strengthen SHG response and structural stability in orthophosphate system.

Unveiling the Superior Optical Properties of Novel Melamine-Based Nonlinear Optical Material with Strong Second-Harmonic Generation and Giant Optical Anisotropy

Two melamine-based metal halides, (C₃ N₆ H₇ )(C₃ N₆ H₆ )HgCl₃ (I) and (C₃ N₆ H₇ )₃ HgCl₅ (II), are synthesized by incorporating the heavy d¹⁰ cation, Hg²⁺ , and the halide anion, Cl^- . The noncentrosymmetric structure of I results from two unique attributes: large asymmetric secondary building units produced by direct covalent coordination of melamine to Hg²⁺ and a small dihedral angle between melamine molecules. The former makes inorganic modules locally acentric, while the latter prevents planar organic groups from forming deleterious antiparallel arrangement. The unique coordination in I results in an enlarged band gap of 4.40 eV. Due to the large polarizability of the heavy Hg²⁺ cation and the π-conjugated system of melamine, I exhibits a strong second-harmonic generation efficiency of 5 × KH₂ PO₄ , larger than any reported melamine-based nonlinear optical materials to date. Density functional theory calculations indicate that I possesses giant optical anisotropy, with a birefringence of 0.246@1064 nm.

Structural Motif Cosubstitution Strategy for Designing Fluoroaluminoborate with the Sr2Be2B2O7-Type Double-Layered Structure

The cosubstitution strategy often was applied to design borate optical crystal materials. Revealingly, a fluoroaluminoborate Sr₂Al_2.18B_5.82O₁₃F₂ with Sr₂Be₂B₂O₇ (SBBO) double-layered like configuration has been rationally designed and successfully synthesized based on structural motif cosubstitution strategy via the high-temperature solution method. In Sr₂Al_2.18B_5.82O₁₃F₂, a structural motif, the [Al₂B₆O₁₄F₄] unit, with edge-sharing [AlO₄F₂] octahedra was filled in interlamination of double-layer structure. The research indicates that Sr₂Al_2.18B_5.82O₁₃F₂ features a short ultraviolet cutoff edge (<200 nm) and moderate birefringence (∼0.058 @ 1064 nm). As the first reported linker in the interlamination of double-layer structures, the [Al₂B₆O₁₄F₄] unit enlightens the synthesis and discovery of new layered structures in borates.

Two Chiral YbIII Enantiomeric Pairs with Distinct Enantiomerically Pure N-Donor Ligands Presenting Significant Differences in Photoluminescence, Circularly Polarized Luminescence, and Second-Harmonic Generation

Using enantiomerically pure bidentate and tridentate N-donor ligands (¹L_R/¹L_S and ²L_R/²L_S) to replace two coordinated H₂O molecules of Yb(tta)₃(H₂O)₂, respectively, two eight- and nine-coordinated Yb^III enantiomeric pairs, namely, Yb(tta)₃¹L_R/Yb(tta)₃¹L_S (Yb-R-1/Yb-S-1) and [Yb(tta)₃²L_R]·CH₃CN/[Yb(tta)₃²L_S]·CH₃CN (Yb-R-2/Yb-S-2), were isolated, in which Htta = 2-thenoyltrifluoroacetone, ¹L_R/¹L_S = (-)/(+)-4,5-pinene-2,2'-bipyridine, and ²L_R/²L_S = (-)/(+)-2,6-bis(4',5'-pinene-2'-pyridyl)pyridine. Interestingly, they not only present distinct degrees of chirality but also show large differences in near-infrared (NIR) photoluminescence (PL), circularly polarized luminescence (CPL), and second-harmonic generation (SHG). Eight-coordinated Yb-R-1 with an asymmetric bidentate ¹L_R ligand has a high NIR-PL quantum yield (1.26%) and a long decay lifetime (20 μs) at room temperature, being more than two times those (0.48%, 8 μs) of nine-coordinated Yb-R-2 with a C₂-symmetric tridentate ²L_R ligand. In addition, Yb-R-1 displays an efficient CPL with a luminescence dissymmetry factor g_lum = 0.077, being 4 × Yb-R-2 (0.018). In particular, Yb-R-1 presents a strong SHG response (0.8 × KDP), which is 8 × Yb-R-2 (0.1 × KDP). More remarkably, the precursor Yb(tta)₃(H₂O)₂ exhibits a strong third-harmonic generation (THG) response (41 × α-SiO₂), while the introduction of chiral N-donors results in the switching of THG to SHG. Our interesting findings provide new insights into both the functional regulation and switching in multifunctional lanthanide molecular materials.

Li3B8O13X (X = Cl and Br): Two New Noncentrosymmetric Crystals with Large Birefringence Induced by BO3 Units

Enthusiasm for the exploration of nonlinear alkali metal borates remains high. Focusing on the Li-B-O-X (X = Cl and Br) system, two examples of noncentrosymmetric borates, Li₃B₈O₁₃Cl and Li₃B₈O₁₃Br, were obtained using a high-temperature solution method under vacuum conditions. Structurally, the Li₃B₈O₁₃X crystals exhibit two independent alternately arranged three-dimensional B-O network structures formed by the basic building block unit B₈O₁₆. The performance measurements demonstrate their short ultraviolet cutoff edges. The theoretical calculation indicates that the BO₃ units dominate the contribution to their large optical anisotropy with the birefringence, 0.094 and 0.088@1064 nm for Li₃B₈O₁₃Cl and Li₃B₈O₁₃Br, respectively.

Two temperature-induced 1D CuII chain enantiomeric pairs showing different magnetic properties and nonlinear optical responses

At different reaction temperatures, using Cu(NO₃)₂·3H₂O to react with enantiomerically pure N-donor ligands (L_S/L_R), respectively, two pairs of chiral one-dimensional (1D) Cu^II chain enantiomers formulated as [Cu(μ₂-NO₃)(NO₃)(L_S)]_n/[Cu(μ₂-NO₃)(NO₃)(L_R)]_n (S-1-Cu/R-1-Cu, formed at 40 °C with an NO₃^- group as a sole bridging ligand) and [Cu(μ₂-L_S)(NO₃)₂]_n/[Cu(μ₂-L_R)(NO₃)₂]_n (S-2-Cu/R-2-Cu, formed at 25 °C with L_S or L_R as a bridging ligand) were prepared, where L_S/L_R = (+)/(-)-4,5-pinenepyridyl-2-pyrazine. Interestingly, such a disparity in bridging ligands leads not only to their distinct structural features but also to their completely different magnetic couplings together with a large difference in their nonlinear optical responses. S-1-Cu with a 1D helical structure shows weak ferromagnetic coupling between Cu^II ions, while S-2-Cu with a 1D stairway-like structure presents weak antiferromagnetic coupling. In particular, they simultaneously possess both second- and third-harmonic generation (SHG and THG) responses in one molecule with large strength differences. More remarkably, S-1-Cu exhibits a very large THG response (162 × α-SiO₂), which is 22.5 times that of S-2-Cu, and the SHG strength of S-1-Cu is more than 3 times that of S-2-Cu. This work demonstrates that reaction temperature has a great impact on the self-assembled structures of coordination polymers and subsequently results in their large performance differences.

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.

Noncentrosymmetric γ-Cs2I4O11 Obtained from IO4 Polyhedral Rearrangements in the Centrosymmetric β-Phase

We report the synthesis and optical properties of noncentrosymmetric (NCS) γ-Cs₂I₄O₁₁ that was obtained through IO₄ polyhedral rearrangements from centrosymmetric (CS) β-Cs₂I₄O₁₁. Trifluoroacetic acid (TFA) acts as a structure-directing agent and plays a key role in the synthesis. It is suggested that the function of TFA is to promote rearrangement reactions found in the organic synthesis of stereoisomers. γ-Cs₂I₄O₁₁ crystallizes in the NCS monoclinic space group P2₁ (No. 4) and exhibits a strong second-harmonic-generation (SHG) response of 5.0 × KDP (KH₂PO₄) under 1064 nm laser radiation. Additional SHG experiments indicate that the material is type I phase matchable. First-principles calculations show that SHG intensity mainly comes from its d₃₄, d₂₁, and d₂₃ SHG tensor components. The synthetic strategy of discovering γ-Cs₂I₄O₁₁ provides a new way for designing novel NCS SHG materials.

Aluminoborates as Nonlinear Optical Materials

Ultraviolet (UV) and Deep-UV (DUV, λ<200 nm) nonlinear optical (NLO) materials play a significant role to convert the wavelength of laser for producing UV/DUV coherent light in the laser industry. The Al [Ne]3s² 3p¹ atom with empty d orbitals can form sp³ , sp³ d, and sp³ d² hybrid orbitals when bonding with O/F atoms, which leads to the construction of [AlO_m F_n ] (m+n=4, 5, 6) units. Therein, aluminoborates have received intensive attention due to the effective structural and functional adjustment effects of the diverse Al-O/F units. Accordingly, numerous aluminoborates as candidates for the next generation of UV/DUV NLO materials were discovered. In this review, recently discovered aluminoborates are overviewed and analyzed, including their syntheses, crystal structure features, and second-order NLO performances. We aim to provide the latest progress and outlook on the crystal chemistry of aluminoborates and boost the finding of the next NLO candidates with high performances.