Rational Design of the Metal-Free KBe2 BO3 F2 ⋅(KBBF) Family Member C(NH2 )3 SO3 F with Ultraviolet Optical Nonlinearity

Sulfate Derivatives with Heteroleptic Tetrahedra: New Deep-Ultraviolet Birefringent Materials in which Weak Interactions Modulate Functional Module Ordering

Deep-ultraviolet (UV) birefringent materials are urgently needed to facilitate light polarization in deep-UV lithography. Maximizing anisotropy by regulating the alignment of functional modules is essential for improving the linear optical performance of birefringent materials. In this work, we proposed a strategy to design deep-UV birefringent materials that achieve functional module ordering via weak interactions. Following this strategy, four compounds CN4H7SO3CF3, CN4H7SO3CH3, C(NH2)3SO3CH3, and C(NH2)3SO3CF3 were identified as high-performance candidates for deep-UV birefringent materials. The millimeter-sized crystals of CN4H7SO3CF3, CN4H7SO3CH3, and C(NH2)3SO3CH3 were grown, and the transmittance spectra show that their cutoff edges are below 200 nm. CN4H7SO3CF3 exhibits the largest birefringence (0.149 @ 546 nm, 0.395 @ 200 nm) in the deep-UV region among reported sulfates and sulfate derivatives. It reveals that the hydrogen bond can modulate the module ordering of the heteroleptic tetrahedra and planar π-conjugated cations, thus greatly enhancing the birefringence. Our study not only discovers new deep-UV birefringent materials but also provides an upgraded strategy for optimizing optical anisotropy to achieve efficient birefringence.

Emergent Mid-Infrared Nonlinear Optical Candidates With Targeted Balance Performances

Infrared nonlinear optical (NLO) crystal materials exert a crucial role in laser technology, which is extensively utilized in the fields of medical laser, long-distance laser communication, infrared laser guidance, etc. Currently, the commercially available infrared NLO crystals are diamond-like structural crystals AgGaQ2 (Q = S, Se) and ZnGeP2. However, their applications are significantly limited owing to their inherent drawbacks, such as low laser damage thresholds and narrow band gaps. Therefore, exploring novel infrared NLO materials with excellent performances is urgent. At present, candidate systems for exploring infrared NLO materials mainly are chalcogenides, pnictides, metal halides for popular systems, and chalcohalides, oxyhalides, heavy metal oxides, oxychalcogenides, nitrides for emergent systems. Notably, among them, pnictides generally exhibited a stronger NLO performance than other systems, but a narrower band gap. Accordingly, after the detailed literature survey, to the best knowledge, ≈139 compounds achieve balanced performances (Eg ≥ 3.0 eV, dij ≥ 0.5 × AgGaS2) in the remaining systems, in which there are 2 metal halides, 9 oxyhalides, 10 heavy metal oxides, 17 nitrides, 19 oxychalcogenides, 22 chalcohalides, and 60 chalcogenides. Thus, the structure-property survey of these compounds produces the practical design strategy to explore emergent infrared NLO crystal materials with balanced properties.

Cationic Coordination Modification Drives Birefringence and Nonlinear Effect Double Lifting in Sulfate

As nonlinear optical (NLO) crystals, sulfates have the superiority of transparency for ultraviolet (UV) light, but they are often troubled by small nonlinear coefficients and birefringence owing to the high symmetry of the [SO4]2- group. By introducing two neutral diethylenetriamine (DETA) molecules to replace the six coordinated water molecules of the [Zn(H2O)6]2+ complex cation in [Zn(H2O)6](SO4)(H2O), a new sulfate with an acentric structure, namely, [Zn(DETA)2](SO4)(H2O)3, has been designed and synthesized. Structural investigation reveals that the coordination modification of Zn2+ ion tremendously enhances its intraoctahedral distortion. The formed distorted [Zn(DETA)2]2+ cations and the [SO4]2- groups feature an optimized arrangement, endowing [Zn(DETA)2](SO4)(H2O)3 with enhancements in both second harmonic generation (SHG) intensity, from undetectable to 0.25 × KH2PO4 (KDP), and birefringence, from 0.014 to 0.042 at 1064 nm. Despite the slight compromise made in the light transmission range, [Zn(DETA)2](SO4)(H2O)3 still possesses a short absorption edge of 220 nm, retaining the majority of the light transmission range in the solar-blind region. Our work provides a novel and applicable approach of cationic coordination modification to improve the nonlinear optical coefficient and birefringence of sulfates.

Two l-Proline Cadmium Thiocyanates with Moderate Second-Harmonic-Generation Responses and Large Birefringences

Two organic-inorganic cadmium thiocyanates, namely, Cd(SCN)(l-C5H8NO2) (1) and Cd(SCN)2(l-C5H9NO2) (2), have been synthesized using l-proline as the bridging ligand. The two compounds have double-chain-like and layered structures, respectively. They feature moderate second-harmonic-generation responses (0.33 × KH2PO4 for 1 and 1.58 × KH2PO4 for 2), wide band gaps (4.68 eV for 1 and 4.59 eV for 2), and large birefringences (0.210 at 546 nm for 1 and 0.295 at 546 nm for 2). The structure-property relationship was explicated by first-principle calculations based on density functional theory.

Screening Strategy Identifies an Overlooked Deep-Ultraviolet Transparent Nonlinear Optical Crystal

In the deep-ultraviolet (DUV) region, nonlinear optical (NLO) crystals must meet stringent requirements, including a large optical band gap and sufficient second harmonic generation (SHG) response. Typically, these criteria are fulfilled by borates, carbonates and nitrates containing π-conjugated groups. In contrast, sulfates and phosphates, with polarizabilities significantly smaller than those of π-conjugated groups, struggle to achieve similar performance. Here, we present the discovery of Mg2PO4Cl, a magnesium-based phosphate, identified from over 10,000 phosphates based on a polar-axial-symmetry screening strategy, which exhibits the highest SHG response (5.2×KH2PO4 (KDP)) with phase-matching ability among non-π-conjugated DUV transparent NLO crystals. This compound belongs to the Pna21 space group, with [PO4] units consistently aligned along the 21 screw axis and glide planes throughout its crystal structure. Theoretical calculations attribute its remarkable SHG effect to the orderly arrangement of heteroanionic [MgO5Cl] and [MgO4Cl2] polyhedra alongside isolated [PO4] tetrahedra, supported by Berry phase analysis. Furthermore, a crystallographic structure analysis of phosphates and sulfates with significant SHG effects validates the effectiveness of our screening strategy. These findings offer valuable insights into the origins of NLO effects in non-π-conjugated compounds from both a material design and structural chemistry perspective, inspiring future efforts to revitalize DUV phosphates.

(C10N2H10)(HI2O6)(HIO3)(IO3): A Birefringent Material Featuring Large π-Conjugated Organic Cation and Two Types of Iodate Anions

Birefringent crystals, which can modulate and polarize light, play a vital role in the development of optical sensors and optoelectronic devices. Herein, we synthesized a new birefringent crystal [(C10N2H10)(HI2O6)(HIO3)(IO3)] with large birefringence by integrating the large π-conjugated 4,4'-bipyridine group with two types of iodates anions ([IO3]- and [HI2O6]-) characterized by high polarizability anisotropy and strong stereochemically active lone pairs (SCALP). The crystal exhibits a large measured birefringence of 0.171 at 550 nm, surpassing most organic-inorganic hybrid iodate birefringent materials reported in the literature. Theoretical analyses reveal that the optical properties of the crystal arise primarily from the synergistic interactions between the π-conjugated 4,4'-bipyridine group and I-O groups. This research presents an effective method for the strategic integration of large π-conjugated organic cations into iodates, facilitating the design of novel birefringent materials and encouraging further exploration of superior birefringent substances.

C2H12N6C4O4·2H2O and Na2C4O4·3H2O: Two Ultraviolet Birefringent Compounds with the [C4O4]2- Group

In modern optics, birefringent materials that can manipulate light polarization play important roles in lasers and information fields. The search for ultraviolet (UV) crystals with large birefringence is the focus of attention in the field of optical materials. In this work, we synthesized two birefringent crystals, C2H12N6C4O4·2H2O and Na2C4O4·3H2O, containing planar π-conjugated [C4O4]2- groups. Attributed to the large structural anisotropy and relatively ordered arrangement of the [C4O4]2- groups, C2H12N6C4O4·2H2O and Na2C4O4·3H2O possess large birefringence of 0.20-0.21 at 1064 nm. Meanwhile, they exhibit short ultraviolet cutoff edges at about 280-300 nm, corresponding to the large band gaps of 4.35 and 4.24 eV, respectively. Using structural analysis and first-principles calculations, the origins of such large birefringence are investigated and discussed. This work provides two potential UV birefringent crystals and prompts the search for novel birefringent materials.

Strategically designed metal-free deep-ultraviolet birefringent crystals with superior optical properties

Finding new birefringent materials with deep-ultraviolet (DUV, λ < 200 nm) transparency is urgent, as current commercial materials cannot meet the rapidly growing demands in related application fields. Herein, three guanidinium-based compounds, C(NH2)3CH3SO3, β-C(NH2)3Cl, and γ-C(NH2)3Cl, all featuring [C(NH2)3·X]∞ (X = CH3SO3 and Cl) pseudo layers, were designed through structural motif tailoring. Theoretical calculations indicate that these metal-free compounds all possess broad bandgaps (6.49-6.71 eV, HSE06) and remarkable birefringence (cal. 0.166-0.211 @ 1064 nm). Centimeter-sized C(NH2)3CH3SO3 crystals have been grown using a feasible aqua-solution method. Subsequently, to further optimize the properties, β/γ-C(NH2)3Cl was remolded by further tailoring the [C(NH2)3]+ cationic unit and the acceptor Cl- anion, and then the fourth compound NH2COF was theoretically constructed. Interestingly, NH2COF exhibits the desired coexistence of a wider bandgap (7.87 eV, HSE06) and giant birefringence (cal. 0.241 @ 1064 nm) attributed to its higher density of well-aligned birefringence-active groups (BAGs). Furthermore, among these four designed compounds, C(NH2)3CH3SO3 has been experimentally synthesized and exhibits a short UV cutoff edge. Centimeter-sized crystals have been grown using a feasible aqueous solution method. This study provides an effective strategy to optimize the density of BAGs for large birefringence and offers valuable insights into the strategic design of metal-free DUV birefringent crystals.

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.

Advancing nonlinear optics: discovery and characterization of new non-centrosymmetric phenazine-based halides

Organic-inorganic metal halides (OIMHs) have drawn considerable attention due to their remarkable optoelectronic properties and substantial promise for nonlinear optical applications. In this research, phenazine has been selected as the organic cation because of its π-conjugated feature. Three compounds, (C12H9N2)PbCl3, (C12H9N2)SbCl4, and (C12H9N2)2InBr4·Br, were synthesized. Initial space group assignments were centrosymmetric for (C12H9N2)PbCl3 and (C12H9N2)SbCl4. However, under 1550 nm laser excitation, (C12H9N2)PbCl3 and (C12H9N2)SbCl4 exhibited second harmonic generation intensities ∼1.7 times greater than that of the benchmark KH2PO4. Structural reevaluation ultimately confirmed non-centrosymmetric P1 and P21 space groups for (C12H9N2)PbCl3 and (C12H9N2)SbCl4, respectively. Upon excitation at 335 nm and 470 nm, (C12H9N2)PbCl3, (C12H9N2)SbCl4, and (C12H9N2)2InBr4·Br emit fluorescence at room temperature. (C12H9N2)2InBr4·Br exhibits reversible phase transitions, showing potential for phase change energy storage. Our research underscores the critical role of comprehensive experimental validation in determining the precise crystallographic space groups and reveals the extensive potential of OIMHs as versatile candidates for advanced optoelectronic applications.

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.

(C4H6N3O)(HSO4): A Cytosinium Bisulfate with Large Birefringence and Moderate Second Harmonic Generation Effect Produced via Combining a Promising Planar Nonlinear Optical-Active Motif with a Tetrahedral Group

Investigating novel nonlinear optical (NLO) active units serves as a valuable method for broadening the research landscape of NLO materials. This study showcases the potential of the cytosinium cation (C4H6N3O)+ as a novel NLO-active motif through theoretical calculations. The title compound exhibited a wide band gap of 3.85 eV, along with a moderate second harmonic generation (SHG) response of 1.65 times that of KH2PO4 (KDP) and significant birefringence of 0.47. Its exceptional optical properties are primarily attributed to the synergy interaction between cations and anionic groups in the asymmetric unit.

Centric Sc(HPO3)(H2PO3)(H2O) and Acentric Sc(H2PO3)3: Two Ultraviolet Scandium Phosphite Optical Crystals

Exploring ultraviolet (UV) nonlinear-optical (NLO) materials is significant for the conversion of a high-frequency laser. Two scandium phosphites, Sc(HPO3)(H2PO3)(H2O) and Sc(H2PO3)3, were successfully synthesized. Centric Sc(HPO3)(H2PO3)(H2O) exhibits a short UV cutoff edge (<200 nm) and a unique double-layer structure of [Sc2(HPO3)2(H2PO3)2(H2O)2]∞. The acentric Sc(H2PO3)3 exhibits a three-dimensional [Sc(H2PO3)3]∞ structure with a large band gap of 4.05 eV, and it demonstrates a moderately phase-matchable second-harmonic-generation response [0.60 × KDP (KH2PO4)] at 1064 nm. The crystal structures, optical properties, and theoretical calculations of the two compounds are discussed. This work will promote the exploration of new NLO phosphite materials.

[C(NH2)3]6Mo7O24: A Guanidinium Molybdate as a UV Nonlinear Optical Crystal with Large Birefringence

The key to searching novel nonlinear optical (NLO) crystals was effectively combining the NLO-active units to obtain a noncentrosymmetric structure. Nevertheless, the present predicament lies in the growing challenge of discovering novel crystals within conventional inorganic frameworks that surpass the properties of the current NLO materials. In view of this, researchers expanded their research focus to the organic-inorganic hybridization system; it is foreseeable to concentrate the advantages from several kinds of NLO-active units to acquire novel NLO crystals with superior properties. We herein report an organic-inorganic hybrid molybdate crystal, namely, [C(NH2)3]6Mo7O24 (GMO). It was successfully obtained via combining inorganic NLO-active MoO6 octahedra and organic π-conjugated [C(NH2)3]+ groups. GMO demonstrates a moderate second-harmonic-generation response, specifically measuring about 1.3 times the value of KDP. Additionally, it exhibits a significant birefringence value of 0.203 at the wavelength of 550 nm and possesses a wide band gap of 3.31 eV. Theoretical calculations suggest that the optical properties of the GMO are primarily influenced by the synergy effect of [C(NH2)3]+ groups between MoO6 octahedra.

Guanidinium Vanadate [C(NH2)3]3VO4·2H2O Revealing Enhanced Second-Harmonic Generation and Wide Band Gaps

A new guanidinium-templated vanadate, [C(NH2)3]3VO4·2H2O, has been synthesized in a phase-pure form. It crystallizes in a noncentrosymmetric polar space group, Cc, and the crystal structure is built upon a framework of guanidinium, vanadate tetrahedra, and water molecules linked by hydrogen bonds. Notably, optical measurements reveal that the material exhibits an approximately 9.6-fold enhancement in second-harmonic generation efficiency compared to its phosphate analogue. The enhancement can be attributed to the increased geometrical distortion of the VO4 tetrahedra. Furthermore, we found that the coordination number of the central vanadium atom significantly affects the optical band gaps. Among various coordination numbers, the 4-coordinate VO4 tetrahedra are found to be more favorable for widening the optical band gap of materials compared to the 5- and 6-coordinate vanadium polyhedra, as demonstrated by this work.

From C4H7N2Ge0.4Sn0.6Br3 to C6H11N2Ge0.4Sn0.6Br3: Effective Modulation of the Second Harmonic Generation Effect and Optical Band Gap by Planar π-Conjugated Organic Cation Size

In the search for nonlinear optical (NLO) materials with excellent overall performance, we have devoted ourselves to organic-inorganic hybrids consisting of anionic groups containing stereochemically active lone-pair (SCALP) electron cations and organic planar π-conjugated group cations. Accordingly, in this paper, two novel organic-inorganic hybrid metal halides, C4H7N2Ge0.4Sn0.6Br3 (I) and C6H11N2Ge0.4Sn0.6Br3 (II), have been synthesized. The powder second-harmonic technique shows that both C4H7N2Ge0.4Sn0.6Br3 and C6H11N2Ge0.4Sn0.6Br3 have moderately strong second-order nonlinear optical effects, which are about 2.03 (I) and 1.16 (II) times that of KH2PO4 (KDP), respectively. They also have different optical band gaps of 2.75 (I) and 2.88 eV (II) due to the different sizes of the organic cations, and their photoluminescent and thermal properties were also investigated. This work provides new structural insights for the design and modulation of organic-inorganic hybrid halide materials with multiple excellent optical properties.

Fluorination Strategy Towards Symmetry Breaking of Boron-centered Tetrahedron for Poly-fluorinated Optical Crystals

Borate crystals can be chemically and functionally modified by the fluorination strategy, which encourages the identification of emerging fluorooxoborates with a structure and set of characteristics not seen in any other oxide parents. However, the bulk of fluorooxoborates have been found accidentally, rational methods of synthesis are required, particularly for the infrequently occurring poly-fluorinated components. Herein, we reported the use of bifluoride salts as a potent source of fluorine to prepare fluorooxoborates that contain rarely tri-fluorinated [BF3 X] (X=O and CH3 ) tetrahedra and eleven compounds were found. We identified the optical properties of the organofluorinated group [CH3 BF3 ] and their potential for nonlinear optics for the first time. Among these, two non-centrosymmetric components hold potential for the production of 266 nm harmonic coherent light for nonlinear optics, and more crucially, have the benefit of growing large size single crystals. Our study establishes experimental conditions for the coexistence of the diverse functional groups, enabling the production of poly-fluorinated optical crystals.

C(NH2)3Cd(C2O4)Cl(H2O)·H2O and BaCd(C2O4)1.5Cl(H2O)2: Two Oxalate Chlorides Obtained by Chemical Scissors Strategy Exhibiting Low-Dimensional Structural Networks and Balanced Overall Optical Properties

Low-dimensional crystalline materials have attracted much attention due to their special physical and chemical properties. Herein, two new oxalate chlorides, C(NH2)3Cd(C2O4)Cl(H2O)·H2O and BaCd(C2O4)1.5Cl(H2O)2, were synthesized. C(NH2)3Cd(C2O4)Cl(H2O)·H2O presents the unique {[Cd(C2O4)Cl(H2O)]-}∞ zigzag chain, while BaCd(C2O4)1.5Cl(H2O)2 shows a novel {[Cd(C2O4)1.5Cl]2-}∞ layer. They showed large measured band gaps, which were 3.76 and 4.53 eV, respectively, and the latter was the largest band gap in the A-M-C2O4-X (A = Monovalent cationic or alkaline earth metals, X = F, Cl, Br, I) family. They exhibit a large calculated birefringence of 0.075 and 0.096 at 1064 nm, respectively. This study promotes the exploration of synthesizing low-dimensional crystalline materials with balanced overall optical performances by a chemical scissors strategy.

Mg(C3 O4 H2 )(H2 O)2 : A New Ultraviolet Nonlinear Optical Material Derived from KBe2 BO3 F2 with High Performance and Excellent Water-Resistance

Acquiring high-performance ultraviolet (UV) nonlinear optical (NLO) materials that simultaneously exhibit a strong second harmonic generation (SHG) coefficients, as short as possible SHG phase-matching (PM) wavelength and non-hygroscopic properties has consistently posed a significant challenge. Herein, through multicomponent modification of KBe2 BO3 F2 (KBBF), an excellent UV NLO crystal, Mg(C3 O4 H2 )(H2 O)2 , was successfully synthesized in malonic system. This material possesses a unique 2D NLO-favorable electroneutral [Mg(C3 O4 H2 )3 (H2 O)2 ]∞ layer, resulting in the rare coexistence of a strong SHG response of 3×KDP (@1064 nm) and short PM wavelength of 200 nm. More importantly, it exhibits exceptional water resistance, which is rare among ionic organic NLO crystals. Theoretical calculations revealed that its excellent water-resistant may be originated from its small available cavity volumes, which is similar to the famous LiB3 O5 (LBO). Therefore, excellent NLO properties and stability against air and moisture indicate it should be a promising UV NLO crystal.

Anisotropic structure building unit involving diverse chemical bonds: a new opportunity for high-performance second-order NLO materials

We define the anisotropic structure building unit that encompasses diverse chemical bonds (ABUCB). The ABUCB is highly likely to cause anisotropy in both crystallographic structure and spatial electron distribution, ultimately resulting in enhanced macroscopic optical anisotropy. Accordingly, the (PO3F)2- or (SO3F)- tetrahedron involving the unique P-F or S-F bond serves as such an ABUCB. The distinct chemical bond effectively alters the microscopic nature of the structure building unit, such as polarizability anisotropy, hyperpolarizability, and geometry distortion; this consequently changes the macroscopic second-order nonlinear optical (2nd-NLO) properties of the materials. In this review, we summarize both typical and newly emerged compounds containing ABUCBs. These compounds encompass approximately 90 examples representing six distinct categories, including phosphates, borates, sulfates, silicates, chalcogenides and oxyhalides. Furthermore, we demonstrate that the presence of ABUCBs in DUV/UV NLO compounds contributes to an increase in birefringence and retention of a large band gap, facilitating phase matching in high-energy short-wavelength spectral ranges. On the other hand, the inclusion of ABUCBs in IR NLO compounds offers a feasible method for increasing the band gap and consequently enhancing the larger laser-induced damage threshold. This review consolidates various trial-and-error explorations and presents a novel strategy for designing 2nd-NLO compounds, potentially offering an opportunity for the development of high-performance 2nd-NLO materials.

Two van der Waals Layered Antimony(III) Phosphites as UV Optical Materials

Herein, two new Sb3+-based phosphites, Sb2O2(HPO3) (I) and Sb2O(HPO3)2 (II), were successfully obtained by ingeniously combining Sb3+-based polyhedra containing stereochemically active lone pair (SCALP) and HPO3 polar groups. Both reported compounds exhibit unique 2D van der Waals layered structures, [Sb4O4(HPO3)2]∞ and [Sb2O(HPO3)2]∞, respectively, which favors compounds with large optical anisotropy. Interestingly, the different curvatures of the two layers resulted in the two title compounds showing significantly different birefringences (0.079@546 and 0.046@546 nm, respectively). Both compounds endow wide optical band gaps (4.32 and 4.54 eV, respectively), which indicates their potential as promising ultraviolet (UV) birefringent crystals. The synthesis of the two title compounds enriched Sb3+-based phosphites in the UV region and provided guidance for the subsequent synthesis of superior optical 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.

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.

Giant Mid-Infrared Second-Harmonic Generation Response in a Densely-Stacked Van Der Waals Transition-Metal Oxychloride

Second-harmonic generation (SHG) is a fundamental optical property of nonlinear optical (NLO) crystals. Thus far, it has proved difficult to engineer large SHG responses, particularly in the mid-infrared region, owing to the difficulty in simultaneously controlling the arrangement and density of functional NLO-active units. Herein, a new assembly strategy employing functional modules only, and aimed at maximizing the density and optimizing the spatial arrangement of highly efficient functional modules, has been applied to the preparation of NLO crystals, affording the van der Waals crystal MoO2 Cl2 . This exhibits the strongest powder SHG response (2.1×KTiOPO4 (KTP) @ 2100 nm) for a transition-metal oxyhalide, a wide optical transparency window, and a sufficient birefringence. MoO2 Cl2 is the first SHG-active transition-metal oxyhalide effective in the infrared region. Theoretical studies and crystal structure analysis suggest that the densely packed, optimally-aligned [MoO4 Cl2 ] modules within the two-dimensional van der Waals layers are responsible for the giant SHG response.

Second-Harmonic Generation in Homochiral Antimony Halides Directed by l-Histidine

Three homochiral organic-inorganic hybrid antimony halides, namely, (L-Hhis)2Sb2Cl8 (1), L-H2his·SbBr5·H2O (2), and (L-H2his)2·Sb3I13·4H2O (3), were prepared to investigate the structure-directing roles of l-histidine (l-his). These compounds feature dimeric, chainlike, and trimeric structures with different optical bandgaps. They display second-harmonic-generation (SHG) responses of 0.1, 2.6, and 0.05 times that of KH2PO4, respectively. Theoretical calculations for compound 2 were carried out to get insights into its structure-property relationship.

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.

Rational Design of Noncentrosymmetric Organic-Inorganic Hybrids with a π-Conjugated Pyridium-Type Cation for High Nonlinear-Optical Performance

Organic-inorganic hybrid materials have attracted increasing attention due to their unique superiority by combining the features of organic parts with inorganic parts. Herein, two organic-inorganic hybrid nonlinear-optical crystals, [C5H6O2N3]2[IO3]2 (I) and [C5H6O2N3][HSO4]·H2O (II), were successfully synthesized in aqueous solution by selecting 2-amino-3-nitropyridine as the cation and different anions of [IO3]- and [HSO4]-. The two compounds crystallized in the noncentrosymmetric space groups of P21 and P212121, respectively. I displays second-harmonic-generation (SHG) effects of 2.4 × KDP (KH2PO4) and a large birefringence (Δncal ∼ 0.22). Moreover, II exhibits a stronger SHG response of 5.2 × KDP, an enhanced band gap (2.81 eV), as well as a large birefringence (Δncal ∼ 0.25). This work points out a new feasible path for the rational design of high-performance organic-inorganic hybrid materials.

Cd2 Nb2 Te4 O15 : A Novel Pseudo-Aurivillius-Type Tellurite with Unprecedented Nonlinear Optical Properties and Excellent Stability

Oxides are emerging candidates for mid-infrared (mid-IR) nonlinear optical (NLO) materials. However, their intrinsically weak second harmonic generation (SHG) effects hinder their further development. A major design challenge is to increase the nonlinear coefficient while maintaining the broad mid-IR transmission and high laser-induced damage threshold (LIDT) of the oxides. In this study, it is reported on a polar NLO tellurite, Cd2 Nb2 Te4 O15 (CNTO), featuring a pseudo-Aurivillius-type perovskite layered structure composed of three types of NLO active groups, including CdO6 octahedra, NbO6 octahedra, and TeO4 seesaws. The uniform orientation of the distorted units induces a giant SHG response that is ≈31 times larger than that of KH2 PO4 , the largest value among all reported metal tellurites. Additionally, CNTO exhibits a large band gap (3.75 eV), a wide optical transparency window (0.33-14.5 µm), superior birefringence (0.12@ 546 nm), high LIDT (23 × AgGaS2 ), and strong acid and alkali resistance, indicating its potential as a promising mid-IR NLO material.

NaCl·CH4N2O·H2O: An Organic-Inorganic Hybrid Ultraviolet Nonlinear Optical Crystal with Optimized Comprehensive Properties

Ultraviolet (UV) nonlinear optical (NLO) crystals are a vital component in laser systems to modulate frequency. Organic groups have the advantages of easy synthesis, structural diversity, and large hyperpolarizations (β), which can be combined with inorganic groups to form unique organic-inorganic hybrid NLO crystal materials. Herein, a urea-containing crystal NaCl·CH4N2O·H2O is screened out and large-size crystals have been grown by a simple aqueous solution method. Compared with most reported organic-inorganic hybrid NLO materials, it has optimized comprehensive properties including a large SHG effect of 1.53 × KDP, a moderate birefringence (0.084@1064 nm), and a short UV cutoff edge of 209 nm. First-principles studies reveal that the dominant contributor to the linear and nonlinear optical properties is the urea molecule.

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.

[C(NH2)2NHNO2][C(NH2)3](NO3)2: A Mixed Organic Cationic Hybrid Nitrate with an Unprecedented Nonlinear-Optical-Active Unit

We herein report a mixed organic cationic hybrid nitrate, namely, [C(NH₂)₂NHNO₂][C(NH₂)₃](NO₃)₂ (1). It was successfully achieved via combining three different planar groups: [(C(NH₂)₂NHNO₂]⁺, C(NH₂)₃⁺, and NO₃^-. First-principles calculations confirm that the [(C(NH₂)₂NHNO₂]⁺ group is an excellent cationic nonlinear-optical (NLO)-active unit. The title compound exhibits a moderate second-harmonic-generation (SHG) response (1.5 × KDP), a wide band gap (3.58 eV), and a suitable birefringence of 0.071 at 550 nm. Theoretical calculations indicate that the synergy effect between the [(C(NH₂)₂NHNO₂]⁺ and C(NH₂)₃⁺ groups dominates the SHG process.

Alignment of Λ-Shaped Basic Building Units to Construct One New KMoO3(IO3) Polar Polymorph

Nonlinear optical (NLO) crystals assume an irreplaceable role in the development of laser science and technologies, yet the reasonable design of a high-performance NLO crystal remains difficult owing to the unpredictability of inorganic structures. In this research, we report the fourth polymorph of KMoO₃(IO₃), i.e., δ-KMoO₃(IO₃), to understand the effect of different packing patterns of basic building units on structures and properties. Among four polymorphs of KMoO₃(IO₃), the different stacking patterns of Λ-shaped cis-MoO₄(IO₃)₂ units result in α- and β-KMoO₃(IO₃) featuring nonpolar layered structures, whereas γ- and δ-KMoO₃(IO₃) display polar frameworks. Theoretical calculations and structure analysis reveal that IO₃ units can be regarded as the main source of its polarization in δ-KMoO₃(IO₃). Further property measurements show that δ-KMoO₃(IO₃) exhibits a large second-harmonic generation response (6.6 × KDP), a wide band gap (3.34 eV), and a broad mid-infrared transparency region (∼10 μm), which demonstrates that adjusting the arrangement of the Λ-shaped basic building units is an effective approach for rationally designing NLO crystals.

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.

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.

M(NH2SO3)2·xH2O (M = Ca, Pb, x = 0, 1, 4): Effect of Hydrogen Bonding on Structural Transformations and Second Harmonic Generation of Metal Sulfamates

Nonlinear optical (NLO) crystals are very important for laser technology, but the performances of available NLO crystals are still insufficient for increasing demand. Recently, the exploration of new NLO crystals in non-π-conjugated systems with the heteroatomic tetrahedra is attracting a lot of interest. In this work, we systematically explore the metal sulfamates containing [NH2SO3] groups and four metal sulfamates, namely, Ca(NH2SO3)2·4H2O, Ca(NH2SO3)2·H2O, Pb(NH2SO3)2·H2O, and Pb(NH2SO3)2 were synthesized by aqueous solution and hydrothermal methods. Notably, these metal sulfamates exhibit different crystal structures and optical properties owing to the diverse arrangement of the functional groups in their structures. In addition, due to hydrogen bond regulation, the centrosymmetric (CS) compound Ca(NH2SO3)2·4H2O can transform into noncentrosymmetric (NCS) Ca(NH2SO3)2·H2O, leading to NLO activity. Experimental characterizations and theoretical analysis reveal that these metal sulfamates are ultraviolet transparent and suitable for developing new NLO materials.

SbHPO3F: 2D van der Waals Layered Phosphite Exhibiting Large Birefringence

A novel antimony(III)-based phosphite, SbHPO₃F, featuring a unique two-dimensional (2D) van der Waals layered structure, has been successfully designed and synthesized via the simultaneous employment of optically active moieties including SbO₃F seesaw and tetrahedral HPO₃ groups. Its optimized layered arrangement formed by the alternating connection of 4-membered rings (4-MRs) and 8-MRs endows the title compound with desirable optical properties including a large birefringence and short ultraviolet (UV) cutoff edge, implying that it is a potential UV birefringent material.

[C(NH2)3]2Zn(CO3)2: A Guanidinium-Templated Ultraviolet Nonlinear Optical Material

A novel guanidinium-templated ultraviolet (UV) nonlinear optical zinc carbonate crystal, [C(NH₂)₃]₂Zn(CO₃)₂ (GZCO), has been synthesized in a closed system at low temperatures. GZCO crystallizing in the tetragonal noncentrosymmetric nonpolar space group, P4₁2₁2 exhibits a three-dimensional anionic framework constructed by interconnected [Zn₆C₆O₃₂] 12-membered ring channels with inorganic CO₃ triangles and ZnO₄ tetrahedra. Notably, the anhydrous GZCO shows a very high thermal stability among guanidine-based hybrid NLO materials benefiting from the confinement effect of the organic cations within inorganic channels. The UV-visible transmittance spectrum reveals that GZCO has a short UV cutoff edge of 210 nm, corresponding to the large band gap of 5.9 eV. GZCO exhibits a mild second-harmonic generation efficiency of 0.5 × KH₂PO₄ with type-I phase-matching behavior.

Homochiral Hybrid Organic-Inorganic Cadmium Chlorides Directed by Enantiopure Amino Acids

Homochiral cadmium chlorides were prepared under mild conditions using enantiopure amino acids as structure-directing agents. They feature a lacunary hexagonal CdCl₂ lattice as well as a one-dimensional perovskite structure. The coexistence of protonated and zwitterionic amino acids between cadmium chloride chains is quite rare. These compounds are nonlinear optically active solids showing a moderate second-harmonic-generation response. Theoretical calculations were performed to reveal the origin of their nonlinear-optical properties.

Directional Construction of New Nonlinear Optical Bifunctional Units through Molecular Engineering Design Inspired by the B3O7-Typed Configuration

Studies on new functional structural units with both large hyperpolarizability and high anisotropy are essentially important for finding high-performance nonlinear optical (NLO) materials and enriching the material systems. Under the guidance of the "structure-analogue" strategy, the work utilizes the molecular engineering approach to direct the construction of target units, BC₂N₅O₂ and B(C₂N₅)₂ units. The BC₂N₅O₂ unit with a highly analogous structure to the B₃O₇ group and its derivate B(C₂N₅)₂ unit with a configuration of B₅O₁₀ group are designed as NLO-active units. Furthermore, two compounds with these new NLO-active units, BC₂N₅H₆(OH)₂·H₂O (I) and B(C₂N₅H_6.5)₂(NO₃)₂ (II), are synthesized, successfully. These compounds exhibit excellent properties with second-harmonic generation (SHG) responses ranging from 0.5 to 5.9 times that of KDP and large birefringence (Δn_I = 0.181 @ 546.1 nm and Δn_II = 0.148 @ 546.1 nm). Theoretical calculations prove that the BC₂N₅O₂ and B(C₂N₅)₂ units make great contributions to the SHG effects and birefringence, which confirms that the BC₂N₅O₂ and B(C₂N₅)₂ units are novel NLO bifunctional units and could be excellent fundamental building blocks to construct amounts of novel NLO and birefringence crystals. Our studies would enlighten the research studies on biguanide complexes of boron.

Noncentrosymmetric (C3H7N6)6(H2PO4)4(HPO4)·4H2O and Centrosymmetric (C3H7N6)2SO4·2H2O: Exploration of Acentric Structure by Combining Planar and Tetrahedral Motifs via Hydrogen Bonds

The combination of organic and inorganic nonlinear active units to obtain organic-inorganic hybrid materials has been proved to be a very effective method to obtain nonlinear optical (NLO) materials with excellent properties. Herein we reported two hybrid melamine-based compounds, namely, acentric (C₃H₇N₆)₆(H₂PO₄)₄(HPO₄)·4H₂O (1) and centrosymmetric (C₃H₇N₆)₂SO₄·2H₂O (2), which were synthesized via facile solvent evaporation method. Compound 1 features a three-dimensional (3D) network structure composed of _∞[(H₂PO₄)₄(HPO₄)(H₂O)₄]^6- layers which are further linked with _∞[(C₃H₇N₆)₆]⁶⁺ layers via hydrogen bonds. Compound 2 displays a 3D structure composed of [(C₃H₇N₆)₂(SO₄)(H₂O)₂]_∞ layers further linked with each other by hydrogen bonds. Compound 1 presents a second harmonic generation signal of about 0.1 × KDP. Furthermore, UV-vis and infrared spectra, thermal analyses, and theoretical calculation were also adopted to evaluate its NLO performance. The theoretical calculations showed that the SHG response and large birefringence of 1 were primarily caused by the (C₃H₇N₆)⁺, (H₂PO₄)^-, and (HPO₄)^2- groups.

A flexible functional module to regulate ultraviolet optical nonlinearity for achieving a balance between a second-harmonic generation response and birefringence

Rigid planar π-conjugated groups are adopted for designing ultraviolet (UV) nonlinear optical (NLO) materials extensively. However, for these UV NLO crystals, the realization of a strong second harmonic generation (SHG) response is commonly accompanied by undesired overlarge birefringence. Herein, we propose a new functional gene, the flexible π-conjugated (C3H2O4)2- group, for designing a UV NLO crystal with a balance between the SHG response and birefringence. Furthermore, the combination of low-coordinated and high-coordinated alkali cations with the flexible (C3H2O4)2- group results in finding a new mixed alkali malonate, KLi(C3H2O4)·H2O (KLMW). As expected, KLMW exhibits a strong SHG efficiency (3 × KDP) and moderate birefringence (0.103 @ 1064 nm). In addition, it has a short UV cut-off edge of 231 nm and can be conveniently grown from solution. More importantly, it realized fourth harmonic generation with type-I phase-matching. Therefore, these excellent properties make KLMW a potential practical UV NLO material.

(N2H6)[HPO3F]2: maximizing the optical anisotropy of deep-ultraviolet fluorophosphates

Although phosphates are a rich source of deep-ultraviolet optical materials, the realization of large optical anisotropy in them still remains a challenge because of the small polarizability anisotropy of [PO₄] units. Inspired by the fluoridation strategy and hydrogen bond interaction, a new metal-free monofluorophosphate, (N₂H₆)[HPO₃F]₂, was synthesized, which exhibits a large birefringence (cal. 0.077) and wide band gap (∼6.51 eV). Such a large birefringence in (N₂H₆)[HPO₃F]₂ sets a new record among available fluorophosphates, and the [HPO₃F] unit is theoretically confirmed to be a new birefringence-active unit.

Noncentrosymmetric Rare-Earth Borate Fluoride La2B5O9F3: A New Ultraviolet Nonlinear Optical Crystal with Enhanced Linear and Nonlinear Performance

In crystal engineering, it is an effective and controllable approach to modify the electronic band structure and optimize crystal performances using rational chemical cosubstitution in a classic structure model. Herein, the noncentrosymmetric (NCS) rare-earth borate fluoride La₂B₅O₉F₃ was designed and synthesized successfully based on the extraordinarily stable M₂B₅O₉X (M = Ca, Sr, Ba, Sn, Pb, and Eu; X = Cl, Br, and I) template. Moreover, all 70 rare-earth borate halides were discussed, and the ratio of crystallization in NCS group is only 17.1%, much lower than 34.9% in all anhydrous borates. Benefiting from the substitution of [MOX] by [LaOF] polyhedra with improved hyperpolarizability and anisotropy of polarizability, compared with the M₂B₅O₉X family, La₂B₅O₉F₃ with optimized band structure exhibits the suitable SHG response (1.2 × KH₂PO₄ (KDP) @ 1064 nm), large band gap (6.58 eV), and moderate birefringence, which well achieves the optimal balance among the three critical parameters mentioned above for nonlinear optical (NLO) applications in the short-wavelength region. This work expands the research field of NLO materials to rare-earth borate fluorides and can lead to a better understanding of the role of rare-earth metal cations.

Variable dimensionality of the anion framework in four new borophosphates and fluoroborophosphates with short cutoff edges

Two new borophosphates, Cs₃B₃P₄O₁₆ and Li(NH₄)₂B₃P₄O₁₆, and two new fluoroborophosphates, K₄B₂P₂O₉F₂ and Rb₃B₂PO₅F₄, have been obtained via the high-temperature solution method. Single-crystal X-ray diffraction indicates that all of them exhibit various anion frameworks although they crystallize in the same space group, P2₁/c. Two-dimensional (2D) [B₃P₄O₁₆]_∞ layers and a 3D [B₃P₄O₁₆]_∞ network can be found in Cs₃B₃P₄O₁₆ and Li(NH₄)₂B₃P₄O₁₆, respectively, while isolated [B₂P₂O₉F₂] and [B₂PO₅F₄] exist in K₄B₂P₂O₉F₂ and Rb₃B₂PO₅F₄, respectively. The effect of the alkali metal cation size on the framework structures of Cs₃B₃P₄O₁₆ and Li(NH₄)₂B₃P₄O₁₆ has been discussed in detail. The IR spectra confirm their structural validity. UV-Vis-NIR diffuse reflectance spectroscopy indicates that the new compounds exhibit short cutoff edges. In addition, theoretical calculations were carried out to understand their electronic structures and optical properties.

Performance of optical materials with derivative planar π-conjugated groups: Recent advances and future prospects

Planar π-conjugated groups which possess not only large hyperpolarizability but also optical anisotropy are proven to be a good functional motif for optical materials with outstanding nonlinear optics and/or birefringence....

PNO: a promising deep-UV nonlinear optical material with the largest second harmonic generation effect

PNO with the largest SHG response in the deep-UV region was discovered by structural prediction methods.