Noncentrosymmetric Cubic Cyanurate K6Cd3(C3N3O3)4 Containing Isolated Planar π-Conjugated (C3N3O3)3– Groups

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.

Hg16O12(NO3)6F2(H2O): A Mercury-Rich Nitrate Oxyfluoride with Diverse Triple-Coordinated Mercury-Based Units That Exhibits an Unparalleled [(Hg16O12F2(H2O))6+]∞ Cationic Framework and a Large Birefringence

Designing new compounds based on anion regulation has been widely favored due to the production of diverse crystal structures and excellent optical properties. Here, a new nitrate oxyfluoride, Hg16O12(NO3)6F2(H2O), has been obtained through a hydrothermal reaction. It crystallizes in the centric Ibca space group and shows a novel three-dimensional [(Hg16O12F2(H2O))6+]∞ cationic framework composed of interconnected HgO2F, HgO3, and HgO2(H2O) units, with isolated NO3- groups as balanced anions to build the whole structure. Notably, the HgO2F and HgO2(H2O) units are first presented here among mercury (Hg)-based compounds. Additionally, Hg16O12(NO3)6F2(H2O) exhibits a large birefringence of 0.17 at 546 nm. This work enriches the multiformity of Hg-based compounds and provides a route for developing promising birefringent materials.

Chiral amino acid-templated tin fluorides tailoring nonlinear optical properties, birefringence, and photoluminescence

In this study, we successfully synthesized two types of new chiral amino acid-templated tin fluoride crystals: (R)-[(C8H10NO3)2]Sn(IV)F6, (S)-[(C8H10NO3)2]Sn(IV)F6, (R)-[C8H10NO3]Sn(II)F3, and (S)-[C8H10NO3]Sn(II)F3, employing a slow evaporation method. The crystal structures of Sn(IV)-compounds were determined to belong to the noncentrosymmetric (NCS) nonpolar space group, P21212. Conversely, the structures of Sn(II)-compounds were found to crystallize in the NCS polar space group, P21, as revealed by single-crystal X-ray diffraction analysis. Remarkably, Sn(IV)-compounds exhibited a larger birefringence (0.328@546.1 nm), attributed to the well-stacked arrangement of planar π-conjugated benzene rings along the b-axis. The ability of tin(IV) fluorides to form more hydrogen bonds with ligands increased the probability of π-π interactions between benzene rings, enabling the growth of centimeter-sized crystals in Sn(IV)-compounds. In contrast, Sn(II)-compounds displayed a stronger second-harmonic generation (SHG) response (0.85 × KDP) than Sn(IV)-compounds (0.46 × KDP). This enhanced SHG response in Sn(II)-compounds was attributed to the increased dipole moments resulting from the presence of lone pairs. Additionally, Sn(II)-compounds exhibited photoluminescent properties due to the transition from the metal-to-ligand charge transfer state, facilitated by the presence of the lone pairs.

KRb2(NO3)2Cl: a new birefringent crystal exhibiting a perovskite-related framework and a short cutoff edge

The combination of π-conjugated groups [NO3] and Cl-centered polyhedra generates a new birefringent crystal with a perovskite-related framework, KRb2(NO3)2Cl, which is the first alkali metal nitrate chloride synthesized by a mild hydrothermal method. It crystallizes in the orthorhombic space group pbam (no. 55). In addition, KRb2(NO3)2Cl crystals with dimensions up to 7 × 1.5 × 1 mm3 were grown. Notably, KRb2(NO3)2Cl has a short UV cut-off edge (below 228 nm) and a significantly enhanced birefringence (Δn = 0.084 at 1064 nm). Theoretical calculations indicate that the birefringence enhancement mainly derives from π-conjugated [NO3] plane triangles.

Introduction of the [B-O/F] Units Enhances the Band Gap and Birefringence from Na6Mg3B10O18F6 to K3NaB10O16F2

The borate family is the main source of deep-ultraviolet (DUV) birefringent crystals, and it has attracted a lot of attention due to versatile [B-O] basic units. Herein, two new borate-based compounds Na₆Mg₃B₁₀O₁₈F₆ and K₃NaB₁₀O₁₆F₂ were discovered. Their fundamental building blocks are [B₅O₁₁] and [B₅O₁₀F] units, respectively. The calculated results showed that the band gap and birefringence of K₃NaB₁₀O₁₆F₂ (E_g = 6.93 eV, Δn = 0.047 at 1064 nm) are greater than those of Na₆Mg₃B₁₀O₁₈F₆ (E_g = 5.40 eV, Δn = 0.039 at 1064 nm). Furthermore, the effects of [B-O/F] units on band gap and birefringence were analyzed by the charge-transfer model and response electron distribution anisotropy method. The results show that introducing the [B-O/F] units can improve the band gap and birefringence. These findings will boost the exploration of DUV birefringent opticals.

Na4[B6O9 (OH)3](H2O) Cl: A Deep-Ultraviolet Transparent Nonlinear Optical Borate Chloride with {[B6O9 (OH)3]3-}∞ Chains

The development of nonlinear-optical (NLO) crystals with short ultraviolet cutoff edges is significant and challenging. Here, a new sodium borate chloride, Na₄[B₆O₉ (OH)₃](H₂O) Cl, was successfully obtained by the mild hydrothermal method, which crystallizes in a polar space group Pca2₁. The structure of the compound is characterized by {[B₆O₉ (OH)₃]^3-}_∞ chains. Measurements of optical properties indicate the compound exhibits a deep-ultraviolet (DUV) cutoff edge (≤200 nm) and moderate second harmonic generation response (0.4 × KH₂PO₄). It presents the first DUV hydrous sodium borate chloride NLO crystal and the first sodium borate chloride possessing a one-dimensional B-O anion framework. Probing into the connection of structure and optical properties has been performed based on theoretical calculations. These results are instructive for designing and obtaining new DUV NLO materials.

K3C6N7O3·2H2O: A Multifunctional Nonlinear Optical Cyamelurate Crystal with Colossal π-Conjugated Orbitals

The delocalized π-conjugated units are considered as an advantageous gene for improving the optical nonlinearity of acentric crystals. For the first time, we synthesized a new acentric SHG-active metal cyamelurate crystal K₃C₆N₇O₃·2H₂O (I) by a facile solution method, containing a colossal planar π-conjugated (C₆N₇O₃)^3- unit. It displays a strong second-order harmonic generation (SHG) of 4 × KDP and a giant anisotropic birefringence of 0.446 at 1064 nm. The theoretical calculations reveal that such substantial improvement is contributed from the strong molecular susceptibility of (C₆N₇O₃)^3- units and their near-perfect coplanar arrangement. Moreover, I exhibits a broadband ultraviolet photoluminescence at 366 nm, suggesting its multifunctional capacity and great potential for compact highly integrated optoelectronic devices.

Cd4InO(BO3)3: A New Nonlinear Optical Crystal Exhibiting Strong Second Harmonic Generation Effect and Moderate Birefringence

A new noncentrosymmetric cadmium indium borate, Cd4InO(BO3)3 (CIBO) has been successfully developed via a standard solid-state reaction. Its crystal structure was confirmed by the single crystal X-ray diffraction, which shows that CIBO belongs to the non-centrosymmetric and polar space group Cm. Its structure contains the distorted InO6 and CdOn (n = 6, 8) polyhedra, which link together by sharing an edge or corner to build a three dimensions framework with BO3 triangles accommodated in tunnels. Benefiting from the approximately parallel configuration of BO3 triangles, CIBO exhibited a strong second harmonic generation (SHG) effect (3 × KDP), and moderate birefringence of 0.077@1064 nm. Further optical and thermal characterizations suggest that CIBO possesses a wide transparent window and good thermal stability. Theoretical calculation reveals that the macroscopic SHG coefficients of CIBO results from the synergistic effect of the parallel arrangement of BO3 groups and d10 Cd2+ cation.

AZn4(OH)4(C3N3O3)2 (A = Mg, Zn): Two Zn-Based Cyanurate Crystals with Various Cation Coordination and Large Birefringence

Cyanurate crystals have recently become a research hot spot in birefringent materials owing to the large structural and optical anisotropy of the planar π-conjugated (H_xC₃N₃O₃)^x-3 (x = 0-3) groups. In this study, two new Zn-based cyanurate crystals, Zn₅(OH)₄(C₃N₃O₃)₂ and MgZn₄(OH)₄(C₃N₃O₃)₂, were synthesized by the hydrothermal method. In Zn₅(OH)₄(C₃N₃O₃)₂, the d¹⁰ Zn²⁺ cations have three different coordinating environments, which has never been found in cyanurates. These compounds have wide band gaps (∼5 eV) and large birefringence (∼0.32 at 400 nm), indicating their potential as ultraviolet birefringence crystals.

K2Pb(H2C3N3O3)4(H2O)4: a potential UV nonlinear optical material with large birefringence

K₂Pb(H₂C₃N₃O₃)₄(H₂O)₄ (I) features a 2D [K₂PbO₈(H₂O)₄]^12- anionic layer and reveals a moderate SHG signal of approximately 2.6 × KDP.

Optimal arrangement of π-conjugated anionic groups in hydro-isocyanurates leads to large optical anisotropy and second-harmonic generation effect

Optimal arrangements of π-conjugated anions in alkaline earth metal hydro-isocyanurates with rich structures result in large birefringence and a giant nonlinear optical effect.

K4Cu3(C3N3O3)2X (X = Cl, Br): strong anisotropic layered semiconductors containing mixed p–p and d–p conjugated π-bonds

Metal cyanurates K₄Cu₃(C₃N₃O₃)₂X (X = Cl, Br) containing π-conjugated anions are synthesized in flame-sealed silica tubes and they exhibit 2D graphene-like layered structures and intriguing semiconductor behaviors.

Hydroisocyanurates X2Y(H2C3N3O3)4·4H2O (X = K, Cs; Y = Zn, Cd) with large birefringence stemming from π-conjugated (H2C3N3O3)− anions

A series of new hydroisocyanurates X₂Y(H₂C₃N₃O₃)₄·4H₂O (X = K, Cs; Y = Zn, Cd) with large birefringence resulting from the π-conjugated (H₂C₃N₃O₃)⁻ unit have been obtained.

M4LiBe4P7O24 and M4Li(Li3P)P7O24 (M = Cs, Rb): deep-ultraviolet nonlinear-optical phosphates with a tetrahedra-substituted paracelsian-like framework

Paracelsian-like beryllophosphates M₄LiBe₄P₇O₂₄ and tetrahedra-substituted phases M₄Li(Li₃P)P₇O₂₄ (M = Cs, Rb) undergo structural transformation originating from the nature of tetrahedral cations.

Inorganic planar π-conjugated groups in nonlinear optical crystals: review and outlook

Nonlinear optical crystals with various inorganic planar π-conjugated groups are reviewed based on their bandgaps.

“Old dog, new tricks”: the lone pair effect inducing divergent optical responses in lead cyanurates containing π-bonds

The stereochemically active lone pair effect of Pb²⁺ and induced narrower bandgaps were elaborated in Pb₃(HC₃N₃O₃)₂(OH)₂ for the first time.

Second-order nonlinear optical materials with a benzene-like conjugated π system

This feature article highlights the strategies used to design UV/DUV NLO materials based on benzene-like π-conjugated units.

KLi(HC3N3O3)·2H2O: Solvent-drop Grinding Method toward the Hydro-isocyanurate Nonlinear Optical Crystal

A novel mixed alkali hydro-isocyanurate, KLi(HC₃N₃O₃)·2H₂O was first prepared in AOH-BOH-H₃C₃N₃O₃ (A/B = Li/Na/K/Rb/Cs) system via a solvent-drop grinding method. KLi(HC₃N₃O₃)·2H₂O shows a large second harmonic generation response (5.3 × KH₂PO₄) with an ultraviolet cutoff edge of 237 nm. More importantly, the bulk single crystal can be readily grown through water solution technique. Characterization of these crystals indicates that KLi(HC₃N₃O₃)·2H₂O has a high laser damage threshold (LDT) (4.76 GW/cm²) and exhibits a large birefringence (Δ n = 0.186@514 nm), which reduces Type I phase-matching to 246 nm.

A new ultraviolet transparent hydra-cyanurate K2(C3N3O3H) with strong optical anisotropy from delocalized π-bonds

A new hydra-cyanurate K₂(C₃N₃O₃H) with high ultraviolet transparency (E_g ∼4.94 eV) and strong optical anisotropy (Δn ∼0.35) was synthesized by a slow evaporation method in aqueous solution.

Cs2CdV2O6Cl2 and Cs3CdV4O12Br: two new non-centrosymmetric oxyhalides containing d0 and d10 cations and exhibiting second harmonic generation activity

Two new non-centrosymmetric oxyhalides containing d⁰ and d¹⁰ cations and exhibiting strong second harmonic generation activity.

LiO4 tetrahedra lock the alignment of π-conjugated layers to maximize optical anisotropy in metal hydroisocyanurates

The “lock-in effect” of LiO₄ tetrahedra contributes to the alignment of π-conjugated layers to maximize optical anisotropy in metal hydroisocyanurates.

Co-crystal LiCl·(H3C3N3O3): a promising solar-blind nonlinear optical crystal with giant nonlinearity from coplanar π-conjugated groups

Co-crystal LiCl·(H₃C₃N₃O₃) with the largest SHG coefficient among all solar-blind nonlinear optical crystals was studied by first principles calculations.

Intrinsic zero thermal expansion in cube cyanurate K6Cd3(C3N3O3)4

The novel cubic cyanurate K₆Cd₃(C₃N₃O₃)₄ exhibits zero thermal expansion behavior with a very low thermal expansion coefficient of 0.06 MK⁻¹ in a broad temperature range from 10 to 130 K.

A rich structural chemistry in π-conjugated hydroisocyanurates: layered structures of A2B(H2C3N3O3)4·nH2O (A = K, Rb, Cs; B = Mg, Ca; n = 4, 10) with high ultraviolet transparency and strong optical anisotropy

Mixed alkali/alkali-earth metal hydroisocyanurates A₂B(H₂C₃N₃O₃)₄·nH₂O (A = K, Rb, Cs; B = Mg, Ca, n = 4, 10) with strong optical anisotropy were grown.

“Two in one”: an unprecedented mixed anion, Ba2(C3N3O3)(CNO), with the coexistence of isolated sp and sp2 π-conjugated groups

The first mixed cyanate–cyanurate double salt Ba₂(C₃N₃O₃)(CNO) containing two types of π-conjugated groups was successfully synthesized and systematically characterized.