Design of the Ionic Organic Nonlinear Optical Material NH4[LiC3H(CH3)O4] with Ultrawide Band Gap and Moderate Birefringence

Ionic organic crystals containing organic planar π-conjugated units has become one of the hot spots as nonlinear optical (NLO) materials. However, although this type of ionic organic NLO crystals commonly have remarkable second harmonic generation (SHG) responses, they also suffer from overlarge birefringences and relatively small band gaps that be hardly beyond 6.2 eV. Herein, a flexible π-conjugated [C₃H(CH₃)O₄]²⁻ unit was theoretically revealed, showing great potential for designing NLO crystals with balanced optical properties. Accordingly, through the reasonable NLO-favourable layered design, a new ionic organic material, NH₄[LiC₃H(CH₃)O₄], was successfully obtained. As expected, it achieves not only a large SHG effect (4×KDP), but also a suitable birefringence (0.06@546 nm) and an ultrawide band gap (>6.5 eV). This study provides a new flexible π-conjugated NLO-active unit, contributing to design more ionic organic NLO materials with excellent balanced optical properties.     

Perfectly Encoding π-Conjugated Anions in the RE5(C3N3O3)(OH)12 (RE=Y,Yb, Lu) Family with Strong Second Harmonic Generation Response and Balanced Birefringence

Nonlinear optical (NLO) crystal, which simultaneously exhibits strong second-harmonic-generation (SHG) response and desired optical anisotropy, is a core optical material accessible to the modern optoelectronics. Accompanied by strong SHG effect in a NLO crystal, a contradictory problem of overlarge birefringence is ignored, leading to low frequency doubling efficiency and poor beam quality. Herein, a series of rare earth cyanurates RE₅(C₃N₃O₃)(OH)₁₂ (RE=Y, Yb, Lu) were successfully characterized by 3D electron diffraction technique. Based on a “three birds with one stone” strategy, they enable the simultaneous fulfillment of strong SHG responses (2.5–4.2× KH₂PO₄), short UV cutoff (ca. 220 nm) and applicable birefringence (ca. 0.15 at 800 nm) by the introduction of rare earth coordination control of π-conjugated (C₃N₃O₃)³⁻ anions. These findings provide high-performance short-wavelength NLO materials and highlight the exploration of cyanurates as a new research area.     

A High-Performance Nonlinear Optical Crystal with a Building Block Containing Expanded π-Delocalization

Common nonlinear optical (NLO) crystals consist of traditional functional building blocks with inherent optical limitation. Herein, inspired by traditional (B₃O₆)³⁻ inorganic building block, we theoretically identified a new type of organic functional building blocks and then successfully synthesized the first cyamelurate NLO crystal, Ba(H₂C₆N₇O₃)₂ ⋅ 8 H₂O. To our surprise, the constituent (H₂C₆N₇O₃)⁻ building block is not in structurally optimal arrangement, but Ba(H₂C₆N₇O₃)₂ ⋅ 8 H₂O exhibits excellent optical properties including wide band gap of 4.10 eV, very large birefringence of 0.24@550 nm, and exceptionally strong second-harmonic generation (SHG) response of about 12×KH₂PO₄. Both the SHG response and birefringence are much larger than those of commercial NLO crystal β-BaB₂O₄ with optimally aligned (B₃O₆)³⁻ building block. Theoretical calculations suggest that the expanded π-conjugation delocalization within (H₂C₆N₇O₃)⁻ vs (B₃O₆)³⁻ should be responsible to the enhanced performance. This work implies that there is still much room to develop new NLO crystals with excellent functional building blocks that may be longly neglected.     

Perfectly Encoding π-Conjugated Anions in the RE5(C3N3O3)(OH)12 (RE=Y,Yb, Lu) Family with Strong Second Harmonic Generation Response and Balanced Birefringence

Nonlinear optical (NLO) crystal, which simultaneously exhibits strong second-harmonic-generation (SHG) response and desired optical anisotropy, is a core optical material accessible to the modern optoelectronics. Accompanied by strong SHG effect in a NLO crystal, a contradictory problem of overlarge birefringence is ignored, leading to low frequency doubling efficiency and poor beam quality. Herein, a series of rare earth cyanurates RE₅(C₃N₃O₃)(OH)₁₂ (RE=Y, Yb, Lu) were successfully characterized by 3D electron diffraction technique. Based on a “three birds with one stone” strategy, they enable the simultaneous fulfillment of strong SHG responses (2.5–4.2× KH₂PO₄), short UV cutoff (ca. 220 nm) and applicable birefringence (ca. 0.15 at 800 nm) by the introduction of rare earth coordination control of π-conjugated (C₃N₃O₃)³⁻ anions. These findings provide high-performance short-wavelength NLO materials and highlight the exploration of cyanurates as a new research area.     

Sb6O7(SO4)2: A Promising Ultraviolet Nonlinear Optical Material with an Enhanced Second-Harmonic-Generation Response Activated by SbIII Lone-Pair Stereoactivity

The stereochemical activity of lone pairs (SCALP) in a cation favors the formation of acentric materials and can enhance the second-harmonic-generation (SHG) response and/or the birefringence. By introducing functional Sb^III into sulfates, an anhydrous sulfate of Sb₆O₇(SO₄)₂ ( 1 ) is explored. Sb³⁺ cations are in seesaw configurations and in-phase aligned in a 3D asymmetric dense structure. Compound 1 exhibits an enhanced phase-matching SHG response, a moderate birefringence, a wide transparency window, and considerable environmental stabilities, which result in it being a promising UV nonlinear optical (NLO) material. Theoretical studies reveal that the stereoactive lone pairs on Sb³⁺ cations make the predominant contribution to the SHG effect. This work will attract more interest from scientists for research into SCALP-cation-based NLO materials.     

To improve the key properties of nonlinear optical crystals assembled with tetrahedral functional building units

Nonlinear optical (NLO) crystals assembled with conventional non-π-conjugated tetrahedral functional building units (FBUs), generally referring to [PO₄] and [BO₄], usually exhibit weak nonlinearity and poor birefringence. It is currently proposed that partially substituting oxygen atoms with fluoride atoms in these FBUs could enhance these crucial properties. Hence, we investigated for the first time the NLO-related properties of NH₄BAsO₄F (ABAF), which was constructed from tetrahedral [BO₃F] and [AsO₄] FBUs, and enhancements of these properties were observed in this material, that is large second-harmonic generation (SHG) response (2 × KDP) and improved birefringence (0.03 at 1064 nm). Notably, both SHG coefficient and birefringence of ABAF exceeded those of a great majority of phosphates, sulfates, or boron phosphates and achieved a preferable balance. It is interesting that ABAF shows vast structural similarities to the typical NLO crystals Sr₂Be₂B₂O₇ (SBBO) and KBe₂BO₃F₂ (KBBF), which might be the partial reason why it showed improvement in these vital properties. This work may afford some inspiration for enhancing the key performances of NLO crystals assembled with non-π-conjugated tetrahedra.

We report a new nonlinear optical crystal assembled exclusively with tetrahedral functional building units in which enhanced birefringence (Δn) and second-harmonic generation (d_eff) were observed.     

Ba4Ca(B2O5)2F2: π-conjugation of B2O5 in the planar pentagonal layer achieving large second harmonic generation of pyro-borate

The nonlinear optical (NLO) crystals that can expand the wavelength of the laser to the deep-ultraviolet (DUV) region by the cascaded second harmonic generation (SHG) are of current research interest. It is well known that borates are the most ideal material class for the design of new DUV NLO crystals owing to the presence of good NLO genes, e.g., BO₃ or B₃O₆ groups. However, the NLO pyro-borates with the B₂O₅ dimers as the sole basic building units are still rarely reported owing to their small SHG responses. In this communication, by constructing a planar pentagonal [Ca(B₂O₅)]_∞ layer, the NLO pyro-borate Ba₄Ca(B₂O₅)₂F₂ with a large SHG response (∼2.2 × KDP, or ∼7 × α-Li₄B₂O₅) and a DUV transparent window has been designed and synthesized. The first-principles calculations show that the large SHG response of Ba₄Ca(B₂O₅)₂F₂ mainly originates from the better π-conjugation of the coplanar B₂O₅ dimers in the [Ca(B₂O₅)]_∞ layer. In addition, the planar pentagonal pattern in the [Ca(B₂O₅)]_∞ layer provides an ideal template for designing the new DUV NLO crystals, apart from those in known DUV borates, e.g., the [Be₂BO₃F₂]_∞ layer in KBe₂BO₃F₂ (KBBF).

A new deep-UV NLO pyro-borate Ba₄Ca(B₂O₅)₂F₂ was synthesized by solid-state reactions. The better π-conjugation of B₂O₅ dimers in the planar pentagonal layer achieves a large SHG response (∼2.2 × KDP), which is the largest among all the known DUV transparent borates with B₂O₅ units.

Deep-ultraviolet (DUV, λ < 200 nm) coherent lights with high photon energy, high spatial resolution, and a small heat-affected zone are of significance for applications in photolithography, high-resolution spectroscopy, laser cooling, and scientific equipment.^1–4 However, it is difficult or well-nigh impossible for solid-state lasers to directly radiate the DUV coherent lights. In contrast, relying on the process of second harmonic generation (SHG) of nonlinear optical (NLO) crystals is a more effective way to generate the DUV coherent lights and causes much attention.^5,6 Therefore, the NLO crystal has become an important material basis of solid-state lasers, which seriously affects the development of all-solid-state laser technology. However, it is still a great challenge to rationally design and synthesize DUV NLO crystals because of the extremely rigorous requirements of structural symmetry and properties.^7–10 Structurally, the DUV NLO crystals must crystallize in the noncentrosymmetric (NCS) space groups which are the prerequisite for the materials to exhibit SHG responses. Moreover, it should possess a broad transparency window, a largely effective NLO coefficient (d_eff ≥ 0.39 pm V⁻¹), and a moderate birefringence (0.05–0.10@1064 nm) to achieve the phase-matching (PM) conditions in the DUV region.¹⁰ Based on these requirements, borates have been considered as the ideal material class for DUV NLO crystals because of their special structure and properties'' virtues, including the rich acentric structural types, large band gaps, and stable physical and chemical properties.⁸ To date, the commercialized borate-based UV NLO crystals consist of β-BaB₂O₄ (BBO), LiB₃O₅ (LBO), CsLiB₆O₁₀ (CLBO),^9,10 and the practical DUV NLO crystal KBe₂BO₃F₂ (KBBF). Especially for KBBF, it has become the sole material that can generate DUV coherent laser light (177.3 nm) by a direct SHG method.⁷ Other excellent borate-based UV NLO crystals also consist of K₃B₆O₁₀Cl,¹¹ SrB₅O₇F₃,¹² Li₂B₆O₉F₂,⁵ CsAlB₃O₆F,¹³ M₂B₁₀O₁₄F₆ (M = Ca, Sr),¹⁴ NH₄B₄O₆F,¹⁵ NaSr₃Be₃B₃O₉F₄,¹⁶ AB₄O₆F (A = K, Rb, and Cs),¹⁷etc.The above borate-based materials have achieved great success as UV and DUV NLO crystals, which are mainly attributed to the ability of boron atoms to coordinate with three or four oxygen anions forming trigonal-planar or tetrahedral building blocks.^18,19 For example, the first borate-based NLO crystal, KB₅O₈·4H₂O (KB₅), has the basic building units (BBUs) of [B₅O₁₀], while the BBUs of β-BBO, LBO, and KBBF are [B₃O₆], [B₃O₇], and isolated [BO₃], respectively.^7,8 Remarkably, although various borate crystals with different types of borate groups have been explored during the past decades, the pyro-borate NLO crystals with B₂O₅ groups as the sole BBUs are rarely reported owing to their weak SHG responses.^20–23 For example, the SHG response of the DUV transparent α-Li₄B₂O₅ (ref. 23) is only ∼0.3 × KDP, which is far smaller than the expected value (0.39 pm V⁻¹, 1 × KDP).Actually, the flexible B₂O₅ groups which are composed of two π-conjugated BO₃ units through corner-sharing may also be capable of generating excellent optical performance if they have benign arrangements. In recent research, Pan''s group has indicated that the B₂O₅ dimers are perfect for the design of DUV birefringent crystals. By the synergistic combination, they have successfully designed a potential pyro-borate birefringent crystal, Li₂Na₂B₂O₅, with a short UV cut-off edge (181 nm) and large birefringence (0.095@532 nm).²¹ And they have also grown Ca(BO₂)₂ crystals exhibiting a short UV cut-off edge and larger birefringence (169 nm; 0.2471@193 nm). Based on the analysis of the structure–property relationship of Ca(BO₂)₂, they stated that the polymerized planar B_nO_2n+1 groups, e.g., B₂O₅, could generate a larger anisotropy than isolated BO₃.²² However, their opposite arrangements of B–O groups make them crystallize in the centrosymmetric (CS) space groups, which limit their further development as NLO compounds. Thus, it is clear that pyro-borates exhibiting a large birefringence and a short UV cut-off edge would also be promising DUV NLO crystals if their SHG responses can be enhanced.Based on the above-mentioned ideas, a systematical investigation has been performed on DUV pyroborates. And finally, we successfully synthesized a new NCS pyro-borate, Ba₄Ca(B₂O₅)₂F₂, which can exhibit not only a large SHG response (∼2.2 × KDP and ∼7 × α-Li₄B₂O₅) but also a short UV cut-off edge (<190 nm). Analyzing its structure, one can find that its excellent NLO properties mainly originate from the unique planar pentagonal [Ca(B₂O₅)]_∞ layer, where the B₂O₅ groups adopt the almost coplanar configurations that favor the structure to generate large SHG response and birefringence,²¹ meanwhile the terminal O atoms of B₂O₅ groups are also linked by the Ca²⁺ cations, which eliminate the dangling bonds of B₂O₅ groups and further blue-shift the UV cut-off edge. More importantly, the adjacent [Ca(B₂O₅)]_∞ layers in Ba₄Ca(B₂O₅)₂F₂ are linked by other B₂O₅ groups to form a 3D framework, which will be favorable for the material to avoid the layer habit that KBBF suffers from. In this sense, the planar pentagonal [Ca(B₂O₅)]_∞ layer is similar to the [Be₂BO₃F₂]_∞ layer in KBBF, and it can be seen as a new structure template for the design of new DUV NLO crystals, especially for the DUV pyro-borates. Herein, we will describe the synthesis, experimental and computational characterization as well as the functional properties of the new DUV NLO material, Ba₄Ca(B₂O₅)₂F₂.A polycrystalline sample of Ba₄Ca(B₂O₅)₂F₂ was synthesized by the conventional solid-state reaction and the purity was confirmed by powder X-ray diffraction (XRD) (Fig. S1^†). With the polycrystalline sample, the thermal behavior of Ba₄Ca(B₂O₅)₂F₂ was studied by the thermogravimetric (TG) and differential scanning calorimetry (DSC) measurements. The heating DSC curve shows a sharp endothermic peak at 815 °C with no obvious weight loss in the TG curve (Fig. S2^†), suggesting that Ba₄Ca(B₂O₅)₂F₂ has good thermal stability. To further investigate the thermal behavior of Ba₄Ca(B₂O₅)₂F₂, the polycrystalline sample was calcined at 840 °C and the XRD analysis showed that the calcined sample was Ba₄Ca(B₂O₅)₂F₂, Ba₂Ca(BO₃)₂ (PDF #01-085-2268), Ba₂CaB₆O₁₂ (PDF #01-075-1401) and other unknown phases (Fig. S3^†). These results illustrate that Ba₄Ca(B₂O₅)₂F₂ melts incongruently and the suitable flux is necessary for the crystal growth.With the Na₂O–PbF₂–B₂O₃ as the flux, millimeter-sized block crystals of Ba₄Ca(B₂O₅)₂F₂ were grown for the single-crystal XRD structure determination. Ba₄Ca(B₂O₅)₂F₂ crystallizes in the NCS and polar space group, P2₁ (Table S1^†). In the asymmetric unit, there are four unique Ba, one Ca, four B, ten O, and two F atom(s), which all fully occupy the 2a Wyckoff positions (Table S2^†). All B atoms are coordinated to three oxygen atoms to form the BO₃ triangles with the B–O distances ranging from 1.312(17) to 1.460(16) Å and O–B–O angles varying from 108.0(13) to 130.2(15)°. The BO₃ triangles are further connected to form two types of B₂O₅ dimers, i.e. plane B(1,3)₂O₅ and twisted B(2,4)₂O₅, which are the BBUs of Ba₄Ca(B₂O₅)₂F₂. The Ca atoms are coordinated to six oxygen atoms to form CaO₆ octahedra with the Ca–O distances ranging from 2.285(9) to 2.325(13) Å. For the Ba²⁺ cations, they exhibit three different coordination environments, Ba(1,2)O₆F₂, Ba(3)O₈F₂, and Ba(4)O₇F₂ (Fig. S4^†) with the Ba–O distances ranging from 2.585(9) to 3.250(11) Å and the Ba–F bond lengths ranging from 2.635(8) to 2.736(8) Å. Remarkably, for the F⁻ anions, each unique fluorine atom serves as a common vertex for four Ba atoms to form the FBa₄ polyhedra (Fig. S5a^†), which could be treated as fluorine-centered secondary building units (SBUs). The Ba–F–Ba angles vary from 99.0 (2) to 120.2 (3)°. The bond valence sum (BVS) calculations show the values of 1.67–1.97, 2.45, 2.88–3.10, 1.78–2.13, and 0.95–1.09, for Ba²⁺, Ca²⁺ B³⁺, O²⁻, and F⁻, respectively (Table S2^†). The BVSs of atoms are consistent with their expected oxidation states except the one from the Ca²⁺ cations. The larger BVSs of Ca²⁺ cations can be attributed to six shorter Ca–O bond lengths, which are also observed in other Ca²⁺-containing borates, such as YCa₃(VO)₃(BO₃)₄ (2.44),²⁴ Rb₂Ca₃B₁₆O₂₈ (2.29), and Cs₂Ca₃B₁₆O₂₈ (2.30).²⁵The structure of Ba₄Ca(B₂O₅)₂F₂ is shown in Fig. 1. In the structure, the plane B(1,3)₂O₅ dimer is first connected with four CaO₆ octahedra, meanwhile, each CaO₆ octahedron is also linked by four B(1,3)₂O₅ dimers through sharing their four equatorial O atoms to form a unique planar pentagonal [Ca(B₂O₅)]_∞ layer in the b–c plane (Fig. 1a, b). Then, these [Ca(B₂O₅)]_∞ layers are further linked by the twisted B(2,4)₂O₅ dimers to construct a 3D framework with Ba²⁺ cations maintaining the charge balance (Fig. 1c). Remarkably, for the arrangements of the Ba²⁺ cations and the F⁻ anions, the fluorine-centered SBU FBa₄ polyhedra are linked to construct the 2D [F₂Ba₄] infinite layer (Fig. S5b^†) with the same orientation, which further fills the apertures in the [Ca(B₂O₅)₂] framework (Fig. S5c^†). The existence of fluorine-centered SBUs would certainly have a strong influence on the local coordinate environments, and finally on the whole structure.²⁶Open in a separate windowFig. 1(a) The [Ca(B₂O₅)] layer is composed of B₂O₅ dimers and CaO₆ octahedra. (b) The planar pentagonal topology layer. The comparison of structures between (c) Ba₄Ca(B₂O₅)₂F₂ and (d) KBBF.It is very interesting that Ba₄Ca(B₂O₅)₂F₂ contains a planar pentagonal [Ca(B₂O₅)]_∞ layer, which is similar to the [Be₂BO₃F₂]_∞ layer in KBBF. The structural evolution from KBBF to Ba₄Ca(B₂O₅)₂F₂ is also shown in Fig. 1c and d. In KBBF, the BBUs are the planar BO₃ triangles, which are connected with BeO₃F in the a–b plane by strong covalent bonds to form the [Be₂BO₃F₂]_∞ layers (Fig. S6c^†) and the [Be₂BO₃F₂]_∞ layers have achieved excellent NLO properties of the KBBF crystal.⁷ However in Ba₄Ca(B₂O₅)₂F₂, the BO₃ triangles are changed into the B₂O₅ dimers, and the BeO₃F tetrahedra are substituted by the CaO₆ polyhedra. These B₂O₅ dimers are also connected by the CaO₆ polyhedra to form the interesting planar pentagonal [Ca(B₂O₅)]_∞ layer (Fig. S6d^†). More importantly, in KBBF, the adjacent [Be₂BO₃F₂]_∞ layers are connected by the weak K⁺-F⁻ ionic bonds that results in the strong layer habit of the KBBF crystals, whereas in Ba₄Ca(B₂O₅)₂F₂, the [Ca(B₂O₅)]_∞ layers are bridged by the strong covalent B–O bonds to form a stable 3D framework, which will greatly overcome the layering tendency of the KBBF crystal and facilitate the crystal growth.In addition, we also notice that the planar pentagonal [Ca(B₂O₅)]_∞ layer maybe helpful for enhancing the SHG responses of pyro-borates because small SHG responses of pyro-borates are attributed to the typical twisted configurations of the B₂O₅ groups, which are unfavorable for forming the π-conjugation and the superposition of the microscopic SHG response. For example, α-Li₄B₂O₅, a DUV transparent pyro-borate with sole B₂O₅ units as the BBUs, has a weak SHG response, which may be derived from the twisted B₂O₅ groups and non-planar arrangements (Fig. S7a^†). However, in Ba₄Ca(B₂O₅)₂F₂, the planar configuration of the pentagonal layers can assist the B₂O₅ groups to adopt a nearly coplanar arrangement (Fig. S7b^†) and effectively enhance the π-conjugation of B₂O₅ groups. The better π-conjugation of the planar B₂O₅ groups in the planar pentagonal [Ca(B₂O₅)]_∞ layer has also been confirmed by the electron orbital calculation based on the first-principles calculations.²⁷ The calculated result is shown in Fig. 2. Clearly, the prominent conjugated interactions are observed in the nearly coplanar B(1,3)₂O₅ dimers of Ba₄Ca(B₂O₅)₂F₂ (Fig. 2a), whereas it does little in the twisted B(2,4)₂O₅ dimers of Ba₄Ca(B₂O₅)₂F₂ (Fig. 2b) and two types of twisted B₂O₅ dimers in α-Li₄B₂O₅ (Fig. 2c and d). It can be expected that the nearly coplanar B₂O₅ dimers are more conducive to the large SHG response than the twisted B₂O₅ dimers. Remarkably, the similar pentagonal layers are also observed in other pyro-phosphates, such as Ba₂NaClP₂O₇, K₂Sb(P₂O₇)F, Rb₃PbBi(P₂O₇)₂, and Rb₃BaBi(P₂O₇)₂. Clearly, as pyro-phosphates are the non-π-conjugated systems, the planar pentagonal layers are only helpful for the orientation of anion groups.^28–31 However, they cannot form the better π-conjugation. Therefore, the better π-conjugation of the nearly coplanar B₂O₅ groups in planar pentagonal layers of pyro-borate Ba₄Ca(B₂O₅)₂F₂ would have a different contributing mechanism to the SHG effect with other non-π-conjugated pyro-phosphates.Open in a separate windowFig. 2The orbitals of the nearly coplanar B(1,3)₂O₅ (a) and twisted B(2,4)₂O₅ dimers (b) in Ba₄Ca(B₂O₅)₂F₂. The orbitals of two twisted B₂O₅ dimers (c and d) in α-Li₄B₂O₅.The presence of BO₃ triangles in Ba₄Ca(B₂O₅)₂F₂ is confirmed by the IR spectral measurements (Fig. S8^†). The peaks at 1362 cm⁻¹ and 1208 cm⁻¹ can be attributed to the asymmetric stretching of BO₃ groups.³² A strong band at 1069 cm⁻¹ in the IR spectrum may be associated with the stretching vibration of B–O–B in B₂O₅.^33,34 The weak absorption bands at 950, and 810 cm⁻¹ correspond to the symmetrical stretching vibrations of BO₃ and B–O–B in B₂O₅, respectively. The peaks at 751 and 615 cm⁻¹ can be attributed to the out-of-plane bending of the BO₃ groups.³⁴ Further, the UV-vis-NIR diffuse reflectance spectrum was also measured (Fig. S9^†), which shows that Ba₄Ca(B₂O₅)₂F₂ is transparent down to the DUV region with a UV cut-off edge less than 190 nm (corresponding to a large band gap of 6.2 eV), which is comparable to the newly developed NLO-active borates, such as RbB₃O₄F₂ (<190 nm), CsZn₂BO₃X₂ (X₂ = F₂,Cl₂, and FCl)) (<190 nm) and so on.^35–38 The short cut-off edge demonstrates the potential application of Ba₄Ca(B₂O₅)₂F₂ as a DUV NLO crystal.As Ba₄Ca(B₂O₅)₂F₂ crystalizes in the NCS space group P2₁, it possesses the SHG response, which was measured by the Kurtz-Perry method with the well-known NLO material KH₂PO₄ (KDP) as a reference.³⁹ As shown in Fig. 3, the SHG intensities of Ba₄Ca(B₂O₅)₂F₂ increase with the increase of particle sizes, indicating that Ba₄Ca(B₂O₅)₂F₂ is type-I phase-matchable. The SHG intensity of Ba₄Ca(B₂O₅)₂F₂ at the particle size of 150–212 μm is about 2.2 times that of KDP, and is larger than that of KBBF (1.2 × KDP) or comparable with those newly reported UV NLO crystals, i.e. γ-Be₂BO₃F (2.3 × KDP),⁶ β-Rb₂Al₂B₂O₇ (2 × KDP),⁴⁰ Li₄Sr(BO₃)₂ (2 × KDP),⁴¹ CsB₄O₆F(∼1.9 × KDP).² In addition, as we know, the SHG response of Ba₄Ca(B₂O₅)₂F₂ is the largest among all the known DUV transparent borates with B₂O₅ units (Table S4^†). Its SHG response (2.2 × KDP) is about seven times larger than that of α-Li₄B₂O₅ (0.3 × KDP), another DUV transparent pyro-borate with sole B₂O₅ units.Open in a separate windowFig. 3(a) Phase-matching curve, i.e., particle size vs. SHG intensity, data for Ba₄Ca(B₂O₅)₂F₂ and KH₂PO₄ (KDP) as reference. The solid curve is a guide for the eye, not a fit to the data. (b) Oscilloscope traces showing SHG intensities for Ba₄Ca(B₂O₅)₂F₂ and KDP.To understand the origin of the excellent optical properties of Ba₄Ca(B₂O₅)₂F₂, we also carried out the first-principles calculations.²⁷ It shows that Ba₄Ca(B₂O₅)₂F₂ has an indirect band gap of 6.34 eV (Figures S10a^†), which is in accordance with the experimental results. The valence band maximum (VBM) of Ba₄Ca(B₂O₅)₂F₂ is mainly composed of the orbitals in Ba, and O atoms, while the conduction band minimum (CBM) is dominantly composed of the orbitals in Ba, B, and O atoms. Therefore, the band gap of Ba₄Ca(B₂O₅)₂F₂ is mainly determined by Ba atoms and B₂O₅ groups. Based on the calculated electron structure, the NLO coefficients of Ba₄Ca(B₂O₅)₂F₂ are also calculated. The largest NLO coefficient of Ba₄Ca(B₂O₅)₂F₂ is d₂₂ = −0.524 pm V⁻¹, which is about 5 times lower than that of α-Li₄B₂O₅ (d₂₄ = −0.102 pm V⁻¹) (Table S5a^†), which is matched with the experimental one. Further, the SHG-weighted density maps of Ba₄Ca(B₂O₅)₂F₂ are shown in Fig. 4. These reveal that B₂O₅ dimers make the dominant contribution (72.7%) to the total SHG effect (Table S5b^†). The band-resolved SHG analysis can also conclude that B–O orbitals in Ba₄Ca(B₂O₅)₂F₂ contribute more to the SHG response than those in α-Li₄B₂O₅ (Fig. S10b, S10c^†), indicating that the arrangements of B₂O₅ dimers in Ba₄Ca(B₂O₅)₂F₂ is more beneficial for the large SHG response. And different from α-Li₄B₂O₅, F-centered secondary building units (SBUs) exist in the structure of Ba₄Ca(B₂O₅)₂F₂, and they are further linked to construct 2D [F₂Ba₄]_∞ infinite layers, which could help B₂O₅ groups arrange in a planar pattern (Fig. S5^†).²⁶ So, based on the above analysis, we can conclude that the nearly coplanar B₂O₅ dimers in the planar pentagonal layer and the SBU FBa₄ tetrahedra make a significant contribution to the SHG response of Ba₄Ca(B₂O₅)₂F₂.Open in a separate windowFig. 4The SHG-weighted density maps of the virtual electron process (a) and virtual hole process (b) of d₂₂ for Ba₄Ca(B₂O₅)₂F₂.     

K3V2O3F4(IO3)3: a high-performance SHG crystal containing both five and six-coordinated V5+ cations

The combination of d⁰ transition metal oxofluorides with iodate anions helps to synthesize polar crystals. Herein, a novel polar crystal, K₃V₂O₃F₄(IO₃)₃, which is the first metal vanadium iodate with two types of V⁵⁺-centered polyhedra (VO₄F₂ octahedron and VO₃F₂ trigonal bipyramid), has been prepared hydrothermally. It crystallizes in the polar space group of Cmc2₁ and its structure displays an unprecedented 0D [V₂O₃F₄(IO₃)₃]³⁻ anion, which is composed of Λ-shaped cis-[VO₂F₂(IO₃)₂]³⁻ and [VO₂F₂(IO₃)]²⁻ anions interconnected via the corner-sharing of one oxo anion. The synergy gained from the VO₄F₂, VO₃F₂ and IO₃ groups resulted in K₃V₂O₃F₄(IO₃)₃ exhibiting both a strong second-harmonic generation (SHG) response (1.3 × KTiOPO₄) under 2050 nm laser irradiation and a large birefringence (0.158 @ 2050 nm). This study provides a facile route for designing SHG materials by assembling various vanadium oxide-fluoride motifs and iodate anions into one compound.

K₃V₂O₃F₄(IO₃)₃, which is the first metal vanadium iodate containing two different V-centered polyhedra, exhibits a strong SHG effect of 1.3 × KTP and a large birefringence of 0.158 @ 2050 nm.     

Rational Design of the Metal-Free KBe2BO3F2⋅(KBBF) Family Member C(NH2)3SO3F with Ultraviolet Optical Nonlinearity

The first fluorosulfonic ultraviolet (UV) nonlinear optical (NLO) material, C(NH₂)₃SO₃F, is rationally designed by taking KBe₂BO₃F₂ (KBBF) as the parent compound. C(NH₂)₃SO₃F features similar topological layers as KBBF by replacing inorganic (BO₃)³⁻ with organic C(NH₂)₃⁺ trigonal units and BeO₃F with SO₃F⁻ tetrahedra. Therefore, C(NH₂)₃SO₃F is a metal-free UV NLO crystal. Benefiting from the coplanar configuration of the C(NH₂)₃⁺ cationic groups, it possesses a large SHG response of 5×KDP and moderate birefringence of 0.133@1064 nm. Besides, it has a short UV cutoff edge of 200 nm. The calculated results reveal the shortest SHG phase-matching wavelengths can reach 200 nm. These findings highlight the exploration of metal-free compounds as nontoxic and low-cost UV NLO materials as a new research area.     

Rational Design of the Metal‐Free KBe2BO3F2⋅(KBBF) Family Member C(NH2)3SO3F with Ultraviolet Optical Nonlinearity

The first fluorosulfonic ultraviolet (UV) nonlinear optical (NLO) material, C(NH₂)₃SO₃F, is rationally designed by taking KBe₂BO₃F₂ (KBBF) as the parent compound. C(NH₂)₃SO₃F features similar topological layers as KBBF by replacing inorganic (BO₃)^3? with organic C(NH₂)₃⁺ trigonal units and BeO₃F with SO₃F^? tetrahedra. Therefore, C(NH₂)₃SO₃F is a metal‐free UV NLO crystal. Benefiting from the coplanar configuration of the C(NH₂)₃⁺ cationic groups, it possesses a large SHG response of 5×KDP and moderate birefringence of 0.133@1064 nm. Besides, it has a short UV cutoff edge of 200 nm. The calculated results reveal the shortest SHG phase‐matching wavelengths can reach 200 nm. These findings highlight the exploration of metal‐free compounds as nontoxic and low‐cost UV NLO materials as a new research area.     

Rational design of a promising oxychalcogenide infrared nonlinear optical crystal

Oxychalcogenides with the performance-advantages of both chalcogenides and oxides are emerging materials class for infrared (IR) nonlinear optical (NLO) crystals that can expand the wavelength of solid-state lasers to IR regions and are of importance in industrial and civil applications. But rationally designing a high-performance oxychalcogenide NLO crystal remains a great challenge. Herein, we chose the melilite-type Sr₂ZnSi₂O₇ as the structure template. Through part isovalent substitution of S²⁻ for O²⁻ anions, the first hetero-anionic thiostannate Sr₂ZnSn₂OS₆ with wide IR transmission has been synthesized. More importantly, compared to the maternal oxide, Sr₂ZnSi₂O₇, the second harmonic generation (SHG) response of Sr₂ZnSn₂OS₆ is enhanced by two orders of magnitude. In addition, Sr₂ZnSn₂OS₆ can exhibit a large band-gap and high laser damage threshold. These advantages make Sr₂ZnSn₂OS₆ a promising IR NLO crystal. Our research will provide insights into the rational design of new IR NLO crystals.

Based on heteroanionic structure engineering, the first heteroanionic thiostannate has been successfully synthesized. It is a promising IR-NLO material exhibiting moderate second-harmonic generation and wide IR transmission.     

Syntheses,characterization and nonlinear optical properties of a bismuth subcarbonate Bi2O2CO3

A novel nonlinear optical (NLO) material Bi₂O₂CO₃ has been successfully developed by the hydrothermal method. It was characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectra, high resolution transmission electron microscopy (HRTEM), UV–vis–NIR diffuse reflectance spectrum (DRS) and photoluminescence (PL) spectra. The band gap was determined to be 3.42 eV, and the PL properties of Eu³⁺ doped Bi₂O₂CO₃ under UV excitation have also been investigated. The powder second-harmonic generation (SHG) measurement performed on the ground crystal indicates that the NLO efficiency is approximately 5 times as large as that of KDP (KH₂PO₄) standard. In addition, the origin of large SHG for Bi₂O₂CO₃ was also researched according to its crystal structure.     

[GaF(H2O)][IO3F]: a promising NLO material obtained by anisotropic polycation substitution

A novel salt-inclusion fluoroiodate [GaF(H₂O)][IO₃F] derived from CsIO₂F₂ was ingeniously obtained through anisotropic polycation substitution. Because the catenulate [GaF(H₂O)]²⁺ framework serves as a template for the favorable assembly of the polar [IO₃F]²⁻ groups and contributes to the nonlinear coefficient, [GaF(H₂O)][IO₃F] exhibits a greatly improved second-harmonic generation (SHG) effect of 10 times that of KH₂PO₄ (KDP) and a considerable band gap of 4.34 eV compared to the parent compound CsIO₂F₂ (3 × KDP, 4.5 eV). Particularly, to the best of our knowledge, [GaF(H₂O)][IO₃F] has the largest laser-induced damage threshold (LDT) of 140 × AgGgS₂ of the reported iodates. All these results signify that [GaF(H₂O)][IO₃F] is a promising nonlinear optical (NLO) crystal. This work also proposes that anisotropic polycation substitution is an effective approach to optimize the SHG effect and develop excellent NLO materials.

A novel salt-inclusion fluoroiodate nonlinear optical material, [GaF(H₂O)][IO₃F], is derived from CsIO₂F₂ through polycation substitution, and it exhibits an intense SHG effect of 10 times that of KH₂PO₄ and improved overall performance.     

The First UV Nonlinear Optical Selenite Material: Fluorination Control in CaYF(SeO3)2 and Y3F(SeO3)4

It is a great challenge to develop UV nonlinear optical (NLO) material due to the demanding conditions of strong second harmonic generation (SHG) intensity and wide band gap. The first ultraviolet NLO selenite material, Y₃F(SeO₃)₄, has been obtained by control of the fluorine content in a centrosymmetric CaYF(SeO₃)₂. The two new compounds represent similar 3D structures composed of 3D yttrium open frameworks strengthened by selenite groups. CaYF(SeO₃)₂ has a large birefringence (0.138@532 nm and 0.127@1064 nm) and a wide optical band gap (5.06 eV). The non-centrosymmetric Y₃F(SeO₃)₄ can exhibit strong SHG intensity (5.5×KDP@1064 nm), wide band gap (5.03 eV), short UV cut-off edge (204 nm) and high thermal stability (690 °C). So, Y₃F(SeO₃)₄ is a new UV NLO material with excellent comprehensive properties. Our work shows that it is an effective method to develop new UV NLO selenite material by fluorination control of the centrosymmetric compounds.     

Syntheses,characterization and nonlinear optical properties of sodium–scandium carbonate Na5Sc(CO3)4·2H2O

A novel nonlinear optical (NLO) material Na₅Sc(CO₃)₄·2H₂O has been synthesized under a subcritical hydrothermal condition. The structure is determined by single-crystal X-ray diffraction and further characterized by TG analyses and UV–vis–NIR diffuse reflectance spectrum. It crystallizes in the tetragonal space group P-421c, with a = b = 7.4622(6) Å, C = 11.5928(15) Å. The Second-harmonic generation (SHG) on polycrystalline samples was measured using the Kurtz and Perry technique, which indicated that Na₅Sc(CO₃)₄·2H₂O was a phase-matchable material, and its measured SHG coefficient was about 1.8 times as large as that of d₃₆ (KDP). The results from the UV–vis diffuse reflectance spectroscopy study of the powder samples indicated that the short-wavelength absorption edges of Na₅Sc(CO₃)₄·2H₂O is about 220 nm, suggesting that this crystal is a promising UV nonlinear optical (NLO) materials.     

Quadruple-Bidentate Nitrate-Ligated A2Hg(NO3)4 (A=K,Rb): Strong Second-Harmonic Generation and Sufficient Birefringence

The design of efficient nonlinear optical (NLO) crystals continues to pose significant challenges due to the difficulty of assembling polar NLO-active modules in an optimal additive fashion. We report herein the first NLO-active mercuric nitrates A₂Hg(NO₃)₄ (A=(KHNO), Rb (RHNO)), for which assembly is induced by ionic polarization of the d¹⁰ cations. The two new crystalline compounds are isostructural, featuring interesting pseudo-diamond-like structures with parallel [Hg(NO₃)₄] modules, and leading to strong powder second-harmonic generation (SHG) responses of 9.2 (KHNO) and 8.8 (RHNO) times that of KH₂PO₄. In combination with the simple solution preparation of centimeter-scale crystals, sufficient birefringence, and short ultraviolet (UV) cutoff edges, these attributes make KHNO and RHNO promising candidates for UV NLO materials. Theoretical calculations and single-crystal structure analysis reveal that the newly-developed highly condensed and distorted [Hg(NO₃)₄] module, with an Hg²⁺ cation that is quadruply bidentate nitrate-ligated, is crucial for the significant SHG responses. This work highlights the potential importance of modules with multiple bidentate ligands for the development of high-performing next-generation NLO materials.     

Two Pyrophosphates with Large Birefringences and Second‐Harmonic Responses as Ultraviolet Nonlinear Optical Materials

Two new pyrophosphates nonlinear optical (NLO) materials, Rb₃PbBi(P₂O₇)₂ ( I ) and Cs₃PbBi(P₂O₇)₂ ( II ), were successfully designed and synthesized. Both compounds exhibit large NLO effects and birefringences. Material I presents the scarce case of possessing the coexistence of large birefringence (0.031 at 1064 nm and 0.037 at 532 nm) and second harmonic generation (SHG) response (2.8× potassium dihydrogen phosphate (KDP)) in ultraviolet NLO phosphates and its SHG is the largest in the phase‐matching (PM) pyrophosphates. Both I and II have three‐dimensional (3D) crystal structures composed of corner‐shared RbO₁₂ (CsO₁₁), RbO₁₀ (CsO₁₀), BiO₆, PbO₇ (PbO₆) and P₂O₇ groups, in which P₂O₇ and PbO₇ (PbO₆) units form an alveolate [PbPO]_∞ skeleton frame. Theoretical calculations reveal that the P?O, Bi?O and Pb?O units are mainly responsible for the moderate birefringence and large SHG efficiency of I .     

Two Pyrophosphates with Large Birefringences and Second-Harmonic Responses as Ultraviolet Nonlinear Optical Materials

Two new pyrophosphates nonlinear optical (NLO) materials, Rb₃PbBi(P₂O₇)₂ ( I ) and Cs₃PbBi(P₂O₇)₂ ( II ), were successfully designed and synthesized. Both compounds exhibit large NLO effects and birefringences. Material I presents the scarce case of possessing the coexistence of large birefringence (0.031 at 1064 nm and 0.037 at 532 nm) and second harmonic generation (SHG) response (2.8× potassium dihydrogen phosphate (KDP)) in ultraviolet NLO phosphates and its SHG is the largest in the phase-matching (PM) pyrophosphates. Both I and II have three-dimensional (3D) crystal structures composed of corner-shared RbO₁₂ (CsO₁₁), RbO₁₀ (CsO₁₀), BiO₆, PbO₇ (PbO₆) and P₂O₇ groups, in which P₂O₇ and PbO₇ (PbO₆) units form an alveolate [PbPO]_∞ skeleton frame. Theoretical calculations reveal that the P−O, Bi−O and Pb−O units are mainly responsible for the moderate birefringence and large SHG efficiency of I .     

Self-Assembly of Antiferromagnetically-Coupled Copper(II) Supramolecular Architectures with Diverse Structural Complexities

The self-assembly of 2,6-diformyl-4-methylphenol (DFMP) and 1-amino-2-propanol (AP)/2-amino-1,3-propanediol (APD) in the presence of copper(II) ions results in the formation of six new supramolecular architectures containing two versatile double Schiff base ligands (H₃L and H₅L1) with one-, two-, or three-dimensional structures involving diverse nuclearities: tetranuclear [Cu₄(HL²⁻)₂(N₃)₄]·4CH₃OH·56H₂O (1) and [Cu₄(L³⁻)₂(OH)₂(H₂O)₂] (2), dinuclear [Cu₂(H₃L1²⁻)(N₃)(H₂O)(NO₃)] (3), polynuclear {[Cu₂(H₃L1²⁻)(H₂O)(BF₄)(N₃)]·H₂O}_n (4), heptanuclear [Cu₇(H₃L1²⁻)₂(O)₂(C₆H₅CO₂)₆]·6CH₃OH·44H₂O (5), and decanuclear [Cu₁₀(H₃L1²⁻)₄(O)₂(OH)₂(C₆H₅CO₂)₄] (C₆H₅CO₂)₂·20H₂O (6). X-ray studies have revealed that the basic building block in 1, 3, and 4 is comprised of two copper centers bridged through one μ-phenolate oxygen atom from HL²⁻ or H₃L1²⁻, and one μ-1,1-azido (N₃⁻) ion and in 2, 5, and 6 by μ-phenoxide oxygen of L³⁻ or H₃L1²⁻ and μ-O²⁻ or μ₃-O²⁻ ions. H-bonding involving coordinated/uncoordinated hydroxy groups of the ligands generates fascinating supramolecular architectures with 1D-single chains (1 and 6), 2D-sheets (3), and 3D-structures (4). In 5, benzoate ions display four different coordination modes, which, in our opinion, is unprecedented and constitutes a new discovery. In 1, 3, and 5, Cu(II) ions in [Cu₂] units are antiferromagnetically coupled, with J ranging from −177 to −278 cm⁻¹.     

Cation‐Tuned Synthesis of Fluorooxoborates: Towards Optimal Deep‐Ultraviolet Nonlinear Optical Materials

The development of new nonlinear optical (NLO) materials for deep‐ultraviolet (DUV) applications is in great demand. However, the synthesis of an ideal DUV NLO crystal is a serious challenge. Herein, three new alkali‐metal fluorooxoborates, AB₄O₆F (A=K, Rb, and Cs, and a mixed cation between two of them), were successfully synthesized by cation regulation. It is found that all reported compounds exhibit short UV absorption edges (<190 nm), and show second harmonic generation (SHG) responses ranging from 0.8 to 1.9 KH₂PO₄ (KDP). Interestingly, by judicious selection of the A‐site alkali‐metal cations, the arrangement of NLO‐active structural units is fine‐tuned to an optimal configuration, which contributes to large SHG responses.