Short Cutoff Edge Research Articles

Flux Growth and Properties of Volatile Bromine-Containing UV Nonlinear Optical Crystal K3B6O10Br

A UV Nonlinear optical (NLO) crystal is one of the key devices in all-solid-state laser technology, and borate halides show outstanding potential due to their abundant structural diversity and short UV cut-off edges. In this article, the sizable UV NLO crystal of K3B6O10Br (KBOB) has been grown with lead-containing and lead-free fluxes systems using the high-temperature top-seeded solution growth (TSSG) method. Energy Dispersive X-ray Spectroscopy (EDS) and transmittance spectra illustrate the influence of Pb2+ ions on the transmittance properties and laser-induced damage threshold (LDT). The thermal property, namely, thermal expansion, thermal conductivity, and thermal diffusivity curves, were characterized. Moreover, a small variation of thermal refractive indexes was analyzed to illustrate the advantage of KBOB in the application for temperature-fluctuated specific regions.

Strong SHG Responses in a Beryllium‐Free Deep‐UV‐Transparent Hydroxyborate via Covalent Bond Modification

Deep-ultraviolet (deep-UV) nonlinear optical (NLO) crystals are key materials in creating tunable deep-UV lasers for frequency conversion technology. However, practical application of the sole usable crystal, KBe2 BO3 F2 , has been hindered by the high toxicity of beryllium and its layering tendency in crystal growth. Herein, we report a beryllium-free deep-UV NLO material NaSr3 (OH)(B9 O16 )[B(OH)4 ] (NSBOH), synthesized by a covalent bond modification strategy under hydrothermal conditions. Moisture-stable NSBOH exhibits strong second-harmonic generation (SHG) at 1064 nm (3.3 × KH2 PO4 ) and 532 nm (0.55 × β-BaB2 O4 ), both amongst the largest powder SHG responses for a deep-UV borate, with good phase-matchability and a short wavelength cutoff edge (below 190 nm). NSBOH possesses a 3D covalent anionic [B9 O19 ]∞ honeycomb-like framework with no layering. The Sr2+ and Na+ ions, residing in the cavities of the anionic framework, act as templates for the assembly and favorable alignment of NLO-active groups, resulting in an optimal balance between strong SHG activities and wide UV transparency. These merits indicate NSBOH is a very attractive candidate for deep-UV NLO applications.

Strong SHG Responses in a Beryllium‐Free Deep‐UV‐Transparent Hydroxyborate via Covalent Bond Modification

AbstractDeep‐ultraviolet (deep‐UV) nonlinear optical (NLO) crystals are key materials in creating tunable deep‐UV lasers for frequency conversion technology. However, practical application of the sole usable crystal, KBe2BO3F2, has been hindered by the high toxicity of beryllium and its layering tendency in crystal growth. Herein, we report a beryllium‐free deep‐UV NLO material NaSr3(OH)(B9O16)[B(OH)4] (NSBOH), synthesized by a covalent bond modification strategy under hydrothermal conditions. Moisture‐stable NSBOH exhibits strong second‐harmonic generation (SHG) at 1064 nm (3.3 × KH2PO4) and 532 nm (0.55 × β‐BaB2O4), both amongst the largest powder SHG responses for a deep‐UV borate, with good phase‐matchability and a short wavelength cutoff edge (below 190 nm). NSBOH possesses a 3D covalent anionic [B9O19]∞ honeycomb‐like framework with no layering. The Sr2+ and Na+ ions, residing in the cavities of the anionic framework, act as templates for the assembly and favorable alignment of NLO‐active groups, resulting in an optimal balance between strong SHG activities and wide UV transparency. These merits indicate NSBOH is a very attractive candidate for deep‐UV NLO applications.

SrAlB3O6F2: A Fluoroaluminoborate with [Al2B6O14F4] Units and Large Birefringence.

The first strontium fluoroaluminoborate SrAlB3O6F2 was synthesized. Interestingly, it exhibits a two-dimensional warped layer composed of the [Al2B6O14F4] fundamental building units, which are first identified in borates. The experimental results indicate that SrAlB3O6F2 has a short ultraviolet cutoff edge (<200 nm) and large birefringence (0.075@546 nm), which are further verified by theoretical calculation. Thus, SrAlB3O6F2 can be used as the deep-ultraviolet birefringence material.

Exploring Deep-UV Nonlinear Optical Materials with Enhanced Second Harmonic Generation Response and Birefringence in Fluoroaluminoborate Crystals.

Deep-UV (DUV) nonlinear optical (NLO) materials are of vital importance to demanding DUV laser applications; however, those with a large NLO coefficient, wide band gap, and sufficient phase-matching ability are rare. In this work, by combining π-conjugated B3O6 groups and distort AlO3F tetrahedra, we successfully designed and synthesized two fluoroaluminoborates, RbAlB3O6F (RABF) and Cs0.5Rb0.5AlB3O6F (CRABF). In addition, the solid solution CsxRb1-xAlB3O6F (0.2 < x < 0.8) polycrystalline samples were obtained by the standard solid-state reaction. It was shown that all these compounds are NLO-active and possess short UV cutoff edges (<200 nm). Interestingly, although they have the same fundamental structural units, their crystal structures and second harmonic generation (SHG) responses are significantly different owing to the cation effect. Theoretical calculations reveal that both B3O6 and AlO3F groups contribute to the SHG responses. Remarkably, it also indicates that RABF exhibits a large birefringence (Δn = 0.0946 at 1064 nm), which is capable of realizing deep-UV phase matching down to 174 nm. The exploration of fluoroaluminoborates provides a new perspective for searching excellent deep-UV NLO materials.

Struvite-type AMgPO4·6H2O (A = NH4, K): Two Natural Deep-Ultraviolet Transparent Nonlinear Optical Crystals.

Deep-ultraviolet (DUV) nonlinear optical (NLO) materials play vital roles in diverse fields. Unfortunately, only the KBe2BO3F2 crystal has found commercial applications so far. Therefore, the discovery of new DUV NLO crystals is still urgent. As we all know, digging into the properties of existing crystals is also an effective way to obtain new NLO crystals. Herein, two natural asymmetric orthophosphates AMgPO4·6H2O (A = NH4, K) are proposed. Although their structures and some properties such as infrared spectra, thermal properties, and dielectric properties have been previously characterized, their NLO properties have not been reported. Thus, in this work, these two natural DUV transparent orthophosphates NH4MgPO4·6H2O (NMP) and KMgPO4·6H2O (KMP) were successfully acquired by a simple slow evaporation method. The single-crystal X-ray diffraction data indicate that NMP and KMP are isomorphic and that both belong to the Pmn21 space group of the orthorhombic system. Remarkably, NMP and KMP possess short cutoff edges below 190 nm, and their second-harmonic generation (SHG) efficiencies are 0.62 and 0.80 times that of KH2PO4(KDP), respectively; furthermore, they can achieve type-I phase matching at 1064 nm.

Ultraviolet nonlinear optical crystals A3SrBi(P2O7)2(A = K, Rb) with large second harmonic generation responses

Two non-centrosymmetric pyro-phosphates of K3SrBi(P2O7)2and Rb3SrBi(P2O7)2have short UV cut-off edges (∼240 nm) and large SHG responses.

AB11O16(OH)2 (A = K and Cs): interpenetrating 2D layers with large birefringence

By a cation substitution strategy, two new hydroxyborates AB11O16(OH)2 (A = K and Cs) were synthesized, and KB11O16(OH)2 exhibits a short DUV cutoff edge (195 nm) and a large birefringence.

Deep-ultraviolet transparent alkali metal–rare earth metal sulfate NaY(SO4)2·H2O as a nonlinear optical crystal: synthesis and characterization

The first SHG-active alkali metal–rare earth metal sulfate NaY(SO4)2·H2O, which manifests a short ultraviolet cutoff edge below 200 nm and exhibits a phase-matching SHG response of 0.6 × KDP, has been obtained using a mild hydrothermal method.

CsZn2 BO3 X2 (X2 =F2 , Cl2 , and FCl): A Series of Beryllium-Free Deep-Ultraviolet Nonlinear-Optical Crystals with Excellent Properties.

Designing deep-ultraviolet (DUV) nonlinear-optical (NLO) crystals is one of the major current research interests, but it faces a great challenge. In order to overcome the problem of crystal growth and the toxicity of BeO raw materials in KBe2 BO3 F2 (KBBF), the only applicable DUV NLO crystal so far, we substitute Be2+ cations with Zn2+ in the KBBF structure and modify the halogen anions, by which three new Zn-containing KBBF-like compounds, CsZn2 BO3 X2 (X2 =F2 , Cl2 , and FCl), have been successfully synthesized. They all exhibit excellent NLO properties, including improved SHG responses (2.8-3.5×KDP) and short UV cut-off edges (<190 nm). In comparison with KBBF, CsZn2 BO3 X2 (X2 =F2 , Cl2 , and FCl) are all chemically benign and have better growth habits, so they are all promising as DUV NLO crystals. Further study on structure-property relationships indicates that the mixing of halogen anions is a feasible strategy to enhance the SHG responses of the KBBF family.

A UV birefringent crystal: RbNH4(H2C3N3O3)2·2H2O

The excellent ultraviolet (UV) birefringent materials are essential to optical systems. Thus, a new UV birefringent crystal RbNH4(H2C3N3O3)2·2H2O has been obtained via the hydrothermal method. It crystallizes in centrosymmetric space group Pnma (No. 62) with the unit cell parameters: a = 6.820(2) Å, b = 16.457(6) Å, c = 11.767(4) Å and Z = 4. The structure of RbNH4(H2C3N3O3)2·2H2O contains the 2∞[H2C3N3O3]− ribbons via hydrogen bonds with the filling of H2O molecules, NH4+ and Rb+ cations. Its calculated birefringence is about 0.40 @ 1064 nm, which comparable or larger than some of the reported largest birefringent values of cyanurates, the experimental UV cutoff edge is down to 223 nm, and the band gap is 5.24 eV. Besides, the first-principles investigation of the electronic properties on the title compound reveals that the parallel alignment of π-conjugated (H2C3N3O3)− anions lead to the short ultraviolet cutoff edge and large birefringence. These results suggest that RbNH4(H2C3N3O3)2·2H2O is a potential UV birefringent material.

A(H3C3N3O3)(NO3) (A = K, Rb): Alkali-Metal Nitrate Isocyanurates with Strong Optical Anisotropy.

The first alkali-metal nitrate isocyanurates, A(H3C3N3O3)(NO3) (A = K, Rb), were synthesized by the tactic of introducing (NO3)- into isocyanurate with a mild hydrothermal technique. They crystallized into the same monoclinic centrosymmetric (CS) space group P21/c, which featured a 2D [(H3C3N3O3)(NO3)]∞ layered structure separated by K+ and Rb+ cations, respectively. Both compounds exhibited short ultraviolet cutoff edges (λcutoff = 228 and 229 nm) and large birefringences (Δn = 0.253 and 0.224 at 546.1 nm). More importantly, in comparison with most of the isocyanurates and nitrates, they have better thermal stability with decomposition temperatures up to 319.8 and 324.4 °C. In addition, our theoretical calculations reveal that the π-conjugated groups play significant roles in improving the optical anisotropy. Remarkably, introducing a π-conjugated inorganic acid radical (NO3)- into isocyanurate is an extremely meaningful strategy to explore new UV birefringent crystals.

Li4Ca2B8O16: A Borate with a Unique Fundamental Building Block and a Short Cutoff Edge.

A complex alkali and alkaline-earth metal borate Li4Ca2B8O16 has been synthesized successfully via the high-temperature solution method. Li4Ca2B8O16 crystallizes into the space group P1̅ (No. 2) of the triclinic crystal system. The crystal structure exhibits a three-dimensional framework consisting of the [B8O16]∞ chains connected by the [LiOn] (n = 3, 4, 6) and [CaO7] polyhedra. The fundamental building block [B8O18] differs from those of other anhydrous octa-borates, which can be identified as a unique one. Furthermore, Li4Ca2B8O16 shows three different Li-O units, which is unique among all non-disordered anhydrous borates. It shows a short UV cutoff edge less than 190 nm. To better study the relationships between the crystal structure and properties, the DFT calculations were used for the evaluations of the optical band gap and birefringence.

Highly porous aluminophosphates with unique three dimensional open framework structures from mild hydrothermal syntheses

Three novel highly porous aluminophosphates were synthesized through mild hydrothermal methods. Interestingly, among them, α-NaAlPO4(OH) is polar and exhibits moderate nonlinear optical properties with a short cutoff edge.

β-Ba(PO3)2 with DUV cut-off edge and large birefringence

Phosphates are another new family of exploring deep-ultraviolet nonlinear optical materials. As one material, the simpler the composition is the more favorable synthesis and application is. In this work, a simple alkaline earth metal phosphate β-Ba(PO3)2 was synthesized by high temperature solid phase method. It is worth mentioning that it has short cut-off edge (<190 nm) and relatively large birefringence (Δn = 0.032 @ 1064 nm) in phosphates and the optical anisotropy and birefringence can be enhanced by optimizing the basic anion arrangement. Combining with theoretical analysis, the relationship between structure and performance is further elaborated. In additional, thermal properties, linear optical and nonlinear optical properties for β-Ba(PO3)2 were also investigated.

Two novel centrosymmetric barium strontium borates with a deep-UV cut-off edge: Ba2Sr3B4O11 and Ba3Sr3B4O12

Two novel strontium barium borates, Ba2Sr3B4O11 and Ba3Sr3B4O12 have been synthesized and their crystal structures were characterized by a single-crystal X-ray diffraction. Ba2Sr3B4O11 is monoclinic, sp.gr. C2/c and based on isolated BO3 triangles and B2O5 pyroborate groups in relation of 1:2. The crystal structure of Ba2Sr3B4O11 has no structural analogues and belongs to a new structural type. Ba3Sr3B4O12 is tetragonal, sp.gr. I4/mcm and based on isolated BO3 triangles oriented in a non-parallel manner. Ba3Sr3B4O12 is a member of the “anti-zeolite” borate family, and structurally close to the Ba3B2O6 borate. Sr and Ba cations are ordered in Ba2Sr3B4O11 and partially disordered in Ba3Sr3B4O12. The structure of Ba3Sr3B4O12 contains BO3 triangles orientationally disordered over the 4 and 8 orientations, while the structure of Ba2Sr3B4O11 contains B2O5 pyroborate groups orientationally disordered over the 2 orientations. Ba2Sr3B4O11 and Ba3Sr3B4O12 exhibit a short cut-off edge lower than 210 and 206 nm, respectively. Thermal behaviour of these borates including Ba5B4O11 was studied by high-temperature X-ray powder diffraction in the 20–800°С temperature range. Thermal expansion is strongly anisotropic for monoclinic Ba2Sr3B4O11 (α11 = 2.8(3), α22 = 5.6(1), α33 = 38.0(5) × 10−6 °С−1 at 200°С), less anisotropic for orthorhombic Ba5B4O11 (α11 = 9.9(2), α22 = 11.7(3), α33 = 20.1(2) × 10−6 °С−1 at 200°С) and close to the isotropic for tetragonal Ba3Sr3B4O12 (α11 = α22 = 12.8(2), α33 = 20.0(1) × 10−6 °С−1 at 200°С). The new compounds were also characterized by thermal analysis, vibrational and UV–Vis–NIR spectroscopy and Ba2Sr3B4O11 was characterized by DFT calculations.

Centrosymmetric Rb2Mg3(SO4)4 and non-centrosymmetric Cs2Mg3(SO4)4 with a phase-matching nonlinear optical response

Non-centrosymmetric structure is the prerequisite of a nonlinear optical (NLO) material. However, most inorganic compounds tend to crystallize in centrosymmetric structures. Here, two new sulfates, Rb2Mg3(SO4)4 (1) and Cs2Mg3(SO4)4 (2), were successfully prepared by high-temperature solution method. Due to cationic size difference, 1 and 2 crystallize in centrosymmetric P21/c and chiral P212121, respectively. As a result, only 2 is NLO-active and exhibits a phase-matching second-harmonic generation response. Furthermore, diffuse reflectance spectra show that both 1 and 2 have short cutoff edges below 200 nm. In addition, they are congruently melting compounds with melting points higher than 780 °C. Calculations of dipole moment are carried out to explain the origin of NLO response. Cation-tuned structural strategies will shed a useful light on the development of new NLO materials.

Cover Feature: β‐CsB9O14: A Triple‐Layered Borate with Edge‐Sharing BO4 Tetrahedra Exhibiting a Short Cutoff Edge and a Large Birefringence (Chem. Eur. J. 50/2019)

Cover Feature: β‐CsB₉O₁₄: A Triple‐Layered Borate with Edge‐Sharing BO₄ Tetrahedra Exhibiting a Short Cutoff Edge and a Large Birefringence (Chem. Eur. J. 50/2019)

β-CsB9 O14 : A Triple-Layered Borate with Edge-Sharing BO4 Tetrahedra Exhibiting a Short Cutoff Edge and a Large Birefringence.

The first triple-layered borate with edge-sharing BO4 tetrahedra, β-CsB9 O14 was obtained under vacuum-sealed condition. It represents a new structure type and enriches the structural diversity of borates. Moreover, β-CsB9 O14 exhibits a short UV cutoff edge and a large birefringence, indicating that it could be regarded as a DUV birefringent crystal.

Four new deep ultraviolet borates with isolated B12O24 groups: Synthesis, structure, and optical properties

Four new deep ultraviolet (DUV) borates Li7Na2KRb2B12O24, Li7.35Na2.36K1.50Cs0.78B12O24, Li6.97Na2.63K1.24Cs1.15B12O24, and Li7.27Na2.67Rb2.06B12O24 have been produced by high temperature solution method in open air. The structural analysis suggests that they all crystallize in the same trigonal space group R3¯ (No. 148), and possess the identical isolated B12O24 groups. Structural comparison among alkali-metal borates with BnO2n in their chemical formulas have been discussed. In addition, infrared spectra were used to verify their structural validity. Furthermore, the UV–Vis–NIR diffuse reflectance spectra reveal that they exhibit short cutoff edges below 180 nm (6.88 eV). The first-principles theoretical studies carried out to increase the understanding of electronic structure and linear optical property.