CsB4O6F: A Congruent‐Melting Deep‐Ultraviolet Nonlinear Optical Material by Combining Superior Functional Units

The discovery of new nonlinear optical (NLO) materials for coherent light generation in the deep‐ultraviolet (DUV, wavelength below 200 nm) region is essential for the development of laser technologies. Herein, we report a new material CsB₄O₆F (CBF), which combines the superior structural properties of two well‐known NLO materials, β‐BaB₂O₄ (BBO) and KBe₂BO₃F₂ (KBBF). CBF exhibits excellent DUV optical properties including a short cutoff edge (155 nm), a large SHG response (≈1.9×KDP), and a suitable birefringence that enables frequency doubling down to 171.6 nm. Remarkably, CBF melts congruently and shows an improved growth habit. In addition, our rational design strategy will contribute to the discovery of DUV NLO materials.     

Beryllium‐Free β‐Rb2Al2B2O7 as a Possible Deep‐Ultraviolet Nonlinear Optical Material Replacement for KBe2BO3F2

A new beryllium‐free deep‐ultraviolet (DUV) nonlinear optical (NLO) material, β‐Rb₂Al₂B₂O₇ (β‐RABO), has been synthesized and characterized. The chiral nonpolar acentric material shows second‐harmonic generation (SHG) activity at both 1064 and 532 nm with efficiencies of 2×KH₂PO₄ and 0.4×β‐BaB₂O₄, respectively, and exhibits a short absorption edge below 200 nm. β‐Rb₂Al₂B₂O₇ has a three‐dimensional structure of corner‐shared Al(BO₃)₃O polyhedra. The discovery of β‐RABO shows that through careful synthesis and characterization, replacement of KBe₂BO₃F₂ (KBBF) by a Be‐free DUV NLO material is possible.     

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.     

Pb2BO3I: A Borate Iodide with the Largest Second‐Harmonic Generation (SHG) Response in the KBe2BO3F2 (KBBF) Family of Nonlinear Optical (NLO) Materials

Borate halides are an ideal materials class from which to design high‐performance nonlinear optical (NLO) materials. Currently, borate fluorides, chlorides, and bromides are extensively investigated while borate iodide materials discovery remains rare because of the perceived synthetic challenges. We report a new borate iodide, Pb₂BO₃I, synthesized by a straightforward hydrothermal method. The Pb₂BO₃I chemical formula conceals that the compound exhibits a structure similar to the well‐established KBe₂BO₃F₂ (KBBF), which we show supports the highest second‐harmonic generation (SHG) at 1064 nm in the KBBF family, 10 × KH₂PO₄ (KDP), arising from the inclusion of Pb²⁺ and I^? and the crystal chemistry. Our work shows that KBBF‐related compounds can be synthesized incorporating iodide and exhibit superior NLO responses.     

NH4Be2BO3F2 and γ‐Be2BO3F: Overcoming the Layering Habit in KBe2BO3F2 for the Next‐Generation Deep‐Ultraviolet Nonlinear Optical Materials

KBe₂BO₃F₂ (KBBF) is still the only practically usable crystal that can generate deep‐ultraviolet (DUV) coherent light by direct second harmonic generation (SHG). However, applications are hindered by layering, leading to difficulty in the growth of thick crystals and compromised mechanical integrity. Despite efforts, it is still a great challenge to discover new nonlinear optical (NLO) materials that overcome the layering while keeping the DUV SHG available. Now, two new DUV NLO beryllium borates have been successfully designed and synthesized, NH₄Be₂BO₃F₂ (ABBF) and γ‐Be₂BO₃F (γ‐BBF), which not only overcome the layering but also can be used as next‐generation DUV NLO materials with the shortest type I phase‐matching second‐harmonic wavelength down to 173.9 nm and 146 nm, respectively. Significantly, γ‐BBF is superior to KBBF in all metrics and would be the most outstanding DUV NLO crystal.     

Pb2BO3Cl: A Tailor‐Made Polar Lead Borate Chloride with Very Strong Second Harmonic Generation

A meticulously designed, polar, non‐centrosymmetric lead borate chloride, Pb₂BO₃Cl, was synthesized using KBe₂BO₃F₂ (KBBF) as a model. Single‐crystal X‐ray diffraction revealed that the structure of Pb₂BO₃Cl consists of cationic [Pb₂(BO₃)]⁺ honeycomb layers and Cl^? anions. Powder second harmonic generation (SHG) measurements on graded polycrystalline Pb₂BO₃Cl indicated that the title compound is phase‐matchable (type I) and exhibits a remarkably strong SHG response, which is approximately nine times stronger than that of potassium dihydrogen phosphate, and the largest efficiency observed in materials with structures similar to KBBF. Further characterization suggested that the compound melts congruently at high temperature and has a wide transparency window from the near‐UV to the mid‐IR region.     

Zwei Fluoridborate des Gadoliniums: Gd2F3[BO3] und Gd3F3[BO3]2

Two Fluoride Borates of Gadolinium: Gd₂F₃[BO₃] and Gd₃F₃[BO₃]₂ By flux‐supported solid‐state reaction of Gd₂O₃ and GdF₃ with B₂O₃ (flux: CsCl, molar ratio: 1 : 1 : 1 : 6, sealed tantalum capsule, 700 °C, 7 d) the new gadolinium fluoride borate Gd₂F₃[BO₃] (monoclinic, P2₁/c; a = 1637.2(1), b = 624.78(4), c = 838.04(6) pm, β = 93.341(8)°; V_m = 64.418(6) cm³/mol, Z = 8) was obtained as colourless, prismatic, face‐rich single crystals. The four crystallographically different Gd³⁺ cations (CN = 9) are all capped square‐antiprismatically surrounded by fluoride and oxide anions, in which the latter represent always components of isolated trigonal planar [BO₃]^3— anions. The six crystallographically independent F^— anions all reside in more or less planar coordination of three Gd³⁺ cations. Thus the constitution of Gd₂F₃[BO₃] can be described as a sequence of alternating layers each of the composition Gd[BO₃] and GdF₃ parallel (100), respectively. The crystal structures of Gd₂F₃[BO₃] and the shortly published Gd₃F₃[BO₃]₂ (monoclinic, C2/c; a = 1253.4(1), b = 623.7(1), c = 836.0(1) pm, β = 97.404(6)°; V_m = 97.571(9) cm³/mol, Z = 4) are compared with each other. Due to the structural analogies between these two gadolinium fluoride borates, a disorder model of the boron atoms frequently found for Gd₂F₃[BO₃] is able to be transferred to Gd₃F₃[BO₃]₂ as well.     

CsZn2BO3X2 (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 KBe₂BO₃F₂ (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, CsZn₂BO₃X₂ (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, CsZn₂BO₃X₂ (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.     

Nonlinear optical crystal BiAlGa2(BO3)4

The new nonlinear optical crystals BiAlGa₂(BO₃)₄ have been grown by spontaneous crystallization with molten flux based on a Bi₂O₃–B₂O₃ solvent. From single crystal X-ray diffraction measurement, BiAlGa₂(BO₃)₄ has been found to crystallize in the trigonal huntite structure type, space group R32, with cell dimensions a = 9.4433(9) and c = 7.4130(10) Å. The diffuse reflectance spectrum on a powder sample indicated that the short-wavelength absorption edge of BiAlGa₂(BO₃)₄ extends to approximately 271 nm. Second-harmonic generation (SHG) on powder samples has been measured using Kurtz and Perry technique, which indicated that BiAlGa₂(BO₃)₄ is a phase-matchable material, and its SHG coefficient is measured to be four times as large as that of KDP.     

Mechanism for linear and nonlinear optical effects in KBe2BO3F2 (KBBF) crystal

Electronic structure calculations of KBe₂BO₃F₂ crystal from first principles have been performed based on a plane-wave pseudopotential method for the first time. Its optical linear and second harmonic generation (SHG) coefficients are also calculated. The SHG coefficient measured early was much higher than that of our calculation, however, recent experiment confirms our calculated results. Moreover, a real-space atom-cutting method is adopted to analyze the respective contributions of the cation and anionic group to optical response. The results show that the contributions to the SHG coefficients from the (BO₃)³⁻ and (BeFO₃)⁵⁻ groups are dominant and comparable.     

ChemInform Abstract: Growth and Characterization of Acentric BaHf(BO3)2 and BaZr(BO3)2.

Single crystals of BaHf(BO₃)₂ are grown on a Pt wire from BaHfO₃ (3.5 mol%) solved in a BaB₄O₇ melt (1050 °C soaking temperature, 1000-940 °C crystallization temperature gradient).     

Reinvestigation and Characterization of the Magnesium Borate Fluoride Mg5(BO3)F

The magnesium borate fluoride Mg₅(BO₃)₃F was grown by spontaneous crystallization with molten flux based on the MgF₂‐LiF‐Na₂CO₃‐H₃BO₃ solvent. Structure solution from single‐crystal X‐ray diffraction shows that the title compound crystallizes in the orthorhombic space group Pnma (No. 62) with cell dimensions of a = 10.068(5) Å, b = 14.858(7) Å, c = 4.540(2) Å and Z = 4. Its structure features a three‐dimensional (3D) Mg‐O‐F framework composed of MgO₅F and MgO₆ polyhedra and isolated BO₃ groups. The detailed structure comparison referred to Mg₅(BO₃)₃F, Mg₃BO₃F₃ and β‐Mg₂BO₃F was carried out. The infrared spectrum (IR) and the bond valence sum (BVS) calculations of Mg₅(BO₃)₃F verify the validity of the structure. The calculated band structure and the density of states of Mg₅(BO₃)₃F suggest that its indirect bandgap is 5.27 eV. The compound was additionally investigated by UV/Vis‐NIR diffuse reflectance spectroscopy and thermal analysis.     

SrB5O7F3 Functionalized with [B5O9F3]6− Chromophores: Accelerating the Rational Design of Deep‐Ultraviolet Nonlinear Optical Materials

Fluorooxoborates, benefiting from the large optical band gap, high anisotropy, and ever‐greater possibility to form non‐centrosymmetric structures activated by the large polarization of [BO_xF_4?x]^(x+1)? building blocks, have been considered as the new fertile fields for searching the ultraviolet (UV) and deep‐UV nonlinear optical (NLO) materials. Herein, we report the first asymmetric alkaline‐earth metal fluorooxoborate SrB₅O₇F₃, which is rationally designed by taking the classic Sr₂Be₂B₂O₇ (SBBO) as a maternal structure. Its [B₅O₉F₃]^6? fundamental building block with near‐planar configuration composed by two edge‐sharing [B₃O₆F₂]^5? rings in SrB₅O₇F₃ has not been reported in any other borates. Solid state ¹⁹F and ¹¹B magic‐angle spinning NMR spectroscopy verifies the presence of covalent B?F bonds in SrB₅O₇F₃. Property characterizations reveal that SrB₅O₇F₃ possesses the optical properties required for deep‐UV NLO applications, which make SrB₅O₇F₃ a promising crystal that could produce deep‐UV coherent light by the direct SHG process.     

Fluorooxoborates: Beryllium‐Free Deep‐Ultraviolet Nonlinear Optical Materials without Layered Growth

Deep‐ultraviolet nonlinear optical (DUV NLO) crystals are the key materials to extend the output range of solid‐state lasers to below 200 nm. The only practical material KBe₂BO₃F₂ suffers high toxicity through beryllium and strong layered growth. Herein, we propose a beryllium‐free material design and synthesis strategy for DUV NLO materials. Introducing the (BO₃F)⁴⁻, (BO₂F₂)³⁻, and (BOF₃)²⁻ groups in borates could break through the fixed 3D B–O network that would produce a larger birefringence without layering and simultaneously keep a short cutoff edge down to DUV. The theoretical and experimental studies on a series of fluorooxoborates confirm this strategy. Li₂B₆O₉F₂ is identified as a DUV NLO material with a large second harmonic generation efficiency (0.9×KDP) and a large predicted birefringence (0.07) without layering. This study provides a feasible way to break down the DUV wall for NLO materials.     

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.     

(NH4)Bi2(IO3)2F5: An Unusual Ammonium‐Containing Metal Iodate Fluoride Showing Strong Second Harmonic Generation Response and Thermochromic Behavior

An ammonium‐containing metal iodate fluoride compound, (NH₄)Bi₂(IO₃)₂F₅, featuring a two‐dimensional double‐layered framework constructed by [BiO₂F₅]^6? and [BiO₄F₄]^9? polyhedra, as well as [IO₃]^? groups, was successfully synthesized. The well‐ordered alignment of these SHG‐active units leads to an extraordinary strong SHG response of 9.2 times that of KDP. Moreover, this compound possesses a large birefringence (Δn=0.0690 at 589.3 nm), a wide energy band gap (E_g=3.88 eV), and a high laser damage threshold (LDT; 40.2×AgGaS₂). In particular, thermochromic behavior was observed for the first time in this type of compound. Such multifunctional crystals will expand the application of nonlinear optical materials.     

KNiB4O6F3: A Layered Fluorooxoborate with Charge-Oriented Ordering

A layered fluorooxoborate, KNiB₄O₆F₃, contains a new (B₄O₆F₄)⁴⁻ group built of one planar (BO₃)³⁻ triangle and three tetrahedral (BO₃F)⁴⁻ units. Those units are joined together by sharing the oxygen atoms to form a 2-dimensinal (BO_3/2F)₃BO_3/2)_∞ layer. K⁺ and Ni²⁺ occupy on the sides with F⁻ and O²⁻ of the (BO_3/2F)₃BO_3/2)_∞ layer with a high (positive charge) to high (negative charge) and low to low coordination. Such kind of charge-oriented ordering is found to be governed by the stabilization energy of Coulomb interaction of the cations in certain sites. It is hoped that this mechanism of ordering may provide an additional tool for designing new structures with favourable properties, such as ferroelectrics or nonlinear optical materials.     

Synthesis of Er5(BO3)2F9 and Properties of RE5(BO3)2F9 (RE = Er,Yb)

Er₅(BO₃)₂F₉ was synthesised under conditions of 3 GPa and 800 °C in a Walker‐type multianvil apparatus. The crystal structure was determined on the basis of single‐crystal X‐ray diffraction data, collected at room temperature. Er₅(BO₃)₂F₉ is isotypic to the recently synthesised Yb₅(BO₃)₂F₉ and crystallises in C2/c with the lattice parameters a = 2031.2(4) pm, b = 609.5(2) pm, c = 824.6(2) pm, and β = 100.29(3)°. The physical properties of RE₅(BO₃)₂F₉ (RE = Er, Yb) including high temperature behaviour and single crystal IR‐ / Raman spectroscopy were investigated.     

A Facile Synthetic Route to a New SHG Material with Two Types of Parallel π‐Conjugated Planar Triangular Units

A new SHG material, namely, Pb₂(BO₃)(NO₃), which contains parallel π‐conjugated nitrate and borate anions, was obtained through a facile hydrothermal reaction by using Pb(NO₃)₂ and Mg(BO₂)₂?H₂O as starting materials. Its structure contains honeycomb [Pb₂(BO₃)]_∞ layers with noncoordination [NO₃]^? anions located at the interlayer space. Pb₂(BO₃)(NO₃) shows a remarkable strong SHG response of approximately 9.0 times that of potassium dihydrogen phosphate (KDP) and the material is also phase‐matchable. The large SHG coefficient of Pb₂(BO₃)(NO₃) arises from the synergistic effect of the stereoactive lone pairs on Pb²⁺ cations and parallel alignment of π‐conjugated BO₃ and NO₃ units. Based on its unique properties, Pb₂(BO₃)(NO₃) may have great potential as a high performance NLO material in photonic applications.     

Rb3Y2(BO3)3 with a noncentrosymmetric structure

Trirubidium diyttrium triborate contains zigzag chains of corner‐sharing [Y₂O₁₀] dimers. The chains are reinforced by one independent BO₃ group and crosslinked by the other two types of BO₃ groups to form a three‐dimensional framework. Channels along the [100] direction accommodate the Rb⁺ cations.