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.     

Polar Fluorooxoborate,NaB4O6F: A Promising Material for Ionic Conduction and Nonlinear Optics

The search of new borates with improved functional properties has attracted considerable attention. Herein, a new polar fluorooxoborate, NaB₄O₆F (NBF) was prepared by high‐temperature solid‐state reaction. NBF belongs to the AB₄O₆F family (A=alkali metal or ammonium), a series of compounds that undergoes significant cation‐dependent structural changes. NBF is of particular interest owing to the special cation position. Temperature‐dependent ionic conductivity measurements show that NBF is a solid ionic conductor, and it has the lowest active energy of 32.5 kJ mol⁻¹ of fluorooxoborates. NBF also shows a second‐harmonic generation (SHG) response of 0.9×KH₂PO₄ and 0.2×β‐BaB₂O₄, at 1064 and 532 nm, respectively, and it has a short UV cutoff edge below 180 nm. Based on bond valence (BV) concepts, symmetry analysis, and the first principles calculation, the unique [B₄O₆F]_∞ layer can be regarded as the “multifunctional unit”, which is responsible for the observed properties of NBF.     

Poly(difluorophosphazene) as the First Deep‐Ultraviolet Nonlinear Optical Polymer: A First‐Principles Prediction

A novel concept to obtain the deep‐ultraviolet (DUV) nonlinear optical (NLO) materials is proposed based on the assembling of one‐dimensional (1D) polar motifs into quasi‐1D polymer patterns. Based on the first‐principles calculations, we have successfully discovered an excellent DUV NLO polymer, i.e., poly(difluorophosphazene), with the chemical formula of (PNF₂)_n. Calculations reveal that PNF₂ has a larger band gap, a stronger second harmonic generation effect, a larger birefringence, and a shorter phase‐matching cutoff than KBe₂BO₃F₂. These findings not only demonstrate that the PNF₂ is the first reported DUV NLO polymer, but also could open a new direction to discover novel DUV NLO materials in polymer systems.     

Prediction of Fluorooxoborates with Colossal Second Harmonic Generation (SHG) Coefficients and Extremely Wide Band Gaps: Towards Modulating Properties by Tuning the BO3/BO3F Ratio in Layers

Fluorooxoborates have inspired investigations of deep‐ultraviolet (DUV) nonlinear optical (NLO) materials that can meet the multiple criteria. Herein, five stable structures with the composition of BaB₂O₃F₂ (I–V) are discovered using the ab initio evolutionary algorithm. Among them, BaB₂O₃F₂‐I has been synthesized experimentally and confirms the reliability of the method. All of the predicted structures possess extremely wide band gaps (8.1–9.0 eV). Moreover, four new structures exhibit giant second harmonic generation (SHG) coefficients (>3×KDP, d₃₆=0.39 pm V^?1). A novel type of the [BOF] layer with BO₃:BO₃F ratio of [1:1] is found in BaB₂O₃F₂‐II and BaB₂O₃F₂‐III. While BaB₂O₃F₂‐IV and BaB₂O₃F₂‐V are solely composed of the BO₃F group and have colossal SHG coefficients (ca. 4×KDP). It gives the direct evidence that the BO₃F group could generate strong SHG effect. Most importantly, the influences of BO₃:BO₃F ratio and their number density on band gap, birefringence and SHG effects are investigated.     

A Facile Route to Nonlinear Optical Materials: Three‐Site Aliovalent Substitution Involving One Cation and Two Anions

Two mixed‐metal gallium iodate fluorides, namely, α‐ and β‐Ba₂[GaF₄(IO₃)₂](IO₃) ( 1 and 2 ), have been designed by the aliovalent substitutions of α‐ and β‐Ba₂[VO₂F₂(IO₃)₂](IO₃) ( 3 and 4 ) involving one cationic and two anionic sites. Both 1 and 2 display large second‐harmonic generation responses (≈6×KH₂PO₄ (KDP)), large energy band gaps (4.61 and 4.35 eV), wide transmittance ranges (≈0.27–12.5 μm), and high relevant laser‐induced damage thresholds (29.7× and 28.3×AgGaS₂, respectively), which indicates that 1 and 2 are potential second‐order nonlinear optical materials in the ultraviolet to mid‐infrared. Our studies propose that three‐site aliovalent substitution is a facile route for the discovery of good NLO materials.     

Ba2Cd(B3O6)2: A Congruent‐Melting Compound with Isolated B3O6 Units

The single crystals of Ba₂Cd(B₃O₆)₂ were grown by the spontaneous crystallization method for the first time. They crystallize in the centrosymmetric trigonal space group R$\bar{3}$ with a = 7.143(3) Å, c = 17.405(16) Å, and Z = 3. The structure is characterized by isolated B₃O₆ units, and the Ba²⁺ and Cd²⁺ cations connect with B₃O₆ rings to form three dimensional structure. The TG/DSC and XRD results reveal that Ba₂Cd(B₃O₆)₂ melts congruently. First‐principles electronic structure calculation performed with the density functional theory (DFT) method shows that the calculated bandgaps are 4.66 eV, which is in good agreement with the UV/Vis/NIR experimental value 4.59 eV. The calculation shows that the Ba₂Cd(B₃O₆)₂ crystal has a large birefringence (Δn = 0.0875–0.0569 from 270 nm to 2600 nm), which demonstrates that Ba₂Cd(B₃O₆)₂ is a potential birefringence crystal.     

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.     

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.     

CsSbF2SO4: An Excellent Ultraviolet Nonlinear Optical Sulfate with a KTiOPO4 (KTP)‐type Structure

A novel noncentrosymmetric (NCS) polar fluoride sulfate, CsSbF₂SO₄, was obtained by ionothermal synthesis. A meticulously designed co‐substitution approach was used to successfully replace the [TiO₆]^8? and [PO₄]^3? functional groups in KTiOPO₄ (KTP) with [SbO₄F₂]^7? and [SO₄]^2? units, respectively. The structure of CsSbF₂SO₄ features a pseudo‐3D framework consisting of interconnected 1D [SbF₂O₂SO₄]^5? chains of corner‐sharing [SbO₄F₂]^7? octahedra and [SO₄]^2? tetrahedra. The title compound exhibits a sharply enlarged band gap compared to its parent compound, KTP, benefitting from the introduction of F^? ions and the displacement of Sb³⁺ cations. Second harmonic generation (SHG) measurements manifested that CsSbF₂SO₄ is phase‐matchable and revealed a strong SHG response of about 3.0 KH₂PO₄ (KDP), which is the highest value reported for any metal sulfate reported to date. The reported fluoride sulfate is a promising near ultraviolet (UV) nonlinear optical (NLO) material.     

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.     

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.     

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.