Where do the Fluorine Atoms Go in Inorganic-Oxide Fluorinations? A Fluorooxoborate Illustration under Terahertz Light.

The substitution of fluorine atoms for oxygen atoms/hydroxyl groups has emerged as a promising strategy to enhance the physical and chemical properties of oxides/hydroxides in fluorine chemistry. However, distinguishing fluorine from oxygen/hydroxyl in the reaction products poses a significant challenge in existing characterization methods. In this study, we illustrate that terahertz (THz) spectroscopy provides a powerful tool for addressing this challenge. To this end, we investigated two fluorination reactions of boric acid, utilizing MHF2 (M=Na, C(NH2)3) as fluorine reagents. Through an interplay between THz spectroscopy and solid-state density functional theory, we have conclusively demonstrated that fluorine atoms exclusively bind with the sp3-boron but not with the sp2-boron in the reaction products of Na[B(OH)3][B3O3F2(OH)2] (NaBOFH) and [C(NH2)3]2B3O3F4OH (GBF2). Based on this evidence, we have proposed a reaction pathway for the fluorinations under investigation, a process previously hindered due to structural ambiguity. This work represents a step forward in gaining a deeper understanding of the precise structures and reaction mechanisms involved in the fluorination of oxides/hydroxides, illuminated by the insights provided by THz spectroscopy.

Fluorination Strategy Towards Symmetry Breaking of Boron-centered Tetrahedron for Poly-fluorinated Optical Crystals.

Borate crystals can be chemically and functionally modified by the fluorination strategy, which encourages the identification of emerging fluorooxoborates with a structure and set of characteristics not seen in any other oxide parents. However, the bulk of fluorooxoborates have been found accidentally, rational methods of synthesis are required, particularly for the infrequently occurring poly-fluorinated components. Herein, we reported the use of bifluoride salts as a potent source of fluorine to prepare fluorooxoborates that contain rarely tri-fluorinated [BF3 X] (X=O and CH3 ) tetrahedra and eleven compounds were found. We identified the optical properties of the organofluorinated group [CH3 BF3 ] and their potential for nonlinear optics for the first time. Among these, two non-centrosymmetric components hold potential for the production of 266 nm harmonic coherent light for nonlinear optics, and more crucially, have the benefit of growing large size single crystals. Our study establishes experimental conditions for the coexistence of the diverse functional groups, enabling the production of poly-fluorinated optical crystals.

Hierarchical Modulation of Optical Anisotropy Driven by Metal Cation Polyhedra in Fluorooxoborates MII B4 O6 F2 (MII =Be, Mg, Pb, Zn, Cd).

The enhancement mechanism of birefringence is very important to modulate optical anisotropy and materials design. Herein, the different cations extending from alkaline-earth to alkaline-earth, d10 electron configuration, and 6s2 lone pair cations are highlighted to explore the influence on the birefringence. A flexible fluorooxoborate framework from AEB4 O6 F2 (AE=Ca, Sr) is adopted for UV/deep-UV birefringent structures, namely, MII B4 O6 F2 (MII =Be, Mg, Pb, Zn, Cd). The maximal enhancement on birefringence can reach 46.6 % with the cation substitution from Ca, Sr to Be, Mg (route-I), Pb (route-II), and Zn, Cd (route-III). The influence of the cation size, the stereochemically active lone pair, and the binding capability of metal cation polyhedra is investigated for the hierarchical improvement on birefringence. Significantly, the BeB4 O6 F2 structure features the shortest UV cutoff edge 146 nm among the available anhydrous beryllium borates with birefringence over 0.1 at 1064 nm, and the PbB4 O6 F2 structure has the shortest UV cutoff edge 194 nm within the reported anhydrous lead borates that hold birefringence larger than 0.1 at 1064 nm. This work sheds light on how metal cation polyhedra modulate birefringence, which suggests a credible design strategy to obtain desirable birefringent structures by cation control.

Na4 B8 O9 F10 : A Deep-Ultraviolet Transparent Nonlinear Optical Fluorooxoborate with Unexpected Short Phase-Matching Wavelength Induced by Optimized Chromatic Dispersion.

Exploring significant ultraviolet/deep-ultraviolet nonlinear optical (NLO) materials is hindered by rigorous and contradictory requirements, especially, possessing a moderate optical birefringence to meet phase-matching (PM). Except for suitable birefringence, small chromatic dispersion is also crucial to blue-shift the PM wavelength. Here, the introduction of a fluorinated tetrahedral boron-centred chromophore strategy was proposed to optimize the chromatic dispersion. Herein, the [BF4 ]- unit with a large HOMO-LUMO band gap was introduced to the Na-B-O-F system and Na4 B8 O9 F10 was designed and synthesized successfully for the first time. Na4 B8 O9 F10 with an optimized chromatic dispersion can achieve a short second harmonic generation PM wavelength of 240 nm with a relatively small birefringence (0.036@1064 nm). Notably, Na4 B8 O9 F10 is the first acentric crystal with [BF4 ]- units among the reported metal-fluorooxoborate systems, involving isolated [BF4 ]- and novel [B7 O10 F6 ]5- fundamental building blocks.

NaRbB3 O4 F3 : A New Fluorooxoborate with a Short UV Cutoff Edge Enriching the Structural Chemistry of Borate.

The combination of RbB3 O4 F2 and NaF generates a new member of fluorooxoborates, NaRbB3 O4 F3 , with a wide transparency range from the IR to DUV region. NaRbB3 O4 F3 shows a three-dimensional (3D) structure composed of 1D [B3 O4 F3 ]∞ chains, [NaO3 F3 ] and [RbO5 F5 ] polyhedra. The structural evolution from NaRbB3 O4 F3 to RbB3 O4 F2 , as well as the structural comparison between NaRbB3 O4 F3 and its identical stoichiometry compound, Li2 B3 O4 F3 were discussed in detail. The IR spectrum verifies its structural validity. The spectral measurement shows that the reflectance has no obvious change in the range of 175-300 nm, and its cutoff edge is below 175 nm. In addition, theoretical calculations are carried out to understand its electronic structure and optical properties.

Cs4 B4 O3 F10 : First Fluorooxoborate with [BF4 ] Involving Heteroanionic Units and Extremely Low Melting Point.

Herein, a new congruently melting mixed-anion compound Cs4 B4 O3 F10 has been characterized as the first fluorooxoborate with [BF4 ] involving heteroanionic units. Compound Cs4 B4 O3 F10 possesses two highly fluorinated anionic clusters and therefore its formula can be expressed as Cs3 (B3 O3 F6 ) ⋅ Cs(BF4 ). The influence of [BF4 ] units on micro-symmetry and structural evolution was discussed based on the parent compound. More importantly, Cs4 B4 O3 F10 shows the lowest melting point among all the available borates and thus sets a new record for such system. This work is of great significance to enrich and tailor the structure of borates using perfluorinated [BF4 ] units.

A Promising Fluorooxoborate Framework with Flexibile Capability for Diverse Cations to Enhance the Second Harmonic Generation.

Fluorooxoborates as potential deep ultraviolet (DUV) nonlinear optical (NLO) materials have exhibited diverse structures and NLO properties with metal cationic changes. Herein, the general mechanisms of metal cations on band gaps and optical properties in a series of typical fluorooxoborates have been clarified. It reveals that the framework of the emblematic 18-membered ring oxyfluorides has the flexibility of being able to contain different cations spanning from alkali to d10 metals by investigating the stability of the artificial CdB5 O7 F3 structure. Besides, introducing d10 metal cations can enhance the second harmonic generation (3.1×KH2 PO4 (KDP), d36 =0.39 pm V-1 ) and also keep a DUV spectral transparency (Eg >6.2 eV). Thus, the d10 -containing fluorooxoborate exhibits a great potential to be a new DUV optical material for nonlinear light-matter interactions.

KNiB4 O6 F3 : A Layered Fluorooxoborate with Charge-Oriented Ordering.

A layered fluorooxoborate, KNiB4 O6 F3 , contains a new (B4 O6 F4 )4- group built of one planar (BO3 )3- triangle and three tetrahedral (BO3 F)4- units. Those units are joined together by sharing the oxygen atoms to form a 2-dimensinal (BO3/2 F)3 BO3/2 )∞ layer. K+ and Ni2+ occupy on the sides with F- and O2- of the (BO3/2 F)3 BO3/2 )∞ 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.

Emergent Deep-Ultraviolet Nonlinear Optical Candidates.

Advances in the generation of coherent lasers at wavelengths below 200 nm as well as the associated equipment and technologies have greatly spurred the development of nonlinear optical (NLO) materials that are capable of phase-matching in the deep-ultraviolet (deep-UV) spectral regions. However, few crystals can meet the technical demands, and studies to date have focused mainly on finding the next competitive candidates. Consequently, several crystals have emerged that hold great potential for the production of deep-UV harmonic coherent light, which benefit from newly developed systems with novel NLO-active units and chromophores. This Review provides a comprehensive overview of the developments of emergent candidates in the field of deep-UV NLO materials, discussing the recent efforts and active advances in the search for superior systems and in understanding their structure-property relationships. The intention is to provide a clear perspective on the current emergent deep-UV NLO candidates, and to accelerate the discovery of next candidates with high performances to meet the urgent demands.

Prediction of Fluorooxoborates with Colossal Second Harmonic Generation (SHG) Coefficients and Extremely Wide Band Gaps: Towards Modulating Properties by Tuning the BO3 /BO3 F 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 BaB2 O3 F2 (I-V) are discovered using the ab initio evolutionary algorithm. Among them, BaB2 O3 F2 -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 :BO3 F ratio of [1:1] is found in BaB2 O3 F2 -II and BaB2 O3 F2 -III. While BaB2 O3 F2 -IV and BaB2 O3 F2 -V are solely composed of the BO3 F group and have colossal SHG coefficients (ca. 4×KDP). It gives the direct evidence that the BO3 F group could generate strong SHG effect. Most importantly, the influences of BO3 :BO3 F ratio and their number density on band gap, birefringence and SHG effects are investigated.

BaB2 O3 F2 : A Barium Fluorooxoborate with a Unique 2 ∞ [B2 O3 F]- Layer and Short Cutoff Edge.

Substitution of oxygen by fluorine in the borate group offers a materials platform from which intriguing structure and functionality may arise. Herein, we report a new fluorooxoborate, BaB2 O3 F2 , synthesized by introducing the F atoms into the BaO-B2 O3 system. BaB2 O3 F2 exhibits a unique oxyfluoride layer 2 ∞ [B2 O3 F]- and a deep-ultraviolet cutoff edge below 180 nm. The effect of the introduction of F atoms on the structure and optical property of the borate group has been investigated; this should be useful to further expand borate chemistry and materials.

Fluorooxoborates: Ushering in a New Era of Deep Ultraviolet Nonlinear Optical Materials.

Borates are one of the most important classes of functional materials, and several hundreds of artificial borates have been synthesized. The substitution of oxygen by fluorine leads to manifold classes of borates. Fluorooxoborates (also known as fluoroborates), in which the F atoms covalently connect with the B atoms, show additional compositional and structural diversity compared to classic borates. Recently, owing to the large polarizability anisotropy, large HOMO-LUMO gaps, and high hyperpolarizability of the oxyfluoride BOx F4-x building blocks, fluorooxoborates have received unprecedented attention in the search for new ultraviolet (UV) and deep-UV (DUV) nonlinear optical (NLO) materials. Specifically, some compounds have excellent NLO properties that are comparable or superior to KBe2 BO3 F2 , which is the only usable crystal that generates coherent light below 200 nm through a direct second harmonic generation (SHG) process. This Minireview illustrates recent progress on the synthesis, crystal structures, structure-properties relationships and applications of fluorooxoborates. This paper concludes by highlighting the outstanding opportunities offered by NLO fluorooxoborate crystals as an innovative avenue for DUV all solid-state coherent light generation.

Designing Deep-UV Birefringent Crystals by Cation Regulation.

Two new alkaline earth metal fluorooxoborates, CaB4 O6 F2 and SrB4 O6 F2 , have been synthesized and characterized. The title compounds are isostructural and crystallize in centrosymmetric space group P1‾ (No. 2). Structurally, the fundamental building blocks, B4 O8 F2 groups, are connected by B-O bonds to form [B4 O6 F2 ]∞ layers, which stack parallelly along the c axis. In addition, BaB4 O6 F2 , a known fluorooxoborate with different crystal structure, was also synthesized and characterized for comparison. The cation size influence on the structure of this MB4 O6 F2 (M=Ca, Sr, Ba) family is discussed in detail. The diffuse-reflectance spectroscopy measurements indicate that the cutoff edges of title compounds are below 190 nm. Moreover, first-principle calculation results demonstrate that MB4 O6 F2 (M=Ca, Sr, Ba) have large birefringence of 0.091, 0.089, and 0.085 at 1064 nm. Both experimental and theoretical studies suggest that MB4 O6 F2 (M=Ca, Sr, Ba) are promising birefringent materials for deep-UV applications.

Polar Fluorooxoborate, NaB4 O6 F: 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, NaB4 O6 F (NBF) was prepared by high-temperature solid-state reaction. NBF belongs to the AB4 O6 F 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×KH2 PO4 and 0.2×ß-BaB2 O4 , 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 [B4 O6 F]∞ layer can be regarded as the "multifunctional unit", which is responsible for the observed properties of NBF.

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, AB4 O6 F (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 KH2 PO4 (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.

CsB4 O6 F: 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 CsB4 O6 F (CBF), which combines the superior structural properties of two well-known NLO materials, ß-BaB2 O4 (BBO) and KBe2 BO3 F2 (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.

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 KBe2 BO3 F2 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 (BO3 F)4- , (BO2 F2 )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. Li2 B6 O9 F2 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.