Polar Fluorooxoborate, NaB 4 O 6 F: A Promising Material for Ionic Conduction and Nonlinear Optics

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 Cs_0.5Rb_0.5AlB₃O₆F (CRABF). In addition, the solid solution Cs_xRb_1-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.

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.

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.

Discovery of First Magnesium Fluorooxoborate with Stable Fluorine Terminated Framework for Deep-UV Nonlinear Optical Application

The generated light can be tuned to cover almost the entire spectral range from deep-ultraviolet to terahertz wavelengths by utilizing the nonlinear optical crystals with a simple frequency doubling process. Among them, the discovery of novel candidates for the production of deep-ultraviolet light is by extension a great challenge toward realizing the vast potential. Actually, the availability for this process mainly depends on whether the critical performance can be well coexisted in one practical crystal. Herein, the first magnesium fluorooxoborate MgB₅ O₇ F₃ was synthesized as a new competitive candidate for deep-ultraviolet nonlinear optical application. It has a sufficiently large nonlinearity and a deep-ultraviolet phase matching wavelength, indicating that it holds great potential for the production of coherent light below 200 nm. The critical performance enhancement of MgB₅ O₇ F₃ when compared with its isomorphic phases was demonstrated and discussed. More importantly, we proposed that fluorooxoborate system with the general formula of MB₅ O₇ F₃ (M=divalent metal) possesses stable fluorine terminated framework, which makes them tend to retain their crystallized space groups unchanged.

Prediction of Novel van der Waals Boron Oxides with Superior Deep-Ultraviolet Nonlinear Optical Performance

Deep-ultraviolet nonlinear optical (DUV NLO) materials are attracting increasing attention because of their structural diversity and complexity. Using the two-dimensional (2D) crystal structure prediction method combined with the first-principles calculations, here we propose layered 18-membered-ring (18MR) boron oxide B₂ O₃ polymorphs as high-performance NLO materials. 18MR-B₂ O₃ with the AA and AB stackings are potential DUV NLO materials. The superior performing 18MR-B₂ O₃ ^AB has an unprecedentedly high second harmonic generation coefficient of 1.63 pm V^-1 , the largest among the DUV NLO materials, three times larger than that of the advanced DUV NLO material KBe₂ BO₃ F₂ and comparable to that of β-BaB₂ O₄ . Its unusually large birefringence of 0.196 at 400 nm guarantees the phase-matching wavelength λ_PM to reach this material's extreme absorption edge of ≈154 nm.

Bi2[B2(SeO3)6]: A Metal Boroselenite with a Unique Zero-Dimensional [B2(SeO3)6]6- Anionic Group and Large Birefringence

Explorations of new types of borates are important because of their promising application in diverse fields. A new bismuth-containing boroselenite, Bi₂[B₂(SeO₃)₆], has been obtained through high-temperature solid-state reaction in a closed system. Bi₂[B₂(SeO₃)₆] possesses a zero-dimensional [B₂(SeO₃)₆]^6- anionic group that does not belong to any types of reported boroselenites. Besides, Bi₂[B₂(SeO₃)₆] is the first boroselenite with lone-pair electrons containing a metal ion as the countercation. More interestingly, on the basis of the first-principles calculations, this compound displays a large birefringence (0.090) at 1064 nm.

SrTi(IO3)6·2H2O and SrSn(IO3)6: distinct arrangements of lone pair electrons leading to large birefringences

Three new iodates SrTi(IO₃)₆·2H₂O, (H₃O)₂Ti(IO₃)₆, and SrSn(IO₃)₆ have been synthesized via a facile hydrothermal method. The three compounds have zero-dimensional crystal structures composed of one [MO₆]⁸⁻ (M = Ti, Sn) octahedron connected with six [IO₃]⁻ trigonal pyramids. However, the particular coordination of Sr²⁺ cations results in distinct arrangements of lone pair electrons in an [IO₃]⁻ trigonal pyramid, which leads to large birefringences. More importantly, this work enriches the species crystal chemistry for [M(IO₃)₆]²⁻ (M = Ti, Sn) clusters-containing iodates.

The distinct arrangements of [IO₃]⁻ trigonal pyramids lead to larger birefringences in SrTi(IO₃)₆·2H₂O and SrSn(IO₃)₆ than that in (H₃O)₂Ti(IO₃)₆.

How Does Bridging Core Modification Alter the Photovoltaic Characteristics of Triphenylamine-Based Hole Transport Materials? Theoretical Understanding and Prediction

Perovskite solar cells have gained immense interest from researchers owing to their good photophysical properties, low-cost production, and high power conversion efficiencies. Hole transport materials (HTMs) play a dominant role in enhancing the power conversion efficiencies (PCEs) and long diffusion length of holes and electrons in perovskite solar cells. In hole transport materials, modification of π-linkers has proved to be an efficient approach for enhancing the overall PCE of perovskite solar cells. In this work, π-linker modification of a recently synthesized H-Bi molecule (R) is achieved with novel π-linkers. After structural modifications, ten novel HTMs (HB1-HB10) with a D-π-D backbone are obtained. The structure-property relationship, and optoelectronic and photovoltaic characteristics of these newly designed hole transport materials are examined comprehensively and compared with reference molecules. In addition, different geometric parameters are also examined with the assistance of density functional theory (DFT) and time-dependent DFT. All the designed molecules exhibit narrow HOMO-LUMO energy gaps (E_g =2.82-2.99 eV) compared with the R molecule (E_g =3.05 eV). The designed molecules express redshifting in their absorption spectra with low values of excitation energy, which in return offer high power conversion efficiencies. Further, density of states and molecular electrostatic potential analysis is performed to locate the different charge sites in the molecules. The reorganizational energies of holes and electrons are found to have good values, suggesting that these novel designed molecules are efficient hole transport materials for perovskite solar cells. In addition, the low binding energy values of the designed molecules (compared with R) offer high current charge density. Finally, complex study of HB9:PC₆₁ BM is also undertaken to understand the charge transfer between the molecules of the complex. The results of all analyses advocate that these novel designed HTMs are promising candidates for the construction of future high-performance perovskite solar cells.

Synergism of multiple functional chromophores significantly enhancing the birefringence in layered non-centrosymmetric chalcohalides

Compared with one or two functional chromophores materials, Hg₃AsQ₄X (Q = S, Se; X = Cl, Br, I) with multiple ones generate extremely large birefringence due to the synergism of the d¹⁰ cation Hg²⁺, lone pair layer of As³⁺ and mixed anions Q²⁻/X⁻.

Two deep-ultraviolet nonlinear optical monolayers obtained by a template-optimized design strategy

We obtained two two-dimentional deep-ultraviolet nonlinear optical layers with a large band gap and proper second harmonic generation by a template-optimized design strategy.

Na2La2B10O19: a new lanthanum sodium borate with infinite 2D layer 2∞[B10O19]8− and moderate birefringence

A new La-based borate Na₂La₂B₁₀O₁₉ with infinite 2D double layers composed of (B₅O₁₂)⁹⁻ FBBs and a moderate birefringence of 0.06 at 1064 nm.

A new acentric borate-nitrate Cs3B8O13(NO3) with interpenetrating porous 3D covalent and ionic lattices

A new acentric borate-nitrate Cs3B8O13(NO3) was synthesized by a molten salt method which consists of interpenetrating porous 3D covalent [B8O13]∞ and ionic [(NO3)Cs3]∞ lattices. It shows low ultraviolet cut-off edge (202 nm) and phase-matching second harmonic generation (SHG) intensity (0.7 KDP @1064 nm). First principles calculations showed that the main source of SHG is the cooperation of the B-O and [NO3]- groups.

BaB4O5F4 with reversible phase transition featuring unprecedented fundamental building blocks of [B16O21F16] in the α-phase and [B4O6F4] in the β-phase

The first fluorooxoborate with reversible phase transition, BaB4O5F4, has been obtained. Interestingly, the two polymorphs, α- and β-BaB4O5F4 with rare [BOF3], feature various one-dimensional chains composed of the unprecedented fundamental building blocks of [B16O21F16] and [B4O6F4], respectively. First-principles calculations were performed to elucidate the structure-property relationships.

Barium fluoroiodate crystals with a large band gap and birefringence

BaI₂O₅F₂ and BaIO₂F₃ have large birefringence of 0.174 and 0.133 at 1064 nm, respectively, which is owing to the rare [IO₃F]²⁻ units with high anisotropic polarizability in BaI₂O₅F₂ and the orderly arranged [IO₂F₂]⁻ units in BaIO₂F₃.

Li5Cs(SO4)3: a potential zero-order wave plate material with short deep-ultraviolet cutoff edge

A reversible phase transition Li5Cs(SO4)3 single crystal with transparency ranging from 180 nm to 4.7 μm has been obtained, which is a potential zero-order DUV wave plate material with small birefringence properties.

From silicates to oxonitridosilicates: improving optical anisotropy for phase-matching as ultraviolet nonlinear optical materials

Oxonitridosilicates are proposed to improve optical anisotropies of silicates as ultraviolet nonlinear optical materials by using the first principles method.

From centrosymmetric to noncentrosymmetric: effect of the cation on the crystal structures and birefringence values of (NH4)n-2AE(PO2F2)n (AE = Mg, Sr and Ba; n = 2, 3 and 4)

Fluorine-containing compounds significantly enrich the diversity of inorganic chemical structures. Here, four alkaline-earth metal difluorophosphates with different space groups, namely NH4Mg(PO2F2)3 (Cmcm) (1), NH4Sr(PO2F2)3 (P1[combining macron]) (2), (NH4)2Ba(PO2F2)4 (P2/n) (3) and Ba(PO2F2)2 (I4[combining macron]2d) (4), were synthesized and their structures were determined for the first time. 4 is not only the first alkaline-earth-metal difluorophosphate, but also the first difluorophosphate reported to crystallize in the non-centrosymmetric space group I4[combining macron]2d of the tetragonal crystal system. All of the three-dimensional (3D) structures of the title compounds consist of isolated (PO2F2)-, AE-O and NH4+ units, while the differences of the cation radius result in various arrangements of the (PO2F2)- units, which leads to diverse crystal structures and the disparate optical anisotropy of the crystals. According to the deep-ultraviolet (DUV) transmittance spectrum, 3 exhibits a short DUV cut off edge (<180 nm). Solid state 19F and 31P magic-angle spinning NMR spectroscopy was used to verify the presence of covalent P-F bonds in 3. The different size effects, as well as the coordination environments of the Mg2+, Sr2+, and Ba2+ cations, on the structures as a whole have been discussed in detail. In addition, the calculated birefringence values were observed to differ from 0.011 to 0.033 for the four compounds.

BaTi(BO3)2: an excellent birefringent material with highly coplanar isolated [BO3] groups

The birefringence of BaTi(BO₃)₂ is equal to or larger than 0.169 at 546 nm (Δn_(cal.) = 0.185 at 546 nm), which mainly originates from the high-density and parallel arrangement of the isolated [BO₃] triangles in the unit cell.

The synthesis, characterization, and theoretical analysis of (NH4)3PbCl5

(NH₄)₃PbCl₅ features a 3D structure with a short UV cutoff edge of 256 nm and simulated birefringence of about 0.050 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)₂ and Rb₃SrBi(P₂O₇)₂ have short UV cut-off edges (∼240 nm) and large SHG responses.

Pb2.28Ba1.72B10O19 featuring a three-dimensional B–O anionic network with edge-sharing [BO4] obtained under ambient pressure

The unique borate Pb_2.28Ba_1.72B₁₀O₁₉ containing a three-dimensional B–O anionic network with edge-sharing [BO₄] was obtained under ambient pressure, featuring an extremely large B–B interatomic distance (d_B–B) and small internal O–B–O angles (∠_{in(O–B–O)}).

Synthesis, crystal structure and theoretical calculations of two rare-earth borates with DUV cut-off edges

Two isomorphic rare-earth borate crystals, YZnB₅O₁₀ and GdZnB₅O₁₀, show three-dimensional structures constructed from two-dimensional {[B₅O₁₀]⁵⁻}_n layers with the Y or Gd, and zinc ions filled in the interlayers, and exhibit DUV cut-off edges (<190 nm).

Recent advances of oxyfluorides for nonlinear optical applications

Oxyfluorides exhibit rich crystal structures that provide a new perspective for designing new NLO materials.

NaKB6O9F2: a new complex alkali metal fluorooxoborate with puckered layers

Introducing F atoms into the 3D borate framework is beneficial to synthesize layered fluorooxoborates, which ensures moderate birefringence.

An antimony(iii) borate with large birefringence exhibiting unwonted [B5O11] fundamental building blocks and dimeric [Sb2O6] clusters

By combining π-conjugated [B₃O₆] groups and Sb³⁺ cations with SCALP, we successfully obtained the second antimony(iii) borate SbB₅O₉ with a large birefringence (0.151 @ 1064 nm), which exhibits unwonted [B₅O₁₁] FBBs and dimeric [Sb₂O₆] clusters.

Sn3B8O15: A Ternary Tin(II) Borate with Flexible [B8O18]12- Fundamental Building Block Formed by [B7O16]11- and [BO3]3- Groups

Owing to the tendency to form the glass state in growing crystals in the ternary tin(II) borate system, hitherto, very few single crystals in ternary tin(II) borate were reported. In this paper, a single crystal of Sn₃B₈O₁₅ was obtained via a high temperature solution method in a closed system. The structure features a flexible fundamental building block [B₈O₁₈]^12- constructed by one [BO₃]^3- unit and one [B₇O₁₆]^11- group that contains three hexatomic rings, which further connect to compose a two-dimensional ²_∞[B₈O₁₅] layer structure. The connection mode and flexibility of the three hexatomic B-O rings were analyzed, verifying the structural diversity of the B-O configuration. Moreover, the synthesis, crystal structure, and various characterizations were comprehensively presented. Also, theoretical calculations were employed to discuss structure-property relationships, and we use the real-space atom-cutting techniques to explain the origin of the birefringence of Sn₃B₈O₁₅.

Experimental and theoretical studies of the ternary thiophosphate PbPS3 featuring ethane-like [P2S6]4- units

The ternary thiophosphate PbPS₃ was synthesized by a high-temperature solid-state reaction using PbS, P and S. Single-crystal X-ray diffraction analysis reveals that the compound crystallizes in the P2₁/c space group. It features a three-dimensional structure, which consists of [PbS₈] hendecahedra and isolated ethane-like [P₂S₆]^4- units. PbPS₃ can be easily obtained by different methods and shows high air stability, which is beneficial to its chemical synthesis and large-sized crystal growth. The crystal structure, optical properties and electronic structure of PbPS₃ have been researched by experimental methods and first-principles calculations. The results show that PbPS₃ has a moderate bandgap (E_{g. exp.} = 2.60 eV) and birefringence (Δn_cal. = 0.094@1064 nm), as well as a wide transparent range. Furthermore, to better understand the origin of the birefringence, structure comparisons and theoretical calculations were carried out. With a relatively high physicochemical stability and easy synthesis, PbPS₃ can be expected to be a prospective birefringent material.

KNa2ZrF7: A Mixed-Metal Fluoride Exhibits Phase-Matchable Second-Harmonic-Generation Effect and High Laser-Induced Damage Threshold

Halides are one type of important IR nonlinear-optical material candidate. Here, a mixed alkali metal and d⁰ transition-metal fluoride, namely, KNa₂ZrF₇, was obtained by a facile hydrothermal method. It crystallizes in an orthorhombic system with the polar space group Pmn2₁, and its pseudo-1D structure features isolated (ZrF₇)^3- moncocapped trigonal prisms, which are ionically linked together by countercations K⁺ and Na⁺, representing a new type of fluoride. The powder sample of KNa₂ZrF₇ exhibits a moderate second-harmonic-generation response and a high laser-induced damage threshold. Besides, it can realize phase matchability and possesses a wide-IR transparent window. Thermal stability analysis suggests that KNa₂ZrF₇ is a congruent compound. Structural comparisons with related fluorides and theoretical calculations are also presented in this work.

NaZnCO3(OH): A High-Performance Carbonate Ultraviolet Nonlinear Optical Crystal Derived from KBe2BO3F2

Currently, to obtain new ultraviolet (UV) nonlinear optical (NLO) materials with large second-harmonic generation (SHG) response and short SHG phase-matching cutoff wavelength (λ_PM) is still very challenging. Herein, through evolving the KBe₂BO₃F₂ (KBBF) structure to a carbonate system, a novel hydroxycarbonate UV NLO crystal, NaZnCO₃(OH), was successfully synthesized. It not only possessed the typical structural characteristics of KBBF but also enhanced the bonding force between functional layers. Remarkably, in this crystal, the rare coexistence of a very large SHG response of 5.2 × KH₂PO₄ (KDP, @1064 nm) and short type-I SHG λ_PM of 201 nm had been achieved, indicating that it was a promising UV NLO crystal. This work showed the great significance of KBBF structure evolution and provided new opportunity to design UV NLO materials with good performance.

RbB3O4F2: a rubidium fluorooxoborate with an unprecedented [B3O5F2]3- functionalized unit and a large birefringence

A new rubidium fluorooxoborate, RbB₃O₄F₂, was structurally designed and synthesized by using the strategy of fluorine-introduction into borates. Interestingly, RbB₃O₄F₂ presents the novel [B₃O₄F₂]^- chains formed by the unprecedented [B₃O₅F₂]^3- fundamental building block. More importantly, RbB₃O₄F₂ has a large birefringence and a deep-UV cutoff edge. The finding of novel [B₃O₅F₂]^3- FBB and [B₃O₄F₂]^- chains in RbB₃O₄F₂ greatly enriches the structural diversity of fluorooxoborates.

Synthesis, Structure, and Characterization of d0 Transition-Metal Iodate: BaTi(IO3)6·0.5H2O

A new noncentrosymmetric (NCS) d⁰ transition-metal (TM) iodate BaTi(IO₃)₆·0.5H₂O has been successfully synthesized by the typical hydrothermal method. The single-crystal X-ray diffraction shows that BaTi(IO₃)₆·0.5H₂O crystallizes in the polar trigonal space group R3 (no. 146) with a = b = 11.1920(12) Å, c = 24.006(3) Å. The reported compound features zero-dimensional (0D) [Ti(IO₃)₆]^2- clusters that are separated by Ba²⁺ cations and bound water in the structure. By comparing BaTi(IO₃)₆·0.5H₂O with the centrosymmetric (CS) α- and β-BaTi(IO₃)₆ that were reported before, it can be found that the bound water plays an important role for the formation of NCS structure for BaTi(IO₃)₆·0.5H₂O. In addition, based on the powder second harmonic (SHG) measurement, BaTi(IO₃)₆·0.5H₂O can achieve type-I phase matching with a moderate SHG effect (∼1 × KDP). The source of SHG effect was analyzed by dipole moment calculation and structure comparison. UV-vis-NIR diffuse reflectance spectroscopy, infrared spectroscopy, and thermogravimetric measurements are also presented.

Finding Optimal Mid-Infrared Nonlinear Optical Materials in Germanates by First-Principles High-Throughput Screening and Experimental Verification

Owing to wide infrared (IR) transparency ranges, high laser damage thresholds, and being easy to grow in open air, germanates are emerging as promising mid-infrared (mid-IR) nonlinear optical (NLO) materials. However, the germanates as NLO materials have not been investigated comprehensively and the crystals with large second harmonic generation (SHG) response have not been identified. Herein, we used the first-principles high-throughput screening pipeline for NLO materials to search for excellent NLO crystals from germanates collected in the inorganic crystal structure database. After two steps of screening, three crystals are picked out from 128 structures based on their predicted energy gaps, birefringences, and SHG coefficients. Subsequently, the three germanates are synthesized and measured. The results show that Pb₃Ga₂Ge₄O₁₄ and Ba₂TiGe₂O₈ exhibit a wide energy gap (>3.1 eV) and a strong phase-matchable SHG intensity that are comparable to the benchmark AgGaS₂ (0.8 and 1.2 × AgGaS₂, respectively). In addition, the statistical analyses of different categories classified according to their cations show that the d⁰-transition metal and lone pair cations are more conducive to achieving a larger SHG response and birefringence compared to other cations in germanates. It gives a guideline for exploring new mid-IR NLO materials.

Cs3B3O3F6 with a Deep-Ultraviolet Cutoff Edge and a Suitable Birefringence as the Potential Zero-Order Waveplate Material

The zero-order waveplates, the essential materials in altering the polarization state of optical waves, are significant in polarimetry and the laser industry. Restricted by birefringence and absorption edge, few materials can have moderately small birefringence (Δn < 0.01) and a deep-ultraviolet (DUV; λ < 200 nm) transparency range. We used the B₃O₃F₆ unit for designing the zero-order waveplate material, which is composed of three BO₂F₂ tetrahedra interconnected by corner sharing. Here we obtained a new Cs₃B₃O₃F₆ compound with the isolated B₃O₃F₆ unit by a high-temperature solution method, which has the DUV cutoff edge and suitable birefringence (0.0069 at 532 nm) based on the experimental and computational results. This demonstrates that the isolated B₃O₃F₆ unit is a potential group, which favors the search for suitable compounds for DUV zero-order waveplate materials.