Designing Deep-UV Birefringent Crystals by Cation Regulation

(C(NH2)3)2(I2O5F)(IO3)(H2O) and C(NH2)3IO2F2: Two Guanidine Fluorooxoiodates with Wide Band Gap and Large Birefringence

Enhancing anisotropy through an effective synergistic arrangement of anionic and cationic groups is crucial for improving the birefringence optical properties of materials. In this work, by transforming I-O into I-F through the fluorination strategy, two metal-free guanidine fluorooxoiodates (C(NH2)3)2(I2O5F)(IO3)(H2O) and C(NH2)3IO2F2 and one guanidine iodate C(NH2)3IO3 were successfully synthesized using the hydrothermal method. An unprecedented dimer [I2O5F] formed by [IO3F] and [IO3] in (C(NH2)3)2(I2O5F)(IO3)(H2O) was found, which greatly enriches the structural diversity of fluorooxoiodates. All three compounds feature a relatively large birefringence (Δn = 0.068, 0.110 and 0.075 at 546 nm) and a short ultraviolet cutoff edge. The theoretical calculation was carried out to understand the electronic structures and linear optical properties.

Chiral amino acid-templated tin fluorides tailoring nonlinear optical properties, birefringence, and photoluminescence

In this study, we successfully synthesized two types of new chiral amino acid-templated tin fluoride crystals: (R)-[(C8H10NO3)2]Sn(IV)F6, (S)-[(C8H10NO3)2]Sn(IV)F6, (R)-[C8H10NO3]Sn(II)F3, and (S)-[C8H10NO3]Sn(II)F3, employing a slow evaporation method. The crystal structures of Sn(IV)-compounds were determined to belong to the noncentrosymmetric (NCS) nonpolar space group, P21212. Conversely, the structures of Sn(II)-compounds were found to crystallize in the NCS polar space group, P21, as revealed by single-crystal X-ray diffraction analysis. Remarkably, Sn(IV)-compounds exhibited a larger birefringence (0.328@546.1 nm), attributed to the well-stacked arrangement of planar π-conjugated benzene rings along the b-axis. The ability of tin(IV) fluorides to form more hydrogen bonds with ligands increased the probability of π-π interactions between benzene rings, enabling the growth of centimeter-sized crystals in Sn(IV)-compounds. In contrast, Sn(II)-compounds displayed a stronger second-harmonic generation (SHG) response (0.85 × KDP) than Sn(IV)-compounds (0.46 × KDP). This enhanced SHG response in Sn(II)-compounds was attributed to the increased dipole moments resulting from the presence of lone pairs. Additionally, Sn(II)-compounds exhibited photoluminescent properties due to the transition from the metal-to-ligand charge transfer state, facilitated by the presence of the lone pairs.

Ba3.75MgB7O14F2.5: A Mixed Alkaline-Earth Metal Borate Fluoride with Short Ultraviolet Cutoff Edge and Large Birefringence

Fluorine-containing borate crystal materials are particularly attractive due to their rich structural chemistry and excellent properties for optical applications. In this work, a new compound Ba3.75MgB7O14F2.5 has been synthesized through the high-temperature solution method. In the crystal structure of Ba3.75MgB7O14F2.5, the [B7O14] basic building unit and the [MgO3F3] octahedra are interconnected to create a complex three-dimensional network. The structural feature of the less commonly observed [B7O14] units is discussed, and it has been found that such units in Ba3.75MgB7O14F2.5 are most conducive to achieving large birefringence. In addition, Ba3.75MgB7O14F2.5 exhibits good thermal stability, a short ultraviolet cutoff of 203 nm, and large birefringence (0.081@546 nm), indicating its potential as a new UV birefringent crystal.

CsB3O4(OH)2: a new deep-ultraviolet birefringent crystal with [B3O4(OH)2] anionic group

A new cesium hydroxyborate CsB₃O₄(OH)₂, was designed and synthesized by a hydrothermal method. Remarkably, CsB₃O₄(OH)₂ presents novel [B₃O₄(OH)₂]_∞ chains formed by [B₃O₄(OH)₂] fundamental building blocks (FBBs). The report of less common [B₃O₄(OH)₂] FBBs and [B₃O₄(OH)₂]_∞ chains in CsB₃O₄(OH)₂ enriches the structural diversity of hydroxyborates. In addition, CsB₃O₄(OH)₂ has a wide transparent window in the DUV spectral range and a large birefringence.

NaBa3[M2B7O16(OH)2]F2 (M = Ge, Si): Two Synthetic F Analogues of Garrelsite with [B7O16(OH)2]13- Polyanions and Deep-Ultraviolet Cutoff Edges

Two new borogermanate/borosilicate fluorides, namely, NaBa₃[M₂B₇O₁₆(OH)₂]F₂ (M = Ge, Si), have been successfully synthesized through a conventional mild hydrothermal method. They represent the first examples of mixed alkali and alkaline-earth borogermanate/borosilicate fluorides. NaBa₃[M₂B₇O₁₆(OH)₂]F₂ (M = Ge, Si) crystallize in the space group of C2/c, and their structures feature a unique 3D anionic framework composed of [B₇O₁₆(OH)₂]^13- polyanions corner-sharing with SiO₄ or GeO₄ tetrahedra, forming 1D 10-membered-ring tunnels along the b axis, which are filled by Na⁺, Ba²⁺, and F^- ions. UV-vis-near-IR absorption spectra identify the title compounds possessing short deep-ultraviolet absorption edges (below 200 nm), while their birefringences were calculated to be 0.021 and 0.016 at 1064 nm, respectively. Optical property, thermal stability, and theoretical calculations have also been conducted on NaBa₃[M₂B₇O₁₆(OH)₂]F₂ (M = Ge, Si).

Cation Regulation to Investigate the Chalcogenide Borate RE6Nb2MgSB8O26 (RE = La-Nd) Family

Chalcogenide borates have been developed and are considered an attractive system due to their favorable physical properties such as magnetism and nonlinear optical effects. Here, isostructural RE₆Nb₂MgSB₈O₂₆ (RE = La-Nd) compounds in the title family have been obtained through cation regulation in rare-earth and VB group metals. This family crystalizes in the centrosymmetric P3̅ space group and features 3D frameworks formed by {[Mg(NbB₄O₁₃)₂]^16-}_∞ polyanionic layers and QRE₆ octahedra. The structural chemistry was characterized and theoretical calculations were performed to understand the structural merit of this family. In addition, RE₆Nb₂MgSB₈O₂₆ possess the largest band gaps among known rare-earth chalcogenide borates, and they all show antiferromagnetic-like behaviors.

MM'B3O4F3 (M = K; M' = Na, K, Cs): Alkali-Metal Fluorooxoborates with ∞1[B3O4F3] Chains and Deep-Ultraviolet Cutoff Edges

Three mixed-alkali-metal fluorooxoborates, KNaB₃O₄F₃ (I), K₂B₃O₄F₃ (II), and KCsB₃O₄F₃ (III), were acquired in a closed system. I-III are isomorphic and adopt orthorhombic structures [Pbcn (No. 60)] with wavy parallelly arranged pseudolayers composed of _∞¹[B₃O₄F₃] chains, which exhibit slight differences in the arrangement modes of the fundamental building blocks. First-principles calculations illustrate that they all have moderate birefringence and large band gaps on the order of 7.0 eV, suggesting deep-ultraviolet (DUV) cutoff edges. In order to investigate the main source of the optical properties, the electronic structure and anisotropy of the response electron distribution were analyzed. Experimental characterizations for I confirm the structure and DUV transparence ability.

A2B6O9F2 (A = NH4, K): new members of A2B6O9F2 family with deep-UV cutoff edges and moderate birefringence

Two new fluorooxoborates A2B6O9F2 (A = NH4, K) with different two-dimensional [B6O9F2]∞ layered structure constructed by the fundamental building block [B6O11F2] were obtained.

Hierarchical Modulation on Optical Anisotropy Driven by Metal Cation Polyhedra in Fluorooxoborates MIIB4O6F2 (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, d 10 electron configuration, and 6s 2 lone pair cations are highlighted to explore the influence on the birefringence. A flexible fluorooxoborate framework from AEB 4 O 6 F 2 (AE = Ca, Sr) is adopted for UV/deep-UV birefringent structures, namely, M II B 4 O 6 F 2 (M II = 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 BeB 4 O 6 F 2 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 PbB 4 O 6 F 2 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.

Fluorine-Driven Enhancement of Birefringence in the Fluorooxosulfate: A Deep Evaluation from a Joint Experimental and Computational Study

Understanding and exploring the functional modules (FMs) consisting of local atomic groups can promote the development of the materials with functional performances. Oxygen-containing tetrahedral modules are popular in deep-ultraviolet (DUV) optical materials, but their weak optical anisotropy is adverse to birefringence. Here, the fluorooxosulfate group is proved as a new birefringence-enhanced FM for the first time. The birefringence of fluorooxosulfates can be 4.8-15.5 times that of sulfates with the same metal cations while maintaining a DUV band gap. The polarizing microscope measurement confirms the birefringence enhancement by using the millimeter crystals experimentally. The theoretical studies from micro and macro levels further reveal a novel universal strategy that the fluorine induced anisotropic electronic distribution in fluorooxo-tetrahedral group is responsible for the enhancement of birefringence. This study will guide the future discovery of DUV optical materials with enlarged birefringence.

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.

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.

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.

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.

Pb3Ba3Zn6(BO3)8 and α-BaZn2(BO3)2: new members of the zincoborates containing two different dimensional Zn–O units

Two new zincoborates, namely, Pb₃Ba₃Zn₆(BO₃)₈ and α-BaZn₂(BO₃)₂, were synthesized by the high-temperature solution method and their structures were determined by single-crystal X-ray diffraction for the first time.

Prediction of ternary fluorooxoborates with coplanar triangular units [BOxF3−x]x− from first-principles

From first-principles prediction, we got all the basic structural units of fluorooxoborates, namely, tetrahedral elements [BO_xF_4−x] (x = 1,2,3) like [BO₄] and triangular elements [BO_xF_3−x] (x = 1,2) like [BO₃].

From BaAl2(BO3)2O to SnAl2(BO3)2F2: structure transformation based on ion regulation

Sn atoms reside in each Al–B–O–F channel consisting of Al₈B₈O₃₂F₁₂ cavities, which makes SnAl₂(BO₃)₂F₂ possess a special structure.

Exploration of New Birefringent Crystals in Bismuth d0 Transition Metal Selenites

The first examples of bismuth fluoride selenites with d⁰ -TM/Te^VI polyhedrons, namely, Bi₄ TiO₂ F₄ (SeO₃ )₄ (1), Bi₄ NbO₃ F₃ (SeO₃ )₄ (2), Bi₄ TeO₄ F₂ (TeO₃ )₂ (SeO₃ )₂ (3), Bi₂ F₂ (MoO₄ )(SeO₃ ) (4) and Bi₂ ZrO₂ F₂ (SeO₃ )₂ (5) have been successfully synthesized under hydrothermal reactions by aliovalent substitution. The five new compounds feature three different types of structures. Compounds 1-3, containing Ti^IV , Nb^V and Te^VI respectively, are isostructural, exhibiting a new 3D framework composed of a 3D bismuth oxyfluoride architecture, with intersecting tunnels occupied by d⁰ -TM/Te^VI octahedrons and selenite/tellurite groups. Interestingly, compound Bi₄ TeO₄ F₂ (TeO₃ )₂ (SeO₃ )₂ (3) is the first structure containing Se^IV and mixed-valent Te^IV /Te^VI cations simultaneously. Compound 4 features a new 3D structure formed by a 3D bismuth oxyfluoride network with MoO₄ tetrahedrons and selenites groups imbedded in the 1D tunnels. Compound 5 displays a novel pillar-layered 3D open framework, consisting of 2D bismuth oxide layers bridged by the [ZrO₂ F₂ (SeO₃ )₂ ]^6- polyanions. Theoretical calculations revealed that the five compounds displayed very strong birefringence. The birefringence values of compounds 1-3, especially, are above 0.19 at 1064 nm, which are larger than the mineral calcite. Based on the structure and property analysis, it was found that the asymmetric SeO₃ groups (and TeO₃ in compound 3) displayed the largest anisotropy, compared with the bismuth cations and the d⁰ -TM/Te polyhedra, which is beneficial to the birefringence.

Ba3B10O17F2·0.1KF: the first mixed alkali/alkaline-earth metal fluorooxoborate with unprecedented double-layered B–O/F anionic arrangement

The first mixed alkali/alkaline-earth metal fluorooxoborate Ba₃B₁₀O₁₇F₂·0.1KF with a unique double-layered B–O/F anionic framework was successfully synthesized and systemically characterized.

A new barium fluorooxoborate BaB5O8F·xH2O with large birefringence and a wide UV transparency window

By a cation substitution strategy, a new barium fluorooxoborate BaB₅O₈F was synthesized, which exhibits two interesting interpenetrating three-dimensional B–O/F frameworks and improved optical properties.

Mg(H2O)6B4O5(OH)4(H2O)3: a new hydrated borate with a short DUV cutoff edge

[B₄O₅(OH)₄] groups are linked together by hydrogen bonds forming undulating pseudo layers, leading to a short cutoff edge.

Chloride carbodiimide K12Pb51(CN2)30Cl54 with an unprecedented 45 Å unit cell axis and a large birefringence

K₁₂Pb₅₁(CN₂)₃₀Cl₅₄ exhibits a giant birefringence of 0.094 at 1064 nm and a large cell axis due to the rotation of Pb²⁺ cations.

BaB8O12F2: a promising deep-UV birefringent material

A new alkaline-earth fluorooxoborate, BaB8O12F2, has been discovered through chemical substitution of a cation. It exhibits a short cutoff edge and a large birefringence.