(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.

Noncentrosymmetric versus Centrosymmetric: Halogen Induced Variable Coordination Modes of Sn2+ and Structural Transition in Sn3B3O7X (X = Cl and Br).

Two new borate halides, Sn3B3O7X (X = Cl and Br), were successfully synthesized via introducing Sn2+ with lone-pair and halogen into borate. Interestingly, halogen-induced variable coordination modes of Sn2+ and anion frameworks make them crystallize in different space groups, from noncentrosymmetric (Pna21) to centrosymmetric (Pbca). Sn3B3O7Cl possesses an SHG response of about 0.5 times that of KDP, while Sn3B3O7Br exhibits a large birefringence (0.123@1064 nm). The theoretical calculations were performed to elucidate the structure-property relationships.

Structural Motif Cosubstitution Strategy for Designing Fluoroaluminoborate with the Sr2Be2B2O7-Type Double-Layered Structure.

The cosubstitution strategy often was applied to design borate optical crystal materials. Revealingly, a fluoroaluminoborate Sr2Al2.18B5.82O13F2 with Sr2Be2B2O7 (SBBO) double-layered like configuration has been rationally designed and successfully synthesized based on structural motif cosubstitution strategy via the high-temperature solution method. In Sr2Al2.18B5.82O13F2, a structural motif, the [Al2B6O14F4] unit, with edge-sharing [AlO4F2] octahedra was filled in interlamination of double-layer structure. The research indicates that Sr2Al2.18B5.82O13F2 features a short ultraviolet cutoff edge (<200 nm) and moderate birefringence (∼0.058 @ 1064 nm). As the first reported linker in the interlamination of double-layer structures, the [Al2B6O14F4] unit enlightens the synthesis and discovery of new layered structures in borates.

Rare-Earth Scandium Borate Fluoride with a Deep-Ultraviolet Cutoff Edge.

Through reasonable selections of raw materials and experimental methods, a new rare-earth borate fluoride K11Sc5(B5O10)4F6 is synthesized successfully by the high-temperature solution method in a closed system, which is the first noncentrosymmetric scandium borate fluoride. It crystallizes in the Fdd2 space group of the orthorhombic crystal system and features an extremely complicated structure constructed by the fundamental building blocks [B5O10] units, Sc-based, and K-based polyhedra. To our knowledge, K11Sc5(B5O10)4F6 is the only rare-earth borate that contains two kinds of [B5O10] groups and crystallizes in the Fdd2 space group, enriching the structural chemistry of rare-earth borates and rare-earth borate fluorides. Additionally, it is discussed in detail how F can significantly improve performance by modifying the modules in a comparison of structures. Discussion on rational synthetic conditions is instructive for obtaining rare-earth borate fluorides. Furthermore, a short cutoff edge (<190 nm) is experimentally confirmed, indicating the potential application of K11Sc5(B5O10)4F6 in ultraviolet/deep-ultraviolet regions.

Introduction of the [B-O/F] Units Enhances the Band Gap and Birefringence from Na6Mg3B10O18F6 to K3NaB10O16F2.

The borate family is the main source of deep-ultraviolet (DUV) birefringent crystals, and it has attracted a lot of attention due to versatile [B-O] basic units. Herein, two new borate-based compounds Na6Mg3B10O18F6 and K3NaB10O16F2 were discovered. Their fundamental building blocks are [B5O11] and [B5O10F] units, respectively. The calculated results showed that the band gap and birefringence of K3NaB10O16F2 (Eg = 6.93 eV, Δn = 0.047 at 1064 nm) are greater than those of Na6Mg3B10O18F6 (Eg = 5.40 eV, Δn = 0.039 at 1064 nm). Furthermore, the effects of [B-O/F] units on band gap and birefringence were analyzed by the charge-transfer model and response electron distribution anisotropy method. The results show that introducing the [B-O/F] units can improve the band gap and birefringence. These findings will boost the exploration of DUV birefringent opticals.

Na2BaB12O18F4: A Mixed Alkali/Alkaline-Earth Metal Fluorooxoborate with Two Unprecedented Interpenetrating Three-Dimensional B-O/F Anionic Networks and a Short Ultraviolet Cutoff Edge.

Fluorooxoborates are promising yet largely untapped crystal materials for linear and nonlinear optical applications. The introduction of a strong electronegative F atom into an oxyboron anionic group offers a virtually unlimited chance for structural engineering and ultimately purposeful tuning of the macroscopic optical properties of the crystal. Herein, a new mixed alkali/alkaline-earth fluorooxoborate, Na2BaB12O18F4, was synthesized in a closed system. Na2BaB12O18F4 features a [B6O11F2] fundamental building unit (FBB), which polymerizes into two new (first example) independent interpenetrating three-dimensional (3D) B-O/F anionic networks constructed entirely from BO3 and BO3F units. Based on optical characterizations and the first-principles calculations, Na2BaB12O18F4 exhibits a moderate birefringence (0.054 @ 1064 nm) and a short ultraviolet (UV) cutoff edge (below 190 nm). The successful synthesis and characterization of Na2BaB12O18F4 may speed up the subsequent discovery of other mixed alkali/alkaline-earth metal fluorooxoborates.

Linear π-conjugated units of the D∞h point group as superior ultraviolet birefringent units.

At present, birefringent materials face a limited selection of large structural anisotropic functional modules (FMs). In this paper, we present a series of linear units which belong to the D∞h point group represented by (BO2)- proposed as novel birefringent active FMs. By analyzing the molecular orbital of the (BO2)- unit, it is found that there are relatively fewer non-bonding orbitals in (BO2)- than in (BO3)3- and the delocalized π bonds in (BO2)- appear in shallow energy levels, which are easily excited. Through first-principles modeling and simulation, it is found that the delocalized π bonds in (BO2)- can still show obvious transition processes, which produce a significant gain to the birefringence. Besides, a series of compounds containing linear anionic frameworks which also belong to the D∞h point group show excellent optical anisotropy in the same way. Therefore, the linear anionic basic units which belong to the D∞h point group have the great potential to become new birefringent FMs.

Recent Development of SnII , SbIII -based Birefringent Material: Crystal Chemistry and Investigation of Birefringence.

The combination of SnII or SbIII with π, non-π-conjugated units has produced birefringent crystals with birefringence ranging from 0.005 to 0.468@1064 nm. It is proven that introducing SnII or SbIII into crystals is a feasible strategy to enlarge the birefringence, which not only promotes the miniaturization of fabricated devices, but also effectively modulates polarized light. Herein, recently discovered SnII , SbIII -based birefringent crystals with birefringence investigated are summarized, including their crystal structure and optical properties, especially birefringence. This review also presents the influence of SnII , SbIII with stereochemically active lone pair on the optical anisotropy. We hope that this work provides a clear perspective on the crystal chemistry of SnII , SbIII -based optical functional crystals and promotes the development of new birefringent crystals with large optical anisotropy.

Birefringence Regulation by Clarifying the Relationship Between Stereochemically Active Lone Pairs and Optical Anisotropy in Tin-based Ternary Halides.

It is important to establish and clarify the relationship between stereochemically active lone pairs and birefringence, since it is one of the significantly effective routes to explore birefringent crystals by introducing Sn-centered polyhedra with stereochemically active lone pairs. Herein, four tin(II)-based ternary halides A3 SnCl5 and ASn2 Cl5 (A=NH4 and Rb) have been synthesized successfully. The experimental birefringence of Rb3 SnCl5 and RbSn2 Cl5 is larger than or equal to 0.046 and 0.123@546 nm, respectively. Through investigating the alkali or alkaline-earth metal tin(II)-based ternary halides, the structure-performance relationship has been concluded between stereochemically active lone pairs and optical anisotropy. It is beneficial to the analysis and prediction of birefringence in tin-based halides and provides a guide for exploring tin(II)-based optoelectronic functional materials.

α-SnF2 : A UV Birefringent Material with Large Birefringence and Easy Crystal Growth.

The simple binary fluoride α-SnF2 is shown to be an excellent birefringent material with outstanding birefringence, about 14 times that of MgF2 . Furthermore, it exhibits a shorter UV cutoff edge and easy crystal growth at ambient temperature compared to YVO4 and TiO2 . A novel theoretical calculation mode was established to analyze the stereochemical-activity lone-pair (SCALP) contribution to the birefringence (SCB) based on the SCALP's strength and arrangement, and it was found that the large birefringence of α-SnF2 mainly benefits from well-aligned [SnF5 ] polyhedra with a strong SCALP. The exploration of the α-SnF2 birefringent crystal points out the direction of the future search for excellent birefringent materials.

Sn2 PO4 I: An Excellent Birefringent Material with Giant Optical Anisotropy in Non π-Conjugated Phosphate.

Exploring non π-conjugated phosphate birefringent crystal with a large birefringence has been a great challenge. Herein, based on the unique two-dimensional layered structure in KBe2 BO3 F2 (KBBF), two new compounds, Sn2 PO4 I and Sn2 BO3 I, were designed and synthesized successfully, maintaining the layer structural feature and enhancing the optical anisotropy of crystals. In particular, the birefringence of Sn2 PO4 I is larger than or equal to 0.664 @546 nm, which is largest among the reported borates and phosphates, even surpassing commercial birefringent crystals YVO4 and TiO2 . This work indicates that a breakthrough in birefringence of inorganic compound was achieved. Also, it provides a guiding idea for exploring large birefringence materials in the future.

La3 B6 O13 (OH): The First Acentric High-Pressure Borate Displaying Edge-Sharing BO4 Tetrahedra.

La3 B6 O13 (OH) was obtained by a high-pressure/high-temperature experiment at 6 GPa and 1673 K. The compound crystallizes in the space group P21 (no. 4) with the lattice parameters a=4.785(2), b=12.880(4), c=7.433(3) Å, and ß=90.36(10)°, and is built up of corner- as well as edge-sharing BO4 tetrahedra. It represents the first acentric high-pressure borate containing these B2 O6 entities. The compound develops borate layers of "sechser"-rings with the La3+ cations positioned between the layers. Single-crystal and powder X-ray diffraction, vibrational and MAS NMR spectroscopy, second-harmonic generation (SHG) and thermoanalytical measurements, as well as computational methods were used to affirm the proposed structure and the B2 O6 entities.

Band-Gap Modulation of Nonlinear-Optical Fluorooxoborates by Controlling the F/B Ratios.

A large band gap is an important prerequisite for deep-ultraviolet materials. Here we provide a strategy to adjust the band gaps by tuning the F/B ratios. By analyzing the molecular orbitals of fluorooxo-functional groups and the electronic structures of fluorooxoborates, we find that the fluorooxo-functional groups with high F/B ratio change the composition of the highest occupied and lowest unoccupied molecular orbitals and the corresponding valence-band maximum/conduction-band minimum.

ß-CsB9 O14 : A Triple-Layered Borate with Edge-Sharing BO4 Tetrahedra Exhibiting a Short Cutoff Edge and a Large Birefringence.

The first triple-layered borate with edge-sharing BO4 tetrahedra, ß-CsB9 O14 was obtained under vacuum-sealed condition. It represents a new structure type and enriches the structural diversity of borates. Moreover, ß-CsB9 O14 exhibits a short UV cutoff edge and a large birefringence, indicating that it could be regarded as a DUV birefringent crystal.

Sn2 B5 O9 Cl: A Material with Large Birefringence Enhancement Activated Prepared via Alkaline-Earth-Metal Substitution by Tin.

The excellent birefringent materials are needed for optical systems. Herein, we reported a new compound, the first tin borate chloride, Sn2 B5 O9 Cl (SBOC) with a large birefringence (0.168 at 546 nm) measured by the polarizing microscope. Its birefringence is 16 times that of the isostructural Ba2 B5 O9 Cl (BBOC) compound (0.010@ at 546 nm). The results show that the birefringence enhancement originates mainly from the Sn2+ polyhedra. We propose that the birefringence can be enlarged by substituting the alkaline-earth metal cation by the Sn2+ cation in the isostructural borate with small birefringence. This strategy will guide the discovery of large birefringent materials in the future.

PbTeB4O9: a lead tellurium borate with unprecedented fundamental building block [B4O10] and large birefringence.

Herein, the first lead tellurium borate, PbTeB4O9, with an unprecedented fundamental building block [B4O10] was successfully synthesized. The near-parallel alignment of [B4O10] groups and [TeO3] polyhedra resulted in a high birefringence (0.099@1064 nm). The structure-property relationship was discussed by using the first-principles calculations.

CsAlB3O6Cl: the rational construction of a KBBF-type structure with aligned 2∞[AlB3O6Cl] layers via introducing unprecedented [AlO3Cl] tetrahedra.

The first chloroaluminoborate, CsAlB3O6Cl, with innovative AlO3Cl tetrahedra and a perfect planar arrangement of [B3O6] groups, was structurally designed and synthesized via chlorination of [AlO4] tetrahedra. Simultaneously, the smooth introduction of the [AlO3Cl] group into borates initiates the development of a chloroaluminoborate and greatly enriches the structural chemistry of aluminoborates.

Target-Driven Design of Deep-UV Nonlinear Optical Materials via Interpretable Machine Learning.

The development of a data-driven science paradigm is greatly revolutionizing the process of materials discovery. Particularly, exploring novel nonlinear optical (NLO) materials with the birefringent phase-matching ability to deep-ultraviolet (UV) region is of vital significance for the field of laser technologies. Herein, a target-driven materials design framework combining high-throughput calculations (HTC), crystal structure prediction, and interpretable machine learning (ML) is proposed to accelerate the discovery of deep-UV NLO materials. Using a dataset generated from HTC, an ML regression model for predicting birefringence is developed for the first time, which exhibits a possibility of achieving fast and accurate prediction. Essentially, crystal structures are adopted as the only known input of this model to establish a close structure-property relationship mapping birefringence. Utilizing the ML-predicted birefringence which can affect the shortest phase-matching wavelength, a full list of potential chemical compositions based on an efficient screening strategy is identified. Further, eight structures with good stability are discovered to show potential applications in the deep-UV region, owing to their promising NLO-related properties. This study provides a new insight into the discovery of NLO materials and this design framework can identify desired materials with high performances in the broad chemical space at a low computational cost.

PNO: a promising deep-UV nonlinear optical material with the largest second harmonic generation effect.

In this work, the polar tetrahedron [PN2O2] was revealed as a new deep-ultraviolet (deep-UV) nonlinear optically active unit. Accordingly, a thermodynamically stable compound (PNO) consisting of the polar [PN2O2] units was predicted and suggested as a promising candidate for deep-UV nonlinear optical (NLO) materials. Compared with other deep-UV materials known to date, PNO possesses the strongest second harmonic generation (SHG) coefficient (about 6 times that of KH2PO4 (KDP)). Moreover, its three-dimensional connectivity endows it with good mechanical and thermal properties. Therefore, PNO should be a new option for non-π-conjugated deep-UV NLO materials.

The Combination of Structure Prediction and Experiment for the Exploration of Alkali-Earth Metal-Contained Chalcopyrite-Like IR Nonlinear Optical Material.

Design and fabrication of new infrared (IR) nonlinear optical (NLO) materials with balanced properties are urgently needed since commercial chalcopyrite-like (CL) NLO crystals are suffering from their intrinsic drawbacks. Herein, the first defect-CL (DCL) alkali-earth metal (AEM) selenide IR NLO material, DCL-MgGa2 Se4 , has been rationally designed and fabricated by a structure prediction and experiment combined strategy. The introduction of AEM tetrahedral unit MgSe4 effectively widens the band gap of DCL compounds. The title compound exhibits a wide band gap of 2.96 eV, resulting in a high laser induced damage threshold (LIDT) of ≈3.0 × AgGaS2 (AGS). Furthermore, the compound shows a suitable second harmonic generation (SHG) response (≈0.9 × AGS) with a type-I phase-matching (PM) behavior and a wide IR transparent range. The results indicate that DCL-MgGa2 Se4 is a promising mid-to-far IR NLO material and give some insights into the design of new CL compound with outstanding IR NLO properties based on the AEM tetrahedra and the structure predication and experiment combined strategy.