PbBeB2O5: A High-Performance Ultraviolet Nonlinear-Optical Crystal with Functional [BeB2O8]8- Group

High-performance nonlinear-optical (NLO) crystals need to simultaneously meet multiple basic and conflicting performance requirements. Here, by using a partial chemical substitution strategy, the first noncentrosymmetric (NCS) PbBeB2O5 crystal with a BeB2O8 group was synthesized, exhibiting a two-dimensional [BeB2O5]∞ layer constructed by interconnecting BeB2O8 groups and bridged PbO4 with an active lone pair. The crystal shows a promising UV NLO functional feature, including a strong SHG effect of 3.5 × KDP (KH2PO4), large birefringence realizing phase matchability in the whole transparency region from 246 to 2500 nm, a short UV absorption edge of 246 nm, and single-crystal easy growth. Remarkably, theoretical studies reveal that the BeB2O8 group has high nonlinear activity, which could stimulate the discovery of a series of excellent NLO beryllium borates.

NaRbB10O14(OH)4 and Na3CsB10O16(OH)2: Two Cases of Hydroxyborates with [B5Om(OH)n] Units and Deep Ultraviolet Cutoff Edges

Two deep ultraviolet (DUV) hydroxylated-alkali-metal borates, NaRbB10O14(OH)4 (I) and Na3CsB10O16(OH)2 (II), have been successfully synthesized by a high-temperature solution and solvothermal method. Both of them feature [B5Om(OH)n] units, which form chains for (I) and bilayers with nine-membered boron rings for (II). It is worth noting that both compounds exhibit very wide theoretical band gaps of 7.33 and 6.55 eV for (I) and (II), respectively, which denotes that they should have desirable DUV transmittance ability. Moreover, the title compounds have moderate birefringence owing to the π-conjugated [BO3], [BO2(OH)] groups, corresponding to 0.070 for (I) and 0.054 for (II) at 1064 nm. The structure characteristics and optical properties were also investigated and discussed. The results make it beneficial for exploring novel DUV hydroxylated borate optical crystals.

Hydroxyfluorooxoborate (NH4)[C(NH2)3][B3O3F4(OH)] for exploring the effects of cation substitution on structure and optical properties

Cation substitution is a straightforward but effective technique for improving the structure and properties; however, controlling directed substitution still poses significant difficulties. Herein, a metal-free hydroxyfluorooxoborate (NH4)[C(NH2)3][B3O3F4(OH)] has been synthesized using the strategy of heterologous substitution based on the template of A2[B3O3F4(OH)]. Tunable structure and optical properties have been achieved via varied A-site cation substitution. The intrinsic mechanism for this tunability was established by crystallography and theoretical research.

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.

(C3N6H7)2SbF5·H2O Exhibiting Strong Optical Anisotropy from the Optimal Arrangement of π-Conjugated (C3N6H7)+ Groups

An antimony fluoride melamine birefringent crystal, (C₃N₆H₇)₂SbF₅·H₂O, was obtained by introducing the π-conjugated delocalized melamine and antimony trifluoride via a simple aqueous solution evaporation method. It features one-dimensional parallel [C₃N₆H₇]_∞ chains further connected by hydrogen bonds originated from [SbF₅]^2- groups with lone pairs. The experimental optical band gap (4.74 eV) allows it to be used in the ultraviolet region. The first-principles calculations suggest that (C₃N₆H₇)₂SbF₅·H₂O exhibits a large birefringence (∼0.38@550 nm), which is twice larger than that of the commercial CaCO₃ crystal. Therefore, introducing the fluoride into π-conjugated melamine may be a good tactic to obtain birefringent crystals with large optical anisotropy.

NaB3O4F(OH): A Hydroxyfluorooxoborate with One-Dimensional Chain Featuring Large Birefringence and Short Ultraviolet Cutoff Edge

A new sodium hydroxyfluorooxoborate, NaB₃O₄F(OH) (NBOFH), was discovered and synthesized. NBOFH features the unprecedented [B₃O₄F(OH)] infinite chain constructed by the novel fundamental building block (FBB) of [B₃O₅F(OH)]. NBOFH has a large birefringence of 0.097 at 1064 nm and short ultraviolet (UV) cutoff edge below 200 nm. First-principles calculations and response electron distribution anisotropy (REDA) were performed to explain the structure-property relationships. This work provides a novel strategy for the synthesis of deep-ultraviolet birefringent crystals and enriches the structural diversity of the emerging hydroxyfluorooxoborates.

Na2SrB16O26: a new borate with independent interpenetrating B-O networks and deep-ultraviolet cutoff edge

A new borate, Na₂SrB₁₆O₂₆, was synthesized by the high-temperature solution method. It exhibits complicated interpenetrating 3D B-O frameworks composed of the functional building block (FBB) [B₈O₁₆]. The UV-vis-NIR diffuse reflectance spectroscopy shows that it has a deep-ultraviolet (DUV) cutoff edge (<200 nm). The relationship between the structures and optical properties was uncovered by theoretical calculations. By the first-principles calculation, the birefringence is estimated to be 0.07 at 1064 nm. The response electron distribution anisotropy (REDA) analysis indicates that the [BO₃] units contribute mainly to the generation of the moderate birefringence.

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.

Lead Tellurite Crystals BaPbTe2O6 and PbVTeO5F with Large Nonlinear-/Linear-Optical Responses due to Active Lone Pairs and Distorted Octahedra

The exploration of nonlinear-/linear-optical crystal materials with high performance is an extremely difficult research project. Herein, the two new lead tellurite crystals BaPbTe₂O₆ and PbVTeO₅F were successfully obtained through a facile hydrothermal synthesis strategy. BaPbTe₂O₆ lies in the noncentrosymmetric (NCS) and chiral orthorhombic space group P2₁2₁2₁, featuring a unique _∞¹[PbTe₂O₆] chain consisting of the PbO₄ and TeO₃ building units, while PbVTeO₅F belonging to the centrosymmetric (CS) orthorhombic space group Pbca manifests a 2D layer made up of _∞¹[PbO₄F₂] chains and novel [V₂Te₂O₁₀F₂] clusters. Further, a systematic analysis of lead tellurites finds that the coordination geometries of the Pb atom exert a considerable influence on the connection modes of Pb-O and Te-O building units. BaPbTe₂O₆ shows a great second-harmonic-generation (SHG) effect of ∼5× the benchmark KH₂PO₄ (KDP) and a large optical birefringence of 0.086 at 590 ± 3 nm. PbVTeO₅F demonstrates a remarkably larger birefringence of 0.142 at 590 ± 3 nm, benefiting from the introduction of the VO₅F octahedral unit. Theoretical studies reveal that the large SHG and birefringence in BaPbTe₂O₆ can be attributed to TeO₃ and PbO₄ polyhedra with active lone pairs, while the remarkably enlarged birefringence in PbVTeO₅F is attributable to the highly distorted octahedral VO₅F. The functional orientations of active building units may offer a practical insight into the design of the desired optical functional materials.

Pb2Al2B3O8F3: Structure and Properties of a New Fluoroaluminoborate with Non-traditional Chain-like B3O8 Group

A new fluoroaluminoborate, Pb2Al2B3O8F3, with a non-traditional chain [B3O8]7- was synthesized by high temperature reactions in closed systems for the first time. Different from the traditional [B3O8]7- ring [(3:Δ +...

(C3N6H7)2SiF6·H2O: an ultraviolet birefringent crystal exceeding the intrinsic energy gap of an organic reagent

A new organic–inorganic hybrid compound (C3N6H7)2SiF6·H2O, in which [C3N6H7]+ groups are uniformly arranged under the regulation of SiF6 octahedra, shows highly polarization anisotropy and a breakthrough in the energy gap.

Sc2F2(B2O5): a deep ultraviolet scandium borate fluoride exhibiting large birefringence induced by the synergistic effect of B2O5 and ScOnF2 groups

A rare earth borate fluoride, Sc2F2(B2O5), displays a deep ultraviolet cutoff edge (<200 nm) and very large birefringence induced by the synergistic effect of B2O5 and ScOnF2 groups.

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

PNO with the largest SHG response in the deep-UV region was discovered by structural prediction methods.

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.

RbMo2P3O14 with large birefringence mainly induced by highly distorted [MoO6] in uncommon [Mo2P3O14]∞ layers

The introduction of d⁰ transition metal Mo⁶⁺ cations into a phosphate generates a new acentric molybdophosphate, RbMo₂P₃O₁₄. It shows uncommon [Mo₂P₃O₁₄]_∞ layers composed of isolated [MoO₆] octahedra and [P₃O₁₀] groups. To the best of our acknowledge, it exhibits the largest birefringence (a calculated value of 0.166 at 546 nm) among reported molybdophosphates. In addition, it also possesses a shorter UV cut-off edge (about 250 nm) than other molybdates and molybdophosphates, indicating that it can be used as a birefringent crystal in the UV optical region. First-principles electronic structure analysis suggests that the large birefringence mainly originates from highly distorted [MoO₆].

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.

Li4 MgGe2 S7 : The First Alkali and Alkaline-Earth Diamond-Like Infrared Nonlinear Optical Material with Exceptional Large Band Gap

Large band gap and strong nonlinear optical (NLO) effect are two valuable but contradictory parameters, which are difficult to balance in one infrared (IR) NLO material. Herein, the first alkali and alkaline-earth metal diamond-like (DL) IR NLO material Li₄ MgGe₂ S₇ , presenting a honeycomb-like 3D framework constructed by 6-membered LiS₄ rings and GeMgS₆ zigzag chains, was rationally designed and synthesized. The introduction of rigid alkali metal and alkaline-earth metal LiS₄ and MgS₄ tetrahedra effectively broadens the band gap of DL compound to 4.12 eV (the largest one in the reported quaternary metal chalcogenides), generating a high laser damage threshold of 7 × AgGaS₂ at 1064 nm. Furthermore, Li₄ MgGe₂ S₇ displays a suitable SHG response (0.7 × AgGaS₂ ) with a type I phase-matching behavior. The results indicate that Li₄ MgGe₂ S₇ is a promising IR NLO material for the high-power laser application and it provides an insight into the design of new DL compound with outstanding IR NLO performances.

The First Mixed Calcium Zinc Borate with a Flexible [B8 O17 ] Fundamental Building Block and Short UV Cutoff Edge

The first mixed calcium zinc borate with a new fundamental building block (FBB) [B₈ O₁₇ ], Ca_1.13 Zn_0.87 B₈ O₁₄ has been successfully synthesized. It exhibits two independent interpenetrating three-dimensional B-O anion networks constructed by [B₈ O₁₇ ] groups, enriching the structural diversity of B-O configurations. In particular, the UV-Vis-NIR diffuse-reflectance spectrum shows that it has a short UV cutoff edge (<195 nm).

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.

Cs2ZnSn3S8: A Sulfide Compound Realizes a Large Birefringence by Modulating the Dimensional Structure

Birefringence, an important optical performance parameter for optoelectronic functional materials, is mainly influenced by the types of anion groups and their spatial arrangement. Inspired by the relationship between the structure and properties of chalcogenides, combined with the dimensional transformation, we successfully synthesized a sulfide compound (Cs₂ZnSn₃S₈) with a two-dimensional layered structure and a large birefringence. The experimental results showed that, compared with Rb₁₀Zn₄Sn₄S₁₇, Cs₂ZnSn₃S₈ achieved the structural transition from a zero-dimensional arrangement to a two-dimensional lamellar arrangement and achieved a breakthrough of birefringence from 0 to 0.12, which was determined by both experiments and first-principles calculations. These findings demonstrated that Cs₂ZnSn₃S₈ was a potential birefringent material and provided instructions for the study of the synthesis of birefringent materials.

Sr2Pb(BeB5O10)(BO3): An Excellent Ultraviolet Nonlinear-Optical Beryllium Borate by the Pb-Modified Construction of a Conjugated System and Lone-Pair Effect

The design of material by chemical and/or crystalline modification of a classic structure model benefits not only the optimized physical properties but also the controllability and efficiency. Herein, a new nonlinear-optical (NLO) beryllium borate crystal, Sr₂Pb(BeB₅O₁₀)(BO₃) (SPBBO), is successfully designed and synthesized by chemical and crystalline modification of the perovskite-like K₃B₆O₁₀Cl NLO crystal. SPBBO displays a 3D BeB₅O₁₀^3- open-framework structure composed of interconnecting BeB₅O₁₃ groups with filled cationic Sr/Pb and anionic BO₃ groups, which exhibits the striking enhancement of the second-harmonic-generation (SHG) response (8 × KDP) and birefringence (0.10) compared to the parent model. Replacement of K by Sr and Pb with a lone pair and replacement of Cl by conjugated BO₃ result in the synergistic conjugation of Pb with host BeB₅O₁₀^3- and filled BO₃ groups, contributing to the striking enhancement of the SHG and birefringence. Single-crystal measurements show that SPBBO has a short UV absorption edge of 280 nm with a wide energy band gap of 4.35 eV and an outstanding laser-induced resistant behavior with a remarkably high laser-induced damage threshold of 2100 MW cm^-2. The excellent properties indicate that the SPBBO crystal is a very promising UV NLO functional material.

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.

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⁻.

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.

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.

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)}).

Hg3(Te3O8)(SO4): a new sulfate tellurite with a novel structure and large birefringence explored from d10 metal compounds

Our exploration of novel inorganic solids with large birefringence in the d10 Metal-Te4+-SO42- system afforded four new sulfate tellurites, Ga2(TeO3)(SO4)(OH)2, In2(TeO3)2(SO4)(H2O), Zn4(Te3O7)2(SO4)2(H2O) and Hg3(Te3O8)(SO4). Notably, Hg3(Te3O8)(SO4) exhibits the largest birefringence at 532 nm (0.184) and 1064 nm (0.166) among the reported metal sulfate tellurites.

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.

[Al(H2O)6](IO3)2(NO3): a material with enhanced birefringence induced by synergism of two superior functional motifs

We report a new strongly birefringent material, [Al(H₂O)₆](IO₃)₂(NO₃) (AINO). Based on a detailed analysis, we concluded that its enhanced birefringence (0.253 at 546 nm) predominantly resulted from a synergy of the effects of its (NO₃)^- and (IO₃)^- functional motifs, combined with an equally important and indispensable optimal arrangement of these two groups in the structure. This work has provided insight for discovering new birefringent materials.

Enhanced optical anisotropy via dimensional control in alkali-metal chalcogenides

Crystals with both large birefringence and wide transparent range are suitable for broad applications in the areas of optical communications, the laser industry and modulation of the light polarization requirement. In this work, to assist the design of urgently needed crystals with large birefringence in the infrared (IR) region, typical alkali-metal chalcogenides, KPSe6, Na2Ge2Se5, and Li2In2GeSe6 have been studied. They exhibit a hierarchical characteristic in the calculated birefringence by about 0.21, 0.11, and 0.04, respectively. To explore the origin of the birefringence difference, the polarizability anisotropy and the effect of electron distribution anisotropy are analyzed. The alkali-metal chalcogenides KPSe6, Na2Ge2Se5, and Li2In2GeSe6 feature infinite one-dimensional (1D) chains of [PSe6], 2D anionic framework of [Ge2Se5] layers and 3D [In2GeSe9] networks, respectively. It is found that the anionic group with low-dimensional configuration could enhance polarizability anisotropy and render large birefringence for the macroscopic structure. This provides evidence that a low-dimensionality configuration in the structure would be beneficial for the enhancement of optical anisotropy, which can motivate the exploration and design of novel IR birefringent materials.

A(H3C3N3O3)(NO3) (A = K, Rb): Alkali-Metal Nitrate Isocyanurates with Strong Optical Anisotropy

The first alkali-metal nitrate isocyanurates, A(H₃C₃N₃O₃)(NO₃) (A = K, Rb), were synthesized by the tactic of introducing (NO₃)^- into isocyanurate with a mild hydrothermal technique. They crystallized into the same monoclinic centrosymmetric (CS) space group P2₁/c, which featured a 2D [(H₃C₃N₃O₃)(NO₃)]_∞ layered structure separated by K⁺ and Rb⁺ cations, respectively. Both compounds exhibited short ultraviolet cutoff edges (λ_cutoff = 228 and 229 nm) and large birefringences (Δn = 0.253 and 0.224 at 546.1 nm). More importantly, in comparison with most of the isocyanurates and nitrates, they have better thermal stability with decomposition temperatures up to 319.8 and 324.4 °C. In addition, our theoretical calculations reveal that the π-conjugated groups play significant roles in improving the optical anisotropy. Remarkably, introducing a π-conjugated inorganic acid radical (NO₃)^- into isocyanurate is an extremely meaningful strategy to explore new UV birefringent crystals.

Gd(NO3)(Se2O5)·3H2O: a nitrate-selenite nonlinear optical material with a short ultraviolet cutoff edge

The first example of a rare-earth metal nitrate-selenite nonlinear optical (NLO) crystal, Gd(NO₃)(Se₂O₅)·3H₂O (1), was synthesized by hydrothermal reaction. This compound crystallizes in the noncentrosymmetric space group P2₁2₁2₁, and its structure features two-dimensional (2D) [Gd(NO₃)(Se₂O₅)]_∞ layers, extending to a pseudo-3D framework linked by hydrogen bonds. 1 exhibits a phase-matchable SHG efficiency about 0.2 times that of KH₂PO₄ (KDP) and a large laser damage threshold of 236.26 MW cm^-2. The UV-Vis-NIR diffuse reflectance spectroscopy study shows that the UV cutoff edge of 1 is 224 nm, which is the shortest value observed among selenite-based NLO materials. The birefringence of 1 is 0.109@1064 nm based on density functional theory calculations. These optical properties make 1 a potential UV NLO material. Theoretical studies using density functional theory have been implemented to further understand the relationship between its optical properties and band structures.

Ba(B2OF3(OH)2)2 with well-ordered OH/F anions and a unique B2OF3(OH)2 dimer

A new centimetre-scale fluorooxoborate crystal Ba(B₂OF₃(OH)₂)₂, with well-ordered OH/F units and a new dimer containing BOF₃ units, has been fabricated.

Refractive properties of the α-BaGeO3 crystal and their origins: a density functional theory study

Density functional theory calculations show that α-BaGeO₃ is a promising birefringent crystal used in the mid-IR region; its unique refractive characteristics are associated with the Ba–O bonds and their spatial orientations.

PbSrSiO4: a new ultraviolet nonlinear optical material with a strong second harmonic generation response and moderate birefringence

PbSrSiO₄ exhibits excellent properties, including a strong second harmonic generation response (∼5.8 × KDP), a short UV cut-off edge (240 nm) and moderate birefringence (0.053@1064 nm).

Rb3Na(H2C3N3O3)4·3H2O with Large Birefringence

A mixed alkali-metal nonlinear optical (NLO) dihydro-cyanurate crystal Rb₃Na(H₂C₃N₃O₃)₄·3H₂O has been synthesized via the hydrothermal method. Its calculated birefringence is about 0.368, which is very large, its ultraviolet (UV) cutoff edge is down to 230 nm, and the powder second harmonic generation (SHG) intensity is about 0.2 × KDP. In addition, a first-principles investigation of the electronic properties on Rb₃Na(H₂C₃N₃O₃)₄·3H₂O was carried out. The calculated band gap and SHG coefficient values agree well with the experimental ones. These results suggest that it could be applied as a UV birefringent material.

[Ln6O8] Cluster-Encapsulating Polyplumbites as New Polyoxometalate Members and Record Inorganic Anion-Exchange Materials for ReO4 - Sequestration

Various types of polyoxometalates (POMs) have been synthesized since the 19th century, but their assortment has been mostly limited to Groups 5 and 6 metals. Herein, a new family of POMs composed of a carbon group element as the addenda atoms with two distinct phases, LnPbOClO4-1 (Ln = Sm to Ho, Y) and LnPbOClO4-2 (Ln = Er and Tm) is reported. Both structures are built from [Ln6O8] rare-earth metal hexamers being incorporated in [Pb18O32]/[Pb12O24] polyplumbites, and unbound perchlorates as charge-balancing anions. Impressively, YPbOClO4-1 and ErPbOClO4-2 exhibit exceptional uptake capacities (434.7 and 427.7 mg g-1) toward ReO4 -, a chemical surrogate for the key radioactive fission product in the nuclear fuel cycle 99TcO4 -, which are the highest values among all inorganic anion-exchange materials reported until now. The sorption mechanism is clearly elucidated and visualized by single-crystal-to-single-crystal structural transformation from ErPbOClO4-2 to a perrhenate-containing complex ErPbOReO4 , revealing a unique ReO4 - uptake selectivity driven by specific interaction within Pb···O-ReO3 - bonds.

RE(H2C3N3O3)2·(OH)·xH2O (RE = La, Y and Gd): potential UV birefringent materials with strong optical anisotropy originating from the (H2C3N3O3)- group

A new series of dihydro-isocyanurates, namely, RE(H₂C₃N₃O₃)₂·(OH)·xH₂O (RE = La, Y and Gd) was readily obtained by a mild hydrothermal technique using lithium hydroxide as a deprotonation agent. La(H₂C₃N₃O₃)₂·OH·2H₂O and Re(H₂C₃N₃O₃)₂·OH·5H₂O (Re = Y, Gd) crystallized in the centrosymmetric (CS) space groups P2₁/c and P1[combining macron], respectively. Their intricate 3D frameworks were built by cationic polyhedrons (LaO₈N/ReO₈) and (H₂C₃N₃O₃)^- groups. The experiments and theoretical studies demonstrated that these compounds possessed short ultraviolet cut-off edges (≤240 nm) and large birefringences (Δn≥ 0.257@546.1 nm). In addition, the theoretical studies further revealed that the (H₂C₃N₃O₃)^2- group plays a pivotal role in strengthening the optical anisotropy for all compounds.

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.