Sn2B5O9Cl: 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, Sn₂B₅O₉Cl (SBOC) with a large birefringence (0.168 at 546 nm) measured by the polarizing microscope. Its birefringence is 16 times that of the isostructural Ba₂B₅O₉Cl (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.     

An Antimony(III) Fluoride Oxalate with Large Birefringence

Birefringent materials play a key role in modulating the polarization of light and thus in optical communication as well as in laser techniques and science. Designing new, excellent birefringent materials remains a challenge. In this work, we designed and synthesized the first antimony(III) fluoride oxalate birefringent material, KSb₂C₂O₄F₅, by a combination of delocalized π-conjugated [C₂O₄]²⁻ groups, stereochemical active Sb³⁺ cations, and the most electronegative element, fluorine. The [C₂O₄]²⁻ groups are not in an optimal arrangement in the crystal structure of KSb₂C₂O₄F₅; nonetheless, KSb₂C₂O₄F₅ exhibits a large birefringence (Δn=0.170 at 546 nm) that is even better than that of the well-known commercial birefringent material α-BaB₂O₄, even though the latter features an optimal arrangement of π-conjugated [B₃O₆]³⁻ groups. Based on first-principles calculations, this prominent birefringence should be attributed to the alliance of planar π-conjugated [C₂O₄]²⁻ anions, highly distorted SbO₂F₂ and SbOF₃ polyhedra with a stereochemically active lone pair. The combination of lone-pair electrons and π-conjugated systems boosts the birefringence to a large extent and will help the development of high-performance birefringent materials.     

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

The combination of Sn^II or Sb^III 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 Sn^II or Sb^III 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 Sn^II, Sb^III-based birefringent crystals with birefringence investigated are summarized, including their crystal structure and optical properties, especially birefringence. This review also presents the influence of Sn^II, Sb^III with stereochemically active lone pair on the optical anisotropy. We hope that this work provides a clear perspective on the crystal chemistry of Sn^II, Sb^III-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 A₃SnCl₅ and ASn₂Cl₅ (A=NH₄ and Rb) have been synthesized successfully. The experimental birefringence of Rb₃SnCl₅ and RbSn₂Cl₅ 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.     

Lone-Pair Enhanced Birefringence in an Alkaline-Earth Metal Tin(II) Phosphate BaSn2(PO4)2

The first alkaline-earth metal tin(II) phosphate, BaSn₂(PO₄)₂, has been discovered, which consists of layered structures constructed from strictly alternating [SnO₃]⁴⁻ and [PO₄]³⁻ moieties. This compound is expected to have a large birefringence with Δn≈0.071 at 1064 nm, owing to the presence of stereochemically active lone pair metal cations.     

Sb6O7(SO4)2: A Promising Ultraviolet Nonlinear Optical Material with an Enhanced Second-Harmonic-Generation Response Activated by SbIII Lone-Pair Stereoactivity

The stereochemical activity of lone pairs (SCALP) in a cation favors the formation of acentric materials and can enhance the second-harmonic-generation (SHG) response and/or the birefringence. By introducing functional Sb^III into sulfates, an anhydrous sulfate of Sb₆O₇(SO₄)₂ ( 1 ) is explored. Sb³⁺ cations are in seesaw configurations and in-phase aligned in a 3D asymmetric dense structure. Compound 1 exhibits an enhanced phase-matching SHG response, a moderate birefringence, a wide transparency window, and considerable environmental stabilities, which result in it being a promising UV nonlinear optical (NLO) material. Theoretical studies reveal that the stereoactive lone pairs on Sb³⁺ cations make the predominant contribution to the SHG effect. This work will attract more interest from scientists for research into SCALP-cation-based NLO materials.     

An Unprecedented Antimony(III) Borate with Strong Linear and Nonlinear Optical Responses

Antimony(III) borates with a stereochemical active lone pair remained unknown, although the first antimony borate was reported more than twenty years ago. Now, the first antimony(III) borate in a closed system is successfully synthesized, namely SbB₃O₆. Remarkably, SbB₃O₆ not only exhibits an exceptional linear optical response, that is, birefringence of Δn=0.290 at the wavelength of 546 nm, which is the largest among borates, but also has a strong nonlinear optical response of 3.5 times larger than the benchmark KH₂PO₄, exceeding those of most borates. Theoretical calculations reveal that the coexistence of strong linear and nonlinear optical responses in SbB₃O₆ should be attributable to the synergistic effect of π‐conjugated B?O anionic groups and Sb³⁺ with stereochemically active lone pair. This work provides a new class of optical bi‐functional materials with potential prospects in integrated optical devices.     

Chain-like [Sx] (x=2–6) Units Realizing Giant Birefringence with Transparency in the Near-Infrared for Optoelectronic Materials

Birefringent crystals are requisite optical devices in laser and modern opto-electronic fields. Development of excellent birefringent materials is still challenging. Herein, the linear or chain-like [S_x] (x=2–6) species were theoretically proved to be the origin of the large birefringence, and could be regarded as birefringent genes. Besides, the metal polysulfide family was first proposed to be rich birefringent materials source, among which Cs₂S₆ realizes giant birefringence 0.58@1064 nm together with a wide band gap of 1.70 eV (based on the generalized gradient approximation). Moreover, the first dual-anion group polysulfide Na₄Ba₃(S₂)₄S₃ was obtained, showing wide infrared transmission range (0.5–6.2 μm), wide band gap (2.3 eV), and large birefringence (0.37 at 1064 nm). This work provides a new guiding thought for exploring large birefringence crystals in the future.     

Polar Fluorooxoborate,NaB4O6F: 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, NaB₄O₆F (NBF) was prepared by high‐temperature solid‐state reaction. NBF belongs to the AB₄O₆F 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×KH₂PO₄ and 0.2×β‐BaB₂O₄, 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 [B₄O₆F]_∞ layer can be regarded as the “multifunctional unit”, which is responsible for the observed properties of NBF.     

Polar Fluorooxoborate,NaB4O6F: 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, NaB₄O₆F (NBF) was prepared by high‐temperature solid‐state reaction. NBF belongs to the AB₄O₆F 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×KH₂PO₄ and 0.2×β‐BaB₂O₄, 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 [B₄O₆F]_∞ layer can be regarded as the “multifunctional unit”, which is responsible for the observed properties of NBF.     

β-CsB9O14: 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, β-CsB₉O₁₄ was obtained under vacuum-sealed condition. It represents a new structure type and enriches the structural diversity of borates. Moreover, β-CsB₉O₁₄ exhibits a short UV cutoff edge and a large birefringence, indicating that it could be regarded as a DUV birefringent crystal.     

Introducing a New d0 Sc3+ Asymmetric Ion for Functional Materials: Large Birefringence Enhancement by ScO6 in Ba3Sc2(BO3)4

Transition metal ions with d⁰ electronic states (Ti⁴⁺, Zr⁴⁺, Nb⁵⁺ and Ta⁵⁺) are widely investigated as functional materials. This work first illustrates that Sc³⁺ ion, long-time ignored, displays a second-order Jahn-Teller (SOJT) effect similar to asymmetric oxide-coordinated transition metal ions, thus providing a new ground to seek for asymmetric functional materials with enhanced performances. In Ba₃Sc₂(BO₃)₄, BO₃ groups are parallelly arranged, satisfying the ideal arrangement to produce large birefringence. Importantly, distorted octahedral ScO₆ with Sc³⁺ ion in its d⁰ electronic state enlarges birefringence unexpectedly up to 0.149 @ 550 nm, which is larger than previously reported borates containing solely BO₃, even to B₃O₆ units. Subsequently, the SOJT influence of distorted ScO₆ octahedra on birefringence is verified by a comparison between experimental data and theoretical calculations. In addition, Ba₃Sc₂(BO₃)₄ also displays a high transmittance in the range of 230 nm–3.5 μm with a UV cut-off wavelength at 198 nm and a large laser induced damage threshold (2.7 GW/cm²), comparable to α-BaB₂O₄. Above characteristics imply that the title compound may be a promising birefringent material.     

Linkage Isomerism and Spin Frustration in Heterometallic Rings: Synthesis,Structural Characterization,and Magnetic and EPR Spectroscopic Studies of Cr7Ni,Cr6Ni2, and Cr7Ni2 Rings Templated About Imidazolium Cations

The synthesis and structural characterization of three heterometallic rings templated about imidazolium cations is reported. The compounds are [2,4‐DiMe‐ImidH][Cr₇Ni^IIF₈(O₂CtBu)₁₆] 1 (2,4‐DiMe‐ImidH=the cation of 2,4‐dimethylimidazole), [ImidH]₂[Cr₆Ni^II₂F₈(O₂CCtBu)₁₆] 2 (ImidH=the cation of imidazole), and [1‐Bz‐ImidH]₂ [Cr₇Ni^II₂F₉(O₂CtBu)₁₈] 3 (1‐Bz‐ImidH=the cation of 1‐benzylimidazole). The structures show the formation of octagonal arrays of metals for 1 and 2 and a nonagon of metal centers for 3 . In all cases the edges of the polygon are bridged by a single fluoride and two pivalate ligands, and the position of the divalent metal centers cannot be distinguished by X‐ray diffraction. Magnetic studies combined with EPR spectroscopy allow the characterization of the magnetic states of the compounds. In each case the exchange is antiferromagnetic with a magnetic exchange parameter J≈?5.8 cm^?1, and it is not possible to differentiate the exchange between two Cr^III centers (J_CrCr) from the exchange between a Cr^III and a Ni^II center (J_CrNi). For 2 there is evidence for the presence of at least two, possibly four, linkage isomers of the heterometallic ring, caused by the presence of two divalent metal centers in the ring. The EPR spectroscopy of 3 suggests an S=1/2 ground state of the ring and that it is likely that only one linkage isomer is present.     

Poly[tetra­aqua­di‐μ3‐malonato‐calcium(II)­zinc(II)]

The title complex, [CaZn(C₃H₂O₄)₂(H₂O)₄]_n, is a two‐dimensional polymer and consists of CaO₈ and ZnO₆ polyhedra linked together by malonate ligands. The Ca^II cation, lying on a twofold axis, is coordinated by two water mol­ecules and six malonate O atoms. The Zn^II cation, which lies on an inversion center in an octa­hedral environment, is coordinated by four malonate O atoms in an equatorial arrangement and two water mol­ecules in axial positions. The Zn—O and Ca—O bond lengths are in the ranges 2.0445 (12)–2.1346 (16) and 2.3831 (13)–2.6630 (13) Å, respectively. The structure comprises alternating layers along the [101] plane, the shortest Zn⋯Zn distance being 6.8172 (8) Å. The whole three‐dimensional structure is maintained and stabilized by the presence of hydrogen bonds.     

Rational Design of the Metal-Free KBe2BO3F2⋅(KBBF) Family Member C(NH2)3SO3F with Ultraviolet Optical Nonlinearity

The first fluorosulfonic ultraviolet (UV) nonlinear optical (NLO) material, C(NH₂)₃SO₃F, is rationally designed by taking KBe₂BO₃F₂ (KBBF) as the parent compound. C(NH₂)₃SO₃F features similar topological layers as KBBF by replacing inorganic (BO₃)³⁻ with organic C(NH₂)₃⁺ trigonal units and BeO₃F with SO₃F⁻ tetrahedra. Therefore, C(NH₂)₃SO₃F is a metal-free UV NLO crystal. Benefiting from the coplanar configuration of the C(NH₂)₃⁺ cationic groups, it possesses a large SHG response of 5×KDP and moderate birefringence of 0.133@1064 nm. Besides, it has a short UV cutoff edge of 200 nm. The calculated results reveal the shortest SHG phase-matching wavelengths can reach 200 nm. These findings highlight the exploration of metal-free compounds as nontoxic and low-cost UV NLO materials as a new research area.     

SrB5O7F3 Functionalized with [B5O9F3]6− Chromophores: Accelerating the Rational Design of Deep‐Ultraviolet Nonlinear Optical Materials

Fluorooxoborates, benefiting from the large optical band gap, high anisotropy, and ever‐greater possibility to form non‐centrosymmetric structures activated by the large polarization of [BO_xF_4?x]^(x+1)? building blocks, have been considered as the new fertile fields for searching the ultraviolet (UV) and deep‐UV nonlinear optical (NLO) materials. Herein, we report the first asymmetric alkaline‐earth metal fluorooxoborate SrB₅O₇F₃, which is rationally designed by taking the classic Sr₂Be₂B₂O₇ (SBBO) as a maternal structure. Its [B₅O₉F₃]^6? fundamental building block with near‐planar configuration composed by two edge‐sharing [B₃O₆F₂]^5? rings in SrB₅O₇F₃ has not been reported in any other borates. Solid state ¹⁹F and ¹¹B magic‐angle spinning NMR spectroscopy verifies the presence of covalent B?F bonds in SrB₅O₇F₃. Property characterizations reveal that SrB₅O₇F₃ possesses the optical properties required for deep‐UV NLO applications, which make SrB₅O₇F₃ a promising crystal that could produce deep‐UV coherent light by the direct SHG process.     

Rational Design of the Metal‐Free KBe2BO3F2⋅(KBBF) Family Member C(NH2)3SO3F with Ultraviolet Optical Nonlinearity

The first fluorosulfonic ultraviolet (UV) nonlinear optical (NLO) material, C(NH₂)₃SO₃F, is rationally designed by taking KBe₂BO₃F₂ (KBBF) as the parent compound. C(NH₂)₃SO₃F features similar topological layers as KBBF by replacing inorganic (BO₃)^3? with organic C(NH₂)₃⁺ trigonal units and BeO₃F with SO₃F^? tetrahedra. Therefore, C(NH₂)₃SO₃F is a metal‐free UV NLO crystal. Benefiting from the coplanar configuration of the C(NH₂)₃⁺ cationic groups, it possesses a large SHG response of 5×KDP and moderate birefringence of 0.133@1064 nm. Besides, it has a short UV cutoff edge of 200 nm. The calculated results reveal the shortest SHG phase‐matching wavelengths can reach 200 nm. These findings highlight the exploration of metal‐free compounds as nontoxic and low‐cost UV NLO materials as a new research area.     

Bright Infrared-to-Ultraviolet/Visible Upconversion in Small Alkaline Earth-Based Nanoparticles with Biocompatible CaF2 Shells

Upconverting nanoparticles (UCNPs) are promising candidates for photon-driven reactions, including light-triggered drug delivery, photodynamic therapy, and photocatalysis. Herein, we investigate the NIR-to-UV/visible emission of sub-15 nm alkaline-earth rare-earth fluoride UCNPs (M_1−xLn_xF_2+x, MLnF) with a CaF₂ shell. We synthesize 8 alkaline-earth host materials doped with Yb³⁺ and Tm³⁺, with alkaline-earth (M) spanning Ca, Sr, and Ba, MgSr, CaSr, CaBa, SrBa, and CaSrBa. We explore UCNP composition, size, and lanthanide doping-dependent emission, focusing on upconversion quantum yield (UCQY) and UV emission. UCQY values of 2.46 % at 250 W cm⁻² are achieved with 14.5 nm SrLuF@CaF₂ particles, with 7.3 % of total emission in the UV. In 10.9 nm SrYbF:1 %Tm³⁺@CaF₂ particles, UV emission increased to 9.9 % with UCQY at 1.14 %. We demonstrate dye degradation under NIR illumination using SrYbF:1 %Tm³⁺@CaF₂, highlighting the efficiency of these UCNPs and their ability to trigger photoprocesses.     

A Hydrogen Bonded Supramolecular Framework Birefringent Crystal

Birefringent crystals could modulate the polarization of light and are widely used as polarizers, waveplates, optical isolators, etc. To date, commercial birefringent crystals have been exclusively limited to purely inorganic compounds such as α-BaB₂O₄ with birefringence of about 0.12. Herein, we report a new hydrogen bonded supramolecular framework, namely, Cd(H₂C₆N₇O₃)₂⋅8 H₂O, which exhibits exceptionally large birefringence up to about 0.60. To the best of our knowledge, the birefringence of Cd(H₂C₆N₇O₃)₂⋅8 H₂O is significantly larger than those of all commercial birefringent crystals and is the largest among hydrogen bonded supramolecular framework crystals. First-principles calculations and structural analyses reveal that the exceptional birefringence is mainly ascribed to strong covalent interactions within (H₂C₆N₇O₃)⁻ organic ligands and the perfect coplanarity between them. Given the rich structural diversity and tunability, hydrogen bonded supramolecular frameworks would offer unprecedented opportunities beyond the traditional purely inorganic oxides for birefringent crystals.     

Structural and spectropscopic studies of a three‐dimensional hydrogen‐bonded copper(II) complex: aqua[bis(pyridin‐2‐ylcarbonyl)amidato]cyanidocopper(II)

The preparation and X‐ray and spectroscopic studies of the title copper(II) complex, [Cu(C₁₂H₈N₃O₂)(CN)(H₂O)], are reported. The Cu^II cation is five‐coordinated, forming a distorted square‐planar pyramid with an Addison τ parameter of 0.14. The UV–vis spectrum shows a d–d transition of the Cu^II centre at 638 nm, and the electron paramagnetic resonance (EPR) spectrum confirms that the Cu^II cation has an axial symmetry coordination and that the unpaired electrons occupy the d_x2–y2 orbital. Cyclic voltammetric studies show two irreversible oxidation and reduction peaks.