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

β-CsHg2I5, a compound with rare [Hg2I5] dimers and large optical anisotropy

Hg-based compounds show abundant structural diversity and distinguished properties. Herein, a new phase transition compound CsHg2I5 was reported. The high-temperature phase β-CsHg2I5 with rare [Hg2I5] dimers was synthesized by the flux method at 573 K, and it shows a reversible phase transition at a low temperature of ∼100 K to form the low-temperature phase α-CsHg2I5. The two phases crystallize in the same P21/c space group, with different crystal structures. β-CsHg2I5 is composed of rare [Hg2I5] dimers and [CsI11] polyhedral units, while α-CsHg2I5 is composed of [Hg4I11] and [CsI10] units. The experimental band gap of β-CsHg2I5 was found to be 2.58 eV. Owing to the presence of [Hg2I5]∞ pseudo-layers, β-CsHg2I5 exhibits large optical anisotropy with a calculated birefringence of 0.132@1064 nm. Meanwhile, β-CsHg2I5 is a congruent compound and the congruent point is ∼481 K. Theoretical calculations indicate that the rare [Hg2I5] dimer is a nonlinear active unit, which can be used as a new fundamental building block for the design of advanced nonlinear optical materials. Moreover, a CsI-HgI2 pseudo-binary diagram was drawn. The results enrich the structural diversity of Hg-based halides and give some insights into the development of new functional materials based on rare [Hg2I5] dimers.

A Sharp Improvement of Sulfate's Birefringence Induced by the Synergistic Effect of Heteroleptic and Dimeric Strategies

As for tetrahedron-based ultraviolet nonlinear optical crystals, it is so difficult to achieve a sufficient birefringence to satisfy the phase-matching condition. Thereover, it is necessary to greatly increase the polarizability anisotropy of tetrahedra. Meanwhile, the tetrahedra should be arranged as uniformly as possible. Based on these ideas, a new strategy that dimerizes the heteroleptic tetrahedra with heteroleptic atoms as bridges is proposed and implemented, resulting in a new compound, Na2S3O6, which crystallizes in a centrosymmetric monoclinic phase (phase-I) and a non-centrosymmetric orthorhombic one (phase-II). In the phase-I, a large birefringence of 0.103 @ 546 nm comparable to those of many triangle-based crystals is confirmed experimentally, indicating the ability of [S3O6] group in improving the birefringence. Besides, the phase-II exhibits an improved birefringence of 0.056 @ 546 nm and a moderate second harmonic generation response of 1.4 × KH2PO4. In particular, the phase-II exhibits a typical phase-matching behavior under the irradiation of 532 nm laser. Moreover, the excellent optical properties of [S3O6] group are further verified by theoretical calculations. These results completely illustrate the validity of the design strategy. Therefore, this work lights a new route for exploring novel ultraviolet nonlinear optical crystals.

Hg4(Te2O5)(SO4): A Giant Birefringent Sulfate Crystal Triggered by a Highly Selective Cation

Sulfate crystals are often criticized for their low birefringence. The small anisotropic SO4 group is becoming the biggest bottleneck hindering the application of sulfates in optical functional materials. In this study, we report a new method to significantly enhance the birefringence of sulfates. The title compound increases the birefringence recording of sulfates to 0.542@546 nm, which is significantly larger than that of the commercial birefringent crystal of TiO2 (0.306@546.1 nm). At the infrared wavelength, the birefringence of Hg4(Te2O5)(SO4) can be up to 0.400@1064 nm, which is also much larger than the infrared birefringent crystal of YVO4 (0.209@1064 nm). In addition, it also has a wide transparency range, high thermal stability, and excellent environmental stability, making it a potential birefringent material. Hg4(Te2O5)(SO4) features a novel two-dimensional layered structure composed of [Hg4(Te2O5)]2+ layers separated by isolated (SO4)2- tetrahedra. This compound was designed by introducing a highly selective cation in a tellurite sulfate system. The low valence low coordination cations connect with tellurite groups only, making the sulfate isolated in the structure. The steric repulsive action of the isolated SO4 tetrahedra may regulate the linear and lone pair groups arranged in a way that favors large birefringence. This method can be proven by theoretical calculations. PAWED studies showed that the large birefringence originated from the synergistic effect of (Hg2O2)2-, (Te2O5)2-, and (SO4)2- units, with a contribution ratio of 42.17, 37.92, and 19.88%, respectively. Our work breaks the limitation of low birefringence in sulfates and opens up new possibilities for their application as birefringent crystals.

AYSO4F2 (A = K, Rb): [YO4F4] Polyhedra Enhancement of Birefringence in Non-π-Conjugated Sulfate Systems

A mild hydrothermal method was employed to successfully synthesize two new sulfate fluorides, namely, AYSO4F2 (A = K, Rb). They are isomorphic, and both contain [YO4F4] polyhedra and [SO4] tetrahedra in the structure. Theoretical calculations and experimental tests show that AYSO4F2 (A = K, Rb) have large band gaps (7.79 and 7.82 eV) and moderate birefringence (0.015 and 0.02 @ 546.1 nm), with significantly enhanced birefringence and band gaps as compared to that of the single alkali metal sulfates A2SO4 (A = K, Rb). Furthermore, theoretical calculations show that [YO4F4] polyhedra are the main reason for the band gap and birefringence enhancement. This work contributes to the advancement of structural chemistry in the field of rare-earth sulfates, offering a novel approach for the design of sulfates characterized by large birefringence.

Designing Sulfate Crystals with Strong Optical Anisotropy through π-Conjugated Tailoring

Sulfate crystals typically exhibit minimal optical anisotropy due to the near-zero polarizability anisotropy (δ) of [SO4 ]2- tetrahedra, arising from highly symmetrical electron clouds. Recent research sought to enhance δ via chemical modifications, such as fluorination. However, the resultant crystals often maintain subpar optical anisotropy, frequently with birefringence values below 0.1. In this study, we have uncovered that δ can be significantly strengthened by chemically tailoring the tetrahedral [SO4 ]2- with anisotropic π-conjugated modules. This has been demonstrated by several newly proposed S-O-Org (Org: π-conjugated organic species) moieties, which show a sharp increase in δ based on theoretical computations. To further validate this experimentally, we synthesized and characterized six new 3-pyridinesulfonate crystals with the formula A(3-C5 H4 NSO3 ) ⋅ xH2 O (A=Li, Ag, K, Rb, Cs, and NH4 ; x=0 and 1). Notably, these materials exhibit strong optical anisotropy, with birefringence values ranging from 0.240 to 0.312 at 546 nm. These values are approximately 23 to 145.5 times greater than those of corresponding sulfates, and they outperform a vast number of sulfate-related optical materials, thus verifying the effectiveness of the proposed strategy. Furthermore, the title compounds exhibit diverse microstructure peculiarities influenced by the size and binding natures of the counter cations.

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.

Anisotropic structure building unit involving diverse chemical bonds: a new opportunity for high-performance second-order NLO materials

We define the anisotropic structure building unit that encompasses diverse chemical bonds (ABUCB). The ABUCB is highly likely to cause anisotropy in both crystallographic structure and spatial electron distribution, ultimately resulting in enhanced macroscopic optical anisotropy. Accordingly, the (PO3F)2- or (SO3F)- tetrahedron involving the unique P-F or S-F bond serves as such an ABUCB. The distinct chemical bond effectively alters the microscopic nature of the structure building unit, such as polarizability anisotropy, hyperpolarizability, and geometry distortion; this consequently changes the macroscopic second-order nonlinear optical (2nd-NLO) properties of the materials. In this review, we summarize both typical and newly emerged compounds containing ABUCBs. These compounds encompass approximately 90 examples representing six distinct categories, including phosphates, borates, sulfates, silicates, chalcogenides and oxyhalides. Furthermore, we demonstrate that the presence of ABUCBs in DUV/UV NLO compounds contributes to an increase in birefringence and retention of a large band gap, facilitating phase matching in high-energy short-wavelength spectral ranges. On the other hand, the inclusion of ABUCBs in IR NLO compounds offers a feasible method for increasing the band gap and consequently enhancing the larger laser-induced damage threshold. This review consolidates various trial-and-error explorations and presents a novel strategy for designing 2nd-NLO compounds, potentially offering an opportunity for the development of high-performance 2nd-NLO materials.

Optically Anisotropic Mixed-Metal Fluoroiodate Ba2[GaF5(IO3F)] with a Wide Optical Transparent Window and a Moderate Birefringence

An optically anisotropic alkali-earth-metal gallium fluoroiodate, Ba2[GaF5(IO3F)] (1), was ingeniously obtained by integrating fluoride and fluoroiodate functional units under moderate hydrothermal conditions. It features a three-dimensional (3D) structure constructed by the highly polarizable fluoroiodate unit [IO3F] and the fluoride groups [GaOF5] and [BaO3Fx] (x = 6, 7). The compound is stable at temperatures up to 500 °C. With the synergistic interaction between [IO3F] and the fluoride groups, the mixed-metal fluoroiodate induces a short ultraviolet cutoff edge at about 230 nm, a medium measured birefringence of 0.068 @ 550 nm, and a wide optical transparent window (0.34-11.9 μm), indicating that 1 has potential applications as a birefringent material from near-UV to mid-infrared. Theoretical calculations prove that the optical characteristics of the compound are mainly attributed to [IO3F] and the fluoride functional groups. This work demonstrates that the presence of various specific functional groups in compounds will help to develop promising inorganic functional materials possessing good optical performance.

YSO4F·H2O: A Deep-Ultraviolet Birefringent Rare-Earth Sulfate Fluoride with Enhanced Birefringence Induced by Fluorinated Y-Centered Polyhedra

Birefringent crystals can modulate and detect the polarization of light and are important optical functional materials. The birefringence is positively correlated to the anisotropy of the structure. By partially substituting sulfate anion with large electronegative fluorine in the parent compound Y2(SO4)3·8H2O, a new fluorinated rare-earth sulfate YSO4F·H2O with enhanced anisotropy was achieved. YSO4F·H2O features a dense 3D structure constructed by the polarizable [YOF] polyhedra and [SO4] tetrahedra. The diffuse reflectance spectrum reveals that it has a short UV absorption edge of below 200 nm. The substitution of the F- ion enhances the optical anisotropy, making the material exhibit an enhanced birefringence (0.0357 at 546 nm), which is 5.1 times that of the parent compound and is also larger than most deep-UV birefringent sulfates. It is expected that this work may shed useful insights in the exploration of deep-UV birefringent materials with enhanced optical performances..

From H12C4N2CdI4 to H11C4N2CdI3: a highly polarizable CdNI3 tetrahedron induced a sharp enhancement of second harmonic generation response and birefringence

In this study, we identify a novel class of second-order nonlinear optical (NLO) crystals, non-π-conjugated piperazine (H10C4N2, PIP) metal halides, represented by two centimeter-sized, noncentrosymmetric organic-inorganic metal halides (OIMHs), namely H12C4N2CdI4 (P212121) and H11C4N2CdI3 (Cc). H12C4N2CdI4 is the first to be prepared, and its structure contains a CdI4 tetrahedron, which led to a poor NLO performance, including a weak and non-phase-matchable second harmonic generation (SHG) response of 0.5 × KH2PO4 (KDP), a small birefringence of 0.047 @1064 nm and a narrow bandgap of 3.86 eV. Moreover, H12C4N2CdI4 is regarded as the model compound, and we further obtain H11C4N2CdI3via the replacement of CdI4 with a highly polarizable CdNI3 tetrahedron, which results in a sharp enhancement of SHG response and birefringence. H11C4N2CdI3 exhibits a promising NLO performance including 6 × KDP, 4.10 eV, Δn = 0.074 @1064 nm and phase matchability, indicating that it is the first OIMH to simultaneously exhibit strong SHG response (>5 × KDP) and a wide bandgap (>4.0 eV). Our work presents a novel direction for designing high-performance NLO crystals based on organic-inorganic halides and provides important insights into the role of the hybridized tetrahedron in enhancing the SHG response and birefringence.

Improved Birefringence Activated by Tetrahedra Decorated with a Single Linear Unit

Performance enhancement induced by structural modification has long been the target in materials science fields. Direct evidence to witness the effectivity of one strategy is challenging and necessary. In this work, a tetrahedra-decoration strategy was proposed to improve the birefringent performance sharply, namely decorating the tetrahedra with a single linear [S2 ] unit. The strategy was verified by comprehensive characterization of two thiogermanates K2 BaGeS4 and K2 BaGeS5 , which crystallize in the same space group, have similar unit cells and the same units arrangements. Theoretical characterization verified that the [GeS5 ] group has much larger polarization anisotropy than [GeS4 ], further demonstrated that the linear [S2 ] led to the sharp birefringence enlargement of K2 BaGeS5 (0.19 vs 0.03 of K2 BaGeS4 ). This work provides a new guiding thought to improve the birefringence performance.

Li2RbSO4Cl with a Short Ultraviolet Absorption Edge and an Acentric Structure through Assembling Heteroleptic [LiO3Cl] Tetrahedra

Currently, short-wavelength nonlinear optical materials are urgently needed. Through substituting homoleptic [LiO4] in centrosymmetric LiRbSO4 with heteroleptic [LiO3Cl] tetrahedra, an acentric sulfate chloride, Li2RbSO4Cl, was designed and synthesized by the high-temperature melting method. Li2RbSO4Cl shows a relatively short ultraviolet absorption edge (<200 nm) among newly reported sulfate chlorides. Millimeter-sized crystals were grown due to the congruent melting behavior and high thermal stability of the compound.

Mixed Alkali Metal and Alkaline Earth Metal Scandium Borate Birefringence Material with Layered Structure and Short Ultraviolet Cutoff Edge

Birefringent crystals are essential in the domains of linear and nonlinear optics that need light wave polarization control. Rare earth borate has become a popular study material for ultraviolet (UV) birefringence crystals due to its short cutoff edge in the UV area. RbBaScB₆O₁₂, a two-dimensional layered structure compound with the B₃O₆ group, was effectively synthesized through spontaneous crystallization. The UV cutoff edge of RbBaScB₆O₁₂ is shorter than 200 nm, and the experimental birefringence is 0.139 @ 550 nm. Theoretical research indicates that the large birefringence originates from the synergistic impact of the B₃O₆ group and the ScO₆ octahedron. RbBaScB₆O₁₂ is an outstanding candidate material for birefringence crystals in the UV and even deep UV regions due to its short UV cutoff edge and significant birefringence.

Intense d-p Hybridization in Nb3 O15 Tripolymer Induced the Largest Second Harmonic Generation Response and Birefringence in Germanates

We herein report two asymmetric germanate crystals, KNbGe₃ O₉ and K₃ Nb₃ Ge₂ O₁₃ , with different structures and optical properties derived from divergent polymerized forms of GeO₄ and NbO₆ groups. Remarkably, K₃ Nb₃ Ge₂ O₁₃ achieved a rare combination of the strongest second harmonic generation (SHG) response of 17.5×KDP @ 1064 nm and the largest birefringence of 0.196 @ 546 nm in germanates. It features unique [Nb₃ O₁₂ ]_∞ tubular chains constructed by circular Nb₃ O₁₅ tripolymers. Theoretical calculations reveal that the d-p interactions in the Nb₃ O₁₅ group are responsible for outstanding optical properties. This work emphasizes the significance of the polymerizable functional units in obtaining high-performance nonlinear optical (NLO) crystals.

Aluminoborates as Nonlinear Optical Materials

Ultraviolet (UV) and Deep-UV (DUV, λ<200 nm) nonlinear optical (NLO) materials play a significant role to convert the wavelength of laser for producing UV/DUV coherent light in the laser industry. The Al [Ne]3s² 3p¹ atom with empty d orbitals can form sp³ , sp³ d, and sp³ d² hybrid orbitals when bonding with O/F atoms, which leads to the construction of [AlO_m F_n ] (m+n=4, 5, 6) units. Therein, aluminoborates have received intensive attention due to the effective structural and functional adjustment effects of the diverse Al-O/F units. Accordingly, numerous aluminoborates as candidates for the next generation of UV/DUV NLO materials were discovered. In this review, recently discovered aluminoborates are overviewed and analyzed, including their syntheses, crystal structure features, and second-order NLO performances. We aim to provide the latest progress and outlook on the crystal chemistry of aluminoborates and boost the finding of the next NLO candidates with high performances.

A High-Performance Nonlinear Optical Crystal with a Building Block Containing Expanded π-Delocalization

Common nonlinear optical (NLO) crystals consist of traditional functional building blocks with inherent optical limitation. Herein, inspired by traditional (B₃ O₆ )^3- inorganic building block, we theoretically identified a new type of organic functional building blocks and then successfully synthesized the first cyamelurate NLO crystal, Ba(H₂ C₆ N₇ O₃ )₂  ⋅ 8 H₂ O. To our surprise, the constituent (H₂ C₆ N₇ O₃ )^- building block is not in structurally optimal arrangement, but Ba(H₂ C₆ N₇ O₃ )₂  ⋅ 8 H₂ O exhibits excellent optical properties including wide band gap of 4.10 eV, very large birefringence of 0.24@550 nm, and exceptionally strong second-harmonic generation (SHG) response of about 12×KH₂ PO₄ . Both the SHG response and birefringence are much larger than those of commercial NLO crystal β-BaB₂ O₄ with optimally aligned (B₃ O₆ )^3- building block. Theoretical calculations suggest that the expanded π-conjugation delocalization within (H₂ C₆ N₇ O₃ )^- vs (B₃ O₆ )^3- should be responsible to the enhanced performance. This work implies that there is still much room to develop new NLO crystals with excellent functional building blocks that may be longly neglected.

Exploring Short-Wavelength Phase-Matching Nonlinear Optical Crystals by Employing KBe2BO3F2 as the Template

Exploration of nonlinear optical (NLO) crystals that are competent in generating short-wavelength ultraviolet (UV, λ ≤ 266 nm, and even deep-UV, λ ≤ 200 nm) coherent light output by direct second harmonic generation (SHG) remains a formidable challenge. Herein, four UV/deep-UV NLO crystals, M₂B₄SO₁₀ (M = K, Rb, and Cs) and Rb₃B₁₁PO₁₉F₃, were successfully synthesized by evolving the KBe₂BO₃F₂ (KBBF) structure into mixed-anionic borosulfate and fluoroborophosphate systems. They display functional [B₄SO₁₀]_∞ or [B₁₁PO₁₉F₃]_∞ KBBF-type layers that are composed of [BO₃], [BO₄], and [SO₄] groups or [BO₃], [BO₄], [BO₃F], and [PO₄] groups, respectively. Experimental characterization and numerical computation results indicate that these crystals possess exceptional NLO performance, including large SHG responses (0.9-1.7 × KDP at 1064 nm and 0.1-0.3 × β-BBO at 532 nm) and adequate birefringence to fulfill the SHG phase-matching (PM) condition at 266 nm. In particular, the shortest type-I PM wavelength (λ_PM) of Rb₃B₁₁PO₁₉F₃ reaches 180 nm, which implies that Rb₃B₁₁PO₁₉F₃ can become a prospective deep-UV NLO crystal. In addition, single crystals of K₂B₄SO₁₀, Rb₂B₄SO₁₀, and Cs₂B₄SO₁₀ are easily obtained by the high-temperature solution approach. This work will facilitate the discovery of short-wavelength PM NLO crystals by using the KBBF structure as the template.

Large optical anisotropy-oriented construction of a carbonate-nitrate chloride compound as a potential ultraviolet birefringent material

The design of new birefringent materials is very significant owing to their indispensable role in modulating the polarization of light and is vital in laser technology. Herein, by applying a large optical anisotropy-oriented construction induced by a synergy effect of multiple anionic groups, a promising carbonate-nitrate chloride, Na₃Rb₆(CO₃)₃(NO₃)₂Cl·(H₂O)₆, has been designed and synthesized successfully by the solvent evaporation method and single crystals of centimeter size were obtained by the recrystallization method in aqueous solution. It crystallizes in the hexagonal P6₃/mcm space group, the RbO₉Cl polyhedra and the NaO₇ polyhedra construct a three-dimensional (3D) framework by sharing O or Cl atoms and trigonal plane units (CO₃ and NO₃). The transmittance spectrum based on a 1 mm thick single-crystal plate shows that its short UV cut-off edge is about 231 nm. And the refractive index differences (0.14 @ 546 nm) measured by using a polarizing microscope on the (101) crystal plane, proves that Na₃Rb₆(CO₃)₃(NO₃)₂Cl·(H₂O)₆ has a large birefringence, which has potential application in the solar blind ultraviolet region. The theoretical calculations reveal that the π-conjugated CO₃ and NO₃ groups are the main cause of the birefringence. It demonstrates that combining π-conjugated CO₃ and NO₃ groups in one structure is an extremely effective strategy to explore new UV birefringent crystals.

SbHPO3F: 2D van der Waals Layered Phosphite Exhibiting Large Birefringence

A novel antimony(III)-based phosphite, SbHPO₃F, featuring a unique two-dimensional (2D) van der Waals layered structure, has been successfully designed and synthesized via the simultaneous employment of optically active moieties including SbO₃F seesaw and tetrahedral HPO₃ groups. Its optimized layered arrangement formed by the alternating connection of 4-membered rings (4-MRs) and 8-MRs endows the title compound with desirable optical properties including a large birefringence and short ultraviolet (UV) cutoff edge, implying that it is a potential UV birefringent material.

LiSrSbS3: parallel configurations of lone pair electrons inducing a large birefringence

Enhancement of birefringence is significant since the birefringent materials can create and control polarized light and be used extensively in various advanced optical systems. By optimizing the arrangement of [SbS₃] units with stereo-chemical active lone pair electrons, a new quaternary thioantimonate LiSrSbS₃ with a large birefringence has been successfully synthesized by a high temperature solid-state reaction method. LiSrSbS₃ crystallizes in the monoclinic space group of P2₁/c. In the structure, the isolated infinite [LiS₄] chains and zigzag [SrS₆] chains are alternately connected with each other to compose a three-dimensional (3D) framework, and the isolated pyramid [SbS₃] units are located between them. To analyze the source of large birefringence, the electronic structure and optical properties of LiSrSbS₃ were further investigated by the first-principles method, and the results show that the optimized arrangement [SbS₃] trigonal pyramid induces a large birefringence.

From Phosphate Fluoride to Fluorophosphate: Design of Novel Ultraviolet/Deep-Ultraviolet Nonlinear Optical Materials for BePO3F with Optical Property Enhancement

Fluorine-containing compounds have stimulated the exploration of ultraviolet/deep-ultraviolet nonlinear optical (NLO) materials. Alkali/alkaline-earth metal phosphates are one of the important potential systems as NLO materials, while the common small birefringence limits the phase-matching (PM) ability in the ultraviolet/deep-ultraviolet region. Herein, by applying a "fluorination synergy-induced enhancement of optical property" strategy, novel structures of phosphate fluoride/fluorophosphate in BePO₃F with good thermodynamic/dynamic stability and promising NLO-related properties are discovered via performing crystal structure prediction combined with first-principles calculations. BePO₃F-I-VI exhibit relatively large birefringence of 0.025, 0.048, 0.049, 0.049, 0.059, and 0.063 at 1064 nm, respectively. Simultaneously, BePO₃F-I (Pc) is a new thermodynamically stable phosphate fluoride which possesses a wide band gap (E_g = 8.03 eV), large second-harmonic generation (SHG) coefficient (d₁₁ = 0.67 pm/V, 1.7 × KDP), and the shortest PM wavelength of 292 nm. Other five thermodynamically metastable noncentrosymmetric (NCS) BePO₃F structures (II-VI) belong to fluorophosphates and exhibit deep-ultraviolet PM wavelengths of 187, 183, 186, 188, and 196 nm. It reveals that the aligned nonbonding O 2p orbitals of [BeO₂F₂] and [PO₄] units lead to a large SHG coefficient in the phosphate fluoride BePO₃F-I. For fluorophosphates (BePO₃F-II-VI), the synergy of [BeO₃] planar units and [PO₃F] units induces relatively large birefringence. Our research results provide an idea for exploring novel high-performance NLO materials.

Ba2Zn2B6O13: coplanar [B2O5] in unnoted U-shaped [B6O13] groups achieving large birefringence

Currently, π-conjugated [B₂O₅] moieties are rarely studied for designing deep-UV birefringent crystals. Here, we report a new deep-UV birefringent crystal Ba₂Zn₂B₆O₁₃ with a deep-UV cut-off edge of 190 nm and large birefringence (Δn = 0.085@ 532 nm), indicating that it can be used as the birefringent material in the DUV area. The first-principles calculation analyses suggest that its large birefringence mainly originates from the coplanar [B₂O₅] dimers in the unnoted U-shaped [B₆O₁₃].

Linear unit BN2: a novel birefringence-enhanced fundamental module with sp orbital hybridization

Inorganic planar π-conjugated groups are advantageous to generate large birefringence in optical functional materials, and many excellent materials contain CO₃, BO₃ or B₃O₆, such as CaCO₃, α/β-BaB₂O₄ (α/β-BBO), and KBe₂BO₃F₂ (KBBF). In view of their microscopic structures, the common characteristics are the planar structures, which are regarded as birefringence-enhanced fundamental modules (FMs). Nowadays, exploring novel birefringence-enhanced FMs is becoming a burning issue. Herein, we investigated the birefringence-enhanced FMs in B–N systems and found that the BN₂ linear unit could produce great birefringence. Through the investigation based on the Inorganic Crystal Structure Database, some compounds with the BN₂ linear group were screened out with the formulas A₃BN₂ (A = Li, Na), A₃BN₃ (A = Mg, Ca), and Ba₃(BN₂)₂. Particularly, Ca₃(BN₂)N exhibits a great birefringence of about 0.411 at 1064 nm, which is 3.5, 2.5 and 2.0 times those of the most commercially used birefringent crystals α-BaB₂O₄ (Δn = 0.116 at 1064 nm), CaCO₃ (Δn = 0.164 at 1064 nm) and YVO₄ (Δn = 0.208 at 1064 nm), respectively. To find the origins of the optical properties of compounds with the BN₂ linear group, the first-principles, REDA and polarizability anisotropy analysis methods were used. Owing to the structural arrangement and the polarization anisotropy of the BN₂ linear group, it can influence the birefringence significantly. This work will provide a general way for exploring birefringence-enhanced FMs in B–N compounds.

The linear unit BN₂ is discovered as a novel birefringence-enhanced fundamental module. Particularly, Ca₃(BN₂)N exhibits a large birefringence (0.411 at 1064 nm), which is about 2.0–3.5 times of the commercial used birefringent crystals, such as α-BaB₂O₄, CaCO₃ and YVO₄.

(N2H6)[HPO3F]2: maximizing the optical anisotropy of deep-ultraviolet fluorophosphates

Although phosphates are a rich source of deep-ultraviolet optical materials, the realization of large optical anisotropy in them still remains a challenge because of the small polarizability anisotropy of [PO₄] units. Inspired by the fluoridation strategy and hydrogen bond interaction, a new metal-free monofluorophosphate, (N₂H₆)[HPO₃F]₂, was synthesized, which exhibits a large birefringence (cal. 0.077) and wide band gap (∼6.51 eV). Such a large birefringence in (N₂H₆)[HPO₃F]₂ sets a new record among available fluorophosphates, and the [HPO₃F] unit is theoretically confirmed to be a new birefringence-active unit.

Phosphogermanate Crystal: A New Ultraviolet-Infrared Nonlinear Optical Crystal with Excellent Optical Performances

The phase matching ability is a key factor for nonlinear optical crystals to realize coherent output. Herein, a new design strategy combining ultraviolet and infrared functional groups into a ferroelectric was put forward. Thus, a phosphogermanate crystal, KGeOPO₄, was designed and studied. It exhibits a wide transparency window (0.22-9.70 μm), a strong second harmonic generation response (5× KH₂PO₄), a high laser-induced damage threshold (1.61 GW/cm²), and the typical ferroelectricity (coercive field ∼ 9.8 kV/cm and remnant polarization ∼7.6 μC/cm²). In the infrared region, it could realize coherent output by the birefringence phase matching method, while it could generate ultraviolet coherent lights by the quasi-phase matching technique. Therefore, this work designs a promising ultraviolet-infrared nonlinear optical crystal and provides a new perspective for exploring nonlinear optical crystals.

Variable dimensionality of the anion framework in four new borophosphates and fluoroborophosphates with short cutoff edges

Two new borophosphates, Cs₃B₃P₄O₁₆ and Li(NH₄)₂B₃P₄O₁₆, and two new fluoroborophosphates, K₄B₂P₂O₉F₂ and Rb₃B₂PO₅F₄, have been obtained via the high-temperature solution method. Single-crystal X-ray diffraction indicates that all of them exhibit various anion frameworks although they crystallize in the same space group, P2₁/c. Two-dimensional (2D) [B₃P₄O₁₆]_∞ layers and a 3D [B₃P₄O₁₆]_∞ network can be found in Cs₃B₃P₄O₁₆ and Li(NH₄)₂B₃P₄O₁₆, respectively, while isolated [B₂P₂O₉F₂] and [B₂PO₅F₄] exist in K₄B₂P₂O₉F₂ and Rb₃B₂PO₅F₄, respectively. The effect of the alkali metal cation size on the framework structures of Cs₃B₃P₄O₁₆ and Li(NH₄)₂B₃P₄O₁₆ has been discussed in detail. The IR spectra confirm their structural validity. UV-Vis-NIR diffuse reflectance spectroscopy indicates that the new compounds exhibit short cutoff edges. In addition, theoretical calculations were carried out to understand their electronic structures and optical properties.

Performance of optical materials with derivative planar π-conjugated groups: Recent advances and future prospects

Planar π-conjugated groups which possess not only large hyperpolarizability but also optical anisotropy are proven to be a good functional motif for optical materials with outstanding nonlinear optics and/or birefringence....

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.

Large optical anisotropy differentiation induced by the anion-directed regulation of structures

The modulation of optical anisotropy for two novel UV birefringent materials [C(NH2)3]2Sb3F3(HPO3)4 and [C(NH2)3]SbFPO4·H2O has been successfully achieved via anion-directing regulation structures.

Pushing KTiOPO4-like Nonlinear Optical Sulfates into the Deep-Ultraviolet Spectral Region

Accurately designing and synthesizing new deep-ultraviolet (deep-UV) nonlinear optical (NLO) crystals that are limited by the so-called "200 nm wall" on their transparency windows remain challenging. On the basis of a bandgap-directed computer-aided material design approach, two new NLO sulfates, KMgSO₄F and KZnSO₄F, are designed and successfully synthesized. They feature three-dimensional frameworks closely related to the commercial NLO crystal, KTiOPO₄ (KTP). Remarkably, the transmittance spectrum based on a single crystal indicates that the transparency window of KZnSO₄F is significantly blue-shifted to <190 nm from 350 nm for KTP. The microscopic origin of this significant transparent window blue shift is illustrated well by first-principles calculations. This work pushes the transparency windows of KTP-like NLO sulfates into the deep-UV spectral region for the first time and will pave a prospective way to the accurate design and synthesis of new deep-UV NLO materials.

Two bismuth iodate sulfates with enhanced optical anisotropy

Two bismuth iodate sulfates crystallizing in the monoclinic space group P2₁/c, namely, Bi(IO₃)(SO₄) and CdBi(IO₃)(SO₄)₂, were synthesized via solvothermal reactions. Bi(IO₃)(SO₄) features 2D [Bi(SO₄)]⁺ layers, which are further linked by the IO₃^- groups to form a 3D network. CdBi(IO₃)(SO₄)₂ exhibits 1D [IO₃]^- chains built from IO₄^3- groups via corner-sharing and is the first example of a polyiodate sulfate as far as we know. These [IO₃]^- chains are interconnected by Bi³⁺ cations into [Bi(IO₃)]²⁺ layers parallel to the bc plane, whereas the neighbouring Cd²⁺ cations are interconnected by bridging SO₄^2- anions into [Cd(SO₄)₂]^2- layers, also parallel to the bc plane. These cationic and anionic 2D layers are held together through Bi-O-S bridges into a complicated 3D framework. Bi(IO₃)(SO₄) and CdBi(IO₃)(SO₄)₂ show wide band gaps of 3.91 and 4.03 eV and large birefringence values of 0.087 and 0.100 at 1064 nm, respectively. Our work indicates that the introduction of iodate group and lone pair cations, such as Bi³⁺, into metal sulfates can greatly enhance their birefringent properties.

Achieving Short-Wavelength Phase-Matching Second Harmonic Generation in Boron-Rich Borosulfate with Planar [BO3 ] Units

Ultraviolet (UV) nonlinear optical (NLO) crystals which can produce short-wavelength lasers via a direct second harmonic generation (SHG) process are of great importance in modern laser technology. Currently, the exploration of UV NLO crystals in borosulfates is nearly stagnant since the non-phase matching (PM) property lies on the small birefringence induced by the intrinsically small optical anisotropy of the tetrahedral groups. Herein, for the first time, the planar [BO₃ ] units were introduced into borosulfates leading to a boron-rich borosulfate (NH₄ )₂ B₄ SO₁₀ with unprecedented [B₄ SO₁₀ ]_∞ layers and evidently enhanced birefringence. To the best of our knowledge, it achieves the shortest SHG PM wavelength of 252 nm in all reported borosulfates with deep UV cutoff edge (184 nm), large SHG response (1.1×KDP at 1064 nm and 0.15×β-BBO at 532 nm) and large birefringence (0.053 at 1064 nm) and is easy to grow single crystals via simple chemical vapor deposition method. These results confirm the feasibility of utilizing planar [BO₃ ] units to optimize birefringence of borosulfates, and also open up broad prospects for UV NLO crystals in boron-rich borosulfates.

Non-π-Conjugated Deep-Ultraviolet Nonlinear Optical Crystal K2Zn3(SO4)(HSO4)2F4

Deep-ultraviolet (deep-UV) (wavelengths of <200 nm) nonlinear optical (NLO) materials are playing an increasingly important role because of their significant technological applications in advanced scientific instruments. In recent years, the non-π-conjugated systems have received extensive attention as new emerging sources of deep-UV NLO materials. Here, a new non-π-conjugated deep-UV NLO material, K₂Zn₃(SO₄)(HSO₄)₂F₄, has been successfully obtained by the hydrothermal method. It has a layered structure formed by [Zn₃(SO₄)(HSO₄)₂F₃]_∞ layers bridged via K-O and K-F bonds. Powder second-harmonic generation shows that K₂Zn₃(SO₄)(HSO₄)₂F₄ can achieve phase matching, and the response is ∼0.3 times that of KH₂PO₄. Remarkably, the single-crystal transmittance spectrum confirms that the absorption edge of K₂Zn₃(SO₄)(HSO₄)₂F₄ is below 200 nm, and the experimentally measured birefringence is 0.0126 at λ = 546 nm. In-depth first-principles calculations illustrate well the microscopic origin of the optical properties. This work enriches the structrual diversity of non-π-conjugated deep-UV NLO materials.

Tetrafluoroborate-Monofluorophosphate (NH4)3[PO3F][BF4]: First Member of Oxyfluoride with B-F and P-F Bonds

Inspired by the strategy of fluorine introduction in borates and phosphates, the inorganic oxyfluoride (NH₄)₃[PO₃F][BF₄] with B-F and P-F bonds has been characterized as the first fluoroborate-fluorophosphate. The International Union of Pure and Applied Chemistry (IUPAC) name for (NH₄)₃[PO₃F][BF₄] should be ammonium tetrafluoroborate-monofluorophosphate according to the structure characteristics. The existence and coordination of fluorine in (NH₄)₃[PO₃F][BF₄] were confirmed by several approaches, including single-crystal structure analysis; bond valence analysis; and X-ray energy dispersive, infrared spectrum, and also nuclear magnetic resonance spectroscopy. This work is of great significance to enrich the solid-state chemistry of borates and phosphates and also open a new branch of mixed anion compound with fluoroborate-fluorophosphates.

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.