Two Short-Wave UV Beryllium Selenites Exhibiting Diverse Optical Properties Stemming from Functional Group Arrangements

The arrangement of functional groups exerts a crucial role in determining the characteristics of compounds. In this study, we synthesized two novel short-wave ultraviolet (UV) nonlinear optical (NLO) crystals: KBe2(SeO3)2(OH)·H2O and K2Be(SeO3)2. Interestingly, the two compounds show the same SeO3 triangular pyramids and K-O polyhedra. However, the two compounds exhibit distinct beryllium-oxygen anion groups: BeO3(OH) for KBe2(SeO3)2(OH)·H2O and BeO4 for K2Be(SeO3)2. This results in the SeO3 groups within the structure having different orientations, ultimately leading to the two compounds exhibiting completely different optical properties. KBe2(SeO3)2(OH)·H2O displays a large second harmonic generation (SHG) effect equivalent to 2× KH2PO4 (KDP), coupled with a large birefringence of 0.078 at 546 nm. In contrast, the SHG effect and birefringence of K2Be(SeO3)2 are only 0.33× that of KDP and 0.024 at 546 nm, respectively. Structural analyses and theoretical calculations indicate that these pronounced differences in optical properties stem from variations in the arrangement of the SeO3 functional groups. This study not only sheds light on the correlation between crystal structure and optical behavior but also presents a hopeful avenue for the advancement of materials in the short-wave UV spectrum.

Na3K6(CO3)3(NO3)2X·6H2O (X = NO3, Cl, Br): Exploring High-Performance UV Birefringent Crystals Induced by Coplanar π-Conjugated CO3 and NO3 Groups

Planar π-conjugated groups, like CO3, NO3, and BO3 triangles, are ideal functional units for designing birefringent materials due to their large optical anisotropy and wide band gap. The key point for designing birefringent crystals is to select appropriate functional building blocks (FBBs) and the proper arrangement mode. It is well known that the substitution strategy has proven to be a promising and accessible approach. In this work, alkali metals were chosen to regulate and control two different π-conjugated groups, CO3 and NO3, to build new compounds with large birefringence. Subsequently, three new compounds, Na3K6(CO3)3(NO3)2X·6H2O (X = NO3, Cl, Br), were successfully synthesized using the hydrothermal method. The aliovalent substitution between the [NO3]- anionic group and halogen anions [Cl]-/[Br]- has been achieved in these compounds. Na3K6(CO3)3(NO3)2X·6H2O feature the well-coplanar CO3 and NO3 groups in their crystal structure. This coplanar arrangement mode may effectively enhance the anisotropic polarizability of Na3K6(CO3)3(NO3)2X·6H2O. And their experimental birefringence can reach 0.094-0.131 at 546 nm. Diffuse reflectance spectra demonstrate that these compounds exhibit short ultraviolet (UV) absorption edges of ∼235 nm. Meanwhile, Na3K6(CO3)3(NO3)2X·6H2O also have an easily grown capacity under facile conditions. This work not only reports three new potential UV birefringent crystals but also provides a strategy to make the π-conjugated MO3 group coplanar.

Tunable Optical Anisotropy in Rare-Earth Borates with Flexible [BO3] Clusters

Borates have garnered a lot of attention in the realm of solid-state chemistry due to their remarkable characteristics, in which the synthesis of borates with isolated [BO3] by adding rare-earth elements is one of the main areas of structural design study. Five new mixed-metal Y-based rare-earth borates, Ba2ZnY2(BO3)4, KNa2Y(BO3)2, Li2CsY4(BO3)5, LiRb2Y(BO3)2, and RbCaY(BO3)2, have been discovered using the high-temperature solution approach. Isolated [BO3] clusters arranged in various configurations comprise their entire anionic framework, allowing for optical anisotropy tuning between 0.024 and 0.081 under 1064 nm. In this study, we characterize the relative placements of their [BO3] groups and examine how their structure affects their characteristics. The origin of their considerable optical anisotropy has been proven theoretically. This study unequivocally demonstrates that even a slight alteration to borates' anionic structure can result in a significant improvement in performance.

Hydrogen bonding bolstered head-to-tail ligation of functional chromophores in a 0D SbF3·glycine adduct for a short-wave ultraviolet nonlinear optical material

The key properties of nonlinear optical (NLO) materials highly rely on the quality of functional chromophores (FCs) and their optimized interarrangement in the lattice. Despite the screening of various FCs, significant challenges persist in optimizing their arrangement within specific structures. Generally, FC alignment is achieved by designing negatively charged 2D layers or 3D frameworks, further regulated by templating cations. In this study, a novel 0D adduct NLO material, SbF3·glycine, is reported. Neutrally charged 0D [SbF3C2H5NO2] FCs, comprising [SbF3] pyramids and zwitterionic glycine, are well-aligned in the structure. The alignment is facilitated by the hydrogen bonding, reinforcing a 'head-to-tail' ligation of [SbF3C2H5NO2] FCs. Consequently, the title compound exhibits favorable NLO properties, including a large second-harmonic generation efficiency (3.6 × KDP) and suitable birefringence (cal. 0.057 @ 1064 nm). Additionally, its short absorption cut-off edge (231 nm) positions it as a promising short-wave ultraviolet NLO material. Importantly, the binary SbF3-amino acid system is expected to serve as a new resource for exploring ultraviolet NLO crystals, owing to the abundance of the amino acid family.

A Noncentrosymmetric Metal-Free Borophosphate: Achieving a Large Birefringence and Excellent Stability by Covalent-Linkage

Organic-inorganic hybrid linear and nonlinear optical (NLO) materials have received increasingly wide spread attention in recent years. Herein, the first hybrid noncentrosymmetric (NCS) borophosphate, (C5H6N)2B2O(HPO4)2 (4PBP), is rationally designed and synthesized by a covalent-linkage strategy. 4-pyridyl-boronic acid (4 PB) is considered as a bifunctional unit, which may effectively improve the optical properties and stability of the resultant material. On the one hand, 4 PB units are covalently linked with PO3(OH) groups via strong B-O-P connections, which significantly enhances the thermal stability of 4PBP (decomposition at 321, vs lower 200 °C of most of hybrid materials). On the other hand, the planar π-conjugated C5H6N units and their uniform layered arrangements represent large structural anisotropy and hyperpolarizability, achieving the largest birefringence (0.156 @ 546 nm) in the reported borophosphates and a second-harmonic generation response (0.7 × KDP). 4PBP also exhibits a wide transparency range (0.27-1.50 µm). This work not only provides a promising birefringent material, but also offers a practical covalent-attachment strategy for the rational design of new high-performance optical materials.

Guanidinium Vanadate [C(NH2)3]3VO4·2H2O Revealing Enhanced Second-Harmonic Generation and Wide Band Gaps

A new guanidinium-templated vanadate, [C(NH2)3]3VO4·2H2O, has been synthesized in a phase-pure form. It crystallizes in a noncentrosymmetric polar space group, Cc, and the crystal structure is built upon a framework of guanidinium, vanadate tetrahedra, and water molecules linked by hydrogen bonds. Notably, optical measurements reveal that the material exhibits an approximately 9.6-fold enhancement in second-harmonic generation efficiency compared to its phosphate analogue. The enhancement can be attributed to the increased geometrical distortion of the VO4 tetrahedra. Furthermore, we found that the coordination number of the central vanadium atom significantly affects the optical band gaps. Among various coordination numbers, the 4-coordinate VO4 tetrahedra are found to be more favorable for widening the optical band gap of materials compared to the 5- and 6-coordinate vanadium polyhedra, as demonstrated by this work.

Designing Optical Anisotropy: Silver-Aminoalkylpyridine Nitrate Complexes with Tunable Structures

To introduce a design strategy for improving optical properties, two silver-amino alkylpyridine nitrate complexes, AgC6H8N3O3 and Ag2C14H20N6O6, were successfully synthesized using a recrystallization method. By employing polarizable π-conjugated [NO3-] ions, two types of pyridine ligands, and silver cations with a high affinity for pyridine, we obtained a one-dimensional chain structure with 4-aminomethylpyridine (AgC6H8N3O3) and a zero-dimensional molecular compound by introducing a relatively flexible aliphatic chain with 4-(2-aminoethyl)pyridine (Ag2C14H20N6O6). The compounds crystallize in the triclinic crystal system with the centrosymmetric P-1 space group, exhibiting a change in orientation between the π-conjugated system and the silver ion. Despite similar optical band gaps (3.69 eV for AgC6H8N3O3 and 3.73 eV for Ag2C14H20N6O6), AgC6H8N3O3 shows higher absorption in the 350-600 nm range. Electronic structure calculations support the ultraviolet absorption findings, suggesting that charge transfer with π-conjugated systems influences birefringence. Ag2C14H20N6O6 exhibits experimental birefringence (0.261@546.1 nm) surpassing that of AgC6H8N3O3 (0.212@546.1 nm), placing it among the highest recorded values within metal-pyridine incorporating nitrate complexes. The nonconventional orientation of π-conjugated [NO3-] ions contributes to this phenomenon, enhancing the action of free π-conjugated orbitals. This design strategy for micromodulating the alignment of the π-conjugated system promises to be an effective approach for enhancing optical properties, such as birefringence.

Fluorination Strategy Towards Symmetry Breaking of Boron-centered Tetrahedron for Poly-fluorinated Optical Crystals

Borate crystals can be chemically and functionally modified by the fluorination strategy, which encourages the identification of emerging fluorooxoborates with a structure and set of characteristics not seen in any other oxide parents. However, the bulk of fluorooxoborates have been found accidentally, rational methods of synthesis are required, particularly for the infrequently occurring poly-fluorinated components. Herein, we reported the use of bifluoride salts as a potent source of fluorine to prepare fluorooxoborates that contain rarely tri-fluorinated [BF3 X] (X=O and CH3 ) tetrahedra and eleven compounds were found. We identified the optical properties of the organofluorinated group [CH3 BF3 ] and their potential for nonlinear optics for the first time. Among these, two non-centrosymmetric components hold potential for the production of 266 nm harmonic coherent light for nonlinear optics, and more crucially, have the benefit of growing large size single crystals. Our study establishes experimental conditions for the coexistence of the diverse functional groups, enabling the production of poly-fluorinated optical crystals.

A2Hgx(SeO3)y (A = K, Rb, Cs): Three Alkali Metal Mercury Selenites Featuring Unique 1D [HgOm(SeO3)n]∞ Chains

Herein, three alkali metal mercury selenites, K2Hg2(SeO3)3, Rb2Hg2(SeO3)3, and Cs2Hg3(SeO3)4, were successfully obtained by a hydrothermal method. The three compounds featured same one-dimensional (1D) [HgOm(SeO3)n]∞ chain structure that consisting of distorted Hg-O polyhedra and SeO3 triangular pyramids with stereochemically active lone pair (SCALP) electrons. Interestingly, the rich coordination environment of Hg atoms and the size difference of alkali metal cations lead to diverse arrangement of SeO3 groups, which makes them exhibit different birefringence. The band gaps of the three compounds indicate that they are potential ultraviolet (UV) optical materials. Detailed theoretical calculations demonstrate that the combined effects of SeO3 triangular pyramids and Hg-O polyhedra are responsible for the optical characteristics of the reported compounds.

Two van der Waals Layered Antimony(III) Phosphites as UV Optical Materials

Herein, two new Sb3+-based phosphites, Sb2O2(HPO3) (I) and Sb2O(HPO3)2 (II), were successfully obtained by ingeniously combining Sb3+-based polyhedra containing stereochemically active lone pair (SCALP) and HPO3 polar groups. Both reported compounds exhibit unique 2D van der Waals layered structures, [Sb4O4(HPO3)2]∞ and [Sb2O(HPO3)2]∞, respectively, which favors compounds with large optical anisotropy. Interestingly, the different curvatures of the two layers resulted in the two title compounds showing significantly different birefringences (0.079@546 and 0.046@546 nm, respectively). Both compounds endow wide optical band gaps (4.32 and 4.54 eV, respectively), which indicates their potential as promising ultraviolet (UV) birefringent crystals. The synthesis of the two title compounds enriched Sb3+-based phosphites in the UV region and provided guidance for the subsequent synthesis of superior optical materials.

Sr(NH2SO3)(NO3)·H2O: An Ultraviolet Nonlinear Optical Material Exhibiting Strong Second-Harmonic Generation Response and Sufficient Birefringence

Second-harmonic generation (SHG) response and birefringence stand as fundamental optical properties for nonlinear optical (NLO) materials. Up to now, engineering a strong SHG response and substantial birefringence in the sulfamate system has proven to be exceedingly challenging. A novel noncentrosymmetric compound, Sr(NH2SO3)(NO3)·H2O, has been meticulously designed by introducing the (NO3)- group characterized by significant polarizability anisotropy and hyperpolarizability into the polar non-π-conjugated (NH2SO3)- system. This compound exhibits a robust SHG response (5.2 × KDP), ample birefringence (0.077 at 546.1 nm), and a short ultraviolet (UV) absorption edge (290 nm). The linear and nonlinear optical properties can be attributed to the (NO3)- group and the (NH2SO3)- group. This study presents an effective approach that contributes to the design of more composite anionic UV NLO materials with excellent and balanced optical properties.

A lithium-scandium sulfate with second-harmonic generation response and deep-ultraviolet transparency

A new beryllium-free deep-UV transparent NLO crystal Li(H2O)2Sc(SO4)2 features a two-dimensional [Sc(SO4)2] framework consisting of twisted [Sc3S4O9] units decorated by [LiO2(H2O)2] groups into a unique layer. Remarkably, Li(H2O)2Sc(SO4)2 exhibits a phase-matching SHG response of 0.7 × KDP and a deep-UV cutoff edge below 190 nm.

KRb2(NO3)2Cl: a new birefringent crystal exhibiting a perovskite-related framework and a short cutoff edge

The combination of π-conjugated groups [NO3] and Cl-centered polyhedra generates a new birefringent crystal with a perovskite-related framework, KRb2(NO3)2Cl, which is the first alkali metal nitrate chloride synthesized by a mild hydrothermal method. It crystallizes in the orthorhombic space group pbam (no. 55). In addition, KRb2(NO3)2Cl crystals with dimensions up to 7 × 1.5 × 1 mm3 were grown. Notably, KRb2(NO3)2Cl has a short UV cut-off edge (below 228 nm) and a significantly enhanced birefringence (Δn = 0.084 at 1064 nm). Theoretical calculations indicate that the birefringence enhancement mainly derives from π-conjugated [NO3] plane triangles.

K5[B3O3F4(OH)]2(NO3): the first hydroxyfluorooxoborate-nitrate with a short ultraviolet cutoff edge and large birefringence

The first hydroxyfluorooxoborate-nitrate mixed anion compound, K5[B3O3F4(OH)]2(NO3), was synthesized by the solution evaporation method. It displays a unique structure built by K+ cations, the hydroxylated and fluorinated six-membered ring [B3O3F4(OH)] and [NO3] groups. It possesses a band gap of 5.68 eV derived from the diffuse reflectance spectrum, which corresponds to an ultraviolet cutoff edge of 218 nm. First-principles calculations show that it has a large birefringence of 0.095 at 532 nm and the result of the response electron distribution anisotropy method indicates that all three anion groups contribute positively to the birefringence, verifying the synergic contributions from the multiple anion groups.

Quadruple-Bidentate Nitrate-Ligated A2 Hg(NO3 )4 (A=K, Rb): Strong Second-Harmonic Generation and Sufficient Birefringence

The design of efficient nonlinear optical (NLO) crystals continues to pose significant challenges due to the difficulty of assembling polar NLO-active modules in an optimal additive fashion. We report herein the first NLO-active mercuric nitrates A2 Hg(NO3 )4 (A=(KHNO), Rb (RHNO)), for which assembly is induced by ionic polarization of the d10 cations. The two new crystalline compounds are isostructural, featuring interesting pseudo-diamond-like structures with parallel [Hg(NO3 )4 ] modules, and leading to strong powder second-harmonic generation (SHG) responses of 9.2 (KHNO) and 8.8 (RHNO) times that of KH2 PO4 . In combination with the simple solution preparation of centimeter-scale crystals, sufficient birefringence, and short ultraviolet (UV) cutoff edges, these attributes make KHNO and RHNO promising candidates for UV NLO materials. Theoretical calculations and single-crystal structure analysis reveal that the newly-developed highly condensed and distorted [Hg(NO3 )4 ] module, with an Hg2+ cation that is quadruply bidentate nitrate-ligated, is crucial for the significant SHG responses. This work highlights the potential importance of modules with multiple bidentate ligands for the development of high-performing next-generation NLO materials.

Annealing-induced phase transformation in In10Se70Te20 thin films and its structural, optical and morphological changes for optoelectronic applications

In2Se3 and In2Te3 have great importance in various device fabrications. The present report is based on the annealing-induced phase formation of both In2Se3 and In2Te3 from In10Se70Te20 thin films at different annealing temperatures as found from the XRD analysis and well supported by the Raman study. The average crystallite size increased with a decrease in the dislocation density. The surface morphology changed with annealing and increased in particle size as noticed from the FESEM images. The uniform distribution and presence of constituent elements in the film were verified using EDX data. The increase in transmittance is accompanied by a decrease in extinction coefficient, optical density and increase in skin depth with annealing. The increase in optical bandgap from 0.418 eV to 0.645 eV upon annealing at 250 °C is associated with a decrease in disorder. The steepness parameter increased and the Se-p value decreased with annealing. The refractive index decreased with an increase in oscillator energy and decrease in dispersion energy. The quality factor, dielectric loss, optical conductivity and electrical susceptibility decreased. The optical electronegativity and plasma frequency increased with annealing. There is a significant change in the non-linear susceptibility and non-linear refractive index with annealing. The observed changes in the film structure and optical behaviour are due to the annealing-induced phase formation from the In10Se70Te20 host matrix upon annealing. Such materials are suitable for optoelectronic and phase change devices.

Influence of time dependent laser-irradiation for tuning the linear-nonlinear optical response of quaternary Ag10In15S15Se60 films for optoelectronic applications

The impact of laser irradiation on thin films results in multiple beneficial modifications of their structural, morphological, nonlinear-linear properties for optoelectronics applications. This work deals with the thermally evaporated Ag10In15S15Se60 films and post-laser irradiation to study the variations in structural and optical properties. The current investigation was carried out for different laser irradiation time durations such as 0, 10, 20, 30, and 60 minutes by 532 nm laser (2.34 eV). According to the X-ray diffraction analysis, all thin films have polycrystalline character. The change in the surface morphology after being exposed to the laser has been checked by FESEM, whereas the presence of constitutional elements has been verified by the EDX study. The related changes with laser irradiation in the optical properties, including both linear and nonlinear, were studied using UV-Vis spectroscopy data. The irradiation caused an enhancement in the transmission, and the absorption edge moved towards a lower wavelength, increasing the bandgap energy from 1.71 eV to 1.88 eV. The refractive index reduced as a result of the film's altered structure. The behaviour of the refractive index satisfies Moss's rule (E g n 4 = const). The nonlinear refractive index, first-order and 3rd order nonlinear susceptibility, is found to be decreased with laser irradiation. The dielectric parameters are also observed to be decreased with irradiation. Considering all the alterations in its properties caused by irradiation, the Ag10In15S15Se60 sample could be a favourable material for various photonic devices.

Pb2Cl2(HPO3)(H2O) and Pb3Br2(HPO3)2: Two Phosphite Halides with 3D Structural Networks and Enlarged Birefringence

Compounds constructed by mixed anions are widely favorable for their diverse crystal structures and physical performances. Here, two lead phosphite halides, Pb₂Cl₂(HPO₃)(H₂O) (1) and Pb₃Br₂(HPO₃)₂ (2), were obtained by facile hydrothermal reactions. Both compounds crystallize in the space group Pnma. 1 exhibits a 3D structure composed of HPO₃ units, PbCl₄O₃ pentagonal bipyramids, and PbO₃ pyramids. 2 also shows a 3D framework built by HPO₃ units, PbO₂Br₄ octahedron, PbO₅ and PbO₃Br₂ square pyramids. By introducing halogen anions to the lead phosphite system, 1 and 2 show enhanced birefringences of 0.083 and 0.072 at 1064 nm, respectively, compared with 0.023@1064 nm for Pb₂(HPO₃)₂. In this work, syntheses, crystal structures, optical properties, and theoretical calculation results have been studied in detail.

(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.

Noncentrosymmetric (C3H7N6)6(H2PO4)4(HPO4)·4H2O and Centrosymmetric (C3H7N6)2SO4·2H2O: Exploration of Acentric Structure by Combining Planar and Tetrahedral Motifs via Hydrogen Bonds

The combination of organic and inorganic nonlinear active units to obtain organic-inorganic hybrid materials has been proved to be a very effective method to obtain nonlinear optical (NLO) materials with excellent properties. Herein we reported two hybrid melamine-based compounds, namely, acentric (C₃H₇N₆)₆(H₂PO₄)₄(HPO₄)·4H₂O (1) and centrosymmetric (C₃H₇N₆)₂SO₄·2H₂O (2), which were synthesized via facile solvent evaporation method. Compound 1 features a three-dimensional (3D) network structure composed of _∞[(H₂PO₄)₄(HPO₄)(H₂O)₄]^6- layers which are further linked with _∞[(C₃H₇N₆)₆]⁶⁺ layers via hydrogen bonds. Compound 2 displays a 3D structure composed of [(C₃H₇N₆)₂(SO₄)(H₂O)₂]_∞ layers further linked with each other by hydrogen bonds. Compound 1 presents a second harmonic generation signal of about 0.1 × KDP. Furthermore, UV-vis and infrared spectra, thermal analyses, and theoretical calculation were also adopted to evaluate its NLO performance. The theoretical calculations showed that the SHG response and large birefringence of 1 were primarily caused by the (C₃H₇N₆)⁺, (H₂PO₄)^-, and (HPO₄)^2- groups.

Unprecedented boat-shaped [Mo2O5(IO3)4]2- polyanions induced a strong second harmonic generation response

The first organic-inorganic hybrid guanidine molybdenyl iodate [C(NH₂)₃]₂Mo₂O₅(IO₃)₄·2H₂O was successfully synthesized via an improved moderate hydrothermal method. It features an unprecedented boat-shaped zero-dimensional [Mo₂O₅(IO₃)₄]^2- polyanion cluster, which induces a wide band gap, moderate birefringence and strong second harmonic generation response, indicating that it is a potential nonlinear optical material.

High-Performance Sulfate Optical Materials Exhibiting Giant Second Harmonic Generation and Large Birefringence

Discovering novel sulfate optical materials with strong second-harmonic generation (SHG) and large birefringence is confronted by a great challenge attributed to the intrinsically weak polarizability and optical anisotropy of tetrahedral SO₄ groups. Herein, two superior-performing sulfate optical materials, namely, noncentrosymmetric Hg₃ O₂ SO₄ and centrosymmetric CsHgClSO₄  ⋅ H₂ O, have been successfully synthesized through the introduction of a highly polarizable d¹⁰ metal cation, Hg²⁺ . The unique component layers in the reported compounds, [Hg₃ O₂ SO₄ ]_∞ layers in Hg₃ O₂ SO₄ and [HgClSO₄ (H₂ O)] ∞ - layers in CsHgClSO₄  ⋅ H₂ O, induce enlarged birefringence in each sulfate. Remarkably, Hg₃ O₂ SO₄ exhibits a very large SHG response (14 times that of KH₂ PO₄ ), which is the strongest efficiency among all the reported nonlinear optical sulfates. Detailed theoretical calculations confirm that the employment of highly polarizable Hg²⁺ is an effective strategy to design superior optical materials with large birefringence and strong SHG response.

CsY(SO4)2·4H2O: A Deep-Ultraviolet Birefringent Crystal Induced by an Edge-Sharing Connection

SO₄ tetrahedral groups have weak polarization anisotropy, which thus results in the small birefringence of sulfates. Here, we report new sulfate CsY(SO₄)₂·4H₂O with unprecedented birefringence among deep-ultraviolet (deep-UV) sulfates. Its single crystal (10 mm × 3.5 mm × 1.5 mm) was simply grown by an aqueous solution evaporation technique, and it features a rare layered structure composed of YO₉ polyhedra, SO₄ tetrahedra, and H₂O molecules. Interestingly, each SO₄ group donates two oxygen atoms to edge-share with one adjacent YO₉ polyhedron and thus causes severe distortion of these groups. The characteristic edge-sharing mode gives CsY(SO₄)₂·4H₂O a large birefringence of ∼0.045@546 nm, which is the maximum among deep-UV sulfates and phosphates with similar non-π-conjugated anionic groups. The ultraviolet-visible-near-infrared diffuse reflection and transmission spectra, infrared spectrum, thermal stability, and theoretical calculations are also presented. The fascinating results will improve our understanding of sulfates and may provide useful insights into the exploration of deep-UV sulfates with large birefringence.

Layered Perovskite-like Nitrate Cs2Pb(NO3)2Br2 as a Multifunctional Optical Material

A novel alkali metal lead halide nitrate, Cs₂Pb(NO₃)₂Br₂, has been successfully synthesized via a hydrothermal method. Interestingly, the title compound features a distinctive Ruddlesden-Popper perovskite-like layered structure, which induces the outstanding multifunctional optical properties, including a large birefringence (0.147@546 nm) and broad light-orange emission. Detailed structural analysis and theoretical calculations revealed that the large birefringence originates from the p-π interaction between the Pb²⁺ cations and NO₃ groups and that the excellent luminescence properties derive from the distortion of PbO₄Br₄ polyhedra. This work not only enriches the variant structure types of layered perovskites but also guides the further exploration of all-inorganic multifunctional optical materials.

(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.

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....

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.

Two molybdenyl carbonates with different dimensional structures exhibiting huge differences in band gaps

Two molybdenyl carbonates with different dimensional structures exhibit huge differences in band gaps, 0D Cs3MoO4(HCO3) exhibiting a much larger band gap than 1D Cs2MoO3(CO3).

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.

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.

Synergistic Effect of π-Conjugated [C(NH2)3] Cation and Sb(III) Lone Pair Stereoactivity on Structural Transformation and Second Harmonic Generation

The search for nonlinear optical (NLO) crystals with excellent comprehensive properties is a formidable challenge. In this work, two guanidine antimony fluorides, C(NH₂)₃Sb₂F₇ and C(NH₂)₃SbF₄, were obtained by conjunction of [C(NH₂)₃] groups with π-conjugated configuration and stereochemically active Sb³⁺ cations. Due to the different coordination modes of Sb-F bonds and H-F hydrogen bonds, the crystal structure of C(NH₂)₃Sb₂F₇ is centrosymmetric (CS), while C(NH₂)₃SbF₄ is noncentrosymmetric (NCS). Optical measurements show that the UV cutoff wavelengths of the title compounds were both less than 240 nm. Thermal studies indicate that these crystals are stable up to 250 °C. In addition, the second harmonic generation (SHG) response of C(NH₂)₃SbF₄ is 2 times that of KH₂PO₄ (KDP) with the phase-matchable capacity. Theoretical calculations reveal that the large SHG effects of C(NH₂)₃SbF₄ were attributed to the synergy between the planar [C(NH₂)₃] units and the distorted [SbF₄] groups. These results demonstrate that the guanidine antimony fluorides will have potential value as UV NLO materials.

A General Principle for DUV NLO Materials: π-Conjugated Confinement Enlarges Band Gap*

Current nonlinear optical materials face a conventional limitation in the trade-off between the band gap and birefringence, especially in the deep UV spectral region. To circumvent this dilemma, we propose a general principle, π-conjugated confinement, to partially decouple the interunit π-conjugated interactions by the separation of non-π-conjugated units. The goal is to further enlarge the band gap to a value larger than that of the singular π-conjugated counterpart and to maintain a suitable density of π-conjugated units to gain a large optical anisotropy. We reveal that π-conjugated confinement is a shared structural feature for all DUV NLO materials known to date, and thus, it provides a novel and essential design criterion for future design synthesis. Guided by this principle, the carbonophosphates are predicted to be a new promising DUV candidate system. Sr₃ Y[PO₄ ][CO₃ ]₃ (1) and Na₃ X[PO₄ ][CO₃ ] (X=Ba, Sr, Ca, Mg, 2-5) exhibit not only greatly enhanced birefringence that is 3-24 times larger than that of singular phosphates but also enhanced band gaps that are 0.2-1.7 eV wider than those of singular carbonates.

Hydroxyfluorooxoborate Na[B3 O3 F2 (OH)2 ]⋅[B(OH)3 ]: Optimizing the Optical Anisotropy with Heteroanionic Units for Deep Ultraviolet Birefringent Crystals

Maximizing the optical anisotropy in birefringent materials has emerged as an efficient route for modulating the polarization-dependent light propagation. Currently, the generation of deep-ultraviolet (deep-UV) polarized light below 200 nm is essential but challenging due to the interdisciplinary significance and insufficiency of high-performing birefringent crystals. Herein, by introducing multiple heteroanionic units, the first sodium difluorodihydroxytriborate-boric acid Na[B₃ O₃ F₂ (OH)₂ ]⋅[B(OH)₃ ] has been characterized as a novel deep-UV birefringent crystal. Two rare heteroanionic units, [B₃ O₃ F₂ (OH)₂ ] and [B(OH)₃ ], optimally align to induce large optical anisotropy and also the dangling bonds are eliminated with hydrogens, which results in an extremely large birefringence and band gap. The well-ordered OH/F anions in [B₃ O₃ F₂ (OH)₂ ] and [B(OH)₃ ] were identified and confirmed by various approaches, and also the origin of large birefringence was theoretically discussed. These results confirm the feasibility of utilizing hydrogen involved heteroanionic units to design crystals with large birefringence, and also expand the alternative system of deep-UV birefringent crystals with new hydroxyfluorooxoborates.

Two Tellurium(IV)-Based Sulfates Exhibiting Strong Second Harmonic Generation and Moderate Birefringence as Promising Ultraviolet Nonlinear Optical Materials

Two tellurium(IV)-based sulfate nonlinear optical (NLO) materials, Te₂O₃(SO₄) and Te(OH)₃(SO₄)·H₃O, were successfully synthesized via the mild hydrothermal method. Te₂O₃SO₄ has a two-dimensional (2D) structure consisting of [Te₆O₁₂]_∞ layers as well as [SO₄] groups. Te(OH)₃(SO₄)·H₃O features a simple 0D structure made up of an isolated [TeO₃] pyramid and a [SO₄] tetrahedra. Both of them are phase-matching materials and show remarkable powder second harmonic generation (SHG) efficiencies about 6 and 3 times that of KH₂PO₄ (KDP), respectively, for Te₂O₃SO₄ and Te(OH)₃(SO₄)·H₃O. Especially for Te(OH)₃(SO₄)·H₃O, in addition to a large SHG response, it possesses a short UV cutoff edge (∼233 nm) as well as moderate birefringence (0.052@546.1 nm). Furthermore, theoretical calculations confirmed that their strong SHG effects are due to the synergistic effect of the [TeO₃] pyramid and [SO₄] tetrahedra.

Tin Chloride Sulfates A3Sn2(SO4)3-xCl1+2x (A = K, Rb, Cs; x = 0, 1) as Multifunctional Optical Materials

The series of alkali-metal tin chloride sulfates A₃Sn₂(SO₄)_3-xCl_1+2x (A = K, Rb, Cs; x = 0, 1), K₃Sn₂(SO₄)₃Cl, Rb₃Sn₂(SO₄)₂Cl₃, and Cs₃Sn₂(SO₄)₂Cl₃, were successfully synthesized through an improved mild hydrothermal method. Interestingly, in addition to the cation size effect, the structure-directing effect of anions induces different symmetries in the three title compounds, with K₃Sn₂(SO₄)₃Cl being noncentrosymmetric, while Rb₃Sn₂(SO₄)₂Cl₃ and Cs₃Sn₂(SO₄)₂Cl₃ are centrosymmetric. Powder second-harmonic generation (SHG) measurements indicate that K₃Sn₂(SO₄)₃Cl is a nonlinear optical material that is type I phase matchable with a weak SHG response (0.1× KDP). Photoluminescence tests reveal that the three title compounds emit strong greenish yellow, orange, and salmon light, respectively, under UV excitation, indicating that they are promising inorganic solid fluorescent materials. Simultaneously, a detailed structural analysis of all the known tin(II) halide sulfates has been performed, which will guide the systematic exploration of high-performance tin(II)-based functional materials in the future.

M(NH2 SO3 )2 (M=Sr, Ba): Two Deep-Ultraviolet Transparent Sulfamates Exhibiting Strong Second Harmonic Generation Responses and Moderate Birefringence

Over the last few decades, the development of each new nonlinear optical (NLO)-active functional unit has led to the discoveries of a series of excellent NLO materials. In the present work, based on first-principles studies, we identified a novel deep-UV (DUV) NLO-active functional unit, a non-π-conjugated group viz. (NH₂ SO₃ )^- . By combining alkaline-earth metals with (NH₂ SO₃ )^- group, two DUV transparent NLO sulfamates, M(NH₂ SO₃ )₂ (M=Sr, Ba) with superior optical properties including strong SHG responses (1.2 and 2.7 × KH₂ PO₄ (KDP)), short UV cut-off edge (<190 nm) and moderate birefringence (0.056@589.3 nm for Sr(NH₂ SO₃ )₂ ) were successfully synthesized. Our work has provided not only two promising DUV transparent NLO crystals, but also an innovative non-π-conjugated unit for developing more DUV transparent NLO materials.

From BaCl2 to Ba(NO3)Cl: significantly enhanced birefringence derived from π-conjugated [NO3]

The chlorine barium nitrate Ba(NO3)Cl was synthesized for the first time to the best of our knowledge and found to exhibit a strong birefringence of 0.178 @ 1064 nm, about 19 times that of BaCl2.

BaTi(BO3)2: an excellent birefringent material with highly coplanar isolated [BO3] groups

The birefringence of BaTi(BO₃)₂ is equal to or larger than 0.169 at 546 nm (Δn_(cal.) = 0.185 at 546 nm), which mainly originates from the high-density and parallel arrangement of the isolated [BO₃] triangles in the unit cell.

Centrosymmetric RbSnF2NO3vs. noncentrosymmetric Rb2SbF3(NO3)2

A new UV nonlinear optical fluoride nitrate Rb₂SbF₃(NO₃)₂ exhibits a strong SHG response of 2.7 times that of benchmark KDP and an appropriate birefringence of 0.06@1064 nm.

The synthesis, characterization, and theoretical analysis of (NH4)3PbCl5

(NH₄)₃PbCl₅ features a 3D structure with a short UV cutoff edge of 256 nm and simulated birefringence of about 0.050 at 1064 nm.