A highly birefringent metal-free crystal assembled by cooperative non-covalent interactions

Birefringent crystals can manipulate the phase and polarization of light, so they are widely used as essential components in various optical devices. Common strategies to construct birefringent crystals are introducing metal cations that are either able to realize favorable coordination with functional anionic units or are susceptible to polarizability anisotropy. Herein, we report a metal-free crystal, NH4(H2C6N7O3)·2H2O, synthesized using the facile solution method. In the crystal structure of NH4(H2C6N7O3)·2H2O, (H2C6N7O3)- functional units are assembled in an optimal manner by cooperative non-covalent interactions, i.e., hydrogen bonding and π-π interactions. As a result, this metal-free crystal possesses exceptional birefringence up to 0.54@550 nm, which is larger than those of most metal-containing birefringent crystals. In addition, the interference color of this crystal does not change obviously from 243 K to 313 K, indicating that the birefringence is robust at different temperatures. This work will inspire useful insights into the role of non-covalent interactions in designing outstanding birefringent crystals for efficient polarized optical devices.

Stereochemically Active Lone Pair Leads to Birefringence in the Vacancy Ordered Cs3Sb2Cl9 Perovskite Single Crystals

Stereochemically active lone pair (SCALP) cations are attractive units for realizing optical anisotropy. Antimony(III) chloride perovskites with the SCALP have remained largely unknown to date. We synthesized a new vacancy ordered Cs3Sb2Cl9 perovskite single crystals with SbCl6 octahedral linkage containing the SCALP. Remarkably, all-inorganic halide perovskite Cs3Sb2Cl9 single crystals exhibit an exceptional birefringence of 0.12 ± 0.01 at 550 nm. The SCALP brings a large local structural distortion of the SbCl6 octahedra promoting birefringence optical responses in Cs3Sb2Cl9 single crystals. Theoretical calculations reveal that the considerable hybridization of Sb 5s and 5p with Cl 3p states largely contribute to the SCALP. Furthermore, the change in the Sb-Cl-Sb bond angle creates distortion in the SbCl6 octahedral arrangement in the apical and equatorial directions within the crystal structure incorporating the required anisotropy for the birefringence. This work explores pristine inorganic halide perovskite single crystals as a potential birefringent material with prospects in integrated optical devices.

AX·(H2SeO3)n (A = K, Cs; X = Cl, Br; n = 1, 2): A Series of Ionic Cocrystals as Promising UV Birefringent Materials with Large Birefringence and Wide Band Gap

The design and syntheses of new birefringent crystals will be of great importance in commercial applications and materials science. A series of ultraviolet (UV) birefringent crystals, AX·(H2SeO3)n (A = K, Cs; X = Cl, Br; n = 1, 2), with large sizes up to 23 × 6 × 3 mm3, was successfully synthesized by simple aqueous solution method. These four compounds belong to three different space groups. Isomorphic KCl·(H2SeO3)2 and CsCl·(H2SeO3)2 crystallize in the P 1 ¯ space group, while CsBr·(H2SeO3)2 and CsCl·H2SeO3 crystallize in the P21/m and P21/c space groups, respectively. They exhibit cocrystal structures composed of [2(H2SeO3)]∞ and [AX]∞ frameworks, ingeniously inheriting the large optical anisotropy of selenite and the wide band gap of alkali metal halide. And it proves that these compounds indeed possess large birefringence (0.1-0.17 at 532 nm) and short UV cutoff edges (227-239 nm), achieving a balance of optical properties. This research affords a simple and viable strategy for the design and syntheses of new UV birefringent crystals. Besides, it is also found that the n value and ionic size (A and X ions) have important influences on the crystal structures and optical properties of AX·(H2SeO3)n. And this will promote further understanding of the alkali metal halide selenite family.

(C(NH2)3)2(I2O5F)(IO3)(H2O) and C(NH2)3IO2F2: Two Guanidine Fluorooxoiodates with Wide Band Gap and Large Birefringence

Enhancing anisotropy through an effective synergistic arrangement of anionic and cationic groups is crucial for improving the birefringence optical properties of materials. In this work, by transforming I-O into I-F through the fluorination strategy, two metal-free guanidine fluorooxoiodates (C(NH2)3)2(I2O5F)(IO3)(H2O) and C(NH2)3IO2F2 and one guanidine iodate C(NH2)3IO3 were successfully synthesized using the hydrothermal method. An unprecedented dimer [I2O5F] formed by [IO3F] and [IO3] in (C(NH2)3)2(I2O5F)(IO3)(H2O) was found, which greatly enriches the structural diversity of fluorooxoiodates. All three compounds feature a relatively large birefringence (Δn = 0.068, 0.110 and 0.075 at 546 nm) and a short ultraviolet cutoff edge. The theoretical calculation was carried out to understand the electronic structures and linear optical properties.

Colossal Optical Anisotropy from Atomic-Scale Modulations

Materials with large birefringence (Δn, where n is the refractive index) are sought after for polarization control (e.g., in wave plates, polarizing beam splitters, etc.), nonlinear optics, micromanipulation, and as a platform for unconventional light-matter coupling, such as hyperbolic phonon polaritons. Layered 2D materials can feature some of the largest optical anisotropy; however, their use in most optical systems is limited because their optical axis is out of the plane of the layers and the layers are weakly attached. This work demonstrates that a bulk crystal with subtle periodic modulations in its structure-Sr9/8 TiS3 -is transparent and positive-uniaxial, with extraordinary index ne = 4.5 and ordinary index no = 2.4 in the mid- to far-infrared. The excess Sr, compared to stoichiometric SrTiS3 , results in the formation of TiS6 trigonal-prismatic units that break the chains of face-sharing TiS6 octahedra in SrTiS3 into periodic blocks of five TiS6 octahedral units. The additional electrons introduced by the excess Sr form highly oriented electron clouds, which selectively boost the extraordinary index ne and result in record birefringence (Δn > 2.1 with low loss). The connection between subtle structural modulations and large changes in refractive index suggests new categories of anisotropic materials and also tunable optical materials with large refractive-index modulation.

Finding a Deep-UV Borate BaZnB4 O8 with Edge-sharing [BO4 ] Tetrahedra and Strong Optical Anisotropy

The polarization modulation of deep-UV light is an important process that incorporates functionality to selectively respond to light-mater interaction. Typically, optical anisotropy is foremost to the use efficiency of deep-UV birefringent crystals. Herein, a new congruently melting polyborate with extremely large birefringence (Δn₍₀₀₁₎ =0.14@589.3 nm) and band gap (6.89 eV) is discovered as a high performance birefringent crystal, which breaks the current deadlock of deep-UV polyborates that usually show small birefringence. The rigid tetrahedra, including [ZnO₄ ] and edge-sharing [BO₄ ] tetrahedra, make all the planar [BO₃ ] triangles in the lattice adopt preferential arrangement and thereby lead to an extraordinary large birefringence that is larger than all the deep-UV borates with experimentally measured values. Structural analyses with the additional theoretical calculations were used to study the origin of strong optical anisotropy in BaZnB₄ O₈ .

Double-Modification Oriented Design of a Deep-UV Birefringent Crystal Functionalized by [B12 O16 F4 (OH)4 ] Clusters

Polarization modulation of deep-UV light is of significance to current technologies, and to this end, the birefringent crystal has emerged as an invaluable material as it allows for effective light modulation. Herein, a double-modification strategy driven by F and OH anions that makes double effects towards the critical property enhancement of deep-UV birefringent crystals is proposed. This leads to a new hydroxyborate (NH₄ )₄ [B₁₂ O₁₆ F₄ (OH)₄ ] with giant cluster as a deep-UV birefringent crystal with large birefringence (Δn_exp. =0.12@546.1 nm). This birefringence is a record among inorganic hydroxyborates with experimentally measured birefringence. Structural analysis shows that the near-plane arrangement of [B₁₂ O₁₆ F₄ (OH)₄ ] cluster is responsible for the large optical anisotropy. Theoretical calculations indicate that its π-conjugated [BO₃ ] and [BO₂ OH] units are the main source of this large optical anisotropy.

Chiral Ligand-Driven Systematic Synthesis of Coordination Polymers with Non-centrosymmetric Structures

Chirality is an important concept in chemistry revealing intriguing optical properties such as circular dichroism (CD), circularly polarized luminescence (CPL), etc. As one of the non-centrosymmetric (NCS) classes, chiral materials with extended structures may exhibit unique nonlinear optical (NLO) properties, such as second-harmonic generation (SHG). In this Concept article, a series of recently discovered NCS coordination polymers (CPs) from use of carefully designed chiral organic ligands are reviewed. Combining several metal cations such as lanthanides, lead, zinc, and cadmium with rigid chiral ligands has resulted in interesting CPs with both polar and nonpolar structures. Detailed structures, SHG properties, and structure-property relationships are provided. The importance of hyperpolarizability formed by intermolecular hydrogen boding interactions to SHG is emphasized.

Two Indium Iodate-Nitrates with Large Birefringence Induced by Hybrid Anionic Functional Groups and Their Favorable Arrangements

Two new indium iodate-nitrates, In(IO₃)₂(NO₃) (1) and [In(IO₃)(OH)(H₂O)](NO₃) (2), were rationally designed through the integration of hybrid anionic functional units. They exhibit large birefringences (1, 0.269; 2, 0.188, at 532 nm) and wide band gaps (1, 4.08 eV; 2, 4.39 eV), which is attributed to the synergistic effect of two types of birefringence-active units, namely, lone-pair IO₃ and π-conjugated NO₃ anionic groups. Through the substitution of OH and H₂O of 2 with IO₃, the hydrogen bonds of 2 are eliminated and the birefringence of 1 is greatly enhanced, highlighting the intriguing role of isovalent substitution in the discovery of fascinating optical materials.

KPb3(o-C5H4NCOO)2Cl5: A Brand-New Stable Lead Chloride with Good Comprehensive Nonlinear Optical Performances

The discovery of nonlinear optical (NLO) crystals with excellent performances has still been a great challenge. Herein, a brand-new stable lead chloride, KPb3(o-C5H4NCOO)2Cl5, has been successfully synthesized and well characterized....

A Lanthanum Ammonium Sulfate Double Salt with a Strong SHG Response and Wide Deep-UV Transparency

The targeted synthesis of deep-ultraviolet (deep-UV) nonlinear optical (NLO) materials, especially those with non-π-conjugated sulfates, has experienced considerable difficulties due to the need to reconcile the oft-competing requirements for deep-UV transparency and strong second-harmonic generation (SHG). We report herein the designed synthesis of the first rare-earth metal-based deep-UV sulfate La(NH₄ )(SO₄ )₂ by a double-salt strategy involving introduction of complementary cations, together with optical studies that reveal a short-wavelength deep-UV absorption edge (below 190 nm) and the strongest SHG response among deep-UV NLO sulfates (2.4×KDP). Theoretical calculations and crystal structure analysis suggest that the excellent balance between SHG response and deep-UV transparency can be attributed to a synergistic interaction of the hetero-cations La³⁺ and [NH₄ ]⁺ , which optimize alignment of the [SO₄ ] tetrahedra and highly polarizable [LaO₈ ] polyhedra.

A New Nonlinear Optical Material with N(CN)2 - Anion

Discovering new functional genes and developing high-performance materials are the goals being pursued by scientists. In this work, we successfully obtained a second-order nonlinear optical (NLO) material via the aqueous solution method, Y[N(CN)₂ ]₄ [NH(C₂ H₅ )₃ ] ⋅ 3H₂ O, which is the first NLO material with the anionic group N(CN)₂ ^- . Remarkably, this material is not only strongly NLO-active at 1064 nm with a response of about 2.8 × KH₂ PO₄ , but also possesses a short UV absorption edge of 250 nm. In-depth first-principles calculations illustrate well that the optical properties are mainly from the strong interaction of N, C and Y atoms. This result indicates that the N(CN)₂ ^- anion may be a new NLO functional gene. This work enriches the diversity of NLO functional genes and materials.

A Lead Mixed Halide with Three Different Coordinated Anions and Strong Second-Harmonic Generation Response

Nonlinear optical (NLO) crystals are widely applied in information technology, micro-manufacturing and medical treatment. Herein, a new lead mixed halide with strong second-harmonic generation (SHG) response, Cs₃ Pb₂ (CH₃ COO)₂ Br₃ I₂ , has been designed and rationally synthesized. Cs₃ Pb₂ (CH₃ COO)₂ Br₃ I₂ represents the rare NLO crystal featuring that three different anions (I^- , Br^- and O^2- ) simultaneously coordinate the Pb(II) atom to form a severely distorted [PbBr₂ I₂ O₂ ] polyhedron with a large polarizability. Remarkably, Cs₃ Pb₂ (CH₃ COO)₂ Br₃ I₂ not only exhibits a very strong phase-matching SHG response of 9×KH₂ PO₄ (KDP), but also possesses a large birefringence (0.27@1064 nm) and high laser damage threshold (LDT). The strong SHG effect of Cs₃ Pb₂ (CH₃ COO)₂ Br₃ I₂ mainly originates from the oriented arrangement of [Pb₂ Br₃ I₂ ] chains. This study points out an effective strategy to develop new NLO crystals with strong SHG response.

Strong SHG Responses in a Beryllium-Free Deep-UV-Transparent Hydroxyborate via Covalent Bond Modification

Deep-ultraviolet (deep-UV) nonlinear optical (NLO) crystals are key materials in creating tunable deep-UV lasers for frequency conversion technology. However, practical application of the sole usable crystal, KBe₂ BO₃ F₂ , has been hindered by the high toxicity of beryllium and its layering tendency in crystal growth. Herein, we report a beryllium-free deep-UV NLO material NaSr₃ (OH)(B₉ O₁₆ )[B(OH)₄ ] (NSBOH), synthesized by a covalent bond modification strategy under hydrothermal conditions. Moisture-stable NSBOH exhibits strong second-harmonic generation (SHG) at 1064 nm (3.3 × KH₂ PO₄ ) and 532 nm (0.55 × β-BaB₂ O₄ ), both amongst the largest powder SHG responses for a deep-UV borate, with good phase-matchability and a short wavelength cutoff edge (below 190 nm). NSBOH possesses a 3D covalent anionic [B₉ O₁₉ ]_∞ honeycomb-like framework with no layering. The Sr²⁺ and Na⁺ ions, residing in the cavities of the anionic framework, act as templates for the assembly and favorable alignment of NLO-active groups, resulting in an optimal balance between strong SHG activities and wide UV transparency. These merits indicate NSBOH is a very attractive candidate for deep-UV NLO applications.

Na7(IO3)(SO4)3: the first noncentrosymmetric alkaline-metal iodate-sulfate with isolated [IO3] and [SO4] units

Metal iodates are of current research interest because of their potential application as nonlinear optical crystals but the exploration of new iodates is mainly concentrated on complex cation iodates. In contrast, iodates with multiple anion groups are rarely reported. In this communication, the first noncentrosymmetric alkaline-metal iodate-sulfate, Na₇(IO₃)(SO₄)₃ has been designed, synthesized and characterized.

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.

A Congruent-Melting Mid-Infrared Nonlinear Optical Vanadate Exhibiting Strong Second-Harmonic Generation

Study of mid-infrared (mid-IR) nonlinear optical (NLO) materials is hindered by the competing requirements of optimized second-harmonic generation (SHG) coefficient d_ij and laser-induced damage threshold (LIDT) as well as the harsh synthetic conditions. Herein, we report facile hydrothermal synthesis of a polar NLO vanadate Cs₄ V₈ O₂₂ (CVO) featuring a quasi-rigid honeycomb-layered structure with [VO₄ ] and [VO₅ ] polyhedra aligned parallel. CVO possesses a wide IR-transparent window, high LIDT, and congruent-melting behavior. It has very strong phase-matchable SHG intensities in metal vanadate family (12.0 × KDP @ 1064 nm and 2.2 × AGS @ 2100 nm). First-principles calculations suggest that the exceptional SHG responses of CVO largely originate from virtual electronic transitions within [V₄ O₁₁ ]_∞ layer; the excellent optical transmittance of CVO arises from the special characteristics of vibrational phonons resulting from the layered structure.

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

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

Be2 (BO3 )(IO3 ): The First Anion-mixed Van der Waals Member in the KBe2 BO3 F2 Family with a Very Strong Second Harmonic Generation Response

To obtain new nonlinear optical (NLO) materials with a large second harmonic generation (SHG) effect has always been a great challenge. We have synthesized the first metal borate-iodate NLO crystal, Be₂ (BO₃ )(IO₃ ) (BBIO), by multicomponent modification of KBe₂ BO₃ F₂ (KBBF), in which the structural features of KBBF were maintained and (IO₃ )^- groups were connected to honeycomb [Be₂ BO₃ O₂ ]_∞ layers. As the first KBBF family member with mixed anionic groups, BBIO benefited from the synergistic effect of (IO₃ )^- , (BO₃ )^3- and (BeO₄ )^6- groups, and exhibited a very strong SHG response of ≈7.2×KH₂ PO₄ (KDP, @1064 nm) and a large birefringence (Δn) of 0.172 at 1064 nm. BBIO may, unexpectedly, optimize growth habits via van der Waals forces. This study presents borate-iodate as a new NLO material and it demonstrates opportunities in KBBF structural engineering.

[o-C5 H4 NHOH]2 [I7 O18 (OH)]⋅3 H2 O: An Organic-Inorganic Hybrid SHG Material Featuring an [I7 O18 (OH)] ∞ 2 - Branched Polyiodate Chain

An organic-inorganic hybrid polyiodate, namely, [o-C₅ H₄ NHOH]₂ [I₇ O₁₈ (OH)]⋅3 H₂ O (I), featuring a novel branched polyiodate chain has been obtained by evaporation method. [o-C₅ H₄ NHOH]₂ [I₇ O₁₈ (OH)]⋅3 H₂ O (I) crystalizes in the polar space group Ia and features an [I₇ O₁₈ (OH)] ∞ 2 - branched polyiodate chain in which [I₃ O₉ ]^3- trimers are grafted on the same side of the one-dimensional (1D) chain based on [I₄ O₁₁ (OH)]^3- tetramers. The asymmetric organic amine groups are beneficial to the polymerization of iodate groups and inducing the formation of the non-centrosymmetric (NCS) structure. Compound I exhibits a rather large Second-Harmonic- Generation (SHG) signal of 8.5×KH₂ PO₄ (KDP) upon 1064 nm laser radiation, a moderate band gap of 3.90 eV and a high laser-induced-damage-threshold (LIDT) of 182.34 MW cm^-2 , hence it is a promising new SHG material. The relationship between the structures of the organic amine groups and the overall structures has been also analyzed.

Discovery of First Magnesium Fluorooxoborate with Stable Fluorine Terminated Framework for Deep-UV Nonlinear Optical Application

The generated light can be tuned to cover almost the entire spectral range from deep-ultraviolet to terahertz wavelengths by utilizing the nonlinear optical crystals with a simple frequency doubling process. Among them, the discovery of novel candidates for the production of deep-ultraviolet light is by extension a great challenge toward realizing the vast potential. Actually, the availability for this process mainly depends on whether the critical performance can be well coexisted in one practical crystal. Herein, the first magnesium fluorooxoborate MgB₅ O₇ F₃ was synthesized as a new competitive candidate for deep-ultraviolet nonlinear optical application. It has a sufficiently large nonlinearity and a deep-ultraviolet phase matching wavelength, indicating that it holds great potential for the production of coherent light below 200 nm. The critical performance enhancement of MgB₅ O₇ F₃ when compared with its isomorphic phases was demonstrated and discussed. More importantly, we proposed that fluorooxoborate system with the general formula of MB₅ O₇ F₃ (M=divalent metal) possesses stable fluorine terminated framework, which makes them tend to retain their crystallized space groups unchanged.

UV Solar-Blind-Region Phase-Matchable Optical Nonlinearity and Anisotropy in a π-Conjugated Cation-Containing Phosphate

Wide ultraviolet (UV) transparency, strong second-harmonic generation (SHG) response, and sufficient optical birefringence for phase-matching (PM) at short SHG wavelengths are vital for practical UV nonlinear optical (NLO) materials. However, simultaneously optimizing these properties is a major challenge, particularly for metal phosphates. Herein, we report a non-traditional π-conjugated cation-based UV NLO phosphate [C(NH₂ )₃ ]₆ (PO₄ )₂ ⋅3 H₂ O (GPO) with a short UV cutoff edge. GPO is SHG active at 1064 nm (3.8 × KH₂ PO₄ @ 1064 nm) and 532 nm (0.3 × β-BaB₂ O₄ @ 532 nm) and also possesses a significant birefringence (0.078 @ 546 nm) with a band gap >6.0 eV. The PM SHG capability of GPO can extend to 250 nm, indicating GPO is a promising UV solar-blind NLO material. Calculations and crystal structure analysis show that the rare coexistence of wide UV transparency, large SHG response, and optical anisotropy is due to the introduction of π-conjugated cations [C(NH₂ )₃ ]⁺ and their favorable arrangement with [PO₄ ]^3- anions.