Toward the Rational Design of Novel Noncentrosymmetric Materials: Factors Influencing the Framework Structures

Targeting the Next Generation of Deep-Ultraviolet Nonlinear Optical Materials: Expanding from Borates to Borate Fluorides to Fluorooxoborates

Coherent light radiation down to the deep-ultraviolet spectral range (λ < 200 nm) produced by common laser sources is extensively used in diverse fields ranging from ultrahigh-resolution photolithography to photochemical synthesis to high-precision microprocessing. Actually, it is hard to immediately obtain certain wavelengths, deep-ultraviolet coherent light in particular, from commercial laser sources. However, the direct second harmonic generation process governed in part by nonlinear optical crystals is a feasible and effective approach to generate deep-ultraviolet coherent light, which motivates chemists and materials scientists to find potential deep-ultraviolet nonlinear optical materials that can practically meet the scientific requirements. The research progress required to go from a new single-crystal structure to final device applications involves many pivotal steps and is highly time-consuming and challenging, and therefore, it is necessary to commence systematic studies aimed at shortening the research cycle and accelerating the rational design of deep-ultraviolet nonlinear optical materials. In this Account, we choose borates as raw materials because they have ever-greater possibilities to form desired noncentrosymmetric structures, wide optical transparency windows, rich structural chemistry, and also large polarizabilities to guarantee the coexistence of large second-order nonlinear optical coefficients and suitable birefringence. Besides, the effects of fluorine atoms on the structural chemistry and optical properties of borates have been summarized and analyzed. On the basis of these favorable influences, three specific rational design strategies, including experimental and theoretical methods, have been proposed in order to shorten the investigational cycle of discovering the new expected compounds with high physicochemical performances required for practical applications. In this way, the progress of searching for candidates for the next generation of deep-ultraviolet nonlinear optical materials was accelerated from borates to borate fluorides to fluorooxoborates with three effective strategies: (1) expansion of the frontier from borates to borate fluorides with the introduction of fluorine to achieve enhanced optical performance; (2) computer-assisted design of new deep-ultraviolet nonlinear optical materials with a newly introduced systematic global structure optimization method; and (3) expansion of the frontier from borate fluorides to fluorooxoborates by proposed functionalized oxyfluoride [BO _xF_{4- x}]^{( x+1)-} ( x = 1, 2, 3) chromophores to balance multiple criteria. The preliminary development of fluorooxoborates exhibiting high performance as a new fertile field to search for deep-ultraviolet nonlinear optical materials is highly encouraging and inspiring and can guide chemists and materials scientists with new directions and thoughts aimed at finding the next generation of practical deep-ultraviolet nonlinear optical materials.

A2SrMIVS4 (A = Li, Na; MIV = Ge, Sn) concurrently exhibiting wide bandgaps and good nonlinear optical responses as new potential infrared nonlinear optical materials

A series of A₂SrM^IVS₄ compounds concurrently exhibiting wide bandgaps and good NLO responses were proven as promising IR NLO materials.

Exploration of new nonlinear optical (NLO) materials is of importance for infrared (IR) applications. However, it is an extremely tough challenge to design and synthesize excellent IR NLO materials with optimal performances (e.g., concurrently a large NLO response and wide bandgap). Herein, four new mixed alkali/alkaline earth metal sulfides, A₂SrM^IVS₄ (A = Li, Na; M^IV = Ge, Sn), were successfully synthesized by a motif-optimization approach using the classical AgGaS₂ as a template. Note that all of them concurrently exhibit wide bandgaps (3.1–3.8 eV) and good NLO responses (0.5–0.8 × AgGaS₂) with phase-matching behavior, which satisfy the balance conditions (E_g ≥ 3.0 eV and d_ij ≥ 0.5 × benchmark AgGaS₂) of optical performances and hence are outstanding IR NLO materials. Remarkably, both of Na₂SrM^IVS₄ have the same structure without the structural transformation (Ge to Sn) in the reported related analogues and an interesting cation-dependent structural change is also found in Na₂M^IISnS₄ (M^II: Sr, R3c vs. Ba, I4[combining macron]2d). These results verify that the above design strategy of motif-optimization provides a feasible guide for the discovery of new IR NLO candidates and the A–AE–M–S (A = alkali metal; AE = alkaline-earth metal; M = Ga, In, Ge, Sn) system was identified as the preferred system for IR NLO materials.

Rational Design via Synergistic Combination Leads to an Outstanding Deep-Ultraviolet Birefringent Li2Na2B2O5 Material with an Unvalued B2O5 Functional Gene

Birefringent materials, the key components in modulating the polarization of light, are of great importance in optical communication and the laser industry. Limited by their transparency range, few birefringent materials can be practically used in the deep ultraviolet (DUV, λ < 200 nm) region. Different from the traditional BO₃- or B₃O₆-based DUV birefringent crystals, we propose a new functional gene, the B₂O₅ unit, for designing birefringent materials. Excitingly, the synergistic combination of Li₄B₂O₅ and Na₄B₂O₅ generates a new compound, Li₂Na₂B₂O₅, with enhanced optical properties. The Li₂Na₂B₂O₅ crystal with a size of up to 35 × 15 × 5 mm³ was grown by the top-seeded solution growth (TSSG) method, and its physicochemical properties were systematically characterized. Li₂Na₂B₂O₅ features a large amount of birefringence (0.095@532 nm), a short DUV cutoff edge (181 nm) with a high laser-induced damage threshold (LDT, 7.5 GW/cm² @1064 nm, 10 ns), favorable anisotropic thermal expansion (α_a/α_b = 5.6), and the lowest crystal growth temperature (<609 °C) among the commercial birefringent crystals. Moreover, the influences of the B₂O₅ structural configurations on the optical anisotropy were explored. The fascinating experimental results will provide a prominent DUV birefringent crystal and an effective synthesis strategy, which can facilitate the design of DUV birefringent materials.

Growth, Crystal Structures, and Characteristics of Li5ASrMB12O24 (A = Zn, Mg; M = Al, Ga) with [MB12O24] Frameworks

Two aluminoborates (Li₅ZnSrAlB₁₂O₂₄ and Li₅MgSrAlB₁₂O₂₄) and one galloborate (Li₅ZnSrGaB₁₂O₂₄) were successfully obtained for the first time, and the centimeter-sized crystals were grown by the Kyropoulos method. These crystals possess the [MB₁₂O₂₄] (M = Al, Ga) basic frameworks in their crystal structures, and the [GaB₁₂O₂₄] framework is a new framework in galloborates. The synthesis, single crystal growth, characterizations, and theoretical calculation are reported for these compounds.

Three Cadmium Tartratoborates with Good Second Harmonic Generation (SHG) or Luminescence Performances

Three new cadmium(II) tartratoborates, namely, Cd₅[(C₄H₂O₆)₂B]₂(H₂O)₈·3H₂O (1), K₂Cd₄[(C₄H₂O₆)₂B]₂(H₂O)₂ (2), and Li_0.92K_1.08Cd_1.5[(C₄H₂O₆)₂B](H₂O)₂ (3), have been successfully synthesized by the hydrothermal method. Compounds 1-3 belong to centric C2/ c, acentric Pc, and the polar C2, respectively. Through based on the same hybrid borate-tartrate [(C₄H₂O₆)₂B]^5- anions, they exhibit different structures. The [(C₄H₂O₆)₂B]^5- anion is composed of two tartrate anions and a B(OH)₄^- unit. Compound 1 features a novel 3D network formed by 2D {Cd₃[(C₄H₂O₆)₂B]₂(H₂O)₂}^4- anionic layers and [Cd₂O₁₀] dimers, forming tunnels of large 14-MRs which are filled by the non-coordination water molecules. Compound 2 has a characteristic 3D network based on {Cd₂[(C₄H₂O₆)₂B]}^- units interlinked via carboxylate groups, forming tunnels of 11-MRs, half of which stuffed with the K⁺ ions. Compound 3 features 2D {Cd₄[(C₄H₂O₆)₄B]}³⁺ layers which are separated by K/Li ions. Luminescent studies suggest that they emit blue light. Compounds 2 and 3 display phase-matchable second harmonic generation signs of about 3.2× and 1.5× KH₂PO₄, respectively.

Y8O(OH)15(CO3)3Cl: an excellent short-wave UV nonlinear optical material exhibiting an infrequent three-dimensional inorganic cationic framework

An excellent short-wave UV nonlinear optical material, Y₈O(OH)₁₅(CO₃)₃Cl, featured an infrequent three-dimensional inorganic cationic framework.

KBi(IO3)3(OH) and NaBi(IO3)4: from the centrosymmetric chain to a noncentrosymmetric double layer

Two bismuth iodates via tuning of alkali–metal ions reveal a structural transition from the centrosymmetric chain to a non-centrosymmetric double layer.

Li4Na2CsB7O14: a new edge-sharing [BO4]5− tetrahedra containing borate with high anisotropic thermal expansion

Li₄Na₂CsB₇O₁₄, a new edge-sharing [BO₄]⁵⁻ tetrahedra containing borate with high anisotropic thermal expansion, was obtained under the atmospheric pressure conditions.

A new ferroelastic hybrid material with a large spontaneous strain: (Me3NOH)2[ZnCl4]

A new organic–inorganic hybrid undergoes a ferroelastic phase transition at 351 K accompanied with a large spontaneous stain.

From centrosymmetric to noncentrosymmetric: cation-directed structural evolution in X3ZnB5O10 (X = Na, K, Rb) and Cs12Zn4(B5O10)4 crystals

A new Cs₁₂Zn₄(B₅O₁₀)₄ compound exhibits a short DUV cutoff edge (lower than 185 nm) and contains a 3D [ZnB₅O₁₀]_∞ network.

Centrosymmetric K2SO4·(SbF3)2 and noncentrosymmetric Rb2SO4·(SbF3)2 resulting from cooperative effects of lone pair and cation size

The cooperative effects of the lone pair and cation size were used to tune the symmetry of two stoichiometrically equivalent compounds, A₂SO₄·(SbF₃)₂ (A = K or Rb).

Sn2B7O12F with a 2∞[B14O24]6− layer constructed from the unprecedented [B7O16]11− fundamental building block

A new tin borate fluoride Sn₂B₇O₁₂F features a complicated 3D network composed of the unique [B₇O₁₆]¹¹⁻ FBBs and [Sn₄O₁₂F₂]¹⁸⁻ clusters.

Ba3B10O17F2·0.1KF: the first mixed alkali/alkaline-earth metal fluorooxoborate with unprecedented double-layered B–O/F anionic arrangement

The first mixed alkali/alkaline-earth metal fluorooxoborate Ba₃B₁₀O₁₇F₂·0.1KF with a unique double-layered B–O/F anionic framework was successfully synthesized and systemically characterized.

The first lithium difluorophosphate LiPO2F2 with a neutral polytetrahedral microporous architecture

The first lithium difluorophosphate LiPO₂F₂ with a unique microporous architecture and improved optical properties was synthesized and characterized.

K2[B4O5(OH)4]·H2O and K2[B4O5(OH)4]: two new hydrated potassium borates with isolated [B4O5(OH)4]2− units and different structural frameworks

Two new hydrated potassium tetraborates with isolated [B₄O₅(OH)₄]²⁻ units were obtained via a mild hydrothermal method.

Mg(H2O)6B4O5(OH)4(H2O)3: a new hydrated borate with a short DUV cutoff edge

[B₄O₅(OH)₄] groups are linked together by hydrogen bonds forming undulating pseudo layers, leading to a short cutoff edge.

Two alkali calcium borates exhibiting second harmonic generation and deep-UV cutoff edges

KCa₄B₃O₉ and K_0.59Rb_0.41Ca₄B₃O₉ crystallize in the polar space group with moderate SHG responses. Calculations were performed to elucidate the structure–property relationships.

Three new phosphates, Cs8Pb4(P2O7)4, CsLi7(P2O7)2 and LiCa(PO3)3: structural comparison, characterization and theoretical calculation

Three new phosphates with a short cutoff edge were obtained, and the structural comparisons were discussed in detail.

ABaSbQ3 (A = Li, Na; Q = S, Se): diverse arrangement modes of isolated SbQ3 ligands regulating the magnitudes of birefringences

The interrelation of arrangement modes of isolated SbQ₃ ligands on structures and birefringences has been systematically studied in series of chalcogenides.

LiGeBO4: a nonlinear optical material with a balance between deep-ultraviolet cut-off edge and large SHG response induced by hand-in-hand tetrahedra

LiGeBO₄ is found to be a material that can balance a large band gap (>6.2 eV) and strong SHG coefficient (2 × KDP), which is rarely seen in tetrahedral-based crystals.

BaB8O12F2: a promising deep-UV birefringent material

A new alkaline-earth fluorooxoborate, BaB8O12F2, has been discovered through chemical substitution of a cation. It exhibits a short cutoff edge and a large birefringence.

Adjustable optical nonlinearity in d10 cations containing chalcogenides via dp hybridization interaction

The role of d¹⁰-cations on band gap and nonlinearity in Cd/Hg-containing metal chalcogenides was investigated using DFT.

Functional layered materials with heavy metal lone pair cations, Pb2+, Bi3+, and Te4+

Syntheses, structures, representative properties, and the structure–property relationships for a series of functional layered materials are presented.

Pb2GaF2(SeO3)2Cl: Band Engineering Strategy by Aliovalent Substitution for Enlarging Bandgap while Keeping Strong Second Harmonic Generation Response

Wide bandgap and strong second-order generation (SHG) effect are two crucial but contradictory conditions for practical nonlinear optical (NLO) materials. Herein, a new NLO crystal Pb₂GaF₂(SeO₃)₂Cl (I) containing novel functional (GaO₃F₃)^6- octahedra is designed and synthesized by a rational band engineering strategy with aliovalent substitution. Benefiting from the removal of transition metal cations and the introduction of bridged F anions, I exhibits the widest bandgap among all reported phase-matchable NLO selenites. Meanwhile, a strong SHG response more than 4.5 times of KH₂PO₄ (KDP) is maintained. The dominate role of the (GaO₃F₃)^6- groups to the enlarged bandgap in I are elucidated by first-principles studies.

Cs3VO(O2)2CO3: an exceptionally thermostable carbonatoperoxovanadate with an extremely large second-harmonic generation response

A highly thermostable carbonatoperoxovanadate, Cs₃VO(O₂)₂CO₃, exhibits a remarkably strong SHG response of approximately 23.0 times that of KDP, which is the largest among those of the reported NLO carbonate materials.

A novel nonlinear optical (NLO) carbonatoperoxovanadate, Cs₃VO(O₂)₂CO₃, with an exceptionally high thermostability was successfully synthesized by introducing highly polarizable Cs⁺ cations and inorganic polydentate carbonate anions into asymmetric peroxovanadates. The structure of Cs₃VO(O₂)₂CO₃ is composed of distorted [VO(O₂)₂CO₃]^3– units and charge balancing Cs⁺ cations. The title compound exhibits the largest NLO intensity ever found in the current carbonate NLO materials, i.e., 23.0 times that of KH₂PO₄ (KDP). The remarkably strong second-harmonic generation (SHG) response originates from the synergistic effect of the exceedingly polarizable Cs⁺ cations, distortive polyhedra of the V⁵⁺ cation, delocalized π orbitals in CO₃ groups, and distorted localized π orbitals in O₂ groups. First-principles calculations indicated that introducing the polarizable cations into peroxovanadates not only induces the enhancement of the SHG response but also improves the thermal stability of the framework.

Ba3Mg3(BO3)3F3 polymorphs with reversible phase transition and high performances as ultraviolet nonlinear optical materials

Nonlinear optical (NLO) materials are the vital components of future photoelectric technologies as they can broaden the tunable wavelength range supplied by common laser sources. However, the necessary prerequisites for a practical NLO material are rather strict. Accordingly, considerable efforts have been focused on finding potential NLO materials. Here we report two asymmetric beryllium-free borates Pna2₁- and P\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar 6$$\end{document}6¯2m-Ba₃Mg₃(BO₃)₃F₃ featuring NLO-favorable ²_∞[Mg₃O₂F₃(BO₃)₂] layered structures. The reversible phase transition among two polymorphs was demonstrated by multiple experimental tests. The optical measurements reveal that Pna2₁-Ba₃Mg₃(BO₃)₃F₃ possesses the optical properties required for ultraviolet NLO applications. Remarkably, Pna2₁-Ba₃Mg₃(BO₃)₃F₃ has a large laser damage threshold, a deep-ultraviolet cutoff edge, a favorable anisotropic thermal expansion as well as the capacity of insolubility in water. These optical properties can be comparable or superior to that of commercial NLO material β-BaB₂O₄, which make Pna2₁-Ba₃Mg₃(BO₃)₃F₃ a promising ultraviolet NLO material.

Nonlinear optical crystals suitable for the UV spectral region could simplify short-wavelength generation and make it more efficient. Here, the authors design and demonstrate that one of two asymmetric borate polymorphs exhibits promising optical and mechanical properties for generating UV light.

New strategy for designing promising mid-infrared nonlinear optical materials: narrowing the band gap for large nonlinear optical efficiencies and reducing the thermal effect for a high laser-induced damage threshold

To circumvent the incompatibility between large nonlinear optical (NLO) efficiencies and high laser-induced damage thresholds (LIDTs) in mid-infrared NLO materials, a new strategy for designing materials with both excellent properties is proposed. This strategy involves narrowing the band gap for large NLO efficiencies and reducing the thermal effect for a high LIDT. To support these proposals, a series of isostructural chalcogenides with various tetrahedral center cations, Na2Ga2MQ6 (M = Ge, Sn; Q = S, Se), were synthesized and studied in detail. Compared with the benchmark AGS, these chalcogenides exhibit significantly narrower band gaps (1.56-1.73 eV, AGS: 2.62 eV) and high NLO efficiencies (1.6-3.9 times that of AGS at 1910 nm), and also outstanding LIDTs of 8.5-13.3 × those of AGS for potential high-power applications, which are contrary to the conventional band gap view but can be attributed to their small thermal expansion anisotropy, surmounting the NLO-LIDT incompatibility. These results shed light on the search for practical IR NLO materials with excellent performance not restricted by NLO-LIDT incompatibility.

RbSe3B2O9(OH) and CsSe3B2O9(OH): one dimensional boroselenite-based anionic frameworks with second harmonic generation properties

Two new alkali boroselenites RbSe3B2O9(OH) and CsSe3B2O9(OH) have been synthesized by traditional solid-state reactions. Single-crystal X-ray diffraction study indicated that they are isostructural and adopt a new type of structure, which crystallizes in the noncentrosymmetric space group P212121. Optical diffuse reflectance spectrum studies emphasized that both are indirect optical transitions with values of 3.79 and 4.17 eV for RbSe3B2O9(OH) and CsSe3B2O9(OH), respectively. Optical analysis revealed a broad transparency window in the 0.3-8.5 μm region for both compounds. In addition, RbSe3B2O9(OH) featured a relatively weak second-harmonic-generation response, and for CsSe3B2O9(OH), the response is 0.8-times that of KH2PO4. Theoretical calculations of band structure, density of state, and linear and nonlinear optical properties were also performed to get insight into the relationships between electronic structures and their optical properties.