Beryllium-Free Rb3Al3B3O10F with Reinforced Interlayer Bonding as a Deep-Ultraviolet Nonlinear Optical Crystal

Assembly of π-Conjugated [B3O6] Units by Mer-Isomer [YO3F3] Octahedra to Design a UV Nonlinear Optical Material, Cs2YB3O6F2

Achieving the extreme balance of the key performance requirements is the crucial to breakthrough the application bottleneck for nonlinear optical (NLO) materials. Herein, by assembly of the π-conjugated [B3O6] functional species with the aid of structure-directing property of mer-isomer [YO3F3] octahedra, a new ultraviolet (UV) NLO material, Cs2YB3O6F2 with aligned arrangement of coplanar [B3O6] groups has been synthesized. The polar material exhibits the rare coexistence of the largest second harmonic generation response of 5.6×KDP, the largest birefringence of 0.091 at 532 nm, the shortest Type I phase-matching down to 200.5 nm and deep-ultraviolet transparency among reported acentric rare-earth borates with [B3O6] groups. Remarkably, benefiting from the enhanced bonding force among functional units [B3O6], a firm three-dimensional framework is constructed, which facilitates the growth of large crystals. This can be proved by a block shape crystal with dimensional of 6×5×4 mm3, indicating that it was a promising UV NLO crystal. This work provides a powerful strategy to design UV NLO materials with good performances.

CsAlB3O6Cl: the rational construction of a KBBF-type structure with aligned 2∞[AlB3O6Cl] layers via introducing unprecedented [AlO3Cl] tetrahedra

The first chloroaluminoborate, CsAlB3O6Cl, with innovative AlO3Cl tetrahedra and a perfect planar arrangement of [B3O6] groups, was structurally designed and synthesized via chlorination of [AlO4] tetrahedra. Simultaneously, the smooth introduction of the [AlO3Cl] group into borates initiates the development of a chloroaluminoborate and greatly enriches the structural chemistry of aluminoborates.

Recent advances in rational structure design for nonlinear optical crystals: leveraging advantageous templates

Nonlinear optical (NLO) crystals that can expand the spectral range of laser outputs have attracted significant attention for their optoelectronic applications. The research progress from the discovery of new single crystal structures to the realization of final device applications involves many key steps and is very time consuming and challenging. Consequently, exploring efficient design strategies to shorten the research period and accelerate the rational design of novel NLO materials has become imperative to address the pressing demand for advanced materials. The recent shift in paradigm toward exploring new NLO crystals involves significant progress from extensive "trial and error" methodologies to strategic approaches. This review proposes the concept of rational structure design for nonlinear optical crystals leveraging advantageous templates. It further discusses their optical characteristics, promising applications as second-order NLO materials, and the relationship between their structure and performance, and highlights urgent issues that need to be addressed in the field of NLO crystals in the future. The review aims to provide ideas and driving impetus to encourage researchers to achieve new breakthroughs in the next generation of NLO materials.

PbBeB2O5: A High-Performance Ultraviolet Nonlinear-Optical Crystal with Functional [BeB2O8]8- Group

High-performance nonlinear-optical (NLO) crystals need to simultaneously meet multiple basic and conflicting performance requirements. Here, by using a partial chemical substitution strategy, the first noncentrosymmetric (NCS) PbBeB2O5 crystal with a BeB2O8 group was synthesized, exhibiting a two-dimensional [BeB2O5]∞ layer constructed by interconnecting BeB2O8 groups and bridged PbO4 with an active lone pair. The crystal shows a promising UV NLO functional feature, including a strong SHG effect of 3.5 × KDP (KH2PO4), large birefringence realizing phase matchability in the whole transparency region from 246 to 2500 nm, a short UV absorption edge of 246 nm, and single-crystal easy growth. Remarkably, theoretical studies reveal that the BeB2O8 group has high nonlinear activity, which could stimulate the discovery of a series of excellent NLO beryllium borates.

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.

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.

Ba3.75MgB7O14F2.5: A Mixed Alkaline-Earth Metal Borate Fluoride with Short Ultraviolet Cutoff Edge and Large Birefringence

Fluorine-containing borate crystal materials are particularly attractive due to their rich structural chemistry and excellent properties for optical applications. In this work, a new compound Ba3.75MgB7O14F2.5 has been synthesized through the high-temperature solution method. In the crystal structure of Ba3.75MgB7O14F2.5, the [B7O14] basic building unit and the [MgO3F3] octahedra are interconnected to create a complex three-dimensional network. The structural feature of the less commonly observed [B7O14] units is discussed, and it has been found that such units in Ba3.75MgB7O14F2.5 are most conducive to achieving large birefringence. In addition, Ba3.75MgB7O14F2.5 exhibits good thermal stability, a short ultraviolet cutoff of 203 nm, and large birefringence (0.081@546 nm), indicating its potential as a new UV birefringent crystal.

Nonlinear Optical Sulfides LiMGa8 S14 (M = Rb/Ba, Cs/Ba) Created by Li+ Driven 2D Centrosymmetric to 3D Noncentrosymmetric Transformation

Cations that can regulate the configuration of anion group are greatly important but regularly unheeded. Herein, the structural transformation from 2D CS to 3D noncentrosymmetric (NCS, which is the prerequisite for second-order NLO effect) is rationally designed to newly afford two sulfides LiMGa8 S14 (M = Rb/Ba, 1; Cs/Ba, 2) by introducing the smallest alkali metal Li+ cation into the interlamination of 2D centrosymmetric (CS) RbGaS2 . The unusual frameworks of 1 and 2 are constructed from C2 -type [Ga4 S11 ] supertetrahedrons in a highly parallel arrangement. 1 and 2 display distinguished NLO performances, including strong phase-matchable second-harmonic generation (SHG) intensities (0.8 and 0.9 × AgGaS2 at 1910 nm), wide optical band gaps (3.24 and 3.32 eV), and low coefficient of thermal expansion for favorable laser-induced damage thresholds (LIDTs, 4.7, and 7.6 × AgGaS2 at 1064 nm), which fulfill the criteria of superior NLO candidates (SHG intensity >0.5 × AGS and band gap >3.0 eV). Remarkably, 1 and 2 melt congruently at 873.8 and 870.5 °C, respectively, which endows them with the potential of growing bulk crystals by the Bridgeman-Stockbarge method. This investigated system provides a new avenue for the structural evolution from layered CS to 3D NCS of NLO materials.

Honeycomb layered topology construction for exceptional long-wave infrared nonlinear optical crystals

Nonlinear optical (NLO) crystals capable of efficient long-wave infrared (8-14 μm) laser output remain scarce, and the exploration of long-wave IR NLO materials with superior comprehensive optical performances is a momentous challenge. Herein, we develop two selenide-halide NLO crystals, Hg3AsSe4Br and Hg3AsSe4I, which are derived from the honeycomb layered topology of prototype GaSe. Remarkably, they exhibit not only strong SHG effects, suitable band gap, large birefringence, broad IR transparency range and low two-photon absorption coefficients but reinforced interlayer interaction and more benign crystal growth habit, compared to those of GaSe, indicating that they are promising long-wave IR NLO materials. Moreover, Hg3AsSe4I achieved better comprehensive optical properties than conventional IR crystals, GaSe, ZnGeP2, CdSe and AgGaSe2. The idea of honeycomb layered topology construction provides a material design heuristic to explore cutting-edge IR NLO materials.

Structural Motif Cosubstitution Strategy for Designing Fluoroaluminoborate with the Sr2Be2B2O7-Type Double-Layered Structure

The cosubstitution strategy often was applied to design borate optical crystal materials. Revealingly, a fluoroaluminoborate Sr₂Al_2.18B_5.82O₁₃F₂ with Sr₂Be₂B₂O₇ (SBBO) double-layered like configuration has been rationally designed and successfully synthesized based on structural motif cosubstitution strategy via the high-temperature solution method. In Sr₂Al_2.18B_5.82O₁₃F₂, a structural motif, the [Al₂B₆O₁₄F₄] unit, with edge-sharing [AlO₄F₂] octahedra was filled in interlamination of double-layer structure. The research indicates that Sr₂Al_2.18B_5.82O₁₃F₂ features a short ultraviolet cutoff edge (<200 nm) and moderate birefringence (∼0.058 @ 1064 nm). As the first reported linker in the interlamination of double-layer structures, the [Al₂B₆O₁₄F₄] unit enlightens the synthesis and discovery of new layered structures in borates.

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.

Two Acentric Aluminoborates Incorporated d10 Cations: Syntheses, Structures, and Nonlinear Optical Properties

Two acentric aluminoborates (ABOs), [Zn(en)₂Al{B₅O₉(OH)}{BO(OH)₂}] (1) and [Cd(en)₂AlB₅O₁₀]·H₂O (2) (en = ethylenediamine), have been solvothermally made. 1 includes a two-dimensional (2D) wavy ABO layer using B₅O₉(OH) clusters and AlO₃{BO(OH)₂} groups, in which both units can be regarded as three-connected nodes, and simplifying the ABO layer to a hcb-type network. 2 features an acentric three-dimensional (3D) porous framework with a unique unc-type network constituted by strictly alternating connected B₅O₁₀ clusters and AlO₄ units. The structural transformation from a 2D layer 1 to a 3D framework 2 was achieved with the elimination of the terminal hydroxyls in layer 1 by adjusting synthetic conditions in the same solvent system. Metal-amine complexes Zn(en)₂/Cd(en)₂ bond to the inorganic walls and are located in the cavity of frameworks 1 and 2, respectively. Compounds 1 and 2 exhibit large second-harmonic generation (SHG) responses that are 2.2 and 2.7 times those of KH₂PO₄ (KDP), respectively, which are among the largest powder SHG responses for all deep-ultraviolet (deep-UV) ABOs. The UV-vis diffuse reflectance spectra of 1 and 2 show a wide transparency window below 190 nm. Density functional theory (DFT) calculations indicate that the B-O units and the introduced distorted d¹⁰ metal polyhedra played a decisive role in the optical properties of both compounds.

Mixed-Anion-Oriented Design of LnMGa3S6O (Ln = La, Pr, and Nd; M = Ca and Sr) Nonlinear Optical Oxysulfides with Targeted Property Balance

Nonlinear optical (NLO) crystals are of importance on extending infrared (IR) laser wavelengths. Considering their performance drawbacks in commercial IR NLO crystals, a recent challenge in exploring new excellent IR NLO crystals is how to break the inherent conflict between a wide bandgap (E_g ≥ 3.0 eV) and large NLO effect (d_ij ≥ 0.5 × AgGaS₂) and simultaneously enlarge the birefringence (Δn) for a requisite phase-matching (PM) behavior. For that reason, rational combination of mixed-anion functional groups into a crystal structure affords the successful design and synthesis of six LnMGa₃S₆O (Ln = La, Pr, and Nd; M = Ca and Sr) NLO oxysulfides. Among them, LaMGa₃S₆O satisfy the property-balance demand (E_g: 3.21-3.27 eV and d_ij: 0.9-1.0 × AgGaS₂) as promising PM NLO crystals through gathering their property advantages between LaMGa₃O₇ and LaMGa₃S₇ by mixed-anion-oriented performance engineering. A study on the structure-property relationship indicates that heteroleptic (Ln/M)S₇O and GaS₃O anionic groups are proven as promising NLO-active units and offer a great synergistic effect on the NLO origin. This work as a visualized model not only provides a first clear cognition on varying properties from oxide to sulfide to oxysulfide but also highlights the feasibility of mixed-anion-oriented design of new NLO candidates with balanced performances.

Deep-ultraviolet nonlinear optical crystals: concept development and materials discovery

Deep-ultraviolet (DUV, wavelength λ < 200 nm) nonlinear optical (NLO) crystal is the core component of frequency conversion to generate DUV laser, which plays an important role in cutting-edge laser technology and fundamental science. Significant progress has been made in both experimental exploration and theoretical design in the field of DUV NLO crystals over the past three decades. In-depth insight into "structure-property correlations", in particular, allows for rigorous and precise identification of DUV NLO crystals. In this article, we reviewed the current experimental and theoretical research progress while elucidating the core concepts and stringent criteria of qualified DUV phase-matched second-harmonic generation crystals. We also discussed the development of the DUV NLO "structure-property correlations" from first principles and how it has sparked interest in related materials, as well as future directions for obtaining potential DUV NLO crystals.

From oxides to oxysulfides: the mixed-anion GeS3O unit induces huge improvement in the nonlinear optical effect and optical anisotropy for potential nonlinear optical materials

Oxysulfides combining intrinsic performance advantages between sulfides (strong NLO response) and oxides (wide optical bandgap) are proposed as potential infrared (IR) NLO materials. Theoretical calculation shows that the mixed-anion GeS₃O tetrahedron has a stronger polarizability anisotropy and hyperpolarizability than that of the typical GeO₄ unit. Based on this, two Sr₂MGe₂S₆O (M = Zn, Cd) oxysulfides with the GeS₃O unit show dozens of times improvement in critical birefringence and the NLO effect compared with those of isostructural Sr₂ZnGe₂O₇. Moreover, structure–property study further verifies that the mixed-anion GeS₃O ligand is a useful NLO-active unit and can offer great influence over the NLO origin. This research result also gives us a feasible design strategy and research system to explore new IR NLO candidates.

The mixed-anion GeS₃O unit induces huge improvement in the optical anisotropy and NLO response from Sr₂ZnGe₂O₇ oxide to Sr₂MGe₂S₆O oxysulfides.

Inducing Large Birefringence by Enhancing Asymmetric Electron Distribution of Y-O Polyhedra

Birefringent crystals can create and control polarized light, which has crucial applications in optical communication, polarimetry, and scientific instrumentation. For most of birefringence phenomenon in borate crystals, optic anisotropy mainly...

Hybrid organic-inorganic triguanidine arsenate dihydrate for ultraviolet nonlinear optical application

Introduction of the planar organic cation [C(NH₂)₃]⁺ into tetrahedronal inorganic arsenate generates a new ultraviolet nonlinear optical crystal triguanidine arsenate dihydrate, [C(NH₂)₃]₃AsO₄·2H₂O, which exhibits a short ultraviolet absorption edge (210 nm), suitable second-harmonic generation response (0.9 × KDP), and moderate birefringene (0.0387 at 546 nm). Notably, by optimizing the growth conditions, transparent and wafer well-developed bulk single crystals can be easily prepared.

Sr3B14O24: a new borate with a [B14O30] fundamental building block and an unwonted 2D double layer

A new alkaline-earth metal borate, Sr₃B₁₄O₂₄, was successfully synthesized by a high-temperature solution method. It crystallizes in the monoclinic space group P2₁/c (no. 14) and features a fundamental building block (FBB) [B₁₄O₃₀] which is composed of eight [BO₃] and six [BO₄] units. The FBBs further condense to form an unwonted infinite 2D double layer extended in the bc plane. The UV-Vis-NIR diffuse reflectance spectrum shows that the cutoff edge of Sr₃B₁₄O₂₄ is less than 200 nm, which indicates its potential use in the deep UV region. In addition, the first principles theoretical study was carried out to better understand the relationship between the structure and performance.

From β-Na2 B6 O10 to Na3 AlB8 O15 and Na3 Al2 B7 O15 : Structural Tuning of Anionic-Group Architectures by Substitution of [BO4 ] by [AlO4 ] Covalent Tetrahedra

Two new sodium aluminum borates, Na₃ AlB₈ O₁₅ and Na₃ Al₂ B₇ O₁₅ , have been successfully synthesized by the high-temperature solution method. They crystallize in the different space groups, P2₁ /c and P2/c, respectively. The B-O configurations of β-Na₂ B₆ O₁₀ , Na₃ AlB₈ O₁₅ and Na₃ Al₂ B₇ O₁₅ are compared to feature complicated different dimensional open-framework structures caused by the substitution of [BO₄ ] by [AlO₄ ] covalent tetrahedra. Moreover, the experimental results indicate that Na₃ AlB₈ O₁₅ and Na₃ Al₂ B₇ O₁₅ have short ultraviolet (UV) cutoff edges (<187 nm). The first-principles calculations show that Na₃ AlB₈ O₁₅ and Na₃ Al₂ B₇ O₁₅ have moderate birefringence (0.075 and 0.041@1064 nm, respectively).

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.

BaZn3(BO3)2F2: a new beryllium-free zincoborate with a KBBF-type structure

A new beryllium-free zincoborate, BaZn₃(BO₃)₂F₂, with a KBBF-type structure has been synthesized for the first time. The electrostatic force of interaction in BaZn₃(BO₃)₂F₂ provides better linkage in neighboring [ZnBO₃F]_∞ single layers. BaZn₃(BO₃)₂F₂ is the first case of borates with both [ZnO₃F] tetrahedra and [ZnO₆] octahedra, enriching the structural chemistry of borate system. All the coplanar [BO₃] triangles align in the same direction with a high density, which endows BaZn₃(BO₃)₂F₂ with a large birefringence of cal. 0.076 at 1064 nm. This work is of great significance to design beryllium-free borates with a KBBF-type structure.

From thiophosphate to chalcohalide: mixed-anion AgSxCly ligands concurrently enhancing nonlinear optical effects and laser-damage threshold

In this study, we propose a new design strategy that introduces unique mixed-anion AgS_xCl_y ligands into thiophosphate to afford a successful synthesis of a promising Ag₅PS₄Cl₂ IR NLO chalcohalide. Compared with chlorine-free Ag₃PS₄, Ag₅PS₄Cl₂ undergoes overall performance enhancement and achieves a good balance between large NLO effect (2.0 × Ag₃PS₄) and high laser damage threshold (3.8 × Ag₃PS₄). Theoretical analysis further indicates that AgS_xCl_y groups are the new superior NLO-active units since they can maintain the wide bandgap while concurrently making a great contribution to the origin of NLO effects. Therefore, the incorporation of AgS_xCl_y groups into the crystal structure can be expected to be one feasible way to design new IR NLO candidates with excellent performance.

Exploring Deep-UV Nonlinear Optical Materials with Enhanced Second Harmonic Generation Response and Birefringence in Fluoroaluminoborate Crystals

Deep-UV (DUV) nonlinear optical (NLO) materials are of vital importance to demanding DUV laser applications; however, those with a large NLO coefficient, wide band gap, and sufficient phase-matching ability are rare. In this work, by combining π-conjugated B₃O₆ groups and distort AlO₃F tetrahedra, we successfully designed and synthesized two fluoroaluminoborates, RbAlB₃O₆F (RABF) and Cs_0.5Rb_0.5AlB₃O₆F (CRABF). In addition, the solid solution Cs_xRb_1-xAlB₃O₆F (0.2 < x < 0.8) polycrystalline samples were obtained by the standard solid-state reaction. It was shown that all these compounds are NLO-active and possess short UV cutoff edges (<200 nm). Interestingly, although they have the same fundamental structural units, their crystal structures and second harmonic generation (SHG) responses are significantly different owing to the cation effect. Theoretical calculations reveal that both B₃O₆ and AlO₃F groups contribute to the SHG responses. Remarkably, it also indicates that RABF exhibits a large birefringence (Δn = 0.0946 at 1064 nm), which is capable of realizing deep-UV phase matching down to 174 nm. The exploration of fluoroaluminoborates provides a new perspective for searching excellent deep-UV NLO materials.

SrAlB3O6F2: A Fluoroaluminoborate with [Al2B6O14F4] Units and Large Birefringence

The first strontium fluoroaluminoborate SrAlB₃O₆F₂ was synthesized. Interestingly, it exhibits a two-dimensional warped layer composed of the [Al₂B₆O₁₄F₄] fundamental building units, which are first identified in borates. The experimental results indicate that SrAlB₃O₆F₂ has a short ultraviolet cutoff edge (<200 nm) and large birefringence (0.075@546 nm), which are further verified by theoretical calculation. Thus, SrAlB₃O₆F₂ can be used as the deep-ultraviolet birefringence material.

RbMT3 (BO3 )2 O3 (M=Ba, Sr; T=Al, Ga): New Double-Layered Oxyborates Constructed from [BO3 ] Triangles and [TO4 ] Tetrahedra

Four new alumino-/galloborates RbMT₃ (BO₃ )₂ O₃ (M=Ba, Sr; T=Al, Ga) have been synthesized for the first time by using a high-temperature solution method. All the title compounds have Sr₂ Be₂ B₂ O₇ -like structures, in which the [BO₃ ] triangles and [TO₄ ] tetrahedra form the final double-layered configurations with the M- and Rb-site atoms located between and in the double layer, respectively. The structure evolution from Sr₂ Be₂ B₂ O₇ to RbMT₃ (BO₃ )₂ O₃ series is discussed. The broader energy bandgaps in Al-based borates when compared with Ga-based ones can be entirely attributed to the location of Al/Ga s orbitals near the Fermi surface. Both experimental and computational approaches were used to study their structure-property relationships.

π-Conjugated Trigonal Planar [C(NH2)3]+ Cationic Group: A Superior Functional Unit for Ultraviolet Nonlinear Optical Materials

Employing π-conjugated anionic groups in molecular construction has been proven to be an effective strategy to find superior ultraviolet (UV) nonlinear optical (NLO) crystals over the decades. Herein, unlike the traditional π-conjugated anionic groups, we identify that a π-conjugated cationic group, viz., [C(NH2)3]+, is also an excellent UV NLO-active functional group in theory. Furthermore, we identify a [C(NH2)3]+-containing compound, C(NH2)3ClO4, as a promising UV NLO candidate due to its short UV cutoff edge (200 nm), remarkable second-harmonic generation effect (∼3 × KDP), and moderate birefringence of 0.076@1064 nm. Additionally, C(NH2)3ClO4 has excellent ferroelectric properties and reversal of domains, which also enables it to produce ultraviolet coherent light as short as 200 nm by a quasi-phase matching technique with a periodically poling method. Our study may provide not only a promising UV NLO crystal but also a new π-conjugated functional unit, [C(NH2)3]+, which will open a path to finding new classes of high-performance UV NLO crystals.

Prediction of Novel van der Waals Boron Oxides with Superior Deep-Ultraviolet Nonlinear Optical Performance

Deep-ultraviolet nonlinear optical (DUV NLO) materials are attracting increasing attention because of their structural diversity and complexity. Using the two-dimensional (2D) crystal structure prediction method combined with the first-principles calculations, here we propose layered 18-membered-ring (18MR) boron oxide B₂ O₃ polymorphs as high-performance NLO materials. 18MR-B₂ O₃ with the AA and AB stackings are potential DUV NLO materials. The superior performing 18MR-B₂ O₃ ^AB has an unprecedentedly high second harmonic generation coefficient of 1.63 pm V^-1 , the largest among the DUV NLO materials, three times larger than that of the advanced DUV NLO material KBe₂ BO₃ F₂ and comparable to that of β-BaB₂ O₄ . Its unusually large birefringence of 0.196 at 400 nm guarantees the phase-matching wavelength λ_PM to reach this material's extreme absorption edge of ≈154 nm.

AgBi(SO4)(IO3)2: aliovalent substitution induces structure dimensional upgrade and second harmonic generation enhancement

A new 3D noncentrosymmetric mixed-metal sulfate iodate, AgBi(SO₄)(IO₃)₂, has been designed based on a 2D iodate of AgBi(IO₃)₄via the aliovalent substitution of [IO₃]^- groups by [SO₄]^2- groups. Enhancement of the second harmonic generation response (3.0 × KDP to 3.9 × KDP) has been achieved while retaining a favorable NLO framework. This is a novel strategy to upgrade structural dimensions.

Cs2AlB5O10: a short-wavelength nonlinear optical crystal with moderate second harmonic generation response

A new compound Cs₂AlB₅O₁₀ was synthesized by a high temperature solution method in open air. It features a DUV cutoff edge below 190 nm and a moderate SHG response (0.8 × KDP at 1064 nm), indicating that it may have certain application prospects in the UV nonlinear optical region.

Hydrogen-Bond-Assisted Alignment of [MCu(SeO3)4Cl(H2O)]4- (M = Fe, Ga) Anionic Layers to Form Two Polar Oxychlorides: Pb2MCu(SeO3)4Cl(H2O)

Two novel selenite oxychlorides Pb₂MCu(SeO₃)₄Cl(H₂O) (M = Fe, Ga) were hydrothermally synthesized and structurally characterized. They are isostructural and crystallize in the two-dimensional [MCu(SeO₃)₄Cl(H₂O)]^4- anionic layer structure mediated with hydrogen bonds and aligned between neighboring layers which assist in building the three-dimensional framework with a polar space group. Optical properties measurements revealed that the optical band gaps are 2.61 and 3.22 eV for Pb₂FeCu(SeO₃)₄Cl(H₂O) (1) and Pb₂GaCu(SeO₃)₄Cl(H₂O) (2) and the SHG responses are about 0.12 and 0.18 times that of KDP, respectively. Furthermore, 1 exhibits an interesting metamagnetic phenomenon under varied applied fields from around 1 to 4 T at 2 K, and 2 behaves with potential ferromagnetic ordering at low temperature.

Recent advances of oxyfluorides for nonlinear optical applications

Oxyfluorides exhibit rich crystal structures that provide a new perspective for designing new NLO materials.