Finding the Next Deep-Ultraviolet Nonlinear Optical Material: NH4B4O6F

From CaBaM2F12 to K2BaM2F12 (M = Zr, Hf): Heterovalent Cation-Substitution-Induced Symmetry Break and Nonlinear-Optical Activity

Designing short-wavelength nonlinear-optical (NLO) crystals is of vital importance for laser applications. Here, the combination of alkaline-earth metals, d0 transition metals, and F atom has generated two new and isostructural fluorides, CaBaZr2F12 (CBZF) and CaBaHf2F12 (CBHF), which adopt centrosymmetric space group I4/mmm. Taking CBZF and CBHF as the parents, two new fluorides, K2BaZr2F12 (KBZF) and K2BaHf2F12 (KBHF), with an Imm2 polar structure were obtained via a heterovalent cation substitution strategy. All four compounds feature ZrF8-dodecahedra-built {[Zr2F12]4-}∞ chains and show short ultraviolet cutoff edges (<200 nm). KBZF and KBHF show phase-matchable behavior with moderate second-harmonic-generation responses [0.6 and 0.35 × KH2PO4 (KDP)] under 1064 nm laser radiation. This work enriches fluorides as promising short-wavelength NLO materials.

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.

Deep-Ultraviolet Bialkali-Rare Earth Metal Anhydrous Sulfate Birefringent Crystal

Birefringence is an important linear optical property of anisotropic crystals that plays a significant role in regulating light polarization. A new bialkali-rare earth metal sulfate, NaRbY2(SO4)4 compound, consisting of non-π-conjugated alkali metals and rare earth metal-centered dodecahedral YO8 has been synthesized. The structure analysis suggests that the three-dimensional (3D) structure of the compound is found to be attributable to the combination of dodecahedral YO8 and tetrahedral SO4 groups with Na+ and Rb+ located in the cavities. The ultraviolet, visible, and near-infrared (UV-vis-NIR) spectra reveal that the compound exhibits transparency at a wavelength of less than 200 nm. The observed birefringence of the compound is 0.045@550 nm, which is comparatively larger than that of most deep-ultraviolet (DUV) birefringent crystals. The birefringence mainly originated from the YO8 dodecahedron, which is suggested by first-principles calculations. This research work can provide a useful perspective to explore new DUV sulfates with excellent birefringence.

Two Hydroxyfluorooxoborates Achieving Deep-Ultraviolet Cutoff Edges and Moderate Birefringence by Assembling Multi-Anionic Groups

Assembling multi-anionic groups is conducive to utilizing respective advantage to achieve the enhancement of optical performance. Two new hydroxyfluorooxoborates, Ama2-Rb2B3O3F4(OH) and K8Cs2B15O14(OH)7F20 ⋅ H2O with [B3O3F4(OH)] six-membered rings were synthesized for the first time. The title compounds exhibit short ultraviolet cutoff edges (<200 nm) and K8Cs2B15O14(OH)7F20 ⋅ H2O possesses a moderate experimental refractive index difference of 0.051@546 nm.

K2MgMoP2O10 and K3Mg2MoP3O14: two new molybdophosphates exhibiting different optical anisotropies induced by variable dimensionality of the anion framework

By introducing the d0 metal cation Mo6+ into phosphates, two new molybdophosphates, K2MgMoP2O10 and K3Mg2MoP3O14, were synthesized by spontaneous crystallization, and their structures were determined by single-crystal X-ray diffraction. K2MgMoP2O10 shows a two-dimensional layer composed of the uncommon eight-membered ring [Mo2P2O16] formed by [MoO6] and [PO4], while K3Mg2MoP3O14 shows isolated [MoP3O14] clusters composed of [MoO5] and [PO4]. K2MgMoP2O10 and K3Mg2MoP3O14 have UV cut-off wavelengths of 277 and 271 nm, respectively, which are significantly shorter than those of most recently published molybdophosphates. To the best of our acknowledge, K2MgMoP2O10 exhibits the largest birefringence (a calculated value of 0.187 at 546 nm) among reported alkali metal or alkaline earth metal molybdophosphates, which provides a way to explore new birefringent materials. First-principles analysis of the electronic structure shows that the large birefringence of K2MgMoP2O10 mainly originates from the [MoO6] units.

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.

Discovery of a new bimetallic borate with strong optical anisotropy activated by π-conjugated [B2O5] units

Birefringent materials with high optical anisotropy have been identified as a research hotspot owing to their significant scientific and technological significance in modern optoelectronics for manipulating light polarization. Researchers studying borate systems have discovered that adding π-conjugated units placed in parallel can significantly increase the birefringence of crystalline solids; some examples include [BO3] units, [B2O5] units, and [B3O6] units. However, there are not many borates with strictly parallel configurations of π-conjugated [B2O5] units. In this study, a new bimetallic borate Sr2Cd4(B2O5)3 with near-parallel arrangement of π-conjugated [B2O5] units was discovered. Sr2Cd4(B2O5)3 possesses the maximum number density of [B2O5] units, shortest dihedral angle of [B2O5] units (between the two [BO3]), and largest degree of [CdO6] octahedral distortion among all the currently known Sr-Cd-B-O tetragonal system borates, making it demonstrate a large birefringence of 0.102 at 532 nm. Theoretical analysis proves that π-conjugated [B2O5] anions are the primary source of the large birefringence of Sr2Cd4(B2O5)3.

Difluorocarbene Enables Access to 2,2-Difluorohydrobenzofurans and 2-Fluorobenzofurans from ortho-Vinylphenols

2-Fluorobenzofurans are the backbone structures of many drug molecules and have many potential therapeutic bioactivities. Despite the potential applications in medicinal chemistry, practical and efficient synthetic methods for the construction of 2-fluorobenzofuran are very limited. Herein, we report an efficient and general method for the construction of 2-fluorobenzofurans. Contrary to the previous functionalizations of the existing backbone of benzofuran, our strategy directly constructs benzofuran scaffolds alongside the incorporation of fluorine atom on C2 position in a formal [4 + 1] cyclization from readily accessible ortho-vinylphenols and difluorocarbene. In our strategy, ClCF2H decomposes into difluorocarbene in the presence of base, which is further captured by the oxygen anion from the hydroxy group in ortho-hydroxychalcones; subsequent intramolecular Michael addition to the α, β-unsaturated system leads to 2,2-difluorohydrobenzofurans, and further fluorine elimination renders 2-fluorobenzofurans by forming one C-O bond and one C-C double bond. Of note, various complex 2,2-difluorohydrobenzofurans and 2-fluorobenzofurans could be readily accessed through our protocol via the late-stage elaborations.

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.

NaRbB10O14(OH)4 and Na3CsB10O16(OH)2: Two Cases of Hydroxyborates with [B5Om(OH)n] Units and Deep Ultraviolet Cutoff Edges

Two deep ultraviolet (DUV) hydroxylated-alkali-metal borates, NaRbB10O14(OH)4 (I) and Na3CsB10O16(OH)2 (II), have been successfully synthesized by a high-temperature solution and solvothermal method. Both of them feature [B5Om(OH)n] units, which form chains for (I) and bilayers with nine-membered boron rings for (II). It is worth noting that both compounds exhibit very wide theoretical band gaps of 7.33 and 6.55 eV for (I) and (II), respectively, which denotes that they should have desirable DUV transmittance ability. Moreover, the title compounds have moderate birefringence owing to the π-conjugated [BO3], [BO2(OH)] groups, corresponding to 0.070 for (I) and 0.054 for (II) at 1064 nm. The structure characteristics and optical properties were also investigated and discussed. The results make it beneficial for exploring novel DUV hydroxylated borate optical crystals.

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.

Wide band gap selenide infrared nonlinear optical materials AIIMg6Ga6Se16 with strong SHG responses and high laser-induced damage thresholds

Infrared (IR) nonlinear optical (NLO) materials with strong NLO response, wide band gap and high laser-induced damage threshold (LIDT) are highly expected in current laser technologies. Herein, by introducing double alkaline-earth metal (AEM) atoms, three wide band gap selenide IR NLO materials AIIMg6Ga6Se16 (AII = Ca, Sr, Ba) with excellent linear and NLO optical properties have been rationally designed and fabricated. AIIMg6Ga6Se16 (AII = Ca, Sr, Ba) are composed of unique [AIISe6] triangular prisms, [MgSe6] octahedra and [GaSe4] tetrahedra. The introduction of double AEMs effectively broadens the band gaps of selenide-based IR NLO materials. Among them, CaMg6Ga6Se16, achieving the best balance between the second-harmonic generation response (∼1.5 × AgGaS2), wide band gap (2.71 eV), high LIDT (∼9 × AgGaS2), and moderate birefringence of 0.052 @ 1064 nm, is a promising NLO candidate for high power IR laser. Theoretical calculations indicate that the NLO responses and band gaps among the three compounds are mainly determined by the NLO-active [GaSe4] units. The results enrich the chemical diversity of chalcogenides, and give some insight into the design of new functional materials based on the rare [AIISe6] prismatic units.

Rational Combination of π-Conjugated and Non-π-Conjugated Groups Achieving Strong Nonlinear Optical Response, Large Optical Anisotropy, and UV Light-Switchable Fluorescence

Combining π-conjugated and non-π-conjugated groups is an important strategy for synthesizing new nonlinear optical (NLO) crystals. However, the second harmonic generation (SHG) response and optical anisotropy can be limited by improper spatial alignment of these functional groups in the crystal structure. In this work, it is revealed that non-π-conjugated [NH2SO3] group acts as both hydrogen bond donor and acceptor, effectively regulating the 2D planar structure formed by π-conjugated [C4N3H6] groups. The resulting organic-inorganic hybrid crystal C4N3H6SO3NH2 exhibits a strong SHG response (2.5 × KDP), large optical anisotropy (0.233@546 nm), and blue-violet and green fluorescence near 360 and 520 nm, respectively. This work expands the methodology for creating new NLO crystals through organic-inorganic hybridization, while also showcasing the potential of C4N3H6SO3NH2 as a multifunctional optical material.

An Unprecedented [BO2]-Based Deep-Ultraviolet Transparent Nonlinear Optical Crystal by Superhalogen Substitution

Solid-state structures with the superhalogen [BO2]- have thus far only been observed with a few compounds whose syntheses require high reaction temperatures and complicated procedures, while their optical properties remain almost completely unexplored. Herein, we report a facile, energy-efficient synthesis of the first [BO2]-based deep-ultraviolet (deep-UV) transparent oxide K9[B4O5(OH)4]3(CO3)(BO2) ⋅ 7H2O (KBCOB). Detailed structural characterization and analysis confirm that KBCOB possesses a rare four-in-one three-dimensional quasi-honeycomb framework, with three π-conjugated anions ([BO2]-, [BO3]3-, and [CO3]2-) and one non-π-conjugated anion ([BO4]5-) in the one crystal. The evolution from the traditional halogenated nonlinear optical (NLO) analogues to KBCOB by superhalogen [BO2]- substitution confers deep-UV transparency (<190 nm), a large second-harmonic generation response (1.0×KH2PO4 @ 1064 nm), and a 15-fold increase in birefringence. This study affords a new route to the facile synthesis of functional [BO2]-based oxides, paving the way for the development of next-generation high-performing deep-UV NLO materials.

Record Second-Harmonic Generation and Birefringence in an Ultraviolet Antimonate by Bond Engineering

Oxides have attracted considerable attention owing to their potential for nonlinear optical (NLO) applications. Although significant progress has been achieved in optimizing the structural characteristics of primitives (corresponding to the simplest constituent groups, namely, cations/anions/neutral molecules) comprising the crystalline oxides, the role of the primitives' interaction in determining the resultant functional structure and optical properties has long been underappreciated and remains unclear. In this study, we employ a π-conjugated organic primitive confinement strategy to manipulate the interactions between primitives in antimonates and thereby significantly enhance the optical nonlinearity. Chemical bonds and relatively weak H-bonding interactions promote the formation of cis- and trans-Sb(III)-based dimer configurations in (C5H5NO)(Sb2OF4) (4-HPYSOF) and (C5H7N2)(Sb2F7) (4-APSF), respectively, resulting in very different second-harmonic generation (SHG) efficiencies and birefringences. In particular, 4-HPYSOF displays an exceptionally strong SHG response (12 × KH2PO4 at 1064 nm) and a large birefringence (0.513 at 546 nm) for a Sb(III)-based NLO oxide as well as a UV cutoff edge. Structural analyses and theoretical studies indicate that polarized ionic bond interactions facilitate the favorable arrangement of both the inorganic and organic primitives, thereby significantly enhancing the optical nonlinearity in 4-HPYSOF. Our findings shed new light on the intricate correlations between the interactions of primitives, inorganic primitive configuration, and SHG properties, and, more broadly, our approach provides a new perspective in the development of advanced NLO materials through the interatomic bond engineering of oxides.

Cs3In(In4Se7)(P2Se6): A Multi-Chromophore Chalcogenide with Excellent Nonlinear Optical Property Designed by Group Grafting

Non-centrosymmetric (NCS) and polar materials capable of exhibiting many important functional properties are indispensable for electro-optical technologies, yet their rational structural design remains a significant challenge. Here, we report a "group grafting" strategy for designing the first multi-chromophore selenophosphate, Cs3In(In4Se7)(P2Se6), that crystallizes in a NCS and polar space group of Cm. The structure features a unique basic building unit (BBU) [In(In4Se10)(P2Se6)], formed through "grafting [In4Se10] supertetrahedra on the root of [In(P2Se6)2] groups". Theoretical calculations confirm that this [In(In4Se10)(P2Se6)] BBU can achieve a "1+1>2" combination of properties from two chromophores, [In4Se10] supertetrahedron and ethane-like [P2Se6] dimer. That makes Cs3In(In4Se7)(P2Se6) exhibit excellent linear and nonlinear optical (NLO) properties, including a strong second harmonic generation (SHG) response (~6×AgGaS2), a large band gap (2.45 eV), broad infrared (IR) transmission (up to 19.5 μm), a significant birefringence (0.26 @1064 nm) as well as the congruently-melting property at ~700 °C. Therefore, Cs3In(In4Se7)(P2Se6) will be a promising NLO crystal, especially in the IR region, and this research also demonstrates that "group grafting" will be an effective strategy for constructing novel polar BBUs with multi-chromophore to design NCS structures and high-performance IR NLO materials.

Where do the Fluorine Atoms Go in Inorganic-Oxide Fluorinations? A Fluorooxoborate Illustration under Terahertz Light

The substitution of fluorine atoms for oxygen atoms/hydroxyl groups has emerged as a promising strategy to enhance the physical and chemical properties of oxides/hydroxides in fluorine chemistry. However, distinguishing fluorine from oxygen/hydroxyl in the reaction products poses a significant challenge in existing characterization methods. In this study, we illustrate that terahertz (THz) spectroscopy provides a powerful tool for addressing this challenge. To this end, we investigated two fluorination reactions of boric acid, utilizing MHF2 (M=Na, C(NH2)3) as fluorine reagents. Through an interplay between THz spectroscopy and solid-state density functional theory, we have conclusively demonstrated that fluorine atoms exclusively bind with the sp3-boron but not with the sp2-boron in the reaction products of Na[B(OH)3][B3O3F2(OH)2] (NaBOFH) and [C(NH2)3]2B3O3F4OH (GBF2). Based on this evidence, we have proposed a reaction pathway for the fluorinations under investigation, a process previously hindered due to structural ambiguity. This work represents a step forward in gaining a deeper understanding of the precise structures and reaction mechanisms involved in the fluorination of oxides/hydroxides, illuminated by the insights provided by THz spectroscopy.

AIB3 IIC2 IIIQ6 VIXVII: A Thioborate Halide Family for Developing Wide Bandgap Infrared Nonlinear Materials by Coupling Planar [BS3] and Polycations

Developing high-performance infrared (IR) nonlinear optical (NLO) materials is urgent but challenging due to the competition between NLO coefficient and bandgap in one compound. Herein, by coupling NLO-active [BS3] planar units and halide-centered polycations, six new metal thioborate halides ABa3B2S6X (A = Rb, Cs; X = Cl, Br, I) composed of zero-dimensional [XBamRbn/Csn] polycations and [BS3] units, belonging to a newA I B 3 II C 2 III Q 6 VI X VII ${\mathrm{A}}^{\mathrm{I}}{\mathrm{B}}_{3}^{\mathrm{II}}{\mathrm{C}}_{2}^{\mathrm{III}}{\mathrm{Q}}_{6}^{\mathrm{VI}}{\mathrm{X}}^{\mathrm{VII}}$ family, are rationally designed and fabricated. The compounds show an interesting structural transition from Pbcn (ABa3B2S6Cl) to Cmc21 (ABa3B2S6Br and ABa3B2S6I) driven by the clamping effect of polycationic frameworks. ABa3B2S6Br and ABa3B2S6I are the first series metal thioborate halide IR NLO materials, and the introduction of [BS3] unit effectively widens the bandgap of planar unit-constructed chalcogenides. ABa3B2S6Br and ABa3B2S6I, exhibiting wide bandgaps (3.55-3.60 eV), high laser-induced damage thresholds (≈ 6 × AgGaS2), and strong SHG effects (0.5-0.6 × AgGaS2) with phase-matching behaviors, are the promising IR NLO candidates for high-power laser applications. The results enrich the chemical and structural diversity of boron chemistry and give some insights into the design of new IR NLO materials with planar units.

Extending Unique 1D Double-Chains to 2D Layers with Birefringent Gain by Introducing a Hydroxyl Group

It remains a significant hurdle for discovering birefringent materials in the deep ultraviolet (DUV, λ < 200 nm). It is well-known that the OH anions are recognized for their capability to eliminate the dangling bonds from terminal oxygen atoms, promoting the ultraviolet (UV) cutoff edge blueshift and regulating the crystal structure. Here, two new barium hydroxyborates, Ba3B11O18(OH)3(H2O) (BaBOH) and Na2BaB10O16(OH)2(H2O)2 (NaBaBOH), were designed and synthesized while displaying different dimensions. Remarkably, BaBOH presents novel one-dimensional (1D) [B22O37(OH)6]∞ double-chains formed by a new fundamental building block (FBB)[B11O21(OH)3]. NaBaBOH possesses a 2D [B10O16(OH)2]∞ layer with a less common FBB [B10O19(OH)2]. They enrich the structural diversity of hydroxyborates. Moreover, NaBaBOH exhibits a broad transparent window within the DUV spectral range (<190 nm) and possesses a favorable birefringence of 0.064. Furthermore, detailed summaries and structural comparisons have been implemented for all hydroxyborates containing alkali and alkaline-earth metals. This reveals that the OH group modulation strategy can be appropriately employed for the structural design.

Molecular Crystals Constructed by Polar Molecular Cages: A Promising System for Exploring High-performance Infrared Nonlinear Optical Crystals

Polar molecular crystals, with their densely stacked polar nonlinear optical (NLO) active units, are favored for their large second harmonic generation (SHG) responses and birefringence. However, their potential for practical applications as Infrared (IR) NLO materials has historically been underappreciated due to the weak inter-molecular interaction forces that may compromise their physicochemical properties. In this study, we propose molecular crystals with polar molecular cages as a treasure-house for the development of superior IR NLO materials and a representative system, binary chalcogenide molecular crystals, composed of [P4 Sn ] (n=3-9) polar molecular cages, is introduced. These crystals may not only achieve wide band gap, large SHG response, and birefringence in a single structure, but also exhibit favorable physicochemical properties. We subsequently obtained a polar molecular crystal, α-P4 S5 , which demonstrated exceptional IR optical properties, including a strong SHG response (1.1×AGS), wide band gap (3.02 eV), large birefringence (0.134@2050 nm), and a broad transmission range (0.41-14.7 μm). Moreover, it showed excellent water resistance and hardness. These findings highlight the potential of polar molecular crystals as a promising platform for the development of high-performance IR NLO materials.

Pb3[O10Pb20](SiO4)4X10 (X = Cl, Br): The First Pb-Containing Halogen Silicates with Mirror-Symmetric Pseudo-Aurivillius Bilayers

Two new congruently melting Pb-containing halogen silicates, Pb3[O10Pb20](SiO4)4X10 (X = Cl, Br), have been synthesized using a high-temperature solution method. Their crystal structures were determined by single-crystal X-ray diffraction, and both compounds crystallize in the orthorhombic space group Cmca. In both structures, the mirror-symmetric bilayer composed of Pb-O polyhedra is observed for the first time in Pb-containing silicates and belongs to α-PbO derivatives and is related to the Aurivillius phase. Thermal behavior analysis, UV-vis diffuse-reflectance spectroscopy, and IR spectroscopy were also performed. The Pb3[O10Pb20](SiO4)4Cl10 matrix was doped with Eu3+ ions as a dopant, and its potential application in fluorescence was confirmed from the resulting orange-red emission.

Theoretical Understanding of Nonlinear Optical Properties in Solids: A Perspective

Nonlinear optical (NLO) crystals have become a hot topic in chemical science and material physics, due to their essential role in laser technology, optical information, optoelectronics, and precision measurements. In this Perspective, we provide an overview of recent advances in second-order nonlinear optics, with a focus on two critical topics: second harmonic generation (SHG) and the bulk photovoltaic effect (BPVE). For SHG, we discuss recent progress in deep-ultraviolet (DUV) materials, highlighting their structural characteristics and nonlinear groups that contribute to their exceptional performance. For BPVE, we concentrate on the emerging field of low-dimensional materials, emphasizing their potential in a shift current. Additionally, we discuss the development of regulation approaches for NLO materials, which is vital for their practical application. Finally, we address the outlook for the field, including the challenges that must be overcome to further advance NLO materials research.

Centrosymmetric CaBaMF8 and Noncentrosymmetric Li2CaMF8 (M = Zr, Hf): Dimension Variation and Nonlinear Optical Activity Resulting from an Isovalent Cation Substitution-Oriented Design

Zirconium/hafnium fluorides have recently been recognized as potential nonlinear optical (NLO) materials with short ultraviolet (UV) cutoff edges, which is significant in laser science and industry. The synthesis of noncentrosymmetric (NCS) materials based on centrosymmetric (CS) compounds by an isovalent cation substitution-oriented design is an emerging strategy in the NLO territory. Here, two isostructural and novel fluorides, CaBaMF8 (M = Zr (1), Hf (2)), have been synthesized through the combination of alkaline earth metals, zirconium/hafnium, and fluorine elements. They feature zero-dimensional and CS structures composed by an isolated MF8 (M = Zr, Hf) dodecahedron and dissociative Ca2+ and Ba2+ cations, and they display short UV cutoff edges (<200 nm) as well. Two three-dimensional fluorides Li2CaMF8 (M = Zr (3), Hf (4)) are obtained by replacing Ba with alkali metal Li atom, which not only represent phase-matchable second-harmonic-generation activities (0.36, 0.30× KH2PO4 (KDP)) at 1064 nm but also maintain short UV cutoff edges with high reflectance. This work has largely enriched the family of NCS zirconium/hafnium fluorides reaching the short UV region.

[C(NH2)3][B(C2O2H4)2]: An Organic-Inorganic Hybrid Borate Containing Nonlinear-Optical Active Unit [B(C2O2H4)2]- with Solar-Blind-Region Optical Nonlinearity

We herein report an unprecedented organic-inorganic hybrid borate incorporating a novel nonlinear-optical (NLO) active unit, namely, [C(NH2)3][B(C2O2H4)2]. The novel NLO active unit was derived from the condensation reaction between two glycol molecules and one (BO4)5- group. The title compound exhibits a moderate second-harmonic-generation effect (0.7 × KDP), a significant band gap (5.76 eV), and a suitable birefringence (0.078 at 550 nm). The optical properties are determined by the synergistic interaction between the C(NH2)3+ cation and the [B(C2O2H4)2]- group, as indicated by theoretical calculations.

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

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

Co(II)-doped hybrid Zn(II) tetraborate complexes, [ZnxCo(1-x)(1,3-dap)B4O7] (1,3-dap = 1,3-diaminopropane): BET analysis and N2/H2O/D2O adsorption studies

A series of mono/bimetallic isostructural hybrid tetraborates of the general formula [ZnxCo(1-x)(1,3-dap)B4O7] has been prepared using a solvothermal method. Their adsorption/desorption curves for H2O and D2O demonstrate that these materials have a stronger affinity for H2O than for D2O and enrich the D2O content of D2O/H2O mixtures.

Ba4B14O25: A Deep Ultraviolet Transparent Nonlinear Optical Crystal with Strong Second Harmonic Generation Response Achieved by a Boron-Rich Closed-Loop Strategy

Discovering new deep ultraviolet (DUV) nonlinear optical (NLO) materials is the current research hotspot. However, how to perfectly integrate several stringent performances into a crystal is a great challenge because of the natural incompatibility among them, particularly wide band gap and large NLO coefficient. To tackle the challenge, a boron-rich closed-loop strategy is supposed, based on which a new barium borate, Ba4B14O25, is designed and synthesized successfully via the high-temperature solid-state melting method. It features a highly polymeric 3D geometry with the closed-loop anionic framework [B14O25]8- constructed by the fundamental building blocks [B14O33]24-. The high-density π-conjugated [BO3]3- groups and the fully closed-loop B-O-B connections make Ba4B14O25 possess excellent NLO properties, including short UV cutoff edge (<200 nm), large second harmonic generation response (3.0 × KDP) and phase-matching capability, being a promising DUV-transparent NLO candidate material. The work provides a creative design strategy for the exploration of DUV NLO crystals.

New antimony fluorooxoborates with strong birefringence and unprecedented structural characterisation

Fluorooxoborates constitute a rich source of optical crystals due to their structural diversity and excellent performance. Antimony fluorooxoborates with stereochemically active lone pairs of electrons still have not been found, although the first antimony borate was discovered several years ago. In this study, we have achieved the successful synthesis of the first antimony(III) fluorooxoborate with an unprecedented [B2O4F]∞ chain, namely SbB2O4F. Remarkably, SbB2O4F shows strong birefringence (0.171@1064 nm) and short UV cutoff edges (about 220 nm) according to calculations. The birefringence of SbB2O4F mainly originates from the highly distorted [SbO4] groups.

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.

Sb4O5I2: Enhancing Birefringence through Optimization of Sb/I Ratio for Alignment of Stereochemically Active Lone Pairs

Birefringent crystals are the key components of functional optics, contributing significantly to scientific and technological advancements. To enhance birefringence, the presence of stereochemically active lone pairs offers a unique opportunity. In fact, strengthening the stereochemical activity and aligning uniformly lone pairs face tough challenges. Herein, an anisotropic layered crystal, Sb4O5I2, is discovered to exhibit enhanced birefringence. The influence of crystal symmetry on the birefringence of Sb4O5X2 (X = Cl, Br, or I) is found to be minor. Instead, the asymmetric nature of ABUCBs (i.e., cis-X3[SbO3]6- and cis-X3[SbO4]8-) plays a crucial role in enhancing the optical anisotropy. And the orientation of these ABUCBs is equally important. We demonstrate that by adjusting the Sb/I ratio from 5:1 to 2:1, all of the intralayer Sb atoms in Sb5O7I-P63 are forced onto the surface position. This structural adjustment leads to strengthened ionic bonding interactions, enhanced activity of the lone pairs, and uniform alignments of the ABUCBs in Sb4O5I2. Consequently, this results in a 6-fold increase in birefringence.

Novel antimony phosphates with enlarged birefringence induced by lone pair cations

Phosphates, whose obvious disadvantage is the relatively small birefringence, can be overcome by the introduction of post-transition metal cations containing stereochemically active lone-pair electrons. In this paper, two new compounds were successfully explored in the A-Sb-P-O system, i.e. Cs2Sb3O(PO4)3 (CsSbPO) and (NH4)2Sb4O2(H2O)(PO4)2[PO3(OH)]2 (NH4SbPOH). Transmission spectra show that CsSbPO has a surprising transmission range with a UV cutoff edge of 213 nm. First-principles calculations show that both compounds have a wide band gap (5.02 eV for CsSbPO and 5.30 eV for NH4SbPOH) and enlarged birefringence (Δn = 0.034@1064 nm for CsSbPO and Δn = 0.045@1064 nm for NH4SbPOH). The results of real-space atom-cutting investigations show that the distorted [SbOx] polyhedra originating from the asymmetric lone pair electrons give the main contribution to the total birefringence and overcome the disadvantage of small birefringence of phosphates but maintain wide transition windows.

Polysulfide Anions [Sx ]2- (x = 2, 3, 4, 5): Promising Functional Building Units for Infrared Nonlinear Optical Materials

Infrared nonlinear optical (IR NLO) materials play significant roles in laser technology. The novel functional building units (FBUs) are of great importance in constructing NLO materials with strong second harmonic generation (SHG). Herein, polysulfide anion [Sx ]2- (x = 2, 3, 4, 5) units are investigated on NLO-related properties and structure-performance relationships. Theoretical calculations uncover that the [Sx ]2- (x = 2, 3, 4, 5) units are potential IR NLO FBUs with large polarizability anisotropy (δ), hyperpolarizability (β) and wide HOMO-LUMO gap. Fourteen crystals including [Sx ]2- (x = 2, 3, 4, 5) units are calculated and analyzed. The results show that these units can result in a wide IR transmittance range, significant SHG effects, wide band gap Eg (Na2 S4 : Eg  = 3.09 eV), and large birefringence Δn [BaS3 (P21 21 2): Δn = 0.70]. More importantly, it is highlighted that the crystal materials including with [Sx ]2- (x = 2, 3, 4, 5) groups are good candidates for the exploration of the outstanding IR NLO materials.

Design of Deep-Ultraviolet Zero-Order Waveplate Materials by Rational Assembly of [AlO2F4] and [SO4] Groups

Zero-order waveplates are widely used in the manufacture of laser polarizer waves, which are important in polarimetry and the laser industry. However, there are still challenges in designing deep-ultraviolet (DUV) waveplate materials that satisfy large band gaps and small optical anisotropy simultaneously. Herein, three cases of aluminum sulfate fluorides: Na2AlSO4F3, Li4NH4Al(SO4)2F4, and Li6K3Al(SO4)4F4, with novel [AlSO4F3] layers or isolated [AlS2O8F4] trimers were designed and synthesized by the rational assembly of [AlO2F4] and [SO4] groups through a hydrothermal method. Experiments and theoretical calculations imply that these three possess short cutoff edges (λ < 200 nm) and small birefringence (0.0014-0.0076 @ 1064 nm), which fulfils the prerequisite for potential DUV zero-order waveplate materials. This work extends the exploration of DUV zero-order waveplate materials to the aluminum sulfate fluoride systems.

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.

Na6Mg3P4S16 and RbMg2PS4Cl2: two Mg-based thiophosphates with ultrawide bandgaps resulting from [MgS6] and [MgSxCl6-x] octahedra

Designing wide-bandgap chalcogenides is one of the most important ways of obtaining high-performance infrared (IR) functional materials. In this work, two Mg-based metal thiophosphates, namely Na6Mg3P4S16 (NMPS) and RbMg2PS4Cl2 (RMPSC), were successfully obtained by introducing [MgS6] and [MgSxCl6-x] octahedra into thiophosphates. In addition, their crystal structures were determined, a first for Mg-containing [PS4]-based thiophosphates to the best of our knowledge. Their bandgaps were investigated in theoretical ways and verified by taking experimental measurements, and determined to be 3.80 eV for NMPS and 3.93 eV for RMPSC, values greater than those of the other investigated thiophosphate halides. The wide bandgaps of NMPS and RMPSC were attributed, based on theoretical calculations, to the [MgSxCl6-x] (x = 0-6) octahedron.

Rb3MgB5O10 and LiBaAl(BO3)2: covalent tetrahedra MO4-containing borates with deep-ultraviolet cutoff edges

Borates are favored by materials scientists and chemists because of the significant electronegativity difference between B and O atoms and their flexible assembly modes resulting in abundant structures and excellent properties. For the design of deep-ultraviolet (DUV) optical crystals with excellent macroscopic performance, it is crucial to choose appropriate cations and anionic groups and microscopically reasonable assembly patterns. Herein, by introducing covalent tetrahedra ([MO4], M = Mg, Al), two new mixed alkali metal and alkaline earth metal borates, Rb3MgB5O10 and LiBaAl(BO3)2, were synthesized using the melt method and high-temperature solution method. They contain M-B-O two-dimensional (2D) layers (2∞[MgB5O10] and 2∞[Al(BO3)2], respectively) composed of isolated B-O groups ([B5O10]5- and [BO3]3-, respectively) and metal-centered tetrahedral connectors ([MgO4]6- and [AlO4]5-, respectively). Combining experiments and theoretical calculations shows that the two compounds have short cutoff edges (<200 nm) and moderate birefringences. Further analysis manifests that the isolated [MO4] covalent tetrahedra can optimize the arrangement of anion groups, guarantee the balanced optical properties of materials, and point out the direction for further exploration of novel borate structures.

(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.

Cd8(BO3)4SiO4: Metal Cation Inducing the Formation of Isolated [BO3] and [SiO4] Units in Borate Silicate

The first compound of cadmium-borate silicate Cd8(BO3)4SiO4, crystallizing in space group P42/n (no. 86), has been successfully synthesized by the conventional high-temperature solution method and melts congruently. The zero-dimensional anionic groups of Cd8(BO3)4SiO4 are isolated [BO3] triangles and isolated [SiO4] tetrahedra which are filled in the framework formed by [CdO6] polyhedra. It has a moderate birefringence (Δn = 0.053 at 546 nm), which is measured by experiment and evaluated by first-principles calculations; meanwhile, the source of birefringence is revealed through the response electronic distribution anisotropy method. The UV-vis-NIR diffuse reflectance spectrum indicates that Cd8(BO3)4SiO4 possesses a wide optical transparency range, with a UV cutoff edge at about 254 nm. This work enriches the structure chemistry of borate silicates, and we discussed the possible methods for the exploration and synthesis of novel optical crystals possessing zero-dimensional anionic groups in the borate silicate system.

K2RbB8PO16: A Borophosphate with Moderate Birefringence and Deep-Ultraviolet Transmission

A new borophosphate, K2RbB8PO16 (KRBPO) was synthesized. It exhibits a bilayer structure consisting of two B-O layers with an 18-membered ring (18-MR) joined by [PO4], which is composed of the π-conjugated group [BO3] and non-π-conjugated groups [BO4] and [PO4]. The UV-vis-NIR diffuse reflectance spectroscopy shows that the cutoff edge is less than 200 nm. The calculation indicates that KRBPO exhibits moderate birefringence of 0.057@1064 nm, and the source of birefringence is mainly from the [BO3] groups.

PbTeB4O9: a lead tellurium borate with unprecedented fundamental building block [B4O10] and large birefringence

Herein, the first lead tellurium borate, PbTeB4O9, with an unprecedented fundamental building block [B4O10] was successfully synthesized. The near-parallel alignment of [B4O10] groups and [TeO3] polyhedra resulted in a high birefringence (0.099@1064 nm). The structure-property relationship was discussed by using the first-principles calculations.

A Non-π-Conjugated Molecular Crystal with Balanced Second-Harmonic Generation, Bandgap, and Birefringence

Traditional nonlinear optical (NLO) crystals are exclusively limited to ionic crystals with π-conjugated groups and it is a great challenge to achieve a subtle balance between second-harmonic generation, bandgap, and birefringence for them, especially in the deep-UV spectrum region (Eg  > 6.20 eV). Herein, a non-π-conjugated molecular crystal, NH3 BH3 , which realizes such balance with a large second-harmonic generation response (2.0 × KH2 PO4 at 1064 nm, and 0.45 × β-BaB2 O4 at 532 nm), deep-UV transparency (Eg > 6.53 eV), and moderate birefringence (Δn = 0.056@550 nm) is reported. As a result, NH3 BH3 exhibits a large quality factor of 0.32, which is evidently larger than those of non-π-conjugated sulfate and phosphate ionic crystals. Using an unpolished NH3 BH3 crystal, effective second-harmonic generation outputs are observed at different wavelengths. These attributes indicate that NH3 BH3 is a promising candidate for deep-UV NLO applications. This work opens up a new door for developing high-performance deep-UV NLO crystals.

[RbSr3X][(BS3)2] (X = Cl, Br): two salt-inclusion thioborates with large birefringence and structure transformation from centrosymmetric to asymmetric

[RbSr3X][(BS3)2] (X = Cl, Br), two salt-inclusion chalcogenides with planar [BS3] as anionic units, were obtained. Structure analysis indicates that the size effect of halogens may adjust the arrangement between the [BS3] units and further lead to the CS-to-NCS structure transformation. Experimental characterizations reveal that they have wide bandgaps (3.64-3.70 eV), large birefringence (0.136-0.144) and high LIDT (12-14 × AgGaS2). This work indicates that the thioborate family is a rich source to explore structure chemistry and promising infrared functional materials.

Mg assists in modulating the dimensionalities of the anionic frameworks of borates

Three Mg-containing borates were obtained by high-temperature spontaneous crystallization. In the (A2O)- or (A2O-MO)-MgO-B2O3 system (A is alkali metal and M is alkaline-earth metal) reported in the ICSD, Li4Mg3SrB12O24 is the first compound that contains one-dimensional infinite anionic chains, and the two examples of the isostructural A2Mg3B16O28 (A = Rb, Cs) exhibit a two-dimensional infinite bilayer structure for the first time, which contributes to the enrichment of the structural chemistry of Mg-containing borates. Besides, the results of comparison and analysis in this system clearly show that Mg not only affects the anionic frameworks of borates to produce low-dimensional structures but, together with the ratio of Ncation/NB, is responsible for the dimensionalities of the anionic frameworks in borates. The optical properties of the three compounds also show that they all have short cutoff edges, and Cs2Mg3B16O28, in particular, could reach the deep-ultraviolet region (<200 nm).

A comparative study of structural and spectroscopic properties of three structurally similar mechanically bending organic single crystals - 2-Amino-3-nitro-5-halo (halo = Cl, Br, or I) pyridine

In recent years, scientists have been very interested in single crystals of monoaromatic compounds with mechanical softness, but they are hard to find. The present work reports a comparative study of structural, spectroscopic, and quantum chemical investigations of three structurally similar mechanically bending monoaromatic compounds, namely, 2-amino-3-nitro-5-chloro pyridine (I), 2-amino-3-nitro-5-bromo pyridine (II), and 2-amino-3-nitro-5-iodo pyridine (III). The mechanical responses of the three organic crystals studied here are very intriguing due to the similarity of their chemical structures, which only differ in the presence of halogen atoms (Cl, Br, and I) at the fifth position of the pyridine ring and are explained through examining intermolecular interaction energies from energy frameworks analysis, slip layer topology, and Hirshfeld surface analysis. The crystals of all the three feature one dimensional ribbons comprising alternating NaminoH⋯Onitro and NaminoH⋯Npyridine hydrogen bonds that form R22(12) and R22(8) dimeric rings, respectively. In (III), weak I⋯I interactions link the adjacent ribbons forming a two dimensional sheet. Layer-like structures are observed in all three crystals, with no significant interactions between the adjacent architectures (ribbons or sheets). Energy framework calculations are used for estimating the bending ability of the three compounds, with the three following the order Cl ≪ Br < I. The iterative electrostatic scheme coupled with the supermolecule approach (SM) at the DFT/CAM-B3LYP/aug-cc-pVTZ level is used to calculate the third-order nonlinear susceptibility (χ3) values in a simulated crystalline environment for the static case as well as two typical electric field frequency values, (λ = 1064 nm) and (λ = 532 nm). In addition, estimates of the topological studies (localized orbital locator and electron localization function) and reactivity characteristics (global reactivity parameters, molecular electrostatic potential, and Fukui function) are made for the compounds under investigation. Docking studies done using AutoDock software with a protein target (PDB ID: 6CM4) revealed that three compounds could be used to treat Alzheimer's disease.

VTe2: Broadband Saturable Absorber for Passively Q-Switched Lasers in the Near- and Mid-Infrared Regions

In this study, we demonstrate the fabrication of a novel 2D transition metal dichalcogenide, VTe2, into a saturable absorber (SA) by using the liquid phase exfoliation method. Furthermore, the first-principles calculations were conducted to elucidate the electronic band structures and absorption spectrum. The nonlinear optical absorption properties of VTe2 at 1.0, 2.0, and 3.0 μm were measured using open-aperture Z-scan and P-scan methods, which showed saturation intensities and modulation depths of 95.57 GW/cm2 and 9.24%, 3.11 GW/cm2 and 7.26%, and 15.8 MW/cm2 and 17.1%, respectively. Furthermore, in the realm of practical implementation, the achievement of stable passively Q-switched (PQS) lasers employing SA composed of few-layered VTe2 nanosheets has manifested itself with broadband operating wavelengths from 1.0 to ∼3.0 μm. Specifically, PQS laser operations from near-infrared to mid-infrared with pulse durations of 195 and 563 ns for 1.0 and 2.0 μm solid-state lasers, respectively, and 749 ns for an Er3+-doped fluoride fiber laser at 3.0 μm were obtained. Our experimental results demonstrate that VTe2 is a potential broadband SA device for achieving PQS lasers. To the best of our knowledge, this is the first demonstration of using VTe2 as an SA in PQS lasers in the near- and mid-infrared regions, which highlights the potential of VTe2 for future research and applications in optoelectronic devices.

Ln2 F2 (OH2 )(MoO3 )2 (SeO3 )2 : Promising Multifunctional Nonlinear Optical Materials Created by Partial Fluorination Strategy under Corrosion Resistant Supercritical Reactions

It has historically been exceedingly challenging to create physically and chemically stable lanthanide compounds with strong second harmonic generation (SHG) due to their strong preference to central symmetry. In this work, five new non-centrosymmetric lanthanide selenites, namely, Ln2 F2 (OH2 )(MoO3 )2 (SeO3 )2 (Ln = Sm, Eu, Gd, Tb and Dy), are achieved by partial fluorination of the lanthanide oxygen polyhedron. An HF corrosion resistant supercritical hydrothermal method is developed, which is a facile and universal method for HF corrosion and high-temperature high-pressure environment. The title compounds displayed a novel 3D framework composed of 1D molybdenum selenite chains bridged by Ln2 F2 O12 (OH2 ) dimers. Their powder SHG responses showed a large difference, ranging from 1.0 to 9.0 × KH2 PO4 (KDP) at 1064 nm. The half-filled Gd compound exhibited very strong SHG efficiency of up to 1.2 × KTP (KTiOPO4 ) at 2050 nm. Compounds Tb and Gd are the first lanthanide selenites with SHG intensity reaching KTP level, which is very rare in this system. Furthermore, these compounds can also possess excellent physicochemical stability and strong luminescence emission, indicating that they are promising multifunctional nonlinear optical materials. This work offered an effective way for design and synthesis of multifunctional and high-performant nonlinear optical materials.

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.

Vanadate Crystals with an Enhanced Birefringence and a Broadened Transparency Spectrum through Controlled [VO3]∞ Chain Arrangements and Alkali Metal Cation Introduction

The vanadate (VO) polyhedron offers a compelling avenue for exploring birefringent materials within the infrared frequency range. Among many potential building blocks, the implementation of [VO3]∞ chains demonstrated great potential as effective birefringent functional units. In this article, we successfully synthesized the Li0.8Na0.2CsV2O6·H2O compound, which exhibits a remarkable birefringence of 0.134 at 546.1 nm, as confirmed by the experiment. Notably, the introduction of alkali metals in this compound led to a significantly shorter cutoff edge at 340 nm. Through a comprehensive investigation, Li0.8Na0.2CsV2O6·H2O has the shortest UV cutoff edge among all vanadates, whose birefringences are larger than 0.1, to the best of our best knowledge. This finding underscores the application potential of this novel material as a birefringent crystal.

Hydroxyl-Driven Enhanced Birefringence in Borophosphates

Borophosphates have become promising candidates for ultraviolet or deep-ultraviolet functional crystals. Through high-temperature solution method, four new borophosphates, K2B2P2O9, (NH4)2BP2O7(OH), K2BP2O7(OH), and P21/c-(NH4)2B2P3O11(OH), were acquired successfully. Single crystal X-ray diffraction suggests that K2B2P2O9, (NH4)2BP2O7(OH), and K2BP2O7(OH) belong to the noncentrosymmetric space group, while P21/c-(NH4)2B2P3O11(OH) belongs to the centrosymmetric compound. It is worth mentioning that K2B2P2O9, (NH4)2BP2O7(OH), and K2BP2O7(OH) present the new fundamental building blocks [B2P2O11], [BP2O10H], and [BP2O9(OH)], respectively, as far as we know. Compared with K2B2P2O9, (NH4)2BP2O7(OH), K2BP2O7(OH), and P21/c-(NH4)2B2P3O11(OH) exhibit a larger optical anisotropy, further confirming the positive effect of hydroxyl groups on birefringence. UV-vis-NIR diffuse reflectance spectra display that K2B2P2O9 and (NH4)2BP2O7(OH) have short UV cutoff edges. Meanwhile, theoretical calculations were conducted to comprehend their optical properties and electronic structures.