Toward Large Second-Harmonic Generation and Deep-UV Transparency in Strongly Electropositive Transition Metal Sulfates

A Wide Band Gap Germanate with the Largest Second Harmonic Generation Response Created by Hypoxic Strategy

Nonlinear optical material is one of the core components for modern laser equipment. The second harmonic generation (SHG) intensity and optical band gap are two key indicators of such materials. Herein, a wide band gap germanate, Li3(OH)PbGeO4, with the largest SHG intensity has been created successfully by hypoxic strategy via traditional hydrothermal reactions. The cations in this structure all coordinate with oxygen anions at their lowest coordination number, forming polar PbO3 triangular pyramid and noncentrosymmetric GeO4, LiO4 tetrahedrons. Li3(OH)PbGeO4 exhibits a remarkable SHG response of about 27×KH2PO4 (KDP), 54 % higher than the previous record. The band gap of Li3(OH)PbGeO4 can reach to 3.74 eV, exceeding most inorganic germanates with SHG intensity larger than 3×KDP. Structural analysis and PAWED calculations indicate that its strong SHG response is due to the synergistic effects from PbO3, GeO4 and LiO4 groups. Furthermore, Li3(OH)PbGeO4 can also present compelling broadband white-light emission with a high color-rendering index of up to 93. Our work not only breaks the record of SHG intensity in germanates but also provides an effective strategy in exploring new inorganic optical functional crystals.

K2SO4(NH3SO3)2 and Cs2SO4(NH3SO3)3: two deep-UV transparent sulfamate-sulfates with moderate birefringence

In this work, polar tetrahedral [NH3SO3] and non-polar tetrahedral [SO4]2- groups were combined to synthesize two deep-UV transparent crystals K2SO4(NH3SO3)2 and Cs2SO4(NH3SO3)3 based on the solution evaporation method. Due to the presence of two types of tetrahedral groups with wide bandgap and alkali metal cations without d-d or f-f transitions, both crystals exhibit a deep-UV cutoff edge, approximately 190 nm. Significantly, the birefringence values of K2SO4(NH3SO3)2 and Cs2SO4(NH3SO3)3 are 0.0123 and 0.0153 @546 nm, respectively, which are remarkably improved compared to those of many conventional sulfate crystals. In addition, theoretical calculations were performed to analyze the relationships between the structures and optical properties.

Potassium Tetraiodatoiodate(III) Iodate(V): A Nonlinear Optical Crystal with Exceptional Second-Harmonic Generation and Full-Wavelength Phase Matching

The development of nonlinear optical materials with large nonlinear optical susceptibilities, wide transmission ranges, and comprehensive full-wavelength phase matching capabilities remains a significant challenge. In this study, we synthesized a novel potassium tetraiodatoiodate(III) iodate(V) via an in-situ reduction and complexation reaction. The resulting compound, K2[IIII(IVO3)4]IVO3, exhibits exceptional second-harmonic generation responses (21.6KH2PO4 at 1064 nm, 1.0KTiOPO4 at 2050 nm), a wide transparency range from the visible to the mid-infrared wavelengths (0.37-6.05 and 7.03-13.40 µm), and a high birefringence (0.358 at 543 nm) to achieve full-wavelength phase matching. These properties are primarily ascribed to the synergistic substantial hyperpolarizability and polarizability anisotropy exhibited by the [IIII(IVO3)4]- complex ions, which comprise square-planar coordinated trivalent iodine and optimally arranged iodate ligands. This study demonstrates the pivotal role of nonmetallic cation centered coordination entities in influencing linear and nonlinear optical properties, a discovery that has significant implications for the development of innovative inorganic functional materials.

Different p-block elements induce C3[111] octahedral distortion in titanium to generate an intense nonlinear effect

Acentric crystalline materials are the cornerstone of numerous cutting-edge technologies and have been highly sought-after, but they are difficult to construct controllably. Herein, by introducing a new p-block element to break the symmetrical environment of the d0 transition metal in the centric matrix TiTe3O8, a novel acentric tellurite sulfate, namely Ti(TeO3)(SO4), was successfully constructed. In its structure, two types of p-block element-centered oxo-anionic groups, i.e. [TeO3] and [SO4], endow [TiO6] with an out-of-center distortion along the local C3[111] direction, which is rare in titanium oxides containing a lone-pair cation. The synergy of the distorted [TiO6] octahedron, lone-pair [TeO3] pyramid and rigid [SO4] tetrahedron within its structure induces a strong second harmonic generation (SHG) response of 11.6 × KDP (KH2PO4), the largest value among mercury-free sulfates. Additionally, Ti(TeO3)(SO4) also shows the largest birefringence (0.145) among sulfates possessing an SHG response that is more than ten times that of KDP, showing huge potential as a nonlinear optical material. The successful implementation of the strategy of inducing intra-octahedral distortion in a d0 transition metal by different p-block elements provides new opportunities for constructing acentric structures and exploiting outstanding nonlinear optically active sulfates.

Rare-Earth Substitution Induced Symmetry Breaking for The First Sc-Based Nonlinear Optical Chalcogenide with High-Performance

Chalcogenides are the most important infrared nonlinear optical (NLO) material candidates, and the exploration of high-performance ones is attractive and challengeable. Hitherto, there is no NLO scandium (Sc) chalcogenides experimentally studied. Here, new quaternary Sc thiophosphate CsScP2S7 (CSPS) was synthesized by the facile metal oxide-boron-sulfur/reactive flux hybrid solid-state method. It crystallizes in the monoclinic chiral space group C2, and the layered structure is composed by the new ScP2S11 functional motifs built by ScS6 octahedra and P2S7 dimers, and the structure-performance analysis reveals that the hyperpolarizability of ScP2S11 is much greater than the assembled units (ScS6 and PS4), which makes the first NLO Sc chalcogenide CSPS exhibits strong NLO response (0.8 × AGS) and high laser-induced damage threshold (LIDT) (4.3 × AGS), and a wide bandgap of 3.10 eV. With the coordination number's reduction of rare-earth (RE) ion and the rearrangement of P2S7 dimers, the centrosymmetric structure of CsREP2S7 family can be broken via substitution with the smallest RE element Sc to form the noncentrosymmetric structure. This work not only discovers a new high-performance infrared NLO material, but also will inspire researchers to explore more potential NLO Sc chalcogenides.

Hafnium-Based Chiral 2D Organic-Inorganic Hybrid Metal Halides: Engineering Polarity and Nonlinear Optical Properties via Para-Substituent Effects

Two-dimensional (2D) organic-inorganic hybrid metal halides (OIMHs), characterized by noncentrosymmetric structures arising from the incorporation of chiral organic molecules that break inversion symmetry, have attracted significant attention. Particularly, chiral-polar 2D OIMHs offer a unique platform for multifunctional applications, as the coexistence of chirality and polarity enables the simultaneous manifestation of distinct properties such as nonlinear optical (NLO) effects, circular dichroism (CD), and ferroelectricity. In this study, we report the first synthesis of hafnium (Hf)-based chiral 2D OIMHs, achieved through the strategic incorporation of para-substituents on the benzene ring of chiral organic components. By tuning the substituents, we successfully modulate the polarity of the crystal structures, resulting in both chiral-nonpolar and chiral-polar systems. Our analysis of structural and optical properties, supported by density functional theory calculations, demonstrates that the polarity of these materials can be systematically tuned, enabling adjustable band gaps and CD in the UV range (200-280 nm). Notably, halogen substitution at the para-position of the benzene ring in the organic layer produces tunable optical band gaps ranging from 4.33 to 4.48 eV, the widest reported to date for chiral-polar 2D OIMHs. Furthermore, these materials exhibit enhanced NLO properties, including a remarkable 3.3-fold increase in second-harmonic generation intensity in chiral-polar compounds compared to their chiral-nonpolar counterparts. These findings position Hf-based chiral 2D OIMHs as promising candidates for UV-region applications, such as UV NLO devices and self-driven circularly polarized light detectors, offering new opportunities for designing multifunctional optoelectronic materials by harnessing the interplay between chirality and polarity.

Sulfate Derivatives with Heteroleptic Tetrahedra: New Deep-Ultraviolet Birefringent Materials in which Weak Interactions Modulate Functional Module Ordering

Deep-ultraviolet (UV) birefringent materials are urgently needed to facilitate light polarization in deep-UV lithography. Maximizing anisotropy by regulating the alignment of functional modules is essential for improving the linear optical performance of birefringent materials. In this work, we proposed a strategy to design deep-UV birefringent materials that achieve functional module ordering via weak interactions. Following this strategy, four compounds CN4H7SO3CF3, CN4H7SO3CH3, C(NH2)3SO3CH3, and C(NH2)3SO3CF3 were identified as high-performance candidates for deep-UV birefringent materials. The millimeter-sized crystals of CN4H7SO3CF3, CN4H7SO3CH3, and C(NH2)3SO3CH3 were grown, and the transmittance spectra show that their cutoff edges are below 200 nm. CN4H7SO3CF3 exhibits the largest birefringence (0.149 @ 546 nm, 0.395 @ 200 nm) in the deep-UV region among reported sulfates and sulfate derivatives. It reveals that the hydrogen bond can modulate the module ordering of the heteroleptic tetrahedra and planar π-conjugated cations, thus greatly enhancing the birefringence. Our study not only discovers new deep-UV birefringent materials but also provides an upgraded strategy for optimizing optical anisotropy to achieve efficient birefringence.

Two l-Proline Cadmium Thiocyanates with Moderate Second-Harmonic-Generation Responses and Large Birefringences

Two organic-inorganic cadmium thiocyanates, namely, Cd(SCN)(l-C5H8NO2) (1) and Cd(SCN)2(l-C5H9NO2) (2), have been synthesized using l-proline as the bridging ligand. The two compounds have double-chain-like and layered structures, respectively. They feature moderate second-harmonic-generation responses (0.33 × KH2PO4 for 1 and 1.58 × KH2PO4 for 2), wide band gaps (4.68 eV for 1 and 4.59 eV for 2), and large birefringences (0.210 at 546 nm for 1 and 0.295 at 546 nm for 2). The structure-property relationship was explicated by first-principle calculations based on density functional theory.

Screening Strategy Identifies an Overlooked Deep-Ultraviolet Transparent Nonlinear Optical Crystal

In the deep-ultraviolet (DUV) region, nonlinear optical (NLO) crystals must meet stringent requirements, including a large optical band gap and sufficient second harmonic generation (SHG) response. Typically, these criteria are fulfilled by borates, carbonates and nitrates containing π-conjugated groups. In contrast, sulfates and phosphates, with polarizabilities significantly smaller than those of π-conjugated groups, struggle to achieve similar performance. Here, we present the discovery of Mg2PO4Cl, a magnesium-based phosphate, identified from over 10,000 phosphates based on a polar-axial-symmetry screening strategy, which exhibits the highest SHG response (5.2×KH2PO4 (KDP)) with phase-matching ability among non-π-conjugated DUV transparent NLO crystals. This compound belongs to the Pna21 space group, with [PO4] units consistently aligned along the 21 screw axis and glide planes throughout its crystal structure. Theoretical calculations attribute its remarkable SHG effect to the orderly arrangement of heteroanionic [MgO5Cl] and [MgO4Cl2] polyhedra alongside isolated [PO4] tetrahedra, supported by Berry phase analysis. Furthermore, a crystallographic structure analysis of phosphates and sulfates with significant SHG effects validates the effectiveness of our screening strategy. These findings offer valuable insights into the origins of NLO effects in non-π-conjugated compounds from both a material design and structural chemistry perspective, inspiring future efforts to revitalize DUV phosphates.

Ultraviolet Nonlinear Optical Single Crystals of A Three-Dimensional Chiral Covalent Framework Containing Te-O-B-O Bonds

Extending covalent organic frameworks (COFs) into crystalline carbon-free covalent backbones is an important strategy to endow these materials with more exotic functions. Integrating metal-free inorganic and organic components into one covalent framework is an effective way to address the issue of poor thermal/solvent stability in the field of nonlinear optics (NLO). However, constructing such structures is very challenging. Here, we linked 3-connected nods (BO3) and 2-connected organic building blocks (Te(Ph)2) together to produce colorless single crystals (size up to 400 μm) of a three-dimensional (3D) chiral covalent framework (CityU-22). The single-crystal X-ray diffraction (SCXRD) analysis reveals that CityU-22 has a non-carbon Te-O-B-O bond-based network with the srs topology. The chiral CityU-22 displays good stability under the treatment of different common solvents or heat (the decomposition temperature above 300 °C). Due to its non-π-conjugated backbone (-Te-O-B-O-), CityU-22 shows an ultraviolet NLO behavior with a second-harmonic generation (SHG) response similar to KH2PO4 (KDP).

Li2NaB3S2O12: A Deep-UV Transparent Borosulfate with Moderate Birefringence Derived from the [B3S2O12]∞ Infinite Chain Designed by the High Boron-to-Sulfur Ratio Strategy

The first mixed alkali metal borosulfate compound, Li2NaB3S2O12 (LNBSO), which contains [BO3] groups, was designed and synthesized by using a high boron-to-sulfur ratio strategy through the high temperature solution method. LNBSO exhibits a birefringence of 0.057@546.1 nm in experiments, which was mainly contributed by the [BO3] groups, and possesses a short absorption edge at 184 nm, and the space group of LNBSO is P21/c. This newly synthesized borosulfate compound holds potential as a promising birefringent material within the deep-ultraviolet wavelength range. Moreover, the investigation on the relationship among the ratio of boron to sulfur, the dimensionality of the anionic framework, and the formation of [BO3] groups has been conducted on available borosulfate, providing insights for the synthesis of borosulfate with desirable performances.

Ba2ScBSi2O9: A Mixed-Coordination Borosilicate with a Low B/Si Ratio Exhibiting Enhanced Second Harmonic Generation Response

Rational chemical substitution is an effective way to regulate structure and enrich property. Herein, a new noncentrosymmetric borosilicate, Ba2ScBSi2O9, was successfully synthesized by substituting CaO6 units in Ba2CaB2Si4O14 with ScO6 octahedra, with comparatively strong covalency. This substitution not only effectively prevents polymerization of the B-O groups, resulting in an intriguing structural transformation from tetrahedral-coordinated borosilicate of Ba2CaB2Si4O14 to mixed-coordinated borosilicate Ba2ScBSi2O9, but also enhances its second harmonic generation response (2 × KDP), that is nearly four times higher than its parent structure while keeping a short ultraviolet (UV) cutoff edge (λcutoff < 190 nm). In addition, the polar space group of Pca21 for Ba2ScBSi2O9 achieves its ferroelectric polarization reversal capability, which makes quasi-phase-matching technology possible to counteract the nonphase-matching caused by small birefringence of silicates. This work indicates the unique role of heterovalent substitution in regulating structure and performance, providing new insights for exploring borosilicate with versatile functionality.

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

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

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.

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.

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.

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

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

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.

UV-Transparent SHG Material Explored in an Alkali Metal Sulfate Selenite System

The first examples of alkali metal selenite sulfates, namely, Na8(SeO3)(SO4)3 (1), Na2(H2SeO3)(SO4) (2), and K4(H2SeO3)(HSO4)2(SO4) (3), were successfully synthesized by hydrothermal reactions. Their structures display three different zero-dimensional configurations composed of isolated sulfate tetrahedra and selenite groups separated by alkali metals. Na8(SeO3)(SO4)3 (1) features a noncentrosymmetric structure, while Na2(H2SeO3)(SO4) (2) and K4(H2SeO3)(HSO4)2(SO4) (3) are centrosymmetric. Powder second-harmonic-generation measurements revealed that Na8(SeO3)(SO4)3 (1) shows a phase-matchable SHG intensity about 1.2 times that of KDP. UV-vis-NIR diffuse reflectance spectroscopic analysis indicated that Na8(SeO3)(SO4)3 (1) has a short UV cutoff edge and a large optical band gap, which makes it a possible UV nonlinear optical material. Theoretical calculations revealed that the birefringence of Na8(SeO3)(SO4)3 (1) is 0.041 at 532 nm, which is suitable for phase-matching condition. This work provides a good experimental foundation for the exploration of new UV nonlinear crystals in an alkali metal selenite sulfate system.

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.

Hg4(Te2O5)(SO4): A Giant Birefringent Sulfate Crystal Triggered by a Highly Selective Cation

Sulfate crystals are often criticized for their low birefringence. The small anisotropic SO4 group is becoming the biggest bottleneck hindering the application of sulfates in optical functional materials. In this study, we report a new method to significantly enhance the birefringence of sulfates. The title compound increases the birefringence recording of sulfates to 0.542@546 nm, which is significantly larger than that of the commercial birefringent crystal of TiO2 (0.306@546.1 nm). At the infrared wavelength, the birefringence of Hg4(Te2O5)(SO4) can be up to 0.400@1064 nm, which is also much larger than the infrared birefringent crystal of YVO4 (0.209@1064 nm). In addition, it also has a wide transparency range, high thermal stability, and excellent environmental stability, making it a potential birefringent material. Hg4(Te2O5)(SO4) features a novel two-dimensional layered structure composed of [Hg4(Te2O5)]2+ layers separated by isolated (SO4)2- tetrahedra. This compound was designed by introducing a highly selective cation in a tellurite sulfate system. The low valence low coordination cations connect with tellurite groups only, making the sulfate isolated in the structure. The steric repulsive action of the isolated SO4 tetrahedra may regulate the linear and lone pair groups arranged in a way that favors large birefringence. This method can be proven by theoretical calculations. PAWED studies showed that the large birefringence originated from the synergistic effect of (Hg2O2)2-, (Te2O5)2-, and (SO4)2- units, with a contribution ratio of 42.17, 37.92, and 19.88%, respectively. Our work breaks the limitation of low birefringence in sulfates and opens up new possibilities for their application as birefringent crystals.

Secondary-Bond-Driven Construction of a Polar Material Exhibiting Strong Broad-Spectrum Second-Harmonic Generation and Large Birefringence

Considerable effort has been invested in the development of non-centrosymmetric (NCS) inorganic solids for ferroelectricity-, piezoelectricity- and, particularly, optical nonlinearity-related applications. While great progress has been made, a persistent problem is the difficulty in constructing NCS materials, which probably stems from non-directionality and unsaturation of the ionic bonds between metal counter-cations and covalent anionic modules. We report herein a secondary-bond-driven approach that circumvents the cancellation of dipole moments between adjacent anionic modules that has plagued second-harmonic generation (SHG) material design, and which thereby affords a polar structure with strong SHG properties. The resultant first NCS counter-cation-free iodate, VO2 (H2 O)(IO3 ) (VIO), a new class of iodate, crystallizes in a polar lattice with∞ 1 [ ${{}_{{\rm { \infty }}}{}^{{\rm { 1}}}{\rm { [}}}$ VO2 (H2 O)(IO3 )] zigzag chains connected by weak hydrogen bonds and intermolecular forces. VIO exhibits very large SHG responses (18 × KH2 PO4 @ 1200 nm, 1.5 × KTiOPO4 @ 2100 nm) and sufficient birefringence (0.184 @ 546 nm). Calculations and crystal structure analysis attribute the large SHG responses to consistent polarization orientations of the∞ 1 [ ${{}_{{\rm { \infty }}}{}^{{\rm { 1}}}{\rm { [}}}$ VO2 (H2 O)(IO3 )] chains controlled by secondary bonds. This study highlights the advantages of manipulating the secondary bonds in inorganic solids to control NCS structure and optical nonlinearity, affording a new perspective in the development of high-performance NLO materials.

Giant Optical Anisotropy in a Covalent Molybdenum Tellurite via Oxyanion Polymerization

Large birefringence is a crucial but hard-to-achieve optical parameter that is a necessity for birefringent crystals in practical applications involving modulation of the polarization of light in modern opto-electronic areas. Herein, an oxyanion polymerization strategy that involves the combination of two different types of second-order Jahn-Teller distorted units is employed to realize giant anisotropy in a covalent molybdenum tellurite. Mo(H2O)Te2O7 (MTO) exhibits a record birefringence value for an inorganic UV-transparent oxide crystalline material of 0.528 @ 546 nm, which is also significantly larger than those of all commercial birefringent crystals. MTO has a UV absorption edge of 366 nm and displays a strong powder second-harmonic generation response of 5.4 times that of KH2PO4. The dominant roles of the condensed polytellurite oxyanions [Te8O20]8- in combination with the [MoO6]6- polyhedra in achieving the giant birefringence in MTO are clarified by structural analysis and first-principles calculations. The results suggest that polymerization of polarizability-anisotropic oxyanions may unlock the promise of birefringent crystals with exceptional birefringence.

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.

Designing Sulfate Crystals with Strong Optical Anisotropy through π-Conjugated Tailoring

Sulfate crystals typically exhibit minimal optical anisotropy due to the near-zero polarizability anisotropy (δ) of [SO4 ]2- tetrahedra, arising from highly symmetrical electron clouds. Recent research sought to enhance δ via chemical modifications, such as fluorination. However, the resultant crystals often maintain subpar optical anisotropy, frequently with birefringence values below 0.1. In this study, we have uncovered that δ can be significantly strengthened by chemically tailoring the tetrahedral [SO4 ]2- with anisotropic π-conjugated modules. This has been demonstrated by several newly proposed S-O-Org (Org: π-conjugated organic species) moieties, which show a sharp increase in δ based on theoretical computations. To further validate this experimentally, we synthesized and characterized six new 3-pyridinesulfonate crystals with the formula A(3-C5 H4 NSO3 ) ⋅ xH2 O (A=Li, Ag, K, Rb, Cs, and NH4 ; x=0 and 1). Notably, these materials exhibit strong optical anisotropy, with birefringence values ranging from 0.240 to 0.312 at 546 nm. These values are approximately 23 to 145.5 times greater than those of corresponding sulfates, and they outperform a vast number of sulfate-related optical materials, thus verifying the effectiveness of the proposed strategy. Furthermore, the title compounds exhibit diverse microstructure peculiarities influenced by the size and binding natures of the counter cations.

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

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

Ultrashort Phase-Matching Wavelength and Strong Second-Harmonic Generation in Deep-UV-Transparent Oxyfluorides by Covalency Reduction

The development of urgently-needed ultraviolet (UV)/deep-UV nonlinear optical (NLO) materials has been hindered by contradictory requirements of the microstructure, in particular the need for a strong second-harmonic generation (SHG) response as well as a short phase-matching (PM) wavelength. We herein employ a "de-covalency" band gap engineering strategy to adjust the optical linearity and nonlinearity. This has been achieved by assembling two types of transition-metal (TM) polyhedra ([TaO2 F4 ] and [TaF7 ]), affording the first tantalum-based deep-UV-transparent NLO materials, A5 Ta3 OF18 (A = K (KTOF), Rb (RTOF)). Experimental and theoretical studies reveal that the highly ionic bonds and strong electropositivity of tantalum in the two oxyfluorides induce record short PM wavelengths (238 (KTOF) and 240 (RTOF) nm) for d0 -TM-centered oxides, in addition to strong SHG responses (2.8 × KH2 PO4 (KTOF) and 2.6 × KH2 PO4 (RTOF)), and sufficient birefringences (0.092 (KTOF) and 0.085 (RTOF) at 546 nm). These results not only broaden the available strategies for achieving deep-UV NLO materials by exploiting the currently neglected d0 -TMs, but also push the shortest PM wavelength into the short-wavelength UV region.

(NH4 )2 (I5 O12 )(IO3 ) and K1.03 (NH4 )0.97 (I5 O12 )(IO3 ): Mixed-Valent Polyiodates with Unprecedented I5 O12 - Unit Exhibiting Strong Second-Harmonic Generation Responses and Giant Birefringence

Second-harmonic generation (SHG) response and birefringence are crucial properties for linear and nonlinear optical (NLO) materials, while it is difficult to further optimize these two key properties by using a single traditional functional building block (FBB) in one compound. Herein, a novel IO4 5- unit is identified, which possesses a square-planar configuration and two stereochemically active lone-pairs (SCALPs). By combining IO4 5- and IO3 - units, the first examples of mixed-valent polyiodates featuring an unprecedented bowl-shaped I5 O12 - polymerized unit, namely (NH4 )2 (I5 O12 )(IO3 ) and K1.03 (NH4 )0.97 (I5 O12 )(IO3 ), are successfully synthesized. Excitingly, both crystals exhibit strong SHG responses (16 × KDP and 19.5 × KDP @1064 nm) as well as giant birefringence (∆nexp  = 0.431 and 0.405 @546 nm). Detailed structure-property analyses reveal that the parallel aligned planar IO4 5- units induce the properly aligned high-density SCALPs, leading to strong SHG response and giant birefringence for both materials. This work not only provides two new potential NLO and birefringent crystals, but also discovers a novel promising FBB (IO4 5- ) for developing high-performance linear and nonlinear optical materials.

Zn2 HgP2 S8 : A Wide Bandgap Hg-Based Infrared Nonlinear Optical Material with Large Second-Harmonic Generation Response

Hg-based chalcogenides, as good candidates for the exploration of high-performance infrared (IR) nonlinear optical (NLO) materials, usually exhibit strong NLO effects, but narrow bandgaps. Herein, an unprecedented wide bandgap Hg-based IR NLO material Zn2 HgP2 S8 (ZHPS) with diamond-like structure is rationally designed and fabricated by a tetrahedron re-organization strategy with the aid of structure and property predictions. ZHPS exhibits a wide bandgap of 3.37 eV, which is the largest one among the reported Hg-based chalcogenide IR NLO materials and first breaks the 3.0 eV bandgap "wall" in this system, resulting in a high laser-induced damage threshold (LIDT) of ≈2.2 × AgGaS2 (AGS). Meanwhile, it shows a large NLO response (1.1 × AGS), achieving a good balance between bandgap (≥3.0 eV) and NLO effect (≥1 × AGS) for an excellent IR NLO material. DFT calculations uncover that, compared to normal [HgS4 ]n , highly distorted [HgS4 ]d tetrahedral units are conducive to generating wide bandgap, and the wide bandgap in ZHPS can be attributed to the strong s-p hybridization between Hg─S bonding in distorted [HgS4 ]d , which gives some insights into the design of Hg-based chalcogenides with excellent properties based on distorted [HgS4 ]d tetrahedra.

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

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

Target-Oriented Synthesis of Borate Derivatives Featuring Isolated [B3O3] Six-Membered Rings as Structural Features

Borates provide an excellent platform for investigating the optical nonlinearity and linearity of crystals as photoelectric functional materials. In our work, borate derivatives with isolated [B₃O₃] six-membered rings as structural features are the preferred system due to their simple functional units and excellent properties. Herein, by utilizing the target-oriented synthesis, a series of borate derivatives, A₂[B₃O₃F₄(OH)] (A= NH₄, Rb, Cs) (ABOFH), K_2.3Cs_0.7B₃O₃F₆ (KCsBOF), and Cs₃[B₃O₃(OH)₃]Cl₃ (CsBOHCl), with novel heteroanionic groups containing [BO_xF_4-x] (x = 0-3) and/or [BO₂(OH)] units were obtained. ABOFH, KCsBOF, and CsBOHCl construct different two-dimensional pesudolayers featuring [B₃O₃F₄(OH)], [B₃O₃F₆], and [B₃O₃(OH)₃] units, respectively. Also, the optical properties and the arrangement information of these anionic groups were studied. Among the total five compounds, (NH₄)₂[B₃O₃F₄(OH)] and Cs₃[B₃O₃(OH)₃]Cl₃ with enlarged birefringence and sufficient band gaps were screened out as promising birefringent crystals due to the optimally aligned configuration of birefringence-active heteroanionic units. The successful results of target-oriented synthesis indicate a more profound conclusion that the borate system now has more diversified structural chemistry, and an effective strategy was proposed to modify the arrangement and species of anionic units to optimize the performance of optical crystals.