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.

Ca2 Ln(BS3 )(SiS4 ) (Ln = La, Ce, and Gd): Mixed Metal Thioborate-Thiosilicates as Well-Performed Infrared Nonlinear Optical Materials.

The first examples of thioborate-thiosilicates, namely Ca2 Ln(BS3 )(SiS4 ) (Ln = La, Ce, and Gd), are synthesized by rationally designed high-temperature solid-state reactions. They crystalize in the polar space group P63 mc and feature a novel three-dimensional crystal structure in which the discrete [BS3 ]3- and [SiS4 ]4- anionic groups are linked by Ca2+ and Ln3+ cations occupying the same atomic site. Remarkably, all three compounds show comprehensive properties required as promising infrared nonlinear optical materials, including phase-matchable strong second harmonic generation (SHG) responses at 2.05 µm (1.1-1.2 times that of AgGaS2 ), high laser-induced damage thresholds (7-10 times that of AgGaS2 ), wide light transmission range (0.45-11 µm), high thermal stabilities (>800 °C), and large calculated birefringence (0.126-0.149 @1064 nm), which justify the material design strategy of combining [BS3 ]3- and [SiS4 ]4- active units. Theoretical calculations suggest that their large SHG effects originate mainly from the synergy effects of the LnS6 , BS3 , and SiS4 groups. This work not only broadens the scope of research on metal chalcogenides but also provides a new synthetic route for mixed anionic thioborates.

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.

Hg3Se(SeO3)(SO4): A Mixed-Valent Selenium Compound with Mid-Infrared Transmittance Obtained by In Situ Reaction.

Exploring new material systems is a highly significant task in the field of inorganic chemistry. A new mixed-valent selenium compound, Hg3Se(SeO3)(SO4), was successfully synthesized through in situ reactions. This compound exhibits a novel three-dimensional structure composed of Hg3Se(SO4) layers bridged by SeO3 trigonal pyramids. It is the first structure containing (SeO3)2-, (SO4)2-, and Se2- simultaneously. In addition, Hg3Se(SeO3)(SO4) possesses a wide bandgap (3.5 eV), moderate birefringence (Cal:0.064@546 nm, Exp:0.069@546 nm), a high laser-induced damage threshold (23.35 MW cm-2), and a wide transmittance window (0.28-6.6 µm). Our work demonstrates that mixed-valent (+4, -2) selenite selenide can be potential optical materials for the mid-infrared region.

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.

Highly Birefringent d0 Transition Metal Fluoroantimonite in the Mid Infrared Band: Order-Disorder Regulation by Cationic Size.

An unusual O/F ordered d0 transition metal fluoroantimonite, namely, K2SbMoO2F7, has been created by the cationic size effect of alkali metals. It features the largest birefringence of 0.220@550 nm among inorganic antimonites with a halogen element, which is an order of magnitude larger than the disordered A2SbMoO2F7 (A = Rb, Cs). These three new compounds exhibit two different structures, although all of the structures were made of [SbMoO2F7]2- chains formed by SbF5 square pyramids and MoO2F4 octahedrons. A transparent single crystal of K2SbMoO2F7 with dimensions of 7.0 × 5.0 × 1.0 mm3 has been successfully grown by the aqueous solution volatilization method. The UV-vis-MIR transmission spectrum showed that K2SbMoO2F7 can display excellent transmittance in the range of 0.5-5.0 µm and 6.0-9.8 µm, indicating its application potential as a birefringent material in the mid infrared band. This work offers a fresh approach to the design and synthesis of mid infrared birefringent materials.

(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.

AAl(Te4O10) (A = Na, Ag) and K2Ga2(HTe6O16)(HTeO3): Three Aluminum/Gallium Tellurites with Large Birefringence and Wide Band Gap.

The exploration of novel inorganic tellurites is significant because of their promising applications in nonlinear-optical materials and birefringent materials. Herein, three new aluminum/gallium tellurites, namely, NaAl(Te4O10) (1), AgAl(Te4O10) (2), and K2Ga2(HTe6O16)(HTeO3) (3), have been successfully obtained by mild hydrothermal reactions. The isostructural compounds 1 and 2 contain the Te3O8 trimer, while compound 3 contains an unprecedented Te6O16 hexamer. Notably, all three compounds exhibit large birefringence values of over 0.1 at 532 nm, which are currently the largest reported birefringence values among tellurium(IV) oxides without additional anionic groups.

Single-Crystalline Li2Sn(IO3)6 Microwires: Combining Optical-Waveguiding and Frequency-Doubling Functions.

High-quality single-crystalline Li2Sn(IO3)6 microwires (MWs) have been successfully prepared by using a facile hydrothermal method. The as-synthesized Li2Sn(IO3)6 MWs exhibit regular hexagonal prism morphology, excellent surface smoothness, and remarkable diameter uniformity. The optical propagation loss has been determined to be as low as 0.026 dB µm-1 at 785 nm wavelength, implying the low-loss optical waveguiding capability of the Li2Sn(IO3)6 MWs. The effective frequency-doubling conversions of the fundamental frequency light source in the wavelength range from 916 to 1560 nm have been observed, and the second-harmonic generation (SHG) conversion efficiency has been measured to be 2.1% with a 1560 nm fundamental pump source (pulse duration of 10 ns, and average power of 9.06 nW) transmitted through a 1.32-µm-diameter and 300-µm-length Li2Sn(IO3)6 MW. These intriguing optical waveguiding and strong SHG conversion capabilities of the Li2Sn(IO3)6 MWs suggest its potential applications for photonic devices in micrometer scale.

CsHfF4(IO3): A Hafnium Iodate Exhibiting a Strong Second-Harmonic-Generation Effect and a Wide Band Gap Achieved by Mixed-Ligand Acentric Coordination.

A new polar hafnium iodate, CsHfF4(IO3), was successfully designed and synthesized by integrating fluorinated hafnium-oxygen polyhedra (HfF6O2) and IO3- anionic functional groups. Owing to the weak electronic effect of Hf4+ and the bond-network-induced out-of-center distortion of the HfF6O2 dodecahedra, CsHfF4(IO3) achieves a good balance between a strong second-harmonic-generation effect (3.5 × KH2PO4) and a rather large band gap (4.47 eV), which is the largest among the d0 transition-metal iodates. In addition, CsHfF4(IO3) possesses a wide transparent region (0.27-9.9 µm), a large birefringence for phase-matching (0.161), and a high laser-induced damage threshold (55.41 MW cm-2, 26 × AgGaS2) and is nonhygroscopic. This work indicates that the integration of mixed-ligand acentric coordination polyhedra and functional groups containing lone electron pairs is an effective strategy for developing novel inorganic nonlinear-optical materials with balanced overall properties.

α- and ß-(C4H5N2O)(IO3)·HIO3: Two SHG Materials Based on Organic-Inorganic Hybrid Iodates.

Organic-inorganic hybrid nonlinear optical (NLO) materials are highly anticipated because of the integration of both merits of the organic and inorganic moieties. Herein, the 2-pyrimidinone cation (C4H5N2O)+ has been incorporated into the iodate system to form two polymorphic organic-inorganic hybrid iodates, namely, α- and ß-(C4H5N2O)(IO3)·HIO3. They crystallize in different polar space groups (Ia and Pca21), and their structures feature one-dimensional (1D) chain structures composed of (C4H5N2O)+ cations, IO3- anions, and HIO3 molecules interconnected via hydrogen bonds. α- and ß-(C4H5N2O) (IO3)·HIO3 exhibit strong and moderate second-harmonic-generation (SHG) responses of 6.4 and 0.9 × KH2PO4 (KDP), respectively, the same band gaps of 3.65 eV, and high powder laser-induced damage threshold (LIDT) values [51 and 57 × AgGaS2 (AGS)]. The results of theoretical calculations revealed that the large SHG effect of α-(C4H5N2O)(IO3)·HIO3 originated from the IO3 and HIO3 groups. This work indicates that (C4H5N2O)+ is a potential group for designing new NLO materials with brilliant optical performances.

Growth and Optical Properties of Large-Sized NaVO2(IO3)2(H2O) Crystals for Second-Harmonic Generation Applications.

Large-sized crystals of the quaternary iodate NaVO2(IO3)2(H2O) (NVIO) with centimeter-scale dimensions (23 mm × 18 mm × 6 mm as a representative) have been successfully grown by the top-seeded hydrothermal method. Linear optical properties have been measured, including the optical transmission spectrum and refractive index. The NVIO crystal possesses an optical window with high transmittance (above 80%) over the range of 500-1410 nm and exhibits strong optical anisotropy with large birefringence Δn (nz - nx) of 0.1522 at 1064 nm and 0.1720 at 532 nm. Based on the measured refractive indices, the phase-matching conditions for second-harmonic generation (SHG) have been calculated, and SHG devices have further been fabricated along the calculated type I and type II phase-matching directions of (θ = 39.0°, φ = 3.8°) and (θ =53.8°, φ = 1.3°). Laser experiments of extra-cavity frequency doubling have been performed on these NVIO devices. It has been confirmed that the effective SHG conversion from 1064 to 532 nm could be achieved with an energy conversion efficiency of 8.1%. Our work demonstrates that large-sized NVIO crystals are promising in the frequency-doubling application.

Ce(IO3)3F and Ce(IO3)2(NO3): Two Mixed-Anion Cerium Iodates with Good Nonlinear Optical and Birefringent Properties.

Two new mixed-anion cerium iodates, namely, Ce(IO3)3F and Ce(IO3)2(NO3), have been rationally designed through the integration of hybrid anionic functional building blocks (FBBs). The structure of Ce(IO3)3F features a novel [Ce(IO3)3F] bilayer, and the material exhibits large birefringence (0.225 @546 nm). The structure of Ce(IO3)2(NO3) features [Ce3(IO3)6]3+ triple layers that are further linked by planar NO3- units. Ce(IO3)2(NO3) shows a moderate SHG response (1 × KDP) and a high laser-induced damage threshold value (22 × AgGaS2). This work demonstrates that the rich coordination geometries of cerium cations facilitate tuning of the structures of related compounds through modulating anionic FBBs.

AKTeO2(CO3) (A = Li, Na): The First Carbonatotellurites Featuring a Zero-Dimensional [Te2C2O10]4- Cluster and a Wide Band Gap.

The first carbonatotellurites, AKTeO2(CO3) (A = Li, Na), have been successfully synthesized by using boric acid as the mineralizer. AKTeO2(CO3) (A = Li, Na) crystallize in the monoclinic space group P21/n (no. 14), and their structures exhibit the novel zero-dimensional (0D) [Te2C2O10]4- clusters, in which two [TeO4]4- groups form a [Te2O6]4- dimer via edge-sharing, with each side of the dimer attached by a [CO3]2- group via a Te-O-C bridge. The alkali metal cations occupy the voids between the 0D clusters and maintain the charge balance. The ultraviolet-visible-near-infrared diffuse reflectance spectra show that the short absorption cut-off edges of LiKTeO2(CO3) (LKTC) and NaKTeO2(CO3) (NKTC) are 248 and 240 nm, respectively, and LKTC exhibits the largest experimental band gap (4.58 eV) among all of the tellurites containing the π-conjugated anionic groups reported. Theoretical calculations revealed that they exhibit moderate birefringences of 0.029 and 0.040@1064 nm, respectively.

The First UV Nonlinear Optical Selenite Material: Fluorination Control in CaYF(SeO3 )2 and Y3 F(SeO3 )4.

It is a great challenge to develop UV nonlinear optical (NLO) material due to the demanding conditions of strong second harmonic generation (SHG) intensity and wide band gap. The first ultraviolet NLO selenite material, Y3 F(SeO3 )4 , has been obtained by control of the fluorine content in a centrosymmetric CaYF(SeO3 )2 . The two new compounds represent similar 3D structures composed of 3D yttrium open frameworks strengthened by selenite groups. CaYF(SeO3 )2 has a large birefringence (0.138@532 nm and 0.127@1064 nm) and a wide optical band gap (5.06 eV). The non-centrosymmetric Y3 F(SeO3 )4 can exhibit strong SHG intensity (5.5×KDP@1064 nm), wide band gap (5.03 eV), short UV cut-off edge (204 nm) and high thermal stability (690 °C). So, Y3 F(SeO3 )4 is a new UV NLO material with excellent comprehensive properties. Our work shows that it is an effective method to develop new UV NLO selenite material by fluorination control of the centrosymmetric compounds.

High-Performance Second-Harmonic-Generation (SHG) Materials: New Developments and New Strategies.

ConspectusSecond-harmonic-generation (SHG) causes the frequency doubling of light, which is very useful for generating high-energy lasers with specific wavelengths. Noncentrosymmetry (NCS) is the first requirement for an SHG process because the SHG coefficient is zero (χ2 = 0) in all centrosymmetric structures. At this stage, developing novel NCS crystals is a crucial scientific topic. Assembling polar units in an addictive fashion can facilely form NCS crystals with outstanding SHG performance. In this way, our group has obtained many different NCS crystals with extremely large SHG intensities (>5 × KDP or 1 × KTP). In this Account, we first provide a brief review of the development of SHG materials and concisely highlight the features of the excellent SHG materials. Then, we present four facile and rational molecular design strategies: (1) Traditional BO33--based crystals feature short absorption edges but usually suffer from relatively weak SHG performance (<5 × KDP). The combination of two types of pure π-conjugated anions (BO33- and NO3-) in a parallel fashion in the same compound has afforded a metal borate nitrate with a strong SHG effect. (2) To overcome the problems of the weak SHG effect and small birefringence in the less anisotropic QO4-based compounds, highly polarizable cations such as Hg2+ and Bi3+ are introduced into these systems, which greatly enhances both SHG effects and birefringence. (3) Iodate anions can be condensed into polynuclear iodate anions with a higher density of I5+ per unit cell, hence polyiodate anions can serve as excellent SHG-active groups. We developed a novel synthesis method for hydrothermal reactions under a phosphoric acid medium and obtained a series of metal polyiodates with strong SHG effects. In addition, as the number of iodate groups increases, the structural configuration of the polyiodate anion changes from linear to bent. (4) We introduce the concept of aliovalent substitution which features site-to-site atomic displacement at the structural level. Such aliovalent substitution led to new materials that have the same chemical stoichiometries or structural features as their parent compounds. Thus, aliovalent substitution can provide more experimental opportunities and afford new high-performance SHG materials. The introduction of a fluoride anion and the replacement of metal cations in the MO6 octahedron can result in new metal iodates with balanced properties including a large SHG effect, a wide band gap, and a high laser-induced damage threshold (LIDT) value. Finally, we briefly discuss several problems associated with the studies of SHG materials and give some prospects for SHG materials in the future.

Explorations of New SHG Materials in Mercury Iodate Sulfate System.

Through systematic experiments, two novel mercury iodate sulfates, namely, Hg2 (IO3 )2 (SO4 )(H2 O) and Hg2 (IO3 )2 (SO4 ) were obtained. They crystallize in monoclinic space group C2 and C2/c, respectively. Hg2 (IO3 )2 (SO4 )(H2 O) exhibits the [Hg(IO3 )]+ polar cationic layer inherited from Hg(IO3 )2 and the three-dimensional (3D) framework inherited from HgSO4 . This enables Hg2 (IO3 )2 (SO4 )(H2 O) to generate a strong second harmonic generation (SHG) response of 6 times that of KH2 PO4 (KDP). The structure of Hg2 (IO3 )2 (SO4 ) is very similar to that of Hg2 (IO3 )2 (SO4 )(H2 O), and they can be transformed into each other. Hg2 (IO3 )2 (SO4 )(H2 O) shows a large optical band gap of 3.98 eV and a high dehydration temperature of 250 °C. This study indicates that by reasonable design, the introduction of multiple functional groups into a compound may combine their advantages to achieve good overall optical performance.

Ba6Zn6(B3O6)6(B6O12): Barium Zinc Borate Contains π-Conjugated [B3O6]3- Anions and [B6O12]6- Anion with Edge-Sharing BO4 Tetrahedra.

A novel barium zinc borate contains π-conjugated [B3O6]3- anions and [B6O12]6- anion with edge-sharing BO4 tetrahedra, Ba6Zn6(B3O6)6(B6O12), has been successfully synthesized via a high-temperature solution reaction. In its structure, the isolated planar π-conjugated B3O6 groups are interconnected by ZnO4 tetrahedra via corner sharing to construct a [Zn3(B3O6)3]3- single layer parallel to the ab plane with the large Zn3B6 9-member rings. Two adjacent [Zn3(B3O6)3]3- single layers are interconnected by [B6O12]6- anions into a two-dimensional [Zn6(B3O6)6(B6O12)]12- double layer with 1D tunnels of Zn4B8 12-member rings along the a-axis. Neighboring such double layers are packed in an A-B-A-B... fashion along the c axis, and the Ba2+ ions act as counterbalance cations filling in the voids of double layers. All of the planar π-conjugated [B3O6]3- groups in Ba6Zn6(B3O6)6(B6O12) are in approximately parallel arrangement, producing large optical anisotropy and birefringence. The UV-vis-NIR absorption spectrum manifests that the UV cutoff edge for the title compound is below 200 nm. Ba6Zn6(B3O6)6(B6O12) possesses the largest birefringence (0.115@1064 nm) among the zincoborates reported. Its thermal stability, infrared spectrum, and theoretical calculations were also performed.

α- and ß-Ag4P2S7: Two Semiconductors with Promising Photocatalytic Hydrogen Production Based on a Density Functional Theory Study.

Herein, a new chiral compound with short Ag-Ag distances, namely, ß-Ag4P2S7 (P3121), has been discovered by a solid-state method. Density functional theory (DFT) calculations show that both α and ß phases exhibit suitable band gaps, low reduction potentials, and large visible-light absorption coefficients, as well as excellent band edges for carrier separation, suggesting their promising application in photocatalytic hydrogen production.

NaBa3[M2B7O16(OH)2]F2 (M = Ge, Si): Two Synthetic F Analogues of Garrelsite with [B7O16(OH)2]13- Polyanions and Deep-Ultraviolet Cutoff Edges.

Two new borogermanate/borosilicate fluorides, namely, NaBa3[M2B7O16(OH)2]F2 (M = Ge, Si), have been successfully synthesized through a conventional mild hydrothermal method. They represent the first examples of mixed alkali and alkaline-earth borogermanate/borosilicate fluorides. NaBa3[M2B7O16(OH)2]F2 (M = Ge, Si) crystallize in the space group of C2/c, and their structures feature a unique 3D anionic framework composed of [B7O16(OH)2]13- polyanions corner-sharing with SiO4 or GeO4 tetrahedra, forming 1D 10-membered-ring tunnels along the b axis, which are filled by Na+, Ba2+, and F- ions. UV-vis-near-IR absorption spectra identify the title compounds possessing short deep-ultraviolet absorption edges (below 200 nm), while their birefringences were calculated to be 0.021 and 0.016 at 1064 nm, respectively. Optical property, thermal stability, and theoretical calculations have also been conducted on NaBa3[M2B7O16(OH)2]F2 (M = Ge, Si).