Pb 2 BO 3 I: A Borate Iodide with the Largest Second‐Harmonic Generation (SHG) Response in the KBe 2 BO 3 F 2 (KBBF) Family of Nonlinear Optical (NLO) Materials

Strong SHG Responses in a Beryllium-Free Deep-UV-Transparent Hydroxyborate via Covalent Bond Modification

Deep-ultraviolet (deep-UV) nonlinear optical (NLO) crystals are key materials in creating tunable deep-UV lasers for frequency conversion technology. However, practical application of the sole usable crystal, KBe₂ BO₃ F₂ , has been hindered by the high toxicity of beryllium and its layering tendency in crystal growth. Herein, we report a beryllium-free deep-UV NLO material NaSr₃ (OH)(B₉ O₁₆ )[B(OH)₄ ] (NSBOH), synthesized by a covalent bond modification strategy under hydrothermal conditions. Moisture-stable NSBOH exhibits strong second-harmonic generation (SHG) at 1064 nm (3.3 × KH₂ PO₄ ) and 532 nm (0.55 × β-BaB₂ O₄ ), both amongst the largest powder SHG responses for a deep-UV borate, with good phase-matchability and a short wavelength cutoff edge (below 190 nm). NSBOH possesses a 3D covalent anionic [B₉ O₁₉ ]_∞ honeycomb-like framework with no layering. The Sr²⁺ and Na⁺ ions, residing in the cavities of the anionic framework, act as templates for the assembly and favorable alignment of NLO-active groups, resulting in an optimal balance between strong SHG activities and wide UV transparency. These merits indicate NSBOH is a very attractive candidate for deep-UV NLO applications.

Synthesis, Crystal Structure, and Nonlinear Optical Property of an Anhydrous Sulfate KTb(SO4)2

A new alkali metal-rare earth metal anhydrous sulfate, KTb(SO₄)₂ (KTSO), was synthesized by a hydrothermal method. It crystallized in the non-centrosymmetric (NCS) orthorhombic space group P2₁2₁2₁ with cell parameters a = 5.43950(10), b = 8.8579(3), c = 13.3962(4), Z = 2. Its structure is composed of [KO₁₁], [TbO₈], and [SO₄] groups, which are interconnected to constitute a one-dimensional (1D) chain, and then further linked to construct a three-dimensional (3D) network structure through [SO₄] units. The results of the powder frequency doubling test show that KTb(SO₄)₂ has a phase-matching second-order nonlinear optical effect of 0.3 times that of KH₂PO₄ (KDP). The measured birefringence of the crystal (0.01) agrees well with the calculated value (0.019). Meanwhile, its infrared and ultraviolet spectrum and thermal properties were studied, respectively. Its optical properties and band gap structure are studied by theoretical calculation. This work provides a NCS structure of anhydrous sulfates and offers guidance for further exploration of new anhydrous sulfates.

Ba2.5Pb1.5B12O22: structural transformation from a centrosymmetric to a noncentrosymmetric space group by introducing Pb into Ba2B6O11

A new congruent-melting lead barium borate, Ba_2.5Pb_1.5B₁₂O₂₂, was synthesized via a high-temperature solid-state reaction under atmospheric pressure. It crystallizes in the noncentrosymmetric space group Cmc2₁ with a unit cell of a = 19.051(8) Å, b = 10.726(3) Å, c = 8.612(3) Å, and Z = 4. It features a new functional building block (FBB), [B₁₂O₂₆]^16-, that is made up of five [B₃O₈]^7- and one [BO₃]^3- connected by sharing vertex O atoms. It is derived by introducing Pb into Ba₂B₆O₁₁, which leads to a structural transformation from a centrosymmetric to a noncentrosymmetric space group. Its second harmonic generation response (SHG) is approximately 0.9 times that of KDP at the fundamental wavelength of 1064 nm and is also phase-matchable. It has a wide transparent region with a UV cutoff edge of about 250 nm according to the reflectance spectra and high thermal stability, which illustrates that it is a potential candidate for ultraviolet nonlinear optical materials.

Linear and Nonlinear Optical Properties of Centrosymmetric Sb4O5SO4 and Noncentrosymmetric Sb4O4(SO4)(OH)2 Induced by Lone Pair Stereoactivity

Introducing stereochemically active lone-pair Sb³⁺ cations into sulfates, two three-dimensional (3D) antimony-sulfates, Sb₄O₅SO₄ (1) and Sb₄O(SO₄)(OH)₂ (2), were achieved under moderate hydrothermal conditions. Both structures are constructed by tetranuclear-{Sb₄}-clusters-based layers and SO₄ tetrahedra. However, owing to the different packing patterns of the layers, they display different characteristics: 1 exhibits a centrosymmetric structure while 2 possesses a noncentrosymmetric structure. UV-vis spectra show that they possess wide band gaps. Sb₄O(SO₄)(OH)₂ is nonlinear optical (NLO) active with a second-harmonic generation (SHG) response of ∼1.2 times of KH₂PO₄, together with the phase-matchable capacity, endowing it a promising UV NLO material. The first-principle calculations were performed to elucidate the structure-property relationships. The results indicate that the lone pair stereoactivity of Sb³⁺ provides the large contribution to the macroscopic SHG effect.

KCs2 [Pb2 Br5 (HCOO)2 ]: A Polar 3D Lead-Bromide Framework Exhibiting Strong Second-Harmonic Generation Response

The discovery of new nonlinear optical (NLO) crystals with excellent properties is in urgently demand because of their ability to generate coherent light. Herein, we report an unique NLO lead bromide formate, KCs₂ [Pb₂ Br₅ (HCOO)₂ ], which has been synthesized by a mix-solvothermal method. KCs₂ [Pb₂ Br₅ (HCOO)₂ ] exhibits strong phase-matching second-harmonic generation (SHG) response (6.5×KDP), large birefringence (0.16@ 1064 nm), and a wide transparent window in most visible light and mid-IR region. Interestingly, KCs₂ [Pb₂ Br₅ (HCOO)₂ ] features a polar 3D lead-bromide framework in which adjacent Pb-Br layers containing coplanar Pb₆ Br₆ rings are not only parallel to each other, but also orient in the same direction. These oriented arrangements are responsible for the strong SHG response and large birefringence that are elucidated by both local dipole moment and theoretical calculations. This research provides a new strategy to explore subsequent NLO crystals.

Strain-Driven Polarized Electric Field-Promoted Photocatalytic Activity in Borate-Based CsCdBO3 Bulk Materials

Piezoelectrically polarized electric field can provide a strong driving force for the separation of the photoinduced charge carriers that has attracted a wide attention in the field of photocatalysis. In this paper, a new type of piezoelectric borate material CsCdBO₃ exhibits a high efficiency for the degradation of typical organic pollutants under the synergistic effects of strain and light conditions. The oxidation rate constant of the synergistic effect is 0.653 min^-1, which is 3.77 times that of just under visible light irradiation. Further, the material shows a higher efficiency when treated both under the clockwise stirring direction and a high stirring speed. A characteristic piezoresponse hysteresis loop was detected using the piezoresponse force microscopy (PFM) approach. The strain-driven polarized electric field facilitates to promote the photoinduced electron-hole pair separation, thus enhancing the photocatalytic activity. The present work provides a new direction of the borate with a noncentrosymmetric structure in the environmental remediation.

Two Tellurium(IV)-Based Sulfates Exhibiting Strong Second Harmonic Generation and Moderate Birefringence as Promising Ultraviolet Nonlinear Optical Materials

Two tellurium(IV)-based sulfate nonlinear optical (NLO) materials, Te₂O₃(SO₄) and Te(OH)₃(SO₄)·H₃O, were successfully synthesized via the mild hydrothermal method. Te₂O₃SO₄ has a two-dimensional (2D) structure consisting of [Te₆O₁₂]_∞ layers as well as [SO₄] groups. Te(OH)₃(SO₄)·H₃O features a simple 0D structure made up of an isolated [TeO₃] pyramid and a [SO₄] tetrahedra. Both of them are phase-matching materials and show remarkable powder second harmonic generation (SHG) efficiencies about 6 and 3 times that of KH₂PO₄ (KDP), respectively, for Te₂O₃SO₄ and Te(OH)₃(SO₄)·H₃O. Especially for Te(OH)₃(SO₄)·H₃O, in addition to a large SHG response, it possesses a short UV cutoff edge (∼233 nm) as well as moderate birefringence (0.052@546.1 nm). Furthermore, theoretical calculations confirmed that their strong SHG effects are due to the synergistic effect of the [TeO₃] pyramid and [SO₄] tetrahedra.

Sr2Pb(BeB5O10)(BO3): An Excellent Ultraviolet Nonlinear-Optical Beryllium Borate by the Pb-Modified Construction of a Conjugated System and Lone-Pair Effect

The design of material by chemical and/or crystalline modification of a classic structure model benefits not only the optimized physical properties but also the controllability and efficiency. Herein, a new nonlinear-optical (NLO) beryllium borate crystal, Sr₂Pb(BeB₅O₁₀)(BO₃) (SPBBO), is successfully designed and synthesized by chemical and crystalline modification of the perovskite-like K₃B₆O₁₀Cl NLO crystal. SPBBO displays a 3D BeB₅O₁₀^3- open-framework structure composed of interconnecting BeB₅O₁₃ groups with filled cationic Sr/Pb and anionic BO₃ groups, which exhibits the striking enhancement of the second-harmonic-generation (SHG) response (8 × KDP) and birefringence (0.10) compared to the parent model. Replacement of K by Sr and Pb with a lone pair and replacement of Cl by conjugated BO₃ result in the synergistic conjugation of Pb with host BeB₅O₁₀^3- and filled BO₃ groups, contributing to the striking enhancement of the SHG and birefringence. Single-crystal measurements show that SPBBO has a short UV absorption edge of 280 nm with a wide energy band gap of 4.35 eV and an outstanding laser-induced resistant behavior with a remarkably high laser-induced damage threshold of 2100 MW cm^-2. The excellent properties indicate that the SPBBO crystal is a very promising UV NLO functional material.

Tin Chloride Sulfates A3Sn2(SO4)3-xCl1+2x (A = K, Rb, Cs; x = 0, 1) as Multifunctional Optical Materials

The series of alkali-metal tin chloride sulfates A₃Sn₂(SO₄)_3-xCl_1+2x (A = K, Rb, Cs; x = 0, 1), K₃Sn₂(SO₄)₃Cl, Rb₃Sn₂(SO₄)₂Cl₃, and Cs₃Sn₂(SO₄)₂Cl₃, were successfully synthesized through an improved mild hydrothermal method. Interestingly, in addition to the cation size effect, the structure-directing effect of anions induces different symmetries in the three title compounds, with K₃Sn₂(SO₄)₃Cl being noncentrosymmetric, while Rb₃Sn₂(SO₄)₂Cl₃ and Cs₃Sn₂(SO₄)₂Cl₃ are centrosymmetric. Powder second-harmonic generation (SHG) measurements indicate that K₃Sn₂(SO₄)₃Cl is a nonlinear optical material that is type I phase matchable with a weak SHG response (0.1× KDP). Photoluminescence tests reveal that the three title compounds emit strong greenish yellow, orange, and salmon light, respectively, under UV excitation, indicating that they are promising inorganic solid fluorescent materials. Simultaneously, a detailed structural analysis of all the known tin(II) halide sulfates has been performed, which will guide the systematic exploration of high-performance tin(II)-based functional materials in the future.

Centrosymmetric RbSnF2NO3vs. noncentrosymmetric Rb2SbF3(NO3)2

A new UV nonlinear optical fluoride nitrate Rb₂SbF₃(NO₃)₂ exhibits a strong SHG response of 2.7 times that of benchmark KDP and an appropriate birefringence of 0.06@1064 nm.

KNa2ZrF7: A Mixed-Metal Fluoride Exhibits Phase-Matchable Second-Harmonic-Generation Effect and High Laser-Induced Damage Threshold

Halides are one type of important IR nonlinear-optical material candidate. Here, a mixed alkali metal and d⁰ transition-metal fluoride, namely, KNa₂ZrF₇, was obtained by a facile hydrothermal method. It crystallizes in an orthorhombic system with the polar space group Pmn2₁, and its pseudo-1D structure features isolated (ZrF₇)^3- moncocapped trigonal prisms, which are ionically linked together by countercations K⁺ and Na⁺, representing a new type of fluoride. The powder sample of KNa₂ZrF₇ exhibits a moderate second-harmonic-generation response and a high laser-induced damage threshold. Besides, it can realize phase matchability and possesses a wide-IR transparent window. Thermal stability analysis suggests that KNa₂ZrF₇ is a congruent compound. Structural comparisons with related fluorides and theoretical calculations are also presented in this work.

The first bismuth borate oxyiodide, Bi4BO7I: commensurate or incommensurate?

The first bismuth borate oxyiodide, Bi₄BO₇I, has been prepared by solid-state reaction in evacuated silica ampoules. Its crystal structure [space group Immm(00γ)000] comprises litharge-related layers of edge-sharing OBi₄ tetrahedra; the interlayer space is filled by I^- and [BO₃]^3- anions. The wavevector, q = 0.242 (3)c*, is very close to the rational value of c*/4, yet refinement based on commensurate modulation faces serious problems indicating the incommensurate nature of the modulation. The I-/[BO₃]^3- anions are ordered in a complex sequence along [001], i.e. -<-BO₃-BO₃-I-I->_n = 28-I-I-I-<-BO₃-BO₃-I-I->_{n = 28}-BO₃-BO₃-BO₃-, leading to a structural modulation. The principal feature of the latter is the presence of -I-I-I- and -BO₃-BO₃-BO₃- sequences that cannot be accounted for in the a × b × 4c supercell. The thermal expansion of Bi₄BO₇I is weakly anisotropic (α_a = 8, α_b = 15 and α_c = 17 × 10^-6 K^-1 at 500 K) which is caused by preferential orientation of the borate groups.

RbB3O4F2: a rubidium fluorooxoborate with an unprecedented [B3O5F2]3- functionalized unit and a large birefringence

A new rubidium fluorooxoborate, RbB₃O₄F₂, was structurally designed and synthesized by using the strategy of fluorine-introduction into borates. Interestingly, RbB₃O₄F₂ presents the novel [B₃O₄F₂]^- chains formed by the unprecedented [B₃O₅F₂]^3- fundamental building block. More importantly, RbB₃O₄F₂ has a large birefringence and a deep-UV cutoff edge. The finding of novel [B₃O₅F₂]^3- FBB and [B₃O₄F₂]^- chains in RbB₃O₄F₂ greatly enriches the structural diversity of fluorooxoborates.

Nonlinear-Optical Crystal Rb3YB6O12 with Condensed B5O10 Blocks That Exhibits an Intriguing Structural Arrangement and a Short Ultraviolet Absorption Edge

Nonlinear-optical (NLO) crystals, which can regulate the laser wavelength through a cascading second-harmonic-generation technique, have been widely utilized in the field of optoelectronics. In this work, we grew the NLO borate crystal Rb₃YB₆O₁₂ (RYBO) using the spontaneous crystallization method. RYBO crystallizes in a chiral trigonal space group of R32 with a new type of structural arrangement built from Y-O short chains and B₅O₁₀ groups. It is significantly different from the known structure of chemical analogues Rb₃REB₆O₁₂ (RE = Nd, Eu) not only in the halved unit cell parameter but also in the Y-O connection manner. The NLO response of RYBO is about 0.8KDP, 8-fold larger than that of KB₅O₈·4H₂O with the same B₅O₁₀ groups because of the coexistence of two NLO-active units of the distorted YO₆ octahedra and B₅O₁₀ anions. Thanks to the short ultraviolet (UV) cutoff, RYBO may have potential NLO applications in the UV and even deep-UV spectral regions.

CsHgNO3Cl2: A New Nitrate UV Birefringent Material Exhibiting an Optimized Layered Structure

Developing effective strategies to design novel, excellent birefringent materials has become an emerging challenge. Herein, we report a new UV nitrate birefringent material, CsHgNO₃Cl₂, which is an analogue of the known UV carbonate nonlinear optical crystal KCaCO₃F. In CsHgNO₃Cl₂, a perfect layered structure was produced owing to the alliance of a planar π-conjugated anion NO₃^- and highly electropolarized Hg²⁺ cation. The optimized layered structure that is beneficial in enlarging the optical anisotropy induced a large birefringence (Δn = 0.145@546 nm) for CsHgNO₃Cl₂, even superior to that of the well-known commercial birefringent material α-BBO in the UV region. The strategy that the optimization of planar functional group arrangement is able to boost birefringence is expected to guide the development of high-performance birefringent materials in the future, especially in the field of UV.

Rational Design of the Metal-Free KBe2 BO3 F2 ⋅(KBBF) Family Member C(NH2 )3 SO3 F with Ultraviolet Optical Nonlinearity

The first fluorosulfonic ultraviolet (UV) nonlinear optical (NLO) material, C(NH₂ )₃ SO₃ F, is rationally designed by taking KBe₂ BO₃ F₂ (KBBF) as the parent compound. C(NH₂ )₃ SO₃ F features similar topological layers as KBBF by replacing inorganic (BO₃ )^3- with organic C(NH₂ )₃ ⁺ trigonal units and BeO₃ F with SO₃ F^- tetrahedra. Therefore, C(NH₂ )₃ SO₃ F is a metal-free UV NLO crystal. Benefiting from the coplanar configuration of the C(NH₂ )₃ ⁺ cationic groups, it possesses a large SHG response of 5×KDP and moderate birefringence of 0.133@1064 nm. Besides, it has a short UV cutoff edge of 200 nm. The calculated results reveal the shortest SHG phase-matching wavelengths can reach 200 nm. These findings highlight the exploration of metal-free compounds as nontoxic and low-cost UV NLO materials as a new research area.

[O2Pb3]2(BO3)I: a new lead borate iodide with [O2Pb3] double chains

A new lead borate iodide, [O2Pb3]2(BO3)I, has been successfully synthesized by a facile hydrothermal reaction. It exhibits an infinite one-dimensional (1D) 1∞[O2Pb3] double chain which is constructed from OPb4 oxygen-centered tetrahedra. The 1∞[O2Pb3] double chains are further bridged by BO3 units via sharing oxygen atoms to form two-dimensional (2D) 2∞[(O2Pb3)2(BO3)] layers with the I- ions situated in the spaces between the layers. First principles calculations indicate that [O2Pb3]2(BO3)I exhibits a birefringence of about 0.072@1064 nm which originates from the synergistic contributions of the Pb-O/I and BO3 units. The IR, UV-vis-NIR and EDS results, as well as TG-DSC curves were also discussed in detail.

Resource-Efficient Low-Temperature Synthesis of Microcrystalline Pb2 B5 O9 X (X = Cl, Br) for Surfaces Studies by Optical Second Harmonic Generation

Optically nonlinear Pb₂ B₅ O₉ X (X = Cl, Br) borate halides are an important group of materials for second harmonic generation (SHG). Additionally, they also possess excellent photocatalytic activity and stability in the process of dechlorination of chlorophenols, which are typical persistent organic pollutants. It would be of great interest to conduct in situ (photo-) catalysis investigations during the whole photocatalytic process by SHG when considering them as photocatalytic materials. In order to get superior photocatalytic efficiency and maximum surface information, small particles are highly desired. Here, a low-cost and fast synthesis route that allows growing microcrystalline optically nonlinear Pb₂ B₅ O₉ X borate halides at large quantities is introduced. When applying the ionothermal growth process at temperatures between 130 and 170 °C, microcrystallites with an average size of about 1 µm precipitate with an orthorhombic hilgardite-like borate halide structure. Thorough examinations using powder X-ray diffraction and scanning electron microscopy, the Pb₂ B₅ O₉ X microcrystals are indicated to be chemically pure and single-phased. Besides, the Pb₂ B₅ O₉ X borate halides' SHG efficiencies are confirmed using confocal SHG microscopy. The low-temperature synthesis route thus makes these borate halides a highly desirable material for surface studies such as monitoring chemical reactions with picosecond time resolution and in situ (photo-) catalysis investigations.

A6Sb4F12(SO4)3 (A = Rb, Cs): Two Novel Antimony Fluoride Sulfates with Unique Crown-like Clusters

Two new antimony fluoride sulfates named A₆Sb₄F₁₂(SO₄)₃ (A = Rb, Cs) with unique crown-like clusters have been successfully synthesized by introducing Sb³⁺ with a stereochemically active lone pair in the sulfate system through a facile hydrothermal method. The structure regulation induced by the cation sizes results in the symmetry difference in these two title compounds, i.e., Rb₆Sb₄F₁₂(SO₄)₃ crystallizing in the polar noncentrosymmetric (NCS) space group of P3 and Cs₆Sb₄F₁₂(SO₄)₃ in the centrosymmetric (CS) space group of P1̅. Detailed characterizations, including XRD, thermal behaviors, optical properties, and the theoretical calculations, for these two compounds have been performed. The discovery of the unique crown-like clusters observed in A₆Sb₄F₁₂(SO₄)₃ (A = Rb, Cs) enriches the diversity of structures for antimony sulfates systems. Simultaneously, a summary and comparison for all reported antimony halide sulfates has been made to illustrate the synergistic effect of the stoichiometric ratio and cation sizes on the macroscopic centricity and the framework structure, which will guide the systematical discovery of antimony(III)-based functional materials in future.

Hydrogen-Bond-Assisted Reinforcement of Interlayer Connections in Zn2BO3X·H2O (X = Cl, Br): Two UV Nonlinear Optical Crystals with KBBF-Type Structure

Two novel zinc borate halides named Zn₂BO₃X·H₂O (X = Cl, Br) were discovered through hydrothermal techniques. Both compounds are isomorphic and feature the layered structure similar to that of KBBF, consisting of the infinite planar [Zn₂BO₃X·OH₂] (X = Cl, Br) layers. Compared with the weak ionic bond between adjacent layers in KBBF, the strong interaction of hydrogen bonds between layers in Zn₂BO₃X·H₂O (X = Cl, Br) effectively enhances the interlayer force, contributing to eliminating the layering growth tendency. Optical measurements on these two NLO crystals revealed that they have a broad wavelength transparency window and a moderate NLO coefficient. Moreover, the theoretical calculations revealed that the linear and NLO properties primarily depended on the [BO₃] and [ZnO₃Cl/Br] groups in both crystals.

Novel ultraviolet (UV) nonlinear optical (NLO) materials discovered by chemical substitution-oriented design

Exploring novel functional materials via chemical substitution-oriented design is an emerging strategy. The method can be expanded to the discovery of high performance ultraviolet (UV) nonlinear optical (NLO) solid state materials by a careful tuning of the substituted atoms. This minireview presents a brief introduction to chemical substitution-oriented design including single-site substitution, dual-site substitution, and multisite substitution. Several state-of-the-art UV NLO materials such as K₃VO(O₂)₂CO₃-type, KBe₂BO₃F₂ (KBBF)-type, Ca₅(PO₄)₃(OH)-type, and KTiOPO₄ (KTP)-type phases successfully discovered by the chemical substitution method are discussed.

A brief introduction to chemical substitution-oriented design to discover high performance ultraviolet (UV) nonlinear optical (NLO) solid state materials is presented.

Pb6O5(NO3)2: A Nonlinear Optical Oxynitrate Structurally Based on Lead Oxide Framework

A high second harmonic generation response is demonstrated by a Pb₆O₅(NO₃)₂ lead oxynitrate whose identity was verified upon reinvestigation of the PbO-Pb(NO₃)₂ system. Its crystal structure exhibits a unique cationic [Pb₆O₅]²⁺ framework hosting orientationally ordered NO₃^- triangles in the channels. Easy preparation and high thermal stability (until ∼500 °C in air) suggest it to be a new promising NLO material.

Gd(NO3)(Se2O5)·3H2O: a nitrate-selenite nonlinear optical material with a short ultraviolet cutoff edge

The first example of a rare-earth metal nitrate-selenite nonlinear optical (NLO) crystal, Gd(NO₃)(Se₂O₅)·3H₂O (1), was synthesized by hydrothermal reaction. This compound crystallizes in the noncentrosymmetric space group P2₁2₁2₁, and its structure features two-dimensional (2D) [Gd(NO₃)(Se₂O₅)]_∞ layers, extending to a pseudo-3D framework linked by hydrogen bonds. 1 exhibits a phase-matchable SHG efficiency about 0.2 times that of KH₂PO₄ (KDP) and a large laser damage threshold of 236.26 MW cm^-2. The UV-Vis-NIR diffuse reflectance spectroscopy study shows that the UV cutoff edge of 1 is 224 nm, which is the shortest value observed among selenite-based NLO materials. The birefringence of 1 is 0.109@1064 nm based on density functional theory calculations. These optical properties make 1 a potential UV NLO material. Theoretical studies using density functional theory have been implemented to further understand the relationship between its optical properties and band structures.

Synthesis, characterization and crystal structure of a new mixed alkali and alkaline-earth metal borate Rb9Ba24(BO3)19

Rb₉Ba₂₄(BO₃)₁₉ features an intricate 3D Ba–O framework with three types of infinite channels where the Rb⁺ cations and isolated BO₃ units are located.

A new iodate-phosphate Pb2(IO3)(PO4) achieving great improvement in birefringence activated by (IO3)− groups

The first divalent-metal iodate-phosphate, Pb₂(IO₃)(PO₄), has been prepared, showing great improvement in birefringence because of the highly anisotropic (IO₃)⁻ groups.

Li3CaB2O5F: a unique sandwich-like structure with diverse and wide Li ion diffusion pathways

A new borate fluoride Li₃CaB₂O₅F has been synthesized.

Two-stage evolution from phosphate to sulfate of new KTP-type family members as UV nonlinear optical materials through chemical cosubstitution-oriented design

KTiOPO4 (KTP) is a classic commercial nonlinear optical (NLO) crystal, but its narrow bandgap (3.52 eV) prevents its practical application in the ultraviolet (UV) region. Many trials to widen the narrow bandgap of KTP have failed in the past few decades. A chemical cosubstitution strategy was implemented to design new members of the KTP-type family as potential UV NLO materials. First, a novel centrosymmetric KTP-type compound NH4SbFPO4·H2O with a sharply enlarged bandgap (5.01 eV) was obtained through three-site aliovalent substitution. Second, the noncentrosymmetric NH4SbF2SO4 was synthesized by the introduction of more F- anions to destroy the crystal symmetry and SO42- to replace PO43- for balancing the charge in NH4SbFPO4·H2O, which realized the transformation from a visible phosphate system to solar blind UV sulfate system for KTP-type family NLO materials. The preliminary experimental results indicated that NH4SbF2SO4 is a promising solar blind UV NLO material. The first-principles calculations revealed that the sharply enlarged bandgap resulted from the substitution of the transition metal cations with the main group metal cations and the introduction of F- anions with high electronegativity.

Inorganic planar π-conjugated groups in nonlinear optical crystals: review and outlook

Nonlinear optical crystals with various inorganic planar π-conjugated groups are reviewed based on their bandgaps.

Nontoxic KBBF Family Member Zn2BO3(OH): Balance between Beneficial Layered Structure and Layer Tendency

The conflict between beneficial layered structure for performances and layered growth habits in KBe2BO3F2 (KBBF) always restricts its practical applications. A beryllium-free KBBF family member, Zn2BO3(OH), is explored to feature the same topological layer with KBBF by replacing [BeO3F]5- with [ZnO3(OH)]5- and excellent UV performances. It exhibits a second harmonic generation response of about 1.5 × KH2PO4 with the UV cutoff edge of 204 nm. The birefringence of Zn2BO3(OH) in the visible region is about 0.067, which is larger than those of commercial UV crystals LiB3O5, CsB3O5, and CsLiB6O10. Additionally, it has excellent thermal and water-resistant stabilities. Owing to the removal of interlayer cations, Zn2BO3(OH) shows better growth habits than KBBF while achieving the balance between beneficial layered structure and layer tendency.

[Ln6O8] Cluster-Encapsulating Polyplumbites as New Polyoxometalate Members and Record Inorganic Anion-Exchange Materials for ReO4 - Sequestration

Various types of polyoxometalates (POMs) have been synthesized since the 19th century, but their assortment has been mostly limited to Groups 5 and 6 metals. Herein, a new family of POMs composed of a carbon group element as the addenda atoms with two distinct phases, LnPbOClO4-1 (Ln = Sm to Ho, Y) and LnPbOClO4-2 (Ln = Er and Tm) is reported. Both structures are built from [Ln6O8] rare-earth metal hexamers being incorporated in [Pb18O32]/[Pb12O24] polyplumbites, and unbound perchlorates as charge-balancing anions. Impressively, YPbOClO4-1 and ErPbOClO4-2 exhibit exceptional uptake capacities (434.7 and 427.7 mg g-1) toward ReO4 -, a chemical surrogate for the key radioactive fission product in the nuclear fuel cycle 99TcO4 -, which are the highest values among all inorganic anion-exchange materials reported until now. The sorption mechanism is clearly elucidated and visualized by single-crystal-to-single-crystal structural transformation from ErPbOClO4-2 to a perrhenate-containing complex ErPbOReO4 , revealing a unique ReO4 - uptake selectivity driven by specific interaction within Pb···O-ReO3 - bonds.

Research and Development of Zincoborates: Crystal Growth, Structural Chemistry and Physicochemical Properties

Borates have been regarded as a rich source of functional materials due to their diverse structures and wide applications. Therein, zincobrates have aroused intensive interest owing to the effective structural and functional regulation effects of the strong-bonded zinc cations. In recent decades, numerous zincoborates with special crystal structures were obtained, such as Cs₃Zn₆B₉O₂₁ and AZn₂BO₃X₂ (A = Na, K, Rb, NH₄; X = Cl, Br) series with KBe₂BO₃F₂-type layered structures were designed via substituting Be with Zn atoms, providing a feasible strategy to design promising non-linear optical materials; KZnB₃O₆ and Ba₄Na₂Zn₄(B₃O₆)₂(B₁₂O₂₄) with novel edge-sharing [BO₄]^5- tetrahedra were obtained under atmospheric pressure conditions, indicating that extreme conditions such as high pressure are not essential to obtain edge-sharing [BO₄]^5--containing borates; Ba₄K₂Zn₅(B₃O₆)₃(B₉O₁₉) and Ba₂KZn₃(B₃O₆)(B₆O₁₃) comprise two kinds of isolated polyborate anionic groups in one borate structure, which is rarely found in borates. Besides, many zincoborates emerged with particular physicochemical properties; for instance, Bi₂ZnOB₂O₆ and BaZnBO₃F are promising non-linear optical (NLO) materials; Zn₄B₆O₁₃ and KZnB₃O₆ possess anomalous thermal expansion properties, etc. In this review, the synthesis, crystal structure features and properties of representative zincoborates are summarized, which could provide significant guidance for the exploration and design of new zincoborates with special structures and excellent performance.