Exploration into the Syntheses of Gallium- and Indiumborates under Extreme Conditions: M5 B12 O25 (OH): Structure, Luminescence, and Surprising Photocatalytic Properties

First-Principles High-Throughput Screening Combined with Experimental Synthesis to Find Vanadate Nonlinear Optical Material BiCa2VO6

Nonlinear optical (NLO) materials that can achieve frequency conversion and coherent light emission have garnered significant interest owing to their promising applications in laser technology and advanced photonic systems. In this study, the first-principles high-throughput screening pipeline (FHSP) was employed to systematically explore nonlinear optical materials within vanadate-based systems. A total of 204 vanadates were calculated in the Inorganic Crystal Structure Database (ICSD). Eventually, BiCa2VO6 was screened out and synthesized by a traditional high-temperature solid-state method. BiCa2VO6 exhibits an outstanding second harmonic generation (SHG) response (11.2 × KDP@1064 nm and 1.67 × AGS@2.09 μm), a wide band gap (3.25 eV), and a suitable birefringence (0.123@1064 nm). The work provides a complete perspective for vanadate as a nonlinear optical material and serves as a valuable reference for future research.

A Rare Inorganic-Organic Hybrid Polyoxoboroniobate Based on Pagoda-Shaped {LiB2Nb29O86} Clusters and {Cd2(cis-en)2(trans-en)} Complexes

A new inorganic-organic hybrid polyoxoboroniobate {H4K2[Cu(cis-en)2(H2O)]9[Cu(cis-en)2]6[Cd2(cis-en)2(trans-en)][LiB2Nb29O86]2·79H2O} (1, en = ethylenediamine), is built from pagoda-shaped {LiB2Nb29O86} clusters, linear {Cd2(cis-en)2(trans-en)} bridging units, and copper-amine complexes. The {LiB2Nb29O86} cluster represents the first example of combining oxoboron clusters with polyoxoniobate clusters (PONbs). It consists of an unusual HPONb fragment {LiNb18O54}, a fused-ring structural boroniobate cluster {B2Nb5O13}, and a classical Lindqvist {Nb6O19} fragment. The {Cd2(cis-en)2(trans-en)} and [Cu(cis-en)2]2+ complexes link the pagoda-shaped {LiB2Nb29O86} clusters into 1D infinite ladder chains. This is the first instance of simultaneous coordination of the cis-en and trans-en ligands with a single metal cation in the inorganic-organic hybrid PONb family. Furthermore, 1 exhibits good proton conductivity.

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.

A Mini Review on Borate Photocatalysts for Water Decomposition: Synthesis, Structure, and Further Challenges

The development of novel photocatalysts, both visible and UV-responsive, for water decomposition reactions is of great importance. Here we focused on the application of the borates as photocatalysts in water decomposition reactions, including water splitting reaction, hydrogen evolution half-reaction, and oxygen evolution half-reaction. In addition, the rates of photocatalytic hydrogen evolution and oxygen evolution by these borate photocatalysts in different water decomposition reactions were summarized. Further, the review summarized the synthetic chemistry and structural features of existing borate photocatalysts for water decomposition reactions. Synthetic chemistry mainly includes high-temperature solid-state method, sol-gel method, precipitation method, hydrothermal method, boric acid flux method, and high-pressure method. Next, we summarized the crystal structures of the borate photocatalysts, with a particular focus on the form of the B-O unit and metal-oxygen polyhedral in the borates, and used this to classify borate photocatalysts, which are rarely mentioned in the current photocatalysis literature. Finally, we analyzed the relationship between the structural features of the borate photocatalysts and photocatalytic performance to discuss the further challenges faced by the borate photocatalysts for water decomposition reactions.

A Lewis acid-base paired InBO3 catalyst: synthesis and high selectivity for isopropanol dehydrogenation

Whilst metal borates are well known as optical materials, their potential in solid catalysis has been less investigated. The calcite structured InBO₃ was selected as the target borate and was prepared using a solvothermal method. High-resolution transmission electron microscopy and powder X-ray diffraction prove that the material has a nanoparticle morphology with an average size ∼50 nm and high crystallinity. Intrinsic surface oxygen vacancies, which are beneficial to catalysis, were detected using X-ray photoelectron spectroscopy. Lewis acidity and basicity were both observed using NH₃-/CO₂-temperature-programmed desorption experiments, and the total acid and base amounts were found to be 46.6 and 123.8 μmol g^-1, respectively. Catalytic dehydration and dehydrogenation reactions for isopropanol at elevated temperatures were conducted in a fixed bed reactor to evaluate the catalytic performance of InBO₃. InBO₃ exhibits a high conversion rate (>90.5%) and, most importantly, a high dehydrogenation selectivity (acetone selectivity >92.5%), whilst the optimal acetone yield achieved was 121.3 mmol h^-1 g^-1_cat at 350 °C. This study on InBO₃ strongly suggests that metal borates have promising applications in heterogeneous catalysis.

Sc2F2(B2O5): a deep ultraviolet scandium borate fluoride exhibiting large birefringence induced by the synergistic effect of B2O5 and ScOnF2 groups

A rare earth borate fluoride, Sc2F2(B2O5), displays a deep ultraviolet cutoff edge (<200 nm) and very large birefringence induced by the synergistic effect of B2O5 and ScOnF2 groups.

M(B(SeO3)3)H2O (M = Al, Ga): the first boroselenites with a unique sandwich like double-layer structure

Exploration of new types of borates is important because of their promising applications in diverse fields. Two new boroselenites, namely, M(B(SeO₃)₃)H₂O (M = Al, Ga), which represent the first IIIA metal boroselenite, were synthesized by hydrothermal reactions. M(B(SeO₃)₃)H₂O (M = Al, Ga) possesses a unique sandwich like double-layer structure formed by two 2D [MSe₂O₈]^5- layers interconnected by 1D [BSeO₅]^3- chains. More interestingly, both compounds display large band gaps (4.86/4.79 eV) and moderate birefringences (Δn = 0.063/0.064 at 1064 nm) based on density functional theory (DFT) calculations.

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.

A Congruent-Melting Mid-Infrared Nonlinear Optical Vanadate Exhibiting Strong Second-Harmonic Generation

Study of mid-infrared (mid-IR) nonlinear optical (NLO) materials is hindered by the competing requirements of optimized second-harmonic generation (SHG) coefficient d_ij and laser-induced damage threshold (LIDT) as well as the harsh synthetic conditions. Herein, we report facile hydrothermal synthesis of a polar NLO vanadate Cs₄ V₈ O₂₂ (CVO) featuring a quasi-rigid honeycomb-layered structure with [VO₄ ] and [VO₅ ] polyhedra aligned parallel. CVO possesses a wide IR-transparent window, high LIDT, and congruent-melting behavior. It has very strong phase-matchable SHG intensities in metal vanadate family (12.0 × KDP @ 1064 nm and 2.2 × AGS @ 2100 nm). First-principles calculations suggest that the exceptional SHG responses of CVO largely originate from virtual electronic transitions within [V₄ O₁₁ ]_∞ layer; the excellent optical transmittance of CVO arises from the special characteristics of vibrational phonons resulting from the layered structure.

Bi2[B2(SeO3)6]: A Metal Boroselenite with a Unique Zero-Dimensional [B2(SeO3)6]6- Anionic Group and Large Birefringence

Explorations of new types of borates are important because of their promising application in diverse fields. A new bismuth-containing boroselenite, Bi₂[B₂(SeO₃)₆], has been obtained through high-temperature solid-state reaction in a closed system. Bi₂[B₂(SeO₃)₆] possesses a zero-dimensional [B₂(SeO₃)₆]^6- anionic group that does not belong to any types of reported boroselenites. Besides, Bi₂[B₂(SeO₃)₆] is the first boroselenite with lone-pair electrons containing a metal ion as the countercation. More interestingly, on the basis of the first-principles calculations, this compound displays a large birefringence (0.090) at 1064 nm.

High-pressure synthesis and crystal structure of HP-Al2B3O7(OH)

Orthorhombic HP-Al2B3O7(OH) was synthesized in a Walker-type multianvil apparatus under high-pressure/high-temperature conditions of 12.4 GPa and 1200 °C, respectively. Its structure is isotypic to that of Ga2B3O7(OH) and has been determined via single-crystal X-ray diffraction at room temperature. HP-Al2B3O7(OH) crystallizes in the space group Cmce (Z = 8) with the lattice parameters a = 10.3124(4), b = 7.3313(3), c = 10.4801(5) Å, and V = 792.33(6) Å3. The compound has also been characterized by IR and Raman spectroscopy.

Al5B12O25(OH) and Ga4InB12O25(OH) – two additional triel borates with the structure type M 5B12O25(OH) (M = Ga, In)

The isotypic triel borates Al5B12O25(OH) and Ga4InB12O25(OH) were synthesized in a Walker-type multianvil apparatus under high-pressure/high-temperature conditions of 12.0 GPa/1400 °C and 12.3 GPa/1200 °C, respectively. The crystal structures of both compounds, determined by single-crystal X-ray diffraction, constitute new representatives of the structure type M 5B12O25(OH) (M = Ga, In) crystallizing in the space group I41/acd. The presence of the hydroxyl groups was confirmed via single-crystal IR spectroscopy.

Sr5TeO2(BO3)4 and NaSr5(BO3)(SiO4)2: two inorganic metal borate derivatives with diverse zero dimensional anions

Two new inorganic metal borate derivatives, Sr₅TeO₂(BO₃)₄ and NaSr₅(BO₃)(SiO₄)₂, have been obtained through a high temperature solid reaction method. Sr₅TeO₂(BO₃)₄ crystallizes in a tetragonal space group of P4/mnc featuring a three-dimensional (3D) structure composed of a porous 3D strontium framework stabilized by the unique zero-dimensional (0D) [TeO₂(BO₃)₄]^10- anions. NaSr₅(BO₃)(SiO₄)₂ crystallizes in a monoclinic space group of P2₁/c with a 3D structure consisting of a splendid strontium 3D framework strengthened by isolated BO₃ and SiO₄ groups. The sodium cations located in the empty space of the structure. Thermal stability analyses, optical property measurements and theoretical studies of the compounds were performed. Refractive index calculations revealed that the birefringence of Sr₅TeO₂(BO₃)₄ and NaSr₅(BO₃)(SiO₄)₂ is 0.048 and 0.029, respectively, at 1064 nm.

High-pressure synthesis and crystal structure of the mixed-cation borate Ga4In4B15O33(OH)3

The mixed cation triel borate Ga4In4B15O33(OH)3 was synthesized in a Walker-type multianvil apparatus at high-pressure/high-temperature conditions of 12.5 GPa and 1300°C. Although the product could not be reproduced in further experiments, its crystal structure could be reliably determined via single-crystal X-ray diffraction data. Ga4In4B15O33(OH)3 crystallizes in the tetragonal space group I41/a (origin choice 2) with the lattice parameters a = 11.382(2), c = 15.244(2) Å, and V = 1974.9(4) Å3. The structure of the quaternary triel borate consists of a complex network of BO4 tetrahedra, edge-sharing InO6 octahedra in dinuclear units, and very dense edge-sharing GaO6 octahedra in tetranuclear units.

(NH4)InB8O14 – a high-pressure borate combining BO3 groups with corner- and edge-sharing BO4 tetrahedra

The first ammonium indium borate (NH4)InB8O14 was synthesized from indium oxide and boric acid under high-pressure/high-temperature conditions of 9 GPa and 1000°C with a Walker-type multianvil apparatus in a BN crucible. Although a reproduction of this synthesis product failed until now, we were able to determine the crystal structure via single-crystal X-ray diffraction data collected at room temperature. (NH4)InB8O14 crystallizes in the monoclinic space group P2/m (Z=2) with a=4.4053(2), b=7.8184(3), c=12.4685(4) Å, β=94.60(1)°, and V=428.06(3) Å3. The structure comprises all three basic structural units of borates, namely corner-sharing BO4 tetrahedra, edge-sharing BO4 tetrahedra, and planar BO3 groups. Large cavities in the borate network give space to disordered, isolated ammonium ions. The presence of nitrogen (confirmed by EDX analysis) appears to originate from the container material.

Exploration into the Syntheses of Gallium- and Indiumborates under Extreme Conditions: M5 B12 O25 (OH): Structure, Luminescence, and Surprising Photocatalytic Properties

Explorative solid-state chemistry led to the discovery of the two new compounds Ga₅ B₁₂ O₂₅ (OH) and In₅ B₁₂ O₂₅ (OH). Extreme synthetic conditions within the range of 12 GPa and a temperature of 1450 °C realized in a Walker-type multianvil apparatus resulted in the formation of an unprecedented tetragonal structure with the exclusive presence of condensed BO₄ tetrahedra, forming cuboctahedral cavities. Doping of these cavities with Eu³⁺ in In₅ B₁₂ O₂₅ (OH) yielded in an orange-red luminescence. Photocatalytic tests of In₅ B₁₂ O₂₅ (OH) revealed a hydrogen production rate comparable to TiO₂ but completely co-catalyst free.