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

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

Na1.5Cs0.5[Al{BO3}{B9O15(OH)3}1/3]: An Acentric Layered Aluminoborate with Nonlinear-Optical Properties

An acentric aluminoborate (ABO), Na1.5Cs0.5[Al{BO3}{B9O15(OH)3}1/3] (1), has been made under solvothermal conditions. The alternations of AlO4 tetrahedra and BO3 triangles first build up the 2D ABO layer with 6-MR (membered-ring) windows. The fanlike B9O15(OH)3 cluster adorns the layer through Al-O-B linkages, producing a pair of unclosed channels with 7-MR aperture on the lateral side of the layers. The novel B9O15(OH)3 cluster represents the largest oxoboron cluster in the acentric ABO family. 1 not only shows a moderate second-harmonic-generation response of 1.2 times that of potassium dihydrogen phosphate but also has a short cutoff edge below 200 nm, indicating its potential applications in deep-UV regions.

Introduction of the [B-O/F] Units Enhances the Band Gap and Birefringence from Na6Mg3B10O18F6 to K3NaB10O16F2

The borate family is the main source of deep-ultraviolet (DUV) birefringent crystals, and it has attracted a lot of attention due to versatile [B-O] basic units. Herein, two new borate-based compounds Na₆Mg₃B₁₀O₁₈F₆ and K₃NaB₁₀O₁₆F₂ were discovered. Their fundamental building blocks are [B₅O₁₁] and [B₅O₁₀F] units, respectively. The calculated results showed that the band gap and birefringence of K₃NaB₁₀O₁₆F₂ (E_g = 6.93 eV, Δn = 0.047 at 1064 nm) are greater than those of Na₆Mg₃B₁₀O₁₈F₆ (E_g = 5.40 eV, Δn = 0.039 at 1064 nm). Furthermore, the effects of [B-O/F] units on band gap and birefringence were analyzed by the charge-transfer model and response electron distribution anisotropy method. The results show that introducing the [B-O/F] units can improve the band gap and birefringence. These findings will boost the exploration of DUV birefringent opticals.

Three Polyborates with High-Symmetry [B12O24] Units Featuring Different Dimensions of Anion Groups

Three polyborates, namely, LiNa₁₁B₂₈O₄₈, Li_1.45Na_7.55B₂₁O₃₆, and Li₂Na₄Ca₇Sr₂B₁₃O₂₇F₉, were synthesized via the high-temperature solution method. All of them feature high-symmetry [B₁₂O₂₄] units, yet their anion groups exhibit distinct dimensions. LiNa₁₁B₂₈O₄₈ features a three-dimensional anionic structure of ³[B₂₈O₄₈]_∞ framework, which is composed of three units: [B₁₂O₂₄], [B₁₅O₃₀], and [BO₃]. Li_1.45Na_7.55B₂₁O₃₆ possesses a one-dimensional anionic structure of ¹[B₂₁O₃₆]_∞ chain consisting of [B₁₂O₂₄] and [B₉O₁₈] units. The anionic structure of Li₂Na₄Ca₇Sr₂B₁₃O₂₇F₉ is composed of two zero-dimensional isolated units, namely, [B₁₂O₂₄] and [BO₃]. The novel FBBs [B₁₅O₃₀] and [B₂₁O₃₉] are present in LiNa₁₁B₂₈O₄₈ and Li_1.45Na_7.55B₂₁O₃₆, respectively. The anionic groups in these compounds exhibit a high degree of polymerization, thereby augmenting the structural diversity of borates. And the crystal structure, synthesis, thermal stability, and optical properties were meticulously discussed to guide the synthesis and characterization of novel polyborates.

Mixed Rare-Earth Chalcogenide Borate Eu9-xRExMgS2B20O41 (RE = Sm, Gd) Featuring a 3D {[B20O41]22-}∞ Framework Connected by [B6O9(O0.5)6]6- and [B7O13(O0.5)3]8- Clusters

Rare-earth (RE) chalcogenide borates are very rarely discovered in view of the difficulties in synthesis though they have demonstrated attractive physical performances. Here, the first mixed RE chalcogenide borates Eu_5.4Sm_3.6MgS₂B₂₀O₄₁ (1) and Eu₃Gd₆MgS₂B₂₀O₄₁ (2) are synthesized by combining RE, sulfur, and borate ions into one structure. They crystallize in the centrosymmetric hexagonal space group P6₃/m, and their 3D honeycomb-like {[B₂₀O₄₁]^22-}_∞ open frameworks are built by [B₆O₉(O_0.5)₆]^6- and [B₇O₁₃(O_0.5)₃]^8- polyanionic clusters and consolidated by Mg²⁺ ions; both of which are formed by BO₄ tetrahedra and BO₃ planar triangles. The coordination modes of RE ions are rare REO₆S₂ bicapped trigonal prisms and REO₈S irregular polyhedra, and their band gaps are determined to be 2.25 and 2.22 eV, respectively. They exhibit antiferromagnetic interactions and distinct photocurrent responses. The corresponding theoretical calculations are also performed. The study of 1 and 2 perhaps stimulates interest in exploring new functional RE chalcogenide borates.

Rational Design of the First Ammonium Magnesium Borate with Deep-Ultraviolet Cutoff Edge and Moderate Birefringence and Further Investigation into the Nature of Ammonium in the Borate System

Through the rational design of the experimental method, the first combination of ammonium and magnesium in the borate system was successfully achieved. In this paper, a case of ammonium magnesium borate, (NH₄)₂{Mg(H₂O)₂[B₆O₇(OH)₆]₂}·2H₂O, was successfully synthesized by a mild hydrothermal method at a relatively low temperature. A brief review was performed to show the participation of NH₄⁺ in the recent development of optical materials. By discussing the optimum synthesis method of ammonium-containing borates and the main factors affecting the dimensionality of B-O anionic groups in their structures, the design strategy for synthesizing ammonium-containing borate and adjusting its structure has been put forward. Relevant experimental measurement results and the first-principles calculation results show that the title compound has a deep-UV cutoff edge (<200 nm) and moderate birefringence (Δn_cal. = 0.064 @546 nm), which indicates its potential application in the deep-UV region.