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

Bi(SO4)F·H2O and Bi(SO4)(NO3)·3H2O: Chemical Substitution-Induced Birefringence Enhancement in Bismuth Sulfates

Two new Bi(III)-based sulfates, namely, Bi(SO4)F·H2O (BSOF) and Bi(SO4)(NO3)·3H2O (BSNO), have been successfully synthesized through aliovalent replacement of partial [SO4]2- groups with F- and [NO3]- anions, respectively, in the parent structure of Bi2(SO4)3. Such chemical replacement altered the coordination environment of Bi3+ cations, facilitating changes in the structure and optical properties. Notably, the birefringence values of BSOF and BSNO are found to be 4.4 and 15.5 times that of parent Bi2(SO4)3. Further investigation into the structure-property relationship revealed that the birefringence enhancement in BSOF and BSNO is attributed to the improvement of the polarizability anisotropy of Bi3+-centered polyhedra in BSOF and BSNO compared to that of Bi2(SO4)3. In addition, the existence and optimized arrangement of planar [NO3]- groups are also indispensable for further birefringence improvement of the BSNO compound.

AX·(H2SeO3)n (A = K, Cs; X = Cl, Br; n = 1, 2): A Series of Ionic Cocrystals as Promising UV Birefringent Materials with Large Birefringence and Wide Band Gap

The design and syntheses of new birefringent crystals will be of great importance in commercial applications and materials science. A series of ultraviolet (UV) birefringent crystals, AX·(H2SeO3)n (A = K, Cs; X = Cl, Br; n = 1, 2), with large sizes up to 23 × 6 × 3 mm3, was successfully synthesized by simple aqueous solution method. These four compounds belong to three different space groups. Isomorphic KCl·(H2SeO3)2 and CsCl·(H2SeO3)2 crystallize in the P 1 ¯ space group, while CsBr·(H2SeO3)2 and CsCl·H2SeO3 crystallize in the P21/m and P21/c space groups, respectively. They exhibit cocrystal structures composed of [2(H2SeO3)]∞ and [AX]∞ frameworks, ingeniously inheriting the large optical anisotropy of selenite and the wide band gap of alkali metal halide. And it proves that these compounds indeed possess large birefringence (0.1-0.17 at 532 nm) and short UV cutoff edges (227-239 nm), achieving a balance of optical properties. This research affords a simple and viable strategy for the design and syntheses of new UV birefringent crystals. Besides, it is also found that the n value and ionic size (A and X ions) have important influences on the crystal structures and optical properties of AX·(H2SeO3)n. And this will promote further understanding of the alkali metal halide selenite family.

RbCl·(H2SeO3)2: A Salt-Inclusion Selenite Featuring Short UV Cut-Off Edge and Large Birefringence

Birefringent materials are key components to control the light polarization in laser science and technology as well as optical communication. However, the performance of current commercial birefringent materials has been limited by the magnitude of birefringence, optical transparency range, or the attainability of large-scale single crystals. To explore new birefringent materials, we strategically incorporated a lone pair cation (Se⁴⁺) with large optical anisotropy, an alkali metal, and halogen ions (Rb⁺ and Cl^-) with superior UV transparent capacity; thus a new compound, RbCl·(H₂SeO₃)₂, was successfully discovered with the aid of the facile hydrothermal method. Interestingly, Rb-Cl chains locate in the [H₂SeO₃]_∞ skeleton, which makes RbCl·(H₂SeO₃)₂ a salt-inclusion selenite. Millimeter-sized single crystals (up to 4 × 2 × 1 mm³) were obtained, and the transmittance spectrum revealed that its UV cut-off edge can be as low as 230 nm. In addition, the calculated birefringence of RbCl·(H₂SeO₃)₂ is 0.14 at 589 nm that is similar to the birefringent value of famous α-BaB₂O₄. Wide UV transparency, large birefringence, and feasible crystal growth make RbCl·(H₂SeO₃)₂ a new member of birefringent materials for UV light applications.

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).

Cation Regulation to Investigate the Chalcogenide Borate RE6Nb2MgSB8O26 (RE = La-Nd) Family

Chalcogenide borates have been developed and are considered an attractive system due to their favorable physical properties such as magnetism and nonlinear optical effects. Here, isostructural RE₆Nb₂MgSB₈O₂₆ (RE = La-Nd) compounds in the title family have been obtained through cation regulation in rare-earth and VB group metals. This family crystalizes in the centrosymmetric P3̅ space group and features 3D frameworks formed by {[Mg(NbB₄O₁₃)₂]^16-}_∞ polyanionic layers and QRE₆ octahedra. The structural chemistry was characterized and theoretical calculations were performed to understand the structural merit of this family. In addition, RE₆Nb₂MgSB₈O₂₆ possess the largest band gaps among known rare-earth chalcogenide borates, and they all show antiferromagnetic-like behaviors.

Layered Perovskite-like Nitrate Cs2Pb(NO3)2Br2 as a Multifunctional Optical Material

A novel alkali metal lead halide nitrate, Cs₂Pb(NO₃)₂Br₂, has been successfully synthesized via a hydrothermal method. Interestingly, the title compound features a distinctive Ruddlesden-Popper perovskite-like layered structure, which induces the outstanding multifunctional optical properties, including a large birefringence (0.147@546 nm) and broad light-orange emission. Detailed structural analysis and theoretical calculations revealed that the large birefringence originates from the p-π interaction between the Pb²⁺ cations and NO₃ groups and that the excellent luminescence properties derive from the distortion of PbO₄Br₄ polyhedra. This work not only enriches the variant structure types of layered perovskites but also guides the further exploration of all-inorganic multifunctional optical materials.

HgTeO2F(OH): A Nonlinear Optical Oxyfluoride Constructed of Active [TeO2F(OH)]2- Pyramids and V-Shaped [HgO2]2- Groups

Oxyhalides possessing the merits of oxides and halides have widely received attention for their comprehensive physical performances, especially as potential nonlinear optical (NLO) crystals. Here, based on conventional strategy for obtaining acentric compounds, a Te⁴⁺ lone-pair cation was introduced into oxyhalides, and one oxyfluoride, HgTeO₂F(OH), was obtained via a hydrothermal reaction. Crystallized in the polar space group Pca2₁, the layered structure of HgTeO₂F(OH) is composed of V-shaped [HgO₂]^2- groups and [TeO₂F(OH)]^2- pyramids, in which the [TeO₂F(OH)]^2- pyramid first served as the NLO functional motif. Its powder sample exhibits a phase-matchable SHG response of 1.1 × KH₂PO₄ at 1064 nm, and its birefringence (0.09@1064 nm) is sufficient for phase-matchable behavior, which manifests its comprehensive capacity as a promising NLO candidate. Theoretical calculations about electronic structure and optical properties are also carried out, revealing that the Te⁴⁺ lone-pair cation makes the predominant contribution to the SHG effect and synergizes with the [HgO₂]^2- groups.

Noncentrosymmetric Ba6In2Q10 (Q = S, Se): Structural Chemistry and Nonlinear-Optical Activity

The indispensable condition for laser frequency-doubling crystal materials is that they crystallize in the noncentrosymmetric (NCS) structures. Here, ternary NCS Ba₆In₂S₁₀ (1) and Ba₆In₂Se₁₀ (2) (P6₃) were synthesized via conventional solid-state reactions. Their zero-dimensional structures feature isolated InQ₄ tetrahedra and Q₂^2- (Q = S, Se) dimers that are separated by Ba²⁺ counter cations. The structural relevance and differences with the centrosymmetric Ba₁₂In₄S₁₉ and Ba₁₂In₄Se₂₀ are analyzed in detail. Both 1 and 2 show obvious laser frequency-doubling activity, and their optical band gaps are 3.72 and 1.78 eV, respectively. This work provides a new type of IR nonlinear-optical materials.

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.

Tin(II)-Induced Large Birefringence Enhancement in Metal Phosphates

Two alkali tin(II) phosphates, namely, Rb[SnF(HPO₄)] and Rb(Sn₃O)₂(PO₄)₃, were synthesized through mild hydrothermal methods. They belong to the orthorhombic Pnma and Pbcn space groups, respectively. Rb[SnF(HPO₄)] features a layered structure based on 1D [SnF(HPO₄)]_∞ chains interconnected by hydrogen bonds, with Rb⁺ cations located at the interlayer space. For Rb(Sn₃O)₂(PO₄)₃, each pair of [Sn₃O]⁴⁺ clusters is bridged by a pair of [P(1)O₄]^3- tetrahedra to build a 1D [Sn-P-O]_∞ chain. These 1D [Sn-P-O]_∞ chains are further cross-linked though [P(2)O₄]^3- tetrahedra to construct a 3D network with 7- and 10-membered-ring channels. The tin(II) ions in Rb[SnF(HPO₄)] and Rb(Sn₃O)₂(PO₄)₃ with stereochemically active lone pairs (SCALPs) significantly enhance the birefringences of metal phosphates: Δn = 0.147@1064 nm for Rb[SnF(HPO₄)] and 0.082@1064 nm for Rb(Sn₃O)₂(PO₄)₃.