New antimony fluorooxoborates with strong birefringence and unprecedented structural characterisation

Fluorooxoborates constitute a rich source of optical crystals due to their structural diversity and excellent performance. Antimony fluorooxoborates with stereochemically active lone pairs of electrons still have not been found, although the first antimony borate was discovered several years ago. In this study, we have achieved the successful synthesis of the first antimony(III) fluorooxoborate with an unprecedented [B2O4F]∞ chain, namely SbB2O4F. Remarkably, SbB2O4F shows strong birefringence (0.171@1064 nm) and short UV cutoff edges (about 220 nm) according to calculations. The birefringence of SbB2O4F mainly originates from the highly distorted [SbO4] groups.

K2RbB8PO16: A Borophosphate with Moderate Birefringence and Deep-Ultraviolet Transmission

A new borophosphate, K2RbB8PO16 (KRBPO) was synthesized. It exhibits a bilayer structure consisting of two B-O layers with an 18-membered ring (18-MR) joined by [PO4], which is composed of the π-conjugated group [BO3] and non-π-conjugated groups [BO4] and [PO4]. The UV-vis-NIR diffuse reflectance spectroscopy shows that the cutoff edge is less than 200 nm. The calculation indicates that KRBPO exhibits moderate birefringence of 0.057@1064 nm, and the source of birefringence is mainly from the [BO3] groups.

Cd2(IO3)(PO4) and Cd1.62Mg0.38(IO3)(PO4): metal iodate-phosphates with large SHG responses and wide band gaps

The first NLO-active metal iodate-phosphates, namely, Cd₂(IO₃)(PO₄) and Cd_1.62Mg_0.38(IO₃)(PO₄) (1 and 2), with three types of NLO groups, have been reported. 1 and 2 are isostructural and the structure of 1 features a 3D network formed by the Cd₄(IO₃)_8/4(PO₄)_6/3 groups. 1 and 2 with strong SHG signals of 4 × and 3.5 × KH₂PO₄ are promising SHG materials in the visible region.

Two Indium Iodate-Nitrates with Large Birefringence Induced by Hybrid Anionic Functional Groups and Their Favorable Arrangements

Two new indium iodate-nitrates, In(IO₃)₂(NO₃) (1) and [In(IO₃)(OH)(H₂O)](NO₃) (2), were rationally designed through the integration of hybrid anionic functional units. They exhibit large birefringences (1, 0.269; 2, 0.188, at 532 nm) and wide band gaps (1, 4.08 eV; 2, 4.39 eV), which is attributed to the synergistic effect of two types of birefringence-active units, namely, lone-pair IO₃ and π-conjugated NO₃ anionic groups. Through the substitution of OH and H₂O of 2 with IO₃, the hydrogen bonds of 2 are eliminated and the birefringence of 1 is greatly enhanced, highlighting the intriguing role of isovalent substitution in the discovery of fascinating optical materials.

A new I3O93− group constructed from IO3− and IO55− anion units in Cs3[Ga2O(I3O9)(IO3)4(HIO3)]

Cs3[Ga2O(I3O9)(IO3)4(HIO3)] with a novel I3O93− fundamental building block (FBB) constituted by two IO3 and one IO5 polyhedra exhibited a wide band gap.

KRE(CO3)2 (RE = Eu, Gd, Tb): new mixed metal carbonates with strong photoluminescence and large birefringence

Three new potassium rare earth carbonates KRE(CO3)2 (RE = Eu, Gd, Tb) with strong photoluminescence and large birefringence were synthesized successfully.

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.

Na7(IO3)(SO4)3: the first noncentrosymmetric alkaline-metal iodate-sulfate with isolated [IO3] and [SO4] units

Metal iodates are of current research interest because of their potential application as nonlinear optical crystals but the exploration of new iodates is mainly concentrated on complex cation iodates. In contrast, iodates with multiple anion groups are rarely reported. In this communication, the first noncentrosymmetric alkaline-metal iodate-sulfate, Na₇(IO₃)(SO₄)₃ has been designed, synthesized and characterized.

NaGaI3O9F: a new alkali metal gallium iodate combined with IO3- and IO3F2- units

Birefringent crystals are very important optical materials, which are widely used in the fields of optics and communication. In this work, we reported a new iodate, NaGaI₃O₉F, synthesized by mild hydrothermal method. NaGaI₃O₉F crystallized in the monoclinic space group P2₁/c (No. 14) and it featured a novel _∞[Ga₂(IO₃F)₂(IO₃)₄]^2- layer stacked with Na⁺ cations located in the void maintaining charge balance. Notably, IO₃^- and IO₃F^2- anionic units appeared in the same time in the A-Ga-I-O/F (A = alkali metal) system. According to the experimental characterization and theoretical calculations, NaGaI₃O₉F showed a wide bandgap (4.27 eV) and large birefringence (Δn_exp∼ 0.203, Δn_cal = 0.197 at 1064 nm), indicating its potential use as a birefringent material.

SrTi(IO3)6·2H2O and SrSn(IO3)6: distinct arrangements of lone pair electrons leading to large birefringences

Three new iodates SrTi(IO₃)₆·2H₂O, (H₃O)₂Ti(IO₃)₆, and SrSn(IO₃)₆ have been synthesized via a facile hydrothermal method. The three compounds have zero-dimensional crystal structures composed of one [MO₆]⁸⁻ (M = Ti, Sn) octahedron connected with six [IO₃]⁻ trigonal pyramids. However, the particular coordination of Sr²⁺ cations results in distinct arrangements of lone pair electrons in an [IO₃]⁻ trigonal pyramid, which leads to large birefringences. More importantly, this work enriches the species crystal chemistry for [M(IO₃)₆]²⁻ (M = Ti, Sn) clusters-containing iodates.

The distinct arrangements of [IO₃]⁻ trigonal pyramids lead to larger birefringences in SrTi(IO₃)₆·2H₂O and SrSn(IO₃)₆ than that in (H₃O)₂Ti(IO₃)₆.

First chiral fluorinated lead vanadate selenite Pb2(V2O4F)(VO2)(SeO3)3 with five asymmetric motifs and large optical properties

The first fluorinated lead vanadate selenite Pb2(V2O4F)(VO2)(SeO3)3 (PVOFS) was successfully synthesized via a mild hydrothermal method. This compound crystallizes in the chiral space group P212121 of the orthorhombic system and it is the first noncentrosymmetric structure in the PbII-VV-SeIV-O-F system. PVOFS is composed of five kinds of second-order Jahn-Teller susceptible asymmetric motifs, including three distinct types of vanadium-centered polyhedral units ([VO5F], [VO6] and [VO5]), [SeO3] pyramids and Pb2+ cations. It features a unique three-dimensional open framework structure displaying three types of tunnels (10-, 8- and 7-membered rings), which enriches the structural diversity for fluorinated vanadate selenite systems. Optical property studies revealed that PVOFS shows a second-harmonic generation response of 0.3 times that of the commercial KH2PO4 with phase matching behavior, a wide transparent region covering IR windows, an optical band gap of 2.35 eV, a high laser damage threshold of 61 times that of AgGaS2, and a large birefringence of 0.105 at 1064 nm. Theoretical calculations have been performed to clarify the correlation between the molecular structure and the optical properties of PVOFS.

Bi4O(I3O10)(IO3)3(SeO4): trimeric condensation of IO43- monomers into the I3O105- polymeric anion observed in a three-component mixed-anion NLO material

A novel mixed-anion NLO crystal, namely, Bi4O(I3O10)(IO3)3(SeO4), containing a brand-new "pinwheel-like" I3O105- polyiodate anion group was constructed from the trimeric condensation of IO43- monomers. Bi4O(I3O10)(IO3)3(SeO4) exhibits a complex 3D topological structure and gives a moderate second harmonic generation (SHG) signal about 1.1 times that of KH2PO4 (KDP).

From Ag2 Zr(IO3 )6 to LaZr(IO3 )5 F2 : A Case of Constructing Wide-band-gap Birefringent Materials through Chemical Cosubstitution

Two mixed-metal zirconium iodates were prepared and studied as a case of chemical cosubstitution. The structure of Ag₂ Zr(IO₃ )₆ (P2₁ /c) features 0D [Zr(IO₃ )₆ ]^2- anion group and 1D [Ag(IO₃ )₂ ]^- anionic chain, with the [Zr(IO₃ )₆ ]^2- anion groups interconnected by Ag⁺ ions into 3D network; LaZr(IO₃ )₅ F₂ (P2₁ /n) features 0D [Zr(IO₃ )₅ F₂ ]^3- anion group and 2D [La(IO₃ )₅ ]^2- anionic layer, with the [Zr(IO₃ )₅ F₂ ]^3- groups interlinked by La³⁺ ions into 3D structure. Notably, LaZr(IO₃ )₅ F₂ is the first zirconium iodate fluoride reported. Wide optical band gaps of 3.77 and 4.13 eV are given for Ag₂ Zr(IO₃ )₆ and LaZr(IO₃ )₅ F₂ , respectively. Theoretical calculations confirmed that the weak d-d transition of Zr⁴⁺ in the band structure leads to a moderate band gap of Ag₂ Zr(IO₃ )₆ , and the introduction of F^- into the zirconium iodate compound results in a large band gap of LaZr(IO₃ )₅ F₂ . Both of the compounds are birefringent materials with birefringences of 0.064 @1064 nm and 0.082 @1064 nm, respectively.

An Exceptional Peroxide Birefringent Material Resulting from d-π Interactions

Birefringent materials, which can modulate the polarization of light, are almost exclusively limited to oxides. Peroxides have long been overlooked as birefringent materials, because they are usually not stable in air. Now, the first peroxide birefringent material Rb₂ VO(O₂ )₂ F is reported, the single crystals of which keep transparency after being exposed in the air for two weeks. Interestingly, Rb₂ VO(O₂ )₂ F does not feature an optimal anisotropic structure, but its birefringence (Δn=0.189 at 546 nm) exceeds those of the majority of oxides. According to the first-principles calculations, this exceptional birefringence should be attributed to the strong electronic interactions between localized π orbital of O₂ ^2- anions and V⁵⁺ 3d orbitals, which may be also favorable to the stability in the air for Rb₂ VO(O₂ )₂ F. These findings distinguish peroxides as a brand-new class of birefringent materials that may possess birefringence superior to the traditional oxides.

Two new ammonium/alkali-rare earth metal difluorophosphates ALa(PO2F2)4 (A = NH4 and K) with moderate birefringence and short cutoff edges

Herein, two new ammonium/alkali-rare-earth metal fluorophosphates ALa(PO2F2)4 (A = NH4 and K) have been successfully obtained via a facile route. The introduction of F- anions with high electronegativity and non-π-conjugated species [PO2F2]- was found in the title compounds. Different from the [PO4]3- unit in phosphates, the (PO2F2)- group retains the merit of wide UV transmittance in phosphates; meanwhile, it has large polarizability anisotropy, and theoretical calculation shows that the calculated birefringences are 0.023 and 0.019 for KLa(PO2F2)4 and NH4La(PO2F2)4, respectively. More importantly, this work will contribute to the structural and functional diversity of phosphate chemistry by the exploration of the fascinating difluorophosphates.

From BiF3 to BiF3·H2O: diverse Bi(iii) coordination for structural transformation and birefringence regulation

BiF₃·H₂O with unprecedented BiF₉ building blocks was synthesized, which exhibits a near-isotropic Bi³⁺ coordination and inert lone pair activity.