BaB2P2O8F2: A Fluoroborophosphate with [B2P2O8F2]∞ Layers and Deep-Ultraviolet Cutoff Edge

A fluoroborophosphate, BaB₂P₂O₈F₂, was successfully obtained. Its structure contains a novel [B₂P₂O₈F₂]_∞ layer containing six-membered rings, which is formed by the fundamental building block composed of three types of non-π-conjugated groups, [PO₄], [BO₄], and [BO₂F₂]. BaB₂P₂O₈F₂ has a deep-ultraviolet (DUV) cutoff edge (λ < 200 nm) and a tiny birefringence (Δn = 0.007 at 532 nm), which originates from the constituent non-π-conjugated groups. The title compound enriches the versatility of the fluoroborophosphates, encouraging further research into DUV materials in fluoroborophosphate systems.

RbMo2P3O14 with large birefringence mainly induced by highly distorted [MoO6] in uncommon [Mo2P3O14]∞ layers

The introduction of d⁰ transition metal Mo⁶⁺ cations into a phosphate generates a new acentric molybdophosphate, RbMo₂P₃O₁₄. It shows uncommon [Mo₂P₃O₁₄]_∞ layers composed of isolated [MoO₆] octahedra and [P₃O₁₀] groups. To the best of our acknowledge, it exhibits the largest birefringence (a calculated value of 0.166 at 546 nm) among reported molybdophosphates. In addition, it also possesses a shorter UV cut-off edge (about 250 nm) than other molybdates and molybdophosphates, indicating that it can be used as a birefringent crystal in the UV optical region. First-principles electronic structure analysis suggests that the large birefringence mainly originates from highly distorted [MoO₆].

BaB4O5F4 with reversible phase transition featuring unprecedented fundamental building blocks of [B16O21F16] in the α-phase and [B4O6F4] in the β-phase

The first fluorooxoborate with reversible phase transition, BaB4O5F4, has been obtained. Interestingly, the two polymorphs, α- and β-BaB4O5F4 with rare [BOF3], feature various one-dimensional chains composed of the unprecedented fundamental building blocks of [B16O21F16] and [B4O6F4], respectively. First-principles calculations were performed to elucidate the structure-property relationships.

From centrosymmetric to noncentrosymmetric: effect of the cation on the crystal structures and birefringence values of (NH4)n-2AE(PO2F2)n (AE = Mg, Sr and Ba; n = 2, 3 and 4)

Fluorine-containing compounds significantly enrich the diversity of inorganic chemical structures. Here, four alkaline-earth metal difluorophosphates with different space groups, namely NH4Mg(PO2F2)3 (Cmcm) (1), NH4Sr(PO2F2)3 (P1[combining macron]) (2), (NH4)2Ba(PO2F2)4 (P2/n) (3) and Ba(PO2F2)2 (I4[combining macron]2d) (4), were synthesized and their structures were determined for the first time. 4 is not only the first alkaline-earth-metal difluorophosphate, but also the first difluorophosphate reported to crystallize in the non-centrosymmetric space group I4[combining macron]2d of the tetragonal crystal system. All of the three-dimensional (3D) structures of the title compounds consist of isolated (PO2F2)-, AE-O and NH4+ units, while the differences of the cation radius result in various arrangements of the (PO2F2)- units, which leads to diverse crystal structures and the disparate optical anisotropy of the crystals. According to the deep-ultraviolet (DUV) transmittance spectrum, 3 exhibits a short DUV cut off edge (<180 nm). Solid state 19F and 31P magic-angle spinning NMR spectroscopy was used to verify the presence of covalent P-F bonds in 3. The different size effects, as well as the coordination environments of the Mg2+, Sr2+, and Ba2+ cations, on the structures as a whole have been discussed in detail. In addition, the calculated birefringence values were observed to differ from 0.011 to 0.033 for the four compounds.

Influence of structure-directing polyhedra and heterocyclic ligands on the chain structures and O/F ordering in a series of zinc vanadium oxyfluorides

Transition metal oxyfluorides composed of distinctive structure-directing vanadium oxyfluoride polyhedra and highly polarizable Zn²⁺ cations were synthesized by mild hydrothermal reactions.

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.

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.

K4(PO2F2)2(S2O7): first fluorooxophosphorsulfate with mixed-anion [S2O7]2- and [PO2F2]- groups

Mixed-anion compounds are among the most promising systems to design functional materials with enhanced properties. Among the phosphate-sulfate species, the [SO4]2- and [PO2F2]- tetrahedra are known and give rise to structural versatility. However, to date, the crystal structures of phosphate-sulfates with the coexistence of two distinct anion groups ([S2O7]2- and [PO2F2]-) in one compound are unknown. Here, a novel type of fluorooxophosphorsulfate, K4(PO2F2)2(S2O7) (KSPOF), is designed and synthesized via a high-temperature method in a closed system. The crystal structure is derived from single-crystal X-ray diffraction (C2/c, a = 13.000(10) Å, b = 7.5430(10) Å, c = 19.010(10) Å, β = 130.070(10)°, and Z = 4). It is the first fluorooxophosphorsulfate with mixed-anion building units ([S2O7]2- and [PO2F2]-) and a unique drum-like cluster was found, which enriches the diversity of structures for fluorooxophosphorsulfate systems. Furthermore, the theoretical calculations indicate that KSPOF possesses moderate birefringence, which mainly originates from the distorted [PO2F2]- tetrahedron.

Ba3B10O17F2·0.1KF: the first mixed alkali/alkaline-earth metal fluorooxoborate with unprecedented double-layered B–O/F anionic arrangement

The first mixed alkali/alkaline-earth metal fluorooxoborate Ba₃B₁₀O₁₇F₂·0.1KF with a unique double-layered B–O/F anionic framework was successfully synthesized and systemically characterized.