Designing Three Fluorooxoborates with a Wide Transmittance Window by Anionic Group Substitution

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.

NH4IO2F2 and (NH4)3(IO2F2)3·H2O: A Series of Ammonium-Containing Fluoroiodates with Wide Band Gaps

A series of ammonium-containing fluoroiodates, NH₄IO₂F₂ and (NH₄)₃(IO₂F₂)₃·H₂O, with isolated [IO₂F₂] units have been fabricated by a fluorine-oxygen substitution strategy from NH₄IO₃. The two compounds crystallize in the orthorhombic system, but in different space groups, noncentrosymmetric Pca2₁ for NH₄IO₂F₂ and centrosymmetric Pnma for (NH₄)₃(IO₂F₂)₃·H₂O, and show wide band gaps of 4.53 eV for (NH₄IO₂F₂) and 4.55 eV for ((NH₄)₃(IO₂F₂)₃·H₂O). In addition, NH₄IO₂F₂ exhibits a 1.2 × KDP second harmonic generation response, a short ultraviolet cutoff edge in iodates, and a good crystal growth habit. The crystal of NH₄IO₂F₂ with a size of 11 × 5 × 2 mm³ was obtained by the aqueous solution method. The results enrich the structural diversity of iodate and supply a greater understanding of the design of new functional materials based on the fluoroiodates.

(NH4)3B11PO19F3: a deep-UV nonlinear optical crystal with unique [B5PO10F]∞ layers

Deep-ultraviolet (DUV) nonlinear optical (NLO) crystals that can extend the output range of coherent light below 200 nm are pivotal materials for solid-state lasers. To date, KBe₂BO₃F₂ (KBBF) is the only usable crystal that can generate DUV coherent light by direct second harmonic generation (SHG), but the layered growth habit and toxic ingredients limit its application. Herein, we report a new fluoroborophosphate, (NH₄)₃B₁₁PO₁₉F₃ (ABPF), containing four different functional units: [BO₃], [BO₄], [BO₃F] and [PO₄]. ABPF exhibits a KBBF-like structure while eliminating the limitations of KBBF crystal. The unique [B₅PO₁₀F]_∞ layers enhance ABPF’s performance; for example, it has a large SHG response (1.2 × KDP) and a sufficient birefringence (0.088 at 1064 nm) that enables the shortest phase-matching wavelength to reach the DUV region. Meanwhile, the introduction of strong B-O-P covalent bonds decreases the layered growth habit. These findings will enrich the structural chemistry of fluoroborophosphate and contribute to the discovery of more excellent DUV NLO crystals.

Variable dimensionality of the anion framework in four new borophosphates and fluoroborophosphates with short cutoff edges

Two new borophosphates, Cs₃B₃P₄O₁₆ and Li(NH₄)₂B₃P₄O₁₆, and two new fluoroborophosphates, K₄B₂P₂O₉F₂ and Rb₃B₂PO₅F₄, have been obtained via the high-temperature solution method. Single-crystal X-ray diffraction indicates that all of them exhibit various anion frameworks although they crystallize in the same space group, P2₁/c. Two-dimensional (2D) [B₃P₄O₁₆]_∞ layers and a 3D [B₃P₄O₁₆]_∞ network can be found in Cs₃B₃P₄O₁₆ and Li(NH₄)₂B₃P₄O₁₆, respectively, while isolated [B₂P₂O₉F₂] and [B₂PO₅F₄] exist in K₄B₂P₂O₉F₂ and Rb₃B₂PO₅F₄, respectively. The effect of the alkali metal cation size on the framework structures of Cs₃B₃P₄O₁₆ and Li(NH₄)₂B₃P₄O₁₆ has been discussed in detail. The IR spectra confirm their structural validity. UV-Vis-NIR diffuse reflectance spectroscopy indicates that the new compounds exhibit short cutoff edges. In addition, theoretical calculations were carried out to understand their electronic structures and optical properties.

NaKB6O9F2: a new complex alkali metal fluorooxoborate with puckered layers

Introducing F atoms into the 3D borate framework is beneficial to synthesize layered fluorooxoborates, which ensures moderate birefringence.

Experimental and Ab Initio Studies of Intrinsic Defects in "Antizeolite" Borates with a Ba12(BO3)66+ Framework and Their Influence on Properties

The porous Ba₁₂(BO₃)₆⁶⁺ framework of the so-called "antizeolite" borates with channels along the c axis is capable of accommodating various guest anionic groups, e.g. [BO₃]^3-, [F₂]^2-, [F₄]^4-, and [(Li,Na)F₄]^3-. Taking as an example the Ba₁₂(BO₃)₆[BO₃][LiF₄] crystal, we put forward the argument that the optical properties of "antizeolite" borates are strongly influenced by the degree of channel packing with anionic groups and, correspondingly, by the conjugated intrinsic defects. With the use of optical, electron-spin resonance, Raman spectroscopy, and ab initio calculations, it was shown that intrinsic defects largely impact the absorption of light in the visible and UV regions (the color of the bulk crystals can change from colorless to dark brown), absorption-edge position, dichroism, and other optical properties. The change in the optical absorption in the visible range is caused by the appearance of new states in the electronic structure inside the band gap, which are associated mainly with the presence of single and double F centers-fluorine vacancies that capture electrons-in [□F₄]^4-, [F₂]^2-, and [LiF₄]^3- groups. The formation of F centers in the [F₂]^2- group is the most energetically favorable. It has been found that Ba₁₂(BO₃)₆[BO₃][LiF₄] crystals are optically active gyrotropic with an isotropic point at 499 nm at 300 K and are of interest for practical application as narrow-band light filters.

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.

Ba(B2OF3(OH)2)2 with well-ordered OH/F anions and a unique B2OF3(OH)2 dimer

A new centimetre-scale fluorooxoborate crystal Ba(B₂OF₃(OH)₂)₂, with well-ordered OH/F units and a new dimer containing BOF₃ units, has been fabricated.