(N2H6)[HPO3F]2: maximizing the optical anisotropy of deep-ultraviolet fluorophosphates

Novel antimony phosphates with enlarged birefringence induced by lone pair cations

Phosphates, whose obvious disadvantage is the relatively small birefringence, can be overcome by the introduction of post-transition metal cations containing stereochemically active lone-pair electrons. In this paper, two new compounds were successfully explored in the A-Sb-P-O system, i.e. Cs2Sb3O(PO4)3 (CsSbPO) and (NH4)2Sb4O2(H2O)(PO4)2[PO3(OH)]2 (NH4SbPOH). Transmission spectra show that CsSbPO has a surprising transmission range with a UV cutoff edge of 213 nm. First-principles calculations show that both compounds have a wide band gap (5.02 eV for CsSbPO and 5.30 eV for NH4SbPOH) and enlarged birefringence (Δn = 0.034@1064 nm for CsSbPO and Δn = 0.045@1064 nm for NH4SbPOH). The results of real-space atom-cutting investigations show that the distorted [SbOx] polyhedra originating from the asymmetric lone pair electrons give the main contribution to the total birefringence and overcome the disadvantage of small birefringence of phosphates but maintain wide transition windows.

Optically Anisotropic Mixed-Metal Fluoroiodate Ba2[GaF5(IO3F)] with a Wide Optical Transparent Window and a Moderate Birefringence

An optically anisotropic alkali-earth-metal gallium fluoroiodate, Ba2[GaF5(IO3F)] (1), was ingeniously obtained by integrating fluoride and fluoroiodate functional units under moderate hydrothermal conditions. It features a three-dimensional (3D) structure constructed by the highly polarizable fluoroiodate unit [IO3F] and the fluoride groups [GaOF5] and [BaO3Fx] (x = 6, 7). The compound is stable at temperatures up to 500 °C. With the synergistic interaction between [IO3F] and the fluoride groups, the mixed-metal fluoroiodate induces a short ultraviolet cutoff edge at about 230 nm, a medium measured birefringence of 0.068 @ 550 nm, and a wide optical transparent window (0.34-11.9 μm), indicating that 1 has potential applications as a birefringent material from near-UV to mid-infrared. Theoretical calculations prove that the optical characteristics of the compound are mainly attributed to [IO3F] and the fluoride functional groups. This work demonstrates that the presence of various specific functional groups in compounds will help to develop promising inorganic functional materials possessing good optical performance.

Stereochemically Active Tin(II)-Induced Enhancement of Birefringence in SnII SnIV (PO4 )2 and SrSn(PO4 )PO2 (OH)2

Two new tin(II) phosphates, SnII SnIV (PO4 )2 and SrSn(PO4 )PO2 (OH)2 , were synthesized by the high-temperature solution method and hydrothermal method, respectively. Theoretical study indicates that by introducing tin(II) with stereochemical activity lone pairs (SCALP) in metal phosphates, the birefringence was enhanced, 0.048@1064 nm for SnII SnIV (PO4 )2 and 0.080@1064 nm for SrSn(PO4 )PO2 (OH)2 .

Mixed Alkali Metal and Alkaline Earth Metal Scandium Borate Birefringence Material with Layered Structure and Short Ultraviolet Cutoff Edge

Birefringent crystals are essential in the domains of linear and nonlinear optics that need light wave polarization control. Rare earth borate has become a popular study material for ultraviolet (UV) birefringence crystals due to its short cutoff edge in the UV area. RbBaScB₆O₁₂, a two-dimensional layered structure compound with the B₃O₆ group, was effectively synthesized through spontaneous crystallization. The UV cutoff edge of RbBaScB₆O₁₂ is shorter than 200 nm, and the experimental birefringence is 0.139 @ 550 nm. Theoretical research indicates that the large birefringence originates from the synergistic impact of the B₃O₆ group and the ScO₆ octahedron. RbBaScB₆O₁₂ is an outstanding candidate material for birefringence crystals in the UV and even deep UV regions due to its short UV cutoff edge and significant birefringence.

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.

SbHPO3F: 2D van der Waals Layered Phosphite Exhibiting Large Birefringence

A novel antimony(III)-based phosphite, SbHPO₃F, featuring a unique two-dimensional (2D) van der Waals layered structure, has been successfully designed and synthesized via the simultaneous employment of optically active moieties including SbO₃F seesaw and tetrahedral HPO₃ groups. Its optimized layered arrangement formed by the alternating connection of 4-membered rings (4-MRs) and 8-MRs endows the title compound with desirable optical properties including a large birefringence and short ultraviolet (UV) cutoff edge, implying that it is a potential UV birefringent material.

Bridging oxygen atoms in trigonal prism units driven strong second-harmonic-generation efficiency in Sr3Ge2O4Te3

Herein, a novel IR NLO oxytelluride Sr₃Ge₂O₄Te₃ was successfully designed and synthesized through a "partial O-to-Te substitution" strategy. Compared with the parent oxide, Sr₃Ge₂O₄Te₃ not only successfully achieves a phase-matchability transition (from NPM to PM), but also greatly improves the linear and NLO performances, including a wide band gap (2.26 eV), strong SHG response (1.3 × AgGaSe₂) and large optical anisotropy (Δn = 0.152@2090 nm). The analyses of the structure-property relationship and SHG-density indicate that the bridging oxygen in the [O₃Ge-O-GeTe₃] prism unit plays the most important role in the multiplication SHG effect. This work provides some insights into the design and exploration of novel IR NLO materials.

Ba2Zn2B6O13: coplanar [B2O5] in unnoted U-shaped [B6O13] groups achieving large birefringence

Currently, π-conjugated [B₂O₅] moieties are rarely studied for designing deep-UV birefringent crystals. Here, we report a new deep-UV birefringent crystal Ba₂Zn₂B₆O₁₃ with a deep-UV cut-off edge of 190 nm and large birefringence (Δn = 0.085@ 532 nm), indicating that it can be used as the birefringent material in the DUV area. The first-principles calculation analyses suggest that its large birefringence mainly originates from the coplanar [B₂O₅] dimers in the unnoted U-shaped [B₆O₁₃].

The Pnictogen Bond, Together with Other Non-Covalent Interactions, in the Rational Design of One-, Two- and Three-Dimensional Organic-Inorganic Hybrid Metal Halide Perovskite Semiconducting Materials, and Beyond

The pnictogen bond, a somewhat overlooked supramolecular chemical synthon known since the middle of the last century, is one of the promising types of non-covalent interactions yet to be fully understood by recognizing and exploiting its properties for the rational design of novel functional materials. Its bonding modes, energy profiles, vibrational structures and charge density topologies, among others, have yet to be comprehensively delineated, both theoretically and experimentally. In this overview, attention is largely centered on the nature of nitrogen-centered pnictogen bonds found in organic-inorganic hybrid metal halide perovskites and closely related structures deposited in the Cambridge Structural Database (CSD) and the Inorganic Chemistry Structural Database (ICSD). Focusing on well-characterized structures, it is shown that it is not merely charge-assisted hydrogen bonds that stabilize the inorganic frameworks, as widely assumed and well-documented, but simultaneously nitrogen-centered pnictogen bonding, and, depending on the atomic constituents of the organic cation, other non-covalent interactions such as halogen bonding and/or tetrel bonding, are also contributors to the stabilizing of a variety of materials in the solid state. We have shown that competition between pnictogen bonding and other interactions plays an important role in determining the tilting of the MX6 (X = a halogen) octahedra of metal halide perovskites in one, two and three-dimensions. The pnictogen interactions are identified to be directional even in zero-dimensional crystals, a structural feature in many engineered ordered materials; hence an interplay between them and other non-covalent interactions drives the structure and the functional properties of perovskite materials and enabling their application in, for example, photovoltaics and optoelectronics. We have demonstrated that nitrogen in ammonium and its derivatives in many chemical systems acts as a pnictogen bond donor and contributes to conferring stability, and hence functionality, to crystalline perovskite systems. The significance of these non-covalent interactions should not be overlooked, especially when the focus is centered on the rationale design and discovery of such highly-valued materials.

Large optical anisotropy differentiation induced by the anion-directed regulation of structures

The modulation of optical anisotropy for two novel UV birefringent materials [C(NH2)3]2Sb3F3(HPO3)4 and [C(NH2)3]SbFPO4·H2O has been successfully achieved via anion-directing regulation structures.