NaRb₆(B₄O₅(OH)₄)₃(BO₂) Featuring Noncentrosymmetry, Chirality, and the Linear Anionic Group BO₂^–

Abstract

Noncentrosymmetric (NCS) structures are of particular interest owing to their symmetry-dependent physical properties, e.g., pyroelectricity, ferroelectricity, piezoelectricity, and nonlinear optical (NLO) behavior. Among them, chiral materials exhibit polarization rotation and host topological properties. Borates often contribute to NCS and chiral structures via their triangular [BO3] and tetrahedral [BO4] units and their numerous superstructure motifs. However, no chiral compound with the linear [BO2] unit has been reported to date. Herein, an NCS and chiral mixed-alkali-metal borate, NaRb6(B4O5(OH)4)3(BO2), with a linear BO2 – unit in the structure was synthesized and characterized. The structure features a combination of three types of basic building units (BBUs), [BO2], [BO3], and [BO4] with sp-, sp2-, and sp3-hybridization of boron atoms, respectively. It crystallizes in the trigonal space group R32 (No. 155), one of the 65 Sohncke space groups. Two enantiomers of NaRb6(B4O5(OH)4)3(BO2) were found, and their crystallographic relationships are discussed. These results not only expand the small family of NCS structures with the rare linear BO2 – unit but also prompt recognition to the fact that NLO materials have generally overlooked the existence of two enantiomers in achiral Sohncke space groups.

Ultraviolet Crystal with Strong Optical Nonlinearity by Creating Halogen-Centered Secondary Building Blocks

“…The ultraviolet (UV) nonlinear optical (NLO) crystal is the core and urgently required device in the all-solid-state laser industry, which is capable of converting the finite incident light source to a shorter wavelength spectral range based on the multistage nonlinear process. − Although numerous efforts have been done for decades, only a few UV NLO crystals have been proven feasible in practical application since three crucial but mutually restrictive optical indexes that are determined by different microstructures are needed to be met simultaneously, − that is, large second-harmonic generation (SHG) response (>1 × KDP) with a noncentrosymmetric crystal structure as a prerequisite, short cutoff edge (<200 nm), and appropriate birefringence to realize phase matching.…”

Beyond π–π Stacking: Understanding Inversion Symmetry Breaking in Crystalline Racemates

“…The rational design of materials calls for a deep understanding of how atoms, units, or motifs in a crystal structure pack together. Noncentrosymmetric (NCS) solids, in which an inversion center is absent in the structure, are known to display a wide range of functional properties such as nonlinear optical activity, piezoelectricity, ferroelectricity, and pyroelectricity. − Functional building units (FBUs), such as polar early transition-metal (ETM) octahedra, − borate clusters, − polyanions, − and π-conjugated units, , are great sources from which NCS solids are constructed with superior properties. However, inversion symmetry is often favorable during crystallization for its efficiency in close packing of the basic building units (BBUs), , and the challenges for synthesizing new NCS solids arises from aligning the FBUs without an inversion center.…”

Target-Oriented Synthesis of Borate Derivatives Featuring Isolated [B3O3] Six-Membered Rings as Structural Features

“…Another branch of borates, hydrated borates with [BOH] as anionic skeletons and terminal oxygen atoms connected to tetra- and tri-coordinated boron atoms, are bound to hydrogen atoms as hydroxyl groups. The presence of terminal side chains including BO 2 (OH) or BO­(OH) 2 feature triangular geometry and π-conjugated orbitals in hydrated borates. , The species containing fluorinated non-π-conjugated tetrahedral [BO x F 4– x ] ( x = 0–3) and π-conjugated [BO x (OH) 3– x ] ( x = 1–2) − are classified as hydroxyfluorooxoborates. Statistical results according to the ICSD database and literature published in recent years show that the anionic skeletons (BOFH) of all hydroxyfluorooxoborates feature 0D clusters.…”

“…The presence of terminal side chains including BO 2 (OH) or BO(OH) 2 feature triangular geometry and π-conjugated orbitals in hydrated borates. 50,51 The species containing fluorinated non-π-conjugated tetrahedral [BO x were obtained by the solution evaporation method. The mixture of H 3 BO 3 and corresponding hydrogen difluorides was loaded into a Teflon beaker in a stoichiometric ratio.…”