Effect of Halogen (Cl, Br) on the Symmetry of Flexible Perovskite-Related Framework

Recent advances in rational structure design for nonlinear optical crystals: leveraging advantageous templates

Nonlinear optical (NLO) crystals that can expand the spectral range of laser outputs have attracted significant attention for their optoelectronic applications. The research progress from the discovery of new single crystal structures to the realization of final device applications involves many key steps and is very time consuming and challenging. Consequently, exploring efficient design strategies to shorten the research period and accelerate the rational design of novel NLO materials has become imperative to address the pressing demand for advanced materials. The recent shift in paradigm toward exploring new NLO crystals involves significant progress from extensive "trial and error" methodologies to strategic approaches. This review proposes the concept of rational structure design for nonlinear optical crystals leveraging advantageous templates. It further discusses their optical characteristics, promising applications as second-order NLO materials, and the relationship between their structure and performance, and highlights urgent issues that need to be addressed in the field of NLO crystals in the future. The review aims to provide ideas and driving impetus to encourage researchers to achieve new breakthroughs in the next generation of NLO materials.

Na4[B6O9 (OH)3](H2O) Cl: A Deep-Ultraviolet Transparent Nonlinear Optical Borate Chloride with {[B6O9 (OH)3]3-}∞ Chains

The development of nonlinear-optical (NLO) crystals with short ultraviolet cutoff edges is significant and challenging. Here, a new sodium borate chloride, Na₄[B₆O₉ (OH)₃](H₂O) Cl, was successfully obtained by the mild hydrothermal method, which crystallizes in a polar space group Pca2₁. The structure of the compound is characterized by {[B₆O₉ (OH)₃]^3-}_∞ chains. Measurements of optical properties indicate the compound exhibits a deep-ultraviolet (DUV) cutoff edge (≤200 nm) and moderate second harmonic generation response (0.4 × KH₂PO₄). It presents the first DUV hydrous sodium borate chloride NLO crystal and the first sodium borate chloride possessing a one-dimensional B-O anion framework. Probing into the connection of structure and optical properties has been performed based on theoretical calculations. These results are instructive for designing and obtaining new DUV NLO materials.

Sn3B8O15: A Ternary Tin(II) Borate with Flexible [B8O18]12- Fundamental Building Block Formed by [B7O16]11- and [BO3]3- Groups

Owing to the tendency to form the glass state in growing crystals in the ternary tin(II) borate system, hitherto, very few single crystals in ternary tin(II) borate were reported. In this paper, a single crystal of Sn₃B₈O₁₅ was obtained via a high temperature solution method in a closed system. The structure features a flexible fundamental building block [B₈O₁₈]^12- constructed by one [BO₃]^3- unit and one [B₇O₁₆]^11- group that contains three hexatomic rings, which further connect to compose a two-dimensional ²_∞[B₈O₁₅] layer structure. The connection mode and flexibility of the three hexatomic B-O rings were analyzed, verifying the structural diversity of the B-O configuration. Moreover, the synthesis, crystal structure, and various characterizations were comprehensively presented. Also, theoretical calculations were employed to discuss structure-property relationships, and we use the real-space atom-cutting techniques to explain the origin of the birefringence of Sn₃B₈O₁₅.

Modulation of perovskite-related frameworks induced by alkaline earth metals in phosphate fluorides A2MPO4F (A = K, Rb; M = Ba, Ca)

A₂CaPO₄F (A = K, Rb) are isomorphs. The 1D zig-zag [FA₄Ca₂]_∞ chains in the two compounds belong to the 1D perovskite-related structure.

Ba3Ca4(BO3)3(SiO4)Cl: a new non-centrosymmetric complex alkaline-earth metal borosilicate chloride with a deep-ultraviolet cut-off edge

The first complex alkaline-earth metal borosilicate chloride, Ba₃Ca₄(BO₃)₃(SiO₄)Cl, as a potential ultraviolet nonlinear optical material.

Improved photo-dechlorination at polar photocatalysts K3B6O10X (X = Cl, Br) by halogen atoms-modulated polarization

The material with larger distortion ability has better photocatalytic activity during the dechlorination of CPs.

Four alkali metal molybdates with two types of Mo–O chains, ABMo3O10 (A = Li, B = Rb; A = Li, Na, K, B = Cs): synthesis, structure comparison and optical properties

The effects of cation size on the framework structures of four stoichiometrically equivalent alkali metal molybdates were discussed in detail.

The structural diversity of halogen-centered secondary building units: two new mixed-metal borate halides with deep-ultraviolet cut-off edges

Categories of halogen-centered SBUs were investigated among borates to elaborate the role of halogen to enrich the structural diversity of borates.

Toward the Rational Design of Novel Noncentrosymmetric Materials: Factors Influencing the Framework Structures

Solid-state materials with extended structures have revealed many interesting structure-related characteristics. Among many, materials crystallizing in noncentrosymmetric (NCS) space groups have attracted massive attention attributable to a variety of superb functional properties such as ferroelectricity, pyroelectricity, piezoelectricity, and nonlinear optical (NLO) properties. In fact, the characteristics are pivotal to many industrial applications such as laser systems, optical communications, photolithography, energy harvesting, detectors, and memories. Thus, for the past several decades, a great deal of synthetic effort has been vigorously made to realize these technologically important properties by improving the occurrence of macroscopic NCS space groups. A bright approach to increase the incidence of NCS structures was combining local asymmetric units during the initial synthesis process. Although a significant improvement has been achieved in obtaining new NCS materials using this strategy, the majority of solid-state materials still crystallize in centrosymmetric (CS) structures as the locally unsymmetrical units are easily lined up in an antiparallel manner. Therefore, discovering an effective method to control the framework structure and the macroscopic symmetry is an imminent ongoing challenge. In order to more effectively control the overall symmetry of solid-state compounds, it is critical to understand how the backbone and the subsequent centricity are affected during the crystallization. In this Account, several factors influencing the framework structure and centricity of solid-state materials are described in order to more systematically discover novel NCS materials. Recent studies on crystalline solid-state materials suggest three factors affecting the local coordination environment as well as the overall symmetry of the framework structure: (1) size variations of the various template cations, (2) a variable backbone arrangement occurring from the hydrogen-bonding interactions, and (3) the presence of framework flexibility. With regard to the first factor, the impact of size of the various metal cations and coordination numbers on the alignment of other adjacent polyhedra, linkers, and lone pairs determining the framework geometries of mixed metal oxides is analyzed. The second factor considers the regulation of crystallographic centricity determined by the availability of hydrogen-bonding interactions between anionic frameworks containing local asymmetric polyhedra and organic cations. Finally, the third factor explores the framework architecture and the space group symmetry influenced by the flexibility of polyhedra revealing variable coordination numbers. The centricity and framework of new solid-state materials might be controlled by using a variety of synthetically controllable asymmetric units such as organic structure-directing cations and linkers with different sizes and functional groups.

Three new phosphates with isolated P2O7units: noncentrosymmetric Cs2Ba3(P2O7)2and centrosymmetric Cs2BaP2O7and LiCsBaP2O7

Three new phosphates, a noncentrosymmetric (NCS) Cs₂Ba₃(P₂O₇)₂and centrosymmetric (CS) Cs₂BaP₂O₇and LiCsBaP₂O₇, have been synthesized from high-temperature solutions for the first time.

“XA6” octahedra influencing the arrangement of anionic groups and optical properties in inverse-perovskite [B6O10]XA3(X = Cl, Br; A = alkali metal)

The distortion and volumes of XA₆(X = Cl, Br; A = alkali metal) octahedra influence the symmetry broken and the arrangement of BO groups, which leads to different second harmonic generation responses in perovskite-type borates.

Na3B4O7X (X = Cl, Br): two new borate halides with a 1D Na-X (X = Cl, Br) chain formed by the face-sharing XNa6octahedra

Two new borate halides consist of a B₄O₉framework and a [Na₃Br] chain which have not been found in other alkali metal borate halides.