Rational Design of the First Lead/Tin Fluorooxoborates MB2O3F2 (M = Pb, Sn), Containing Flexible Two-Dimensional [B6O12F6]∞ Single Layers with Widely Divergent Second Harmonic Generation Effects

Large nonlinear optical effect in tungsten bronze structures via Li/Na cross-substitutions

By a simple cross-substitution of A-site Li/Na in tetragonal tungsten bronze (TTB) structures, we successfully synthesized a new niobate compound, Pb2.15(Li0.25Na0.75)0.7Nb5O15, with a superstructure. This compound exhibits a strong second harmonic generation (SHG) up to ∼47 × KDP. The large SHG response is related to strengthened local distortion, manifesting cross-substitution as a possibly general route to improve the NLO effect in stiff and low symmetric structures.

Rational Design of the Metal-Free KBe2 BO3 F2 ⋅(KBBF) Family Member C(NH2 )3 SO3 F with Ultraviolet Optical Nonlinearity

The first fluorosulfonic ultraviolet (UV) nonlinear optical (NLO) material, C(NH₂ )₃ SO₃ F, is rationally designed by taking KBe₂ BO₃ F₂ (KBBF) as the parent compound. C(NH₂ )₃ SO₃ F features similar topological layers as KBBF by replacing inorganic (BO₃ )^3- with organic C(NH₂ )₃ ⁺ trigonal units and BeO₃ F with SO₃ F^- tetrahedra. Therefore, C(NH₂ )₃ SO₃ F is a metal-free UV NLO crystal. Benefiting from the coplanar configuration of the C(NH₂ )₃ ⁺ cationic groups, it possesses a large SHG response of 5×KDP and moderate birefringence of 0.133@1064 nm. Besides, it has a short UV cutoff edge of 200 nm. The calculated results reveal the shortest SHG phase-matching wavelengths can reach 200 nm. These findings highlight the exploration of metal-free compounds as nontoxic and low-cost UV NLO materials as a new research area.

[O2Pb3]2(BO3)I: a new lead borate iodide with [O2Pb3] double chains

A new lead borate iodide, [O2Pb3]2(BO3)I, has been successfully synthesized by a facile hydrothermal reaction. It exhibits an infinite one-dimensional (1D) 1∞[O2Pb3] double chain which is constructed from OPb4 oxygen-centered tetrahedra. The 1∞[O2Pb3] double chains are further bridged by BO3 units via sharing oxygen atoms to form two-dimensional (2D) 2∞[(O2Pb3)2(BO3)] layers with the I- ions situated in the spaces between the layers. First principles calculations indicate that [O2Pb3]2(BO3)I exhibits a birefringence of about 0.072@1064 nm which originates from the synergistic contributions of the Pb-O/I and BO3 units. The IR, UV-vis-NIR and EDS results, as well as TG-DSC curves were also discussed in detail.

Hydrogen-Bond-Assisted Reinforcement of Interlayer Connections in Zn2BO3X·H2O (X = Cl, Br): Two UV Nonlinear Optical Crystals with KBBF-Type Structure

Two novel zinc borate halides named Zn₂BO₃X·H₂O (X = Cl, Br) were discovered through hydrothermal techniques. Both compounds are isomorphic and feature the layered structure similar to that of KBBF, consisting of the infinite planar [Zn₂BO₃X·OH₂] (X = Cl, Br) layers. Compared with the weak ionic bond between adjacent layers in KBBF, the strong interaction of hydrogen bonds between layers in Zn₂BO₃X·H₂O (X = Cl, Br) effectively enhances the interlayer force, contributing to eliminating the layering growth tendency. Optical measurements on these two NLO crystals revealed that they have a broad wavelength transparency window and a moderate NLO coefficient. Moreover, the theoretical calculations revealed that the linear and NLO properties primarily depended on the [BO₃] and [ZnO₃Cl/Br] groups in both crystals.

Ba8SrPb24O24Cl18: the first alkali-earth metal lead(ii) oxyhalide with an intriguing multimember-ring layer

The first alkali-earth metal lead(ii) oxyhalide Ba₈SrPb₂₄O₂₄Cl₁₈ characterized by fascinating multimember-ring layers has been discovered. Theoretical and experimental investigations illustrate that Ba₈SrPb₂₄O₂₄Cl₁₈ exhibits a moderate band gap of 3.09 eV, incongruent melting behavior and birefringence of 0.014@1064 nm. This discovery may offer new ideas for regulating the optical properties of oxyhalides and broadening their structural diversity.

Large optical polarizability causing positive effects on the birefringence of planar-triangular BO3 groups in ternary borates

The structure-property relationship of photoelectric functional materials has been recognized as a hot topic. The study of the inner link between the band gaps and birefringence of optical materials is extremely crucial for the design and creation of novel optical devices, but still remains rather unexplored. In this work, taking a series of borates with only planar-triangular BO₃ groups, α-/β-TM₃(BO₃)₂ (TM = Zn, Cd), Cd₂B₂O₅, and M₃(BO₃)₂ (M = Hg, Mg, Ca, Sr) as the research subject, the comprehensive relationship between their electronic structures and linear optical properties has been systematically investigated. Through combining experimental measurements and theoretical calculations, the effect of optical polarizability on the birefringence of these borates was clarified. Based on the present discussion, the relationship between the O (2p) bandwidth of the highest valence band and the HSE06 band gaps is opposite. Meanwhile, a method involving the determination of so-called optical permittivity Δε to evaluate the magnitude of birefringence Δn is found to be feasible. A large Δε makes a positive contribution to Δn. In addition, the experimentally measured band gaps and IR vibrations are in good agreement with theoretical results for the compounds α-Cd₃(BO₃)₂ and Cd₂B₂O₅.

(C3H9NI)4AgBiI8: a direct-bandgap layered double perovskite based on a short-chain spacer cation for light absorption

A new iodide layered double perovskite (C₃H₉NI)₄AgBiI₈ (IPAB) has been developed based on a short-chain spacer cation, which is the first homologous compound in iodide double perovskites that adopt the Ruddlesden–Popper structure type.

Synthesis, characterization and crystal structure of a new mixed alkali and alkaline-earth metal borate Rb9Ba24(BO3)19

Rb₉Ba₂₄(BO₃)₁₉ features an intricate 3D Ba–O framework with three types of infinite channels where the Rb⁺ cations and isolated BO₃ units are located.

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.

Li3CaB2O5F: a unique sandwich-like structure with diverse and wide Li ion diffusion pathways

A new borate fluoride Li₃CaB₂O₅F has been synthesized.

Two-stage evolution from phosphate to sulfate of new KTP-type family members as UV nonlinear optical materials through chemical cosubstitution-oriented design

KTiOPO4 (KTP) is a classic commercial nonlinear optical (NLO) crystal, but its narrow bandgap (3.52 eV) prevents its practical application in the ultraviolet (UV) region. Many trials to widen the narrow bandgap of KTP have failed in the past few decades. A chemical cosubstitution strategy was implemented to design new members of the KTP-type family as potential UV NLO materials. First, a novel centrosymmetric KTP-type compound NH4SbFPO4·H2O with a sharply enlarged bandgap (5.01 eV) was obtained through three-site aliovalent substitution. Second, the noncentrosymmetric NH4SbF2SO4 was synthesized by the introduction of more F- anions to destroy the crystal symmetry and SO42- to replace PO43- for balancing the charge in NH4SbFPO4·H2O, which realized the transformation from a visible phosphate system to solar blind UV sulfate system for KTP-type family NLO materials. The preliminary experimental results indicated that NH4SbF2SO4 is a promising solar blind UV NLO material. The first-principles calculations revealed that the sharply enlarged bandgap resulted from the substitution of the transition metal cations with the main group metal cations and the introduction of F- anions with high electronegativity.

Coordinated regulation on critical physiochemical performances activated from mixed tetrahedral anionic ligands in new series of Sr6A4M4S16 (A = Ag, Cu; M = Ge, Sn) nonlinear optical materials

Coordinated regulation on physiochemical performances activated from mixed anionic ligands in a new series of IR NLO materials was systematically investigated.

Ba6BO3Cl9 and Pb6BO4Cl7: structural insights into ortho-borates with uncondensed BO4 tetrahedra

Two new halogen-rich ortho-borates, Ba₆BO₃Cl₉ and Pb₆BO₄Cl₇, were synthesized and characterized. Interestingly, Pb₆BO₄Cl₇ contains rare uncondensed BO₄ tetrahedra.

Pb3Ba3Zn6(BO3)8 and α-BaZn2(BO3)2: new members of the zincoborates containing two different dimensional Zn–O units

Two new zincoborates, namely, Pb₃Ba₃Zn₆(BO₃)₈ and α-BaZn₂(BO₃)₂, were synthesized by the high-temperature solution method and their structures were determined by single-crystal X-ray diffraction for the first time.

Surface Nonlinear Optics on Centrosymmetric Dirac Nodal-Line Semimetal ZrSiS

Gapless surface states (SSs) are features of topological semimetals and are extensively observed. Nowadays, the emerging question is whether the SSs possess exotic and applicable properties. Here, associated with the symmetrical selection rule for nonlinear optical materials, the surface nonlinear optics on a centrosymmetric Dirac nodal-line semimetal ZrSiS crystal is studied and it is found that the SSs bring record nonlinear susceptibilities. The unprecedented conversion efficiencies for second and third harmonic generations are 0.11‰ and 0.43‰, respectively, more than ten orders of magnitude larger than the typical surface second harmonic generation. This work discovers a new route toward studying the SSs for applications in nonlinear photonics.

Prediction of ternary fluorooxoborates with coplanar triangular units [BOxF3−x]x− from first-principles

From first-principles prediction, we got all the basic structural units of fluorooxoborates, namely, tetrahedral elements [BO_xF_4−x] (x = 1,2,3) like [BO₄] and triangular elements [BO_xF_3−x] (x = 1,2) like [BO₃].

Alkali-earth metal lead(ii) oxyhalide Ba27Pb8O8Cl54 exhibiting interesting [Pb4Ba4O4]8+ species

A novel oxyhalide Ba₂₇Pb₈O₈Cl₅₄ exhibits interesting [Pb₄Ba₄O₄]⁸⁺ species.

From BiF3 to BiF3·H2O: diverse Bi(iii) coordination for structural transformation and birefringence regulation

BiF₃·H₂O with unprecedented BiF₉ building blocks was synthesized, which exhibits a near-isotropic Bi³⁺ coordination and inert lone pair activity.

PbSrSiO4: a new ultraviolet nonlinear optical material with a strong second harmonic generation response and moderate birefringence

PbSrSiO₄ exhibits excellent properties, including a strong second harmonic generation response (∼5.8 × KDP), a short UV cut-off edge (240 nm) and moderate birefringence (0.053@1064 nm).

From BaAl2(BO3)2O to SnAl2(BO3)2F2: structure transformation based on ion regulation

Sn atoms reside in each Al–B–O–F channel consisting of Al₈B₈O₃₂F₁₂ cavities, which makes SnAl₂(BO₃)₂F₂ possess a special structure.

Lone pair electron effect induced differences in linear and nonlinear optical properties of bismuth borates

We analyzed systematically each phase of BIBO, and we found that the synergistic effect between bismuth oxygen polyhedra and boron–oxygen groups codetermines the optical properties of BIBO.

“Old dog, new tricks”: the lone pair effect inducing divergent optical responses in lead cyanurates containing π-bonds

The stereochemically active lone pair effect of Pb²⁺ and induced narrower bandgaps were elaborated in Pb₃(HC₃N₃O₃)₂(OH)₂ for the first time.

CsAlB3O6F: a beryllium-free deep-ultraviolet nonlinear optical material with enhanced thermal stability

The design of new beryllium-free deep-ultraviolet nonlinear optical materials is important but challenging. Here, we describe a new strategy to search for such materials based on rational selection of fundamental structural units. By combining asymmetric AlO₃F tetrahedra and π-conjugated B₃O₆ rings, a new aluminum borate fluoride, CsAlB₃O₆F was obtained. It exhibits excellent linear and nonlinear optical properties including a high optical transmittance with a cut-off edge shorter than 190 nm, large second harmonic generation intensities (2.0× KH₂PO₄, KDP), and suitable birefringence for phase-matching under 200 nm. It also has good thermal stability and can be synthesized easily in an open system.

A new potential deep-ultraviolet nonlinear optical material CsAlB₃O₆F was designed by a rational selection of fundamental structural units. This material does not require toxic raw materials and can be grown in an open system.

Tb(HSO4)(SO4) - a green emitting hydrogensulfate sulfate with second harmonic generation response

Recently, sulfates have attracted attention as materials for non-linear optical applications. This compound class is extended by Tb(HSO₄)(SO₄), which is solvothermally synthesised from Tb₄O₇ and sulfuric acid. The compound crystallises in the non-centrosymmetric space group P2₁ (Z = 2, a = 665.03(5) pm, b = 659.41(5) pm, c = 680.24(5) pm, and β = 104.640(2)°) and is homeotypic with Ni₂In. The terbium ions adopt the indium sites and the sulfate and hydrogen sulfate anions are situated on the nickel sites. The compound shows green luminescence based on f-f-transitions and the positions of the f-d-excitation bands reveal a weak coordination behaviour of the sulfate anions. Tb(HSO₄)(SO₄) exhibits a second harmonic generation response comparable to KH₂PO₄ (KDP). Furthermore, the material is characterised by electrostatic calculations, infrared spectroscopy and thermal analysis.

RE(H2C3N3O3)2·(OH)·xH2O (RE = La, Y and Gd): potential UV birefringent materials with strong optical anisotropy originating from the (H2C3N3O3)- group

A new series of dihydro-isocyanurates, namely, RE(H₂C₃N₃O₃)₂·(OH)·xH₂O (RE = La, Y and Gd) was readily obtained by a mild hydrothermal technique using lithium hydroxide as a deprotonation agent. La(H₂C₃N₃O₃)₂·OH·2H₂O and Re(H₂C₃N₃O₃)₂·OH·5H₂O (Re = Y, Gd) crystallized in the centrosymmetric (CS) space groups P2₁/c and P1[combining macron], respectively. Their intricate 3D frameworks were built by cationic polyhedrons (LaO₈N/ReO₈) and (H₂C₃N₃O₃)^- groups. The experiments and theoretical studies demonstrated that these compounds possessed short ultraviolet cut-off edges (≤240 nm) and large birefringences (Δn≥ 0.257@546.1 nm). In addition, the theoretical studies further revealed that the (H₂C₃N₃O₃)^2- group plays a pivotal role in strengthening the optical anisotropy for all compounds.