M2B10O14F6 (M = Ca, Sr): Two Noncentrosymmetric Alkaline Earth Fluorooxoborates as Promising Next-Generation Deep-Ultraviolet Nonlinear Optical Materials

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.

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.

KNa4B2P3O13: A Deep-Ultraviolet Transparent Borophosphate Exhibiting Second-Harmonic Generation Response

A new deep-ultraviolet transparent alkali metal borophosphate KNa₄B₂P₃O₁₃ is synthesized by high-temperature flux method, and its properties were characterized by a combination of experimental and ab initio methods. This compound crystallizes in the noncentrosymmetric Pna2₁ space group and its framework is formed of one-dimensional infinite [B₂P₃O₁₃]_∞ chains consisting of apex-sharing [BO₄] and [PO₄] tetrahedra. KNa₄B₂P₃O₁₃ had a short ultraviolet edge (<190 nm) and exhibited a moderate powder second-harmonic generation signal (0.4 × KDP). The discovery of KNa₄B₂P₃O₁₃ enriches the structural diversity of borophosphates and would bring a more comprehensive understanding of the structure-properties relationship in deep-ultraviolet nonlinear optical crystals.

Be2CO3F2 Monolayer: A Flexible Ultraviolet Nonlinear Optical Material via Rational Design

Multifunctional monolayer materials with attractive properties and novel applications are of present research interest. In this contribution, we design a new monolayer Be₂CO₃F₂ (BCF) by taking a KBe₂BO₃F₂ unique crystal that is able to produce a high energetic laser by a direct second harmonic generation method, as a parent. The cohesive energy, positive phonon modes, and elastic constants reveal that the BCF monolayer is dynamically and mechanically stable, and the appropriate cleavage energy predicts the experimental realization possibility. The property investigations demonstrated that the monolayer BCF has the significantly superiority in flexibility over the representative flexible optoelectronic material MoS₂ based on the calculation of Young's modulus. Additionally, the monolayer BCF possesses both a large band gap (5.2 eV) and a second harmonic generation response. These results demonstrate that the monolayer BCF may provide better applications as a promising multifunctional material in the flexible nonlinear optical fields. We hope that this research will pave a new way for designing new generation multifunctional devices.

Rational Design and Synthesis of a Deep-Ultraviolet Nonlinear Optical Fluorinated Orthophosphate: BaZn(PO4)F

Rational design and tailored synthesis of noncentrosymmetric compounds with nonlinear optical (NLO) properties, especially in the deep-ultraviolet (deep-UV) region, remains a great challenge. Herein, we report on the development of a modified fluoro-solvo-hydrothermal method with two additive reagents (trimethylamine and NaF solution) as the solvents, using BaFe(PO₄)(OH) ( P2₁2₁2₁) as the prototype, for the rational design and tailored synthesis of the first deep-UV fluorinated orthophosphate, BaZn(PO₄)F. It crystallizes in the polar space group Pna2₁ and exhibits transparency down to deep-UV region (<190 nm) with SHG effect at 0.26 × KH₂(PO₄). Its structure is built from strictly alternating ZnO₄F trigonal bipyramids and PO₄ tetrahedra, resulting in a four-connected ABW-type zeolite framework. First-principles calculations confirm the deep-UV absorption edge and reveal that ZnO₄F plays an essential role in the NLO properties. The synergetic effect of Zn and F atoms leads to its more polar crystal structure, much deeper absorption edge, and better SHG effect than the prototype.

Targeting the Next Generation of Deep-Ultraviolet Nonlinear Optical Materials: Expanding from Borates to Borate Fluorides to Fluorooxoborates

Coherent light radiation down to the deep-ultraviolet spectral range (λ < 200 nm) produced by common laser sources is extensively used in diverse fields ranging from ultrahigh-resolution photolithography to photochemical synthesis to high-precision microprocessing. Actually, it is hard to immediately obtain certain wavelengths, deep-ultraviolet coherent light in particular, from commercial laser sources. However, the direct second harmonic generation process governed in part by nonlinear optical crystals is a feasible and effective approach to generate deep-ultraviolet coherent light, which motivates chemists and materials scientists to find potential deep-ultraviolet nonlinear optical materials that can practically meet the scientific requirements. The research progress required to go from a new single-crystal structure to final device applications involves many pivotal steps and is highly time-consuming and challenging, and therefore, it is necessary to commence systematic studies aimed at shortening the research cycle and accelerating the rational design of deep-ultraviolet nonlinear optical materials. In this Account, we choose borates as raw materials because they have ever-greater possibilities to form desired noncentrosymmetric structures, wide optical transparency windows, rich structural chemistry, and also large polarizabilities to guarantee the coexistence of large second-order nonlinear optical coefficients and suitable birefringence. Besides, the effects of fluorine atoms on the structural chemistry and optical properties of borates have been summarized and analyzed. On the basis of these favorable influences, three specific rational design strategies, including experimental and theoretical methods, have been proposed in order to shorten the investigational cycle of discovering the new expected compounds with high physicochemical performances required for practical applications. In this way, the progress of searching for candidates for the next generation of deep-ultraviolet nonlinear optical materials was accelerated from borates to borate fluorides to fluorooxoborates with three effective strategies: (1) expansion of the frontier from borates to borate fluorides with the introduction of fluorine to achieve enhanced optical performance; (2) computer-assisted design of new deep-ultraviolet nonlinear optical materials with a newly introduced systematic global structure optimization method; and (3) expansion of the frontier from borate fluorides to fluorooxoborates by proposed functionalized oxyfluoride [BO _xF_{4- x}]^{( x+1)-} ( x = 1, 2, 3) chromophores to balance multiple criteria. The preliminary development of fluorooxoborates exhibiting high performance as a new fertile field to search for deep-ultraviolet nonlinear optical materials is highly encouraging and inspiring and can guide chemists and materials scientists with new directions and thoughts aimed at finding the next generation of practical deep-ultraviolet nonlinear optical materials.

KLi(HC3N3O3)·2H2O: Solvent-drop Grinding Method toward the Hydro-isocyanurate Nonlinear Optical Crystal

A novel mixed alkali hydro-isocyanurate, KLi(HC₃N₃O₃)·2H₂O was first prepared in AOH-BOH-H₃C₃N₃O₃ (A/B = Li/Na/K/Rb/Cs) system via a solvent-drop grinding method. KLi(HC₃N₃O₃)·2H₂O shows a large second harmonic generation response (5.3 × KH₂PO₄) with an ultraviolet cutoff edge of 237 nm. More importantly, the bulk single crystal can be readily grown through water solution technique. Characterization of these crystals indicates that KLi(HC₃N₃O₃)·2H₂O has a high laser damage threshold (LDT) (4.76 GW/cm²) and exhibits a large birefringence (Δ n = 0.186@514 nm), which reduces Type I phase-matching to 246 nm.

A new ultraviolet transparent hydra-cyanurate K2(C3N3O3H) with strong optical anisotropy from delocalized π-bonds

A new hydra-cyanurate K₂(C₃N₃O₃H) with high ultraviolet transparency (E_g ∼4.94 eV) and strong optical anisotropy (Δn ∼0.35) was synthesized by a slow evaporation method in aqueous solution.

Rb3SbF3(NO3)3: an excellent antimony nitrate nonlinear optical material with a strong second harmonic generation response fabricated by a rational multi-component design

Rb₃SbF₃(NO₃)₃ fabricated by a rational multi-component design exhibits a strong phase-matching SHG response and a wide transmission window.

Pb10O4(BO3)3I3: a new noncentrosymmetric oxyborate iodide synthesized by the straightforward hydrothermal method

A new first reported oxyborate iodide Pb₁₀O₄(BO₃)₃I₃ has been synthesized by a straightforward hydrothermal method.

Centrosymmetric K2SO4·(SbF3)2 and noncentrosymmetric Rb2SO4·(SbF3)2 resulting from cooperative effects of lone pair and cation size

The cooperative effects of the lone pair and cation size were used to tune the symmetry of two stoichiometrically equivalent compounds, A₂SO₄·(SbF₃)₂ (A = K or Rb).

Na3B7O11F2: a new sodium-rich fluorooxoborate with a unique [B14O24F4] ring and a short ultraviolet absorption edge

A new sodium-rich fluorooxoborate Na₃B₇O₁₁F₂ features a novel large [B₁₄O₂₄F₄] ring formed by two corner-shared [B₇O₁₃F₂] groups, each containing two [B₃O₇] anionic units linked by a [BO₂F₂] group.

Chloride carbodiimide K12Pb51(CN2)30Cl54 with an unprecedented 45 Å unit cell axis and a large birefringence

K₁₂Pb₅₁(CN₂)₃₀Cl₅₄ exhibits a giant birefringence of 0.094 at 1064 nm and a large cell axis due to the rotation of Pb²⁺ cations.

Fluoroborophosphates: a family of potential deep ultraviolet NLO materials

A series of new fluoroborophosphates obtained by combining BO₃F or BO₂F₂ with a PO₄ tetrahedron exhibit short UV absorption edges and display second-harmonic generation signals.

LiO4 tetrahedra lock the alignment of π-conjugated layers to maximize optical anisotropy in metal hydroisocyanurates

The “lock-in effect” of LiO₄ tetrahedra contributes to the alignment of π-conjugated layers to maximize optical anisotropy in metal hydroisocyanurates.

K2SrP4O12: a deep-UV transparent cyclophosphate as a nonlinear optical crystal

Benefiting from high transmittance of PO4 tetrahedra in the deep-ultraviolet (deep-UV) region, phosphates have been investigated as deep-UV nonlinear optical (NLO) candidates for years. So far, reported deep-UV NLO phosphates are orthophosphates and polyphosphates while cyclophosphates, important members of the phosphates family, have usually been neglected for use as deep-UV NLO crystals. Here, we first report a novel cyclotetraphosphate K2SrP4O12 (KSPO) as a deep-UV transparent NLO crystal. In its structure, the fundamental building groups are ring [P4O12]4- groups. KSPO exhibits a moderate second-harmonic generation (SHG) response that is 0.5 times that of KH2PO4 (KDP) and a short deep-ultraviolet absorption edge smaller than 200 nm.

M2(SeO3)F2 (M = Zn, Cd): understanding the structure directing effect of [SeO3]2− groups on constructing ordered oxyfluorides

The structure directing effect of [SeO₃]²⁻ anionic groups play a crucial role in the configuration of ordered [MO₃F₃] octahedral and its large anisotropy.

LiGeBO4: a nonlinear optical material with a balance between deep-ultraviolet cut-off edge and large SHG response induced by hand-in-hand tetrahedra

LiGeBO₄ is found to be a material that can balance a large band gap (>6.2 eV) and strong SHG coefficient (2 × KDP), which is rarely seen in tetrahedral-based crystals.

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.

A rich structural chemistry in π-conjugated hydroisocyanurates: layered structures of A2B(H2C3N3O3)4·nH2O (A = K, Rb, Cs; B = Mg, Ca; n = 4, 10) with high ultraviolet transparency and strong optical anisotropy

Mixed alkali/alkali-earth metal hydroisocyanurates A₂B(H₂C₃N₃O₃)₄·nH₂O (A = K, Rb, Cs; B = Mg, Ca, n = 4, 10) with strong optical anisotropy were grown.

“Two in one”: an unprecedented mixed anion, Ba2(C3N3O3)(CNO), with the coexistence of isolated sp and sp2 π-conjugated groups

The first mixed cyanate–cyanurate double salt Ba₂(C₃N₃O₃)(CNO) containing two types of π-conjugated groups was successfully synthesized and systematically characterized.

Prediction of MCO [M = S, (Cl2B)3B] Systems with Giant Optical Birefringence and Nonlinearity in the Deep-Ultraviolet Region

Deep-ultraviolet (DUV) materials with giant optical birefringence and nonlinearity are scarce, and their discovery is very challenging. In this Communication, we predict that the MCO [M = S, (Cl₂B)₃B] compounds containing the polar C-O motif, which have been synthesized in the experiments, could be the first material systems to achieve this rare capability. First-principles calculations demonstrate that (Cl₂B)₃BCO and SCO exhibit the largest second-harmonic-generation effect (∼4.4 pm/V) and the largest birefringence (∼0.6) in all known DUV nonlinear-optical (NLO) crystals. In addition, SCO might be a DUV NLO material for the practical 193.7 nm laser output. Our discovery could enrich the structural chemistry of NLO crystals and advance the development of functional DUV optical materials.

Pb2GaF2(SeO3)2Cl: Band Engineering Strategy by Aliovalent Substitution for Enlarging Bandgap while Keeping Strong Second Harmonic Generation Response

Wide bandgap and strong second-order generation (SHG) effect are two crucial but contradictory conditions for practical nonlinear optical (NLO) materials. Herein, a new NLO crystal Pb₂GaF₂(SeO₃)₂Cl (I) containing novel functional (GaO₃F₃)^6- octahedra is designed and synthesized by a rational band engineering strategy with aliovalent substitution. Benefiting from the removal of transition metal cations and the introduction of bridged F anions, I exhibits the widest bandgap among all reported phase-matchable NLO selenites. Meanwhile, a strong SHG response more than 4.5 times of KH₂PO₄ (KDP) is maintained. The dominate role of the (GaO₃F₃)^6- groups to the enlarged bandgap in I are elucidated by first-principles studies.

Na4Yb(CO3)3F: A New UV Nonlinear Optical Material with a Large Second Harmonic Generation Response

A new nonlinear optical crystals fluoride carbonate, Na4Yb(CO3)3F, has been synthesized by mild hydrothermal method. The Na4Yb(CO3)3F crystallizes in the noncentrosymmetric space group Cc (no. 9) with the lattice parameters a = 8.018(3), b = 15.929(5), c = 13.950(5) Å and β = 101.425(6)°. The compound Na4Yb(CO3)3F has a high density of [CO3] groups. The structure can be described as one-dimensional [Na5Yb(CO3)2F2] chains connected by [CO3] groups, forming an intricate three-dimensional (3D) framework. Other Na+ and Yb3+ cations are located in the cavities of 3D network. The powder second harmonic generation (SHG) measurement shows that Na4Yb(CO3)3F features a large SHG response, about 4.3 times that of KH2PO4 (KDP), and is a phase-matchable material. In addition, its UV-Vis-NIR diffuse reflectance spectral data indicate that Na4Yb(CO3)3F has a large optical gap about 4.72 eV, which corresponds to the UV cut-off edge of 263 nm.

Cs3VO(O2)2CO3: an exceptionally thermostable carbonatoperoxovanadate with an extremely large second-harmonic generation response

A highly thermostable carbonatoperoxovanadate, Cs₃VO(O₂)₂CO₃, exhibits a remarkably strong SHG response of approximately 23.0 times that of KDP, which is the largest among those of the reported NLO carbonate materials.

A novel nonlinear optical (NLO) carbonatoperoxovanadate, Cs₃VO(O₂)₂CO₃, with an exceptionally high thermostability was successfully synthesized by introducing highly polarizable Cs⁺ cations and inorganic polydentate carbonate anions into asymmetric peroxovanadates. The structure of Cs₃VO(O₂)₂CO₃ is composed of distorted [VO(O₂)₂CO₃]^3– units and charge balancing Cs⁺ cations. The title compound exhibits the largest NLO intensity ever found in the current carbonate NLO materials, i.e., 23.0 times that of KH₂PO₄ (KDP). The remarkably strong second-harmonic generation (SHG) response originates from the synergistic effect of the exceedingly polarizable Cs⁺ cations, distortive polyhedra of the V⁵⁺ cation, delocalized π orbitals in CO₃ groups, and distorted localized π orbitals in O₂ groups. First-principles calculations indicated that introducing the polarizable cations into peroxovanadates not only induces the enhancement of the SHG response but also improves the thermal stability of the framework.

Why Some Noncentrosymmetric Borates Do Not Make Good Nonlinear Optical Materials: A Case Study with K3B5O8(OH)2

Synthesis of deep-ultraviolet (DUV) transparent materials, especially those with noncentrosymmetric structures, remains a great challenge in the solid-state chemistry community. A new DUV transparent borate, K₃B₅O₈(OH)₂, was discovered with a short absorption edge below 200 nm. The title compound crystallizes in a noncentrosymmetric, polar space group, Fdd2 (point group mm2), with the following cell parameters: a = 13.736(9) Å, b = 19.317(12) Å, c = 7.606(5) Å. The structure of K₃B₅O₈(OH)₂ features a 3D framework composed of [B₅O₈(OH)₂]^3- basic building units that are linked by contacts with K⁺ cations and O-H···O hydrogen bonds. Second harmonic generation (SHG) measurements were performed, and an SHG efficiency of 0.5 × SiO₂ was observed. Symmetry dictates that the χ₁₄ component of the macroscopic NLO susceptibility is equal to zero in mm2, which prevents the maximal component of the microscopic NLO susceptibility for [B₅O₈(OH)₂]^3- units (χ₁₄) from contributing to the macroscopic NLO susceptibility of the crystal and therefore limits the SHG efficiency. In contrast, large SHG effects can be observed from compounds containing [B₅O₁₀] units that crystallize in point groups with nonzero χ₁₄, such as 222. These findings provide insight into understanding the relationship between crystal structure and SHG efficiency in [B₅O₁₀]-based compounds and discovering other borate-based DUV materials.

Removal of A-Site Alkali and Alkaline Earth Metal Cations in KBe2BO3F2-Type Layered Structures To Enhance the Deep-Ultraviolet Nonlinear Optical Capability

In order to generate deep-ultraviolet (DUV, λ < 200 nm) coherent light, many DUV-transparent nonlinear optical (NLO) compounds have been synthesized experimentally over the past few decades, most of which contain alkali or/and alkaline earth metal cations. However, to date, practical DUV NLO materials beyond KBe₂BO₃F₂ (KBBF) are still very scarce. In this work, based on analysis of the DUV NLO effect induced by the A-site alkali and alkaline earth metal cations, we attempt to expand the options for DUV NLO compounds from a molecular engineering point of view. Accordingly, a useful strategy is proposed to design densely stacked layered structures without A-site cations as new and promising DUV NLO materials. Along with the available experimental and first-principles calculation results, it is demonstrated that layered structures in which the A-site cations are removed, such as Be₂BO₃F, Be₂CO₃F₂, AlCO₃F, SiCO₃F₂, AlNO₃F₂, PBO₃F₂, and PB₃O₆F₂, whether real or hypothetical, exhibit excellent DUV NLO performances. Based on the findings, our strategy may open up new opportunities for the design and exploration of high-performance DUV NLO materials beyond KBBF.