Beryllium-Free Rb3Al3B3O10F with Reinforced Interlayer Bonding as a Deep-Ultraviolet Nonlinear Optical Crystal

NaZnCO3(OH): A High-Performance Carbonate Ultraviolet Nonlinear Optical Crystal Derived from KBe2BO3F2

Currently, to obtain new ultraviolet (UV) nonlinear optical (NLO) materials with large second-harmonic generation (SHG) response and short SHG phase-matching cutoff wavelength (λ_PM) is still very challenging. Herein, through evolving the KBe₂BO₃F₂ (KBBF) structure to a carbonate system, a novel hydroxycarbonate UV NLO crystal, NaZnCO₃(OH), was successfully synthesized. It not only possessed the typical structural characteristics of KBBF but also enhanced the bonding force between functional layers. Remarkably, in this crystal, the rare coexistence of a very large SHG response of 5.2 × KH₂PO₄ (KDP, @1064 nm) and short type-I SHG λ_PM of 201 nm had been achieved, indicating that it was a promising UV NLO crystal. This work showed the great significance of KBBF structure evolution and provided new opportunity to design UV NLO materials with good performance.

CsHgNO3Cl2: A New Nitrate UV Birefringent Material Exhibiting an Optimized Layered Structure

Developing effective strategies to design novel, excellent birefringent materials has become an emerging challenge. Herein, we report a new UV nitrate birefringent material, CsHgNO₃Cl₂, which is an analogue of the known UV carbonate nonlinear optical crystal KCaCO₃F. In CsHgNO₃Cl₂, a perfect layered structure was produced owing to the alliance of a planar π-conjugated anion NO₃^- and highly electropolarized Hg²⁺ cation. The optimized layered structure that is beneficial in enlarging the optical anisotropy induced a large birefringence (Δn = 0.145@546 nm) for CsHgNO₃Cl₂, even superior to that of the well-known commercial birefringent material α-BBO in the UV region. The strategy that the optimization of planar functional group arrangement is able to boost birefringence is expected to guide the development of high-performance birefringent materials in the future, especially in the field of UV.

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.

Ca2B5O9Cl and Sr2B5O9Cl: Nonlinear Optical Crystals with Deep-Ultraviolet Transparency Windows

M₂B₅O₉X is a prominent family with excellent nonlinear optical (NLO) responses, just as the Pb₂B₅O₉I crystal with a large second harmonic generation (SHG) of 13.5 times that of KH₂PO₄. However, most of these compounds are limited to ultraviolet and visible regions because of their long absorption edge (small band gap). Here, we report two members of this family, which change the situation. Using a high-temperature solution method, we obtain Ca₂B₅O₉Cl and Sr₂B₅O₉Cl crystals, which exhibit a deep-ultraviolet (DUV) absorption edge of 170 nm (band gap ≈ 7.29 eV). It is an important breakthrough in the DUV transparency of the M₂B₅O₉X family. Furthermore, Ca₂B₅O₉Cl crystals display a phase-matching SHG response under a 1064 nm laser, which is further confirmed by the balance between the suitable birefringence and the small dispersion of refractive indexes in the wavelength range of 1064-532 nm. Therefore, they are promising DUV transparency windows and NLO candidates.

Rb3BaTeB7O15: a novel [B7O16] fundamental building block in a new telluroborate with [TeO3] polyhedra

A new telluroborate Rb₃BaTeB₇O₁₅ with a new [B₇O₁₆] fundamental building block has been synthesized, and it is the first telluroborate that is constructed only by using [TeO₃] polyhedra.

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.

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.

Second-order nonlinear optical materials with a benzene-like conjugated π system

This feature article highlights the strategies used to design UV/DUV NLO materials based on benzene-like π-conjugated units.

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.

Nontoxic KBBF Family Member Zn2BO3(OH): Balance between Beneficial Layered Structure and Layer Tendency

The conflict between beneficial layered structure for performances and layered growth habits in KBe2BO3F2 (KBBF) always restricts its practical applications. A beryllium-free KBBF family member, Zn2BO3(OH), is explored to feature the same topological layer with KBBF by replacing [BeO3F]5- with [ZnO3(OH)]5- and excellent UV performances. It exhibits a second harmonic generation response of about 1.5 × KH2PO4 with the UV cutoff edge of 204 nm. The birefringence of Zn2BO3(OH) in the visible region is about 0.067, which is larger than those of commercial UV crystals LiB3O5, CsB3O5, and CsLiB6O10. Additionally, it has excellent thermal and water-resistant stabilities. Owing to the removal of interlayer cations, Zn2BO3(OH) shows better growth habits than KBBF while achieving the balance between beneficial layered structure and layer tendency.

Sr[B(OH)4](IO3) and Li4Sr5[B12O22(OH)4](IO3)2: two unprecedented metal borate-iodates showing a subtle balance of enlarged band gap and birefringence

Two unprecedented metal borate-iodates, Sr[B(OH)₄](IO₃) (SBIO) and Li₄Sr₅[B₁₂O₂₂(OH)₄](IO₃)₂ (LSBIO), were synthesized via the strategy of introducing (IO₃)^- into borates. In particular, the LSBIO possessed a new B₁₂O₂₆ fundamental building block (FBB). The two compounds integrated the advantages of both pure iodates and borates, showing enlarged band gaps and birefringence. Moreover, diverse B-O FBBs achieved the modulation of birefringence. This work reported the first two examples of borate-iodates.

Research and Development of Zincoborates: Crystal Growth, Structural Chemistry and Physicochemical Properties

Borates have been regarded as a rich source of functional materials due to their diverse structures and wide applications. Therein, zincobrates have aroused intensive interest owing to the effective structural and functional regulation effects of the strong-bonded zinc cations. In recent decades, numerous zincoborates with special crystal structures were obtained, such as Cs₃Zn₆B₉O₂₁ and AZn₂BO₃X₂ (A = Na, K, Rb, NH₄; X = Cl, Br) series with KBe₂BO₃F₂-type layered structures were designed via substituting Be with Zn atoms, providing a feasible strategy to design promising non-linear optical materials; KZnB₃O₆ and Ba₄Na₂Zn₄(B₃O₆)₂(B₁₂O₂₄) with novel edge-sharing [BO₄]^5- tetrahedra were obtained under atmospheric pressure conditions, indicating that extreme conditions such as high pressure are not essential to obtain edge-sharing [BO₄]^5--containing borates; Ba₄K₂Zn₅(B₃O₆)₃(B₉O₁₉) and Ba₂KZn₃(B₃O₆)(B₆O₁₃) comprise two kinds of isolated polyborate anionic groups in one borate structure, which is rarely found in borates. Besides, many zincoborates emerged with particular physicochemical properties; for instance, Bi₂ZnOB₂O₆ and BaZnBO₃F are promising non-linear optical (NLO) materials; Zn₄B₆O₁₃ and KZnB₃O₆ possess anomalous thermal expansion properties, etc. In this review, the synthesis, crystal structure features and properties of representative zincoborates are summarized, which could provide significant guidance for the exploration and design of new zincoborates with special structures and excellent performance.

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.

A2SrMIVS4 (A = Li, Na; MIV = Ge, Sn) concurrently exhibiting wide bandgaps and good nonlinear optical responses as new potential infrared nonlinear optical materials

A series of A₂SrM^IVS₄ compounds concurrently exhibiting wide bandgaps and good NLO responses were proven as promising IR NLO materials.

Exploration of new nonlinear optical (NLO) materials is of importance for infrared (IR) applications. However, it is an extremely tough challenge to design and synthesize excellent IR NLO materials with optimal performances (e.g., concurrently a large NLO response and wide bandgap). Herein, four new mixed alkali/alkaline earth metal sulfides, A₂SrM^IVS₄ (A = Li, Na; M^IV = Ge, Sn), were successfully synthesized by a motif-optimization approach using the classical AgGaS₂ as a template. Note that all of them concurrently exhibit wide bandgaps (3.1–3.8 eV) and good NLO responses (0.5–0.8 × AgGaS₂) with phase-matching behavior, which satisfy the balance conditions (E_g ≥ 3.0 eV and d_ij ≥ 0.5 × benchmark AgGaS₂) of optical performances and hence are outstanding IR NLO materials. Remarkably, both of Na₂SrM^IVS₄ have the same structure without the structural transformation (Ge to Sn) in the reported related analogues and an interesting cation-dependent structural change is also found in Na₂M^IISnS₄ (M^II: Sr, R3c vs. Ba, I4[combining macron]2d). These results verify that the above design strategy of motif-optimization provides a feasible guide for the discovery of new IR NLO candidates and the A–AE–M–S (A = alkali metal; AE = alkaline-earth metal; M = Ga, In, Ge, Sn) system was identified as the preferred system for IR NLO materials.

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.

Sn2B7O12F with a 2∞[B14O24]6− layer constructed from the unprecedented [B7O16]11− fundamental building block

A new tin borate fluoride Sn₂B₇O₁₂F features a complicated 3D network composed of the unique [B₇O₁₆]¹¹⁻ FBBs and [Sn₄O₁₂F₂]¹⁸⁻ clusters.

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.

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.

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.