New lead-iodide formates with a strong second-harmonic generation response and suitable birefringence obtained by the substitution strategy

Nonlinear optical (NLO) crystals featuring a strong second-harmonic generation (SHG) response and suitable birefringence to achieve phase-matching are in urgent demand in industrial and commercial applications. Based on the substitution strategy, two new NLO lead-iodide formates, K₂[PbI₂(HCOO)₂] and Rb₂[PbI₂(HCOO)₂], have been successfully synthesized using a moderate mixed-solvothermal method. K₂[PbI₂(HCOO)₂] and Rb₂[PbI₂(HCOO)₂] exhibit strong phase-matching SHG responses of 8 and 6.8 × KDP, respectively, a suitable birefringence and transparent window covering most of the visible light and mid-IR region. Crystal structures and theoretical calculations unveil that the origins of the strong SHG response and suitable birefringence can be credibly attributed to the oriented arrangement of the highly distorted [PbI₂O₄] hexa-coordinated polyhedra, which are consistent in their local dipole moments, as well. This research provides a new strategy to explore high-performance NLO crystals.

SbHPO3F: 2D van der Waals Layered Phosphite Exhibiting Large Birefringence

A novel antimony(III)-based phosphite, SbHPO₃F, featuring a unique two-dimensional (2D) van der Waals layered structure, has been successfully designed and synthesized via the simultaneous employment of optically active moieties including SbO₃F seesaw and tetrahedral HPO₃ groups. Its optimized layered arrangement formed by the alternating connection of 4-membered rings (4-MRs) and 8-MRs endows the title compound with desirable optical properties including a large birefringence and short ultraviolet (UV) cutoff edge, implying that it is a potential UV birefringent material.

Isoreticular Design of KTiOPO4-Like Deep-Ultraviolet Transparent Materials Exhibiting Strong Second-Harmonic Generation

Second-harmonic generation (SHG) is of great technological importance for applications in nonlinear optics, but it remains challenging to engineer large SHG responses in the short-wavelength ultraviolet (UV) region owing to competing microstructure requirements. Herein, we report the first examples of d⁰ transition-metal-based (TM-based) deep-UV-transparent nonlinear optical (NLO) crystals MOF₄H₂ (M = Zr (ZOF), Hf (HOF)), which exhibit unprecedented short UV absorption edges (below 190 nm). Evolving from the KTiOPO₄ (KTP) structure by an isoreticular node substitution strategy, the three-dimensional frameworks of ZOF and HOF consist of corner-sharing [MO₂F₆] moieties that are new functional units in deep-UV NLO material design, conferring wide UV transparency and strong phase-matchable SHG response (2.2 × KH₂PO₄ (ZOF) and 1.8 × KH₂PO₄ (HOF) at 1064 nm). Such d⁰-TM-based [MO₂F₆] polyhedra preclude deleterious d-d electronic transitions, resulting in significantly blue-shifted UV absorption edges of ZOF and HOF (<190 nm). The d⁰-TM-based [MO₂F₆] polyhedra introduced in this work offer a new perspective in the construction of deep-UV transparent NLO materials, demonstrating the feasibility of an isoreticular design strategy in developing functional NLO materials.

Oxychalcogenides as Promising Ultraviolet Nonlinear Optical Candidates: Experimental and Theoretical Studies of AEGeOS2 (AE = Sr and Ba)

Oxychalcogenides have gained widespread attention as promising infrared nonlinear optical (IR-NLO) candidates. However, high-performance oxychalcogenides have rarely been reported in the ultraviolet (UV) region owing to the low energy gaps (E_g < 4.0 eV). Herein, two non-centrosymmetric (NCS) oxychalcogenides with one-dimensional (1D) chain structures and wide E_g (>4.3 eV), namely, AEGeOS₂ (AE = Sr and Ba), have been discovered by combined experiments and theory calculations as a new source of UV-NLO materials. Significantly, they exhibit excellent comprehensive performance comparable to the commercial UV-NLO material KH₂PO₄ (KDP), including large phase-matching ranges (>380 nm), sufficient second harmonic generation intensities (0.7-1.1 × KDP), high laser-induced damage thresholds (1.2 × KDP), wide transparent regions (0.26-12.2 μm), and good thermal stability (up to 1100 K). Moreover, systematic structure-activity relationship analysis illustrates that the 1D homochiral helical [GeOS₂]^2- chains composed of heteroanionic [GeS₂O₂] units make major contribution to the desirable UV-NLO performance. This work makes the two compounds shine out as new energy in the UV-NLO field and offers a new perspective for the exploration of structure-driven functional oxychalcogenides.

[C(NH2)3]2MoO2F4·H2O: Unique Chinese-Knot Structure Revealing Superior Nonlinear-Optical Properties

A novel organic-inorganic hybrid guanidine fluoromolybdate, [C(NH₂)₃]₂MoO₂F₄·H₂O, was successfully synthesized via our proposed cation-anion synergetic interaction strategy. The title compound features a unique Chinese-knot structure constructed by hydrogen-bonding interactions, which induces an all-around improvement of the optical band gap, second-harmonic-generation effect, and phase-matchable ability compared with the reported fluoromolybdates, demonstrating that it is a promising UV nonlinear-optical material.

Sb5O7I: Exploration of Ternary Antimony-Based Oxyhalide as a Nonlinear-Optical Material

Metal oxyhalides are attracting extensive interest for their enchanting structures and diverse properties. Herein, a ternary antimony oxyiodide, Sb₅O₇I with the new hexagonal noncentrosymmetric P6₃ structure is systematically surveyed by focusing on its nonlinear-optical (NLO) behavior. Its two-dimensional structure is constructed by {Sb₂[Sb₃O₇]}_∞⁺ layers separated by charge-balanced I^- anions. The second-harmonic-generation measurement result suggests that Sb₅O₇I is NLO-active, and the effect is assigned to the [SbO₃]^3- triangular pyramids' contribution. Sb₅O₇I shows a direct optical energy gap of 3.22 eV, which is the largest among all reported ternary oxyiodides. This work is the first investigation of ternary NLO Sb-based oxyhalides and enriches the study of metal oxyhalides as promising NLO materials.

(C3N6H7)2SbF5·H2O Exhibiting Strong Optical Anisotropy from the Optimal Arrangement of π-Conjugated (C3N6H7)+ Groups

An antimony fluoride melamine birefringent crystal, (C₃N₆H₇)₂SbF₅·H₂O, was obtained by introducing the π-conjugated delocalized melamine and antimony trifluoride via a simple aqueous solution evaporation method. It features one-dimensional parallel [C₃N₆H₇]_∞ chains further connected by hydrogen bonds originated from [SbF₅]^2- groups with lone pairs. The experimental optical band gap (4.74 eV) allows it to be used in the ultraviolet region. The first-principles calculations suggest that (C₃N₆H₇)₂SbF₅·H₂O exhibits a large birefringence (∼0.38@550 nm), which is twice larger than that of the commercial CaCO₃ crystal. Therefore, introducing the fluoride into π-conjugated melamine may be a good tactic to obtain birefringent crystals with large optical anisotropy.

Sharp Enhancement of Birefringence in Antimony Oxalates Achieved by the Cation-Anion Synergetic Interaction Strategy

Birefringent materials with large birefringence play an important role in in laser science and technology owing to their ability to modulate polarized light. However, the lack of systematic and effective synthesis strategies severely hinders the development of novel superior birefringent materials. Herein, the cation-anion synergetic interaction strategy was proposed to successfully synthesize two excellent UV birefringent materials, RbSb(C₂O₄)F₂·H₂O and [C(NH₂)₃]Sb(C₂O₄)F₂·H₂O. Both compounds feature unprecedented [Sb(C₂O₄)F₂]_∞^- anionic chains composed of planar π-conjugated [C₂O₄]^2- units and a distorted SbO₄F₂ complex with stereochemically active lone pairs, which induce a large optical anisotropy. Remarkably, further enhancement of birefringence in [C(NH₂)₃]Sb(C₂O₄)F₂·H₂O was achieved via cation-anion synergetic interactions between the [C(NH₂)₃]⁺ cationic groups and [Sb(C₂O₄)F₂]_∞^- anionic chains. It exhibited a giant birefringence of 0.323@546 nm, twice larger than that of its analogue RbSb(C₂O₄)F₂·H₂O (0.162@546 nm). A detailed structural analysis and theoretical calculations revealed that the cation-anion synergetic interaction strategy is an effective strategy for the efficient exploration of superior birefringent materials, which will guide the further exploration of new structure-driven functional materials.

[C(NH2)3]2Zn(CO3)2: A Guanidinium-Templated Ultraviolet Nonlinear Optical Material

A novel guanidinium-templated ultraviolet (UV) nonlinear optical zinc carbonate crystal, [C(NH₂)₃]₂Zn(CO₃)₂ (GZCO), has been synthesized in a closed system at low temperatures. GZCO crystallizing in the tetragonal noncentrosymmetric nonpolar space group, P4₁2₁2 exhibits a three-dimensional anionic framework constructed by interconnected [Zn₆C₆O₃₂] 12-membered ring channels with inorganic CO₃ triangles and ZnO₄ tetrahedra. Notably, the anhydrous GZCO shows a very high thermal stability among guanidine-based hybrid NLO materials benefiting from the confinement effect of the organic cations within inorganic channels. The UV-visible transmittance spectrum reveals that GZCO has a short UV cutoff edge of 210 nm, corresponding to the large band gap of 5.9 eV. GZCO exhibits a mild second-harmonic generation efficiency of 0.5 × KH₂PO₄ with type-I phase-matching behavior.

A Hybrid Halide Perovskite Birefringent Crystal

Birefringent crystals that can modulate the polarization of light play a significant role in modern optical devices including polarizing microscopes, optical isolators, and achromatic quarter-wave plates. To date, commercial birefringent crystals are exclusively limited to purely inorganic compounds. Here we report a new organic-inorganic hybrid halide, MLAPbBr₄ (MLA=melamine), which features a (110)-oriented layered perovskite structure. Although the 6s² lone-pair electrons of Pb²⁺ cations are stereochemically inert, MLAPbBr₄ exhibits a birefringence of 0.322@550 nm, which exceeds those of all commercial birefringent crystals. The first-principles calculations reveal that this birefringence should be ascribed to the highly dislocated π-conjugation of MLA cations and high distortion of PbBr₆ octahedra. This work highlights the persistently neglected great potential of hybrid halide perovskites as birefringent crystals.

Uncovering a Vital Band Gap Mechanism of Pnictides

Pnictides are superior infrared (IR) nonlinear optical (NLO) material candidates, but the exploration of NLO pnictides is still tardy due to lack of rational material design strategies. An in-depth understanding structure-performance relationship is urgent for designing novel and eminent pnictide NLO materials. Herein, this work unravels a vital band gap mechanism of pnictides, namely P atom with low coordination numbers (2 CN) will cause the decrease of band gap due to the delocalization of non-bonding electron pairs. Accordingly, a general design paradigm for NLO pnictides, ionicity-covalency-metallicity regulation is proposed for designing wide-band gap NLO pnictides with maintained SHG effect. Driven by this idea, millimeter-level crystals of MgSiP2 are synthesized with a wide band gap (2.34 eV), a strong NLO performance (3.5 x AgGaS2 ), and a wide IR transparency range (0.53-10.3 µm). This work provides an essential guidance for the future design and synthesis of NLO pnictides, and also opens a new perspective at Zintl chemistry important for other material fields.

Metal oxyhalides: an emerging family of nonlinear optical materials

Second-order nonlinear optical (NLO) materials have drawn enormous academic and technological attention attributable to their indispensable role in laser frequency conversion and other greatly facilitated applications. The exploration of new NLO materials with high performances thus has long been an intriguing research field for chemists and material scientists. However, an ideal NLO material should simultaneously satisfy quite a few fundamental yet rigorous criteria including a noncentrosymmetric structure, large NLO coefficients, desired transparent range, large birefringence, high laser damage threshold, and availability of a large-size single crystal. Therefore, the identification of promising compound systems, targeted design, and experience-based syntheses are crucial to discover novel NLO materials working in the spectral region of interest. As an important family of mixed-anion compounds, versatile metal oxyhalides containing metal-centered oxyhalide functional units ([MO m X n ] (X = F, Cl, Br, and I)) are becoming a marvelous branch for interesting NLO materials. Especially, when the central metals are d0/d10 transition metals or heavy post-transition metals, a number of novel NLO materials with superior functionalities are expected. Our thorough review on the recent achievements of metal oxyhalides for NLO materials are divided into the fast-growing NLO metal oxyhalides with single type halogen anions and the newly identified NLO metal oxyhalides with mixed halogen anions. Here we mainly focus on the design strategy, structural chemistry, NLO-related properties, and structure-property correlation of the metal oxyhalides with relatively large NLO responses. We hope this review can provide an insight on the rational design and future development of emerging metal oxyhalides for NLO and other applications.

NaGa3Se5: An Infrared Nonlinear Optical Material with Balanced Performance Contributed by Complex {[Ga3Se5]-}∞ Anionic Network

Second-order nonlinear optical (NLO) materials are extensively applied in laser-related techniques. For developing IR NLO materials, chalcogenides are the main candidates. Here, NaGa₃Se₅ was explored as inspired by its unique anionic structure. It crystallizes with the orthorhombic chiral P2₁2₁2₁ structure, featuring 12 types of GaSe₄ tetrahedra built into a three-dimensional {[Ga₃Se₅]^-}_∞ anionic network, representing a new NLO-functional motif. NaGa₃Se₅ exhibits large and phase-matchable NLO response 1.37 × AgGaS₂. It has the largest band gap among the noncentrosymmetric A-M^III-Se (A = alkali metal; M = Ga, In) compounds. The NLO properties' origin is explored via theoretical analysis. The success of NaGa₃Se₅ contributes a practical case for exploring new NLO materials.

K3ZrF4(SbF4)(SbF5) and K8(ZrF6)(Sb2Zr2F20): Two Zirconium Fluoroantimonites with Low Dimensional Structures and Wide Transparency Range

The first examples of zirconium fluoroantimonites, namely, K₃ZrF₄(SbF₄)(SbF₅) and K₈(ZrF₆)(Sb₂Zr₂F₂₀), have been successfully synthesized by facial hydrothermal reactions. K₃ZrF₄(SbF₄)(SbF₅) features a unique 1D (ZrSb₂F₁₃)^3- double-chain structure, while K₈(ZrF₆)(Sb₂Zr₂F₂₀) displays a special 0D construction composed of Zr₂Sb₂F₂₀ tetranuclear clusters and isolated ZrF₆ octahedra. The two fluorides can exhibit a broad transparency range with almost no absorption peaks from ultraviolet to near-IR. For K₈(ZrF₆)(Sb₂Zr₂F₂₀), a phase transformation was found before decomposition. The band structures, density of states, and linear-optical properties for the title compounds were also obtained.

Corrugated 1D Hybrid Metal Halide [C6H7ClN]CdCl3 Exhibiting Broadband White-Light Emission

Organic-inorganic hybrid metal halides (OIMHs) exhibiting white-light emission are a splendid class of emitters and are regarded as desired phosphors for solid-state lighting applications. Here we report a single-component white-light-emitting hybrid metal halide, namely, [C₆H₇ClN]CdCl₃ (C₆H₇ClN = 4-(chloromethyl)pyridinium), which features a corrugated 1D anionic double chain composed of edge-shared CdCl₆ octahedrons and exhibits broadband white-light emission with a photoluminescence quantum yield of 12.3% under 365 nm UV light irradiation. Density functional theory calculations and temperature-dependent emission spectral analysis unveil that the broadband emission of [C₆H₇ClN]CdCl₃ is ascribed to self-trapped excitons. Moreover, a single-component white-light-emitting diode device with a correlated color temperature of 5214 K and color rendering index of 83.7 can be fabricated via coating [C₆H₇ClN]CdCl₃ on a 365 nm UV light-emitting diode chip. Such a promising luminescent material provides guidance for the design and synthesis of OIMHs with unique structures and desired properties.

Unprecedented boat-shaped [Mo2O5(IO3)4]2- polyanions induced a strong second harmonic generation response

The first organic-inorganic hybrid guanidine molybdenyl iodate [C(NH₂)₃]₂Mo₂O₅(IO₃)₄·2H₂O was successfully synthesized via an improved moderate hydrothermal method. It features an unprecedented boat-shaped zero-dimensional [Mo₂O₅(IO₃)₄]^2- polyanion cluster, which induces a wide band gap, moderate birefringence and strong second harmonic generation response, indicating that it is a potential nonlinear optical material.

High-Performance Sulfate Optical Materials Exhibiting Giant Second Harmonic Generation and Large Birefringence

Discovering novel sulfate optical materials with strong second-harmonic generation (SHG) and large birefringence is confronted by a great challenge attributed to the intrinsically weak polarizability and optical anisotropy of tetrahedral SO₄ groups. Herein, two superior-performing sulfate optical materials, namely, noncentrosymmetric Hg₃ O₂ SO₄ and centrosymmetric CsHgClSO₄  ⋅ H₂ O, have been successfully synthesized through the introduction of a highly polarizable d¹⁰ metal cation, Hg²⁺ . The unique component layers in the reported compounds, [Hg₃ O₂ SO₄ ]_∞ layers in Hg₃ O₂ SO₄ and [HgClSO₄ (H₂ O)] ∞ - layers in CsHgClSO₄  ⋅ H₂ O, induce enlarged birefringence in each sulfate. Remarkably, Hg₃ O₂ SO₄ exhibits a very large SHG response (14 times that of KH₂ PO₄ ), which is the strongest efficiency among all the reported nonlinear optical sulfates. Detailed theoretical calculations confirm that the employment of highly polarizable Hg²⁺ is an effective strategy to design superior optical materials with large birefringence and strong SHG response.

CsY(SO4)2·4H2O: A Deep-Ultraviolet Birefringent Crystal Induced by an Edge-Sharing Connection

SO₄ tetrahedral groups have weak polarization anisotropy, which thus results in the small birefringence of sulfates. Here, we report new sulfate CsY(SO₄)₂·4H₂O with unprecedented birefringence among deep-ultraviolet (deep-UV) sulfates. Its single crystal (10 mm × 3.5 mm × 1.5 mm) was simply grown by an aqueous solution evaporation technique, and it features a rare layered structure composed of YO₉ polyhedra, SO₄ tetrahedra, and H₂O molecules. Interestingly, each SO₄ group donates two oxygen atoms to edge-share with one adjacent YO₉ polyhedron and thus causes severe distortion of these groups. The characteristic edge-sharing mode gives CsY(SO₄)₂·4H₂O a large birefringence of ∼0.045@546 nm, which is the maximum among deep-UV sulfates and phosphates with similar non-π-conjugated anionic groups. The ultraviolet-visible-near-infrared diffuse reflection and transmission spectra, infrared spectrum, thermal stability, and theoretical calculations are also presented. The fascinating results will improve our understanding of sulfates and may provide useful insights into the exploration of deep-UV sulfates with large birefringence.

Layered Perovskite-like Nitrate Cs2Pb(NO3)2Br2 as a Multifunctional Optical Material

A novel alkali metal lead halide nitrate, Cs₂Pb(NO₃)₂Br₂, has been successfully synthesized via a hydrothermal method. Interestingly, the title compound features a distinctive Ruddlesden-Popper perovskite-like layered structure, which induces the outstanding multifunctional optical properties, including a large birefringence (0.147@546 nm) and broad light-orange emission. Detailed structural analysis and theoretical calculations revealed that the large birefringence originates from the p-π interaction between the Pb²⁺ cations and NO₃ groups and that the excellent luminescence properties derive from the distortion of PbO₄Br₄ polyhedra. This work not only enriches the variant structure types of layered perovskites but also guides the further exploration of all-inorganic multifunctional optical materials.

PO(CH2CH2CF3)3: an organic ultraviolet nonlinear optical material without any anionic group

Researchers have focused on inorganic compounds to design deep ultraviolet (UV) nonlinear optical (NLO) materials according to anionic group theory, such as CO₃²⁻, NO₃⁻, SO₄²⁻ and PO₄³⁻ anions. Here we provide a new route to design an UV NLO material using a pure organic compound without any anions. Compound PO(CH₂CH₂CF₃)₃1, an UV NLO material, has light transmittance up to 83% in the UV spectral region which is larger than most inorganic UV NLO materials. It also displays a wide transparent band, a SHG response of 0.30 × KH₂PO₄, and a cut-off edge below 200 nm. It displays ladder-like nonlinear optical properties weakened by 1.4 times at around T_c, making compound 1 a potential temperature-controlled UV NLO material. Theoretical analyses reveal that the nonlinear optical properties are primarily due to O-2p, P-2p and F-2p.

A new route was provided to design ultraviolet nonlinear optical material using a pure organic compound without any anions. As a potential temperature-controlled UV NLO material, its light transmittance is up to 83% in the UV spectral region.

[C(NH2)3]BiCl2SO4: an excellent birefringent material obtained by multifunctional group synergy

A multifunctional group synergistic strategy was proposed to successfully develop an excellent organic–inorganic hybrid guanidine sulfate birefringent material [C(NH2)3]BiCl2SO4.

Halogen regulation triggers structural transformation from centrosymmetric to noncentrosymmetric switches in tin phosphate halides Sn2PO4X (X = F, Cl)

A tin phosphate halide, centrosymmetric Sn2PO4F, was successfully transformed into noncentrosymmetric Sn2PO4Cl with excellent comprehensive performance by the substitution of F− with Cl−.

Hexameric poly-fluoroberyllophosphate Na4Be2PO4F5 with moderate birefringence and deep-ultraviolet transmission as a potential zero-order-waveplate crystal

Na4Be2PO4F5 features novel [Be4P2O8F10] hexameric isolated groups, caused due to a cut-off effect from F atoms, and it is an optimal choice to act as a deep-UV zero-order waveplate material.

Centrosymmetric Rb[Te2O4(OH)5] and noncentrosymmetric K2[Te3O8(OH)4]: metal tellurates with corner and edge-sharing (Te4O18)12- anion group

New nonlinear optical (NLO) crystals have become urgent need for extending laser wavelengths. By cation substitution between alkali metal (K+, Rb+), anion group arrangement transition were realized, and centrosymmetric Rb[Te2O4(OH)5]...

KLi2CO3F: a Beryllium-free KBBF-type Deep-UV Carbonate with Enhanced Interlayer Interaction and Large Birefringence

KBe2BO3F2 (KBBF) with perfect honeycomb hexagonal layered structure is the sole practical deep-ultraviolet (DUV) nonlinear optical material which could generate DUV coherent light through direct frequency doubling. However, the layered...

K2(BeS)O4F2: a novel fluorosulfate with unprecedented 1D [(BeO3F)–(SO3F)]∞ chains exhibiting large birefringence

By introducing the strongly electronegative F− ions into [SO4] tetrahedral units, polar NLO-active units [(BeS)O4F2] were obtained, thus forming 1D zigzag [(BeO3F)–(SO3F)]∞ chains.

Large optical anisotropy differentiation induced by the anion-directed regulation of structures

The modulation of optical anisotropy for two novel UV birefringent materials [C(NH2)3]2Sb3F3(HPO3)4 and [C(NH2)3]SbFPO4·H2O has been successfully achieved via anion-directing regulation structures.

Hierarchical Modulation on Optical Anisotropy Driven by Metal Cation Polyhedra in Fluorooxoborates MIIB4O6F2 (MII = Be, Mg, Pb, Zn, Cd)

The enhancement mechanism of birefringence is very important to modulate optical anisotropy and materials design. Herein, the different cations extending from alkaline-earth to alkaline-earth, d 10 electron configuration, and 6s 2 lone pair cations are highlighted to explore the influence on the birefringence. A flexible fluorooxoborate framework from AEB 4 O 6 F 2 (AE = Ca, Sr) is adopted for UV/deep-UV birefringent structures, namely, M II B 4 O 6 F 2 (M II = Be, Mg, Pb, Zn, Cd). The maximal enhancement on birefringence can reach 46.6% with the cation substitution from Ca, Sr to Be, Mg ( route-I ), Pb (route-II), and Zn, Cd (route-III). The influence of the cation size, the stereochemically active lone pair, and the binding capability of metal cation polyhedra is investigated for the hierarchical improvement on birefringence. Significantly, the BeB 4 O 6 F 2 structure features the shortest UV cutoff edge 146 nm among the available anhydrous beryllium borates with birefringence over 0.1 at 1064 nm , and the PbB 4 O 6 F 2 structure has the shortest UV cutoff edge 194 nm within the reported anhydrous lead borates that hold birefringence larger than 0.1 at 1064 nm. This work sheds light on how metal cation polyhedra modulate birefringence, which suggests a credible design strategy to obtain desirable birefringent structures by cation control.

Pushing KTiOPO4-like Nonlinear Optical Sulfates into the Deep-Ultraviolet Spectral Region

Accurately designing and synthesizing new deep-ultraviolet (deep-UV) nonlinear optical (NLO) crystals that are limited by the so-called "200 nm wall" on their transparency windows remain challenging. On the basis of a bandgap-directed computer-aided material design approach, two new NLO sulfates, KMgSO₄F and KZnSO₄F, are designed and successfully synthesized. They feature three-dimensional frameworks closely related to the commercial NLO crystal, KTiOPO₄ (KTP). Remarkably, the transmittance spectrum based on a single crystal indicates that the transparency window of KZnSO₄F is significantly blue-shifted to <190 nm from 350 nm for KTP. The microscopic origin of this significant transparent window blue shift is illustrated well by first-principles calculations. This work pushes the transparency windows of KTP-like NLO sulfates into the deep-UV spectral region for the first time and will pave a prospective way to the accurate design and synthesis of new deep-UV NLO materials.

La3 Ga3 Ge2 S3 O10 : An Ultraviolet Nonlinear Optical Oxysulfide Designed by Anion-Directed Band Gap Engineering

Chalcogenide-containing compounds have been widely studied as infrared nonlinear optical (NLO) materials. However, they have never been applied in the ultraviolet (UV) region owing to the high energy levels of chalcogen anions, leading to band gap narrowing. We report the synthesis of a new UV NLO oxysulfide La₃ Ga₃ Ge₂ S₃ O₁₀ with an exceptionally wide band gap of 4.70 eV due to from the unique anion-ordered frameworks comprising 1D ¹ _∞ [(Ga_3/5 Ge_2/5 )₃ S₃ O₃ ] triangular tubes and 0D (Ga_3/5 Ge_2/5 )₂ O₇ dimers of corner-sharing (Ga/Ge)S₂ O₂ and (Ga/Ge)O₄ tetrahedra, respectively. Second harmonic generation (SHG) measurements revealed that La₃ Ga₃ Ge₂ S₃ O₁₀ was phase matchable with twice the SHG response of KH₂ PO₄ . The results of theoretical calculations suggest that the strong SHG response is mainly attributable to the S-3p and O-2p orbitals in the occupied states. The anion-directed band-gap engineering may give insights into the application of NLO oxychalcogenides in the UV regions.

A Lanthanum Ammonium Sulfate Double Salt with a Strong SHG Response and Wide Deep-UV Transparency

The targeted synthesis of deep-ultraviolet (deep-UV) nonlinear optical (NLO) materials, especially those with non-π-conjugated sulfates, has experienced considerable difficulties due to the need to reconcile the oft-competing requirements for deep-UV transparency and strong second-harmonic generation (SHG). We report herein the designed synthesis of the first rare-earth metal-based deep-UV sulfate La(NH₄ )(SO₄ )₂ by a double-salt strategy involving introduction of complementary cations, together with optical studies that reveal a short-wavelength deep-UV absorption edge (below 190 nm) and the strongest SHG response among deep-UV NLO sulfates (2.4×KDP). Theoretical calculations and crystal structure analysis suggest that the excellent balance between SHG response and deep-UV transparency can be attributed to a synergistic interaction of the hetero-cations La³⁺ and [NH₄ ]⁺ , which optimize alignment of the [SO₄ ] tetrahedra and highly polarizable [LaO₈ ] polyhedra.

Synergistic Effect of π-Conjugated [C(NH2)3] Cation and Sb(III) Lone Pair Stereoactivity on Structural Transformation and Second Harmonic Generation

The search for nonlinear optical (NLO) crystals with excellent comprehensive properties is a formidable challenge. In this work, two guanidine antimony fluorides, C(NH₂)₃Sb₂F₇ and C(NH₂)₃SbF₄, were obtained by conjunction of [C(NH₂)₃] groups with π-conjugated configuration and stereochemically active Sb³⁺ cations. Due to the different coordination modes of Sb-F bonds and H-F hydrogen bonds, the crystal structure of C(NH₂)₃Sb₂F₇ is centrosymmetric (CS), while C(NH₂)₃SbF₄ is noncentrosymmetric (NCS). Optical measurements show that the UV cutoff wavelengths of the title compounds were both less than 240 nm. Thermal studies indicate that these crystals are stable up to 250 °C. In addition, the second harmonic generation (SHG) response of C(NH₂)₃SbF₄ is 2 times that of KH₂PO₄ (KDP) with the phase-matchable capacity. Theoretical calculations reveal that the large SHG effects of C(NH₂)₃SbF₄ were attributed to the synergy between the planar [C(NH₂)₃] units and the distorted [SbF₄] groups. These results demonstrate that the guanidine antimony fluorides will have potential value as UV NLO materials.

A Lead Mixed Halide with Three Different Coordinated Anions and Strong Second-Harmonic Generation Response

Nonlinear optical (NLO) crystals are widely applied in information technology, micro-manufacturing and medical treatment. Herein, a new lead mixed halide with strong second-harmonic generation (SHG) response, Cs₃ Pb₂ (CH₃ COO)₂ Br₃ I₂ , has been designed and rationally synthesized. Cs₃ Pb₂ (CH₃ COO)₂ Br₃ I₂ represents the rare NLO crystal featuring that three different anions (I^- , Br^- and O^2- ) simultaneously coordinate the Pb(II) atom to form a severely distorted [PbBr₂ I₂ O₂ ] polyhedron with a large polarizability. Remarkably, Cs₃ Pb₂ (CH₃ COO)₂ Br₃ I₂ not only exhibits a very strong phase-matching SHG response of 9×KH₂ PO₄ (KDP), but also possesses a large birefringence (0.27@1064 nm) and high laser damage threshold (LDT). The strong SHG effect of Cs₃ Pb₂ (CH₃ COO)₂ Br₃ I₂ mainly originates from the oriented arrangement of [Pb₂ Br₃ I₂ ] chains. This study points out an effective strategy to develop new NLO crystals with strong SHG response.

Strong SHG Responses in a Beryllium-Free Deep-UV-Transparent Hydroxyborate via Covalent Bond Modification

Deep-ultraviolet (deep-UV) nonlinear optical (NLO) crystals are key materials in creating tunable deep-UV lasers for frequency conversion technology. However, practical application of the sole usable crystal, KBe₂ BO₃ F₂ , has been hindered by the high toxicity of beryllium and its layering tendency in crystal growth. Herein, we report a beryllium-free deep-UV NLO material NaSr₃ (OH)(B₉ O₁₆ )[B(OH)₄ ] (NSBOH), synthesized by a covalent bond modification strategy under hydrothermal conditions. Moisture-stable NSBOH exhibits strong second-harmonic generation (SHG) at 1064 nm (3.3 × KH₂ PO₄ ) and 532 nm (0.55 × β-BaB₂ O₄ ), both amongst the largest powder SHG responses for a deep-UV borate, with good phase-matchability and a short wavelength cutoff edge (below 190 nm). NSBOH possesses a 3D covalent anionic [B₉ O₁₉ ]_∞ honeycomb-like framework with no layering. The Sr²⁺ and Na⁺ ions, residing in the cavities of the anionic framework, act as templates for the assembly and favorable alignment of NLO-active groups, resulting in an optimal balance between strong SHG activities and wide UV transparency. These merits indicate NSBOH is a very attractive candidate for deep-UV NLO applications.

Achieving Short-Wavelength Phase-Matching Second Harmonic Generation in Boron-Rich Borosulfate with Planar [BO3 ] Units

Ultraviolet (UV) nonlinear optical (NLO) crystals which can produce short-wavelength lasers via a direct second harmonic generation (SHG) process are of great importance in modern laser technology. Currently, the exploration of UV NLO crystals in borosulfates is nearly stagnant since the non-phase matching (PM) property lies on the small birefringence induced by the intrinsically small optical anisotropy of the tetrahedral groups. Herein, for the first time, the planar [BO₃ ] units were introduced into borosulfates leading to a boron-rich borosulfate (NH₄ )₂ B₄ SO₁₀ with unprecedented [B₄ SO₁₀ ]_∞ layers and evidently enhanced birefringence. To the best of our knowledge, it achieves the shortest SHG PM wavelength of 252 nm in all reported borosulfates with deep UV cutoff edge (184 nm), large SHG response (1.1×KDP at 1064 nm and 0.15×β-BBO at 532 nm) and large birefringence (0.053 at 1064 nm) and is easy to grow single crystals via simple chemical vapor deposition method. These results confirm the feasibility of utilizing planar [BO₃ ] units to optimize birefringence of borosulfates, and also open up broad prospects for UV NLO crystals in boron-rich borosulfates.

2D van der Waals Layered [C(NH2)3]2SO3S Exhibits Desirable UV Nonlinear-Optical Trade-Off

It remains a challenge to develop UV nonlinear optical (NLO) crystals that can achieve a desirable trade-off on UV absorption edge, second harmonic generation (SHG), and birefringence. Here we report a thiosulfate UV NLO crystal of a 2D van der Waals layered structure, [C(NH₂)₃]₂SO₃S. Remarkably, this thiosulfate realizes the desired trade-off, with a short absorption edge of 254 nm, a strong SHG response of approximately 2.8 times that of the benchmark KH₂PO₄, and a sufficient birefringence of 0.073 at the wavelength of 546 nm. In addition, it exhibits strong in-plane anisotropy of the SHG intensity. According to the first-principles calculations, the non-π-conjugated [SO₃S]^2- anion is the dominant SHG functional gene, while the π-conjugated [C(NH₂)₃]⁺ cation serves as the functional gene of birefringence. This is different from common UV NLO materials whose functionals of SHG and birefringence are the same. These findings indicate that combining different function genes may be an effective strategy to develop outstanding NLO materials with the desirable property trade-off.

The tin sulfates Sn(SO4)2 and Sn2(SO4)3: crystal structures, optical and thermal properties

We report the crystal structures of two tin(IV) sulfate polymorphs Sn(SO4)2-I (P2₁/c (no. 14), a = 504.34(3), b = 1065.43(6), c = 1065.47(6) pm, β = 91.991(2)°, 4617 independent reflections, 104 refined parameters, wR₂ = 0.096) and Sn(SO4)2-II (P2₁/n (no. 14), a = 753.90(3), b = 802.39(3), c = 914.47(3) pm, β = 92.496(2)°, 3970 independent reflections, 101 refined parameters, wR₂ = 0.033). Moreover, the first heterovalent tin sulfate Sn2(SO4)3 is reported which adopts space group P1̄ (no. 2) (a = 483.78(9), b = 809.9(2), c = 1210.7(2) pm, α = 89.007(7)°, β = 86.381(7)°, γ = 73.344(7)°, 1602 independent reflections, 152 refined parameters, wR₂ = 0.059). Finally, SnSO4 - the only tin sulfate with known crystal structure - was revised and information complemented. The optical and thermal properties of all tin sulfates are investigated by FTIR, UV-vis, luminescence and ¹¹⁹Sn Mössbauer spectroscopy as well as thermogravimetry and compared.