CsSbF 2 SO 4 : An Excellent Ultraviolet Nonlinear Optical Sulfate with a KTiOPO 4 (KTP)‐type Structure

The First UV Nonlinear Optical Selenite Material: Fluorination Control in CaYF(SeO3 )2 and Y3 F(SeO3 )4

It is a great challenge to develop UV nonlinear optical (NLO) material due to the demanding conditions of strong second harmonic generation (SHG) intensity and wide band gap. The first ultraviolet NLO selenite material, Y₃ F(SeO₃ )₄ , has been obtained by control of the fluorine content in a centrosymmetric CaYF(SeO₃ )₂ . The two new compounds represent similar 3D structures composed of 3D yttrium open frameworks strengthened by selenite groups. CaYF(SeO₃ )₂ has a large birefringence (0.138@532 nm and 0.127@1064 nm) and a wide optical band gap (5.06 eV). The non-centrosymmetric Y₃ F(SeO₃ )₄ can exhibit strong SHG intensity (5.5×KDP@1064 nm), wide band gap (5.03 eV), short UV cut-off edge (204 nm) and high thermal stability (690 °C). So, Y₃ F(SeO₃ )₄ is a new UV NLO material with excellent comprehensive properties. Our work shows that it is an effective method to develop new UV NLO selenite material by fluorination control of the centrosymmetric compounds.

An efficient calcium-based sorbent for flue gas dry-desulfurization: promotion roles of nitrogen oxide and oxygen

The development of sorbents for flue gas desulfurization in a dry mode is essential to control emission of sulfur dioxide. Based on the novel concept of "treating waste with waste", a low-cost and highly activated calcium-based sorbent (ACS) was prepared using coal fly ash, CaO and waste gypsum as the raw materials via the one-step incipient wetness impregnation method. Based on characterization using scanning electron microscopy and nitrogen adsorption-desorption, the ACS possessed a fibrous and netted structure with high porosity, which improved SO₂ adsorption greatly. The SO₂ adsorption capacity of ACS with coal fly ash/CaO/CaSO₄ = 1/2/1 was high, up to 44.26 mg g^-1, with 100% removal efficiency at 150 °C. In the absence of O₂, SO₂ was rapidly adsorbed on the sorbent to form CaSO₃ according to in situ DRIFTS analysis, while when O₂ was present in the flue gas, SO₂/SO₃ ^2- tended to be oxidized into SO₄ ^2- species. Moreover, the presence of NO can further enhance the SO₂ adsorption capacity of the ACS due to the formation of adsorbed NO₂ or nitrate species with strong oxidizing properties. Therefore, the ACS can be considered as a sustainable sorbent with the advantage of employing fly ash for the removal of sulfur dioxide.

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.

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.

Crystal Structure and Theoretical Analysis of Cs2Ca3(SO4)4

Using the homovalent cation substitution strategy, a new sulfate, Cs2Ca3(SO4)4, was successfully prepared using the spontaneous crystallization technique. A single-crystal structure measurement suggested that it crystallizes in space group P21/c, with lattice parameters and molecules per unit cell of a = 9.9153(8), b = 9.3760(6), c = 9.8044(9), β = 118.365(3)°, V = 802.04(11), and Z = 2. In the structure of Cs2Ca3(SO4)4, CaO6 octahedra and SO4 tetrahedra are interconnected via a corner-sharing mode to form a three-dimensional framework comprising large cavities filled with Cs+ cations. First-principles calculations and diffuse reflectance spectra indicated that Cs2Ca3(SO4)4 has a large energy band gap. Moreover, structural comparisons with similar compounds were conducted to explain the role of cations in tuning structural symmetry and birefringence. This paper helps to explain the size effect of cations on structural evolution.

KPb3(o-C5H4NCOO)2Cl5: A Brand-New Stable Lead Chloride with Good Comprehensive Nonlinear Optical Performances

The discovery of nonlinear optical (NLO) crystals with excellent performances has still been a great challenge. Herein, a brand-new stable lead chloride, KPb3(o-C5H4NCOO)2Cl5, has been successfully synthesized and well characterized....

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.

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.

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.

Na7(IO3)(SO4)3: the first noncentrosymmetric alkaline-metal iodate-sulfate with isolated [IO3] and [SO4] units

Metal iodates are of current research interest because of their potential application as nonlinear optical crystals but the exploration of new iodates is mainly concentrated on complex cation iodates. In contrast, iodates with multiple anion groups are rarely reported. In this communication, the first noncentrosymmetric alkaline-metal iodate-sulfate, Na₇(IO₃)(SO₄)₃ has been designed, synthesized and characterized.

[Ag(NH3 )2 ]2 SO4 : A Strategy for the Coordination of Cationic Moieties to Design Nonlinear Optical Materials*

Over recent decades, guided by the anionic group theory of nonlinear optical (NLO) materials, rational design strategies have been primarily focused on anionic moieties; consequently, structural modification and design of cationic moieties have long been neglected. Herein, we report a strategy for the coordination of cationic moieties that substantially enhances the optical properties of NLO materials. For an example with well-known crystal structure, [Ag(NH₃ )₂ ]₂ SO₄ , we demonstrate that the coordination of the Ag⁺ cation by the neutral ligand drives the formation of a noncentrosymmetric tetragonal P 4 ‾ 2₁ c structure as a positive uniaxial crystal. The bending of the [Ag(NH₃ )₂ ]⁺ cationic moiety parallel to the z-axis generates an anisotropic arrangement of the dipoles, i.e., a dipole of 0.12 D along the z-direction, yet zero dipole in the xy-plane, which interacts anisotropically with the incident light oscillating electromagnetic wave, leading to optical anisotropy with a large birefringence. The incident beam of 589.3 nm normal to the (110) crystal plane measures Δn_obv. =0.08, and [Ag(NH₃ )₂ ]₂ SO₄ also exhibits a phase-matching NLO response 1.4 times that of KH₂ PO₄ (KDP) (obv. 1.4×KDP @1064 nm; cal. d₃₆ =1.50 pm V^-1 ).

KCs2 [Pb2 Br5 (HCOO)2 ]: A Polar 3D Lead-Bromide Framework Exhibiting Strong Second-Harmonic Generation Response

The discovery of new nonlinear optical (NLO) crystals with excellent properties is in urgently demand because of their ability to generate coherent light. Herein, we report an unique NLO lead bromide formate, KCs₂ [Pb₂ Br₅ (HCOO)₂ ], which has been synthesized by a mix-solvothermal method. KCs₂ [Pb₂ Br₅ (HCOO)₂ ] exhibits strong phase-matching second-harmonic generation (SHG) response (6.5×KDP), large birefringence (0.16@ 1064 nm), and a wide transparent window in most visible light and mid-IR region. Interestingly, KCs₂ [Pb₂ Br₅ (HCOO)₂ ] features a polar 3D lead-bromide framework in which adjacent Pb-Br layers containing coplanar Pb₆ Br₆ rings are not only parallel to each other, but also orient in the same direction. These oriented arrangements are responsible for the strong SHG response and large birefringence that are elucidated by both local dipole moment and theoretical calculations. This research provides a new strategy to explore subsequent NLO crystals.

Be2 (BO3 )(IO3 ): The First Anion-mixed Van der Waals Member in the KBe2 BO3 F2 Family with a Very Strong Second Harmonic Generation Response

To obtain new nonlinear optical (NLO) materials with a large second harmonic generation (SHG) effect has always been a great challenge. We have synthesized the first metal borate-iodate NLO crystal, Be₂ (BO₃ )(IO₃ ) (BBIO), by multicomponent modification of KBe₂ BO₃ F₂ (KBBF), in which the structural features of KBBF were maintained and (IO₃ )^- groups were connected to honeycomb [Be₂ BO₃ O₂ ]_∞ layers. As the first KBBF family member with mixed anionic groups, BBIO benefited from the synergistic effect of (IO₃ )^- , (BO₃ )^3- and (BeO₄ )^6- groups, and exhibited a very strong SHG response of ≈7.2×KH₂ PO₄ (KDP, @1064 nm) and a large birefringence (Δn) of 0.172 at 1064 nm. BBIO may, unexpectedly, optimize growth habits via van der Waals forces. This study presents borate-iodate as a new NLO material and it demonstrates opportunities in KBBF structural engineering.

UV Solar-Blind-Region Phase-Matchable Optical Nonlinearity and Anisotropy in a π-Conjugated Cation-Containing Phosphate

Wide ultraviolet (UV) transparency, strong second-harmonic generation (SHG) response, and sufficient optical birefringence for phase-matching (PM) at short SHG wavelengths are vital for practical UV nonlinear optical (NLO) materials. However, simultaneously optimizing these properties is a major challenge, particularly for metal phosphates. Herein, we report a non-traditional π-conjugated cation-based UV NLO phosphate [C(NH₂ )₃ ]₆ (PO₄ )₂ ⋅3 H₂ O (GPO) with a short UV cutoff edge. GPO is SHG active at 1064 nm (3.8 × KH₂ PO₄ @ 1064 nm) and 532 nm (0.3 × β-BaB₂ O₄ @ 532 nm) and also possesses a significant birefringence (0.078 @ 546 nm) with a band gap >6.0 eV. The PM SHG capability of GPO can extend to 250 nm, indicating GPO is a promising UV solar-blind NLO material. Calculations and crystal structure analysis show that the rare coexistence of wide UV transparency, large SHG response, and optical anisotropy is due to the introduction of π-conjugated cations [C(NH₂ )₃ ]⁺ and their favorable arrangement with [PO₄ ]^3- anions.

Na6 MQ4 (M=Zn, Cd; Q=S, Se): Promising New Ternary Infrared Nonlinear Optical Materials

Four sodium-based ternary IR nonlinear optical (NLO) materials, Na₆ MQ₄ (M=Zn, Cd; Q=S, Se), were prepared through a high-temperature flux method. The crystal structure of the compounds is built up of isolated [MQ₄ ] tetrahedra and a 3D framework formed by the NaQ_n (n=4, 5) units. The two selenides, Na₆ MSe₄ (M=Zn, Cd), as promising IR NLO materials, show moderate second-harmonic generation (SHG) responses (0.9 and 0.5×AgGaS₂ ) with good phase-matching behavior, as well as high laser damage thresholds (2 and 1.9×AgGaS₂ ). The two sulfides, Na₆ MS₄ (M=Zn, Cd), exhibit higher laser damage thresholds (13 and 4×AgGaS₂ ), but smaller SHG responses (0.3 and 0.2×AgGaS₂ ). Theoretical calculations and statistical analyses indicate that the SHG effect and band gap in the compounds originate mainly from the distorted NaQ₄ NLO-active units with a short Na-S bond length, which provides a new insight into the design of novel IR NLO materials.

An Antimony(III) Fluoride Oxalate with Large Birefringence

Birefringent materials play a key role in modulating the polarization of light and thus in optical communication as well as in laser techniques and science. Designing new, excellent birefringent materials remains a challenge. In this work, we designed and synthesized the first antimony(III) fluoride oxalate birefringent material, KSb₂ C₂ O₄ F₅ , by a combination of delocalized π-conjugated [C₂ O₄ ]^2- groups, stereochemical active Sb³⁺ cations, and the most electronegative element, fluorine. The [C₂ O₄ ]^2- groups are not in an optimal arrangement in the crystal structure of KSb₂ C₂ O₄ F₅ ; nonetheless, KSb₂ C₂ O₄ F₅ exhibits a large birefringence (Δn=0.170 at 546 nm) that is even better than that of the well-known commercial birefringent material α-BaB₂ O₄ , even though the latter features an optimal arrangement of π-conjugated [B₃ O₆ ]^3- groups. Based on first-principles calculations, this prominent birefringence should be attributed to the alliance of planar π-conjugated [C₂ O₄ ]^2- anions, highly distorted SbO₂ F₂ and SbOF₃ polyhedra with a stereochemically active lone pair. The combination of lone-pair electrons and π-conjugated systems boosts the birefringence to a large extent and will help the development of high-performance birefringent materials.

Reply to the Correspondence on "K2 Sb(P2 O7 )F: Cairo Pentagonal Layer with Bifunctional Genes Reveal Optical Performance"

In their Correspondence, Sergey V. Krivovichev and Luca Bindi argue that the novel excellent nonlinear optical material antimony pyrophosphate, K₂ Sb(P₂ O₇ )F, reported in our Research Article published in this journal belongs to the fresnoite Ba₂ [TiO(Si₂ O₇ )] group and its structure is not novel in terms of either topology or local coordination geometry around Sb³⁺ cations. We clarify that the statements regarding structural features of our original paper are valid in context.

Anionic Aliovalent Substitution from Structure Models of ZnS: Novel Defect Diamond-like Halopnictide Infrared Nonlinear Optical Materials with Wide Band Gaps and Large SHG Effects

To design pnictide nonlinear optical materials with wide band gap and large second-harmonic generation, the heavy halogen I was introduced into pnictides through anionic aliovalent substitution with diamond-like ZnS as templates. Thus, four excellent halopnictide-based infrared nonlinear optical crystals, M^II ₃ PnI₃ (M^II =Zn, Cd; Pn=P, As), were obtained. They all exhibited defect diamond-like structures with highly parallel-oriented [M^II PnI₃ ] mixed-anionic tetrahedral groups, leading to excellent physical properties including wide band gaps (2.38-2.85 eV), large second harmonic generation responses (2.7-5.1×AgGaS₂ ), high laser-induced damage thresholds (5.5-10.7×AgGaS₂ ), and good IR transparency. In particular, Cd₃ PI₃ and Cd₃ AsI₃ achieved phase-matching (Δn=0.035 and 0.031) that their template β-ZnS could not do. Anionic aliovalent substitution provides a feasible strategy to design novel promising halopnictide IR NLO materials.

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.