Phase-matching Behavior Research Articles

(NH4)2Cd2Cl3F3 and (NH4)2Cd2Br3F3: The First Fluoride-Containing d10 Metal Mixed Halides Exhibiting Superior Ultraviolet Nonlinear Optical Properties.

In the search for new ultraviolet (UV) nonlinear optical (NLO) materials, two novel cadmium mixed halide compounds, (NH4)2Cd2Cl3F3 and (NH4)2Cd2Br3F3, are successfully synthesized via hydrothermal methods. These compounds crystallize in the noncentrosymmetric (NCS) space group, R32, and are composed of distorted octahedral [CdX3F3] (X═Cl or Br) units, which extend into a 3D framework. Remarkably, both compounds demonstrate strong second-harmonic generation (SHG) efficiencies-3.0 and 8.0 times that of KH2PO4 for the Cl- and Br-containing analogs, respectively-with phase-matching behavior observed. The SHG efficiency is attributed to the highly distorted coordination environment of the polarizable d10 Cd2+ ions, with (NH4)2Cd2Br3F3 benefiting further from Br's greater polarizability. Furthermore, these compounds exhibit wide bandgaps exceeding 4.2eV, making them the first d10 metal mixed halide systems incorporating fluoride that are suitable for UV NLO applications. With UV absorption cut-off edges as short as 203nm for (NH4)2Cd2Cl3F3 and 243nm for (NH4)2Cd2Br3F3, these materials represent a significant advancement in the development of UV-transparent NLO materials. This study introduces a novel synthetic strategy for the design of d10 mixed halide systems with enhanced optical properties, offering promising candidates for future UV NLO technologies.

Deep-Ultraviolet Transparent Nonlinear Optical Phosphates: A New Alternative to Conventional Borates.

Deep ultraviolet (DUV) nonlinear optical (NLO) crystals with balanced performance are crucial for extending laser wavelengths into the DUV region, essential for various laser applications. However, developing ideal DUV crystals is challenging due to stringent requirements: strong second-harmonic generation (SHG) response, short cut-off wavelength, and effective phase-matching behavior. DUV NLO borates, which feature π-conjugated groups, have garnered attention for their higher SHG coefficients compared to phosphates. However, phosphates typically show short UV cut-off edges due to the large HOMO-LUMO gap (approximately 9.6 eV) of the [PO₄] units. To enhance the SHG effect while maintaining DUV transparency in phosphates, several strategies have been proposed: 1) using distorted polymerized P-O groups like isolated C₁-[P₃O₁₀]⁵- and [P₂O₇]⁴-; 2) aligning isolated [PO₄]3- tetrahedra along the polar screw axis; 3) introducing additional NLO-active units; and 4) exploring new units derived from [PO₄]3-, such as [PO₃F]2- tetrahedra. These strategies have led to the successful development of various non-centrosymmetric phosphates, highlighting their potential as DUV NLO candidates. This review explores the relationship between their crystal structures and DUV NLO performance, and proposes future directions for developing ideal DUV NLO materials.

Phase-Matching Second-Harmonic Generation and Enhanced Laser-Induced Damage Threshold Induced by Cs Substitution: Cu3PSe4 vs Cs2CuP3S9.

Chalcophosphates are an important type of infrared nonlinear optical (NLO) candidates in view of their rich anionic motifs. Here, two copper chalcophosphates Cu3PSe4 (CPSe) and Cs2CuP3S9 (CCPS) were synthesized and studied as IR NLO materials. They both feature three-dimensional polyanionic frameworks constructed by similar T2-supertetrahedra, and the structure of CCPS can be derived from CPSe via introducing Cs and substituting Se with S. This structural evolution results in phase-matchable NLO behavior, enlarged optical band gap, and enhanced laser-induced damage threshold for CCPS. These results are elucidated by structure analysis and theoretical calculations, and the increased structural anisotropy contributes to the phase matchable behavior of CCPS. This work presents a case on how to adjust NLO properties via certain structure considerations, which may be extended to more systems for obtaining high-performance NLO materials.

From CaBaM2F12 to K2BaM2F12 (M = Zr, Hf): Heterovalent Cation-Substitution-Induced Symmetry Break and Nonlinear-Optical Activity.

Designing short-wavelength nonlinear-optical (NLO) crystals is of vital importance for laser applications. Here, the combination of alkaline-earth metals, d0 transition metals, and F atom has generated two new and isostructural fluorides, CaBaZr2F12 (CBZF) and CaBaHf2F12 (CBHF), which adopt centrosymmetric space group I4/mmm. Taking CBZF and CBHF as the parents, two new fluorides, K2BaZr2F12 (KBZF) and K2BaHf2F12 (KBHF), with an Imm2 polar structure were obtained via a heterovalent cation substitution strategy. All four compounds feature ZrF8-dodecahedra-built {[Zr2F12]4-}∞ chains and show short ultraviolet cutoff edges (<200 nm). KBZF and KBHF show phase-matchable behavior with moderate second-harmonic-generation responses [0.6 and 0.35 × KH2PO4 (KDP)] under 1064 nm laser radiation. This work enriches fluorides as promising short-wavelength NLO materials.

Trithionic guanidine: A novel semi-organic short-wave ultraviolet nonlinear optical sulfate with dimeric heteroleptic tetrahedra

Sulfates are always promising short-wave ultraviolet (UV) nonlinear optical (NLO) candidates, if their birefringence could be greatly improved. Here, in terms of the insufficient birefringence, the unity of heteroleptic tetrahedral groups and triangular ones was proposed and implemented. Thus, a new semi-organic crystal, [C(NH2)3]S3O6 (G2S3O6), was obtained, which is composed of [S3O6]2− and [C(NH2)3]+ groups. It exhibits excellent optical properties with a short absorption cutoff edge of 218 nm, a strong NLO response of 1.4 × KH2PO4, and more especially, a large birefringence of 0.097@546 nm. This birefringence leap makes the G2S3O6 crystal achieve a phase-matching behavior under a 532 nm laser. Thus, the synergy of [S3O6]2− and [C(NH2)3]+ groups results in excellent optical performances. This finding opens a new horizon for exploring novel UV NLO crystals.

Rational Design of Novel Promising Infrared Nonlinear Optical Materials: Structural Chemistry and Balanced Performances.

ConspectusSecond-order nonlinear optical (NLO) materials are currently a hot topic in modern solid-state chemistry and optics because they can produce coherent light by frequency conversion. Noncentrosymmetric (NCS) structure is not only the prerequisite for NLO materials but also a challengeable issue because materials tend to be centrosymmetric (CS) in terms of thermodynamical stability. Among NLO materials, an excellent infrared (IR) candidate should simultaneously meet several strict key conditions including a large NLO coefficient, high laser-induced damage threshold (LIDT), phase-matchable (PM) behavior, and so on. Achieving a balance between the large NLO effect and high LIDT is difficult, as they have contradictory requirements for chemical bonds. Considering the urgent need of the high-power IR laser market and the drawbacks of the available ones, exploring new high-performance IR NLO crystals is necessary while challenging. In this Account, we first briefly introduce the status and advancement of IR NLO crystals and emphasize the criteria of an excellent candidate. Then, we will introduce five simple methods developed by us to discover practical NLO candidates through understanding of the chemical composition-structure-NLO performance relationship. (1) A rarely investigated system with simple chemical compositions as new-type NLO crystals, namely, adducts containing S8 molecules, are developed. Combining a chairlike S8 unit with other units through van der Waals forces has successfully obtained several high-performance NLO adducts. (2) The main trend in exploring new NLO crystals is that the chemical composition is more and more diversified and the structure is more and more complex, and expensive and chemically active alkaline and alkaline earth metals are usually introduced as counter cations. In contrast, the research on systems with simple chemical compositions, simple structures, and low costs has been continuously ignored. The binary M2Q3 (M = Ga, In; Q = S, Se) family with rich acentric modifications has been systematically investigated, and they all exhibit strong SHG effects and high LIDTs. (3) We first proposed the concept of inducing CS structures transformed to NCS ones by partial cation substitution to design novel NLO crystals. Considering the huge number of CS structures in the database compared to the number of NCS structures, it is an attractive method to apply CS structures as the parents to obtain potential NLO materials via partial-substitution-induced symmetry breaking. A series of chalcogenides with high NLO performances have been successfully obtained by us in this way. (4) We investigated the first NLO-active rare earth (RE) chalcophosphates and developed this family systematically, and they demonstrate wonderful comprehensive NLO properties. (5) We created a novel mixed-anion system for NLO applications, namely, chalcogenide borates. Usually, mixed-anion compounds can engender a synergistic effect to obtain desired IR NLO properties. Our recent progress on this system suggests that chalcogenide borates are potential candidates for IR NLO applications, although the study is still in its infancy. Finally, we state the current problems of IR NLO materials and give some perspectives for their future development.

Synthesis, crystal and electronic structures, linear and nonlinear optical properties, and photocurrent response of oxyhalides CeHaVIO4 (Ha = Cl, Br; VI = Mo, W).

Four heteroanionic oxyhalides, CeClMoO4, CeBrMoO4, CeClWO4, and CeBrWO4, have been studied as multifunctional materials, which show a combination of good second harmonic generation (SHG) response and photocurrent signals. Millimeter-sized CeHaVIO4 (Ha = Cl, Br; VI = Mo, W) crystals were grown by halide salt flux. The crystal structure of CeHaVIO4 crystals was accurately determined by single-crystal X-ray diffraction. CeClMoO4, CeBrMoO4, and CeBrWO4 are isostructural to each other, and crystallize in the acentric LaBrMoO4 structure type. CeClWO4 crystallizes in a new structure type with unit cell parameters of a = 19.6059(2) Å, b = 5.89450(10) Å, c = 7.80090(10) Å, and β = 101.4746(8)°. The bandgaps of CeHaVIO4 fall into the range of 2.8(1)-3.1(1) eV, which are much smaller than those of isotypic LaHaVIO4 (Ha = Cl, Br; VI = Mo, W) in the range of 3.9(1)-4.3(1) eV. The narrowing of bandgaps in CeHaVIO4 originates from the presence of partially filled 4f orbitals of cerium atoms, which was confirmed by density functional theory (DFT) calculations. The moderate bandgaps make CeHaVIO4 suitable for infrared nonlinear optical (IR NLO) applications. CeBrMoO4 and CeBrWO4 exhibit moderate SHG responses of 0.58× AGS and 0.46× AGS, respectively, and are both type-I phase-matching materials. Moderate SHG response, easy growth of crystals, high ambient stability, and type-I phase-matching behavior make CeBrMoO4 and CeBrWO4 great materials for IR NLO applications. CeHaVIO4 films also exhibited good photocurrent response upon light radiation. This work demonstrates the rich structural chemistry of the REHaVIO4 (RE = Y, La-Lu; Ha = Cl, Br; VI = Mo, W) family and the potential presence of more multifunctional materials.

A Sharp Improvement of Sulfate's Birefringence Induced by the Synergistic Effect of Heteroleptic and Dimeric Strategies.

As for tetrahedron-based ultraviolet nonlinear optical crystals, it is so difficult to achieve a sufficient birefringence to satisfy the phase-matching condition. Thereover, it is necessary to greatly increase the polarizability anisotropy of tetrahedra. Meanwhile, the tetrahedra should be arranged as uniformly as possible. Based on these ideas, a new strategy that dimerizes the heteroleptic tetrahedra with heteroleptic atoms as bridges is proposed and implemented, resulting in a new compound, Na2S3O6, which crystallizes in a centrosymmetric monoclinic phase (phase-I) and a non-centrosymmetric orthorhombic one (phase-II). In the phase-I, a large birefringence of 0.103 @ 546nm comparable to those of many triangle-based crystals is confirmed experimentally, indicating the ability of [S3O6] group in improving the birefringence. Besides, the phase-II exhibits an improved birefringence of 0.056 @ 546nm and a moderate second harmonic generation response of 1.4×KH2PO4. In particular, the phase-II exhibits a typical phase-matching behavior under the irradiation of 532nm laser. Moreover, the excellent optical properties of [S3O6] group are further verified by theoretical calculations. These results completely illustrate the validity of the design strategy. Therefore, this work lights a new route for exploring novel ultraviolet nonlinear optical crystals.

Noncentrosymmetric Na6Pb3P4S16 and Centrosymmetric K2MIIP2S6 (MII = Mg and Zn) Displaying Multiple Membered-Ring Configurations and Strong Optical Anisotropy.

Three thiophosphates including noncentrosymmetric Na6Pb3P4S16 and centrosymmetric K2MIIP2S6 (MII = Mg and Zn) were successfully synthesized in vacuum-sealed silica tubes. Note that interesting multiple six membered-rings (6-MRs) including 6-NaS6-MRs and 6-KSn-MRs (n = 6 and 7) formed by A+-centered polyhedra were discovered in the structures of title thiophosphates and these MR-composed three-dimensional (3D) tunnels show great possibility to facilitate the filling of various structural blocks (such as zero-dimensional (0D) Pb3S10 trimers or one-dimensional (1D) (MIISn)n chains). Na6Pb3P4S16 exhibits the strongest nonlinear optical (NLO) response (5.4 × AgGaS2) with phase-matching (PM) behavior among the known Pb-based PM NLO sulfides, which is much larger than that of Pb3P2S8 (3.5 × AgGaS2); it was verified that such large second harmonic generation (SHG) response in Na6Pb3P4S16 can be attributed to the huge contribution of stereochemically active PbS4 units based on the SHG-density and dipole-moment calculations. Moreover, title thiophosphates show large birefringences (Δn = 0.102-0.21), which indicates that incorporation of [P2S6] dimers or polarized PbS4 units into structures provides positive benefits for the onset of strong optical anisotropy.

Crystal structure, electronic structure, and optical properties of Cu4ZnGe2S7 and Cu4CdGe2S7: Polar diamond-like semiconductors exhibiting second harmonic generation

The novel I4-II-IV2-VI7 quaternary diamond-like semiconductors (DLSs), Cu4ZnGe2S7 and Cu4CdGe2S7, were prepared from the constituent elemental powders at elevated temperatures. Cu4ZnGe2S7 crystallizes in the polar, chiral space group C2 and adopts the Cu4NiSi2S7 structure-type, which can be considered a derivative of the cubic diamond structure. The cadmium analog takes on the Cu5Si2S7 structure-type, which is related to the hexagonal diamond structure, and displays polar, Cc space group symmetry. Both compounds contain [Ge2S7]6- anions created by corner-sharing GeS4 tetrahedra. The crystal structures were carefully evaluated using: 1) extended connectivity tables, 2) minimum bounding ellipsoid analysis, and 3) bond valence sum (BVS) and global instability index (G) calculations. Both compounds are thermally stable up to temperatures exceeding 950 °C and melt incongruently. As assessed using diffuse reflectance spectroscopy, Cu4ZnGe2S7 and Cu4CdGe2S7 possess optical bandgaps of 1.65 eV and 1.90 eV, respectively, which are narrower than the analogous I2-II-IV-VI4 DLSs. Electronic structure calculations show that both compounds are direct-bandgap semiconductors with significant contributions of Cu-d and S-p orbitals to the states at the top of the valence band. While Cu4CdGe2S7 displays a weak second harmonic generation (SHG) response, the second-order nonlinear optical susceptibility, χ(2), of Cu4ZnGe2S7 is 14.8 ± 1.5 pm V-1, assessed using the Kurtz-Perry powder method and an optical-grade AgGaSe2 (AGSe) reference. Both compounds exhibit laser-induced damage threshold values similar to AGSe, ∼0.1 GW cm-2, for λ = 1.064 µm using picosecond laser pulses. Most importantly, Cu4ZnGe2S7 exhibits a phase-matching behavior at λ = 3.1 µm. These results suggest that Cu4ZnGe2S7 holds potential for use in low-powered laser applications involving efficient wave mixing in the mid-IR region.

Cs4 Zn5 P6 S18 I2 : the Largest Birefringence in Chalcohalide Achieved by Highly Polarizable Nonlinear Optical Functional Motifs.

Chalcohalides not only keep the balance between the nonlinear optical (NLO) coefficient and wide band gap, but also provide a promising solution to achieve sufficient birefringence for phase-matching ability in NLO crystals. In this study, a novel chalcohalide, Cs4 Zn5 P6 S18 I2 (1) is successfully synthesized, by incorporating the highly electropositive Cs and the large electronegative I element into the zinc thiophosphate. Its 3D open framework features an edge-shared by distorted [ZnS4 ], ethanol-like [P2 S6 ], and unusual [ZnS2 I2 ] polyhedrons, which is inconsistent with the soft-hard-acids-bases theory. Remarkably, compound 1 simultaneously exhibits the large second-harmonic generation (SHG, 1.1×AgGaS2 , @1.3µm) and a wide band gap (3.75eV) toward a high laser-induced damage threshold (16.7×AgGaS2 , @1.06µm), satisfying the rigorous requirements for a prominent infrared NLO material with concurrent SHG intensity (≥0.5×AGS) and band gap (≥3.5eV). Moreover, to the best of the knowledge, the experimental result shows that phase 1 has the largest birefringence (0.108, @546nm) in chalcohalide and meets phase-matching behavior demand originating from the polarizable anisotropy of NLO-functional motifs. This finding may provide great opportunities for designing birefringent chalcohalides.

Synthesis and characterization of a new rare earth borate nonlinear optical crystal K7PbLu2B15O30.

A new complex rare earth borate K7PbLu2B15O30 was prepared by the spontaneous crystallization method. K7PbLu2B15O30 is crystallized in the chiral trigonal space group R32 with cell parameters a = b = 13.0893(3) Å, c = 15.2379(6) Å, α = β = 90°, γ = 120°, and Z = 3. The basic structure of the crystal can be seen as composed of B5O10 groups and LuO6 polyhedra sharing oxygen atoms, while K+ and Pb2+ fill the space to balance the charge. The UV transmission cut-off edge of K7PbLu2B15O30 was less than 300 nm, and the powder SHG response was roughly 1.1 times that of KDP. Furthermore, a first-principles analysis was performed to see more about the relationship between the crystal structure and optical characteristics.

A series of novel phosphites/phosphates with deep-ultraviolet cut-off edges from 0D polar to 3D non-polar architectures

Four new phosphites/phosphates were designed and prepared; three of them exhibit a deep UV cutoff edge of ∼190 nm and NLO material Ca[HPO3] shows a phase-matching behavior.

Growth, Structure, and Optical Properties of a Nonlinear Optical Niobium Borate Crystal CsNbOB2O5 with Distorted NbO5 Square Pyramids.

In this paper, a revised structure determination of an already known compound CsNbOB2O5 with new structural insights was performed and the detailed characterization of its optical properties was reported for the first time. CsNbOB2O5 was synthesized by spontaneous crystallization. It crystallizes in the Pmn21 space group, and the unit lattice parameters are a = 7.5220(7) Å, b = 3.9881(4) Å, c = 9.7167(9) Å, and Z = 2. In the structure of CsNbOB2O5, a [NbO5] square pyramid and [B2O5] unit are linked to constitute an infinitely extended two-dimensional ∞2[NbOB2O5] layer via sharing oxygen atoms. Between these two-dimensional layers, there are no covalent bonds perpendicular to their planes based on Mulliken bond order analysis. CsNbOB2O5 has a wide band gap (4.52 eV) and a large second-harmonic-generation (SHG) response (1.2 × KDP) and demonstrates type-I phase-matchable behavior. First-principles simulations reveal that the birefringence is approximately 0.10. Moreover, SHG-weighted charge density analysis shows that the primary source of the nonlinearity of the title compound is the distorted NbO5 square pyramids.

Novel structural transformation in K3ReP2S8 thiophosphates originating from the rare-earth (Re) cation sizes induced local coordination asymmetry

Noncentrosymmetric (NCS) structure is a prerequisite condition for nonlinear optical (NLO) crystals and searching for feasible design strategy on the discovery of new NCS compounds has become extremely important. For that reason, new series of rare-earth (Re) based NCS K3ReP2S8 (Re = Y, Ho, Er; P21 space group) and CS K3ReP2S8 (Re = Pr, Sm, Gd; P21/c) thiophosphates were successfully prepared and this interesting CS to NCS structural remodeling shows the interior relationship with cation-size effect of Re metals. Systematic structural analysis indicates that several of varying influence factors including Re's ionic radii, average Re–S bond lengths and distortion degrees of ReS8 units induce the born of local coordination asymmetry between ReS8 and PS4 units, which facilitates the transformation from CS to NCS structures in K3ReP2S8 system. Moreover, excellent performances including strong NLO responses (1.1–1.4 times that of commercial AgGaS2) and required phase-matching behavior in P21–K3ReP2S8 were determined. Note that P21–K3ReP2S8 can be viewed as first NLO examples in the quaternary rare-earth thiophosphates. Moreover, NCS K3ReP2S8 also satisfy the well-balance between large NLO effects (1.1–1.4 × AgGaS2) and high laser damage thresholds (2.5–7.0 × AgGaS2). Theoretical study demonstrates that synergetic contributions of ReS8 and PS4 units guarantee the intrinsic origin of NLO response and ReS8 units provide the major NLO contribution. Therefore, this work not only affords one effective pathway to design new NCS materials through breaking the local symmetry in CS structures based on the size-directing effect of Re cations, but also verifies that bridging ReSn units into thiophosphates is beneficial to the huge enhancement of intrinsic NLO performances.

Dual-Ligand-Oriented Design of Noncentrosymmetric Complexes with Nonlinear-Optical Activity.

When the N- and O-donor ligands are combined as coligands, two noncentrosymmetric (NCS) complexes of [Ni(p-bdc)(tipa)(H2O)2]2·H2O (1) and Ni(npdc)(tipa)H2O (2) [tipa = tris[4-(1H-imidazol-1-yl)phenyl]amine, p-H2bdc = 1,4-benzenedicarboxylic acid, and H2npdc = 2,6-naphthalenedicarboxylic acid] were achieved under solvothermal conditions. For both structures, N-donor ligands are responsible for the generation of a layered structure, while the O-donor ligands are hung on the layers and are responsible for enhancing the polarity, giving rise to the NCS structures. Because of the different connection modes between the metal centers and different carboxylate ligands (p-bdc2- in 1 and npdc2- in 2), 1 and 2 show some structural differences. The p-bdc2- ligands in 1 are suspended on the upper and lower sides of the [Ni(tipa)]n layers, while all of the npdc2- ligands in 2 hang on one side of the [Ni(tipa)]n layers and point in the same direction, which makes the two NCS complexes show phase-matchable behavior with different second-harmonic-generation (SHG) responses of about 0.9 and 1.5 times that of KH2PO4 (KDP), respectively. Theoretical studies reveal that charge transfers between Ni2+ and carboxylate ligands make the dominant contribution to the optical properties. It is expected that a dual-ligand strategy may guide the design of novel superior-performing NCS complexes.

Growth and Characterization of a Nonlinear Optical Crystal Cd3Te2WO10 Decorated by WO4 Tetrahedra Showing a Large Band Gap

A noncentrosymmetric nonlinear optical (NLO) crystal Cd3Te2WO10 with a large band gap has been synthesized and grown using the high-temperature solution method with TeO2 as a flux. Cd3Te2WO10, crystallized in P21, contains two classes of d0 W6+ and lone-pair Te4+ cations holding asymmetric coordination environments which are associated with the second-order Jahn–Teller effect. Importantly, it has a large band gap of 4.28 eV, which is profitable for high-power application, and wide transparency region of 0.29–5.63 μm, especially in 1–5 μm possessing tremendous transmittance that exceeded 80% and fully covering the 3–5 μm range, which is one of the vital atmospheric windows, and a good Raman performance that arises from the WO4 polyhedra. In addition, Cd3Te2WO10 features phase-matching behavior owing to large birefringence and considerable second-harmonic generation response equivalent to 1.1 times that of KH2PO4, which is further elucidated by the theoretical calculation. Therefore, our experimental results demonstrate that Cd3Te2WO10 could be used as a promising functional NLO crystal.

KC9H5O6(H2O): A Promising UV Nonlinear-Optical Material with Large Birefringence Based on a π-Conjugated (C9H5O6)- Group.

In order to meet the growing needs for the laser technology and optics industries, the goal is to find suitable fundamental building blocks with large nonlinear-optical (NLO) coefficients and birefringence for an excellent-performance NLO or birefringent system. Via preliminary investigations and calculations, it has been found that the planar π-conjugated group (C9H5O6)- possesses large polarizability anisotropy (δ) and hyperpolarizability (βmax), comparable to well-known groups such as (B3O6)3-, (C3N3O3)3-, etc. Herein, we report a new alkali-metal 3,5-dicarboxybenzoate, KC9H5O6(H2O) (KH2BTC), which crystallized in the acentric space group Pna21. Second-harmonic-generation (SHG) measurements of KH2BTC under 1064 nm laser radiation show that the SHG response of KH2BTC is 1.2 times that of KDP with type I phase-matching behavior. Birefringence measurements show that KH2BTC owns a large birefringence of about 0.372 at 550 nm. The band gap of KH2BTC obtained by ultraviolet (UV) diffuse-reflectance spectroscopy is 3.91 eV, indicating that KH2BTC has potential applications as UV NLO or birefringent materials. Theoretical calculation further confirmed that the impressive optical properties of KH2BTC are derived from the large polarizability anisotropy of the (C9H5O6)- anions.

Mixed-Anion-Oriented Design of LnMGa3S6O (Ln = La, Pr, and Nd; M = Ca and Sr) Nonlinear Optical Oxysulfides with Targeted Property Balance

Nonlinear optical (NLO) crystals are of importance on extending infrared (IR) laser wavelengths. Considering their performance drawbacks in commercial IR NLO crystals, a recent challenge in exploring new excellent IR NLO crystals is how to break the inherent conflict between a wide bandgap (Eg ≥ 3.0 eV) and large NLO effect (dij ≥ 0.5 × AgGaS2) and simultaneously enlarge the birefringence (Δn) for a requisite phase-matching (PM) behavior. For that reason, rational combination of mixed-anion functional groups into a crystal structure affords the successful design and synthesis of six LnMGa3S6O (Ln = La, Pr, and Nd; M = Ca and Sr) NLO oxysulfides. Among them, LaMGa3S6O satisfy the property-balance demand (Eg: 3.21-3.27 eV and dij: 0.9-1.0 × AgGaS2) as promising PM NLO crystals through gathering their property advantages between LaMGa3O7 and LaMGa3S7 by mixed-anion-oriented performance engineering. A study on the structure-property relationship indicates that heteroleptic (Ln/M)S7O and GaS3O anionic groups are proven as promising NLO-active units and offer a great synergistic effect on the NLO origin. This work as a visualized model not only provides a first clear cognition on varying properties from oxide to sulfide to oxysulfide but also highlights the feasibility of mixed-anion-oriented design of new NLO candidates with balanced performances.

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.