Excellent Nonlinear Optical M[M4 Cl][Ga11 S20 ] (M = A/Ba, A = K, Rb) Achieved by Unusual Cationic Substitution Strategy.

The typical chalcopyrite AgGaQ2 (Q = S, Se) are commercial infrared (IR) second-order nonlinear optical (NLO) materials; however, they suffer from unexpected laser-induced damage thresholds (LIDTs) primairy due to their narrow band gaps. Herein, what sets this apart from previously reported chemical substitutions is the utilization of an unusual cationic substitution strategy, represented by [[SZn4 ]S12 + [S4 Zn13 ]S24 + 11ZnS4 ⇒ MS12 + [M4 Cl]S24 + 11GaS4 ], in which the covalent Sx Zny units in the diamond-like sphalerite ZnS are synergistically replaced by cationic Mx Cly units, resulting in two novel salt-inclusion sulfides, M[M4 Cl][Ga11 S20 ] (M = A/Ba, A = K, 1; Rb, 2). As expected, the introduction of mixed cations in the GaS4 anionic frameworks of 1 and 2 leads to wide band gaps (3.04 and 3.01 eV), which exceeds the value of AgGaS2 , facilitating the improvement of high LIDTs (9.4 and 10.3 × AgGaS2 @1.06 µm, respectively). Furthermore, compounds 1 and 2 exhibit moderate second-harmonic generation intensities (0.84 and 0.78 × AgGaS2 @2.9 µm, respectively), mainly originating from the orderly packing tetrahedral GaS4 units. Importantly, this study demonstrates the successful application of the cationic substitution strategy based on diamond-like structures to provide a feasible chemical design insight for constructing high-performance NLO materials.

Nonlinear Optical Sulfides LiMGa8 S14 (M = Rb/Ba, Cs/Ba) Created by Li+ Driven 2D Centrosymmetric to 3D Noncentrosymmetric Transformation.

Cations that can regulate the configuration of anion group are greatly important but regularly unheeded. Herein, the structural transformation from 2D CS to 3D noncentrosymmetric (NCS, which is the prerequisite for second-order NLO effect) is rationally designed to newly afford two sulfides LiMGa8 S14 (M = Rb/Ba, 1; Cs/Ba, 2) by introducing the smallest alkali metal Li+ cation into the interlamination of 2D centrosymmetric (CS) RbGaS2 . The unusual frameworks of 1 and 2 are constructed from C2 -type [Ga4 S11 ] supertetrahedrons in a highly parallel arrangement. 1 and 2 display distinguished NLO performances, including strong phase-matchable second-harmonic generation (SHG) intensities (0.8 and 0.9 × AgGaS2 at 1910 nm), wide optical band gaps (3.24 and 3.32 eV), and low coefficient of thermal expansion for favorable laser-induced damage thresholds (LIDTs, 4.7, and 7.6 × AgGaS2 at 1064 nm), which fulfill the criteria of superior NLO candidates (SHG intensity >0.5 × AGS and band gap >3.0 eV). Remarkably, 1 and 2 melt congruently at 873.8 and 870.5 °C, respectively, which endows them with the potential of growing bulk crystals by the Bridgeman-Stockbarge method. This investigated system provides a new avenue for the structural evolution from layered CS to 3D NCS of NLO materials.

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 , @1.3 µm) and a wide band gap (3.75 eV) toward a high laser-induced damage threshold (16.7×AgGaS2 , @1.06 µm), satisfying the rigorous requirements for a prominent infrared NLO material with concurrent SHG intensity (≥0.5×AGS) and band gap (≥3.5 eV). Moreover, to the best of the knowledge, the experimental result shows that phase 1 has the largest birefringence (0.108, @546 nm) 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.

Photochromic Semiconductors: Bottom-Up Strategy to Construct Type II-Stacking Viologen π-Aggregates.

Semiconductor conductivities depend largely on the crystal structures and the associated electronic structures. If the electronic structures can be switched reversibly in the same crystal structure, then a drastic conductivity change may be controllable. The effect of electron transfer (ET) on semiconductor conductivity remained elusive so far. In this work, a series of two pillared inorganicorganic hybrid photochromic semiconductors (PSCs), [(CQ)Pb3X6(H2O)]·2H2O [X = Cl (1) and Br (2), CQ = N-4,4'-bipyridiniopropionate (viologen)], with II-stacking viologen π-aggregates, are constructed by a bottom-up self-assembly strategy through inorganic skeleton-directed intercalation and intermolecular noncovalent interaction. The conductivities are abnormally "invariant" after photoinduced ET, breaking the convention that the generation of radicals favors conductivity. The abnormally "invariant" conductivities are mainly derived from approximate electronic couplings before and after ET between II-stacking viologen π-aggregates.

Uncovering a Functional Motif of Nonlinear Optical Materials by In Situ Electron Density and Wavefunction Studies Under Laser Irradiation.

Exploring nonlinear optical (NLO) functional motifs (FM, the structural origin of NLO efficiency) is vital for the rational design of NLO materials. Normal spectrum techniques applied in studying photon exciting materials are invalid for NLO materials, in which electrons are not excited substantially but only distorted under laser. A general strategy of determining NLO FM is proposed by comparative studies of experimental electron density (ED) without and under the laser. The in situ experimental ED and wavefunction of typical NLO material LiB3 O5 (LBO) under dark and 360 and 1064 nm lasers are investigated. Compared with the initial state under dark, the ED of [B3 O5 ]- unit at functional states under laser irradiation exhibits remarkable changes of topological atomic and bond properties, confirming the NLO FM being [B3 O5 ]- . The work extracts for the first time the FM of a NLO material experimentally and highlights the crucial role of in situ ED analysis in studying NLO mechanisms.

[ABa2Cl][Ga4S8] (A = Rb, Cs): Wide-Spectrum Nonlinear Optical Materials Obtained by Polycation-Substitution-Induced Nonlinear Optical (NLO)-Functional Motif Ordering.

Noncentrosymmetry (NCS) is a prerequisite for second-order nonlinear optical (NLO) materials. In this work, a new polycation-substitution-induced centrosymmetry (CS)-to-NCS transformation strategy was applied in CS RbGaS2, affording two novel NCS salt-inclusion chalcogenides: [ABa2Cl][Ga4S8] (A = Rb, 1; Cs, 2). Remarkably, they exhibit the key merits of both Vis and IR NLO candidates, including strong phase-matchable second harmonic generation intensities (10.4-15.3 × KH2PO4 at 1064 nm; 0.9-1.0 × AgGaS2 at 1910 nm), high laser-induced damage thresholds (11-12 × AgGaS2), and a wide transparent window. Their prominent NLO performances originate from the orderly packing of T2-supertetrahedral Ga4S10 motifs resulting from the template effect of acentric polycation [ClA2Ba3]7+. The CS-to-NCS transformation first realized by polycation-substitution-induced NLO-functional motif ordering provides an effective approach for designing new NLO materials.

Li[LiCs2 Cl][Ga3 S6 ]: A Nanoporous Framework of GaS4 Tetrahedra with Excellent Nonlinear Optical Performance.

A large nonlinear optical (NLO) coefficient and a wide band gap are two crucial but contradictory parameters that are difficult to achieve simultaneously in a single infrared (IR) NLO compound. A salt-inclusion chalcogenide (SIC), Li[LiCs2 Cl][Ga3 S6 ] (1), was prepared that presents a nanosized tunnel framework constructed from monotype chalcogenide tetrahedra. Highly oriented covalent GaS4 tetrahedra in the host lead to a moderate second harmonic generation response (0.7 AgGaS2 ), and ionic guests effectively broaden the band gap to the widest value (4.18 eV) among all IR NLO chalcogenides, thereby achieving a remarkable balance between NLO efficiency and band gap.

Large Second Harmonic Generation (SHG) Effect and High Laser-Induced Damage Threshold (LIDT) Observed Coexisting in Gallium Selenide.

A big challenge for nonlinear optical (NLO) materials is the application in high power lasers, which needs the simultaneous occurrence of large second harmonic generation (SHG) and high laser induced damage threshold (LIDT). Herein we report the preparation of a new Ga2 Se3 phase, which shows the SHG intensities of around 2.3 times and the LIDT of around 16.7 times those of AgGaS2 (AGS), respectively. In addition, its IR transparent window ca. 0.59-25 µm is also significantly wider than that of AGS (ca. 0.48-≈11.4 µm). The occurrence of the strong SHG responses and good phase-matching indicate that the structure of the new Ga2 Se3 phase can only be non-centrosymmetric and have a lower symmetry than the cubic γ-phase. The observed excellent SHG and phase-matching properties are consistent with our diffraction experiments and can be well explained by using the orthorhombic models obtained through our high throughput simulations.

Strong Infrared Nonlinear Optical Efficiency and High Laser Damage Threshold Realized in Quaternary Alkali Metal Sulfides Na2Ga2MS6 (M = Ge, Sn) Containing Mixed Nonlinear Optically Active Motifs.

Two new infrared (IR) nonlinear optical (NLO) sulfides, Na2Ga2GeS6 and Na2Ga2SnS6, were obtained by mixing different typical NLO-active motifs GaS4 and GeS4/SnS4 in the alkali metal-containing system. The IR NLO sulfides present laser-induced damage thresholds that are 18.1 and 17.9 times that of the reference AgGaS2 (AGS) and second-harmonic generation efficiencies that are 0.8 and 1.1 times that of AGS. These properties originate from the GaS4, GeS4, and SnS4 tetrahedral blocks in the structures of the sulfides. Both compounds also exhibit a broad transparency range and type-I phase-matching behavior, which support their high potential in high-power laser applications. This work sheds new light on the development of promising mid-IR NLO materials by combining different NLO-active motifs.

Phase Transition and Second Harmonic Generation in Thiophosphates Ag2Cd(P2S6) and AgCd3(PS4)S2 Containing Two Second-Order Jahn-Teller Distorted Cations.

Two new phases in the Ag-Cd-P-S system containing two second-order Jahn-Teller (SOJT) distorted d10 cations (Cd2+ and Ag+), namely, Ag2Cd(P2S6) (1) and AgCd3(PS4)S2 (2), are obtained via medium-temperature solid-state synthesis. Compound 1 exhibits a two-dimensional layered structure and undergoes a first-order structural phase transition at approximately 280 °C. This outcome can be ascribed to the significant mismatch in the expansion coefficients between Cd-S (Ag-S) and P-P (P-S) bonds evaluated through bond valence theory. The three-dimensional non-centrosymmetric (NCS) framework of 2 is constructed by two types of tetrahedral layers consisting of corner-shared CdS4, AgS4, and PS4 tetrahedra. Compound 2 exhibits second harmonic generation (SHG) intensity of 0.45 times that of commercial AgGaS2 (AGS) at a laser irradiation of 1.85 µm and an optical band gap of 2.56 eV, and no intrinsic vibrational absorption of chemical bonds is observed in the range of 2.5-18.2 µm. Both phase transition in 1 and SHG properties in 2 are closely related to the SOJT distorted d10 cations and diverse phosphorus-sulfur polyanions (PaSb)n-, which together can easily result in NCS distorted structures and interesting properties.

Thiophosphates Containing Ag+ and Lone-Pair Cations with Interchiral Double Helix Show Both Ionic Conductivity and Phase Transition.

Quaternary metal thiophosphates containing second-order Jahn-Teller distorted d10 Ag+ and lone-pair cations, Ag3Bi(PS4)2 (1), Ag7Sn(PS4)3 (2), and Ag7Pb(PS4)3 (3), were obtained by solid-state synthesis. The structural frameworks of 2 and 3 feature an infinite 1-D interchiral double helix ∞1(Ag3P2S11), which is rare in inorganic compounds. Compound 3 undergoes a significant first-order structural phase transition from monoclinic to hexagonal at ∼204 °C. This can be ascribed to the significant mismatch in the expansion coefficients between Pb-S (Ag-S) and P-S bonds evaluated by bond valence theory. The three compounds are Ag+ ionic conductors, and Ag+ ion migration pathways are proposed by calculating maps of low bond valence mismatch. Moreover, the optical properties of the three compounds were studied, and electronic structure calculations were performed. The combination of second-order Jahn-Teller distorted d10 cation and lone-pair cation provides a new strategy to explore new metal thiophosphates with interesting structures and promising properties.

Syntheses, Structures, and Nonlinear Optical Properties of Two Sulfides Na2In2MS6 (M = Si, Ge).

The first two new Na-containing sulfides Na2In2MS6 (M = Si (1), Ge (2)) in the Na2Q-B2Q3-CQ2 (B = Ga, In; C = Si, Ge, Sn; Q = S, Se) system were prepared for the first time through conventional high-temperature solid-state reaction. They are isostructural with space group Cc (No. 9) in monoclinic phases and feature three-dimensional frameworks built by the (∞)¹[In2MS6]²â» (M = Si, Ge) chains through corner-sharing InS4 tetrahedra and MS4 (M = Si, Ge) tetrahedra, with Na⁺ cation located in the cavities. They display moderate second harmonic generation (SHG) conversion efficiencies compared with commercial AgGaS2, with phase-matching behavior at 1800 nm and laser-induced damage thresholds 6.9 and 4.0 times higher than that of AgGaS2, respectively. Therefore, the output SHG intensities of 1 and 2 will be ∼4.3 and 4.0 times larger than that of AgGaS2, when the intensity of incident laser increased to close the damage energy of 1 and 2, indicating their potential for high-power nonlinear optical application.

Semiconductive Nanotube Array Constructed from Giant [Pb(II)18I54(I2)9] Wheel Clusters.

Crystalline nanotube array would create great opportunity for novel electrical application. Herein we report the first example of a metal halide based crystalline nanotube array which is constructed from an unprecedented giant [Pb(II)18I54(I2)9] wheel cluster, as determined by synchrotron X-ray diffraction. The electrical properties of the single crystal were studied and the present compound shows typical semiconductivity and highly anisotropic conductivity.

Syntheses, structures, and nonlinear-optical properties of metal sulfides Ba2Ga8MS16 (M = Si, Ge).

Two new metal sulfides, Ba(2)Ga(8)MS(16) (M = Si, Ge), have been synthesized by high-temperature solid-state reactions. They are isostructural and crystallize in the noncentrosymmetric space group P6(3)mc (No. 186) with a = 10.866(5) Å, c = 11.919(8) Å, and z = 2 for Ba(2)Ga(8)SiS(16) (1) and a = 10.886(8) Å, c = 11.915(3) Å, and z = 2 for Ba(2)Ga(8)GeS(16) (2). Their three-dimensional frameworks are constructed by corner-sharing mixed (Ga/M)S(4) (M = Si, Ge) and pure GaS(4) tetrahedra, with Ba(2+) cations filling in the tunnels. Compounds 1 and 2 are transparent over 0.42-20 µm and have wide band gaps of around 3.4 and 3.0 eV, respectively. Polycrystalline 2 displays strong nonlinear second-harmonic-generation (SHG) intensities that are comparable to that of the benchmark AgGaS(2) (AGS) with phase-matching behavior at a laser irradiation of 1950 nm. Of particular interest, compound 2 also possesses a high powder laser-induced damage threshold of ∼22 times that of AGS. The alternate stacking of the mixed (Ga/M)S(4) (M = Si, Ge) tetrahedral layer with the pure GaS(4) tetrahedral layer along the c axis and the alignment of these two types of tetrahedra in the same direction may be responsible for the large SHG signals observed.

A highly stable 3D acentric zinc metal-organic framework based on two symmetrical flexible ligands: high second-harmonic-generation efficiency and tunable photoluminescence.

A 3D metal-organic framework (MOF), [Zn(BPHY)(SA)]n (1; BPHY = 1,2-bis(4-pyridyl)hydrazine, H2SA = succinic acid), which crystallizes in a noncentrosysmmetric space group (Cc), has been solvothermally obtained and testified to be a good nonlinear-optical material with the largest second-harmonic-generation response among the known MOFs based on sysmmetric ligands and high stability. Ultraviolet-to-visible tunable emission for 1 is observed.

Nonlinear Optical Phosphide CuInSi2P4: The Inaugural Member of Diamond-Like Family I-III-IV2-V4 Inspired by ZnGeP2.

Noncentrosymmetric phosphides have garnered significant attention as promising systems of infrared (IR) nonlinear optical (NLO) materials. Herein, a new quaternary diamond-like phosphide family I-III-IV2-V4 and its inaugural member, namely, CuInSi2P4 (CISP), were successfully fabricated by isovalent and aliovalent substitution based on ZnGeP2. First-principles calculations revealed that CISP has a large NLO coefficient (d14 = 110.8 pm/V), which can be attributed to the well-aligned tetrahedral [CuP4], [InP4], and [SiP4] units. Remarkably, the extremely small thermal expansion anisotropy (0.09) of CISP enables it to exhibit a considerable laser-induced damage threshold (LIDT, 5.0 × AgGaS2@1.06 µm) despite the relatively narrow band gap (0.81 eV). This work improves the chemical diversity of inorganic phosphide and promotes the development of phosphide systems, which may provide valuable perspectives for future exploration of IR NLO materials.

HgBr2: an easily growing wide-spectrum birefringent crystal.

Birefringent materials are of great significance to the development of modern optical technology; however, research on halide birefringent crystals with a wide transparent range remains limited. In this work, mercuric bromide (HgBr2) has been investigated for the first time as a promising birefringent material with a wide transparent window spanning from ultraviolet (UV) to far-infrared (far-IR) spectral regions (0.34-22.9 µm). HgBr2 has an exceptionally large birefringence (Δn, 0.235 @ 546 nm), which is 19.6 times that of commercial MgF2. The ordered linear motif [Br-Hg-Br] with high polarizability anisotropy within the molecule is the inherent source of excellent birefringence, making it an efficient building block for birefringent materials. In addition, HgBr2 can be easily grown under mild conditions and remain stable in air for prolonged periods. Studying the birefringent properties of HgBr2 crystals would provide new ideas for future exploration of wide-spectrum birefringent materials.

[K2 PbX][Ga7 S12 ] (X = Cl, Br, I): The First Lead-Containing Cationic Moieties with Ultrahigh Second-Harmonic Generation and Band Gaps Exceeding the Criterion of 2.33 eV.

In contrast to anionic group theory of nonlinear optical (NLO) materials that second-harmonic generation (SHG) responses mainly originate from anionic groups, structural regulation on the cationic groups of salt-inclusion chalcogenides (SICs) is performed to make them also contribute to the NLO effects. Herein, the stereochemically active lone-electron-pair Pb2+ cation is first introduced to the cationic groups of NLO SICs, and the resultant [K2 PbX][Ga7 S12 ] (X = Cl, Br, I) are isolated via solid-state method. The features of their three-dimensional structures comprise highly oriented [Ga7 S12 ]3- and [K2 PbX]3+ frameworks derived from AgGaS2 , which display the largest phase-matching SHG intensities (2.5-2.7 × AgGaS2 @1800 nm) among all SICs. Concurrently, three compounds manifest band gap values of 2.54, 2.49, and 2.41 eV (exceeding the criterion of 2.33 eV), which can avoid two-photon absorption under the fundamental laser of 1064 nm, along with the relatively low anisotropy of thermal expansion coefficients, leading to improved laser-induced damage thresholds (LIDTs) values of 2.3, 3.8, and 4.0 times that of AgGaS2 . In addition, the density of states and SHG coefficient calculations demonstrate that the Pb2+ cations narrow the band gaps and benefit SHG responses.

Breaking the bottleneck of simultaneously wide band gap and large nonlinear optical coefficient by a "pore reconstruction" strategy in a salt-inclusion chalcogenide.

The template-based design of the crystal structure is a direct and highly efficient method to achieve optimal nonlinear optical (NLO, meaning second-order NLO) performances. The structural flexibility of porous salt-inclusion chalcogenides (SICs) provides an alternative platform for modulating the enlargement of the band gap (that is generally positive with laser-induced damage threshold) and second harmonic generation (SHG) response simultaneously. By applying the "pore reconstruction" strategy to SIC [K3Cl][Mn2Ga6S12] (1), a new derivative K3Rb3[K3Cl][Li2Mn4Ga12S27] (2) is successfully isolated, which unusually features a heterologous nanopore framework with inner diameters of 8.90 and 9.16 Å. Guided by such a strategy, compound 2 possesses the widest band gap (3.31 eV) among the magnetic NLO chalcogenides; this finding is dominantly attributed to the porous structure and the "dimensional deduction" effect. Moreover, phase 2 displays a remarkable phase-matchable SHG intensity (1.1 × AgGaS2 at the incident laser of 1910 nm) that originated from the oriented alignment of NLO-functional motifs, as well as the rich terminal S atoms in the nanopore structure. Furthermore, the "pore reconstruction" strategy offers an efficient pathway to explore potential NLO candidates with excellent comprehensive performances; in particular, it settles the conflicting issue of enhancing the band gap (>3.0 eV) and SHG intensity (>1.0 × AgGaS2) concurrently.

The first polyanion-substitution-driven centrosymmetric-to-noncentrosymmetric structural transformation realizing an excellent nonlinear optical supramolecule [Cd4P2][CdBr4].

Crystallographically, noncentrosymmetricity (NCS) is an essential precondition and foundation of achieving nonlinear optical (NLO), pyroelectric, ferroelectric, and piezoelectric materials. Herein, structurally, octahedral [SmCl6]3- is substituted by the acentric tetrahedral polyanion [CdBr4]2-, which is employed as a templating agent to induce centrosymmetric (CS)-to-NCS transformation based on the new CS supramolecule [Cd5P2][SmCl6]Cl (1), thereby providing the NCS supramolecule [Cd4P2][CdBr4] (2). Meanwhile, this replacement further results in the host 2D ∞2[Cd5P2]4+ layers converting to yield the twisted 3D ∞3[Cd4P2]2+ framework, which promotes the growth of bulk crystals. Additionally, phase 2 possesses well-balanced NLO properties, enabling considerable second-harmonic generation (SHG) responses (0.8-2.7 × AgGaS2) in broadband spectra, the thermal expansion anisotropy (2.30) together with suitable band gap (2.37 eV) primarily leading to the favorable laser-induced damage threshold (3.33 × AgGaS2), broad transparent window, and sufficient calculated birefringence (0.0433) for phase-matching ability. Furthermore, the first polyanion substitution of the supramolecule plays the role of templating agent to realize the CS-to-NCS transformation, which offers an effective method to rationally design promising NCS-based functional materials.