Rb2Na(NO3)3: A Congruently Melting UV-NLO Crystal with a Very Strong Second-Harmonic Generation Response

Designing Optical Anisotropy: Silver-Aminoalkylpyridine Nitrate Complexes with Tunable Structures

To introduce a design strategy for improving optical properties, two silver-amino alkylpyridine nitrate complexes, AgC6H8N3O3 and Ag2C14H20N6O6, were successfully synthesized using a recrystallization method. By employing polarizable π-conjugated [NO3-] ions, two types of pyridine ligands, and silver cations with a high affinity for pyridine, we obtained a one-dimensional chain structure with 4-aminomethylpyridine (AgC6H8N3O3) and a zero-dimensional molecular compound by introducing a relatively flexible aliphatic chain with 4-(2-aminoethyl)pyridine (Ag2C14H20N6O6). The compounds crystallize in the triclinic crystal system with the centrosymmetric P-1 space group, exhibiting a change in orientation between the π-conjugated system and the silver ion. Despite similar optical band gaps (3.69 eV for AgC6H8N3O3 and 3.73 eV for Ag2C14H20N6O6), AgC6H8N3O3 shows higher absorption in the 350-600 nm range. Electronic structure calculations support the ultraviolet absorption findings, suggesting that charge transfer with π-conjugated systems influences birefringence. Ag2C14H20N6O6 exhibits experimental birefringence (0.261@546.1 nm) surpassing that of AgC6H8N3O3 (0.212@546.1 nm), placing it among the highest recorded values within metal-pyridine incorporating nitrate complexes. The nonconventional orientation of π-conjugated [NO3-] ions contributes to this phenomenon, enhancing the action of free π-conjugated orbitals. This design strategy for micromodulating the alignment of the π-conjugated system promises to be an effective approach for enhancing optical properties, such as birefringence.

Ultrashort Phase-Matching Wavelength and Strong Second-Harmonic Generation in Deep-UV-Transparent Oxyfluorides by Covalency Reduction

The development of urgently-needed ultraviolet (UV)/deep-UV nonlinear optical (NLO) materials has been hindered by contradictory requirements of the microstructure, in particular the need for a strong second-harmonic generation (SHG) response as well as a short phase-matching (PM) wavelength. We herein employ a "de-covalency" band gap engineering strategy to adjust the optical linearity and nonlinearity. This has been achieved by assembling two types of transition-metal (TM) polyhedra ([TaO2 F4 ] and [TaF7 ]), affording the first tantalum-based deep-UV-transparent NLO materials, A5 Ta3 OF18 (A = K (KTOF), Rb (RTOF)). Experimental and theoretical studies reveal that the highly ionic bonds and strong electropositivity of tantalum in the two oxyfluorides induce record short PM wavelengths (238 (KTOF) and 240 (RTOF) nm) for d0 -TM-centered oxides, in addition to strong SHG responses (2.8 × KH2 PO4 (KTOF) and 2.6 × KH2 PO4 (RTOF)), and sufficient birefringences (0.092 (KTOF) and 0.085 (RTOF) at 546 nm). These results not only broaden the available strategies for achieving deep-UV NLO materials by exploiting the currently neglected d0 -TMs, but also push the shortest PM wavelength into the short-wavelength UV region.

KRb2(NO3)2Cl: a new birefringent crystal exhibiting a perovskite-related framework and a short cutoff edge

The combination of π-conjugated groups [NO3] and Cl-centered polyhedra generates a new birefringent crystal with a perovskite-related framework, KRb2(NO3)2Cl, which is the first alkali metal nitrate chloride synthesized by a mild hydrothermal method. It crystallizes in the orthorhombic space group pbam (no. 55). In addition, KRb2(NO3)2Cl crystals with dimensions up to 7 × 1.5 × 1 mm3 were grown. Notably, KRb2(NO3)2Cl has a short UV cut-off edge (below 228 nm) and a significantly enhanced birefringence (Δn = 0.084 at 1064 nm). Theoretical calculations indicate that the birefringence enhancement mainly derives from π-conjugated [NO3] plane triangles.

K5[B3O3F4(OH)]2(NO3): the first hydroxyfluorooxoborate-nitrate with a short ultraviolet cutoff edge and large birefringence

The first hydroxyfluorooxoborate-nitrate mixed anion compound, K5[B3O3F4(OH)]2(NO3), was synthesized by the solution evaporation method. It displays a unique structure built by K+ cations, the hydroxylated and fluorinated six-membered ring [B3O3F4(OH)] and [NO3] groups. It possesses a band gap of 5.68 eV derived from the diffuse reflectance spectrum, which corresponds to an ultraviolet cutoff edge of 218 nm. First-principles calculations show that it has a large birefringence of 0.095 at 532 nm and the result of the response electron distribution anisotropy method indicates that all three anion groups contribute positively to the birefringence, verifying the synergic contributions from the multiple anion groups.

M(NH2SO3)2·xH2O (M = Ca, Pb, x = 0, 1, 4): Effect of Hydrogen Bonding on Structural Transformations and Second Harmonic Generation of Metal Sulfamates

Nonlinear optical (NLO) crystals are very important for laser technology, but the performances of available NLO crystals are still insufficient for increasing demand. Recently, the exploration of new NLO crystals in non-π-conjugated systems with the heteroatomic tetrahedra is attracting a lot of interest. In this work, we systematically explore the metal sulfamates containing [NH₂SO₃] groups and four metal sulfamates, namely, Ca(NH₂SO₃)₂·4H₂O, Ca(NH₂SO₃)₂·H₂O, Pb(NH₂SO₃)₂·H₂O, and Pb(NH₂SO₃)₂ were synthesized by aqueous solution and hydrothermal methods. Notably, these metal sulfamates exhibit different crystal structures and optical properties owing to the diverse arrangement of the functional groups in their structures. In addition, due to hydrogen bond regulation, the centrosymmetric (CS) compound Ca(NH₂SO₃)₂·4H₂O can transform into noncentrosymmetric (NCS) Ca(NH₂SO₃)₂·H₂O, leading to NLO activity. Experimental characterizations and theoretical analysis reveal that these metal sulfamates are ultraviolet transparent and suitable for developing new 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.

Chiral Ligand-Driven Systematic Synthesis of Coordination Polymers with Non-centrosymmetric Structures

Chirality is an important concept in chemistry revealing intriguing optical properties such as circular dichroism (CD), circularly polarized luminescence (CPL), etc. As one of the non-centrosymmetric (NCS) classes, chiral materials with extended structures may exhibit unique nonlinear optical (NLO) properties, such as second-harmonic generation (SHG). In this Concept article, a series of recently discovered NCS coordination polymers (CPs) from use of carefully designed chiral organic ligands are reviewed. Combining several metal cations such as lanthanides, lead, zinc, and cadmium with rigid chiral ligands has resulted in interesting CPs with both polar and nonpolar structures. Detailed structures, SHG properties, and structure-property relationships are provided. The importance of hyperpolarizability formed by intermolecular hydrogen boding interactions to SHG is emphasized.

Flux Growth and Properties of Volatile Bromine-Containing UV Nonlinear Optical Crystal K3B6O10Br

A UV Nonlinear optical (NLO) crystal is one of the key devices in all-solid-state laser technology, and borate halides show outstanding potential due to their abundant structural diversity and short UV cut-off edges. In this article, the sizable UV NLO crystal of K3B6O10Br (KBOB) has been grown with lead-containing and lead-free fluxes systems using the high-temperature top-seeded solution growth (TSSG) method. Energy Dispersive X-ray Spectroscopy (EDS) and transmittance spectra illustrate the influence of Pb2+ ions on the transmittance properties and laser-induced damage threshold (LDT). The thermal property, namely, thermal expansion, thermal conductivity, and thermal diffusivity curves, were characterized. Moreover, a small variation of thermal refractive indexes was analyzed to illustrate the advantage of KBOB in the application for temperature-fluctuated specific regions.

A General Principle for DUV NLO Materials: π-Conjugated Confinement Enlarges Band Gap*

Current nonlinear optical materials face a conventional limitation in the trade-off between the band gap and birefringence, especially in the deep UV spectral region. To circumvent this dilemma, we propose a general principle, π-conjugated confinement, to partially decouple the interunit π-conjugated interactions by the separation of non-π-conjugated units. The goal is to further enlarge the band gap to a value larger than that of the singular π-conjugated counterpart and to maintain a suitable density of π-conjugated units to gain a large optical anisotropy. We reveal that π-conjugated confinement is a shared structural feature for all DUV NLO materials known to date, and thus, it provides a novel and essential design criterion for future design synthesis. Guided by this principle, the carbonophosphates are predicted to be a new promising DUV candidate system. Sr₃ Y[PO₄ ][CO₃ ]₃ (1) and Na₃ X[PO₄ ][CO₃ ] (X=Ba, Sr, Ca, Mg, 2-5) exhibit not only greatly enhanced birefringence that is 3-24 times larger than that of singular phosphates but also enhanced band gaps that are 0.2-1.7 eV wider than those of singular carbonates.

From BaCl2 to Ba(NO3)Cl: significantly enhanced birefringence derived from π-conjugated [NO3]

The chlorine barium nitrate Ba(NO3)Cl was synthesized for the first time to the best of our knowledge and found to exhibit a strong birefringence of 0.178 @ 1064 nm, about 19 times that of BaCl2.

Ba2B7O12F with novel FBB [B7O16F] and deep-ultraviolet cut-off edge

A new fluorooxoborate Ba₂B₇O₁₂F features novel FBB [B₇O₁₆F] and two different sizes of channels in its anionic structure, which possesses a deep-ultraviolet cutoff edge of 180 nm and a large bandgap of 6.67 eV.

Explorations of Second-Order Nonlinear Optical Materials in the Alkaline Earth Barbiturate System: Noncentrosymmetric Ca(H3C4N2O3)2·H2O and Centrosymmetric Sr(H5C8N4O5)2·4H2O

Two new alkaline earth barbiturates, Ca(H₃C₄N₂O₃)₂·H₂O and Sr(H₅C₈N₄O₅)₂·4H₂O, were synthesized via the mild hydrothermal technique. For Ca(H₃C₄N₂O₃)₂·H₂O, the stacking (H₃C₄N₂O₃)^- anions along the c axis are interconnected by CaO₇ polydedra forming a three-dimensional structure. Also, for Sr(H₅C₈N₄O₅)₂·4H₂O, it has two-dimensional layers composed by SrO₇ polyhedra and (H₅C₈N₄O₅)^- anions. The (H₅C₈N₄O₅)^- anions can be seen as two (H₃C₄N₂O₃)^- anions connected each other via the N-C bond. Powder second-harmonic generation (SHG) measurements revealed that Ca(H₃C₄N₂O₃)₂·H₂O is a phase-matchable material with a moderate SHG response (ca. 1.15× that of KDP). Furthermore, the birefringence values of the two crystals were measured as 0.49 and 0.475 at 546.1 nm, respectively. The theoretical calculations showed that the SHG response and large birefringence were primarily caused by the (H₃C₄N₂O₃)^- and (H₅C₈N₄O₅)^- groups.

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

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

(NH4)3[B(OH)3]2(COOH)3: a graphite-like UV nonlinear optical material with a large birefringence via structural optimization

A novel noncentrosymmetric (NCS) metal-free formic-borate, (NH4)3[B(OH)3]2(COOH)3, has been discovered, which exhibits an infrequent graphite-like structure. The alliance of two types of π-conjugated planar anions BO33- and COOH- produced an optimized layered structure to maximize the anisotropic polarizability, resulting in an extremely large birefringence (0.156@546 nm), larger than that of the classical commercial UV birefringent material α-BBO (0.122@546 nm). This strategy that structural optimization could enhance birefringence will guide the discovery of large birefringence materials, especially in the UV region.

A(H3C3N3O3)(NO3) (A = K, Rb): Alkali-Metal Nitrate Isocyanurates with Strong Optical Anisotropy

The first alkali-metal nitrate isocyanurates, A(H₃C₃N₃O₃)(NO₃) (A = K, Rb), were synthesized by the tactic of introducing (NO₃)^- into isocyanurate with a mild hydrothermal technique. They crystallized into the same monoclinic centrosymmetric (CS) space group P2₁/c, which featured a 2D [(H₃C₃N₃O₃)(NO₃)]_∞ layered structure separated by K⁺ and Rb⁺ cations, respectively. Both compounds exhibited short ultraviolet cutoff edges (λ_cutoff = 228 and 229 nm) and large birefringences (Δn = 0.253 and 0.224 at 546.1 nm). More importantly, in comparison with most of the isocyanurates and nitrates, they have better thermal stability with decomposition temperatures up to 319.8 and 324.4 °C. In addition, our theoretical calculations reveal that the π-conjugated groups play significant roles in improving the optical anisotropy. Remarkably, introducing a π-conjugated inorganic acid radical (NO₃)^- into isocyanurate is an extremely meaningful strategy to explore new UV birefringent crystals.

A6Sb4F12(SO4)3 (A = Rb, Cs): Two Novel Antimony Fluoride Sulfates with Unique Crown-like Clusters

Two new antimony fluoride sulfates named A₆Sb₄F₁₂(SO₄)₃ (A = Rb, Cs) with unique crown-like clusters have been successfully synthesized by introducing Sb³⁺ with a stereochemically active lone pair in the sulfate system through a facile hydrothermal method. The structure regulation induced by the cation sizes results in the symmetry difference in these two title compounds, i.e., Rb₆Sb₄F₁₂(SO₄)₃ crystallizing in the polar noncentrosymmetric (NCS) space group of P3 and Cs₆Sb₄F₁₂(SO₄)₃ in the centrosymmetric (CS) space group of P1̅. Detailed characterizations, including XRD, thermal behaviors, optical properties, and the theoretical calculations, for these two compounds have been performed. The discovery of the unique crown-like clusters observed in A₆Sb₄F₁₂(SO₄)₃ (A = Rb, Cs) enriches the diversity of structures for antimony sulfates systems. Simultaneously, a summary and comparison for all reported antimony halide sulfates has been made to illustrate the synergistic effect of the stoichiometric ratio and cation sizes on the macroscopic centricity and the framework structure, which will guide the systematical discovery of antimony(III)-based functional materials in future.

Effect of anion dimensionality on optical properties: the ∞[B7O10(OH)2] layer in CsB7O10(OH)2vs. the ∞[B7O12] framework in CsBaB7O12

CsB₇O₁₀(OH)₂ with new FBB features a 2D anionic structure whose band gap is about 6.6 eV and CsBaB₇O₁₂ features a 3D anionic structure whose band gap is 5.6 eV. And their simulated birefringence are about 0.08 and 0.05 at 1064 nm, respectively.

Three diphosphates, α-Li2Na2P2O7, Li8Pb3Ba(P2O7)4 and Li7Rb(P2O7)2: influences of co-substitution on the crystal structure

We reported the synthesis, structure, characterization and first-principles calculations of diphosphates, α-Li₂Na₂P₂O₇, Li₈Pb₃Ba(P₂O₇)₄ and Li₇Rb(P₂O₇)₂, and studied the influences of co-substitution on the crystal structure.

Inorganic planar π-conjugated groups in nonlinear optical crystals: review and outlook

Nonlinear optical crystals with various inorganic planar π-conjugated groups are reviewed based on their bandgaps.

Three non-centrosymmetric bismuth phosphates, Li2ABi(PO4)2 (A = K, Rb, and Cs): effects of cations on the crystal structure and SHG response

We reported the synthesis, structure and nonlinear properties of three phosphates, Li₂ABi(PO₄)₂ (A = K, Rb, and Cs), and the effects of cations on the crystal structure and SHG response.

Experimental characterization and first principles calculations of linear and nonlinear optical properties of two orthophosphates A3Al2(PO4)3 (A = Rb, K)

We report the experimental characterization and first principles calculation of linear and nonlinear optical properties of two orthophosphates A₃Al₂(PO₄)₃ (A = Rb, K).

Polar phase transitions in heteroepitaxial stabilized La0.5Y0.5AlO3 thin films

We report on the fabrication of epitaxial La_0.5Y_0.5AlO₃ ultrathin films on (001) LaAlO₃ substrates. Structural characterizations by scanning transmission electron microscopy and x-ray diffraction confirm the high quality of the film with a ^- b ⁺ c ^- AlO₆ octahedral tilt pattern. Unlike either of the nonpolar parent compound, LaAlO₃ and YAlO₃, second harmonic generation measurements on the thin films suggest a nonpolar-polar phase transition at T _c near 500 K, and a polar-polar phase transition at T _a near 160 K. By fitting the angular dependence of the second harmonic intensities, we further propose that the two polar structures can be assigned to the Pmc2 ₁ and Pmn2 ₁ space group, while the high temperature nonpolar structure belongs to the Pbnm space group.

Ag3I[(MoO3)2(IO3)2]: a new polar material synthesized from the structural modulation of molybdenyl iodate architectures by polarizable cation (Ag+) and anion (I−)

A new noncentrosymmetric compound Ag₃I[(MoO₃)₂(IO₃)₂] has been hydrothermally synthesized and characterized.