BiFSeO3: An Excellent SHG Material Designed by Aliovalent Substitution

Controlling the Orientation-Dependent Second Harmonic Generation in Hybrid Germanium Perovskites

Manipulating the crystal orientation plays a crucial role in the conversion efficiency during second harmonic generation (SHG). Here, we provide a new strategy in controlling the surface-dependent anisotropic SHG with the precise design of (101) and (21 ‾ ${\bar 1}$ 0) MAGeI3 facets. Based on the SHG measurement, the (101) MAGeI3 single crystal exhibits larger SHG (1.3×(21 ‾ ${\bar 1}$ 0) MAGeI3). Kelvin probe force microscopy imaging shows a smaller work function for the (101) MAGeI3 compared with the (21 ‾ ${\bar 1}$ 0), which indirectly demonstrates the stronger intrinsic polarization on the (101) surface. X-ray photoelectron spectroscopy confirms the band bending within the (101) facet. Temperature-dependent steady-state and time-resolved photoluminescence spectroscopy show shorter lifetime and wider emission band in the (101) MAGeI3 single crystal, revealing the higher defect states. Additionally, powder X-ray diffraction patterns show the (101) MAGeI3 possesses larger in-plane polar units [GeI3]- density, which could directly enhance the spontaneous polarization in the (101) facet. Density functional theory (DFT) calculation further demonstrates the higher intrinsic polarization in the (101) facet compared with the (21 ‾ ${\bar 1}$ 0) facet, and the larger built-in electric field in the (101) facet facilitates surface vacancy defect accumulation. Our work provides a new angle in tuning and optimizing hybrid perovskite-based nonlinear optical materials.

An Unprecedented [BO2]-Based Deep-Ultraviolet Transparent Nonlinear Optical Crystal by Superhalogen Substitution

Solid-state structures with the superhalogen [BO2]- have thus far only been observed with a few compounds whose syntheses require high reaction temperatures and complicated procedures, while their optical properties remain almost completely unexplored. Herein, we report a facile, energy-efficient synthesis of the first [BO2]-based deep-ultraviolet (deep-UV) transparent oxide K9[B4O5(OH)4]3(CO3)(BO2) ⋅ 7H2O (KBCOB). Detailed structural characterization and analysis confirm that KBCOB possesses a rare four-in-one three-dimensional quasi-honeycomb framework, with three π-conjugated anions ([BO2]-, [BO3]3-, and [CO3]2-) and one non-π-conjugated anion ([BO4]5-) in the one crystal. The evolution from the traditional halogenated nonlinear optical (NLO) analogues to KBCOB by superhalogen [BO2]- substitution confers deep-UV transparency (<190 nm), a large second-harmonic generation response (1.0×KH2PO4 @ 1064 nm), and a 15-fold increase in birefringence. This study affords a new route to the facile synthesis of functional [BO2]-based oxides, paving the way for the development of next-generation high-performing deep-UV 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.

Two Short-Wave UV Beryllium Selenites Exhibiting Diverse Optical Properties Stemming from Functional Group Arrangements

The arrangement of functional groups exerts a crucial role in determining the characteristics of compounds. In this study, we synthesized two novel short-wave ultraviolet (UV) nonlinear optical (NLO) crystals: KBe2(SeO3)2(OH)·H2O and K2Be(SeO3)2. Interestingly, the two compounds show the same SeO3 triangular pyramids and K-O polyhedra. However, the two compounds exhibit distinct beryllium-oxygen anion groups: BeO3(OH) for KBe2(SeO3)2(OH)·H2O and BeO4 for K2Be(SeO3)2. This results in the SeO3 groups within the structure having different orientations, ultimately leading to the two compounds exhibiting completely different optical properties. KBe2(SeO3)2(OH)·H2O displays a large second harmonic generation (SHG) effect equivalent to 2× KH2PO4 (KDP), coupled with a large birefringence of 0.078 at 546 nm. In contrast, the SHG effect and birefringence of K2Be(SeO3)2 are only 0.33× that of KDP and 0.024 at 546 nm, respectively. Structural analyses and theoretical calculations indicate that these pronounced differences in optical properties stem from variations in the arrangement of the SeO3 functional groups. This study not only sheds light on the correlation between crystal structure and optical behavior but also presents a hopeful avenue for the advancement of materials in the short-wave UV spectrum.

Exceptional Optical Anisotropy Enhancement Achieved Through Dual-Ions Cosubstitution Strategy in Novel Hybrid Bismuth Halides

Guided by a superb dual-ions cosubstitution strategy, two novel, highly optically anisotropic hybrid bismuth halides are designed and synthesized. The first compound, Gu3Bi2NO3Cl8 (Gu = C(NH2)3), is developed using the 2D perovskite halide Cs3Bi2Cl9 as the maternal structure. This involved substituting all Cs+ cations with organic Gu+ and replacing some Cl- anions with [NO3]-. Further substitution of Cl- with additional [NO3]- resulted in the formation of nitrate-rich Gu2Bi(NO3)3Cl2 crystal, exhibiting a 3.4-fold increase in [NO3]- per unit volume. Both compounds have a structurally 0D nature, comprising bismuth-centered polyhedra formed by coordinated chlorides and monodentate/bidentate nitrate moieties, with Gu+ serving as a separator and linker. Notably, the presence of superb optically anisotropic dual-ions, i.e., planar Gu+ and [NO3]-, enables these crystals to possess sharply enhanced optical anisotropy, with birefringence values more than 1 order of magnitude higher than that of the initial crystal Cs3Bi2Cl9 (0.162/0.186vs 0.011 at 546 nm). The discovery and characterization of Gu3Bi2NO3Cl8 and Gu2Bi(NO3)3Cl2 crystals provide new insights into achieving expected modifications in optical properties through the utilization of a dual-ions cosubstitution strategy.

The enhanced bandgap and birefringence of rare-earth phosphates XPO4 (X = Sc, Y, La, and Lu): a first-principles investigation

Rare-earth phosphates were thought to be good candidates as ultraviolet/deep ultraviolet optical materials due to their relatively large bandgap and optical properties. In this paper, the authors screened out a family of XPO4 (X = Sc, Y, La, and Lu) compounds with an enhanced bandgap (HSE06 bandgap ≥ 7.61 eV) and birefringence (0.0934-0.2003@1064 nm) using first-principles calculations. The origin of enhanced optical properties was investigated using projected density of states, distortion indices, and Born effective charges. The results show that the PO4 anionic groups and X-O polyhedra give the main contribution in determining the optical properties, and the PO4 anionic groups give more contribution than other functional basic units. The spin-orbit interaction was also investigated. Similar band structures were found after spin-orbit coupling (SOC) was considered, and slightly enhanced birefringence was found when SOC was applied to these rare-earth phosphates.

Guanidinium Vanadate [C(NH2)3]3VO4·2H2O Revealing Enhanced Second-Harmonic Generation and Wide Band Gaps

A new guanidinium-templated vanadate, [C(NH2)3]3VO4·2H2O, has been synthesized in a phase-pure form. It crystallizes in a noncentrosymmetric polar space group, Cc, and the crystal structure is built upon a framework of guanidinium, vanadate tetrahedra, and water molecules linked by hydrogen bonds. Notably, optical measurements reveal that the material exhibits an approximately 9.6-fold enhancement in second-harmonic generation efficiency compared to its phosphate analogue. The enhancement can be attributed to the increased geometrical distortion of the VO4 tetrahedra. Furthermore, we found that the coordination number of the central vanadium atom significantly affects the optical band gaps. Among various coordination numbers, the 4-coordinate VO4 tetrahedra are found to be more favorable for widening the optical band gap of materials compared to the 5- and 6-coordinate vanadium polyhedra, as demonstrated by this work.

LiGa(IO3 )4 : An Excellent NLO Material with Unprecedented 2D [Ga(IO3 )4 ]∞ - Layer Synthesized by Aliovalent Substitution

Nonlinear optical (NLO) crystals are indispensable for the solid-state lasers for their ability to expand wavelength spectral to the regions where the directing lasing is difficult or even impossible, yet the rational design of a high-performance NLO crystal remains a great challenge owing to the severe structural and properties' requirements. Herein, a new noncentrosymmetric (NCS) and polar gallium iodate, LiGa(IO3 )4 , with a novel 2D anionic layer, is successfully designed and synthesized by the aliovalent substitution strategy based on classic α-LiIO3 . The 2D [Ga(IO3 )4 ]∞ - layer in LiGa(IO3 )4 is built from the GaO6 octahedra and highly polarizable units IO3 . Compared with its parent compound, the partial replacement of A-site Li+ cation with main group Ga3+ cation facilitates LiGa(IO3 )4 to possess excellent NLO properties, including the large second-harmonic generation (SHG) response (14 × KH2 PO4 (KDP) @ 1064 nm), wide bandgap (4.25 eV), large birefringence (0.23 @ 1064 nm), and wide optical transparency from UV to mid-IR. These reveal that LiGa(IO3 )4 will be a promising NLO crystal.

Ln2 F2 (OH2 )(MoO3 )2 (SeO3 )2 : Promising Multifunctional Nonlinear Optical Materials Created by Partial Fluorination Strategy under Corrosion Resistant Supercritical Reactions

It has historically been exceedingly challenging to create physically and chemically stable lanthanide compounds with strong second harmonic generation (SHG) due to their strong preference to central symmetry. In this work, five new non-centrosymmetric lanthanide selenites, namely, Ln2 F2 (OH2 )(MoO3 )2 (SeO3 )2 (Ln = Sm, Eu, Gd, Tb and Dy), are achieved by partial fluorination of the lanthanide oxygen polyhedron. An HF corrosion resistant supercritical hydrothermal method is developed, which is a facile and universal method for HF corrosion and high-temperature high-pressure environment. The title compounds displayed a novel 3D framework composed of 1D molybdenum selenite chains bridged by Ln2 F2 O12 (OH2 ) dimers. Their powder SHG responses showed a large difference, ranging from 1.0 to 9.0 × KH2 PO4 (KDP) at 1064 nm. The half-filled Gd compound exhibited very strong SHG efficiency of up to 1.2 × KTP (KTiOPO4 ) at 2050 nm. Compounds Tb and Gd are the first lanthanide selenites with SHG intensity reaching KTP level, which is very rare in this system. Furthermore, these compounds can also possess excellent physicochemical stability and strong luminescence emission, indicating that they are promising multifunctional nonlinear optical materials. This work offered an effective way for design and synthesis of multifunctional and high-performant nonlinear optical materials.

Synthesis, structures and properties of two new selenite optical materials: K2Zn3Se4O12 and K4Zn3V4Se2O19

Two new selenites, K2Zn3Se4O12 (compound 1) and K4Zn3V4Se2O19 (compound 2), have been successfully synthesized by solid-state reactions in vacuum tubes. Compound 1 consists of a three-dimensional (3D) framework with [SeO3] triangular pyramids and [ZnO4] tetrahedra in the monoclinic space group P21/c (No. 14). Compound 1's cut-off edge is below 344 nm, based on its UV-Vis-NIR diffuse reflectance studies, and theoretical calculations indicate a birefringence of around 0.043 at 1064 nm. The two-dimensional layer of compound 2, in contrast, is made up of [SeO3] triangular pyramids, [ZnO4] tetrahedra, and [V4O13] tetrahedra. It crystallizes in the monoclinic space group C2/c (No. 15). Its UV-Vis-NIR diffuse reflectance studies demonstrate that the compound's cut-off edge is lower than 330 nm.

Unveiling the Superior Optical Properties of Novel Melamine-Based Nonlinear Optical Material with Strong Second-Harmonic Generation and Giant Optical Anisotropy

Two melamine-based metal halides, (C₃ N₆ H₇ )(C₃ N₆ H₆ )HgCl₃ (I) and (C₃ N₆ H₇ )₃ HgCl₅ (II), are synthesized by incorporating the heavy d¹⁰ cation, Hg²⁺ , and the halide anion, Cl^- . The noncentrosymmetric structure of I results from two unique attributes: large asymmetric secondary building units produced by direct covalent coordination of melamine to Hg²⁺ and a small dihedral angle between melamine molecules. The former makes inorganic modules locally acentric, while the latter prevents planar organic groups from forming deleterious antiparallel arrangement. The unique coordination in I results in an enlarged band gap of 4.40 eV. Due to the large polarizability of the heavy Hg²⁺ cation and the π-conjugated system of melamine, I exhibits a strong second-harmonic generation efficiency of 5 × KH₂ PO₄ , larger than any reported melamine-based nonlinear optical materials to date. Density functional theory calculations indicate that I possesses giant optical anisotropy, with a birefringence of 0.246@1064 nm.

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.

Noncentrosymmetric γ-Cs2I4O11 Obtained from IO4 Polyhedral Rearrangements in the Centrosymmetric β-Phase

We report the synthesis and optical properties of noncentrosymmetric (NCS) γ-Cs₂I₄O₁₁ that was obtained through IO₄ polyhedral rearrangements from centrosymmetric (CS) β-Cs₂I₄O₁₁. Trifluoroacetic acid (TFA) acts as a structure-directing agent and plays a key role in the synthesis. It is suggested that the function of TFA is to promote rearrangement reactions found in the organic synthesis of stereoisomers. γ-Cs₂I₄O₁₁ crystallizes in the NCS monoclinic space group P2₁ (No. 4) and exhibits a strong second-harmonic-generation (SHG) response of 5.0 × KDP (KH₂PO₄) under 1064 nm laser radiation. Additional SHG experiments indicate that the material is type I phase matchable. First-principles calculations show that SHG intensity mainly comes from its d₃₄, d₂₁, and d₂₃ SHG tensor components. The synthetic strategy of discovering γ-Cs₂I₄O₁₁ provides a new way for designing novel NCS SHG materials.

PbBi(SeO3)2F and Pb2Bi(SeO3)2Cl3: Coexistence of Three Kinds of Stereochemically Active Lone-Pair Cations Exhibiting Excellent Nonlinear Optical Properties

Stereochemically active lone-pair (SCALP) cations are one attractive type of nonlinear optical (NLO)-active units because of their large microcosmic polarizability and anisotropy. Currently, the single and/or dual lone-pair cation-based noncentrosymmetric (NCS) oxides have been extensively investigated and verified to be one class of outstanding NLO materials. From the perspective of function optimization, the integration of three kinds of SCALP cations into one crystal may synergistically improve the NLO properties, which is greatly expected but unexplored to date. Herein, by introducing flexible metal halide bonds to guarantee the stereochemical activity and overcome the energetically favorable antiparallel arrangements of lone-pair cations, the first type of three lone-pair-cation (Pb2+, Bi3+, and Se4+)-coexisting NCS oxides PbBi(SeO3)2F (I) and Pb2Bi(SeO3)2Cl3 (II) was obtained. As expected, both compounds show outstanding NLO properties, such as the strong second-harmonic-generation signal (10.5× and 13.5 × KDP), large birefringence (0.103 and 0.186), relatively wide energy band gaps (3.75 and 3.45 eV), and good physicochemical stability. Theoretical calculations demonstrated the effect of three lone-pair-cation-based polyhedra and the halide anion on NLO properties.

Second-Harmonic Generation-Positive Na2Ga2SiS6 with a Broad Band Gap and a High Laser Damage Threshold

The development of high-power solid-state lasers is in urgent need of new infrared nonlinear optical (IR NLO) materials with a wide band gap and a high laser-induced damage threshold. A new infrared nonlinear optical material Na₂Ga₂SiS₆ has been synthesized for the first time, crystallizing in the Fdd2 (no. 43) noncentrosymmetric space group. Its three-dimensional tunnel framework consists of two typical NLO active motifs [GaS₄] and [SiS₄], with Na⁺ cations located inside the tunnels. Na₂Ga₂SiS₆ exhibits comprehensive optical properties, namely, a wide transmission range, a high laser-induced damage threshold (10 × AgGaS₂), a type-I phase-matching second-harmonic generation response (0.2 × AgGaS₂), and especially a wide band gap (3.93 eV), which is the largest in the A₂M^III₂M^IVQ₆ (A = alkali metals; M^III = IIIA elements; M^IV = IVA elements; Q = S and Se) family. Therefore, Na₂Ga₂SiS₆ does not produce two-photon absorption under a 1064 nm laser pump and could be used in high-energy laser systems, which makes Na₂Ga₂SiS₆ a promising candidate for high-energy IR NLO applications.

Metal oxyhalides: an emerging family of nonlinear optical materials

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

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

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

Halogen-Ion-Induced Structural Phase Transition Giving a Polymorph of HgBr2 with Balanced Nonlinear Optical Properties

The key to developing new infrared nonlinear optical (NLO) materials is balancing second-order nonlinear optical effects and the laser-induced damage threshold (LIDT). In this paper, a new polymorph of HgBr₂ (P2₁2₁2₁) was synthesized by a "halogen-ion-induced effect" in solution, which features a crystal structure different from that of the original phase (Cmc2₁) and exhibits better NLO properties. Its powders show a strong SHG effect (9 × KDP), a high LIDT (30 × AgGaS₂), a wide infrared transparent range, and stability in air, making it a prospective NLO material in the IR region. In addition, the above excellent NLO characteristics are well illustrated in DFT theoretical calculations. More importantly, experimental results show that the new infrared NLO polymorph with excellent comprehensive properties could be controllably obtained by using the halogen-ion-induced strategy.

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.

Synthesis and Characterization of Novel Nanoparticles of Lithium Aluminum Iodate LiAl(IO3)4, and DFT Calculations of the Crystal Structure and Physical Properties

Here we report on the non-hydrothermal aqueous synthesis and characterization of nanocrystalline lithium aluminum iodate, LiAl(IO₃)₄. Morphological and compositional analyses were carried out by using scanning electron microscopy (SEM) and energy-dispersive X-ray measurements (EDX). The optical and vibrational properties of LiAl(IO₃)₄ have been studied by UV-Vis and IR spectroscopy. LiAl(IO₃)₄ is found to crystallize in the non-centrosymmetric, monoclinic P2₁ space group, contrary to what was reported previously. Theoretical simulations and Rietveld refinements of crystal structure support this finding, together with the relatively high Second Harmonic Generation (SGH) response that was observed. Electronic band structure calculations show that LiAl(IO₃)₄ crystal has an indirect band gap Egap=3.68 eV, in agreement with the experimental optical band gap Egap=3.433 eV. The complex relative permittivity and the refraction index of LiAl(IO₃)₄ have also been calculated as a function of energy, as well as its elastic constants and mechanical parameters. LiAl(IO₃)₄ is found to be a very compressible and ductile material. Our findings imply that LiAl(IO₃)₄ is a promising material for optoelectronic and non -linear optical applications.

Role of fluorine on the structure and second-harmonic-generation property of inorganic selenites and tellurites

Fluorine, as the most electronegative element, can replace the oxygen ligands of functional groups under given conditions. These fluoride groups are more or less different from the pure oxide groups in composition, symmetry, polarizability, transmittancy, etc. The rational use of these differences is expected to improve the probability of noncentrosymmetric structures and the comprehensive performance of second-harmonic-generation (SHG) materials. In this feature article, we introduce the recent developments in fluoride selenite and tellurite SHG materials together with highlighting our contributions, including Se(IV) and Te(IV) compounds with (i) d⁰ transition metal oxyfluoride octahedron, (ii) IIIA metal oxyfluoride octahedron, (iii) fluoride lone pair cation polyhedron, and (iv) other fluoride polyhedron. The future perspectives of fluoride selenite and tellurite SHG materials are also discussed.

M(B(SeO3)3)H2O (M = Al, Ga): the first boroselenites with a unique sandwich like double-layer structure

Exploration of new types of borates is important because of their promising applications in diverse fields. Two new boroselenites, namely, M(B(SeO₃)₃)H₂O (M = Al, Ga), which represent the first IIIA metal boroselenite, were synthesized by hydrothermal reactions. M(B(SeO₃)₃)H₂O (M = Al, Ga) possesses a unique sandwich like double-layer structure formed by two 2D [MSe₂O₈]^5- layers interconnected by 1D [BSeO₅]^3- chains. More interestingly, both compounds display large band gaps (4.86/4.79 eV) and moderate birefringences (Δn = 0.063/0.064 at 1064 nm) based on density functional theory (DFT) calculations.

Ba2[FeF4(IO3)2]IO3: a promising nonlinear optical material achieved by chemical-tailoring-induced structure evolution

A new noncentrosymmetric iron-iodate-fluoride Ba₂[FeF₄(IO₃)₂]IO₃ was ingeniously obtained based on the centrosymmetric Ba[FeF₄(IO₃)] through chemical tailoring. Ba₂[FeF₄(IO₃)₂]IO₃ exhibits a strong phase-matchable second-harmonic generation effect, a large band gap, and a wide mid-infrared transparent window. The chemical tailoring design based on oxide-fluoride anions affords a feasible approach to design nonlinear optical materials.

Controllable Syntheses, Crystal Structure Evolution, and Photoluminescence of Polymorphic Zirconium Oxyfluorides

Precise synthesis of polymorphic phases with similar components but distinct crystal structures is one of the key problems in inorganic chemistry. In this work, we report a fluorination method adopting ZrO₂ as the starting material and NH₄F as the fluoridation agent that can afford multiphases in the Zr-O-F system, including Zr₇O₉F₁₀, Zr₃O₂F₈, ZrO_0.46F_3.08, ZrO_0.33F_3.33, β-ZrF₄, NH₄Zr₂F₉, and NH₄ZrF₅. A preliminary phase formation diagram was established as a function of the fluorination temperature (T), reaction time (t), and F/Zr ratio after systematic optimization of the preparation conditions. Among the as-obtained phases, the detailed crystal structures of Zr₇O₉F₁₀ and ZrO_0.33F_3.33 were refined based on the powder X-ray diffraction patterns. As the F/O ratio increases, the crystal structures of Zr-O-F phases transform gradually from an anion-deficient α-UO₃-related structure of Zr₇O₉F₁₀ to an anion-excess ReO₃-related structure of ZrO_0.33F_3.33. At last, we also prepared Ti-doped ZrO₂, Zr₇O₉F₁₀, ZrO_0.46F_3.08, and ZrO_0.33F_3.33 to study the host-lattice-dependent photoluminescence properties of zirconium oxyfluorides. The four materials show distinct photoluminescence in the UV and visible regions due to different local coordination environments of Zr/Ti. This work demonstrates the low-temperature fluorination method as an efficient route to phase-selective polymorphic metal oxyfluorides, which can be employed in further structure-property relationship studies.

Hydrogen-Bond-Driven Synergistically Enhanced Hyperpolarizability: Chiral Coordination Polymers with Nonpolar Structures Exhibiting Unusually Strong Second-Harmonic Generation

Four chiral coordination polymers (CPs), M[(S,S)-C₁₄ H₁₄ N₂ O₆ ] and M[(R,R)-C₁₄ H₁₄ N₂ O₆ ] (M=Zn or Cd), have been exclusively synthesized in high yields with the aid of newly designed chiral ligand under hydrothermal condition. The CPs crystallizing in the orthorhombic nonpolar space group, C222₁ , reveal three-dimensional framework structures composed of MO₄ tetrahedra and the corresponding homochiral linkers. Powder second-harmonic generation (SHG) measurements indicate that the nonpolar CPs reveal very strong SHG efficiency of ca. 5-9 times that of KH₂ PO₄ and exhibit type-I phase-matching behavior. Density functional theory calculations suggest that the unusually large SHG efficiency found from the nonpolar CPs should be attributable to the synergistic effect of polarizable metal cations and enhanced hyperpolarizability in the donor-acceptor system originating from the hydrogen bonding in the coordinated linkers.

Giant Second-Harmonic Generation Response and Large Band Gap in the Partially Fluorinated Mid-Infrared Oxide RbTeMo2O8F

Strong second-harmonic generation (SHG) and a wide band gap are two crucial but often conflicting parameters that must be optimized for practical nonlinear optical (NLO) materials. We report herein the first d⁰-transition-metal (TM) tellurite with half of the d⁰-TM-octahedra partially fluorinated, namely, quinary RbTeMo₂O₈F, which exhibits giant SHG responses (27 times that of KH₂PO₄ (KDP) and 2.2 times that of KTiOPO₄ (KTP) with 1064 and 2100 nm laser radiation, respectively), the largest SHG values among all reported metal tellurites. RbTeMo₂O₈F also possesses a large band gap (3.63 eV), a wide optical transparency window (0.34-5.40 μm), and a significant birefringence (Δn = 0.263 at 546 nm). Theoretical calculations and crystal structure analysis demonstrate that the outstanding SHG responses can be definitively attributed to the uniform alignment of the polarized [MoO₅F]/[MoO₆] octahedra and the seesaw-like [TeO₄], and the consequent favorable summative polarization of the three distinct SHG-active polyhedra, both induced by partial fluorine substitution on the [MoO₆] octahedra.

Linear and Nonlinear Optical Properties of Centrosymmetric Sb4O5SO4 and Noncentrosymmetric Sb4O4(SO4)(OH)2 Induced by Lone Pair Stereoactivity

Introducing stereochemically active lone-pair Sb³⁺ cations into sulfates, two three-dimensional (3D) antimony-sulfates, Sb₄O₅SO₄ (1) and Sb₄O(SO₄)(OH)₂ (2), were achieved under moderate hydrothermal conditions. Both structures are constructed by tetranuclear-{Sb₄}-clusters-based layers and SO₄ tetrahedra. However, owing to the different packing patterns of the layers, they display different characteristics: 1 exhibits a centrosymmetric structure while 2 possesses a noncentrosymmetric structure. UV-vis spectra show that they possess wide band gaps. Sb₄O(SO₄)(OH)₂ is nonlinear optical (NLO) active with a second-harmonic generation (SHG) response of ∼1.2 times of KH₂PO₄, together with the phase-matchable capacity, endowing it a promising UV NLO material. The first-principle calculations were performed to elucidate the structure-property relationships. The results indicate that the lone pair stereoactivity of Sb³⁺ provides the large contribution to the macroscopic SHG effect.

High-Performance Second-Harmonic-Generation (SHG) Materials: New Developments and New Strategies

ConspectusSecond-harmonic-generation (SHG) causes the frequency doubling of light, which is very useful for generating high-energy lasers with specific wavelengths. Noncentrosymmetry (NCS) is the first requirement for an SHG process because the SHG coefficient is zero (χ² = 0) in all centrosymmetric structures. At this stage, developing novel NCS crystals is a crucial scientific topic. Assembling polar units in an addictive fashion can facilely form NCS crystals with outstanding SHG performance. In this way, our group has obtained many different NCS crystals with extremely large SHG intensities (>5 × KDP or 1 × KTP). In this Account, we first provide a brief review of the development of SHG materials and concisely highlight the features of the excellent SHG materials. Then, we present four facile and rational molecular design strategies: (1) Traditional BO₃^3--based crystals feature short absorption edges but usually suffer from relatively weak SHG performance (<5 × KDP). The combination of two types of pure π-conjugated anions (BO₃^3- and NO₃^-) in a parallel fashion in the same compound has afforded a metal borate nitrate with a strong SHG effect. (2) To overcome the problems of the weak SHG effect and small birefringence in the less anisotropic QO₄-based compounds, highly polarizable cations such as Hg²⁺ and Bi³⁺ are introduced into these systems, which greatly enhances both SHG effects and birefringence. (3) Iodate anions can be condensed into polynuclear iodate anions with a higher density of I⁵⁺ per unit cell, hence polyiodate anions can serve as excellent SHG-active groups. We developed a novel synthesis method for hydrothermal reactions under a phosphoric acid medium and obtained a series of metal polyiodates with strong SHG effects. In addition, as the number of iodate groups increases, the structural configuration of the polyiodate anion changes from linear to bent. (4) We introduce the concept of aliovalent substitution which features site-to-site atomic displacement at the structural level. Such aliovalent substitution led to new materials that have the same chemical stoichiometries or structural features as their parent compounds. Thus, aliovalent substitution can provide more experimental opportunities and afford new high-performance SHG materials. The introduction of a fluoride anion and the replacement of metal cations in the MO₆ octahedron can result in new metal iodates with balanced properties including a large SHG effect, a wide band gap, and a high laser-induced damage threshold (LIDT) value. Finally, we briefly discuss several problems associated with the studies of SHG materials and give some prospects for SHG materials in the future.

Tin Chloride Sulfates A3Sn2(SO4)3-xCl1+2x (A = K, Rb, Cs; x = 0, 1) as Multifunctional Optical Materials

The series of alkali-metal tin chloride sulfates A₃Sn₂(SO₄)_3-xCl_1+2x (A = K, Rb, Cs; x = 0, 1), K₃Sn₂(SO₄)₃Cl, Rb₃Sn₂(SO₄)₂Cl₃, and Cs₃Sn₂(SO₄)₂Cl₃, were successfully synthesized through an improved mild hydrothermal method. Interestingly, in addition to the cation size effect, the structure-directing effect of anions induces different symmetries in the three title compounds, with K₃Sn₂(SO₄)₃Cl being noncentrosymmetric, while Rb₃Sn₂(SO₄)₂Cl₃ and Cs₃Sn₂(SO₄)₂Cl₃ are centrosymmetric. Powder second-harmonic generation (SHG) measurements indicate that K₃Sn₂(SO₄)₃Cl is a nonlinear optical material that is type I phase matchable with a weak SHG response (0.1× KDP). Photoluminescence tests reveal that the three title compounds emit strong greenish yellow, orange, and salmon light, respectively, under UV excitation, indicating that they are promising inorganic solid fluorescent materials. Simultaneously, a detailed structural analysis of all the known tin(II) halide sulfates has been performed, which will guide the systematic exploration of high-performance tin(II)-based functional materials in the future.

AgBi(SO4)(IO3)2: aliovalent substitution induces structure dimensional upgrade and second harmonic generation enhancement

A new 3D noncentrosymmetric mixed-metal sulfate iodate, AgBi(SO₄)(IO₃)₂, has been designed based on a 2D iodate of AgBi(IO₃)₄via the aliovalent substitution of [IO₃]^- groups by [SO₄]^2- groups. Enhancement of the second harmonic generation response (3.0 × KDP to 3.9 × KDP) has been achieved while retaining a favorable NLO framework. This is a novel strategy to upgrade structural dimensions.

Bi2[B2(SeO3)6]: A Metal Boroselenite with a Unique Zero-Dimensional [B2(SeO3)6]6- Anionic Group and Large Birefringence

Explorations of new types of borates are important because of their promising application in diverse fields. A new bismuth-containing boroselenite, Bi₂[B₂(SeO₃)₆], has been obtained through high-temperature solid-state reaction in a closed system. Bi₂[B₂(SeO₃)₆] possesses a zero-dimensional [B₂(SeO₃)₆]^6- anionic group that does not belong to any types of reported boroselenites. Besides, Bi₂[B₂(SeO₃)₆] is the first boroselenite with lone-pair electrons containing a metal ion as the countercation. More interestingly, on the basis of the first-principles calculations, this compound displays a large birefringence (0.090) at 1064 nm.

M(NH2 SO3 )2 (M=Sr, Ba): Two Deep-Ultraviolet Transparent Sulfamates Exhibiting Strong Second Harmonic Generation Responses and Moderate Birefringence

Over the last few decades, the development of each new nonlinear optical (NLO)-active functional unit has led to the discoveries of a series of excellent NLO materials. In the present work, based on first-principles studies, we identified a novel deep-UV (DUV) NLO-active functional unit, a non-π-conjugated group viz. (NH₂ SO₃ )^- . By combining alkaline-earth metals with (NH₂ SO₃ )^- group, two DUV transparent NLO sulfamates, M(NH₂ SO₃ )₂ (M=Sr, Ba) with superior optical properties including strong SHG responses (1.2 and 2.7 × KH₂ PO₄ (KDP)), short UV cut-off edge (<190 nm) and moderate birefringence (0.056@589.3 nm for Sr(NH₂ SO₃ )₂ ) were successfully synthesized. Our work has provided not only two promising DUV transparent NLO crystals, but also an innovative non-π-conjugated unit for developing more DUV transparent NLO materials.

A Series of Pentanary Salt-Inclusion Chalcogenoborates Containing a B12Q12 (Q = S, Se) Cluster Exhibiting a Kleinman-Forbidden Frequency-Doubling Effect

A series of pentanary chalcogenoborates (A₃X)[InB₁₂(InQ₄)₃] (A = K, Rb, Cs; X = Cl, Br, I; Q = S, Se) were obtained by high-temperature solid-state reactions. These salt-inclusion chalcogenides crystallize in the hexagonal space group P6₃22, a type of Kleinman-forbidden noncentrosymmetric structure. Their structures feature a [InB₁₂(InQ₄)₃] open framework built by InQ₆ octahedra and InQ₄ tetrahedra consolidated B₁₂ cluster, which accomodates octahedral cavities for XA₆ units. They are second-harmonic generation active, and their optical properties were studied experimentally and theoretically. This work can evoke more interest in chalcogenoborate-based second-order nonlinear optical materials.

Unexpected aliovalent cation substitution between two NLO materials LiBa3Bi6(SeO3)7F11 and Ba3Bi6.5(SeO3)7F10.5O0.5

One new combination, alkali-metal and alkaline earth-metal selenite fluoride, LiBa₃Bi₆(SeO₃)₇F₁₁ (LBBSF) and its analogue Ba₃Bi_6.5(SeO₃)₇F_10.5O_0.5 (BBSF) were reported here for the first time. Unusual aliovalent cation substitution between them affected the layer thickness and made the bond strains of [SeO₃] enhanced, thereby inducing greater distortions and affecting the SHG efficiencies. This work may provide thoughts for exploring new NLO materials.

Hydrogen-Bond-Assisted Alignment of [MCu(SeO3)4Cl(H2O)]4- (M = Fe, Ga) Anionic Layers to Form Two Polar Oxychlorides: Pb2MCu(SeO3)4Cl(H2O)

Two novel selenite oxychlorides Pb₂MCu(SeO₃)₄Cl(H₂O) (M = Fe, Ga) were hydrothermally synthesized and structurally characterized. They are isostructural and crystallize in the two-dimensional [MCu(SeO₃)₄Cl(H₂O)]^4- anionic layer structure mediated with hydrogen bonds and aligned between neighboring layers which assist in building the three-dimensional framework with a polar space group. Optical properties measurements revealed that the optical band gaps are 2.61 and 3.22 eV for Pb₂FeCu(SeO₃)₄Cl(H₂O) (1) and Pb₂GaCu(SeO₃)₄Cl(H₂O) (2) and the SHG responses are about 0.12 and 0.18 times that of KDP, respectively. Furthermore, 1 exhibits an interesting metamagnetic phenomenon under varied applied fields from around 1 to 4 T at 2 K, and 2 behaves with potential ferromagnetic ordering at low temperature.

First chiral fluorinated lead vanadate selenite Pb2(V2O4F)(VO2)(SeO3)3 with five asymmetric motifs and large optical properties

The first fluorinated lead vanadate selenite Pb2(V2O4F)(VO2)(SeO3)3 (PVOFS) was successfully synthesized via a mild hydrothermal method. This compound crystallizes in the chiral space group P212121 of the orthorhombic system and it is the first noncentrosymmetric structure in the PbII-VV-SeIV-O-F system. PVOFS is composed of five kinds of second-order Jahn-Teller susceptible asymmetric motifs, including three distinct types of vanadium-centered polyhedral units ([VO5F], [VO6] and [VO5]), [SeO3] pyramids and Pb2+ cations. It features a unique three-dimensional open framework structure displaying three types of tunnels (10-, 8- and 7-membered rings), which enriches the structural diversity for fluorinated vanadate selenite systems. Optical property studies revealed that PVOFS shows a second-harmonic generation response of 0.3 times that of the commercial KH2PO4 with phase matching behavior, a wide transparent region covering IR windows, an optical band gap of 2.35 eV, a high laser damage threshold of 61 times that of AgGaS2, and a large birefringence of 0.105 at 1064 nm. Theoretical calculations have been performed to clarify the correlation between the molecular structure and the optical properties of PVOFS.

Centrosymmetric RbSnF2NO3vs. noncentrosymmetric Rb2SbF3(NO3)2

A new UV nonlinear optical fluoride nitrate Rb₂SbF₃(NO₃)₂ exhibits a strong SHG response of 2.7 times that of benchmark KDP and an appropriate birefringence of 0.06@1064 nm.

Simple New Method for the Preparation of La(IO3)3 Nanoparticles

We present a cost- and time-efficient method for the controlled preparation of single phase La(IO₃)₃ nanoparticles via a simple soft-chemical route, which takes a matter of hours, thereby providing an alternative to the common hydrothermal method, which takes days. Nanoparticles of pure α-La(IO₃)₃ and pure δ-La(IO₃)₃ were synthesised via the new method depending on the source of iodate ions, thereby demonstrating the versatility of the synthesis route. The crystal structure, nanoparticle size-dispersal, and chemical composition were characterised via angle- and energy-dispersive powder X-ray diffraction, scanning electron microscopy, and Fourier-transform infrared spectroscopy.

Selenite bromide nonlinear optical materials Pb2GaF2(SeO3)2Br and Pb2NbO2(SeO3)2Br: synthesis and characterization

Two new isomorphic selenite bromides, namely Pb2GaF2(SeO3)2Br (1) and Pb2NbO2(SeO3)2Br (2), were synthesized by mild hydrothermal reactions. These two compounds crystallize in the noncentrosymmertric space group P21 and feature 3D structures consisting of 1D (Pb2Br)3+ cationic chains and 2D [Ga2F4(SeO3)4]6-/[Nb2O4(SeO3)4]6- anonic chains. As measured, 1 and 2 are phase-matchable and exhibit excellent second harmonic generation (SHG) responses of 4.5 and 1.4 times that of KDP, respectively. Moreover, these two compounds possess wide bandgaps (>3.17 eV) and high thermal stabilities (>425 °C), indicating their suitable performances as potential nonlinear optical materials. First-principle density functional theory (DFT) calculations were also performed to explicate the structure-property relationship.