Breaking Through the Trade-Off Between Wide Band Gap and Large SHG Coefficient in Mercury-Based Chalcogenides for IR Nonlinear Optical Application.

It is substantially challenging for non-centrosymmetric (NCS) Hg-based chalcogenides for infrared nonlinear optical (IR-NLO) applications to realize wide band gap (Eg > 3.0 eV) and sufficient phase-matching (PM) second-harmonic-generation intensity (deff > 1.0 × benchmark AgGaS2 ) simultaneously due to the inherent incompatibility. To address this issue, this work presents a diagonal synergetic substitution strategy for creating two new NCS quaternary Hg-based chalcogenides, AEHgGeS4 (AE = Sr and Ba), based on the centrosymmetric (CS) AEIn2 S4 . The derived AEHgGeS4 displays excellent NLO properties such as a wide Eg (≈3.04-3.07 eV), large PM deff (≈2.2-3.0 × AgGaS2 ), ultra-high laser-induced damage threshold (≈14.8-15 × AgGaS2 ), and suitable Δn (≈0.19-0.24@2050 nm), making them highly promising candidates for IR-NLO applications. Importantly, such excellent second-order NLO properties are primarily attributed to the synergistic combination of tetrahedral [HgS4 ] and [GeS4 ] functional primitives, as supported by detailed theoretical calculations. This study reports the first two NCS Hg-based materials with well-balanced comprehensive properties (i.e., Eg > 3.0 eV and deff > 1.0 × benchmark AgGaS2 ) and puts forward a new design avenue for the construction of more efficient IR-NLO candidates.

Hg3Se(SeO3)(SO4): A Mixed-Valent Selenium Compound with Mid-Infrared Transmittance Obtained by In Situ Reaction.

Exploring new material systems is a highly significant task in the field of inorganic chemistry. A new mixed-valent selenium compound, Hg3Se(SeO3)(SO4), was successfully synthesized through in situ reactions. This compound exhibits a novel three-dimensional structure composed of Hg3Se(SO4) layers bridged by SeO3 trigonal pyramids. It is the first structure containing (SeO3)2-, (SO4)2-, and Se2- simultaneously. In addition, Hg3Se(SeO3)(SO4) possesses a wide bandgap (3.5 eV), moderate birefringence (Cal:0.064@546 nm, Exp:0.069@546 nm), a high laser-induced damage threshold (23.35 MW cm-2), and a wide transmittance window (0.28-6.6 µm). Our work demonstrates that mixed-valent (+4, -2) selenite selenide can be potential optical materials for the mid-infrared region.

(NH4 )2 (I5 O12 )(IO3 ) and K1.03 (NH4 )0.97 (I5 O12 )(IO3 ): Mixed-Valent Polyiodates with Unprecedented I5 O12 - Unit Exhibiting Strong Second-Harmonic Generation Responses and Giant Birefringence.

Second-harmonic generation (SHG) response and birefringence are crucial properties for linear and nonlinear optical (NLO) materials, while it is difficult to further optimize these two key properties by using a single traditional functional building block (FBB) in one compound. Herein, a novel IO4 5- unit is identified, which possesses a square-planar configuration and two stereochemically active lone-pairs (SCALPs). By combining IO4 5- and IO3 - units, the first examples of mixed-valent polyiodates featuring an unprecedented bowl-shaped I5 O12 - polymerized unit, namely (NH4 )2 (I5 O12 )(IO3 ) and K1.03 (NH4 )0.97 (I5 O12 )(IO3 ), are successfully synthesized. Excitingly, both crystals exhibit strong SHG responses (16 × KDP and 19.5 × KDP @1064 nm) as well as giant birefringence (∆nexp  = 0.431 and 0.405 @546 nm). Detailed structure-property analyses reveal that the parallel aligned planar IO4 5- units induce the properly aligned high-density SCALPs, leading to strong SHG response and giant birefringence for both materials. This work not only provides two new potential NLO and birefringent crystals, but also discovers a novel promising FBB (IO4 5- ) for developing high-performance linear and nonlinear optical materials.

Rational Design of a Rare-Earth Oxychalcogenide Nd3 [Ga3 O3 S3 ][Ge2 O7 ] with Superior Infrared Nonlinear Optical Performance.

Inorganic chalcogenides have been studied as the most promising infrared (IR) nonlinear optical (NLO) candidates for the past decades. However, it is proven difficult to discover high-performance materials that combine the often-incompatible properties of large energy gap (Eg ) and strong second harmonic generation (SHG) response (deff ), especially for rare-earth chalcogenides. Herein, centrosymmetric Cs3 [Sb3 O6 ][Ge2 O7 ] is selected as a maternal structure and a new noncentrosymmetric rare-earth oxychalcogenide, namely, Nd3 [Ga3 O3 S3 ][Ge2 O7 ], is successfully designed and obtained by the module substitution strategy for the first time. Especially, Nd3 [Ga3 O3 S3 ][Ge2 O7 ] is the first case of breaking the trade-off relationship between wide Eg (>3.5 eV) and large deff (>0.5 × AgGaS2 ) in rare-earth chalcogenide system, and thus displays an outstanding IR-NLO comprehensive performance. Detailed structure analyses and theoretical studies reveal that the NLO effect originates mainly from the cooperation of heteroanionic [GaO2 S2 ] and [NdO2 S6 ] asymmetric building blocks. This work not only presents an excellent rare-earth IR-NLO candidate, but also plays a crucial role in the rational structure design of other NLO materials in which both large Eg and strong deff are pursued.

CsHfF4(IO3): A Hafnium Iodate Exhibiting a Strong Second-Harmonic-Generation Effect and a Wide Band Gap Achieved by Mixed-Ligand Acentric Coordination.

A new polar hafnium iodate, CsHfF4(IO3), was successfully designed and synthesized by integrating fluorinated hafnium-oxygen polyhedra (HfF6O2) and IO3- anionic functional groups. Owing to the weak electronic effect of Hf4+ and the bond-network-induced out-of-center distortion of the HfF6O2 dodecahedra, CsHfF4(IO3) achieves a good balance between a strong second-harmonic-generation effect (3.5 × KH2PO4) and a rather large band gap (4.47 eV), which is the largest among the d0 transition-metal iodates. In addition, CsHfF4(IO3) possesses a wide transparent region (0.27-9.9 µm), a large birefringence for phase-matching (0.161), and a high laser-induced damage threshold (55.41 MW cm-2, 26 × AgGaS2) and is nonhygroscopic. This work indicates that the integration of mixed-ligand acentric coordination polyhedra and functional groups containing lone electron pairs is an effective strategy for developing novel inorganic nonlinear-optical materials with balanced overall properties.

α- and ß-(C4H5N2O)(IO3)·HIO3: Two SHG Materials Based on Organic-Inorganic Hybrid Iodates.

Organic-inorganic hybrid nonlinear optical (NLO) materials are highly anticipated because of the integration of both merits of the organic and inorganic moieties. Herein, the 2-pyrimidinone cation (C4H5N2O)+ has been incorporated into the iodate system to form two polymorphic organic-inorganic hybrid iodates, namely, α- and ß-(C4H5N2O)(IO3)·HIO3. They crystallize in different polar space groups (Ia and Pca21), and their structures feature one-dimensional (1D) chain structures composed of (C4H5N2O)+ cations, IO3- anions, and HIO3 molecules interconnected via hydrogen bonds. α- and ß-(C4H5N2O) (IO3)·HIO3 exhibit strong and moderate second-harmonic-generation (SHG) responses of 6.4 and 0.9 × KH2PO4 (KDP), respectively, the same band gaps of 3.65 eV, and high powder laser-induced damage threshold (LIDT) values [51 and 57 × AgGaS2 (AGS)]. The results of theoretical calculations revealed that the large SHG effect of α-(C4H5N2O)(IO3)·HIO3 originated from the IO3 and HIO3 groups. This work indicates that (C4H5N2O)+ is a potential group for designing new NLO materials with brilliant optical performances.

Growth and Optical Properties of Large-Sized NaVO2(IO3)2(H2O) Crystals for Second-Harmonic Generation Applications.

Large-sized crystals of the quaternary iodate NaVO2(IO3)2(H2O) (NVIO) with centimeter-scale dimensions (23 mm × 18 mm × 6 mm as a representative) have been successfully grown by the top-seeded hydrothermal method. Linear optical properties have been measured, including the optical transmission spectrum and refractive index. The NVIO crystal possesses an optical window with high transmittance (above 80%) over the range of 500-1410 nm and exhibits strong optical anisotropy with large birefringence Δn (nz - nx) of 0.1522 at 1064 nm and 0.1720 at 532 nm. Based on the measured refractive indices, the phase-matching conditions for second-harmonic generation (SHG) have been calculated, and SHG devices have further been fabricated along the calculated type I and type II phase-matching directions of (θ = 39.0°, φ = 3.8°) and (θ =53.8°, φ = 1.3°). Laser experiments of extra-cavity frequency doubling have been performed on these NVIO devices. It has been confirmed that the effective SHG conversion from 1064 to 532 nm could be achieved with an energy conversion efficiency of 8.1%. Our work demonstrates that large-sized NVIO crystals are promising in the frequency-doubling application.

Ce(IO3)3F and Ce(IO3)2(NO3): Two Mixed-Anion Cerium Iodates with Good Nonlinear Optical and Birefringent Properties.

Two new mixed-anion cerium iodates, namely, Ce(IO3)3F and Ce(IO3)2(NO3), have been rationally designed through the integration of hybrid anionic functional building blocks (FBBs). The structure of Ce(IO3)3F features a novel [Ce(IO3)3F] bilayer, and the material exhibits large birefringence (0.225 @546 nm). The structure of Ce(IO3)2(NO3) features [Ce3(IO3)6]3+ triple layers that are further linked by planar NO3- units. Ce(IO3)2(NO3) shows a moderate SHG response (1 × KDP) and a high laser-induced damage threshold value (22 × AgGaS2). This work demonstrates that the rich coordination geometries of cerium cations facilitate tuning of the structures of related compounds through modulating anionic FBBs.

Explorations of New SHG Materials in Mercury Iodate Sulfate System.

Through systematic experiments, two novel mercury iodate sulfates, namely, Hg2 (IO3 )2 (SO4 )(H2 O) and Hg2 (IO3 )2 (SO4 ) were obtained. They crystallize in monoclinic space group C2 and C2/c, respectively. Hg2 (IO3 )2 (SO4 )(H2 O) exhibits the [Hg(IO3 )]+ polar cationic layer inherited from Hg(IO3 )2 and the three-dimensional (3D) framework inherited from HgSO4 . This enables Hg2 (IO3 )2 (SO4 )(H2 O) to generate a strong second harmonic generation (SHG) response of 6 times that of KH2 PO4 (KDP). The structure of Hg2 (IO3 )2 (SO4 ) is very similar to that of Hg2 (IO3 )2 (SO4 )(H2 O), and they can be transformed into each other. Hg2 (IO3 )2 (SO4 )(H2 O) shows a large optical band gap of 3.98 eV and a high dehydration temperature of 250 °C. This study indicates that by reasonable design, the introduction of multiple functional groups into a compound may combine their advantages to achieve good overall optical performance.

Na3Ti3O3(SeO3)4F: A Phase-Matchable Nonlinear-Optical Crystal with Enlarged Second-Harmonic-Generation Intensity and Band Gap.

A new phase-matchable nonlinear-optical material, Na3Ti3O3(SeO3)4F, has been achieved by a facile hydrothermal reaction. The anionic skeleton displays a honeycombed 3D structure with Ti6Se6 12-member polyhedral ring tunnels along the c axis. Na3Ti3O3(SeO3)4F presents a strong second-harmonic-generation (SHG) response of about 6 × KDP, which is twice that of its isostructural Ag compound. Interestingly, the band gap of Na3Ti3O3(SeO3)4F is also broader than that of the isomorph. Also, the powder laser-induced damage threshold of Na3Ti3O3(SeO3)4F is even 5.5 times larger than that of Ag3Ti3O3(SeO3)4F. Theoretical calculations revealed that the fully filled Ag 4d and empty Ag 5s states have made the difference in the band gap and SHG response of the two isostructural compounds. Our work has provided a practical way of improving the SHG efficiency and band gap simultaneously, which is usually considered to be a pair of negatively correlated parameters.

[o-C5 H4 NHOH]2 [I7 O18 (OH)]⋠3 H2 O: An Organic-Inorganic Hybrid SHG Material Featuring an [I7 O18 (OH)] ∞ 2 - Branched Polyiodate Chain.

An organic-inorganic hybrid polyiodate, namely, [o-C5 H4 NHOH]2 [I7 O18 (OH)]⋅3 H2 O (I), featuring a novel branched polyiodate chain has been obtained by evaporation method. [o-C5 H4 NHOH]2 [I7 O18 (OH)]⋅3 H2 O (I) crystalizes in the polar space group Ia and features an [I7 O18 (OH)] ∞ 2 - branched polyiodate chain in which [I3 O9 ]3- trimers are grafted on the same side of the one-dimensional (1D) chain based on [I4 O11 (OH)]3- tetramers. The asymmetric organic amine groups are beneficial to the polymerization of iodate groups and inducing the formation of the non-centrosymmetric (NCS) structure. Compound I exhibits a rather large Second-Harmonic- Generation (SHG) signal of 8.5×KH2 PO4 (KDP) upon 1064 nm laser radiation, a moderate band gap of 3.90 eV and a high laser-induced-damage-threshold (LIDT) of 182.34 MW cm-2 , hence it is a promising new SHG material. The relationship between the structures of the organic amine groups and the overall structures has been also analyzed.

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.

CsVO2 F(IO3 ): An Excellent SHG Material Featuring an Unprecedented 3D [VO2 F(IO3 )]- Anionic Framework.

The first alkali-metal vanadium iodate fluoride, CsVO2 F(IO3 ), with a novel 3D anionic framework, has been rationally designed and hydrothermally synthesized. The 3D [VO2 F(IO3 )]- framework in CsVO2 F(IO3 ) is built from 0D Λ-shaped cis-[VO3 F(IO3 )2 ]4- polyanions via corner-sharing of oxo anions and bridging of the iodate groups. CsVO2 F(IO3 ) displays both a strong second-harmonic generation (SHG) 1.1 times as strong as KTiOPO4 (KTP) under 2.05 µm laser radiation and high laser-induced damage threshold (LIDT) of 107.9 MW cm-2 . This work provides a new route to design SHG crystals with stable 3D anionic structures from low-dimensional structural building units.

PbCdF(SeO3)(NO3): A Nonlinear Optical Material Produced by Synergistic Effect of Four Functional Units.

A new nonlinear optical material, the first fluoride selenite nitrate PbCdF(SeO3)(NO3), was successfully synthesized by traditional solid state reactions. This compound crystallizes in the polar space group of Pca21, and its structure features a novel 2D layered structure consisting of 1D lead nitrate chains and 2D cadmium selenite layers. PbCdF(SeO3)(NO3) exhibits a phase-matchable SHG efficiency of about 2.6 times that of KDP and a wide band gap of 4.42 eV. Its laser damage threshold was measured to be 135.6 MW/cm2, which is comparative to that of the reported Li7(TeO3)3F with a short ultraviolet cutoff edge. Theoretical calculation confirmed that the synergistic effects of all the four functional units make PbCdF(SeO3)(NO3) a remarkable SHG material.

Ternary Mixed-Metal Cd4GeS6: Remarkable Nonlinear-Optical Properties Based on a Tetrahedral-Stacking Framework.

Noncentrosymmetric (NCS) mixed-metal chalcogenides have attracted significant attention lately because of their structural multiplicities, strong second-harmonic-generation (SHG) efficiencies ( d ij) and large laser-induced damage thresholds (LIDTs), which make them promising nonlinear-optical (NLO) materials in mid- and far-IR (MFIR) applications. In this work, a ternary mixed-metal material, Cd4GeS6, has been synthesized by reacting CdS with GeS2 via a solid-state method at 1273 K. It exhibits a unique tetrahedral-stacking NCS framework structure consisting of two types of [Cd2S7] asymmetric groups and dispersed GeS4 tetrahedra. Remarkably, Cd4GeS6 shows type-I phase-matching ability and achieves a desired balance between strong d ij (about 1.1AgGaS2) and large LIDT (about 3.6AgGaS2), demonstrating that this material satisfies the essential requirements as a promising MFIR NLO candidate. Moreover, theoretical calculations were performed for a better understanding of the structure-property relationships.

AgGa2 PS6 : A New Mid-Infrared Nonlinear Optical Material with a High Laser Damage Threshold and a Large Second Harmonic Generation Response.

To develop new mid-infrared (MIR) nonlinear optical (NLO) materials, which can overcome the low laser damage threshold (LDT) of the commercial MIR-NLO crystals (AgGaS2 , AgGaSe2 and ZnGeP2 ) and simultaneously keep the large NLO susceptibility, is necessary for high-power MIR laser frequency conversion technology. To improve the LDT, a new strategy of increasing lattice stability was adopted. Here, the strongly covalent structural unit of the PS4 tetrahedron was introduced into AgGaS2 (AGS), and that led to the isolation of the first compound in AgI -GaIII -PV -S system, namely, AgGa2 PS6 (Cc). It retains a large SHG efficiency (1.0×AGS) with phase-matchable ability, and also exhibits an improved LDT (5.1×AGS), indicating AgGa2 PS6 is a new promising MIR-NLO crystal. Moreover, a novel 3D framework of [Ga2 PS6 ]- , with triangular-shaped channels, as well as interesting single triangular geometry of AgS3 -both of which are very rare in reported sulfides-was discovered in AgGa2 PS6 . Furthermore, theoretical calculations, and lattice energy and thermal expansions analyses suggest that the PS4 group makes a large contribution to the large SHG efficiency and high LDT.

Acentric La3(IO3)8(OH) and La(IO3)2(NO3): Partial Substitution of Iodate Anions in La(IO3)3 by Hydroxide or Nitrate Anion.

Partial substitution of iodate anions in La(IO3)3 by OH- or NO3- anion led to acentric La3(IO3)8(OH) and chiral La(IO3)2(NO3). The structure of La3(IO3)8(OH) can be seen as a complex three-dimensional (3D) network composed of two-dimensional [La3(IO3)2(OH)]6+ cationic layers that are further bridged by remaining iodate anions, or alternatively as a 3D network composed of one-dimensional [La3(IO3)6(OH)]2+ cationic columns being further interconnected by additional iodate anions, while the structure of La(IO3)2(NO3) can be seen as a novel 3D structure with planar NO3 groups serving as linkage between the [La3(IO3)6]3+ triple layers. Compared to La(IO3)3, both compounds show considerably wide band gaps and enhanced thermal stability. La(IO3)2(NO3) shows a moderate second harmonic generation (SHG) response of ∼0.6 times that of KDP (KH2PO4), a wide band gap of 4.23 eV, and a high LDT value (22 × AgGaS2). Optical property measurements, thermal analysis, as well as theoretical calculations on SHG origin, were performed. It can be deduced that partial substitution of iodate anions can be a facile route to design new noncentrosymmetric metal iodates with novel structure and potential application.

K2 Au(IO3)5 and ß-KAu(IO3)4: Polar Materials with Strong SHG Responses Originating from Synergistic Effect of AuO4 and IO3 Units.

Two new polar potassium gold iodates, namely, K2 Au(IO3)5 (Cmc21) and ß-KAu(IO3)4 (C2), have been synthesized and structurally characterized. Both compounds feature zero-dimensional polar [Au(IO3)4](-) units composed of an AuO4 square-planar unit coordinated by four IO3(-) ions in a monodentate fashion. In ß-KAu(IO3)4, isolated [Au(IO3)4](-) ions are separated by K(+) ions, whereas in K2 Au(IO3)5, isolated [Au(IO3)4](-) ions and non-coordinated IO3(-) units are separated by K(+) ions. Both compounds are thermally stable up to 400 °C and exhibit high transmittance in the NIR region (λ=800-2500 nm) with measured optical band gaps of 2.65 eV for K2 Au(IO3 )5 and 2.75 eV for ß-KAu(IO3)4. Powder second-harmonic generation measurements by using λ=2.05 µm laser radiation indicate that K2 Au(IO3)5 and ß-KAu(IO3)4 are both phase-matchable materials with strong SHG responses of approximately 1.0 and 1.3 times that of KTiOPO4, respectively. Theoretical calculations based on DFT methods confirm that such strong SHG responses originate from a synergistic effect of the AuO4 and IO3 units.

Syntheses, Characterization, and Optical Properties of Centrosymmetric Ba3(BS3)1.5(MS3)0.5 and Noncentrosymmetric Ba3(BQ3)(SbQ3).

The most advanced UV-vis and IR NLO materials are usually borates and chalcogenides, respectively. But thioborates, especially thio-borometalates, are extremely rare. Here, four new such compounds are discovered by solid state reactions representing 0D structures constructed by isolated BQ3 trigonal planes and discrete MQ3 pyramids with Ba(2+) cations filling among them, centrosymmetric monoclinic P21/c Ba3(BS3)1.5(MS3)0.5 (M = Sb, Bi) 1, 2 with a = 12.9255(9), 12.946(2) Å; b = 21.139(2), 21.170(2)Å; c = 8.4194(6), 8.4207(8) Å; ß = 101.739(5), 101.688(7)°; V = 2252.3(3), 2259.9(3) Å(3) and noncentrosymmetric hexagonal P6̅2m Ba3(BQ3)(SbQ3) (Q = S, Se) 3, 4 with a = b = 17.0560(9), 17.720(4) Å; c = 10.9040(9), 11.251(3) Å; V = 2747.1(3), 3060(2) Å(3). 3 exhibits the strongest SHG among thioborates that is about three times that of the benchmark AgGaS2 at 2.05 µm. 1 and 3 also show an interesting structure relationship correlated to the size mismatching of the anionic building units that can be controlled by the experimental loading ratio of B:Sb. Syntheses, structure characterizations, and electronic structures based on the density functional theory calculations are reported.