Structure-property relationship in nonlinear optical materials with π-conjugated CO3 triangles

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.

Theoretical design of alkaline earthides M+(36 adz) Be- (M+ = V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) with excellent nonlinear optical response and ultraviolet transparency

A novel series of alkaline earthides containing eight complexes based upon 36adz complexant are designed by placing carefully transition metals (V-Zn) on inner side and alkaline earth metal outer side of the complexant i.e., M+(36adz) Be- (M+ = V, Cr, Mn, Fe, Co, Ni, Cu and Zn). All the designed compounds are electronically and thermodynamically stable as evaluated by their interaction energy and vertical ionization potential respectively. Moreover, the true nature of alkaline earthides is verified through NBOs and FMO study, showing negative charge and excess electrons on alkaline earth metal respectively. Furthermore, true alkaline earthides characteristics are evaluated graphically by spectra of partial density state (PDOS). The energy gap (HOMO -LUMO gap) is very small (ranging 2.95 eV-1.89 eV), when it is compared with pure cage 36adz HOMO-LUMO gap i.e., 8.50 eV. All the complexes show a very small value of transition energy ranging from 1.68eV to 0.89eV. Also, these possess higher hyper polarizability values up to 2.8 x 105au (for Co+(36adz) Be-). Furthermore, an increase in hyper polarizability was observed by applying external electric field on complexes. The remarkable increase of 100fold in hyper polarizability of Zn+(36adz) Be- complex is determined after application of external electric field i.e., from 1.7 x 104 au to 1.7 x 106 au when complex is subjected to external electric field of 0.001 au strength. So, when external electric field is applied on complexes it enhances the charge transfer, polarizability and hyper polarizability of complexes and proves to be effective for designing of true alkaline earthides with remarkable NLO response.

Na3K6(CO3)3(NO3)2X·6H2O (X = NO3, Cl, Br): Exploring High-Performance UV Birefringent Crystals Induced by Coplanar π-Conjugated CO3 and NO3 Groups

Planar π-conjugated groups, like CO3, NO3, and BO3 triangles, are ideal functional units for designing birefringent materials due to their large optical anisotropy and wide band gap. The key point for designing birefringent crystals is to select appropriate functional building blocks (FBBs) and the proper arrangement mode. It is well known that the substitution strategy has proven to be a promising and accessible approach. In this work, alkali metals were chosen to regulate and control two different π-conjugated groups, CO3 and NO3, to build new compounds with large birefringence. Subsequently, three new compounds, Na3K6(CO3)3(NO3)2X·6H2O (X = NO3, Cl, Br), were successfully synthesized using the hydrothermal method. The aliovalent substitution between the [NO3]- anionic group and halogen anions [Cl]-/[Br]- has been achieved in these compounds. Na3K6(CO3)3(NO3)2X·6H2O feature the well-coplanar CO3 and NO3 groups in their crystal structure. This coplanar arrangement mode may effectively enhance the anisotropic polarizability of Na3K6(CO3)3(NO3)2X·6H2O. And their experimental birefringence can reach 0.094-0.131 at 546 nm. Diffuse reflectance spectra demonstrate that these compounds exhibit short ultraviolet (UV) absorption edges of ∼235 nm. Meanwhile, Na3K6(CO3)3(NO3)2X·6H2O also have an easily grown capacity under facile conditions. This work not only reports three new potential UV birefringent crystals but also provides a strategy to make the π-conjugated MO3 group coplanar.

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

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

Rare-earth-based chalcogenides and their derivatives: an encouraging IR nonlinear optical material candidate

With the continuous development of laser technology and the increasing demand for lasers of different frequencies in the infrared (IR) spectrum, research on infrared nonlinear optical (NLO) crystals has garnered growing attention. Currently, the three main commercially available types of borate materials each have their drawbacks, which limit their applications in various areas. Rare-earth (RE)-based chalcogenide compounds, characterized by the unique f-electron configuration, strong positive charges, and high coordination numbers of RE cations, often exhibit distinctive optical responses. In the field of IR-NLO crystals, they have a research history spanning several decades, with increasing interest. However, there is currently no comprehensive review summarizing and analyzing these promising compounds. In this review, we categorize 85 representative examples out of more than 400 non-centrosymmetric (NCS) compounds into four classes based on the connection of different asymmetric building motifs: (1) RE-based chalcogenides containing tetrahedral motifs; (2) RE-based chalcogenides containing lone-pair-electron motifs; (3) RE-based chalcogenides containing [BS3] and [P2Q6] motifs; and (4) RE-based chalcohalides and oxychalcogenides. We provide detailed discussions on their synthesis methods, structures, optical properties, and structure-performance relationships. Finally, we present several favorable suggestions to further explore RE-based chalcogenide compounds. These suggestions aim to approach these compounds from a new perspective in the field of structural chemistry and potentially uncover hidden treasures within the extensive accumulation of previous research.

A Noncentrosymmetric Metal-Free Borophosphate: Achieving a Large Birefringence and Excellent Stability by Covalent-Linkage

Organic-inorganic hybrid linear and nonlinear optical (NLO) materials have received increasingly wide spread attention in recent years. Herein, the first hybrid noncentrosymmetric (NCS) borophosphate, (C5H6N)2B2O(HPO4)2 (4PBP), is rationally designed and synthesized by a covalent-linkage strategy. 4-pyridyl-boronic acid (4 PB) is considered as a bifunctional unit, which may effectively improve the optical properties and stability of the resultant material. On the one hand, 4 PB units are covalently linked with PO3(OH) groups via strong B-O-P connections, which significantly enhances the thermal stability of 4PBP (decomposition at 321, vs lower 200 °C of most of hybrid materials). On the other hand, the planar π-conjugated C5H6N units and their uniform layered arrangements represent large structural anisotropy and hyperpolarizability, achieving the largest birefringence (0.156 @ 546 nm) in the reported borophosphates and a second-harmonic generation response (0.7 × KDP). 4PBP also exhibits a wide transparency range (0.27-1.50 µm). This work not only provides a promising birefringent material, but also offers a practical covalent-attachment strategy for the rational design of new high-performance optical materials.

Soaking the Rare-Earth Carbonates for a Change: An Alternative Approach to Explore Carbonate Nonlinear Optical Crystals

Alkali-metal rare-earth carbonates (ARECs) find great potential in nonlinear optical applications. As the most common method, the hydrothermal reaction is widely used in synthesizing ARECs. The black-box nature of the hydrothermal reaction makes it difficult for understanding the formation processes and therefore may slow down the pace of structural discovery. Here, by simply soaking the rare-earth carbonates in Na2CO3 solutions, we successfully obtain a series of noncentrosymmetric Na3RE(CO3)3·6H2O (RE = Tb 1, Sm 2, Eu 3, Gd 4, Dy 5, Ho 6, and Er 7) compounds without using the high-temperature hydrothermal method. The transformation process, investigated by powder X-ray diffraction and scanning electron microscopy, is governed by the concentration of the soaking solutions. Na3Tb(CO3)3·6H2O, as an example, is studied structurally, and its physical properties are characterized. It exhibits a second harmonic generation effect of 0.5 × KDP and a short UV cutoff edge of 222 nm (5.8 eV). Our study provides insights for exploring new AREC structures, which may further advance the development of carbonate nonlinear optical crystals.

Centrosymmetric CaBaMF8 and Noncentrosymmetric Li2CaMF8 (M = Zr, Hf): Dimension Variation and Nonlinear Optical Activity Resulting from an Isovalent Cation Substitution-Oriented Design

Zirconium/hafnium fluorides have recently been recognized as potential nonlinear optical (NLO) materials with short ultraviolet (UV) cutoff edges, which is significant in laser science and industry. The synthesis of noncentrosymmetric (NCS) materials based on centrosymmetric (CS) compounds by an isovalent cation substitution-oriented design is an emerging strategy in the NLO territory. Here, two isostructural and novel fluorides, CaBaMF8 (M = Zr (1), Hf (2)), have been synthesized through the combination of alkaline earth metals, zirconium/hafnium, and fluorine elements. They feature zero-dimensional and CS structures composed by an isolated MF8 (M = Zr, Hf) dodecahedron and dissociative Ca2+ and Ba2+ cations, and they display short UV cutoff edges (<200 nm) as well. Two three-dimensional fluorides Li2CaMF8 (M = Zr (3), Hf (4)) are obtained by replacing Ba with alkali metal Li atom, which not only represent phase-matchable second-harmonic-generation activities (0.36, 0.30× KH2PO4 (KDP)) at 1064 nm but also maintain short UV cutoff edges with high reflectance. This work has largely enriched the family of NCS zirconium/hafnium fluorides reaching the short UV region.

Secondary-Bond-Driven Construction of a Polar Material Exhibiting Strong Broad-Spectrum Second-Harmonic Generation and Large Birefringence

Considerable effort has been invested in the development of non-centrosymmetric (NCS) inorganic solids for ferroelectricity-, piezoelectricity- and, particularly, optical nonlinearity-related applications. While great progress has been made, a persistent problem is the difficulty in constructing NCS materials, which probably stems from non-directionality and unsaturation of the ionic bonds between metal counter-cations and covalent anionic modules. We report herein a secondary-bond-driven approach that circumvents the cancellation of dipole moments between adjacent anionic modules that has plagued second-harmonic generation (SHG) material design, and which thereby affords a polar structure with strong SHG properties. The resultant first NCS counter-cation-free iodate, VO2 (H2 O)(IO3 ) (VIO), a new class of iodate, crystallizes in a polar lattice with∞ 1 [ ${{}_{{\rm { \infty }}}{}^{{\rm { 1}}}{\rm { [}}}$ VO2 (H2 O)(IO3 )] zigzag chains connected by weak hydrogen bonds and intermolecular forces. VIO exhibits very large SHG responses (18 × KH2 PO4 @ 1200 nm, 1.5 × KTiOPO4 @ 2100 nm) and sufficient birefringence (0.184 @ 546 nm). Calculations and crystal structure analysis attribute the large SHG responses to consistent polarization orientations of the∞ 1 [ ${{}_{{\rm { \infty }}}{}^{{\rm { 1}}}{\rm { [}}}$ VO2 (H2 O)(IO3 )] chains controlled by secondary bonds. This study highlights the advantages of manipulating the secondary bonds in inorganic solids to control NCS structure and optical nonlinearity, affording a new perspective in the development of high-performance NLO materials.

Exploring a new short-wavelength nonlinear optical fluoride material featuring unprecedented polar cis-[Zr6F34]10- clusters

Traditional fluorides are rarely reported as candidates for nonlinear optical (NLO) materials featuring a deep-ultraviolet cutoff edge. Theoretical investigations suggest that the ZrF8 dodecahedron shows large polarizability anisotropy and benefits for large birefringence. Herein, a new fluorine-rich fluoride, K3Ba2Zr6F31, was synthesized by coupling the ZrF8 group, featuring acentric cis-[Zr6F34]10- clusters with a 63-screw axis. Significantly, K3Ba2Zr6F31 exhibits a short UV cutoff edge (below 200 nm) and moderate second-harmonic generation (SHG) response (0.5 × KH2PO4). It also possesses a relatively large birefringence (0.08@1064 nm), together with a broad transparency window (2.5-21.1 μm). First-principles calculations suggest that the cis-[Zr6F34]10- cluster built by ZrF8 dodecahedra are the dominant contributors to the large band gap (7.89 eV, cal.) and SHG response simultaneously. Such systematic work highlights that Zr-based fluorides afford a new paradigm for the development of efficient NLO materials with a short UV cutoff edge.

New Functional Groups Design toward High Performance Ultraviolet Nonlinear Optical Materials

ConspectusThe invention of the laser is a pivotal milestone in the evolution of modern science and technology. Second-order nonlinear optical (NLO) crystals, which possess the ability to convert frequencies, have found widespread applications in laser science, information transmission, industrial Internet, and other cutting-edge fields within materials and optics. As modern science and technology continue to advance at a rapid pace, existing ultraviolet (UV) and deep ultraviolet (DUV) NLO crystals struggle to meet the ever-growing demands of various applications. Consequently, the development of novel UV and DUV NLO crystals has become an urgent necessity. For a UV NLO crystal to be considered outstanding in the UV/DUV range, it must exhibit three fundamental yet crucial properties: large second-order NLO coefficients, suitable birefringence, and short UV cutoff edge corresponding to a wide band gap. However, these key factors often conflict with one another, making it challenging to achieve a harmonious balance within a single crystal. It is widely believed that these mutually constrained optical properties are codetermined by microscopic NLO-active units and macroscopic structure features. Therefore, how to design high performance UV NLO-active groups to balance these three key properties is an essential scientifically question and serious challenge. In this Account, we present three strategies for designing high-performance UV NLO-active groups: (1) The "tetrahedron partial substitution" strategy by employing various substituents to replace one or more atoms in the traditional nonpolar tetrahedral groups, might achieve the aim of increasing the polarizability anisotropy and hyperpolarizability of the newly formed polar tetrahedral functional groups, such as from SO4 to SO3NH2 or SO3CH3 groups. (2) The "structure-analogue" strategy to develop a range of organic functional groups exhibiting more strong polarizability anisotropy and hyperpolarizability by using inorganic π-conjugated groups, such as BO3 and B3O6 groups, as templates. (3) The "two in one" strategy for integrating groups featuring planar triangle configurations and tetrahedrons to create NLO-active functional groups possessing large band gaps, strong hyperpolarizability, and moderate polarizability anisotropy. These three strategies successfully guide us to design and explore various kinds of organic-inorganic composite NLO crystal materials with excellent performances, like Ba(SO3CH3)2, M(SO3NH2)2 (M = Sr, Ba), C(NH2)3SO3F, KLi(HC3N3O3)·2H2O, KLi(C3H2O4)·H2O, and so on. Finally, we briefly conclude these strategies and propose some prospects for exploring new excellent UV/DUV NLO materials with practical applications. These findings could inspire novel thoughts for researchers designing new UV/DUV NLO materials and providing abundant materials used in UV/DUV regions.

Introduction of the [B-O/F] Units Enhances the Band Gap and Birefringence from Na6Mg3B10O18F6 to K3NaB10O16F2

The borate family is the main source of deep-ultraviolet (DUV) birefringent crystals, and it has attracted a lot of attention due to versatile [B-O] basic units. Herein, two new borate-based compounds Na₆Mg₃B₁₀O₁₈F₆ and K₃NaB₁₀O₁₆F₂ were discovered. Their fundamental building blocks are [B₅O₁₁] and [B₅O₁₀F] units, respectively. The calculated results showed that the band gap and birefringence of K₃NaB₁₀O₁₆F₂ (E_g = 6.93 eV, Δn = 0.047 at 1064 nm) are greater than those of Na₆Mg₃B₁₀O₁₈F₆ (E_g = 5.40 eV, Δn = 0.039 at 1064 nm). Furthermore, the effects of [B-O/F] units on band gap and birefringence were analyzed by the charge-transfer model and response electron distribution anisotropy method. The results show that introducing the [B-O/F] units can improve the band gap and birefringence. These findings will boost the exploration of DUV birefringent opticals.

From Centrosymmetry to Noncentrosymmetry: Precise Structural Regulation and Characterization on ZnHPO3·2H2O Polymorphs

Polymorphs of ZnHPO₃·2H₂O with centrosymmetry (Cmcm) and noncentrosymmetry (C2) structures were prepared by modified solution evaporation and seed-crystal-induced secondary nucleation methods. In Cmcm-ZnHPO₃·2H₂O, the zinc atoms are only octahedrally coordinated, while in C2-ZnHPO₃·2H₂O, they feature both tetrahedral and octahedral coordination. As a result, Cmcm-ZnHPO₃·2H₂O features a 2D layered structure with lattice water molecules located in the interlayer space, while C2-ZnHPO₃·2H₂O features a 3D electroneutral framework of tfa topology connected by Zn(1)O₄, Zn(2)O₆, and HPO₃ units. The UV-visible diffuse reflectance spectra associated with Tauc's analyses give a direct bandgap of 4.24 and 4.33 eV for Cmcm-ZnHPO₃·2H₂O and C2-ZnHPO₃·2H₂O, respectively. Moreover, C2-ZnHPO₃·2H₂O exhibits a weak second harmonic generation (SHG) response and a moderate birefringence for phase matching, indicating its potential as a nonlinear optical material. Detailed dipole moment calculation and analysis confirmed that the SHG response mainly derived from the HPO₃ pseudo-tetrahedra.

A three orders of magnitude increase in nonlinear optical response by external electric field on Cryptand[2.2.2] (C222) based alkaline earthides

A new series of alkaline earthides based on Cryptand [2.2.2] (C222) containing nine complexes is designed by carefully placing alkali metals and alkaline earth metals inside and outside the C222 complexant, respectively i.e., M1(C222)M2 (M1 = Li, Na, K; M2 = Be, Mg, Ca). The designed complexes are reasonably stable both electronically and thermodynamically, as revealed through their vertical ionization potentials (VIPs) and interaction energies, respectively. Moreover, the true alkaline earthide nature of the complexes is confirmed through NBO and FMO analyses showing the negative charges and HOMOs over the alkaline earth metals, respectively. The further validity of true earthide characteristic is represented graphically by the spectra of partial density of states (PDOS). HOMO-LUMO gaps of the compounds are also very small (from 2.23 to 2.83 eV) when compared with pure cage's (C222) H-L gap i.e., 5.63 eV. All these features award these complexes with very small values of transition energies (ΔE) ranging from 0.68 to 2.06 eV ultimately resulting in remarkably high hyperpolarizability values up to 2.7 × 10⁵ au (for Na⁺(C222)Mg^-). Furthermore, applying external electric field (EEF) on the complexes enhances hyperpolarizability further. A remarkable increase of 1000 folds has been seen when hyperpolarizability of K⁺(C222)Ca^- is calculated after EEF application i.e., from 8.79 × 10⁴ au to 2.48 × 10⁷ au; when subjected to 0.001 au external electric field.

Synthesis, Crystal Structure, and Nonlinear-Optical Properties of a Diamond-Like Chalcogenide Cu2GeS3

Metal sulfides with diamond-like (DL) structures generally exhibit excellent mid-IR nonlinear-optical (NLO) properties. Here, Cu₂GeS₃ (CGS) as a member of the DL chalcogenides was synthesized by a high-temperature solid-state method, and the optical properties were carefully studied experimentally and theoretically. The results revealed that CGS has a large second harmonic generation (0.8 × AgGaSe₂) and a moderate birefringence of 0.067 at 1064 nm. In addition, the linear and NLO properties of the A₂MS₃ (A = Cu, Li; M = Ge, Si) series of compounds were evaluated and compared with the help of first-principles calculations.

Sb4O3(TeO3)2(HSO4)(OH): An Antimony Tellurite Sulfate Exhibiting Large Optical Anisotropy Activated by Lone Pair Stereoactivity

A new birefringent crystal of Sb₄O₃(TeO₃)₂(HSO₄)(OH) was achieved by incorporating two stereochemically active lone pair (SCALP) cations of Sb(III) and Te(IV) into sulfates simultaneously. The Sb³⁺ and Te⁴⁺ ions display highly distorted coordination environments due to the SCALP effect. Sb₄O₃(TeO₃)₂(HSO₄)(OH) displays a 3D structure composed of [Sb₄O₃(TeO₃)₂(OH)]_∞⁺ layers bridged by [SO₃(OH)]^- tetrahedra. It possesses a large birefringence and a wide optical transparent range, making it a new UV birefringent crystal. First-principles calculation analysis suggests that the synergistic effect of the cooperation of SCALP effect of Sb³⁺ and Te⁴⁺ cations make a dominant contribution to the birefringence. The work highlights that units with SCALP cations have advantages in generating large optical anisotropy and are preferable structural units for designing novel birefringent materials.

Toward Large Second-Harmonic Generation and Deep-UV Transparency in Strongly Electropositive Transition Metal Sulfates

The development of deep-ultraviolet (DUV)/solar-blind UV nonlinear optical (NLO) crystals simultaneously possessing wide UV transparency, strong second-harmonic generation (SHG) response, and suitable birefringence is a major challenge in advanced laser technology. We herein propose a "cation compensation" strategy for strong optical nonlinearity in inorganic solids that is exemplified by the introduction of strongly electropositive transition metals (TMs). Following this strategy, the first d⁰ TM UV-transparent NLO sulfates, MF₂(SO₄) (M = Zr (ZFSO), Hf (HFSO)), have been synthesized. Short UV cutoff edges of 206 nm and below 190 nm are observed for bulk ZFSO and HFSO crystals, respectively, together with the strongest powder SHG responses (3.2 × (ZFSO) and 2.5 × KDP (HFSO)) for solar-blind UV/DUV NLO sulfates, as well as suitable birefringence. This work provides a new and efficient approach to the development of urgently needed high-performance NLO materials for applications in the short-wavelength UV region.

Ba5Ga2SiO4S6: a Phase-Matching Nonlinear Optical Oxychalcogenide Design via Structural Regulation Originated from Heteroanion Introduction

Tailored structural regulation to obtain a new non-centrosymmetric (NCS) compound with excellent optical properties is highly desirable but remains a challenge for nonlinear optical (NLO) material design. In this work, centrosymmetric celsian-type BaGa₂Si₂O₈ was selected as a template structure, and a novel NCS oxychalcogenide, namely, Ba₅Ga₂SiO₄S₆, was successfully designed via the introduction of heteroanions under high-temperature solid-state conditions. Ba₅Ga₂SiO₄S₆ adopts the monoclinic space group of Cc (no. 9) and is formed by charges balancing Ba²⁺ cations and discrete [Ga₂SiO₄S₆] clusters made of corner-sharing [SiO₄] and [GaOS₃] tetrahedra. Notably, Ba₅Ga₂SiO₄S₆ exhibits the critical requirements as a potential UV NLO candidate, including a phase-matching second-harmonic generation intensity (∼1.0 × KDP), a beneficial laser-induced damage threshold (1.2 × KDP), a large birefringence (Δn = 0.10@546 nm), and a short UV absorption cutoff edge (ca. 0.26 μm). Furthermore, the theoretical calculation is implemented to provide a deeper analysis of the structure-activity relationship. The investigated example of structural regulation originated from heteroanion introduction in this study may offer a feasible strategy for high-performance NLO candidate design.

d6versus d10, Which Is Better for Second Harmonic Generation Susceptibility? A Case Study of K2TGe3Ch8 (T = Fe, Cd; Ch = S, Se)

Two acentric chalcogenide compounds, K₂CdGe₃S₈ and K₂CdGe₃Se₈, were synthesized via conventional high-temperature solid-state reactions. The crystal structures of K₂CdGe₃S₈ and K₂CdGe₃Se₈ were accurately determined by single-crystal X-ray diffraction and crystallize in the K₂FeGe₃S₈ structure type. K₂CdGe₃S₈ is isostructural to K₂FeGe₃S₈ with superior nonlinear optical properties. For the second harmonic generation (SHG) response, K₂CdGe₃S₈ is 18× K₂FeGe₃S₈ for samples of particle size of 38-55 μm. The superior nonlinear optical properties of K₂CdGe₃S₈ over K₂FeGe₃S₈ are mainly contributed by the chemical characteristics of Cd compared with Fe, which are elucidated by nonlinear optical property measurements, electronic structure calculations, and density functional theory calculations. The [CdS₄] tetrahedra within K₂CdGe₃S₈ exhibit a higher degree of distortion and larger volume compared to the [FeS₄] tetrahedra in K₂FeGe₃S₈. This study possesses a good platform to investigate how d-block elements contribute to the SHG response. The fully occupied d¹⁰-elements are better for SHG susceptibility than d⁶-elements in this study. K₂CdGe₃S₈ is a good candidate as an infrared nonlinear optical material of high SHG response (2.1× AgGaS₂, samples of particle size of 200-250 μm), type-I phase-matching capability, high laser damage threshold (6.2× AgGaS₂), and good stability.

Perfectly Encoding π-Conjugated Anions in the RE5 (C3 N3 O3 )(OH)12 (RE=Y, Yb, Lu) Family with Strong Second Harmonic Generation Response and Balanced Birefringence

Nonlinear optical (NLO) crystal, which simultaneously exhibits strong second-harmonic-generation (SHG) response and desired optical anisotropy, is a core optical material accessible to the modern optoelectronics. Accompanied by strong SHG effect in a NLO crystal, a contradictory problem of overlarge birefringence is ignored, leading to low frequency doubling efficiency and poor beam quality. Herein, a series of rare earth cyanurates RE₅ (C₃ N₃ O₃ )(OH)₁₂ (RE=Y, Yb, Lu) were successfully characterized by 3D electron diffraction technique. Based on a "three birds with one stone" strategy, they enable the simultaneous fulfillment of strong SHG responses (2.5-4.2× KH₂ PO₄ ), short UV cutoff (ca. 220 nm) and applicable birefringence (ca. 0.15 at 800 nm) by the introduction of rare earth coordination control of π-conjugated (C₃ N₃ O₃ )^3- anions. These findings provide high-performance short-wavelength NLO materials and highlight the exploration of cyanurates as a new research area.

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.

New lead-iodide formates with a strong second-harmonic generation response and suitable birefringence obtained by the substitution strategy

Nonlinear optical (NLO) crystals featuring a strong second-harmonic generation (SHG) response and suitable birefringence to achieve phase-matching are in urgent demand in industrial and commercial applications. Based on the substitution strategy, two new NLO lead-iodide formates, K₂[PbI₂(HCOO)₂] and Rb₂[PbI₂(HCOO)₂], have been successfully synthesized using a moderate mixed-solvothermal method. K₂[PbI₂(HCOO)₂] and Rb₂[PbI₂(HCOO)₂] exhibit strong phase-matching SHG responses of 8 and 6.8 × KDP, respectively, a suitable birefringence and transparent window covering most of the visible light and mid-IR region. Crystal structures and theoretical calculations unveil that the origins of the strong SHG response and suitable birefringence can be credibly attributed to the oriented arrangement of the highly distorted [PbI₂O₄] hexa-coordinated polyhedra, which are consistent in their local dipole moments, as well. This research provides a new strategy to explore high-performance NLO crystals.

Chemical Bonding Effects and Physical Properties of Noncentrosymmetric Hexagonal Fluorocarbonates ABCO3F (A: K, Rb, Cs; B: Mg, Ca, Sr, Zn, Cd)

The present work applied the methods of density functional theory and the van der Waals interaction PBE + D3(BJ) on the basis of localized orbitals of the CRYSTAL17 package. It featured the effect of interactions between structural elements of fluorocarbonates ABCO₃F (A: K, Rb, Cs; B: Mg, Ca, Sr, Zn, Cd) on their elastic and vibrational properties. The hexagonal structures proved to consist of alternating ···B-CO₃··· and ···A-F··· layers in planes ab, interconnected along axis c by infinite chains ···F-B-F···, where cations formed polyhedra AO_nF₃ and BO_mF₂. The calculations included the band energy structure, the total and partial density of electron states, the energy and band widths of the upper ns- and np-states of alkali and alkaline-earth metals, as well as nd-zinc and nd-cadmium. For hydrostatic compression, we calculated the parameters of the Birch–Murnaghan equation of state and the linear compressibility moduli along the crystal axes and bond lines. We also defined the elastic constants of single crystals to obtain the Voigt–Reuss–Hill approximations for the elastic moduli of polycrystalline materials. The study also revealed the relationship between the elastic properties and the nature of the chemical bond. Hybrid functional B3LYP made it possible to calculate the modes of normal long-wavelength oscillations, which provided the spectra of infrared absorption and Raman scattering. Intramolecular modes ν1 and ν4 with one or two maxima were found to be intense, and their relative positions depended on the lengths of nonequivalent C–O bonds.

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.

KF·B(OH)3: a KBBF-type material with large birefringence and remarkable deep-ultraviolet transparency

A designed material with large birefringence and remarkable deep-ultraviolet transparency, KF·B(OH)₃, has been discovered. The well-aligned [F·B(OH)₃]^- layers endow a very large bandgap (7.63 eV), comparably large birefringence (0.114 @1064 nm), and the reinforced interlayer bonding of ca. 2.16 × KBe₂BO₃F₂.

Directional Construction of New Nonlinear Optical Bifunctional Units through Molecular Engineering Design Inspired by the B3O7-Typed Configuration

Studies on new functional structural units with both large hyperpolarizability and high anisotropy are essentially important for finding high-performance nonlinear optical (NLO) materials and enriching the material systems. Under the guidance of the "structure-analogue" strategy, the work utilizes the molecular engineering approach to direct the construction of target units, BC₂N₅O₂ and B(C₂N₅)₂ units. The BC₂N₅O₂ unit with a highly analogous structure to the B₃O₇ group and its derivate B(C₂N₅)₂ unit with a configuration of B₅O₁₀ group are designed as NLO-active units. Furthermore, two compounds with these new NLO-active units, BC₂N₅H₆(OH)₂·H₂O (I) and B(C₂N₅H_6.5)₂(NO₃)₂ (II), are synthesized, successfully. These compounds exhibit excellent properties with second-harmonic generation (SHG) responses ranging from 0.5 to 5.9 times that of KDP and large birefringence (Δn_I = 0.181 @ 546.1 nm and Δn_II = 0.148 @ 546.1 nm). Theoretical calculations prove that the BC₂N₅O₂ and B(C₂N₅)₂ units make great contributions to the SHG effects and birefringence, which confirms that the BC₂N₅O₂ and B(C₂N₅)₂ units are novel NLO bifunctional units and could be excellent fundamental building blocks to construct amounts of novel NLO and birefringence crystals. Our studies would enlighten the research studies on biguanide complexes of boron.

Promising Deep-Ultraviolet Birefringent Materials via Rational Design and Assembly of Planar π-Conjugated [B(OH)3 ] and [B3 O3 (OH)3 ] Functional Species

Birefringent materials play a significant role in modulating polarized light in optical communication and the laser industry. However, the discovery of deep ultraviolet (DUV, λ<200 nm) birefringent materials still faces a serious challenge. Herein, we propose hydroxylated π-conjugated [B(OH)₃ ] and [B₃ O₃ (OH)₃ ] units for designing DUV birefringent materials. Innovatively, four new hydroxyborates have been synthesized under mild synthesis conditions. They present four novel pseudo layers that benefit from the large degree of freedom assembly modes of [B(OH)₃ ] and [B₃ O₃ (OH)₃ ] genes and large birefringence (0.057-0.123@532 nm). Moreover, the Cs₃ [B(OH)₃ ]₂ Cl₃ crystal features a short DUV cutoff edge (180 nm), which further indicates that the reported compounds are potential DUV birefringent crystals. Free and flexible assembly modes of π-conjugated [B(OH)₃ ] and [B₃ O₃ (OH)₃ ] groups endow them a particular advantage as significant genes for exploring promising DUV birefringent materials.

Linear unit BN2: a novel birefringence-enhanced fundamental module with sp orbital hybridization

Inorganic planar π-conjugated groups are advantageous to generate large birefringence in optical functional materials, and many excellent materials contain CO₃, BO₃ or B₃O₆, such as CaCO₃, α/β-BaB₂O₄ (α/β-BBO), and KBe₂BO₃F₂ (KBBF). In view of their microscopic structures, the common characteristics are the planar structures, which are regarded as birefringence-enhanced fundamental modules (FMs). Nowadays, exploring novel birefringence-enhanced FMs is becoming a burning issue. Herein, we investigated the birefringence-enhanced FMs in B–N systems and found that the BN₂ linear unit could produce great birefringence. Through the investigation based on the Inorganic Crystal Structure Database, some compounds with the BN₂ linear group were screened out with the formulas A₃BN₂ (A = Li, Na), A₃BN₃ (A = Mg, Ca), and Ba₃(BN₂)₂. Particularly, Ca₃(BN₂)N exhibits a great birefringence of about 0.411 at 1064 nm, which is 3.5, 2.5 and 2.0 times those of the most commercially used birefringent crystals α-BaB₂O₄ (Δn = 0.116 at 1064 nm), CaCO₃ (Δn = 0.164 at 1064 nm) and YVO₄ (Δn = 0.208 at 1064 nm), respectively. To find the origins of the optical properties of compounds with the BN₂ linear group, the first-principles, REDA and polarizability anisotropy analysis methods were used. Owing to the structural arrangement and the polarization anisotropy of the BN₂ linear group, it can influence the birefringence significantly. This work will provide a general way for exploring birefringence-enhanced FMs in B–N compounds.

The linear unit BN₂ is discovered as a novel birefringence-enhanced fundamental module. Particularly, Ca₃(BN₂)N exhibits a large birefringence (0.411 at 1064 nm), which is about 2.0–3.5 times of the commercial used birefringent crystals, such as α-BaB₂O₄, CaCO₃ and YVO₄.

Ba6(CuxZy)Sn4S16 (Z = Mg, Mn, Zn, Cd, In, Bi, Sn): High Chemical Flexibility Resulting in Good Nonlinear-Optical Properties

Seven acentric sulfides Ba₆(Cu_xZ_y)Sn₄S₁₆ (Z = Mg, Mn, Zn, Cd, In, Bi, Sn) were grown by a high-temperature salt flux method. The crystal structures of the Ba₆(Cu_xZ_y)Sn₄S₁₆ (Z = Mg, Mn, Zn, Cd, In, Bi, Sn) compounds were determined by single-crystal X-ray diffraction with the aid of solid-state NMR spectroscopy. The Ba₆(Cu_xZ_y)Sn₄S₁₆ (Z = Mg, Mn, Zn, Cd, In, Bi) compounds are isostructural and crystallize in the Ba₆Ag₄Sn₄S₁₆ structure type. The Sn-containing compound exhibits high structural similarity to Ba₆(Cu_xZ_y)Sn₄S₁₆ (Z = Mg, Mn, Zn, Cd, In, Bi) with the presence of an interstitial atomic position partially occupied by Sn atoms. The chemical bonding characteristics of Ba₆(Cu_2.9Sn_0.4)Sn₄S₁₆ were understood with electron localization function calculations coupled with crystal orbital Hamilton population calculations. The Ba-S and Cu-S interactions are dominantly ionic, but the Sn-S interactions consist of strong covalent bonding characteristics in Ba₆(Cu_2.9Sn_0.4)Sn₄S₁₆. The monovalent Cu atoms, mixed with certain metals with various oxidation states, significantly shift the optical properties of the Ba₆(Cu_xZ_y)Sn₄S₁₆ (Z = Mg, Mn, Zn, Cd, In, Bi) compounds. This results in a good balance between the second-harmonic-generation (SHG) response and laser damage threshold (LDT). Ba₆(Cu_1.9Zn_1.1)Sn₄S₁₆ possesses a high SHG response and a high LDT of 2.8 × AGS and 3 × AGS, respectively. A density functional theory calculation revealed that CuS₄ and SnS₄ tetrahedra significantly contribute to the SHG response in Ba₆(Cu₂Mg)Sn₄S₁₆, which also confirmed that CuS₄ tetrahedra are crucial for the stability and optical properties of the Ba₆(Cu_xZ_y)Sn₄S₁₆ (Z = Mg, Mn, Zn, Cd, In, Bi, Sn) compounds revealed by electronic structure analysis.

Performance of optical materials with derivative planar π-conjugated groups: Recent advances and future prospects

Planar π-conjugated groups which possess not only large hyperpolarizability but also optical anisotropy are proven to be a good functional motif for optical materials with outstanding nonlinear optics and/or birefringence....

A2P2S6 (A=Ba, Pb): A Good Platform to Study Polymorph Effect and Lone Pairs Effect to Form Acentric Structure

Three ternary thiophosphates α-Ba2P2S6, β-Ba2P2S6, and Pb2P2S6, were synthesized via a high temperature salt flux method or an I2 transport reaction. β-Ba2P2S6 and Pb2P2S6 were previously structurally characterized without investigating...

Enhancement of band gap and birefringence induced via π-conjugated chromophore with “tail effect”

The enhancement of band gap and birefringence of template compounds can be attributed to this “tail effect” caused by the extended electronic distribution from [CO3]2−/[BO3]3−/[C3N3O3]3− to [HCO3]−/[HBO3]2−/[HC3N3O3]2−.

KRE(CO3)2 (RE = Eu, Gd, Tb): new mixed metal carbonates with strong photoluminescence and large birefringence

Three new potassium rare earth carbonates KRE(CO3)2 (RE = Eu, Gd, Tb) with strong photoluminescence and large birefringence were synthesized successfully.

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.

Distinctive modulation of optical anisotropy by halogens in α/β-Cd-P-X (X = Cl, Br, and I)

Birefringent materials are widely applied as photoelectric functional field devices to modulate the polarization of lasers. The introduction of a halogen into the structure of crystals could balance the relationship between the band gap E_g and nonlinear optical (NLO) coefficient owing to their outstanding electronegativity and control the optical anisotropy. In this work, the optical properties of phosphohalides α/β-Cd₂P₃X (X = Cl, Br, I) were studied. It was found that the birefringences of α/β-Cd₂P₃Cl (0.23/0.24 @ 1064 nm) are unexpectedly 8 times larger than those of α/β-Cd₂P₃I (0.04/0.03 @ 1064 nm). To find the optical property origins and explore the contributions of microscopic groups to the optical anisotropy and NLO responses in Cd-P-X (X = Cl, Br, I), the first-principles, real-space atom-cutting, and polarizability anisotropy analysis methods were used. This reveals that the electron distribution is susceptible to halogen electronegativity. Halogen atoms can modulate the polarization anisotropy of the active polyhedron and influence the birefringence significantly, owing to the synergistic effect of the anion size and strong covalent interactions between halogens and metal cations. This work clarifies the optical anisotropy origin mechanism and provides a general strategy for finding promising birefringent crystals in phosphohalide systems.

Group 12 Carbonates and their Binary Complexes with Nitrogen Bases and FH2 Z Molecules (Z=P, As, Sb): Synergism in Forming Ternary Complexes

MCO₃ (M=Zn, Cd, Hg) forms a spodium bond with nitrogen-containing bases (HCN, NHCH₂ , NH₃ ) and a pnicogen bond with FH₂ Z (Z=P, As, Sb). The spodium bond is very strong with the interaction energy ranging from -31 kcal/mol to -56 kcal/mol. Both NHCH₂ and NH₃ have an equal electrostatic potential on the N atom, but the corresponding interaction energy is differentiated by 1.5-4 kcal/mol due to the existence of spodium and hydrogen bonds in the complex with NHCH₂ as the electron donor. The spodium bond is weakest in the HCN complex, which is not consistent with the change of the binding distance. The spodium bond becomes stronger in the CdCO₃ <ZnCO₃ <HgCO₃ sequence although the positive electrostatic potential on the Hg atom is smallest. This is because the electrostatic interaction is dominant in the spodium-bonded complexes of CdCO₃ and ZnCO₃ but the polarization interaction in that of HgCO₃ . The pnicogen bond is much weaker than the spodium bond and the former has a larger enhancement than the latter in the FH₂ Z⋅⋅⋅OCO₂ M⋅⋅⋅N-base ternary complexes.

KCs2 [Pb2 Br5 (HCOO)2 ]: A Polar 3D Lead-Bromide Framework Exhibiting Strong Second-Harmonic Generation Response

The discovery of new nonlinear optical (NLO) crystals with excellent properties is in urgently demand because of their ability to generate coherent light. Herein, we report an unique NLO lead bromide formate, KCs₂ [Pb₂ Br₅ (HCOO)₂ ], which has been synthesized by a mix-solvothermal method. KCs₂ [Pb₂ Br₅ (HCOO)₂ ] exhibits strong phase-matching second-harmonic generation (SHG) response (6.5×KDP), large birefringence (0.16@ 1064 nm), and a wide transparent window in most visible light and mid-IR region. Interestingly, KCs₂ [Pb₂ Br₅ (HCOO)₂ ] features a polar 3D lead-bromide framework in which adjacent Pb-Br layers containing coplanar Pb₆ Br₆ rings are not only parallel to each other, but also orient in the same direction. These oriented arrangements are responsible for the strong SHG response and large birefringence that are elucidated by both local dipole moment and theoretical calculations. This research provides a new strategy to explore subsequent NLO crystals.

Be2 (BO3 )(IO3 ): The First Anion-mixed Van der Waals Member in the KBe2 BO3 F2 Family with a Very Strong Second Harmonic Generation Response

To obtain new nonlinear optical (NLO) materials with a large second harmonic generation (SHG) effect has always been a great challenge. We have synthesized the first metal borate-iodate NLO crystal, Be₂ (BO₃ )(IO₃ ) (BBIO), by multicomponent modification of KBe₂ BO₃ F₂ (KBBF), in which the structural features of KBBF were maintained and (IO₃ )^- groups were connected to honeycomb [Be₂ BO₃ O₂ ]_∞ layers. As the first KBBF family member with mixed anionic groups, BBIO benefited from the synergistic effect of (IO₃ )^- , (BO₃ )^3- and (BeO₄ )^6- groups, and exhibited a very strong SHG response of ≈7.2×KH₂ PO₄ (KDP, @1064 nm) and a large birefringence (Δn) of 0.172 at 1064 nm. BBIO may, unexpectedly, optimize growth habits via van der Waals forces. This study presents borate-iodate as a new NLO material and it demonstrates opportunities in KBBF structural engineering.