Non-Centrosymmetric RbNaMgP2O7 with Unprecedented Thermo-Induced Enhancement of Second Harmonic Generation

Unearthing Superior Inorganic UV Second-Order Nonlinear Optical Materials: A Mineral-Inspired Method Integrating First-Principles High-Throughput Screening and Crystal Engineering

Natural minerals, with their adaptable framework structures exemplified by perovskite and lyonsite, have sparked substantial interest as potential templates for the design of advanced functional solid-state materials. Nonetheless, the quest for new materials with desired properties remains a substantial challenge, primarily due to the scarcity of effective and practical synthetic approaches. In this study, we have harnessed a synergistic approach that seamlessly integrates first-principles high-throughput screening and crystal engineering to reinvigorate the often-overlooked fresnoite mineral, Ba2 TiOSi2 O7 . This innovative strategy has culminated in the successful synthesis of two superior inorganic UV nonlinear optical materials, namely Rb2 TeOP2 O7 and Rb2 SbFP2 O7 . Notably, Rb2 SbFP2 O7 demonstrates a comprehensive enhancement in nonlinear optical performance, featuring a shortened UV absorption edge (260 nm) and a more robust second-harmonic generation response (5.1×KDP). Particularly striking is its significantly increased birefringence (0.15@546 nm), which is approximately 30 times higher than the prototype Ba2 TiOSi2 O7 (0.005@546 nm). Our research has not only revitalized the potential of the fresnoite mineral for the development of new high-performance UV nonlinear optical materials but has also provided a clearly defined roadmap for the efficient exploration of novel structure-driven functional materials with targeted properties.

Novel antimony phosphates with enlarged birefringence induced by lone pair cations

Phosphates, whose obvious disadvantage is the relatively small birefringence, can be overcome by the introduction of post-transition metal cations containing stereochemically active lone-pair electrons. In this paper, two new compounds were successfully explored in the A-Sb-P-O system, i.e. Cs2Sb3O(PO4)3 (CsSbPO) and (NH4)2Sb4O2(H2O)(PO4)2[PO3(OH)]2 (NH4SbPOH). Transmission spectra show that CsSbPO has a surprising transmission range with a UV cutoff edge of 213 nm. First-principles calculations show that both compounds have a wide band gap (5.02 eV for CsSbPO and 5.30 eV for NH4SbPOH) and enlarged birefringence (Δn = 0.034@1064 nm for CsSbPO and Δn = 0.045@1064 nm for NH4SbPOH). The results of real-space atom-cutting investigations show that the distorted [SbOx] polyhedra originating from the asymmetric lone pair electrons give the main contribution to the total birefringence and overcome the disadvantage of small birefringence of phosphates but maintain wide transition windows.

K(NH4)Zn2(PO4)2: a Beryllium-Free Sr2Be2B2O7 Derivative with Enhanced Interlayer Connectivity

A novel zinc phosphate derivative of Sr2Be2B2O7 (SBBO), K(NH4)Zn2(PO4)2 (KNZP), featuring [Zn2P2O8]∞2- double layers akin to the [Be2B2O7]∞4- layers in SBBO, was successfully synthesized via a moderate hydrothermal method. Through the substitution of BeO4 and BO3 with ZnO4 and PO4, the issue of toxicity has been effectively resolved, while the enhanced interlayer interactions facilitated by covalent and hydrogen bonding in KNZP overcome the inherent structural instability. Notably, KNZP exhibits a wide transparent window and a moderate second-harmonic generation (SHG) intensity, reaching 0.7 times that of KH2PO4 (KDP), rendering it type-I phase-matchable, indicating that it is a promising UV nonlinear optical (NLO) material.

A2Hgx(SeO3)y (A = K, Rb, Cs): Three Alkali Metal Mercury Selenites Featuring Unique 1D [HgOm(SeO3)n]∞ Chains

Herein, three alkali metal mercury selenites, K2Hg2(SeO3)3, Rb2Hg2(SeO3)3, and Cs2Hg3(SeO3)4, were successfully obtained by a hydrothermal method. The three compounds featured same one-dimensional (1D) [HgOm(SeO3)n]∞ chain structure that consisting of distorted Hg-O polyhedra and SeO3 triangular pyramids with stereochemically active lone pair (SCALP) electrons. Interestingly, the rich coordination environment of Hg atoms and the size difference of alkali metal cations lead to diverse arrangement of SeO3 groups, which makes them exhibit different birefringence. The band gaps of the three compounds indicate that they are potential ultraviolet (UV) optical materials. Detailed theoretical calculations demonstrate that the combined effects of SeO3 triangular pyramids and Hg-O polyhedra are responsible for the optical characteristics of the reported compounds.

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

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

Two van der Waals Layered Antimony(III) Phosphites as UV Optical Materials

Herein, two new Sb3+-based phosphites, Sb2O2(HPO3) (I) and Sb2O(HPO3)2 (II), were successfully obtained by ingeniously combining Sb3+-based polyhedra containing stereochemically active lone pair (SCALP) and HPO3 polar groups. Both reported compounds exhibit unique 2D van der Waals layered structures, [Sb4O4(HPO3)2]∞ and [Sb2O(HPO3)2]∞, respectively, which favors compounds with large optical anisotropy. Interestingly, the different curvatures of the two layers resulted in the two title compounds showing significantly different birefringences (0.079@546 and 0.046@546 nm, respectively). Both compounds endow wide optical band gaps (4.32 and 4.54 eV, respectively), which indicates their potential as promising ultraviolet (UV) birefringent crystals. The synthesis of the two title compounds enriched Sb3+-based phosphites in the UV region and provided guidance for the subsequent synthesis of superior optical materials.

Giant Mid-Infrared Second-Harmonic Generation Response in a Densely-Stacked Van Der Waals Transition-Metal Oxychloride

Second-harmonic generation (SHG) is a fundamental optical property of nonlinear optical (NLO) crystals. Thus far, it has proved difficult to engineer large SHG responses, particularly in the mid-infrared region, owing to the difficulty in simultaneously controlling the arrangement and density of functional NLO-active units. Herein, a new assembly strategy employing functional modules only, and aimed at maximizing the density and optimizing the spatial arrangement of highly efficient functional modules, has been applied to the preparation of NLO crystals, affording the van der Waals crystal MoO2 Cl2 . This exhibits the strongest powder SHG response (2.1×KTiOPO4 (KTP) @ 2100 nm) for a transition-metal oxyhalide, a wide optical transparency window, and a sufficient birefringence. MoO2 Cl2 is the first SHG-active transition-metal oxyhalide effective in the infrared region. Theoretical studies and crystal structure analysis suggest that the densely packed, optimally-aligned [MoO4 Cl2 ] modules within the two-dimensional van der Waals layers are responsible for the giant SHG response.

Phase Transition-Promoted Rapid Photomechanical Motions of Single Crystals of a Triene Coordination Polymer

Molecular crystals with the ability to transform light energy into macroscopic mechanical motions are a promising class of materials with potential applications in actuating and photonic devices. In regard to such materials, coordination polymers that exhibit dynamic photomechanical motion, associated with a phase transition, are unknown. Herein, we report an intriguing photoactive, one-dimensional ZnII coordination polymer, 1, derived from 1,3,5-tri-4-pyridyl-1,2-ethenylbenzene and 3,5-difluorobenzoate. Single crystals of 1 under UV light irradiation exhibit rapid shrinking and bending, violent bursting-jumping, splitting, and cracking behavior. Single-crystal X-ray diffraction analysis and 1 H NMR spectroscopy reveal an unusual photoinduced phase transition involving a single-crystal-to-single-crystal [2+2] cycloaddition reaction that results in photomechanical responses. Interestingly, crystals of 1, which are triclinic with space groupP 1 ‾ ${P\bar{1}}$ , are transformed into a higher symmetry, monoclinic cell with space group C2/c. This process represents a rare example of symmetry enhancement upon photoirradiation. The photomechanical activity is likely due to the sudden release of stress associated with strained molecular geometries and significant solid-state molecular movement arising from cleavage and formation of chemical bonds. A composite membrane fabricated from 1 and polyvinyl alcohol (PVA) also displays interesting photomechanical behavior under UV light illumination, indicating the material's potential as a photoactuator.

K2Sr4(PO3)10: A Polyphosphate with Deep-UV Cutoff Edge and Enlarged Birefringence

A new polyphosphate K2Sr4(PO3)10 is synthesized by a high-temperature solution method. This compound crystallizes in the triclinic space group of P1̅, consisting of the 1D infinite [PO3]∞ chains and K and Sr ions between the chains. Compared with AM2(PO3)5 (A = K, Rb, Cs; M = Ba, Pb), K2Sr4(PO3)10 exhibits a more complex [PO3]∞ chain structure and more diverse metal cationic coordination environment. More importantly, K2Sr4(PO3)10 has both a deep-UV cutoff edge (<200 nm) and a significantly enlarged birefringence. First-principles calculations indicate that the birefringence of K2Sr4(PO3)10 is 0.017 at 1064 nm, about 2 times that of RbBa2(PO3)5 (0.008 at 1064 nm), which reaches a new height among the reported mixed alkali metal and alkaline earth metal phosphate. Theoretical calculations and structural analyses show that the enlarged birefringence of K2Sr4(PO3)10 mainly originates from the [PO3]∞ chains arranged in an inverted zigzag. This discovery introduces a new strategy for devising novel phosphate deep-UV optical crystals with a large birefringence.

Li2RbSO4Cl with a Short Ultraviolet Absorption Edge and an Acentric Structure through Assembling Heteroleptic [LiO3Cl] Tetrahedra

Currently, short-wavelength nonlinear optical materials are urgently needed. Through substituting homoleptic [LiO4] in centrosymmetric LiRbSO4 with heteroleptic [LiO3Cl] tetrahedra, an acentric sulfate chloride, Li2RbSO4Cl, was designed and synthesized by the high-temperature melting method. Li2RbSO4Cl shows a relatively short ultraviolet absorption edge (<200 nm) among newly reported sulfate chlorides. Millimeter-sized crystals were grown due to the congruent melting behavior and high thermal stability of the compound.

Stereochemically Active Tin(II)-Induced Enhancement of Birefringence in SnII SnIV (PO4 )2 and SrSn(PO4 )PO2 (OH)2

Two new tin(II) phosphates, SnII SnIV (PO4 )2 and SrSn(PO4 )PO2 (OH)2 , were synthesized by the high-temperature solution method and hydrothermal method, respectively. Theoretical study indicates that by introducing tin(II) with stereochemical activity lone pairs (SCALP) in metal phosphates, the birefringence was enhanced, 0.048@1064 nm for SnII SnIV (PO4 )2 and 0.080@1064 nm for SrSn(PO4 )PO2 (OH)2 .

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.

Phosphates with Two Types of Isolated P-O Groups: Noncentrosymmetric Na6Sr2Bi3(PO4)(P2O7)4 and Centrosymmetric Cs2CaBi2(PO4)2(P2O7)

Inorganic phosphates are of great interest, because of their rich structural chemistry and multiple functional properties. Compared with the phosphates that only contain the solely condensed P-O groups, the phosphates with various condensed P-O groups are less reported, especially for the noncentrosymmetric (NCS) ones. Here, two new bismuth phosphates, Na₆Sr₂Bi₃(PO₄)(P₂O₇)₄ and Cs₂CaBi₂(PO₄)₂(P₂O₇), were synthesized by the solid-state reaction and both structures contain two types of isolated P-O groups. Remarkably, Na₆Sr₂Bi₃(PO₄)(P₂O₇)₄ crystallizes in the tetragonal space group P4̅2₁c, which represents the first NCS bismuth phosphate with PO₄ and P₂O₇ groups. Detailed structural comparisons among Bi³⁺-containing alkali/alkaline-earth metal phosphates show that the ratios of cations/phosphorus profoundly influence the condensed degree of P-O groups. Ultraviolet-visible-near-infrared (UV-vis-NIR) diffusion spectra show that both compounds have the relatively short UV cutoff edges. And Na₆Sr₂Bi₃(PO₄)(P₂O₇)₄ has a second-harmonic generation response of 1.1 × KDP. The first-principles calculations are carried out to understand the structure-performance relationship.

Controlling the Nonlinear Optical Behavior and Structural Transformation with A-Site Cation in α-AZnPO4 (A = Li, K)

Regulating the crystal structure by A-site cation substitution is one of the effective methods to explore high-performance nonlinear optical (NLO) materials. Herein, two non-centrosymmetric (NCS) compounds, α-MZnPO₄ (M = Li, K) with short UV absorption edges 221 and 225 nm, are obtained by performing A-site cation substitution method. It is noteworthy that α-LiZnPO₄ (α-LZPO) achieves >10 times second harmonic generation (SHG) response (2.3 × KDP) enhancement compared with that of α-KZnPO₄ (α-KZPO) (0.2 × KDP), which is the only case among phosphates with different A-site cations. By structural comparison, it is found that the A-site cations play important roles for anion rearrangements, and further the structure features of the two compounds by designing two suppositional crystal models as well as performing other theoretical calculations are analyzed. The study confirms the feasibility to design promising NLO materials with strengthen SHG response and structural stability in orthophosphate system.

A Rare-Earth Selenite with Unexpectedly Well-Balanced Ultraviolet Nonlinear Optical Functionality, Sc(HSeO3 )3

Establishing high performance ultraviolet (UV) nonlinear optical (NLO) selenite crystals with well-balanced properties is very challenging attributable to their strong absorption for UV light. Here a rare-earth selenite, Sc(HSeO₃ )₃ , with excellent UV NLO properties is introduced. Sc(HSeO₃ )₃ crystallizing in the polar NCS space group, Cc, features a 3D archetiture built up by interconnected ScO₆ octahedra and HSeO₃ groups. The crystal exhibits remarkably well-balanced UV-NLO functionality, namely, the shortest absorption edge (214 nm) among NLO-active selenites, wide bandgap (5.28 eV), large phase-matchable SHG response (5 × KDP), and sufficiently large birefringence (cal. 0.105 @1064 nm). Detailed DFT calculations have been performed to elucidate the structure-property relationships. This work provides a new example of discovering novel UV NLO selenite materials.

Two Mixed-Alkali-Metal Selenates as Short-Wave Ultraviolet Nonlinear-Optical Materials

Two novel mixed-alkali-metal selenate nonlinear-optical (NLO) crystals, Na₃Li(H₂O)₃(SeO₄)₂·3H₂O (I) and CsLi₃(H₂O)(SeO₄)₂ (II), have been successfully synthesized by an aqueous solution evaporation method. Both compounds feature the unique layers constructed of the same functional moieties including SeO₄ and LiO₄ tetrahedra: [Li(H₂O)₃(SeO₄)₂·3H₂O]_∞^3- layers in I and [Li₃(H₂O)(SeO₄)₂]_∞^- layers in II. The titled compounds display wide optical band gaps of 5.62 and 5.66 eV, respectively, according to the UV-vis spectra. Interestingly, they exhibit significantly different second-order nonlinear coefficients (0.34 × KDP and 0.70 × KDP, respectively). Detailed dipole moment calculations manifest that the large disparity can be attributed to the difference in the dipole moment of the crystallographically independent SeO₄ and LiO₄ groups. This work confirms that alkali-metal selenate system is an excellent candidate for short-wave ultraviolet NLO materials.

Structure and Characterization of K2Na3B2P3O13, a New Nonlinear Optical Borophosphate with One-Dimensional Chain Structure and Short Ultraviolet Cutoff Edge

Nonlinear optical (NLO) crystals, being the primary medium for laser wavelength conversion, are crucial in all-solid-state lasers. Borophosphates offer more structural varieties than pure borates and phosphates, and they have become popular as NLO crystal candidates. Through spontaneous crystallization, we acquired a noncentrosymmetric alkali metal borophosphate crystal material, K₂Na₃B₂P₃O₁₃ (KNBPO). KNBPO crystallizes in the orthorhombic Cmc2₁ space group with the following unit cell parameters: a = 13.9238(18) Å, b = 6.7673(8) Å, c = 12.1298(15) Å, and Z = 4, and its structure is characterized by a fundamental building unit 1_∞ [B₂P₃O₁₃] chain structure made up of bridging oxygen linkages between BO₄ and PO₄ tetrahedra. KNBPO has a short ultraviolet (UV) cut-off edge (<186 nm), a congruent melting characteristic, good thermal stability, and a moderate second harmonic generation response roughly 0.42 times that of KH₂PO₄. Theoretical calculations reveal that the optical properties of the compound mainly originate from BO₄ and PO₄ units. Due to the short UV cut-off edge, KNBPO can be used as a potential NLO material in the UV and even deep UV regions, and it enhances the structural variety of borophosphates, which has a reference value for scholars investigating similar materials.

Cadmium phosphates with two types of fundamental building units and a wide ultraviolet transparency window

Owing to diverse crystal structures and intriguing properties, metal phosphates have attracted much attention in recent studies. In this work, two cadmium phosphates, KCd₆P₇O₂₄ and KCd₃P₃O₁₁, were obtained by the high-temperature solution method. In the crystal structure, both the title compounds contain two types of P-O fundamental building units. Thermal and spectral analyses were carried out on the title compounds, indicating that both of them have good thermal stability and short UV cut-off edges. In addition, the relationship between the electronic structures and optical properties was studied by theoretical calculations.

A Cation-Driven Approach toward Deep-Ultraviolet Nonlinear Optical Materials

The design of new materials with special performances is still a great challenge, especially for the deep-ultraviolet nonlinear optical materials in which it is difficult to balance large bandgaps and strong second harmonic generation responses due to their inverse relationship. Cation variation not only influences the whole structure frameworks but also directly participates in the formation of electronic structures, both of which could lead to the uncontrollability of the properties of the designed materials. Here, a novel approach, aiming at purposeful and foreseeable material designs, is proposed to characterize the role of cations. By the verification of several series of borates, the influences of cation variation on property changes are explored systematically. Accordingly, a feasible strategy of designing deep-ultraviolet nonlinear optical materials by substituting barium for lead has been concluded, which could obviously blue-shift the ultraviolet cutoff edge and maintain the relatively strong second harmonic generation response (more than 2 times of KH₂PO₄), achieving the property optimization, and especially works efficiently in fluorooxoborates. The property optimization design strategy and the cation characterization method are not only helpful in exploring nonlinear optical materials but also enlightening in material design and selection.

Engineering hydrogen bonding to align molecular dipoles in organic solids for efficient second harmonic generation

Considering nearly infinite design possibilities, organic second harmonic generation (SHG) molecules are believed to have long-term promise. However, because of the tendency to form dipole-antiparallel crystals that lead to zero macroscopic polarization, it is difficult to design a nonlinear optical (NLO) material based on organic molecules. In this manuscript, we report a new molecule motif that can form asymmetric organic solids by controlling the degree of hydrogen bonding through protonation. A conjugated polar organic molecule was prepared with a triple bond connecting an electron-withdrawing pyridine ring and an electron-donating thiophene ring. By controlling the degree of hydrogen bonding through protonation, two different crystal packing motifs are achieved. One crystallizes into the common dipole-antiparallel nonpolar P1̄ space group. The second crystallizes into the uncommon dipole-parallel polar P1 space group, in which the molecular dipoles are aligned along a single axis and thus exhibit a high macroscopic polarization in its solid-state form. Due to the P1 polar packing, the sample can generate second harmonic light efficiently, about three times the intensity of the benchmark potassium dihydrogen phosphate. Our findings show that crystal engineering by hydrogen bonding in a single molecular backbone can be used for controlling the macroscopic NLO properties.

Transition metal complexes with strong and long-lived excited state absorption: from molecular design to optical power limiting behavior

Transition metal complexes (TMCs) with strong and long-lived excited state absorption (ESA) usually exhibit high-performance optical power limiting (OPL) response. Several techniques, such as transmission vs. incident fluence curves and Z-scan have been widely used to assess the OPL performance of typical TMCs. The OPL performance of TMCs is highly molecular structure-dependent. Special emphasis is placed on the structure-OPL response relationships of Pt(II), Ir(III), Ru(II), and other metal complexes. This review concludes with perspectives on the current status of OPL field, as well as opportunities that lie just beyond its frontier.

[C(NH2)3]2Zn(CO3)2: A Guanidinium-Templated Ultraviolet Nonlinear Optical Material

A novel guanidinium-templated ultraviolet (UV) nonlinear optical zinc carbonate crystal, [C(NH₂)₃]₂Zn(CO₃)₂ (GZCO), has been synthesized in a closed system at low temperatures. GZCO crystallizing in the tetragonal noncentrosymmetric nonpolar space group, P4₁2₁2 exhibits a three-dimensional anionic framework constructed by interconnected [Zn₆C₆O₃₂] 12-membered ring channels with inorganic CO₃ triangles and ZnO₄ tetrahedra. Notably, the anhydrous GZCO shows a very high thermal stability among guanidine-based hybrid NLO materials benefiting from the confinement effect of the organic cations within inorganic channels. The UV-visible transmittance spectrum reveals that GZCO has a short UV cutoff edge of 210 nm, corresponding to the large band gap of 5.9 eV. GZCO exhibits a mild second-harmonic generation efficiency of 0.5 × KH₂PO₄ with type-I phase-matching behavior.

CsY(SO4)2·4H2O: A Deep-Ultraviolet Birefringent Crystal Induced by an Edge-Sharing Connection

SO₄ tetrahedral groups have weak polarization anisotropy, which thus results in the small birefringence of sulfates. Here, we report new sulfate CsY(SO₄)₂·4H₂O with unprecedented birefringence among deep-ultraviolet (deep-UV) sulfates. Its single crystal (10 mm × 3.5 mm × 1.5 mm) was simply grown by an aqueous solution evaporation technique, and it features a rare layered structure composed of YO₉ polyhedra, SO₄ tetrahedra, and H₂O molecules. Interestingly, each SO₄ group donates two oxygen atoms to edge-share with one adjacent YO₉ polyhedron and thus causes severe distortion of these groups. The characteristic edge-sharing mode gives CsY(SO₄)₂·4H₂O a large birefringence of ∼0.045@546 nm, which is the maximum among deep-UV sulfates and phosphates with similar non-π-conjugated anionic groups. The ultraviolet-visible-near-infrared diffuse reflection and transmission spectra, infrared spectrum, thermal stability, and theoretical calculations are also presented. The fascinating results will improve our understanding of sulfates and may provide useful insights into the exploration of deep-UV sulfates with large birefringence.

Ba2Bi2[Zn(PO4)4]: ionic-isolation-activated first barium bismuth zincophosphate with unprecedented ∞[Zn(PO4)4]10− chains and balanced overall performance

The combination of ion-mixing and anionic condensation has activated the first barium bismuth zincophosphate of Ba2Bi2[Zn(PO4)4] with unprecedented ∞[Zn(PO4)4]10− chains and balanced overall performance.

Thermal-driven unusual dual SHG switching with wide SHG-active step triggered by inverse symmetry breaking

Solid nonlinear optical (NLO) materials with switchable second-harmonic generation (SHG) on/off state show a strong potential application in photo-electronic devices. Sustained progress has been made through the persistent exploration of...

BaSc2(HPO3)4(H2O)2: A New Nonlinear Optical Phosphite Exhibiting 3D [Sc2(HPO3)4]2- Anionic Framework and Phase Matchable SHG Effect

Phosphite materials have aroused widely attention for their diverse open-frameworks and promising magnetic and optical performances. Here, the first barium scandium phosphite, BaSc2(HPO3)4(H2O)2, was obtained via a facile hydrothermal route....

Li3[Al(PO4)2(H2O)1.5] and Na[AlP2O7]: from 2D layered polar to 3D centrosymmetric framework structures

Two new aluminophosphates with moderate NLO property or a short UV cut-off edge (∼190 nm) are reported.

K2(BeS)O4F2: a novel fluorosulfate with unprecedented 1D [(BeO3F)–(SO3F)]∞ chains exhibiting large birefringence

By introducing the strongly electronegative F− ions into [SO4] tetrahedral units, polar NLO-active units [(BeS)O4F2] were obtained, thus forming 1D zigzag [(BeO3F)–(SO3F)]∞ chains.

Tellurate Polymorphs with High-performance Nonlinear Optical Switch Property and Wide Mid-IR Transparency

Here, we report two tellurate polymorphs, α- and β-Li2HfTeO6, undergoes a fast thermally induced phase transition that originates from symmetry breaking of HfO6 and TeO6 octahedra, behaves as a potential...

Investigations on the Synthesis, Crystal Structure, Linear- and Nonlinear-Optical Properties of the Zinc Germanate Rb2ZnGe2O6

A quaternary nonlinear-optical (NLO) zinc germanate, Rb₂ZnGe₂O₆ (RZGO), was prepared from its stoichiometric melts, which belongs to the noncentrosymmetric space group C222₁ (No. 20) and features a 3D framework formed by GeO₄ tetrahedra, ZnO₄ tetrahedra, and Rb⁺ cations filling into the void space. RZGO displays a good phase-matchable second-harmonic-generation (SHG) intensity similar to that of KH₂PO₄. The UV cutoff edge for RZGO was found to be approximately 0.37 μm with a broad optical band gap of 3.22 eV, indicating a large laser-induced damage threshold. Further characterizations suggested that this compound displays an excellent thermal stability (1014 °C) and melts congruently. Besides, theoretical analysis was also implemented to investigate the electronic structures and effective NLO coefficients as well as the origin of the observed SHG response for RZGO. The present study can enrich the diversity of acentric germanate structures and pave the way for the synthesis of new NLO compounds.

Pushing KTiOPO4-like Nonlinear Optical Sulfates into the Deep-Ultraviolet Spectral Region

Accurately designing and synthesizing new deep-ultraviolet (deep-UV) nonlinear optical (NLO) crystals that are limited by the so-called "200 nm wall" on their transparency windows remain challenging. On the basis of a bandgap-directed computer-aided material design approach, two new NLO sulfates, KMgSO₄F and KZnSO₄F, are designed and successfully synthesized. They feature three-dimensional frameworks closely related to the commercial NLO crystal, KTiOPO₄ (KTP). Remarkably, the transmittance spectrum based on a single crystal indicates that the transparency window of KZnSO₄F is significantly blue-shifted to <190 nm from 350 nm for KTP. The microscopic origin of this significant transparent window blue shift is illustrated well by first-principles calculations. This work pushes the transparency windows of KTP-like NLO sulfates into the deep-UV spectral region for the first time and will pave a prospective way to the accurate design and synthesis of new deep-UV NLO materials.

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.

RbMo2P3O14 with large birefringence mainly induced by highly distorted [MoO6] in uncommon [Mo2P3O14]∞ layers

The introduction of d⁰ transition metal Mo⁶⁺ cations into a phosphate generates a new acentric molybdophosphate, RbMo₂P₃O₁₄. It shows uncommon [Mo₂P₃O₁₄]_∞ layers composed of isolated [MoO₆] octahedra and [P₃O₁₀] groups. To the best of our acknowledge, it exhibits the largest birefringence (a calculated value of 0.166 at 546 nm) among reported molybdophosphates. In addition, it also possesses a shorter UV cut-off edge (about 250 nm) than other molybdates and molybdophosphates, indicating that it can be used as a birefringent crystal in the UV optical region. First-principles electronic structure analysis suggests that the large birefringence mainly originates from highly distorted [MoO₆].

Tin(II)-Induced Large Birefringence Enhancement in Metal Phosphates

Two alkali tin(II) phosphates, namely, Rb[SnF(HPO₄)] and Rb(Sn₃O)₂(PO₄)₃, were synthesized through mild hydrothermal methods. They belong to the orthorhombic Pnma and Pbcn space groups, respectively. Rb[SnF(HPO₄)] features a layered structure based on 1D [SnF(HPO₄)]_∞ chains interconnected by hydrogen bonds, with Rb⁺ cations located at the interlayer space. For Rb(Sn₃O)₂(PO₄)₃, each pair of [Sn₃O]⁴⁺ clusters is bridged by a pair of [P(1)O₄]^3- tetrahedra to build a 1D [Sn-P-O]_∞ chain. These 1D [Sn-P-O]_∞ chains are further cross-linked though [P(2)O₄]^3- tetrahedra to construct a 3D network with 7- and 10-membered-ring channels. The tin(II) ions in Rb[SnF(HPO₄)] and Rb(Sn₃O)₂(PO₄)₃ with stereochemically active lone pairs (SCALPs) significantly enhance the birefringences of metal phosphates: Δn = 0.147@1064 nm for Rb[SnF(HPO₄)] and 0.082@1064 nm for Rb(Sn₃O)₂(PO₄)₃.

Vibrational and optical studies of a nonlinear optical crystal, Cs2Bi2O(Ge2O7)

Single crystals of a non-centrosymmetric and polar Cs₂Bi₂O(Ge₂O₇) compound were grown from melt. This material was characterized by polarized Raman scattering at room temperature. The comparison to the polycrystalline IR and Raman spectra has been made and assignment of all observed modes to the respective vibrations and their symmetry has been proposed. Our results show that this crystal is promising SRS-active nonlinear optical material for up- and down-Raman laser-frequency converters with the most intense lasing line at 481 cm^-1. Optical studies show that Cs₂Bi₂O(Ge₂O₇) exhibits intense absorption bands with peaks maxima at 260 and 293 nm (77 K) typical for ¹S₀→^1,3P₁ transitions of Bi³⁺ ions, which are parity-allowed due to spin-orbit coupling as well as unexpected band centered at 360 nm that was assigned to intervalence charge transfer transition.