Toward the Rational Design of Novel Noncentrosymmetric Materials: Factors Influencing the Framework Structures

Tuning the first hyperpolarizability of hexaphyrins with different connections of mislinked pyrrole units: a theoretical study

In the satisfactory design and synthesis of high-performance nonlinear optical (NLO) materials, for meeting the rapidly expanding demands of optoelectronic devices, a deeper understanding of the relationship between the structures and NLO properties has become a key issue. Herein, five novel mislinked expanded hexaphyrins with different connections of pyrrole units are selected to study the relationship between the structures and NLO properties. These five mislinked expanded hexaphyrins are neo-fused, neo-confused hexaphyrins, singly, doubly, and triply N-confused hexaphyrins. From theoretical results, the order of the static first hyperpolarizability (β₀) values is found to be: neo-fused hexaphyrin (β₀ = 4163 a.u.) < neo-confused hexaphyrin (β₀ = 5494 a.u.) < singly N-confused hexaphyrin (β₀ = 6510 a.u.) < doubly N-confused hexaphyrin (β₀ = 15 130 a.u.) < triply N-confused hexaphyrin (β₀ = 26 095 a.u.). Furthermore, β₀ values of the doubly and triply N-confused hexaphyrins are improved 2.1 and 3.7 times over that of their usual parent hexaphyrin (β₀ = 7120 a.u.), respectively. It is worth noting that increasing mislinked connection numbers and changing mislinked connection ways of the pyrrole units in these mislinked expanded hexaphyrins plays a crucial role in the tune of their second-order NLO responses.

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

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

New Solids in As-O-Mo, As(P)-O-Mo(W) and As(P)-O-Nb(W) Systems That Exhibit Nonlinear Optical Properties

Interactions between well-mixed fine powders of As₂O₃, P₂O₅, MoO₃, WO₃ and Nb₂O₅ at different stoichiometry in quartz ampoules under vacuum at ~1000 °C in the presence of metallic molybdenum (or niobium), over several weeks, led to shiny dichroic crystalline materials being formed in cooler parts of the reaction vessel. An addition of small quantities of metals-Mo or Nb-was made with the aim of partially reducing their highly oxidized Mo(VI), W(VI) or Nb(V) species to corresponding Mo(V), W(V) and Nb(IV) centers, in order to form mixed valence solids. Sublimed crystals of four new compounds were investigated using a variety of techniques, with prime emphasis on the X-ray analysis, followed by spectroscopy (diffusion reflectance, IR, Raman and EPR), second harmonic generation (SHG), thermal analysis under N₂ and air atmosphere, and single crystals electrical conductivity studies. The results evidenced the formation of new complex solids of previously unknown compositions and structures. Three out of four compounds crystallized in non-centrosymmetric space groups and represent layered 2D polymeric puckered structures that being stacked on each other form 3D lattices. All new solids exhibit strong second-harmonic-generation (SHG effect; based on YAG 1064 nm tests with detection of 532 nm photons), and a rare photosalient effect when crystals physically move in the laser beam. Single crystals' electrical conductivity of the four new synthesized compounds was measured, and the results showed their semiconductor behavior. Values of band gaps of these new solids were determined using diffusion reflectance spectroscopy in the visible region. Aspects of new solids' practical usefulness are discussed.

SrTi(IO3)6·2H2O and SrSn(IO3)6: distinct arrangements of lone pair electrons leading to large birefringences

Three new iodates SrTi(IO₃)₆·2H₂O, (H₃O)₂Ti(IO₃)₆, and SrSn(IO₃)₆ have been synthesized via a facile hydrothermal method. The three compounds have zero-dimensional crystal structures composed of one [MO₆]⁸⁻ (M = Ti, Sn) octahedron connected with six [IO₃]⁻ trigonal pyramids. However, the particular coordination of Sr²⁺ cations results in distinct arrangements of lone pair electrons in an [IO₃]⁻ trigonal pyramid, which leads to large birefringences. More importantly, this work enriches the species crystal chemistry for [M(IO₃)₆]²⁻ (M = Ti, Sn) clusters-containing iodates.

The distinct arrangements of [IO₃]⁻ trigonal pyramids lead to larger birefringences in SrTi(IO₃)₆·2H₂O and SrSn(IO₃)₆ than that in (H₃O)₂Ti(IO₃)₆.

Cs2AlB5O10: a short-wavelength nonlinear optical crystal with moderate second harmonic generation response

A new compound Cs₂AlB₅O₁₀ was synthesized by a high temperature solution method in open air. It features a DUV cutoff edge below 190 nm and a moderate SHG response (0.8 × KDP at 1064 nm), indicating that it may have certain application prospects in the UV nonlinear optical region.

Synergism of multiple functional chromophores significantly enhancing the birefringence in layered non-centrosymmetric chalcohalides

Compared with one or two functional chromophores materials, Hg₃AsQ₄X (Q = S, Se; X = Cl, Br, I) with multiple ones generate extremely large birefringence due to the synergism of the d¹⁰ cation Hg²⁺, lone pair layer of As³⁺ and mixed anions Q²⁻/X⁻.

AB11O16(OH)2 (A = K and Cs): interpenetrating 2D layers with large birefringence

By a cation substitution strategy, two new hydroxyborates AB₁₁O₁₆(OH)₂ (A = K and Cs) were synthesized, and KB₁₁O₁₆(OH)₂ exhibits a short DUV cutoff edge (195 nm) and a large birefringence.

Two deep-ultraviolet nonlinear optical monolayers obtained by a template-optimized design strategy

We obtained two two-dimentional deep-ultraviolet nonlinear optical layers with a large band gap and proper second harmonic generation by a template-optimized design strategy.

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

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

In situ hydrothermal synthesis of polar second-order nonlinear optical selenate Na5(SeO4)(HSeO4)3(H2O)2

An alkali–metal selenate nonlinear optical (NLO) crystal, Na₅(SeO₄)(HSeO₄)₃(H₂O)₂, which possesses good second-order nonlinear optical properties, has been obtained by mild in situ hydrothermal synthesis.

BaB4O5F4 with reversible phase transition featuring unprecedented fundamental building blocks of [B16O21F16] in the α-phase and [B4O6F4] in the β-phase

The first fluorooxoborate with reversible phase transition, BaB4O5F4, has been obtained. Interestingly, the two polymorphs, α- and β-BaB4O5F4 with rare [BOF3], feature various one-dimensional chains composed of the unprecedented fundamental building blocks of [B16O21F16] and [B4O6F4], respectively. First-principles calculations were performed to elucidate the structure-property relationships.

Na4SnS4 and Na4SnSe4 exhibiting multifunctional physicochemical performances as potential infrared nonlinear optical crystals and sodium ion conductors

Na₄SnS₄ and Na₄SnSe₄ exhibiting excellent physicochemical performances as potential IR NLO crystals and sodium ion conductors were systematically studied.

Homochiral three-dimensional noncentrosymmetric lanthanide coordination polymers directed by chiral linkers: syntheses, crystal structures, and optical properties

Novel enantiomorphic lanthanide–organic coordination polymers directed by the chiral organic linkers exhibit second-harmonic generation.

Pb2.28Ba1.72B10O19 featuring a three-dimensional B–O anionic network with edge-sharing [BO4] obtained under ambient pressure

The unique borate Pb_2.28Ba_1.72B₁₀O₁₉ containing a three-dimensional B–O anionic network with edge-sharing [BO₄] was obtained under ambient pressure, featuring an extremely large B–B interatomic distance (d_B–B) and small internal O–B–O angles (∠_{in(O–B–O)}).

Recent advances of oxyfluorides for nonlinear optical applications

Oxyfluorides exhibit rich crystal structures that provide a new perspective for designing new NLO materials.

Pressure-dependent modifications in the optical and electronic properties of Fe(IO3)3: the role of Fe 3d and I 5p lone–pair electrons

The electronic and transport properties of Fe(IO3)3 have been characterized under compression. A nice correlation of bandgaps of iodates to orbital configuration is proposed giving an explanation for the 2.1 eV bandgap of Fe(IO3)3.

NaKB6O9F2: a new complex alkali metal fluorooxoborate with puckered layers

Introducing F atoms into the 3D borate framework is beneficial to synthesize layered fluorooxoborates, which ensures moderate birefringence.

AMnAs3S6 (A = Cs, Rb): Phase-Matchable Infrared Nonlinear Optical Functional Motif [As3S6]3- Obtained via Surfactant-Thermal Method

Exploration of a new nonlinear optical (NLO)-active functional motif is important in the rational design of promising infrared (IR) NLO materials. Compared with typical tetrahedral MQ₄ (M = IIB, III, IV metals; Q = S, Se) motifs, MQ₃ (M = As, Sb) pyramids favor high second-harmonic generation (SHG) efficiency while frequently hindering phase matching (PM) because of excessively large optical anisotropy. The surfactant-thermal method was first adopted to achieve PM in MQ₃-containing systems and synthesize mixed covalent-ionic IR NLO materials. Two new thioarsenates of AMnAs₃S₆ (A = Cs, Rb) exhibiting strong PM SHG efficiencies comparable to commercial AGS and laser-induced damage thresholds of one order higher than AGS were obtained. The [As₃S₆]^3- unit in their structures is an unprecedented NLO-active functional motif, which can be useful in designing new IR NLO compounds with large SHG efficiency. In addition, the surfactant-thermal method provides a new general strategy for synthesizing new IR NLO materials.

Ba(MoO2F)2(XO3)2 (X = Se and Te): First Cases of Noncentrosymmetric Fluorinated Molybdenum Oxide Selenite/Tellurite Through Unary Substitution for Enlarging Band Gaps and Second Harmonic Generation

Nonlinear optical (NLO) materials have critically important applications in advanced laser technologies. However, achieving a good balance between the mutual competing NLO properties and band gap within one molecular structure remains a great challenge. In this study, two alkaline earth metal fluorinated molybdenum oxide selenite/tellurite, Ba(MoO₂F)₂(XO₃)₂ [X = Se (BMFS) and Te (BMFT)], were synthesized through a facile unary substitution: BMFS was obtained by partial substitution of oxygen atoms with highly electronegative fluorine in the parent compound BaMo₂O₅(SeO₃)₂ (BMS), while BMFT was achieved by further replacing lone-pair Se⁴⁺ cations in BMFS with heavier Te⁴⁺ cations in the same main group. By partially replacing oxygen with fluorine, BMFS shows a broadened band gap and enhanced second harmonic generation (SHG) response compared to BMS owing to the high electronegativity of fluorine anions and the favorable orientation and alignment of NLO-active [MoO₅F]^5- and [SeO₃]^2- groups, which is relatively rare for unary anion substitution. BMFS and BMFT are isostructural and both belong to the polar space group Aba2, featuring a three-dimensional (3D) double-layered framework composed of 2D [MoO₄F(XO₃)]_∞ anionic layers interconnected by divalent barium cations. Both BMFS and BMFT exhibit good optical performance, including large SHG responses (3× and 4× KH₂PO₄), wide band gaps (3.30 and 3.27 eV) and optical transparency window, and high laser damage thresholds (60× and 53× AgGaS₂), demonstrating their potential applications as promising second-order NLO crystals. DFT calculations have elucidated the crucial role of the [MoO₅F]^5- groups in the enlarged band gaps and enhanced SHG responses in BMFS and BMFT. This work proposes a feasible unary substitution strategy for synthesizing the first polar fluorinated molybdenum oxide selenite/tellurite with synchronously enlarged band gaps and SHG efficiency.

From Ag2 Zr(IO3 )6 to LaZr(IO3 )5 F2 : A Case of Constructing Wide-band-gap Birefringent Materials through Chemical Cosubstitution

Two mixed-metal zirconium iodates were prepared and studied as a case of chemical cosubstitution. The structure of Ag₂ Zr(IO₃ )₆ (P2₁ /c) features 0D [Zr(IO₃ )₆ ]^2- anion group and 1D [Ag(IO₃ )₂ ]^- anionic chain, with the [Zr(IO₃ )₆ ]^2- anion groups interconnected by Ag⁺ ions into 3D network; LaZr(IO₃ )₅ F₂ (P2₁ /n) features 0D [Zr(IO₃ )₅ F₂ ]^3- anion group and 2D [La(IO₃ )₅ ]^2- anionic layer, with the [Zr(IO₃ )₅ F₂ ]^3- groups interlinked by La³⁺ ions into 3D structure. Notably, LaZr(IO₃ )₅ F₂ is the first zirconium iodate fluoride reported. Wide optical band gaps of 3.77 and 4.13 eV are given for Ag₂ Zr(IO₃ )₆ and LaZr(IO₃ )₅ F₂ , respectively. Theoretical calculations confirmed that the weak d-d transition of Zr⁴⁺ in the band structure leads to a moderate band gap of Ag₂ Zr(IO₃ )₆ , and the introduction of F^- into the zirconium iodate compound results in a large band gap of LaZr(IO₃ )₅ F₂ . Both of the compounds are birefringent materials with birefringences of 0.064 @1064 nm and 0.082 @1064 nm, respectively.

Cs3B3O3F6 with a Deep-Ultraviolet Cutoff Edge and a Suitable Birefringence as the Potential Zero-Order Waveplate Material

The zero-order waveplates, the essential materials in altering the polarization state of optical waves, are significant in polarimetry and the laser industry. Restricted by birefringence and absorption edge, few materials can have moderately small birefringence (Δn < 0.01) and a deep-ultraviolet (DUV; λ < 200 nm) transparency range. We used the B₃O₃F₆ unit for designing the zero-order waveplate material, which is composed of three BO₂F₂ tetrahedra interconnected by corner sharing. Here we obtained a new Cs₃B₃O₃F₆ compound with the isolated B₃O₃F₆ unit by a high-temperature solution method, which has the DUV cutoff edge and suitable birefringence (0.0069 at 532 nm) based on the experimental and computational results. This demonstrates that the isolated B₃O₃F₆ unit is a potential group, which favors the search for suitable compounds for DUV zero-order waveplate materials.

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

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

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

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

[O2Pb3]2(BO3)I: a new lead borate iodide with [O2Pb3] double chains

A new lead borate iodide, [O2Pb3]2(BO3)I, has been successfully synthesized by a facile hydrothermal reaction. It exhibits an infinite one-dimensional (1D) 1∞[O2Pb3] double chain which is constructed from OPb4 oxygen-centered tetrahedra. The 1∞[O2Pb3] double chains are further bridged by BO3 units via sharing oxygen atoms to form two-dimensional (2D) 2∞[(O2Pb3)2(BO3)] layers with the I- ions situated in the spaces between the layers. First principles calculations indicate that [O2Pb3]2(BO3)I exhibits a birefringence of about 0.072@1064 nm which originates from the synergistic contributions of the Pb-O/I and BO3 units. The IR, UV-vis-NIR and EDS results, as well as TG-DSC curves were also discussed in detail.

Bulk polarity of 3,5,7-trinitro-1-azaadamantane mediated by asymmetric NO2(lone pair)...NO2(π-hole) supramolecular bonding

Molecular crystals exhibiting polar symmetry are important paradigms for developing new electrooptical materials. Though accessing bulk polarity still presents a significant challenge, in some cases it may be rationalized as being associated with the specific molecular shapes and symmetries and subtle features of supramolecular interactions. In the crystal structure of 3,5,7-trinitro-1-azaadamantane, C₉H₁₂N₄O₆, the polar symmetry of the molecular arrangement is a result of complementary prerequisites, namely the C_3v symmetry of the molecules is suited to the generation of polar stacks and the inherent asymmetry of the principal supramolecular bonding, as is provided by NO₂(lone pair)...NO₂(π-hole) interactions. These bonds arrange the molecules into a trigonal network. In spite of the apparent simplicity, the structure comprises three unique molecules (Z' = 1/3 + 1/3 + 1/3), two of which are donors and acceptors of three N...O interactions and the third being primarily important for weak C-H...O hydrogen bonding. These distinct structural roles agree with the results of Hirshfeld surface analysis. A set of weak C-H...O and C-H...N hydrogen bonds yields three kinds of stacks. The orientation of the stacks is identical and therefore the polarity of each molecule contributes additively to the net dipole moment of the crystal. This suggests a special potential of asymmetric NO₂(lone pair)...NO₂(π-hole) interactions for the supramolecular synthesis of acentric materials.

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

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

Ba(IO3)F: An Alkaline-Earth-Metal Iodate Fluoride Crystal with Large Band Gap and Birefringence

An alkaline-earth-metal iodate fluoride, Ba(IO₃)F (1), was discovered by introducing F atoms into alkaline-earth-metal iodates via a hydrothermal method. The pseudo-two-dimensional (2D) [BaF]⁺ layers and [IO₃]^- anionic groups built the crystal structure, which obviously exhibited large anisotropies. The birefringence of 1 was initially tested to be 0.1256 at 589.3 nm. Besides, compared with most reported metal iodate crystals, 1 possesses a large band gap (E_g = 4.32 eV). Theoretical calculations proved that the excellent optical characteristics can be attributed to [IO₃]^- anionic groups and [BaO₄F₄]^10- polyhedra.

Novel ultraviolet (UV) nonlinear optical (NLO) materials discovered by chemical substitution-oriented design

Exploring novel functional materials via chemical substitution-oriented design is an emerging strategy. The method can be expanded to the discovery of high performance ultraviolet (UV) nonlinear optical (NLO) solid state materials by a careful tuning of the substituted atoms. This minireview presents a brief introduction to chemical substitution-oriented design including single-site substitution, dual-site substitution, and multisite substitution. Several state-of-the-art UV NLO materials such as K₃VO(O₂)₂CO₃-type, KBe₂BO₃F₂ (KBBF)-type, Ca₅(PO₄)₃(OH)-type, and KTiOPO₄ (KTP)-type phases successfully discovered by the chemical substitution method are discussed.

A brief introduction to chemical substitution-oriented design to discover high performance ultraviolet (UV) nonlinear optical (NLO) solid state materials is presented.