BiFSeO3: An Excellent SHG Material Designed by Aliovalent Substitution

[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.

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.

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.

Synthesis and Characterization of Two New Second Harmonic Generation Active Iodates: K3Sc(IO3)6 and KSc(IO3)3Cl

Transparent single crystals of two new iodates K₃Sc(IO₃)₆ and KSc(IO₃)₃Cl have been synthesized hydrothermally. Single-crystal X-ray diffraction was used to determine their crystal structures. Both compounds crystallize in non-centrosymmetric space groups. The compound K₃Sc(IO₃)₆ crystallizes in the orthorhombic space group Fdd2. The crystal structure is made up of [ScO₆] octahedra, [IO₃] trigonal pyramids, and [KO₈] distorted cubes. The compound KSc(IO₃)₃Cl crystallizes in the trigonal space group R3. The building blocks are [ScO₆] octahedra, [KO₁₂] polyhedra, and [IO₃] trigonal pyramids. The Cl^- ions act as counter ions and reside in tunnels in the crystal structure. The second harmonic generation (SHG) measurements at room temperature, using 1064 nm radiation, on polycrystalline samples show that the SHG intensities of K₃Sc(IO₃)₆ and KSc(IO₃)₃Cl are around 2.8 and 2.5 times that of KH₂PO₄ (KDP), respectively. In addition, K₃Sc(IO₃)₆ and KSc(IO₃)₃Cl are phase-matchable at the fundamental wavelength of 1064 nm. The large anharmonicity in the optical response of both compounds is further supported by an anomalous temperature dependence of optical phonon frequencies as well as their enlarged intensities in Raman scattering. The latter corresponds to a very large electronic polarizability.

Large optical polarizability causing positive effects on the birefringence of planar-triangular BO3 groups in ternary borates

The structure-property relationship of photoelectric functional materials has been recognized as a hot topic. The study of the inner link between the band gaps and birefringence of optical materials is extremely crucial for the design and creation of novel optical devices, but still remains rather unexplored. In this work, taking a series of borates with only planar-triangular BO₃ groups, α-/β-TM₃(BO₃)₂ (TM = Zn, Cd), Cd₂B₂O₅, and M₃(BO₃)₂ (M = Hg, Mg, Ca, Sr) as the research subject, the comprehensive relationship between their electronic structures and linear optical properties has been systematically investigated. Through combining experimental measurements and theoretical calculations, the effect of optical polarizability on the birefringence of these borates was clarified. Based on the present discussion, the relationship between the O (2p) bandwidth of the highest valence band and the HSE06 band gaps is opposite. Meanwhile, a method involving the determination of so-called optical permittivity Δε to evaluate the magnitude of birefringence Δn is found to be feasible. A large Δε makes a positive contribution to Δn. In addition, the experimentally measured band gaps and IR vibrations are in good agreement with theoretical results for the compounds α-Cd₃(BO₃)₂ and Cd₂B₂O₅.

Ba(B2OF3(OH)2)2 with well-ordered OH/F anions and a unique B2OF3(OH)2 dimer

A new centimetre-scale fluorooxoborate crystal Ba(B₂OF₃(OH)₂)₂, with well-ordered OH/F units and a new dimer containing BOF₃ units, has been fabricated.

The functionality of tetrahedral units in fluoroxoiodates: fluorine-modulating effects in iodates with lone pair electrons

The relationship between performance and properties can be modulated by the F element in iodates. This strategy greatly enhances the application of iodates in nonlinear optical crystals.

Two-stage evolution from phosphate to sulfate of new KTP-type family members as UV nonlinear optical materials through chemical cosubstitution-oriented design

KTiOPO4 (KTP) is a classic commercial nonlinear optical (NLO) crystal, but its narrow bandgap (3.52 eV) prevents its practical application in the ultraviolet (UV) region. Many trials to widen the narrow bandgap of KTP have failed in the past few decades. A chemical cosubstitution strategy was implemented to design new members of the KTP-type family as potential UV NLO materials. First, a novel centrosymmetric KTP-type compound NH4SbFPO4·H2O with a sharply enlarged bandgap (5.01 eV) was obtained through three-site aliovalent substitution. Second, the noncentrosymmetric NH4SbF2SO4 was synthesized by the introduction of more F- anions to destroy the crystal symmetry and SO42- to replace PO43- for balancing the charge in NH4SbFPO4·H2O, which realized the transformation from a visible phosphate system to solar blind UV sulfate system for KTP-type family NLO materials. The preliminary experimental results indicated that NH4SbF2SO4 is a promising solar blind UV NLO material. The first-principles calculations revealed that the sharply enlarged bandgap resulted from the substitution of the transition metal cations with the main group metal cations and the introduction of F- anions with high electronegativity.

Evidence of reaction intermediates in microwave-assisted synthesis of SHG active α-La(IO3)3nanocrystals

Highlighting the lanthanum iodates compounds and phase transformations through microwave-assisted hydrothermal syntheses.

From BiF3 to BiF3·H2O: diverse Bi(iii) coordination for structural transformation and birefringence regulation

BiF₃·H₂O with unprecedented BiF₉ building blocks was synthesized, which exhibits a near-isotropic Bi³⁺ coordination and inert lone pair activity.

Recent progress in selenite and tellurite based SHG materials

Fluorination and bandgaps have attracted more attention than d⁰ TM and SHG efficiency recently in metal selenites and tellurites.

From centrosymmetric to noncentrosymmetric: intriguing structure evolution in d10-transition metal iodate fluorides

Two new d10-transition metal iodate fluorides, centrosymmetric ZnIO3F and noncentrosymmetric CdIO3F, were successfully synthesized by intriguing structure evolution. Both compounds have higher thermal stability and CdIO3F has a large SHG response and wide band gap. Our study may afford a viable route to design materials with excellent NLO functions.

LiMg(IO3)3: an excellent SHG material designed by single-site aliovalent substitution

An excellent second harmonic generation (SHG) material, LiMg(IO₃)₃ (LMIO), has been elaborately designed from Li₂M^IV(IO₃)₆ (M^IV = Ti, Sn, and Ge) by aliovalent substitution of the central M^IV cation followed by Wyckoff position exchange. The new structure sustains the ideal-alignment of (IO₃)^- groups. Importantly, LMIO exhibits an extremely strong SHG effect of roughly 24 × KH₂PO₄ (KDP) under 1064 nm laser radiation or 1.5 × AgGaS₂ (AGS) under 2.05 μm laser radiation, which is larger than that of α-LiIO₃ (18 × KDP). The replacement of M^IV with Mg²⁺ without d-d electronic transitions induces an obviously larger band gap (4.34 eV) with a short absorption edge (285 nm). This study shows that single-site aliovalent substitution provides a new synthetic route for designing SHG materials.

Exploration of New Birefringent Crystals in Bismuth d0 Transition Metal Selenites

The first examples of bismuth fluoride selenites with d⁰ -TM/Te^VI polyhedrons, namely, Bi₄ TiO₂ F₄ (SeO₃ )₄ (1), Bi₄ NbO₃ F₃ (SeO₃ )₄ (2), Bi₄ TeO₄ F₂ (TeO₃ )₂ (SeO₃ )₂ (3), Bi₂ F₂ (MoO₄ )(SeO₃ ) (4) and Bi₂ ZrO₂ F₂ (SeO₃ )₂ (5) have been successfully synthesized under hydrothermal reactions by aliovalent substitution. The five new compounds feature three different types of structures. Compounds 1-3, containing Ti^IV , Nb^V and Te^VI respectively, are isostructural, exhibiting a new 3D framework composed of a 3D bismuth oxyfluoride architecture, with intersecting tunnels occupied by d⁰ -TM/Te^VI octahedrons and selenite/tellurite groups. Interestingly, compound Bi₄ TeO₄ F₂ (TeO₃ )₂ (SeO₃ )₂ (3) is the first structure containing Se^IV and mixed-valent Te^IV /Te^VI cations simultaneously. Compound 4 features a new 3D structure formed by a 3D bismuth oxyfluoride network with MoO₄ tetrahedrons and selenites groups imbedded in the 1D tunnels. Compound 5 displays a novel pillar-layered 3D open framework, consisting of 2D bismuth oxide layers bridged by the [ZrO₂ F₂ (SeO₃ )₂ ]^6- polyanions. Theoretical calculations revealed that the five compounds displayed very strong birefringence. The birefringence values of compounds 1-3, especially, are above 0.19 at 1064 nm, which are larger than the mineral calcite. Based on the structure and property analysis, it was found that the asymmetric SeO₃ groups (and TeO₃ in compound 3) displayed the largest anisotropy, compared with the bismuth cations and the d⁰ -TM/Te polyhedra, which is beneficial to the birefringence.

Pb2Cd(SeO3)2X2 (X = Cl and Br): two halogenated selenites with phase matchable second harmonic generation

The second harmonic generation (SHG) efficiencies of Pb₂Cd(SeO₃)₂X₂ (X = Cl and Br) are higher than that of commercial KDP (KH₂PO₄) and their laser damage thresholds are 30 times more than that of AGS (AgGaS₂).

Rb3SbF3(NO3)3: an excellent antimony nitrate nonlinear optical material with a strong second harmonic generation response fabricated by a rational multi-component design

Rb₃SbF₃(NO₃)₃ fabricated by a rational multi-component design exhibits a strong phase-matching SHG response and a wide transmission window.

From centrosymmetric to noncentrosymmetric: cation-directed structural evolution in X3ZnB5O10 (X = Na, K, Rb) and Cs12Zn4(B5O10)4 crystals

A new Cs₁₂Zn₄(B₅O₁₀)₄ compound exhibits a short DUV cutoff edge (lower than 185 nm) and contains a 3D [ZnB₅O₁₀]_∞ network.

Pb7F12Cl2: a promising infrared nonlinear optical material with high laser damage threshold

This study reports, for the first time, the second-harmonic generation (SHG) property of Pb₇F₁₂Cl₂ as a promising infrared nonlinear optical (NLO) material.

M2(SeO3)F2 (M = Zn, Cd): understanding the structure directing effect of [SeO3]2− groups on constructing ordered oxyfluorides

The structure directing effect of [SeO₃]²⁻ anionic groups play a crucial role in the configuration of ordered [MO₃F₃] octahedral and its large anisotropy.

Pb2GaF2(SeO3)2Cl: Band Engineering Strategy by Aliovalent Substitution for Enlarging Bandgap while Keeping Strong Second Harmonic Generation Response

Wide bandgap and strong second-order generation (SHG) effect are two crucial but contradictory conditions for practical nonlinear optical (NLO) materials. Herein, a new NLO crystal Pb₂GaF₂(SeO₃)₂Cl (I) containing novel functional (GaO₃F₃)^6- octahedra is designed and synthesized by a rational band engineering strategy with aliovalent substitution. Benefiting from the removal of transition metal cations and the introduction of bridged F anions, I exhibits the widest bandgap among all reported phase-matchable NLO selenites. Meanwhile, a strong SHG response more than 4.5 times of KH₂PO₄ (KDP) is maintained. The dominate role of the (GaO₃F₃)^6- groups to the enlarged bandgap in I are elucidated by first-principles studies.

Synthesis and structural variety of first Mn and Bi selenites and selenite chlorides

Single crystals of new Mn2[Bi2O](SeO3)4 (I), MnBi(SeO3)2Cl (II), MnIIMnIII(SeO3)2Cl (III), Mn5(SeO3)2Cl6 (IV), and Mn4(Mn5,Bi)(SeO3)8Cl5 (V) have been synthesized by chemical vapour transport and hydrothermal methods. They have been structurally characterized by single crystal X-ray diffraction analysis. The compounds II–V are the first Mn selenite chlorides, while the I, II and V compounds are the first Bi-containing Mn oxoselenites. Structural relationships of the new phases with other compounds are discussed. An overview of the mixed-ligand MnO m Cl n polyhedra in inorganic compounds is given.

Synthesis and characterization of a series of transition metal oxychlorides: MBi(SeO3)2(H2O)Cl (M = Co, Ni, Cu)

Three new transition metal oxychlorides MBi(SeO3)2(H2O)Cl (M = Co, Ni, Cu) have been firstly synthesized through a hydrothermal reaction method at 200 °C. They were structurally determined to be isostructural with the Pbca space group of the orthorhombic system. They feature a 3D framework topology with two-dimensional tunnels intersected and filled with Cl anions parallel to the crystallographic bc-plane. Two neighboring MO5Cl octahedra are connected by a sharing edge into a M2O8Cl2 dimer which serves as the structural knots to knit the 2D [Bi(SeO3)2]∞ layers in the ab-plane together into the total 3D crystal architecture. The optical band gaps of CoBi(SeO3)2(H2O)Cl (1), NiBi(SeO3)2(H2O)Cl (2) and CuBi(SeO3)2(H2O)Cl (3) were evaluated to be 3.7 eV, 3.5 eV and 3.2 eV, respectively, through extrapolating the UV-vis-NIR optical absorption spectra. Besides, the spin-allowed d-d transition absorption spectra of the transition ion centers are observed in three compounds exhibiting different colors. Compounds 1, 2 and 3 can stay thermally stable below 370 °C, 400 °C and 300 °C, respectively, as found from the results of thermal analysis based on the simultaneous thermogravimetry and differential scanning calorimetry techniques. 1, 2 and 3 exhibit antiferromagnetic properties below Néel temperatures 6 K, 18 K and 52 K, respectively. Above the Néel temperatures, Curie-Weiss behavior dominates in the M-T process for the three compounds. The cell parameters are listed: a = 14.056 Å, b = 7.582 Å, c = 14.996 Å, and Z = 8 for 1, a = 14.083 Å, b = 7.575 Å, c = 14.860 Å, and Z = 8 for 2, and a = 14.576 Å, b = 7.371 Å, c = 14.656 Å, and Z = 8 for 3.

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

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

Advantageous Units in Antimony Sulfides: Exploration and Design of Infrared Nonlinear Optical Materials

The exploration of infrared (IR) nonlinear optical (NLO) materials remains attractive because of the urgent requirements in the laser field. Meanwhile, the deepened cognition of structure-property relationships is necessary to help guide the exploration of IR NLO materials. So far, the family of antimony sulfides is an important system with a lot of attention, and a series of antimony sulfides are reported. However, it is urgent to reveal how different Sb-S units, like SbS₃, SbS₄, and more complex combinations, affect apparent properties. Here, taking ternary metal antimony sulfides as examples, the sources of some essential optical properties, such as second harmonic generation (SHG) and birefringence, are systematically analyzed through first-principles calculations, and the mechanisms of the performances with various magnitudes are also presented to clarify the structure-property relationships. The results indicate that the SbS₄ unit among antimony sulfides is an advantageous NLO-active unit, which can balance the contradiction between the band gap and SHG response. Introduction of transition metals in the Sb-S anionic frameworks can tune the magnitude of birefringence. Besides, the substitution of a cation from a transition metal to an alkali metal can notably enlarge the band gap and maintain a large SHG response. These design strategies are beneficial to explore potential IR NLO materials with Sb-S units.

Rational Design of the First Lead/Tin Fluorooxoborates MB2O3F2 (M = Pb, Sn), Containing Flexible Two-Dimensional [B6O12F6]∞ Single Layers with Widely Divergent Second Harmonic Generation Effects

Molecular engineering design is a productive atomic-scale strategy to optimize crystal structure and develop new functional materials. Herein, the first lead/tin fluorooxoborates, MB₂O₃F₂ (M = Pb, Sn), were rationally designed by employing the nonlinear optical crystal Sr₂Be₂B₂O₇ (SBBO) as a parent model. Compared with the rigid [Be₆B₆O₁₅]_∞ double layers in SBBO, MB₂O₃F₂ have flexible two-dimensional [B₆O₁₂F₆]_∞ single layer, which not only keeps the NLO-favorable layered structure but also overcomes the structural instability issues of SBBO. Both compounds exhibited desired short UV cutoff edge. Interestingly, MB₂O₃F₂ exhibit widely divergent second harmonic responses, although they are isostructural and both contain stereochemically active lone-pair cations. Our first-principles calculations revealed that the SHG difference is mainly attributed to the different anisotropies of Pb and Sn SHG-active orbitals, which make constructive and destructive contributions to the SHG effects in PbB₂O₃F₂ and SnB₂O₃F₂, respectively.