Electronic, Crystal Chemistry, and Nonlinear Optical Property Relationships in the Dugganite A3B3CD2O14 Family

Structural characterisation and optical properties of new alkaline earth phosphate CaMg(P4O12) and alkaline alumophosphate Cs3Al4(PO4)5

Two new family members of mixed alkali-earth metal phosphate and aluminophosphate CaMg(P4O12) and Cs3Al4(PO4)5 were prepared from a phosphate system using a high-temperature solution method. The structural analysis results show that two compounds crystallize in the monoclinic space group C2/c and P21/n and feature a three-dimensional (3D) network. The 3D structure of CaMg(P4O12) consists of [CaO6], [MgO6] octahedra and [P4O12] rings, in which the [MgO6] and [P4O12] rings link to form a 3D structure and Ca2+ cations are filled within the structure. Interestingly, for compound Cs3Al4(PO4)5, its structure features 4, 8, and 12-ring channels with [Al2O4O4P2O4], [Al4O8O8P4O8] and [Al6O12O12P6O12] units as BBUs, respectively; the Cs+ cations are located in the cavities. Furthermore, IR spectral analysis and thermal properties are discussed. UV-vis-NIR diffuse reflectance spectroscopy data show that the UV cutoff edges of CaMg(P4O12) are below 200 nm. Remarkably, in order to determine optical properties and the structure-properties relationship, theoretical calculations were adopted. Electronic structure calculations demonstrate that CaMg(P4O12) has an indirect band gap with the value of 5.86 eV, and Cs3Al4(PO4)5 has a direct band gap of 5.21 eV.

[C(NH2)3][B(C2O2H4)2]: An Organic-Inorganic Hybrid Borate Containing Nonlinear-Optical Active Unit [B(C2O2H4)2]- with Solar-Blind-Region Optical Nonlinearity

We herein report an unprecedented organic-inorganic hybrid borate incorporating a novel nonlinear-optical (NLO) active unit, namely, [C(NH2)3][B(C2O2H4)2]. The novel NLO active unit was derived from the condensation reaction between two glycol molecules and one (BO4)5- group. The title compound exhibits a moderate second-harmonic-generation effect (0.7 × KDP), a significant band gap (5.76 eV), and a suitable birefringence (0.078 at 550 nm). The optical properties are determined by the synergistic interaction between the C(NH2)3+ cation and the [B(C2O2H4)2]- group, as indicated by theoretical calculations.

[C(NH2)3]6Mo7O24: A Guanidinium Molybdate as a UV Nonlinear Optical Crystal with Large Birefringence

The key to searching novel nonlinear optical (NLO) crystals was effectively combining the NLO-active units to obtain a noncentrosymmetric structure. Nevertheless, the present predicament lies in the growing challenge of discovering novel crystals within conventional inorganic frameworks that surpass the properties of the current NLO materials. In view of this, researchers expanded their research focus to the organic-inorganic hybridization system; it is foreseeable to concentrate the advantages from several kinds of NLO-active units to acquire novel NLO crystals with superior properties. We herein report an organic-inorganic hybrid molybdate crystal, namely, [C(NH2)3]6Mo7O24 (GMO). It was successfully obtained via combining inorganic NLO-active MoO6 octahedra and organic π-conjugated [C(NH2)3]+ groups. GMO demonstrates a moderate second-harmonic-generation response, specifically measuring about 1.3 times the value of KDP. Additionally, it exhibits a significant birefringence value of 0.203 at the wavelength of 550 nm and possesses a wide band gap of 3.31 eV. Theoretical calculations suggest that the optical properties of the GMO are primarily influenced by the synergy effect of [C(NH2)3]+ groups between MoO6 octahedra.

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.

Cd2 Nb2 Te4 O15 : A Novel Pseudo-Aurivillius-Type Tellurite with Unprecedented Nonlinear Optical Properties and Excellent Stability

Oxides are emerging candidates for mid-infrared (mid-IR) nonlinear optical (NLO) materials. However, their intrinsically weak second harmonic generation (SHG) effects hinder their further development. A major design challenge is to increase the nonlinear coefficient while maintaining the broad mid-IR transmission and high laser-induced damage threshold (LIDT) of the oxides. In this study, it is reported on a polar NLO tellurite, Cd2 Nb2 Te4 O15 (CNTO), featuring a pseudo-Aurivillius-type perovskite layered structure composed of three types of NLO active groups, including CdO6 octahedra, NbO6 octahedra, and TeO4 seesaws. The uniform orientation of the distorted units induces a giant SHG response that is ≈31 times larger than that of KH2 PO4 , the largest value among all reported metal tellurites. Additionally, CNTO exhibits a large band gap (3.75 eV), a wide optical transparency window (0.33-14.5 µm), superior birefringence (0.12@ 546 nm), high LIDT (23 × AgGaS2 ), and strong acid and alkali resistance, indicating its potential as a promising mid-IR NLO material.

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.

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.

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.

NaRb6(B4O5(OH)4)3(BO2) Featuring Noncentrosymmetry, Chirality, and the Linear Anionic Group BO2. J Am Chem Soc 2023;145:4928-4933. [PMID: 36811389 DOI: 10.1021/jacs.2c12069]

Noncentrosymmetric (NCS) structures are of particular interest owing to their symmetry-dependent physical properties, e.g., pyroelectricity, ferroelectricity, piezoelectricity, and nonlinear optical (NLO) behavior. Among them, chiral materials exhibit polarization rotation and host topological properties. Borates often contribute to NCS and chiral structures via their triangular [BO₃] and tetrahedral [BO₄] units and their numerous superstructure motifs. However, no chiral compound with the linear [BO₂] unit has been reported to date. Herein, an NCS and chiral mixed-alkali-metal borate, NaRb₆(B₄O₅(OH)₄)₃(BO₂), with a linear BO₂^- unit in the structure was synthesized and characterized. The structure features a combination of three types of basic building units (BBUs), [BO₂], [BO₃], and [BO₄] with sp-, sp²-, and sp³-hybridization of boron atoms, respectively. It crystallizes in the trigonal space group R32 (No. 155), one of the 65 Sohncke space groups. Two enantiomers of NaRb₆(B₄O₅(OH)₄)₃(BO₂) were found, and their crystallographic relationships are discussed. These results not only expand the small family of NCS structures with the rare linear BO₂^- unit but also prompt recognition to the fact that NLO materials have generally overlooked the existence of two enantiomers in achiral Sohncke space groups.

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.

Pb5(GeO4)(Ge2O7) and Pb3.32Ca1.68(GeO4)(Ge2O7): Two Nonlinear Optical Germanates Induced by Diverse PbOx Polyhedra

Oxide nonlinear optical (NLO) crystals have drawn wide interest for their comprehensive physical performances including wide infrared (IR) transparency ranges, large band gaps, and good stability in open air. Here, two isostructural germanate oxides, Pb₅(GeO₄)(Ge₂O₇) (1) and Pb_3.32Ca_1.68(GeO₄)(Ge₂O₇) (2), adopting the noncentrosymmetric (NCS) space group P6̅, were composed via a conventional solid-state reaction. The latter was designed by the partial cation substitution strategy based on parent 1. The whole structures of 1 and 2 are composed of isolated distorted GeO₄ tetrahedra, Ge₂O₇ dimers, and diverse M (M = Pb, Ca, or Pb/Ca)-centered polyhedra. They exhibit second-harmonic generation (SHG) responses around 3.3 and 1.4 times that of KH₂PO₄ (KDP) under 1.064 μm laser radiation, respectively. Theoretical calculation results reveal that the Pb²⁺ cations with stereo-active long pair (SCALP) electrons of 1 favor the large SHG response, while Pb-based polyhedra showing inert SCALP electrons make predominant contributions to the moderate SHG effect of 2.

AgGaGeSe4: An Infrared Nonlinear Quaternary Selenide with Good Performance

The symmetry of crystals is an extremely important property of crystals. Crystals can be divided into centrosymmetric and non-centrosymmetric crystals. In this paper, an infrared (IR) nonlinear optical (NLO) material AgGaGeSe4 was synthesized. The related performance analysis, nonlinear optical properties, and first-principle calculation of AgGaGeSe4 were also introduced in detail. In the AgGaGeSe4 structure, Ge4+ was replaced with Ga3+ and produced the same number of vacancies at the Ag+ position. The low content of Ge doping kept the original chalcopyrite structure and improved its optical properties such as the band gap. The UV-Vis diffuse reflection spectrum shows that the experimental energy band gap of AgGaGeSe4 is 2.27 eV, which is 0.48 eV larger than that of AgGaSe2 (1.79 eV). From the perspective of charge-transfer engineering strategy, the introduction of Group IV Ge elements into the crystal structure of AgGaSe2 effectively improves its band gap. The second harmonic generation (SHG) effect of AgGaGeSe4 is similar to that of AgGaSe2, and at 1064 nm wavelength, the birefringence of AgGaGeSe4 is 0.03, which is greater than that of AgGaSe2 (∆n = 0.02). The results show that AgGaGeSe4 possessed better optical properties than AgGaSe2, and can been broadly applied as a good infrared NLO material.

Lead Tellurite Crystals BaPbTe2O6 and PbVTeO5F with Large Nonlinear-/Linear-Optical Responses due to Active Lone Pairs and Distorted Octahedra

The exploration of nonlinear-/linear-optical crystal materials with high performance is an extremely difficult research project. Herein, the two new lead tellurite crystals BaPbTe₂O₆ and PbVTeO₅F were successfully obtained through a facile hydrothermal synthesis strategy. BaPbTe₂O₆ lies in the noncentrosymmetric (NCS) and chiral orthorhombic space group P2₁2₁2₁, featuring a unique _∞¹[PbTe₂O₆] chain consisting of the PbO₄ and TeO₃ building units, while PbVTeO₅F belonging to the centrosymmetric (CS) orthorhombic space group Pbca manifests a 2D layer made up of _∞¹[PbO₄F₂] chains and novel [V₂Te₂O₁₀F₂] clusters. Further, a systematic analysis of lead tellurites finds that the coordination geometries of the Pb atom exert a considerable influence on the connection modes of Pb-O and Te-O building units. BaPbTe₂O₆ shows a great second-harmonic-generation (SHG) effect of ∼5× the benchmark KH₂PO₄ (KDP) and a large optical birefringence of 0.086 at 590 ± 3 nm. PbVTeO₅F demonstrates a remarkably larger birefringence of 0.142 at 590 ± 3 nm, benefiting from the introduction of the VO₅F octahedral unit. Theoretical studies reveal that the large SHG and birefringence in BaPbTe₂O₆ can be attributed to TeO₃ and PbO₄ polyhedra with active lone pairs, while the remarkably enlarged birefringence in PbVTeO₅F is attributable to the highly distorted octahedral VO₅F. The functional orientations of active building units may offer a practical insight into the design of the desired optical functional materials.

HgTeO2F(OH): A Nonlinear Optical Oxyfluoride Constructed of Active [TeO2F(OH)]2- Pyramids and V-Shaped [HgO2]2- Groups

Oxyhalides possessing the merits of oxides and halides have widely received attention for their comprehensive physical performances, especially as potential nonlinear optical (NLO) crystals. Here, based on conventional strategy for obtaining acentric compounds, a Te⁴⁺ lone-pair cation was introduced into oxyhalides, and one oxyfluoride, HgTeO₂F(OH), was obtained via a hydrothermal reaction. Crystallized in the polar space group Pca2₁, the layered structure of HgTeO₂F(OH) is composed of V-shaped [HgO₂]^2- groups and [TeO₂F(OH)]^2- pyramids, in which the [TeO₂F(OH)]^2- pyramid first served as the NLO functional motif. Its powder sample exhibits a phase-matchable SHG response of 1.1 × KH₂PO₄ at 1064 nm, and its birefringence (0.09@1064 nm) is sufficient for phase-matchable behavior, which manifests its comprehensive capacity as a promising NLO candidate. Theoretical calculations about electronic structure and optical properties are also carried out, revealing that the Te⁴⁺ lone-pair cation makes the predominant contribution to the SHG effect and synergizes with the [HgO₂]^2- groups.

Wide Bandgaps and Strong SHG Responses of Hetero-Oxyfluorides by Dual-Fluorination-Directed Bandgap Engineering

A wide bandgap is an essential requirement for a nonlinear optical (NLO) material. However, it is very challenging to simultaneously engineer a wide bandgap and a strong second-harmonic generation (SHG) response, particularly in NLO materials containing second-order Jahn–Teller (SOJT) distorted units. Herein, we employ a bandgap engineering strategy that involves the dual fluorination of two different types of SOJT distorted units to realize remarkably wide bandgaps in the first examples of 5d⁰-transition metal (TM) fluoroiodates. Crystalline A₂WO₂F₃(IO₂F₂) (A = Rb (RWOFI) and Cs (CWOFI)) exhibit the largest bandgaps yet observed in d⁰-TM iodates (4.42 (RWOFI) and 4.29 eV (CWOFI)), strong phase-matching SHG responses of 3.8 (RWOFI) and 3.5 (CWOFI) × KH₂PO₄, and wide optical transparency windows. Computational studies have shown that the excellent optical responses result from synergism involving the two fluorinated SOJT distorted units ([WO₃F₃]³⁻ and [IO₂F₂]⁻). This work provides not only an efficient strategy for bandgap modulation of NLO materials, but also affords insight into the relationship between the electronic structure of the various fluorinated SOJT distorted units and the optical properties of crystalline materials.

Wide bandgaps, strong SHG responses, and sufficient birefringence are observed in the first examples of 5d⁰-transition metal fluoroiodates, A₂WO₂F₃(IO₂F₂) (A = Rb, and Cs), which were constructed by dual-fluorination-directed bandgap engineering.

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...

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...

A Congruent-Melting Mid-Infrared Nonlinear Optical Vanadate Exhibiting Strong Second-Harmonic Generation

Study of mid-infrared (mid-IR) nonlinear optical (NLO) materials is hindered by the competing requirements of optimized second-harmonic generation (SHG) coefficient d_ij and laser-induced damage threshold (LIDT) as well as the harsh synthetic conditions. Herein, we report facile hydrothermal synthesis of a polar NLO vanadate Cs₄ V₈ O₂₂ (CVO) featuring a quasi-rigid honeycomb-layered structure with [VO₄ ] and [VO₅ ] polyhedra aligned parallel. CVO possesses a wide IR-transparent window, high LIDT, and congruent-melting behavior. It has very strong phase-matchable SHG intensities in metal vanadate family (12.0 × KDP @ 1064 nm and 2.2 × AGS @ 2100 nm). First-principles calculations suggest that the exceptional SHG responses of CVO largely originate from virtual electronic transitions within [V₄ O₁₁ ]_∞ layer; the excellent optical transmittance of CVO arises from the special characteristics of vibrational phonons resulting from the layered structure.

Centrosymmetric or Noncentrosymmetric? Transition Metals Talking in K2TGe3S8(T = Co, Fe)

Two new quaternary sulfides K₂TGe₃S₈(T = Co, Fe) have been synthesized by a high-temperature solid-state routine and flux growth method. The crystal growth process of K₂TGe₃S₈(T = Co, Fe) was elucidated by in situ powder X-ray diffraction and DSC thermal analysis. The millimeter-sized crystals of K₂TGe₃S₈(T = Co, Fe) were grown. K₂CoGe₃S₈ crystallizes in a new structure type in centrosymmetric space group P1 (no. 2) with unit cell parameters of a = 7.016(1) Å, b= 7.770(1) Å, c = 14.342(1) Å, α = 93.80(1)°, β = 92.65(1)°, γ = 114.04(1)°. K₂FeGe₃S₈ crystallizes in the K₂FeGe₃Se₈ structure type and the noncentrosymmetric space group P2₁ (no. 4) with unit cell parameters of a = 7.1089(5)Å, b = 11.8823(8) Å, c = 16.7588(11) Å, β = 96.604(2)°. There is a high structural similarity between K₂CoGe₃S₈ and K₂FeGe₃S₈. The larger volume coupled with higher degrees of distortion of the [FeS₄] tetrahedra compared to the [CoS₄] tetrahedra accounts for the structure's shift from centrosymmetric to noncentrosymmetric. The theory simulation confirms that [TS₄]_{T= Co or Fe} tetrahedra play a crucial role in controlling the structure and properties of K₂TGe₃S₈(T = Co, Fe). The measured optical bandgaps of K₂CoGe₃S₈ and K₂FeGe₃S₈ are 2.1(1) eV and 2.6(1) eV, respectively. K₂FeGe₃S₈ shows antiferromagnetic ordering at 24 K while no magnetic ordering was detected in K₂CoGe₃S₈. The magnetic measurements also demonstrate the divalent nature of transition metals in K₂TGe₃S₈(T = Co, Fe).

UV Solar-Blind-Region Phase-Matchable Optical Nonlinearity and Anisotropy in a π-Conjugated Cation-Containing Phosphate

Wide ultraviolet (UV) transparency, strong second-harmonic generation (SHG) response, and sufficient optical birefringence for phase-matching (PM) at short SHG wavelengths are vital for practical UV nonlinear optical (NLO) materials. However, simultaneously optimizing these properties is a major challenge, particularly for metal phosphates. Herein, we report a non-traditional π-conjugated cation-based UV NLO phosphate [C(NH₂ )₃ ]₆ (PO₄ )₂ ⋅3 H₂ O (GPO) with a short UV cutoff edge. GPO is SHG active at 1064 nm (3.8 × KH₂ PO₄ @ 1064 nm) and 532 nm (0.3 × β-BaB₂ O₄ @ 532 nm) and also possesses a significant birefringence (0.078 @ 546 nm) with a band gap >6.0 eV. The PM SHG capability of GPO can extend to 250 nm, indicating GPO is a promising UV solar-blind NLO material. Calculations and crystal structure analysis show that the rare coexistence of wide UV transparency, large SHG response, and optical anisotropy is due to the introduction of π-conjugated cations [C(NH₂ )₃ ]⁺ and their favorable arrangement with [PO₄ ]^3- anions.

Cation Substitution of Hexagonal Triple Perovskites: A Case in Trimetallic Tellurates A2A'BTe2O9

Three new tellurates, namely, K₂BaPb_0.62Ba_0.38Te₂O₉ (1), Rb₂BaPb_0.7Ba_0.3Te₂O₉ (2), and Ba₂KLiTe₂O₉ (3), with hexagonal triple-perovskite structures have been designed via a cation substitution strategy. All three centrosymmetric compounds crystallize in the same hexagonal space group P6₃/mmc. In their structures, the [BTe₂O₉] (B = Pb/Ba for 1 and 2 and Li for 3) groups construct the final three-dimensional framework using face-sharing [Te₂O₉] dimmers and vertex-linked [BO₆] linkages. Meanwhile, the theoretical calculations demonstrate the numerical differences of their birefringence. Thermal stability analysis and differential scanning calorimetry were performed, and UV-vis-NIR diffuse reflectance spectra and infrared spectra of the three title compounds were also characterized and analyzed.

Titanium Iodates with a Second-Harmonic-Generation Response

Nonlinear-optical (NLO) crystal is an important photoelectric functional material. In this work, a new NLO titanium iodate, (H₃O)₂Ti(IO₃)₆, along with Ti(IO₃)₄ has been synthesized under facile conditions. The space group of (H₃O)₂Ti(IO₃)₆ is the chiral noncentrosymmetric group R3 (No. 146), with an interesting three-dimensional framework, while that of Ti(IO₃)₄ is the centrosymmetric space group P1̅ (No. 2) containing one-dimensional chains. Thermogravimetric analysis shows that (H₃O)₂Ti(IO₃)₆ and Ti(IO₃)₄ have no weight loss below 220 and 390 °C, respectively. In addition, (H₃O)₂Ti(IO₃)₆ not only is thermally stable up to 200 °C in an air atmosphere but also is stable in water. (H₃O)₂Ti(IO₃)₆ has a moderate-intensity second-harmonic-generation (SHG) response (1.4×KDP), a large laser-induced damage threshold (46×AgGaS₂), and high transmittance in the wavelength ranges of 0.5-1.4 and 2.5-10 μm. Both local dipole moment and systematic theoretical calculations reveal that the SHG response of (H₃O)₂Ti(IO₃)₆ is mainly because of the combined effect of [TiO₆] octahedra and IO₃ and IO₄ units. In a word, (H₃O)₂Ti(IO₃)₆ exhibits good NLO performances, as well as water resistance and facile growth of a single crystal with high quality, indicating its potential application as NLO materials in the visible and mid-IR regions, especially the visible region.

Y2(Te4O10)(SO4): a new sulfate tellurite with a unique Te4O10 polyanion and large birefringence

Y₂(Te₄O₁₀)(SO₄), featuring a unique (Te₄O₁₀)⁴⁻ polyanion, exhibits a large birefringence (0.124@1064), wide energy band gap (4.1 eV) and high thermal stability (>700 °C), which make it a potential birefringent material.

Sn3B8O15: A Ternary Tin(II) Borate with Flexible [B8O18]12- Fundamental Building Block Formed by [B7O16]11- and [BO3]3- Groups

Owing to the tendency to form the glass state in growing crystals in the ternary tin(II) borate system, hitherto, very few single crystals in ternary tin(II) borate were reported. In this paper, a single crystal of Sn₃B₈O₁₅ was obtained via a high temperature solution method in a closed system. The structure features a flexible fundamental building block [B₈O₁₈]^12- constructed by one [BO₃]^3- unit and one [B₇O₁₆]^11- group that contains three hexatomic rings, which further connect to compose a two-dimensional ²_∞[B₈O₁₅] layer structure. The connection mode and flexibility of the three hexatomic B-O rings were analyzed, verifying the structural diversity of the B-O configuration. Moreover, the synthesis, crystal structure, and various characterizations were comprehensively presented. Also, theoretical calculations were employed to discuss structure-property relationships, and we use the real-space atom-cutting techniques to explain the origin of the birefringence of Sn₃B₈O₁₅.

Tuning the Langasite, La3SbZn3Si2O14, towards white light emission: synthesis, structure, SHG and photoluminescence studies

A series of single-phase color-tunable Langasite, La3SbZn3Si2O14:RE3+ (RE = Eu, Tb, and Tm) phosphors have been prepared and characterized employing X-ray diffraction (XRD), Raman, and photoluminescence (PL) studies. The substituted compounds, La3SbZn3Si2O14:Eu3+, La3SbZn3Si2O14:Tb3+ and La3SbZn3Si2O14:Tm3+ exhibit the characteristic emissions of Eu3+ (red), Tb3+ (green) and Tm3+ (blue), respectively. By carefully adjusting the concentration of Eu3+ ions in the La3SbZn3Si2O14:1% Tm3+, 2% Tb3+, x% Eu3+ system, we achieved a white emission with a reasonable quantum yield. The compounds are found to be SHG (second-harmonic generation) active with the highest values of 3.6 × KDP (potassium dihydrogen phosphate). In addition, the compounds exhibit reasonable dielectric constants with low loss. The present study demonstrates the color tunability of the Langasite family of compounds and establishes it as an adaptable structure.

Synthesis, Structure, and Characterization of d0 Transition-Metal Iodate: BaTi(IO3)6·0.5H2O

A new noncentrosymmetric (NCS) d⁰ transition-metal (TM) iodate BaTi(IO₃)₆·0.5H₂O has been successfully synthesized by the typical hydrothermal method. The single-crystal X-ray diffraction shows that BaTi(IO₃)₆·0.5H₂O crystallizes in the polar trigonal space group R3 (no. 146) with a = b = 11.1920(12) Å, c = 24.006(3) Å. The reported compound features zero-dimensional (0D) [Ti(IO₃)₆]^2- clusters that are separated by Ba²⁺ cations and bound water in the structure. By comparing BaTi(IO₃)₆·0.5H₂O with the centrosymmetric (CS) α- and β-BaTi(IO₃)₆ that were reported before, it can be found that the bound water plays an important role for the formation of NCS structure for BaTi(IO₃)₆·0.5H₂O. In addition, based on the powder second harmonic (SHG) measurement, BaTi(IO₃)₆·0.5H₂O can achieve type-I phase matching with a moderate SHG effect (∼1 × KDP). The source of SHG effect was analyzed by dipole moment calculation and structure comparison. UV-vis-NIR diffuse reflectance spectroscopy, infrared spectroscopy, and thermogravimetric measurements are also presented.

A novel approach to the structural distortions of U/Th snub-disphenoids and their control on zircon → reidite type phase transitions of U1-x Th x SiO4

Coffinite (USiO₄) and thorite (ThSiO₄) are conspicuous radiogenic silicates in the geonomy. They form U_1-x Th _x SiO₄ (uranothorite) solid solutions in zircon-type phase. Investigating the phase-evolution of these minerals is of utmost significance in realizing their applicability in the front-as well as at the back-end of nuclear industries. We carried out a systematic study of zircon- to reidite-type (tetragonal I4₁/amd to I4₁/a) structural transitions of U_1-x Th _x SiO₄ solid solution, and investigated their mechanical behaviour. We found a unique behaviour of transition pressure with the change in U-Th concentration in the solid solution. The phase transition pressure (p _t) is found to be minimum for x  =  0.5. We develop the necessary formalism and present an efficient method to estimate the longitudinal and angular distortions of U/ThO₈-triangular dodecahedra (snub-disphenoids). We have parameterized two new factors: δ (longitudinal distortions) and σ ² (angular distortions) to quantify the polyhedral distortions. A detailed analysis of U/ThO₈ snub-disphenoidal distortions is presented to address such variation of p _t with U and Th concentration. We argue that our approach is independent of polyhedral volume and can be used for any AB₈ (A: cation, B: anion) type snub-disphenoidal system.

Gd(NO3)(Se2O5)·3H2O: a nitrate-selenite nonlinear optical material with a short ultraviolet cutoff edge

The first example of a rare-earth metal nitrate-selenite nonlinear optical (NLO) crystal, Gd(NO₃)(Se₂O₅)·3H₂O (1), was synthesized by hydrothermal reaction. This compound crystallizes in the noncentrosymmetric space group P2₁2₁2₁, and its structure features two-dimensional (2D) [Gd(NO₃)(Se₂O₅)]_∞ layers, extending to a pseudo-3D framework linked by hydrogen bonds. 1 exhibits a phase-matchable SHG efficiency about 0.2 times that of KH₂PO₄ (KDP) and a large laser damage threshold of 236.26 MW cm^-2. The UV-Vis-NIR diffuse reflectance spectroscopy study shows that the UV cutoff edge of 1 is 224 nm, which is the shortest value observed among selenite-based NLO materials. The birefringence of 1 is 0.109@1064 nm based on density functional theory calculations. These optical properties make 1 a potential UV NLO material. Theoretical studies using density functional theory have been implemented to further understand the relationship between its optical properties and band structures.

Effect of anion dimensionality on optical properties: the ∞[B7O10(OH)2] layer in CsB7O10(OH)2vs. the ∞[B7O12] framework in CsBaB7O12

CsB₇O₁₀(OH)₂ with new FBB features a 2D anionic structure whose band gap is about 6.6 eV and CsBaB₇O₁₂ features a 3D anionic structure whose band gap is 5.6 eV. And their simulated birefringence are about 0.08 and 0.05 at 1064 nm, respectively.

NaK15[B4O5(OH)4]6(NO2)2(CO3)·7H2O: assembly of an unprecedented mixed anion inorganic compound via a facile hydrothermal route

The first borate–nitrite–carbonate that simultaneously possesses [B₄O₅(OH)₄]²⁻, [NO₂]⁻, and [CO₃]²⁻ fundamental building blocks, has been obtained by a hydrothermal method.

Synthesis, structure and characterization of three new Mg-containing phosphates with deep-UV cut-off edges

Three new Mg-containing phosphates, AMg₂P₃O₁₀ (A = Li, Cs) and Rb₂MgP₂O₇, have been successfully synthesized using a high-temperature solution method.

In[Ba3Cl3F6]: a novel infrared-transparent molecular sieve constructed by halides

A new metal halide molecular sieve In[Ba₃Cl₃F₆] has been synthesized by the hydrothermal method. It possesses a wide transparent window from 0.366 to 22 μm and can exhibit the good adsorption–desorption property.

The first silver bismuth borate, AgBi2B5O11

The first silver bismuth borate, AgBi₂B₅O₁₁ (silver dibismuth pentaborate), has been prepared via glass crystallization in the Ag₂O-Bi₂O₃-B₂O₃ system and characterized by single-crystal X-ray diffraction. Its structure is derived from that of centrosymmetric Bi₃B₅O₁₂ by ordered substitution of one Bi³⁺ ion for Ag⁺, which results in the disappearance of the mirror plane and inversion centre. Second harmonic generation (SHG) measurements confirm the acentric crystal structure. It is formed by [Bi₂B₅O₁₁]_∞ layers stretched along c and comprised of vertex-sharing B₅O₁₀ and BiO₃ groups which incorporate the Ag⁺ cations. The new compound was characterized by thermal analysis, high-temperature powder X-ray diffraction, and vibrational and UV-Vis-NIR (near infrared) spectroscopy. Its thermal expansion is strongly anisotropic due to the presence of rigid B₅O₁₀ groups aligned in a parallel manner. The minimal value is observed along their axis [parallel to c, α_c = 3.1 (1) × 10^-6 K^-1], while maximal values are observed in the ab plane [α_a = 20.4 (2) and α_b = 7.8 (2) × 10^-6 K^-1]. Upon heating, AgBi₂B₅O₁₁ starts to decay above 684 K due to partial reduction of silver; incongruent melting is observed at 861 K. According to density functional theory (DFT) band-structure calculations, the new compound is a semiconductor with an indirect energy gap of 3.57 eV, which agrees with the experimental data (absorption onset at 380 nm).

Syntheses, structures, anomalous phase transition and optical properties of two new polymorphic α- and β-LiMoPO6

Two new polymorphic α- and β-LiMoPO₆ were synthesized, and they exhibit anomalous phase transition caused by the stretching of the [MoO₃O₃PO]_∞ chain.