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.

BaMo3O10 Polymorphs with Tunable Symmetries and Properties

Polymorphism is a well-known but important phenomenon in the field of solid-state chemistry. Crystalline materials can form various polymorphs and present drastically varied physical and chemical properties. Herein, systematic exploration of the BaO-MoO₃ binary system leads to the discovery of a new barium molybdate, α-BaMo₃O₁₀. The temperature-dependent phase transition from α-BaMo₃O₁₀ to β-BaMo₃O₁₀ is confirmed. The tunable linear and nonlinear optical properties induced by the phase transition are confirmed by both experimental and theoretical approaches. Also, β-BaMo₃O₁₀ is identified as a nonlinear-optical crystal for the first time. The origin of linear- and nonlinear-optical properties of BaMo₃O₁₀ polymorphs is confirmed by the additional theoretical means. This work indicates that a small change in the structure can induce tunable symmetries and thereby widely divergent optical properties.

Crystal growth and thermal properties of a LiY(MoO4)2 crystal

A high-quality tetragonal LiY(MoO4)2 single crystal was grown by a top-seeded solution growth method. Based on the quality evaluation, the optical transmission and thermal properties of the LiY(MoO4)2 single crystal were fully characterized.

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.

Two New Tellurite Halides with Cationic Layers: Syntheses, Structures, and Characterizations of CdPb2Te3O8Cl2 and Cd13Pb8Te14O42Cl14

Two new tellurite halides, CdPb2Te3O8Cl2 and Cd13Pb8Te14O42Cl14 with mixed cationic layered structures, have been synthesized by high-temperature solution method. CdPb2Te3O8Cl2 crystallizes in the noncentrosymmetric Aba2 space group, built by [CdPb2Te3O8]...

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...

Ca3(TeO3)2(MO4) (M = Mo, W): Mid-Infrared Nonlinear Optical Tellurates with Ultrawide Transparency Ranges and Superhigh Laser-Induced mage ThreDasholds

Intense interests in mid-infrared (MIR) nonlinear optical (NLO) crystals have erupted in recent years due to the development of optoelectronic applications ranging from remote monitoring to molecular spectroscopy. Here, two polar crystals Ca₃(TeO₃)₂(MO₄) (M = Mo, W) were grown from TeO₂-MO₃ flux by high-temperature solution methods. Ca₃(TeO₃)₂(MoO₄) and Ca₃(TeO₃)₂(WO₄) are isostructural, which feature novel structures consisting of asymmetric MO₄ tetrahedra and TeO₃ trigonal pyramids. Optical characterizations show that both crystals display ultrawide transparency ranges (279 nm to 5.78 μm and 290 nm to 5.62 μm), especially high optical transmittance over 80% in the important atmospheric transparent window of 3-5 μm, and superhigh laser damage thresholds (1.63 GW/cm² and 1.50 GW/cm²), 54.3 and 50 times larger than that of state-of-the-art MIR NLO AgGaS₂, respectively. Notably, they exhibit the widest band gaps and the loftiest laser-induced threshold damages among the reported tellurates so far. Moreover, Ca₃(TeO₃)₂(MO₄) exhibit type I phase matching at two working wavelengths owing to their large birefringence and strong second-harmonic generation responses from the distorted anions, as further elucidated by the first-principles calculations. The above characteristics indicate that Ca₃(TeO₃)₂(MO₄) crystals are high-performance MIR NLO materials, especially applying in high-power MIR laser operations.

Ba2[FeF4(IO3)2]IO3: a promising nonlinear optical material achieved by chemical-tailoring-induced structure evolution

A new noncentrosymmetric iron-iodate-fluoride Ba₂[FeF₄(IO₃)₂]IO₃ was ingeniously obtained based on the centrosymmetric Ba[FeF₄(IO₃)] through chemical tailoring. Ba₂[FeF₄(IO₃)₂]IO₃ exhibits a strong phase-matchable second-harmonic generation effect, a large band gap, and a wide mid-infrared transparent window. The chemical tailoring design based on oxide-fluoride anions affords a feasible approach to design nonlinear optical materials.

Li2 MTeO6 (M=Ti, Sn): Mid-Infrared Nonlinear Optical Crystal with Strong Second Harmonic Generation Response and Wide Transparency Range

Oxide crystals have been widely used in nonlinear optics (NLO) in the ultraviolet-visible and near-infrared regions. Most traditional oxide crystals are restricted to the mid-infrared region due to their narrow transmission window. Hence, attempting to extend infrared cutoff wavelength of oxides has attracted much attention. Herein, we report two new tellurates Li₂ TiTeO₆ (LTT) and Li₂ SnTeO₆ (LST) with broad transparent regions of 0.38-6.72 and 0.38-6.86 μm, respectively, as excellent candidates for mid-infrared NLO applications. Both LTT and LST crystallize in the orthorhombic space group Pnn2. The LTT crystal exhibits intense powder second-order generation efficiency (26×KDP) under the fundamental wavelength of 1064 nm. First-principles calculations and dipole moments were used to illustrate the results of the powder second-harmonic generations based on the crystal structures. Our results provide a novel oxide NLO crystal with a strong SHG and wide transparency range. They also pave a way for the design of new oxide mid-IR NLO crystals.

Giant Second-Harmonic Generation Response and Large Band Gap in the Partially Fluorinated Mid-Infrared Oxide RbTeMo2O8F

Strong second-harmonic generation (SHG) and a wide band gap are two crucial but often conflicting parameters that must be optimized for practical nonlinear optical (NLO) materials. We report herein the first d⁰-transition-metal (TM) tellurite with half of the d⁰-TM-octahedra partially fluorinated, namely, quinary RbTeMo₂O₈F, which exhibits giant SHG responses (27 times that of KH₂PO₄ (KDP) and 2.2 times that of KTiOPO₄ (KTP) with 1064 and 2100 nm laser radiation, respectively), the largest SHG values among all reported metal tellurites. RbTeMo₂O₈F also possesses a large band gap (3.63 eV), a wide optical transparency window (0.34-5.40 μm), and a significant birefringence (Δn = 0.263 at 546 nm). Theoretical calculations and crystal structure analysis demonstrate that the outstanding SHG responses can be definitively attributed to the uniform alignment of the polarized [MoO₅F]/[MoO₆] octahedra and the seesaw-like [TeO₄], and the consequent favorable summative polarization of the three distinct SHG-active polyhedra, both induced by partial fluorine substitution on the [MoO₆] octahedra.

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.

A powder method for the high-efficacy evaluation of electro-optic crystals

The electro-optic crystal holds great promise for extensive applications in optoelectronics and optical communication. However, the discovery of novel electro-optic crystals is sporadic due to the difficulties of large-sized crystal growth for electro-optic coefficient measurement. Herein, to address this issue, a high-efficacy evaluation method using accessible powder samples is proposed in which the second-harmonic-generation effect, infrared reflectance spectrum and Raman spectrum are introduced to predict the magnitude of the electro-optic coefficient. The calculated electro-optic coefficients of numerous reported electro-optic crystals through this approach give universal agreement to the experimental values, evidencing the validity of the strategy. Based on this method, CsLiMoO₄ is screened as a novel potential electro-optic crystal and a high-quality crystal is grown by the Czochralski technique for electro-optic coefficient measurement using the half-wave voltage method, the result of which is also comparable to the calculated value. Consequently, the evaluation strategy presented here will pave a new way to explore promising electro-optic crystals efficiently.

An Exceptional Peroxide Birefringent Material Resulting from d-π Interactions

Birefringent materials, which can modulate the polarization of light, are almost exclusively limited to oxides. Peroxides have long been overlooked as birefringent materials, because they are usually not stable in air. Now, the first peroxide birefringent material Rb₂ VO(O₂ )₂ F is reported, the single crystals of which keep transparency after being exposed in the air for two weeks. Interestingly, Rb₂ VO(O₂ )₂ F does not feature an optimal anisotropic structure, but its birefringence (Δn=0.189 at 546 nm) exceeds those of the majority of oxides. According to the first-principles calculations, this exceptional birefringence should be attributed to the strong electronic interactions between localized π orbital of O₂ ^2- anions and V⁵⁺ 3d orbitals, which may be also favorable to the stability in the air for Rb₂ VO(O₂ )₂ F. These findings distinguish peroxides as a brand-new class of birefringent materials that may possess birefringence superior to the traditional oxides.

Structural Diversity of Molybdate Iodate and Fluoromolybdate: Syntheses, Structures, and Calculations on Na3(MoO4)(IO3) and Na3Cs(MoO2F4)2

The alkali-metal molybdate iodate Na₃(MoO₄)(IO₃) (I) and mixed-alkali-metal fluoromolybdate Na₃Cs(MoO₂F₄)₂ (II) were obtained via a mild hydrothermal reaction using a "Teflon-pouch" method. I crystallizes in the triclinic space group P1, whose structure comprises a 3D backbone made up of isolated [IO₃]^- pyramids and [MoO₄]^2- tetrahedra connected via 5- and 6-fold coordinated sodium cations. II crystallizes in the monoclinic space group P2₁/c and comprises isolated [MoO₂F₄]^2- octahedra with strong out-of-center distortions and the Na⁺ as well as Cs⁺ cations acting as interstitial ions. Both compounds have been characterized by infrared (IR) spectra and ultraviolet-visible-near-infrared (UV-vis-NIR) diffuse reflectance spectra. First-principles calculations respectively reveal that they exhibit birefringence values with Δn = 0.078 and 0.210 at 1064 nm for I and II, and the origin of the birefringence is discussed.

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.

CsBP2O6(OH)2: a second-order NLO material with a unique borophosphate anionic partial structure and strong SHG effect

A new NCS alkali-metal borophosphate CsBP₂O₆(OH)₂ (CBPO) possessing a wide UV-vis transparent range and showing a strong SHG response [1.2 × KDP (KH₂PO₄)].

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.

Investigation into the optimized growth, anisotropic properties and theoretical calculations of the polar material Cs2TeW3O12

Bulk single crystals of Cs₂TeW₃O₁₂ were grown successfully by a modified TSSG method and physical properties were systematically investigated.

Four alkali metal molybdates with two types of Mo–O chains, ABMo3O10 (A = Li, B = Rb; A = Li, Na, K, B = Cs): synthesis, structure comparison and optical properties

The effects of cation size on the framework structures of four stoichiometrically equivalent alkali metal molybdates were discussed in detail.

Electric–elastic properties of a novel high-quality CdTeMoO6 piezoelectric crystal

Bulk single crystals of CdTeMoO₆ and its optimized growth parameters and the complete sets of dielectric, piezoelectric and elastic coefficients have been investigated.

PbTeGeO6: polar rosiaite-type germanate featuring a two dimensional layered structure

Polar rosiaite-type germanate PbTeGeO₆ with a layered structure shows a moderate second harmonic generation response.

Growth, Properties, and Theoretical Analysis of M2LiVO4 (M = Rb, Cs) Crystals: Two Potential Mid-Infrared Nonlinear Optical Materials

Mid-Infrared nonlinear optical (Mid-IR NLO) crystals with excellent performances play a particularly important role for applications in areas such as telecommunications, laser guidance, and explosives detection. However, the design and growth of high performance Mid-IR NLO crystals with large NLO efficiency and high laser-damage threshold (LDT) still face numerous fundamental challenge. In this study, two potential Mid-IR NLO materials, Rb₂LiVO₄ (RLVO) and Cs₂LiVO₄ (CLVO) with noncentrosymmetric structures (Orthorhombic, Cmc2₁) were synthesized by high-temperature solution method. Thermal analysis and powder X-ray diffraction demonstrate that RLVO and CLVO melt congruently. Centimeter sized crystals of CLVO have been grown by the top-seeded solution growth method. RLVO and CLVO exhibit strong second harmonic generation (SHG) effects (about 4 and 5 times that of KH₂PO₄, respectively) with a phase-matching behavior at 1.064 μm, and a wide transparency range (0.33–6.0 μm for CLVO). More importantly, RLVO and CLVO possess a high LDT value (~28 × AgGaS₂). In addition, the density functional theory (DFT) and dipole moments studies indicate that the VO₄ anionic groups have a dominant contribution to the SHG effects in RLVO and CLVO. These results suggest that the title compounds are promising NLO candidate crystals applied in the Mid-IR region.

Ag3I[(MoO3)2(IO3)2]: a new polar material synthesized from the structural modulation of molybdenyl iodate architectures by polarizable cation (Ag+) and anion (I−)

A new noncentrosymmetric compound Ag₃I[(MoO₃)₂(IO₃)₂] has been hydrothermally synthesized and characterized.

Sb-Based antiferromagnetic oxychlorides: MSb2O3(OH)Cl (M = Mn, Fe, Co) with 2D spin-dimer structures

Three new Sb-based antiferromagnetic oxychlorides: MnSb₂O₃(OH)Cl (1), FeSb₂O₃(OH)Cl (2), and CoSb₂O₃(OH)Cl (3) have been synthesized by using hydrothermal reaction methods. Their crystal structures are isostructural and incorporate two types of structural units: sphenoid SbO₄ polyhedra and M₂O₈Cl₂ (M = Mn, Fe, Co) dimers, that share edges forming a two dimensional (2D) MSb₂O₃(OH)Cl network. The neutral MSb₂O₃(OH)Cl 2D layers are arranged from ABAB-type for compounds 1 and 2 to AA'B'B-type for 3 with smaller cation radii. Magnetic measurements show that 1 belongs to an antiferromagnetic material, while 2 and 3 have weak ferromagnetic properties with unsaturated ferromagnetism originating from spin-canting below the transition temperature. In addition, the optical absorption band gaps were determined to be 3.8, 2.4, and 3.6 eV for 1, 2, and 3, respectively. This work demonstrates that utilizing both halide ions and lone-pair elements simultaneously in one structure is a feasible and practical route to reduce framework connectivity in order to synthesize new low-dimensional oxyhalides.

Nonlinear optical and self-activated luminescent properties of A2W3O10 (A = Rb and Cs)

The nonlinear optical and luminescent properties of Rb₂W₃O₁₀ and Cs₂W₃O₁₀ show that the two compounds are candidates for NLO materials in wavelengths from the visible to mid-infrared and also potential UV-based self-activated luminescent materials.

A series of new silver selenites with d0-TM cations

Four silver selenites containing d⁰-TM cations have been reported, one of them displays a moderately strong SHG response.

Top-seeded solution growth and characterization of Raman crystal LiVMoO6

Single crystal LiVMoO₆, with dimensions up to 20 mm × 15 mm × 12 mm, has been grown successfully for the first time from a high-temperature solution with MoO₃ as a flux.

Explorations of New SHG Materials in the Alkali-Metal-Nb(5+)-Selenite System

Standard high-temperature solid-state reactions of NaCl, Nb2O5, and SeO2 resulted in two new sodium selenites containing a second-order Jahn-Teller (SOJT) distorted Nb(5+) cation, namely, Na2Nb4O7(SeO3)4 (P1̅; 1) and NaNbO(SeO3)2 (Cmc21; 2). Compound 1 exhibits an unusual 3D [Nb4O7(SeO3)4](2-) anionic network composed of 2D [Nb4O11(SeO3)2](6-) layers which are further bridged by additional SeO3(2-) anions via corner sharing; the 2D [Nb4O11(SeO3)2](6-) layer is formed by unusual quadruple [Nb4O17](14-) niobium oxide chains of corner-sharing NbO6 octahedra being further interconnected by selenite anions via Nb-O-Se bridges. The polar compound 2 features a 1D [NbO(SeO3)2](-) anionic chain in which two neighboring Nb(5+) cations are bridged by one oxo and two selenite anions. The alignments of the polarizations from the NbO6 octahedra in 2 led to a strong SHG response of ∼7.8 × KDP (∼360 × α-SiO2), which is the largest among all phases found in metal-Nb(5+)-Se(4+)/metal-Nb(5+)-Te(4+)-O systems. Furthermore, the material is also type I phase matchable. The above experimental results are consistent with those based on DFT theoretical calculations. Thermal stabilities and optical properties for both compounds are also reported.

Bi2Te(IO3)O5Cl: a novel polar iodate oxychloride exhibiting a second-order nonlinear optical response

A novel iodate oxychloride Bi₂Te(IO₃)O₅Cl with a second-order nonlinear optical response originating from constructive stacking of dipole moments of IO₃.