Modulation of Framework and Centricity: Cation Size Effect in New Quaternary Selenites, ASc(SeO3)2 (A = Na, K, Rb, and Cs)

A UV non-hydrogen pure selenite nonlinear optical material for achieving balanced properties through framework-optimized structural transformation

For non-centrosymmetric (NCS) oxides intended for ultraviolet (UV) nonlinear optical (NLO) applications, achieving a wide band gap, large second harmonic generation (SHG) intensity, and sufficient birefringence to satisfy phase matching is a significant challenge due to their inherent incompatibility. To address this issue, this study proposes a strategy called framework-optimized structural transformation. Building upon centrosymmetric (CS) NaGa(SeO3)2 as a foundation, an original UV selenite NLO material, NaLu(SeO3)2, was successfully synthesized. The derived NaLu(SeO3)2 exhibits a balanced comprehensive performance, including a band gap (5.3 eV), an SHG response (2.7 × KDP), a UV cut-off edge (210 nm), a laser-induced damage threshold (LIDT) (151.69 MW cm-2), birefringence (Cal: 0.138@546 nm, Exp: 0.153@546 nm), thermal stability (∼575 °C) and environmental stability. Notably, its SHG effect, band gap, LIDT, and birefringence are all the largest among UV non-hydrogen pure selenite materials. Such progress can be attributed to the successful arrangement of the SeO3 groups by optimizing the cations on the framework of the parent compound.

UV-Transparent SHG Material Explored in an Alkali Metal Sulfate Selenite System

The first examples of alkali metal selenite sulfates, namely, Na8(SeO3)(SO4)3 (1), Na2(H2SeO3)(SO4) (2), and K4(H2SeO3)(HSO4)2(SO4) (3), were successfully synthesized by hydrothermal reactions. Their structures display three different zero-dimensional configurations composed of isolated sulfate tetrahedra and selenite groups separated by alkali metals. Na8(SeO3)(SO4)3 (1) features a noncentrosymmetric structure, while Na2(H2SeO3)(SO4) (2) and K4(H2SeO3)(HSO4)2(SO4) (3) are centrosymmetric. Powder second-harmonic-generation measurements revealed that Na8(SeO3)(SO4)3 (1) shows a phase-matchable SHG intensity about 1.2 times that of KDP. UV-vis-NIR diffuse reflectance spectroscopic analysis indicated that Na8(SeO3)(SO4)3 (1) has a short UV cutoff edge and a large optical band gap, which makes it a possible UV nonlinear optical material. Theoretical calculations revealed that the birefringence of Na8(SeO3)(SO4)3 (1) is 0.041 at 532 nm, which is suitable for phase-matching condition. This work provides a good experimental foundation for the exploration of new UV nonlinear crystals in an alkali metal selenite sulfate system.

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

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

The First UV Nonlinear Optical Selenite Material: Fluorination Control in CaYF(SeO3 )2 and Y3 F(SeO3 )4

It is a great challenge to develop UV nonlinear optical (NLO) material due to the demanding conditions of strong second harmonic generation (SHG) intensity and wide band gap. The first ultraviolet NLO selenite material, Y₃ F(SeO₃ )₄ , has been obtained by control of the fluorine content in a centrosymmetric CaYF(SeO₃ )₂ . The two new compounds represent similar 3D structures composed of 3D yttrium open frameworks strengthened by selenite groups. CaYF(SeO₃ )₂ has a large birefringence (0.138@532 nm and 0.127@1064 nm) and a wide optical band gap (5.06 eV). The non-centrosymmetric Y₃ F(SeO₃ )₄ can exhibit strong SHG intensity (5.5×KDP@1064 nm), wide band gap (5.03 eV), short UV cut-off edge (204 nm) and high thermal stability (690 °C). So, Y₃ F(SeO₃ )₄ is a new UV NLO material with excellent comprehensive properties. Our work shows that it is an effective method to develop new UV NLO selenite material by fluorination control of the centrosymmetric compounds.

Hydrogen-Bond-Assisted Alignment of [MCu(SeO3)4Cl(H2O)]4- (M = Fe, Ga) Anionic Layers to Form Two Polar Oxychlorides: Pb2MCu(SeO3)4Cl(H2O)

Two novel selenite oxychlorides Pb₂MCu(SeO₃)₄Cl(H₂O) (M = Fe, Ga) were hydrothermally synthesized and structurally characterized. They are isostructural and crystallize in the two-dimensional [MCu(SeO₃)₄Cl(H₂O)]^4- anionic layer structure mediated with hydrogen bonds and aligned between neighboring layers which assist in building the three-dimensional framework with a polar space group. Optical properties measurements revealed that the optical band gaps are 2.61 and 3.22 eV for Pb₂FeCu(SeO₃)₄Cl(H₂O) (1) and Pb₂GaCu(SeO₃)₄Cl(H₂O) (2) and the SHG responses are about 0.12 and 0.18 times that of KDP, respectively. Furthermore, 1 exhibits an interesting metamagnetic phenomenon under varied applied fields from around 1 to 4 T at 2 K, and 2 behaves with potential ferromagnetic ordering at low temperature.

[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.

Hexagonal tungsten oxides with large bandgaps synthesized by a chemical substitution method

A novel hexagonal tungsten oxide designed by a chemical substitution method, NH₄(GaF₂)₃(SeO₃)₂, exhibits the largest bandgap among all known phase-matchable NLO selenite compounds.

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.

A cation size effect on the framework structures in ABi2SeO3F5 (A = K and Rb): first examples of alkali metal bismuth selenite fluorides

Two new centric alkali metal bismuth selenite fluorides, ABi2SeO3F5 (A = K and Rb), have been synthesized through a soft hydrothermal method. KBi2SeO3F5 crystallizes in an orthorhombic crystal system with the centric space group Pbcm possessing a three-dimensional (3D) network made up of SeO3 trigonal pyramids and asymmetric BiO3F5 polyhedra. As regards compound RbBi2SeO3F5, it crystallizes in a triclinic crystal system with a centric space group of P1[combining macron], which contains alternant two-dimensional (2D) [Bi2SeO3F5]∞1- layers separated by Rb+ cations. In terms of structure, the differences between KBi2SeO3F5 and RbBi2SeO3F5 originate from the size of alkali metal cations, which has a great influence on the framework geometry and the dimensionalities of crystals. The UV-vis diffuse reflectance spectroscopy study of powder samples indicates that the band gaps of KBi2SeO3F5 and RbBi2SeO3F5 are approximately 4.08 and 4.18 eV, respectively, which are the largest ones among the known bismuth selenites owing to the existence of alkali metal cations as well as fluorine anions in crystals. Furthermore, theoretical calculations show that the optical absorption of two compounds is mainly attributed to the contribution of BiOxFy polyhedra and [SeO3]2- anionic groups.

RbSe3B2O9(OH) and CsSe3B2O9(OH): one dimensional boroselenite-based anionic frameworks with second harmonic generation properties

Two new alkali boroselenites RbSe3B2O9(OH) and CsSe3B2O9(OH) have been synthesized by traditional solid-state reactions. Single-crystal X-ray diffraction study indicated that they are isostructural and adopt a new type of structure, which crystallizes in the noncentrosymmetric space group P212121. Optical diffuse reflectance spectrum studies emphasized that both are indirect optical transitions with values of 3.79 and 4.17 eV for RbSe3B2O9(OH) and CsSe3B2O9(OH), respectively. Optical analysis revealed a broad transparency window in the 0.3-8.5 μm region for both compounds. In addition, RbSe3B2O9(OH) featured a relatively weak second-harmonic-generation response, and for CsSe3B2O9(OH), the response is 0.8-times that of KH2PO4. Theoretical calculations of band structure, density of state, and linear and nonlinear optical properties were also performed to get insight into the relationships between electronic structures and their optical properties.

Noncentrosymmetric (NCS) solid solutions: elucidating the structure-nonlinear optical (NLO) property relationship and beyond

A systematic approach toward the discovering of novel functional noncentrosymmetric (NCS) materials revealing technologically useful applications is an ongoing challenge. This Frontiers article investigates a series of NCS solid solutions with respect to their crystal structure and second-harmonic generation (SHG) response. The solid solutions include NCS polar aluminoborates, Al_5-xGa_xBO₉ (0.0 ≤ x ≤ 0.5), rare earth element-doped bismuth tellurites, Bi_2-xRE_xTeO₅ (RE = Y, Ce, and Eu; 0.0 ≤ x ≤ 0.2), layered perovskites, Bi_4-xLa_xTi₃O₁₂ (0.0 ≤ x ≤ 0.75: Aurivillius phases) and CsBi_1-xEu_xNb₂O₇ (0.0 ≤ x ≤ 0.2: Dion-Jacobson phases), calcium bismuth oxides, Ca₄Bi_6-xLn_xO₁₃ (Ln = La and Eu; x = 0, 0.06 and 0.12), and sodium lanthanide iodates, NaLa_1-xLn_x(IO₃)₄ (Ln = Sm and Eu; 0 ≤ x ≤ 1). The origin of SHG for all the NCS solid solutions is discussed and the detailed structure-nonlinear optical (NLO) property relationships are elucidated. In addition, photoluminescence (PL) properties and subsequent energy transfer mechanisms for NCS solid solutions are provided.

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.

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.

Structural diversities induced by cation sizes in a series of fluorogermanophosphates: A2[GeF2(HPO4)2] (A = Na, K, Rb, NH4, and Cs)

Germanophosphates, in comparison with other metal phosphates, have been less studied but potentially exhibit more diverse structural chemistry with wide applications. Herein we applied a hydro-/solvo-fluorothermal route to make use of both the "tailor effect" of fluoride for the formation of low dimensional anionic clusters and the presence of alkali cations of different sizes to align the anionic clusters to control the overall crystal symmetries of germanophosphates. The synergetic effects of fluoride and alkali cations led to structural changes from chain-like structures to layered structures in a series of five novel fluorogermanophosphates: A₂[GeF₂(HPO₄)₂] (A = Na, K, Rb, NH₄, and Cs, denoted as Na, K, Rb, NH4, and Cs). Although these fluorogermanophosphates have stoichiometrically equivalent formulas, they feature different anionic clusters, diverse structural dimensionalities, and contrasting crystal symmetries. Chain-like structures were observed for the compounds with the smaller sized alkali ions (Na⁺, K⁺, and Rb⁺), whereas layered structures were found for those containing the larger sized cations ((NH₄)⁺ and Cs⁺). Specifically, monoclinic space groups were observed for the Na, K, Rb, and NH4 compounds, whereas a tetragonal space group P4/mbm was found for the Cs compound. These compounds provide new insights into the effects of cation sizes on the anionic clusters built from GeO₄F₂ octahedra and HPO₄ tetrahedra as well as their influences on the overall structural symmetries in germanophosphates. Further characterization including IR spectroscopy and thermal analyses for all five compounds is also presented.

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

Solid-state materials with extended structures have revealed many interesting structure-related characteristics. Among many, materials crystallizing in noncentrosymmetric (NCS) space groups have attracted massive attention attributable to a variety of superb functional properties such as ferroelectricity, pyroelectricity, piezoelectricity, and nonlinear optical (NLO) properties. In fact, the characteristics are pivotal to many industrial applications such as laser systems, optical communications, photolithography, energy harvesting, detectors, and memories. Thus, for the past several decades, a great deal of synthetic effort has been vigorously made to realize these technologically important properties by improving the occurrence of macroscopic NCS space groups. A bright approach to increase the incidence of NCS structures was combining local asymmetric units during the initial synthesis process. Although a significant improvement has been achieved in obtaining new NCS materials using this strategy, the majority of solid-state materials still crystallize in centrosymmetric (CS) structures as the locally unsymmetrical units are easily lined up in an antiparallel manner. Therefore, discovering an effective method to control the framework structure and the macroscopic symmetry is an imminent ongoing challenge. In order to more effectively control the overall symmetry of solid-state compounds, it is critical to understand how the backbone and the subsequent centricity are affected during the crystallization. In this Account, several factors influencing the framework structure and centricity of solid-state materials are described in order to more systematically discover novel NCS materials. Recent studies on crystalline solid-state materials suggest three factors affecting the local coordination environment as well as the overall symmetry of the framework structure: (1) size variations of the various template cations, (2) a variable backbone arrangement occurring from the hydrogen-bonding interactions, and (3) the presence of framework flexibility. With regard to the first factor, the impact of size of the various metal cations and coordination numbers on the alignment of other adjacent polyhedra, linkers, and lone pairs determining the framework geometries of mixed metal oxides is analyzed. The second factor considers the regulation of crystallographic centricity determined by the availability of hydrogen-bonding interactions between anionic frameworks containing local asymmetric polyhedra and organic cations. Finally, the third factor explores the framework architecture and the space group symmetry influenced by the flexibility of polyhedra revealing variable coordination numbers. The centricity and framework of new solid-state materials might be controlled by using a variety of synthetically controllable asymmetric units such as organic structure-directing cations and linkers with different sizes and functional groups.

New Series of Polar and Nonpolar Platinum Iodates A2Pt(IO3)6 (A = H3O, Na, K, Rb, Cs)

A new series of platinum iodates, namely, α-(H3O)2Pt(IO3)6, β-(H3O)2Pt(IO3)6, and A2Pt(IO3)6 (A = Na, K, Rb, Cs), have been synthesized. Interestingly, among these six stoichiometrically identical compounds, α-(H3O)2Pt(IO3)6 is polar, whereas other compounds are nonpolar and centrosymmetric. They all consist of zero-dimensional [Pt(IO3)6](2-) molecular units separated by H3O(+) or A(+) cations. All of the lone electron pairs of the IO3(-) groups are aligned and almost point to one direction for α-(H3O)2Pt(IO3)6, whereas IO3(-) groups are located trans to each other in other compounds. The material, α-(H3O)2Pt(IO3)6, exhibits very strong second harmonic generation (SHG) effects, approximately 1.2 × KTiOPO4 (KTP), and is phase-matchable. Thermogravimetric analysis, elemental analysis, infrared spectra, UV-vis spectra, nonlinear optical properties, and theoretical calculations are also reported.

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.

Nonlinear optical response mechanism of noncentrosymmetric lead borate Pb6[B4O7(OH)2]3with three crystallographically independent [B4O7(OH)2]4−chains

Pb₆[B₄O₇(OH)₂]₃features three unique [B₄O₇(OH)₂]⁴⁻chains while lead cations and π-conjugated BO₃groups make contribution to the nonlinear optical effect.