New Alkali-Metal Gallium Selenites, AGa(SeO3)2 (A = Li, Na, K, and Cs): Effect of Cation Size on the Framework Structures and Macroscopic Centricities

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.

A2Hgx(SeO3)y (A = K, Rb, Cs): Three Alkali Metal Mercury Selenites Featuring Unique 1D [HgOm(SeO3)n]∞ Chains

Herein, three alkali metal mercury selenites, K2Hg2(SeO3)3, Rb2Hg2(SeO3)3, and Cs2Hg3(SeO3)4, were successfully obtained by a hydrothermal method. The three compounds featured same one-dimensional (1D) [HgOm(SeO3)n]∞ chain structure that consisting of distorted Hg-O polyhedra and SeO3 triangular pyramids with stereochemically active lone pair (SCALP) electrons. Interestingly, the rich coordination environment of Hg atoms and the size difference of alkali metal cations lead to diverse arrangement of SeO3 groups, which makes them exhibit different birefringence. The band gaps of the three compounds indicate that they are potential ultraviolet (UV) optical materials. Detailed theoretical calculations demonstrate that the combined effects of SeO3 triangular pyramids and Hg-O polyhedra are responsible for the optical characteristics of the reported compounds.

Noncentrosymmetric γ-Cs2I4O11 Obtained from IO4 Polyhedral Rearrangements in the Centrosymmetric β-Phase

We report the synthesis and optical properties of noncentrosymmetric (NCS) γ-Cs₂I₄O₁₁ that was obtained through IO₄ polyhedral rearrangements from centrosymmetric (CS) β-Cs₂I₄O₁₁. Trifluoroacetic acid (TFA) acts as a structure-directing agent and plays a key role in the synthesis. It is suggested that the function of TFA is to promote rearrangement reactions found in the organic synthesis of stereoisomers. γ-Cs₂I₄O₁₁ crystallizes in the NCS monoclinic space group P2₁ (No. 4) and exhibits a strong second-harmonic-generation (SHG) response of 5.0 × KDP (KH₂PO₄) under 1064 nm laser radiation. Additional SHG experiments indicate that the material is type I phase matchable. First-principles calculations show that SHG intensity mainly comes from its d₃₄, d₂₁, and d₂₃ SHG tensor components. The synthetic strategy of discovering γ-Cs₂I₄O₁₁ provides a new way for designing novel NCS SHG materials.

Lithium superionic conductors with corner-sharing frameworks

Superionic lithium conductivity has only been discovered in a few classes of materials, mostly found in thiophosphates and rarely in oxides. Herein, we reveal that corner-sharing connectivity of the oxide crystal structure framework promotes superionic conductivity, which we rationalize from the distorted lithium environment and reduced interaction between lithium and non-lithium cations. By performing a high-throughput search for materials with this feature, we discover ten new oxide frameworks predicted to exhibit superionic conductivity-from which we experimentally demonstrate LiGa(SeO₃)₂ with a bulk ionic conductivity of 0.11 mS cm^-1 and an activation energy of 0.17 eV. Our findings provide insight into the factors that govern fast lithium mobility in oxide materials and will accelerate the development of new oxide electrolytes for all-solid-state batteries.

Halogen regulation triggers structural transformation from centrosymmetric to noncentrosymmetric switches in tin phosphate halides Sn2PO4X (X = F, Cl)

A tin phosphate halide, centrosymmetric Sn2PO4F, was successfully transformed into noncentrosymmetric Sn2PO4Cl with excellent comprehensive performance by the substitution of F− with Cl−.

Unexpected aliovalent cation substitution between two NLO materials LiBa3Bi6(SeO3)7F11 and Ba3Bi6.5(SeO3)7F10.5O0.5

One new combination, alkali-metal and alkaline earth-metal selenite fluoride, LiBa₃Bi₆(SeO₃)₇F₁₁ (LBBSF) and its analogue Ba₃Bi_6.5(SeO₃)₇F_10.5O_0.5 (BBSF) were reported here for the first time. Unusual aliovalent cation substitution between them affected the layer thickness and made the bond strains of [SeO₃] enhanced, thereby inducing greater distortions and affecting the SHG efficiencies. This work may provide thoughts for exploring new NLO materials.

Selenite bromide nonlinear optical materials Pb2GaF2(SeO3)2Br and Pb2NbO2(SeO3)2Br: synthesis and characterization

Two new isomorphic selenite bromides, namely Pb2GaF2(SeO3)2Br (1) and Pb2NbO2(SeO3)2Br (2), were synthesized by mild hydrothermal reactions. These two compounds crystallize in the noncentrosymmertric space group P21 and feature 3D structures consisting of 1D (Pb2Br)3+ cationic chains and 2D [Ga2F4(SeO3)4]6-/[Nb2O4(SeO3)4]6- anonic chains. As measured, 1 and 2 are phase-matchable and exhibit excellent second harmonic generation (SHG) responses of 4.5 and 1.4 times that of KDP, respectively. Moreover, these two compounds possess wide bandgaps (>3.17 eV) and high thermal stabilities (>425 °C), indicating their suitable performances as potential nonlinear optical materials. First-principle density functional theory (DFT) calculations were also performed to explicate the structure-property relationship.

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.

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.

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.

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.

Cation-tuned synthesis of the A2SO4·SbF3 (A = Na+, NH4+, K+, Rb+) family with nonlinear optical properties

Four stoichiometrically equivalent compounds have been successfully synthesized through A⁺ cations by tuning the framework structures and macroscopic centricities.

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.

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.

Three new phosphates with isolated P2O7units: noncentrosymmetric Cs2Ba3(P2O7)2and centrosymmetric Cs2BaP2O7and LiCsBaP2O7

Three new phosphates, a noncentrosymmetric (NCS) Cs₂Ba₃(P₂O₇)₂and centrosymmetric (CS) Cs₂BaP₂O₇and LiCsBaP₂O₇, have been synthesized from high-temperature solutions for the first time.

Structural and magnetic studies on three new mixed metal copper(II) selenites and tellurites

Three new transition metal copper(II) selenites or tellurites, namely, CdCu(SeO3)2 (1), HgCu(SeO3)2 ()2, and Hg2Cu3(Te3O8)2 (3), have been obtained by conventional hydrothermal reactions of CdO (or Hg2Cl2), CuO and SeO2 (or TeO2). Compounds 1 and 2 are isostructural and crystallize in P2(1)/c. Their structures feature a 3D anionic framework of Cu(SeO3)2(2-) with 1D channels of eight-membered rings (MRs) along the c-axis and a-axis, respectively, which are filled by Cd(2+) or Hg(2+) cations. Compound 3 crystallizes in a tetragonal system of space group P42(1)2. Its structure is characterized by a [Cu3(Te3O8)2](2-) honeycomb layer composed of [Te3O8](4-) anions interconnected by Cu(2+) ions with 1D channels of 8-MRs along the c-axis. TOPOS analysis indicates that the copper(ii) tellurite layer exhibits a new topological structure with a Schläfli symbol of {4(6)·8(9)}(2){4(6)}(3). The above anionic copper(II) tellurite layers are further linked by dumbbell Hg2(2+) cations to form a novel 3D framework. Magnetic measurements based on magnetic susceptibility and heat capacity indicate that compounds 1 and 2 show a spin-singlet ground state with a spin gap based on the [Cu2O8](12-) dimeric model, whereas compound e3 xhibits a 2D spin-system with an antiferromagnetic ordering around 25 K correlated with its honeycomb [Cu3(Te3O8)2](2-) layer. Furthermore, crystalline structures, thermal stabilities, IR spectra and UV-Vis diffuse reflectance spectra have also been studied.

Effect of the cation size on the framework structures of magnesium tungstate, A4Mg(WO4)3 (A = Na, K), R2Mg2(WO4)3 (R = Rb, Cs)

A series of alkali metal magnesium tungstates, A₄Mg(WO₄)₃ (A = Na, K), R₂Mg₂(WO₄)₃ (R = Rb, Cs), were synthesized from a high temperature solution, and their structures were determined by single-crystal X-ray diffraction.

Noncentrosymmetric YVSe2O8 and centrosymmetric YVTe2O8: macroscopic centricities influenced by the size of lone pair cation linkers

Two new quaternary vanadium selenite and tellurite, i.e., YVSe2O8 and YVTe2O8, have been synthesized through hydrothermal and solid-state reactions. Both of the reported materials exhibit three-dimensional framework structures that are composed of layers of corner-shared VO6 octahedra, layers of edge-shared YO8 groups, and SeO3 or TeO3 linkers. While YVSe2O8 crystallizes in the orthorhombic noncentrosymmetric (NCS) space group, Abm2, YVTe2O8 reveals the monoclinic centrosymmetric (CS) space group, C2/m. The band gaps for YVSe2O8 and YVTe2O8 are calculated to be 2.7 and 2.2 eV, respectively, from the UV-vis diffuse reflectance spectra. Powder second harmonic generation (SHG) measurements using 1064 nm radiation reveals that NCS YVSe2O8 has a similar SHG efficiency to that of NH4H2PO4 (ADP). Detailed explanations of the structure-property relationships, effect of lone pair cation size on the macroscopic centricities, and full characterizations including infrared spectroscopy, thermal analyses, and elemental analyses are also presented.

Homo- and heteropolymetallic 3-(2-pyridyl)pyrazolate manganese and rhenium complexes

Heteropolymetallic complexes with pyridylpyrazolates bridging Mn and Na or K are tetrametallic and contain bridging pyridylpyrazolates with an unprecedented coordination mode, whereas the Mn/Li complex is bimetallic.