Effect of the Framework Flexibility on the Centricities in Centrosymmetric In2Zn(SeO3)4 and Noncentrosymmetric Ga2Zn(TeO3)4

Shedding Light on the Structure and Characterization of K2ZnGe2O6: A Phase-Matchable Nonlinear Optical Crystal

Nonlinear optical (NLO) materials have recently aroused great interest owing to their capability of frequency conversion in solid-state lasers. Herein, we report an acentric zinc germanate K₂ZnGe₂O₆ obtained successfully through spontaneous crystallization methods. It affords a novel three-dimensional (3D) framework comprised of [GeO₄] and [ZnO₄] motifs with K atoms located in the tunnels. K₂ZnGe₂O₆ displays a moderate second-harmonic-generation (SHG) intensity (0.73 × KDP) with phase-matchable behavior. Optical characterization demonstrated that it has a UV cutoff edge located at 368 nm with a large energy band of 3.23 eV, accompanied by a wide transmission window, covering a 3-5 μm atmospheric window. Moreover, thermal properties implied that it possesses intriguing thermal stability of 987 °C and a congruent melting nature. Additionally, first-principles calculations unveiled that the NLO performance was primarily attributed to the collective effect of [GeO₄] and [ZnO₄] building units. These findings indicate that K₂ZnGe₂O₆ is a potential NLO crystal.

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.

Investigations on the Synthesis, Crystal Structure, Linear- and Nonlinear-Optical Properties of the Zinc Germanate Rb2ZnGe2O6

A quaternary nonlinear-optical (NLO) zinc germanate, Rb₂ZnGe₂O₆ (RZGO), was prepared from its stoichiometric melts, which belongs to the noncentrosymmetric space group C222₁ (No. 20) and features a 3D framework formed by GeO₄ tetrahedra, ZnO₄ tetrahedra, and Rb⁺ cations filling into the void space. RZGO displays a good phase-matchable second-harmonic-generation (SHG) intensity similar to that of KH₂PO₄. The UV cutoff edge for RZGO was found to be approximately 0.37 μm with a broad optical band gap of 3.22 eV, indicating a large laser-induced damage threshold. Further characterizations suggested that this compound displays an excellent thermal stability (1014 °C) and melts congruently. Besides, theoretical analysis was also implemented to investigate the electronic structures and effective NLO coefficients as well as the origin of the observed SHG response for RZGO. The present study can enrich the diversity of acentric germanate structures and pave the way for the synthesis of new NLO compounds.

KIn0.33IIITe0.67VITe2IVO7: Zirconolite-like Mixed-Valent Metal Oxide with a 3D Framework

We provide the material synthesis method, crystal structure information, and characterization of a novel mixed-valent metal oxide KIn_0.33^IIITe_0.67^VITe₂^IVO₇, closely related to zirconolite (CaZrTi₂O₇), a radioactive waste immobilized material, having a 3D framework. The reported metal oxide containing an alkali-metal cation (K⁺), main-group cation (In³⁺), tellurate, and tellurite has been synthesized as both single crystals and a pure polycrystalline phase through a hydrothermal synthesis method. Single-crystal X-ray diffraction indicates that KIn_0.33Te_2.67O₇ crystallizing in the orthorhombic space group Cmcm (No. 63) reveals a 3D framework structure with a 1D channel consisting of Te/InO₆ octahedra and TeO₄ polyhedra. An interesting transition reaction from KIn_0.33Te_2.67O₇ to KIn(TeO₃)₂ under hydrothermal conditions at 230 °C is discussed.

Ba3Sb2(PO4)4 and Cd3Sb2(PO4)4(H2O)2: Two New Antimonous Phosphates with Distinct [Sb(PO4)2] Structure Types and Enhanced Birefringence

Two new antimonous phosphates, namely Ba₃Sb₂(PO₄)₄ and Cd₃Sb₂(PO₄)₄(H₂O)₂, have been successfully prepared through mild hydrothermal reactions. Ba₃Sb₂(PO₄)₄ features a 1D [Sb(PO₄)₂]^3- chain structure separated by Ba²⁺ cations while Cd₃Sb₂(PO₄)₄(H₂O)₂ presents a 2D [Sb(PO₄)₂]^3- layered structure with Cd²⁺ located at the interlayer space. The [Sb(PO₄)₂]^3- chain in Ba₃Sb₂(PO₄)₄ is the first example of 1D antimonous phosphate structure, and Cd₃Sb₂(PO₄)₄(H₂O)₂ represents the first d¹⁰ transition metal antimonous phosphate. Based on UV-vis-NIR spectra, both Ba₃Sb₂(PO₄)₄ and Cd₃Sb₂(PO₄)₄(H₂O)₂ can display large optical band gaps (4.30 and 4.36 eV, respectively). But their transparent ranges are quite different because of the coordination water of Cd₃Sb₂(PO₄)₄(H₂O)₂ (500-2000 and 500-1300 nm for Ba and Cd compounds). The anhydrous Ba₃Sb₂(PO₄)₄ shows high thermal stability in the nitrogen atmosphere (900 °C). Because of the incorporation of the lone pair cation of Sb(III), the birefringence of Ba₃Sb₂(PO₄)₄ and Cd₃Sb₂(PO₄)₄(H₂O)₂ has been enhanced to 0.086 and 0.053 at 532 nm, respectively.

Lone-pair self-containment in pyritohedron-shaped closed cavities: optimized hydrothermal synthesis, structure, magnetism and lattice thermal conductivity of Co15F2(TeO3)14

Co₁₅F₂(TeO₃)₁₄ features an extremely rare example of the Te(iv) lone pairs self-containment in pyritohedron-shaped [(TeO₃)₁₄]²⁸⁻ units, which allows Te atom to vibrate with large amplitude, leading to extremely low lattice thermal conductivity.

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.

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.

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.

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.