Polar Noncentrosymmetric ZnMoSb2O7 and Nonpolar Centrosymmetric CdMoSb4O10: d10 Transition Metal Size Effect Influencing the Stoichiometry and the Centricity

Influence of Cation-Size Effects of Alkaline Earth (Ae) Metals on Dimensionality and Optical Anisotropy Regulation in K4AeP2S8 Thiophosphates

Cationic substitution demonstrates significant potential for regulating structural dimensionality and physicochemical performance owing to the cation-size effect. Leveraging this characteristic, this study synthesized a new family of K4AeP2S8 (Ae = alkaline earth elements: Mg, Ca, Sr, and Ba) thiophosphates, involving the substitution of Ae2+ cations. The synthesized compounds crystallized in distinct space groups, monoclinic P2/c (Ae = Mg) versus orthorhombic Ibam (Ae = Ca, Sr, and Ba), exhibiting intriguing dimensionality transformations from zero-dimensional (0D) [Mg2P4S16]8- clusters in K4MgP2S8 to 1D ∞[AeP2S8]4- chains in other K4AeP2S8 thiophosphates owing to the varying ionic radii of Ae2+ cations, Ae-S bond lengths, and coordination numbers of AeSn (Mg: n = 6 versus other: n = 8). Experimental investigations revealed that K4AeP2S8 thiophosphates featured wide optical bandgaps (3.37-3.64 eV), and their optical absorptions were predominantly influenced by the S 3p and P 3s orbitals, with negligible contributions from the K and Ae cations. Notably, within the K4AeP2S8 series, birefringence (Δn) increased from K4MgP2S8 (Δn = 0.034) to other K4AeP2S8 (Δn = 0.050-0.079) compounds, suggesting that infinite 1D chains more significantly influence Δn origins than 0D clusters, thus offering a feasible approach for enhancing optical anisotropy and exploring potential new birefringent materials.

Two Short-Wave UV Antimony(III) Sulfates Exhibiting Large Birefringence

In the present work, we have successfully obtained two new UV antimony-based sulfates, NH4Sb(SO4)2 and Ca2Sb2O(SO4)4, by a conventional hydrothermal method. Interestingly, both compounds share similar structural building blocks, such as SbO4 seesaws and SO4 tetrahedra, yet they endow discrepant birefringence values measured at 546 nm with values of 0.150 and 0.114, respectively, owing to the different distortions of the SbO4 groups with SCALP electrons. Moreover, both compounds display large band gaps (4.32 and 4.43 eV, respectively), so they can be used as short-wavelength UV birefringent materials. Moreover, NH4Sb(SO4)2 is a noncentrosymmetric compound, showing a frequency doubling effect of 0.2 × KDP. Detailed structural analyses and calculations confirm the source of superior optical performance and the reasons for the different birefringence of the two compounds. This work provides ideas for the following discovery of antimony-based optical materials with excellent properties.

Mixed-valence compounds based on heterometallic-oxo-clusters containing Sb(III,V): crystal structures and proton conduction

Two isostructural Co(Cd)-antimony-oxo tartrate cluster-based compounds with a one-dimensional (1-D) belt-like structure, namely H9.2[Co(H2O)6]{M0.5(H2O)3.5{M'(H2O)4[SbVO6[Co4.2(H2O)5SbIII6(μ3-O)2(tta)6]]}}2·nH2O (M = Co, M' = Co, n = 9 (1); M = Cd0.39/Co0.61, M' = Cd0.24/Co0.76, n = 7 (2); H4tta = tartaric acid), have been synthesized by solvothermal methods. It is noteworthy that the relatively rare mixed-valence Sb(III,V) exists in the structures. The anionic clusters in both compounds appear to be in a sandwich configuration; the top and bottom layers are based on {Sb3(μ3-O)(tta)3} brackets, and the intermediate layer is occupied by {SbVO6[Co4.2(H2O)5]}. The title compounds have been characterized by single-crystal X-ray diffraction, powder X-ray diffraction, elemental analyses, thermogravimetric analyses, and UV-Vis spectroscopy. We chose compound 2 as a representative to test its proton conductivity, and the results show that the conductivity can reach 1.42 × 10-3 S cm-1 at 85 °C under 98% relative humidity.

{[In2S7]8-}∞ Chain and Isolated HgS4 Planar Unit Constructed One-Dimensional Pentanary Sulfide K2Ba7HgIn4S16 Exhibiting Nonlinear-Optical Activity

Hg-based chalcogenides possess diverse structures, large nonlinear-optical (NLO) responses, and suitable birefringences, making them potentially suitable for numerous crucial criteria of practical application as infrared (IR) NLO crystals. Here, a new pentanary Hg-based sulfide K₂Ba₇HgIn₄S₁₆ has been discovered by a high-temperature solid-state method. It crystallizes in the orthorhombic P2₁2₁2 space group, and its one-dimensional structure is constructed by {[In₂S₇]^8-}_∞ chains and isolated [HgS₄]^6- planar quadrilateral units located bewteeen the chains, representing a novel type of chalcogenide. K₂Ba₇HgIn₄S₁₆ exhibits a moderate NLO effect of 0.5 × AGS at 2.1 μm and a high laser-induced damage threshold of ∼5.8 × AGS, as well as a band gap of 2.98 eV, demonstrating that K₂Ba₇HgIn₄S₁₆ is a potential IR NLO material. This work enriches the structural chemistry of chalcogenides and the family of Hg-based IR NLO chalcogenides.

Sb5O7I: Exploration of Ternary Antimony-Based Oxyhalide as a Nonlinear-Optical Material

Metal oxyhalides are attracting extensive interest for their enchanting structures and diverse properties. Herein, a ternary antimony oxyiodide, Sb₅O₇I with the new hexagonal noncentrosymmetric P6₃ structure is systematically surveyed by focusing on its nonlinear-optical (NLO) behavior. Its two-dimensional structure is constructed by {Sb₂[Sb₃O₇]}_∞⁺ layers separated by charge-balanced I^- anions. The second-harmonic-generation measurement result suggests that Sb₅O₇I is NLO-active, and the effect is assigned to the [SbO₃]^3- triangular pyramids' contribution. Sb₅O₇I shows a direct optical energy gap of 3.22 eV, which is the largest among all reported ternary oxyiodides. This work is the first investigation of ternary NLO Sb-based oxyhalides and enriches the study of metal oxyhalides as promising NLO materials.

A6Mo2P4O19 (A = Rb, Cs) and Rb2MoP2O9: new molybdophosphates with distinct polyanionic configurations

The exploration of A-Mo-P-O (A = Rb, Cs) systems has allowed several new Mo(V) phosphates, Rb₂MoP₂O₉, Rb₆Mo₂P₄O₁₉ and Cs₆Mo₂P₄O₁₉, to be synthesized through the spontaneous nucleation method. Single-crystal X-ray diffraction analysis reveals that the identical stoichiometry compounds Rb₂MoP₂O₉ and Cs₂MoP₂O₉ belong to different space groups C2/c and Pbca, respectively. Both compounds consist of dissimilar 1D [Mo-O-P]_∞ chains with different repeated building units, while monovalent cations fill in spaces to form 3D structures. However, Rb₆Mo₂P₄O₁₉ and Cs₆Mo₂P₄O₁₉ are isostructural and crystallize in the same space group of P2₁/c. They exhibit a 3D framework structure with 0D Mo₂O₅O₆P₄O₈ groups, which are separated by Rb/Cs atoms. Interestingly, structural relationships between the different monophosphates of the A-Mo-P-O (A = Rb, Cs) systems are presented in which distinct polyanionic configurations appear owing to the A/P ratios, as well as the size of the univalent cations. Further, detailed structural comparisons, optical properties and theoretical calculations are also discussed.

(R)- and (S)-[C8H10NO3]2[NbOF5]: Noncentrosymmetric niobium oxyfluorides with large optical anisotropy

Huge crystals of noncentrosymmetric (NCS) organic-inorganic hybrid niobium oxyfluorides, (R)-[C8H10NO3]2[NbOF5] [(R)-Nb] and (S)-[C8H10NO3]2[NbOF5] [(S)-Nb], have been easily grown via a slow evaporation method in high yields through the systematic driving...

Second-Harmonic-Generation-Active Oxyhalides: CuSb2O3X (X = Cl, Br)

Metal oxyhalides have attracted broad interest recently because of their diverse structures and versatile properties. Here, two oxyhalides, CuSb₂O₃Cl (1) and CuSb₂O₃Br (2), were studied by focusing on their nonlinear-optical properties. They are crystallized in the noncentrosymmetric monoclinic Cc structure, and the layered structures could be derived from a 1:1 combination of CuX- (X = Cl, Br) and Sb₂O₃-type slabs. Their energy gaps were determined to be 2.76 and 2.64 eV. The second-harmonic-generation (SHG) test suggests that they are nonlinear-optical-active, and the effects are ascribed to the contribution of CuX₃O units. Meanwhile, the SbO₃ units' arrangement has a small contribution to the SHG effects. This work is the pioneer SHG investigation of the M^I-M^III-O-X (M^I = Cu, Ag; M^III = trivalent As, Sb, Bi; X = halogen) family.

Systematic centricity control using a chiral template: novel noncentrosymmetric polar niobium oxyfluorides and tantalum fluorides directed by chiral histidinium cations, [(l-hisH2)NbOF5], [(d-hisH2)NbOF5], [(l-hisH2)TaF7], and [(d-hisH2)TaF7]

Novel noncentrosymmetric polar niobium and tantalum (oxy)fluorides have been systematically synthesized by using a chiral histidinium template.

Two-stage evolution from phosphate to sulfate of new KTP-type family members as UV nonlinear optical materials through chemical cosubstitution-oriented design

KTiOPO4 (KTP) is a classic commercial nonlinear optical (NLO) crystal, but its narrow bandgap (3.52 eV) prevents its practical application in the ultraviolet (UV) region. Many trials to widen the narrow bandgap of KTP have failed in the past few decades. A chemical cosubstitution strategy was implemented to design new members of the KTP-type family as potential UV NLO materials. First, a novel centrosymmetric KTP-type compound NH4SbFPO4·H2O with a sharply enlarged bandgap (5.01 eV) was obtained through three-site aliovalent substitution. Second, the noncentrosymmetric NH4SbF2SO4 was synthesized by the introduction of more F- anions to destroy the crystal symmetry and SO42- to replace PO43- for balancing the charge in NH4SbFPO4·H2O, which realized the transformation from a visible phosphate system to solar blind UV sulfate system for KTP-type family NLO materials. The preliminary experimental results indicated that NH4SbF2SO4 is a promising solar blind UV NLO material. The first-principles calculations revealed that the sharply enlarged bandgap resulted from the substitution of the transition metal cations with the main group metal cations and the introduction of F- anions with high electronegativity.

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.

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.

Alkali earth MOx (x = 6, 7, 9, 12) polyhedra tuned cadmium selenites with different dimensions and diverse SeO32− coordinations

Alkali earth cation-tuned framework of cadmium selenites resulting in evolvement of the structures from high dimension to low dimension.

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.

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.