Structure-property relationship in nonlinear optical materials with π-conjugated CO3 triangles

Centrosymmetric or Noncentrosymmetric? Transition Metals Talking in K2TGe3S8(T = Co, Fe)

Two new quaternary sulfides K₂TGe₃S₈(T = Co, Fe) have been synthesized by a high-temperature solid-state routine and flux growth method. The crystal growth process of K₂TGe₃S₈(T = Co, Fe) was elucidated by in situ powder X-ray diffraction and DSC thermal analysis. The millimeter-sized crystals of K₂TGe₃S₈(T = Co, Fe) were grown. K₂CoGe₃S₈ crystallizes in a new structure type in centrosymmetric space group P1 (no. 2) with unit cell parameters of a = 7.016(1) Å, b= 7.770(1) Å, c = 14.342(1) Å, α = 93.80(1)°, β = 92.65(1)°, γ = 114.04(1)°. K₂FeGe₃S₈ crystallizes in the K₂FeGe₃Se₈ structure type and the noncentrosymmetric space group P2₁ (no. 4) with unit cell parameters of a = 7.1089(5)Å, b = 11.8823(8) Å, c = 16.7588(11) Å, β = 96.604(2)°. There is a high structural similarity between K₂CoGe₃S₈ and K₂FeGe₃S₈. The larger volume coupled with higher degrees of distortion of the [FeS₄] tetrahedra compared to the [CoS₄] tetrahedra accounts for the structure's shift from centrosymmetric to noncentrosymmetric. The theory simulation confirms that [TS₄]_{T= Co or Fe} tetrahedra play a crucial role in controlling the structure and properties of K₂TGe₃S₈(T = Co, Fe). The measured optical bandgaps of K₂CoGe₃S₈ and K₂FeGe₃S₈ are 2.1(1) eV and 2.6(1) eV, respectively. K₂FeGe₃S₈ shows antiferromagnetic ordering at 24 K while no magnetic ordering was detected in K₂CoGe₃S₈. The magnetic measurements also demonstrate the divalent nature of transition metals in K₂TGe₃S₈(T = Co, Fe).

High-Performance Second-Harmonic-Generation (SHG) Materials: New Developments and New Strategies

ConspectusSecond-harmonic-generation (SHG) causes the frequency doubling of light, which is very useful for generating high-energy lasers with specific wavelengths. Noncentrosymmetry (NCS) is the first requirement for an SHG process because the SHG coefficient is zero (χ² = 0) in all centrosymmetric structures. At this stage, developing novel NCS crystals is a crucial scientific topic. Assembling polar units in an addictive fashion can facilely form NCS crystals with outstanding SHG performance. In this way, our group has obtained many different NCS crystals with extremely large SHG intensities (>5 × KDP or 1 × KTP). In this Account, we first provide a brief review of the development of SHG materials and concisely highlight the features of the excellent SHG materials. Then, we present four facile and rational molecular design strategies: (1) Traditional BO₃^3--based crystals feature short absorption edges but usually suffer from relatively weak SHG performance (<5 × KDP). The combination of two types of pure π-conjugated anions (BO₃^3- and NO₃^-) in a parallel fashion in the same compound has afforded a metal borate nitrate with a strong SHG effect. (2) To overcome the problems of the weak SHG effect and small birefringence in the less anisotropic QO₄-based compounds, highly polarizable cations such as Hg²⁺ and Bi³⁺ are introduced into these systems, which greatly enhances both SHG effects and birefringence. (3) Iodate anions can be condensed into polynuclear iodate anions with a higher density of I⁵⁺ per unit cell, hence polyiodate anions can serve as excellent SHG-active groups. We developed a novel synthesis method for hydrothermal reactions under a phosphoric acid medium and obtained a series of metal polyiodates with strong SHG effects. In addition, as the number of iodate groups increases, the structural configuration of the polyiodate anion changes from linear to bent. (4) We introduce the concept of aliovalent substitution which features site-to-site atomic displacement at the structural level. Such aliovalent substitution led to new materials that have the same chemical stoichiometries or structural features as their parent compounds. Thus, aliovalent substitution can provide more experimental opportunities and afford new high-performance SHG materials. The introduction of a fluoride anion and the replacement of metal cations in the MO₆ octahedron can result in new metal iodates with balanced properties including a large SHG effect, a wide band gap, and a high laser-induced damage threshold (LIDT) value. Finally, we briefly discuss several problems associated with the studies of SHG materials and give some prospects for SHG materials in the future.

Molecular Engineering toward an Enlarged Optical Band Gap in a Bismuth Sulfate via Homovalent Cation Substitution

Materials capable of generating coherent short-wave (<300 nm) light have attracted extensive scientific and technical interest due to their wide utilization in laser research. In this study, a the rare-earth-metal sulfate NaCe(SO₄)₂(H₂O) (NCSO) was synthesized through a hydrothermal method, while NaBi(SO₄)₂(H₂O) (NBSO) was successfully obtained via a homovalent cation substitution of the parent compound NCSO under hydrothermal conditions. The space groups of crystalline NCSO and NBSO are P3₁2₁ and P3₂2₁, respectively. Both compounds have similar connectivities which feature a three-dimensional channel structure formed by asymmetric [CeO₉]^15-/[BiO₉]^15- tricapped trigonal prisms and distorted [SO₄]^2- tetrahedra. The introduction of Bi³⁺ with larger ionic radii and stereochemically active lone-pair electrons simultaneously enhanced the SHG effect and band gap of NBSO in comparison to its parent compound NCSO. In contrast to NCSO, which possesses a narrow energy band gap (2.46 eV), NBSO displays the largest energy band gap (4.54 eV) among the reported bismuth sulfate NLO materials. Powder frequency-doubling-effect measurements exhibit that NCSO and NBSO possess phase-matchable SHG responses of 0.2 × KDP and 0.38 × KDP at 1064 nm, respectively. Theoretical studies have been implemented to further elucidate the structure-performance relationships of the two compounds. Experimental and theoretical studies both demonstrate that NBSO may be a promising nonlinear material applied in the short-wavelength region.

Synergism of multiple functional chromophores significantly enhancing the birefringence in layered non-centrosymmetric chalcohalides

Compared with one or two functional chromophores materials, Hg₃AsQ₄X (Q = S, Se; X = Cl, Br, I) with multiple ones generate extremely large birefringence due to the synergism of the d¹⁰ cation Hg²⁺, lone pair layer of As³⁺ and mixed anions Q²⁻/X⁻.

High-pressure synthesis and crystal structure of non-centrosymmetric K2Ca3(CO3)4

A new non-centrosymmetric K2Ca3(CO3)4 crystal (P212121, a = 7.39134(18) Å, b = 8.8153(2) Å, c = 16.4803(4) Å) was synthesized in a multi-anvil press at a pressure of 3 GPa and temperature of 975 °C.

From [B6O13]8− to [GaB5O13]8− to [Ga{B5O9(OH)}{BO(OH)2}]2−: synthesis, structure and nonlinear optical properties of new metal borates

By tuning the synthetic conditions, a 3D gallo-borate with a high second-harmonic generation response was obtained.

Comprehensive Density Functional Theory Studies of Vibrational Spectra of Carbonates

Within the framework of the density functional theory (DFT) and the hybrid functional B3LYP by means of the CRYSTAL17 program code, the wavenumbers and intensities of normal oscillations of MgCO3, CaCO3, ZnCO3, CdCO3 in the structure of calcite; CaMg(CO3)2, CdMg(CO3)2, CaMn(CO3)2, CaZn(CO3)2 in the structure of dolomite; BaMg(CO3)2 in the structure of the norsethite type; and CaCO3, SrCO3, BaCO3, and PbCO3 in the structure of aragonite were calculated. Infrared absorption and Raman spectra were compared with the known experimental data of synthetic and natural crystals. For lattice and intramolecular modes, linear dependences on the radius and mass of the metal cation are established. The obtained dependences have predictive power and can be used to study solid carbonate solutions. For trigonal and orthorhombic carbonates, the linear dependence of wavenumbers on the cation radius RM (or M-O distance) is established for the infrared in-plane bending mode: 786.2-65.88·RM and Raman in-plane stretching mode: 768.5-53.24·RM, with a correlation coefficient of 0.87.