BiGa(SeO3)3: A Phase Matchable SHG Material Achieved by Cation Substitution

RE(HSeO3)(SeO3)(H2O)·(H2O) (RE = Y, Gd): Two Rare-Earth Selenite Nonlinear Optical Crystals with Wide Band Gaps

The band gap is one of the significant parameters for nonlinear optical (NLO) materials, which is the key factor in their application band in laser technology. This study reports the synthesis of two rare-earth selenite NLO crystals, namely, RE(HSeO3)(SeO3)(H2O)·(H2O) (RE = Y, Gd), utilizing the hydrothermal technique. The two compounds are isostructural, and both belong to the orthorhombic chiral space group of P212121 and possess three-dimensional structures consisting of two-dimensional rare-earth selenite layers connected by hydrogen bonds (O-H··O). These two compounds exhibit a moderate second harmonic generation intensity of 0.5/0.8 × KDP and wide energy band gaps reaching 4.8 eV. Comprehensive density functional theory analyses based on first-principles were carried out to unravel the relationship between the structural and corresponding properties. This work provides an approach for band gap enlargement in rare-earth selenite systems and is beneficial to the design of future NLO materials.

KBa3M2F14Cl (M = Zr, Hf): novel short-wavelength mixed metal halides with the largest second-harmonic generation responses contributed by mixed functional moieties

The development of short-wavelength nonlinear optical (NLO) materials is indispensable and urgently required for further applications. Halides have been disregarded as potential NLO materials with deep-ultraviolet (DUV) cutoff edges due to their weak second-harmonic generation (SHG) response and poor birefringence. Here, two novel and isostructural halides, KBa3M2F14Cl (M = Zr (KBZFC), Hf (KBHFC)), possess structures that are formed by isolated MF7 monocapped triangular prisms and dissociative K+, Ba2+, and Cl- ions. Compared with reported metal halides that are transparent to the DUV region, KBZFC and KBHFC possess the strongest SHG responses (approximately 1, 0.9 × KH2PO4), which are contributed by the synergistic effect of MF7 (M = Zr, Hf) groups, Ba2+ cations, and Cl- ions. The zero-dimensional structures favour sufficient birefringences (0.12, 0.10 @ 1064 nm) for phase-matchable (PM) behaviours. The discovery of KBZFC and KBHFC showcases the potential of NLO mixed metal halides transparent to the DUV region.

Polar Bismuth Selenite Iodate Oxide BiSeIO6 with Three Types of Lone Pair Cations in One Structure

Novel bismuth selenite iodate oxide BiSeIO₆ was synthesized in a mild hydrothermal condition. BiSeIO₆ was crystallized in the polar space group Pna2₁ of an orthorhombic system. The crystal structure features a three-dimensional framework composed of three types of lone pair cations with distorted BiO₇ polyhedra, SeO₃ pyramids, and IO₃ pyramids in one structure. Interestingly, BiSeIO₆ exhibits a strong and phase-matchable second-harmonic generation (SHG) of ∼6 times that of KH₂PO₄ (KDP). Dipole moment analysis shows that all three local acentric groups of BiO₇, SeO₃, and IO₃ cooperatively contribute to the large macroscopic polarization and thereby strong SHG efficiency of BiSeIO₆. In addition, BiSeIO₆ has a broad transparency range from 0.35 to 11 μm, indicating its promising nonlinear optical applications from visible to mid-infrared bands.

Towards a relationship between photoluminescence emissions and photocatalytic activity of Ag2SeO4: combining experimental data and theoretical insights

A systematic theoretical and experimental study was carried out to find a relationship between photoluminescence emissions and photocatalytic activity of Ag₂SeO₄ obtained by different synthesis methods (sonochemistry, ultrasonic probe, coprecipitation and microwave assisted hydrothermal synthesis). Experimental characterization techniques (XRD with Rietveld refinement, Raman, FTIR, UV-vis, XPS and photoluminescence spectroscopy) were used to elucidate its structural order at short, medium, and long ranges. Morphological analysis performed by FE-SEM showed distinct morphologies due to the different methods of synthesis. Based on density functional theory (DFT) calculations, it was possible to study in detail the Ag₂SeO₄ surface properties, including its surface energy, geometry, and electronic structure for the (100), (010), (001), (101), (011), (110), (111), (021), (012) and (121) surfaces. The equilibrium morphology of Ag₂SeO₄ was predicted as a truncated octahedron with exposed (111), (001), (010) and (011) surfaces. Photoluminescence emissions showed a band covering the visible spectrum, and the Ag₂SeO₄ obtained by the coprecipitation method presented the most intense band with a maximum in the red region. Photocatalytic results confirmed that Ag₂SeO₄ synthesized by the sonochemistry method is the best photocatalyst for rhodamine B degradation under UV light irradiation.

HgTeO2F(OH): A Nonlinear Optical Oxyfluoride Constructed of Active [TeO2F(OH)]2- Pyramids and V-Shaped [HgO2]2- Groups

Oxyhalides possessing the merits of oxides and halides have widely received attention for their comprehensive physical performances, especially as potential nonlinear optical (NLO) crystals. Here, based on conventional strategy for obtaining acentric compounds, a Te⁴⁺ lone-pair cation was introduced into oxyhalides, and one oxyfluoride, HgTeO₂F(OH), was obtained via a hydrothermal reaction. Crystallized in the polar space group Pca2₁, the layered structure of HgTeO₂F(OH) is composed of V-shaped [HgO₂]^2- groups and [TeO₂F(OH)]^2- pyramids, in which the [TeO₂F(OH)]^2- pyramid first served as the NLO functional motif. Its powder sample exhibits a phase-matchable SHG response of 1.1 × KH₂PO₄ at 1064 nm, and its birefringence (0.09@1064 nm) is sufficient for phase-matchable behavior, which manifests its comprehensive capacity as a promising NLO candidate. Theoretical calculations about electronic structure and optical properties are also carried out, revealing that the Te⁴⁺ lone-pair cation makes the predominant contribution to the SHG effect and synergizes with the [HgO₂]^2- groups.

Structure, Photoluminescence Emissions, and Photocatalytic Activity of Ag2SeO3: A Joint Experimental and Theoretical Investigation

In this paper, we report the synthesis of silver selenite (Ag₂SeO₃) by different methods [sonochemistry, ultrasonic probe, coprecipitation, and microwave-assisted hydrothermal methods]. These microcrystals presented a structural long-range order as confirmed by X-ray diffraction (XRD) and Rietveld refinements and a structural short-range order as confirmed by Fourier transform infrared (FTIR) and Raman spectroscopies. X-ray photoelectron spectroscopy (XPS) provided information about the surface of the samples indicating that they were pure. The microcrystals presented different morphologies and sizes due to the synthesis method as observed by field emission scanning electron microscopy (FE-SEM). The optical properties of these microcrystals were evaluated by ultraviolet-visible (UV-vis) spectroscopy and photoluminescence (PL) measurements. Thermal analysis confirmed the temperature stability of the as-synthetized samples. Further trapping experiments prove that the holes and hydroxyl radicals, to a minor extent, are responsible for the photocatalytic reactions. The experimental results are sustained by first-principles calculations, at the density functional theory (DFT) level, to decipher the structural parameters, electronic properties of the bulk, and surfaces of Ag₂SeO₃. By matching the experimental FE-SEM images and theoretical morphologies, we are capable of finding a correlation between the morphology and photocatalytic activity, along with photodegradation of the Rhodamine B dye under UV light, based on the different numbers of unsaturated superficial Ag and Se cations (local coordination, i.e., clusters) of each surface.

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.

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.