Pb2(SeO3)(SiF6): the first selenite fluorosilicate with a wide bandgap and large birefringence achieved through perfluorinated group modification

Birefringent crystals serve as the core elements of polarizing optical devices. However, the inherent challenge of balancing bandgap and birefringence poses a significant hurdle in designing crystals with excellent overall performance. In this study, we propose a novel approach, namely modification with perfluorinated groups, to achieve dual enhancement of the bandgap and birefringence of selenite materials. We have successfully synthesized the first selenite fluorosilicate, namely, Pb2(SeO3)(SiF6). This compound exhibits a three-dimensional structure composed of two-dimensional lead selenite layers bridged by SiF6 octahedrons. Notably, by introducing a perfluorinated SiF6 group, the bandgap of the lead selenite compound has been expanded to 4.4 eV. Furthermore, Pb2(SeO3)(SiF6) demonstrates a large birefringence (0.161 @ 546 nm), surpassing most of the selenite compounds with a bandgap larger than 4.2 eV. Theoretical calculations suggest that the large birefringence of Pb2(SeO3)(SiF6) can be attributed to the synergistic effects of SeO3, PbO4 and PbO3F4 polyhedrons. Our research not only pioneers a new system for selenite materials, enriching the diversity of selenite structures, but also provides a design methodology for obtaining wide bandgap birefringent selenite.

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.

The First UV Nonlinear Optical Selenite Material: Fluorination Control in CaYF(SeO3 )2 and Y3 F(SeO3 )4

It is a great challenge to develop UV nonlinear optical (NLO) material due to the demanding conditions of strong second harmonic generation (SHG) intensity and wide band gap. The first ultraviolet NLO selenite material, Y₃ F(SeO₃ )₄ , has been obtained by control of the fluorine content in a centrosymmetric CaYF(SeO₃ )₂ . The two new compounds represent similar 3D structures composed of 3D yttrium open frameworks strengthened by selenite groups. CaYF(SeO₃ )₂ has a large birefringence (0.138@532 nm and 0.127@1064 nm) and a wide optical band gap (5.06 eV). The non-centrosymmetric Y₃ F(SeO₃ )₄ can exhibit strong SHG intensity (5.5×KDP@1064 nm), wide band gap (5.03 eV), short UV cut-off edge (204 nm) and high thermal stability (690 °C). So, Y₃ F(SeO₃ )₄ is a new UV NLO material with excellent comprehensive properties. Our work shows that it is an effective method to develop new UV NLO selenite material by fluorination control of the centrosymmetric compounds.

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.

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.

KCs2 [Pb2 Br5 (HCOO)2 ]: A Polar 3D Lead-Bromide Framework Exhibiting Strong Second-Harmonic Generation Response

The discovery of new nonlinear optical (NLO) crystals with excellent properties is in urgently demand because of their ability to generate coherent light. Herein, we report an unique NLO lead bromide formate, KCs₂ [Pb₂ Br₅ (HCOO)₂ ], which has been synthesized by a mix-solvothermal method. KCs₂ [Pb₂ Br₅ (HCOO)₂ ] exhibits strong phase-matching second-harmonic generation (SHG) response (6.5×KDP), large birefringence (0.16@ 1064 nm), and a wide transparent window in most visible light and mid-IR region. Interestingly, KCs₂ [Pb₂ Br₅ (HCOO)₂ ] features a polar 3D lead-bromide framework in which adjacent Pb-Br layers containing coplanar Pb₆ Br₆ rings are not only parallel to each other, but also orient in the same direction. These oriented arrangements are responsible for the strong SHG response and large birefringence that are elucidated by both local dipole moment and theoretical calculations. This research provides a new strategy to explore subsequent NLO crystals.

AgBi(SO4)(IO3)2: aliovalent substitution induces structure dimensional upgrade and second harmonic generation enhancement

A new 3D noncentrosymmetric mixed-metal sulfate iodate, AgBi(SO₄)(IO₃)₂, has been designed based on a 2D iodate of AgBi(IO₃)₄via the aliovalent substitution of [IO₃]^- groups by [SO₄]^2- groups. Enhancement of the second harmonic generation response (3.0 × KDP to 3.9 × KDP) has been achieved while retaining a favorable NLO framework. This is a novel strategy to upgrade structural dimensions.

First chiral fluorinated lead vanadate selenite Pb2(V2O4F)(VO2)(SeO3)3 with five asymmetric motifs and large optical properties

The first fluorinated lead vanadate selenite Pb2(V2O4F)(VO2)(SeO3)3 (PVOFS) was successfully synthesized via a mild hydrothermal method. This compound crystallizes in the chiral space group P212121 of the orthorhombic system and it is the first noncentrosymmetric structure in the PbII-VV-SeIV-O-F system. PVOFS is composed of five kinds of second-order Jahn-Teller susceptible asymmetric motifs, including three distinct types of vanadium-centered polyhedral units ([VO5F], [VO6] and [VO5]), [SeO3] pyramids and Pb2+ cations. It features a unique three-dimensional open framework structure displaying three types of tunnels (10-, 8- and 7-membered rings), which enriches the structural diversity for fluorinated vanadate selenite systems. Optical property studies revealed that PVOFS shows a second-harmonic generation response of 0.3 times that of the commercial KH2PO4 with phase matching behavior, a wide transparent region covering IR windows, an optical band gap of 2.35 eV, a high laser damage threshold of 61 times that of AgGaS2, and a large birefringence of 0.105 at 1064 nm. Theoretical calculations have been performed to clarify the correlation between the molecular structure and the optical properties of PVOFS.

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.

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.

Pb2Cd(SeO3)2X2 (X = Cl and Br): two halogenated selenites with phase matchable second harmonic generation

The second harmonic generation (SHG) efficiencies of Pb₂Cd(SeO₃)₂X₂ (X = Cl and Br) are higher than that of commercial KDP (KH₂PO₄) and their laser damage thresholds are 30 times more than that of AGS (AgGaS₂).

Cs2CdV2O6Cl2 and Cs3CdV4O12Br: two new non-centrosymmetric oxyhalides containing d0 and d10 cations and exhibiting second harmonic generation activity

Two new non-centrosymmetric oxyhalides containing d⁰ and d¹⁰ cations and exhibiting strong second harmonic generation activity.

Pb2GaF2(SeO3)2Cl: Band Engineering Strategy by Aliovalent Substitution for Enlarging Bandgap while Keeping Strong Second Harmonic Generation Response

Wide bandgap and strong second-order generation (SHG) effect are two crucial but contradictory conditions for practical nonlinear optical (NLO) materials. Herein, a new NLO crystal Pb₂GaF₂(SeO₃)₂Cl (I) containing novel functional (GaO₃F₃)^6- octahedra is designed and synthesized by a rational band engineering strategy with aliovalent substitution. Benefiting from the removal of transition metal cations and the introduction of bridged F anions, I exhibits the widest bandgap among all reported phase-matchable NLO selenites. Meanwhile, a strong SHG response more than 4.5 times of KH₂PO₄ (KDP) is maintained. The dominate role of the (GaO₃F₃)^6- groups to the enlarged bandgap in I are elucidated by first-principles studies.