AX·(H₂SeO₃)_n (A = K, Cs; X = Cl, Br; n = 1, 2): A Series of Ionic Cocrystals as Promising UV Birefringent Materials with Large Birefringence and Wide Band Gap

Abstract

The design and syntheses of new birefringent crystals will be of great importance in commercial applications and materials science. A series of ultraviolet (UV) birefringent crystals, AX•(H 2 SeO 3 ) n (A = K, Cs; X = Cl, Br; n = 1, 2), with large sizes up to 23 × 6 × 3 mm 3 , was successfully synthesized by simple aqueous solution method. These four compounds belong to three different space groups. Isomorphic KCl•(H 2 SeO 3 ) 2 and CsCl•(H 2 SeO 3 ) 2 crystallize in the P 1 space group, while CsBr•(H 2 SeO 3 ) 2 and CsCl•H 2 SeO 3 crystallize in the P2 1 /m and P2 1 /c space groups, respectively. They exhibit cocrystal structures composed of [2(H 2 SeO 3 )] ∞ and [AX] ∞ frameworks, ingeniously inheriting the large optical anisotropy of selenite and the wide band gap of alkali metal halide. And it proves that these compounds indeed possess large birefringence (0.1−0.17 at 532 nm) and short UV cutoff edges (227−239 nm), achieving a balance of optical properties. This research affords a simple and viable strategy for the design and syntheses of new UV birefringent crystals. Besides, it is also found that the n value and ionic size (A and X ions) have important influences on the crystal structures and optical properties of AX•(H 2 SeO 3 ) n . And this will promote further understanding of the alkali metal halide selenite family.

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

“… 18 It can be observed that Pb 2 (SeO 3 )(SiF 6 ) exhibits a large birefringence in these selenites, second only to KCl·(H 2 SeO 3 ) 2 (0.17 @ 532 nm). 64 However, Pb 2 (SeO 3 )(SiF 6 ) demonstrates the largest birefringence among the selenites without hydrogen, and it is the sole example of lead selenite with a large birefringence (Δ n ≥ 0.1) and wide bandgap ( E g ≥ 4.2 eV). This result highlights the effectiveness of perfluorinated group modification in achieving dual enhancement of the bandgap and birefringence in selenite systems.…”

Designing excellent UV birefringent materials through the synergistic interaction of two highly distorted functional groups

“…However, the introduction of SCALP often leads to a significant redshift in the cut off edge of compounds. In current research, to ensure the bandgap of compounds, two strategies are commonly employed: (1) Introducing alkali metals and alkaline earth metals without d–d and f–f transitions to prevent the redshift of the cut-off edges of the compounds, 30 such as K 2 Sr 4 (PO 3 ) 10 (6.20 eV), 31 Rb 3 MgB 5 O 10 (6.49 eV), 32 Cs 6 Mg 6 (PO 3 ) 18 (6.52 eV), 33 KBa 2( PO 3 ) 5 (7.43 eV); 34 (2) Introducing halogen ions with high electronegativity, which facilitates a blueshift in the cut-off edge of the compounds, 35 such as Rb 3 Pb 2 (CH 3 COO) 2 Br 5 (3.12 eV), 36 (NH 4 )SbCl 2 (SO 4 ) (4.54 eV), 37 CsBr·(H 2 SeO 3 ) 2 (5.19 eV), 38 KCl·(H 2 SeO 3 ) 2 (5.44 eV), 38 Na 2 B 6 O 9 F 2 (7.33 eV). 39…”

NaVSeO5: synergistic combinations to synthesize excellent birefringent materials

“…1,2 It is essential to examine artificial crystals’ birefringence in optical functional materials. 3–7 Birefringent crystal materials, a type of linear optical material, are widely used in high-precision scientific research tools, optical communication, and various other scientific and technological disciplines. 8,9 They are also capable of modulating and detecting the polarization state of light.…”