Hierarchical Modulation of Optical Anisotropy Driven by Metal Cation Polyhedra in Fluorooxoborates M^IIB₄O₆F₂ (M^II=Be, Mg, Pb, Zn, Cd)

Abstract

The enhancement mechanism of birefringence is very important to modulate optical anisotropy and materials design. Herein, the different cations extending from alkaline-earth to alkaline-earth, d 10 electron configuration, and 6s 2 lone pair cations are highlighted to explore the influence on the birefringence. A flexible fluorooxoborate framework from AEB 4 O 6 F 2 (AE = Ca, Sr) is adopted for UV/ deep-UV birefringent structures, namely, M II B 4 O 6 F 2 (M II = Be, Mg, Pb, Zn, Cd). The maximal enhancement on birefringence can reach 46.6 % with the cation substitution from Ca, Sr to Be, Mg (route-I), Pb (route-II), and Zn, Cd (route-III). The influence of the cation size, the stereochemically active lone pair, and the binding capability of metal cation polyhedra is investigated for the hierarchical improvement on birefringence. Significantly, the BeB 4 O 6 F 2 structure features the shortest UV cutoff edge 146 nm among the available anhydrous beryllium borates with birefringence over 0.1 at 1064 nm, and the PbB 4 O 6 F 2 structure has the shortest UV cutoff edge 194 nm within the reported anhydrous lead borates that hold birefringence larger than 0.1 at 1064 nm. This work sheds light on how metal cation polyhedra modulate birefringence, which suggests a credible design strategy to obtain desirable birefringent structures by cation control.

Zn(C6H4NO2)2·4H2O: A Solar-Blind Birefringent Crystal

“…With the advancement of corresponding experiments, the intricate interplay between metal cations and functional anionic primitives in manipulating crystal birefringence has been elucidated with increasing precision. Highly polarized d 10 transition metal cations (e.g., Zn 2+ , Cd 2+ ) and cations possessing stereochemically active lone pairs (e.g., Pb 2+ , Sn 2+ ) have found extensive application in fabricating novel birefringent crystals. − Zhou’s team successfully synthesized Cs 2 Pb­(NO 3 ) 2 Br 2 , which exhibits a Rudelsden–Popper perovskite layered structure with a remarkable birefringence value of 0.147@546 nm . Similarly, Pan’s group employed a strategy involving rigid tetrahedron-induced planar triangular anion prioritization to synthesize deep ultraviolet borate BaZnB 4 O 8 , achieving a breakthrough birefringence value of 0.14@589.3 nm within the same category .…”