Li 2 PbB 2 O 5 : A Pyroborate with Large Birefringence Induced by the Synergistic Effect of Stereochemical Active Lone Pair Cations and π-Conjugated [B 2 O 5 ] Groups.

Through the inclusion of two kinds of birefringence reactive groups, that is, Pb 2+ cations with stereochemically active lone pairs and π-conjugated [B 2 O 5 ] groups, a new pyroborate Li 2 PbB 2 O 5 used to be acquired by the usage of the high-temperature solution method. The three-dimensional structural framework of Li 2 PbB 2 O 5 is made up of highly distorted [PbO 5 ] polyhedra and parallelly arranged π-conjugated [B 2 O 5 ] groups via the connection of four- and five-coordinated Li + cations. The birefringence of Li 2 PbB 2 O 5 is 0.091 at 1064 nm, which is relatively large and revealed by the first-principles calculations. The contribution from the synergistic effect of the two kinds of birefringence reactive groups was substantiated via the analysis of the real-space atom cutting method. Also, the diffuse reflectance spectrum suggests that Li 2 PbB 2 O 5 features a band gap of 4.0 eV, which corresponds to an ultraviolet (UV) cutoff edge. We hope that this work will foster further improvements in the design of UV birefringent crystal materials in the borate system.