Design of alkali lead oxybromides with a strong second-harmonic generation response and large birefringence†

Nonlinear optical (NLO) crystals are of importance in modern lasers, high-precision micromachining, ultrahigh resolution photolithography and advanced scientific equipment. Herein, we have rationally obtained two new lead oxybromides, A₃[Pb₂Br₅(OOC(CH₂)₃COO)] (A = Rb, Cs), with a strong second-harmonic generation (SHG) response and large birefringence. Two neighboring highly distorted [PbBr₄O₂] polyhedrons with high polarizability and anisotropic polarization are bridged by the flexible glutarate group and one Br⁻ anion to form a large [Pb₂Br₇(OOC(CH₂)₃COO)] group. Interestingly, the large [Pb₂Br₇(OOC(CH₂)₃COO)] groups are induced by Rb⁺/Cs⁺ cations into an oriented alignment, leading to the effective superimposition of their microscopic second-order susceptibility and the enhancement of optical anisotropy. Notably, Rb₃[Pb₂Br₅(OOC(CH₂)₃COO)] exhibits good comprehensive NLO performance, including a strong phase-matching SHG response of 3.1 × KDP, a large birefringence of 0.207@546 nm, a wide high transparency window, easy growth of large single crystals, as well as good thermal stability up to 240 °C under an air atmosphere. On the basis of their crystal structures and theoretical calculations, their strong SHG responses mainly stem from the highly distorted [PbBr₄O₂] polyhedron. This research provides an effective strategy for the design and pursuit of high-performance NLO crystals in the future.