A Hybrid Halide Perovskite Birefringent Crystal

Birefringent crystals that can modulate the polarization of light play a significant role in modern optical devices including polarizing microscopes, optical isolators, and achromatic quarter-wave plates. To date, commercial birefringent crystals are exclusively limited to purely inorganic compounds. Here we report a new organic-inorganic hybrid halide, MLAPbBr₄ (MLA=melamine), which features a (110)-oriented layered perovskite structure. Although the 6s² lone-pair electrons of Pb²⁺ cations are stereochemically inert, MLAPbBr₄ exhibits a birefringence of 0.322@550 nm, which exceeds those of all commercial birefringent crystals. The first-principles calculations reveal that this birefringence should be ascribed to the highly dislocated π-conjugation of MLA cations and high distortion of PbBr₆ octahedra. This work highlights the persistently neglected great potential of hybrid halide perovskites as birefringent crystals.