A hybrid halide lead-free pseudo-perovskite with large birefringence

Birefringent crystals have important applications in optoelectronics due to their ability to modulate and polarize light. Hybrid halide perovskites were not considered as promising candidates for birefringent crystals until quite recently, when we reported a hybrid lead bromine perovskite with large birefringence. Herein, we report a novel lead-free hybrid pseudo-perovskite layered structure of MLASnCl4 (MLA = melamine). Remarkably, MLASnCl4 reveals a large birefringence of 0.294@550 nm, which is comparable to that of the previously reported hybrid lead bromine perovskite MLAPbBr4. Furthermore, the surface morphologies of the reported crystals exhibit excellent air stability investigated by their exposure to air at room temperature for various time ranges. The observed birefringence is generated from the delocalized π-conjugations of melamine cations and stereochemically active lone pair electrons on the Sn2+ cations of highly distorted SnCl4 tetrahedra, as suggested by theoretical calculations. This current research effort may open new windows for the design of hybrid perovskite materials for polarization-dependent optical applications.