Sn₂B₅O₉Cl: A Material with Large Birefringence Enhancement Activated Prepared via Alkaline‐Earth‐Metal Substitution by Tin

The excellent birefringent materials are needed for optical systems. Herein, we reported a new compound, the first tin borate chloride, Sn 2 B 5 O 9 Cl (SBOC) with a large birefringence (0.168 at 546 nm) measured by the polarizing microscope. Its birefringence is 16 times that of the isostructural Ba 2 B 5 O 9 Cl (BBOC) compound (0.010@ at 546 nm). The results show that the birefringence enhancement originates mainly from the Sn 2+ polyhedra. We propose that the birefringence can be enlarged by substituting the alkaline‐earth metal cation by the Sn 2+ cation in the isostructural borate with small birefringence. This strategy will guide the discovery of large birefringent materials in the future.