Owing to the capability of modulating polarized light, birefringent materials are expected to serve as plentiful polarization devices, which are generally applied in linear optic device, fiber optic sensor, laser processing industry and advanced optical communication system. Over the past few decades, a number of birefringent materials with applications have been synthesized and developed, including YVO₄, TiO₂, CaCO₃, MgF₂ and α-BaB₂O₄ (α-BBO) crystals. Even so, scientists have not stopped searching for better birefringent materials, and struggled to hunt for excellent birefringent functional units to gain large birefringence.

    Recently, a research group led by Prof. ShiliePan at Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Science,hassuccessfully reporteda new compound, the first tin borate chloride, Sn₂B₅O₉Cl (SBOC) with a brilliant birefringence (0.168@546 nm) measured by the polarizing microscope. And its birefringence is 16 times that of isostructural Ba₂B₅O₉Cl (BBOC) compound (0.010@546 nm).The study was published in Angew. Chem. Int. Ed.as a very important paper (DOI: 10.1002/anie.201911187).

    In fact, previous researchers proposed that Pb²⁺ and Sn²⁺ with stereochemical activity lone pair (SCALP) could enhance birefringence based on theoretical calculation. However, the enhancement of birefringence caused by Sn²⁺ has no experimental confirmation.But we synthesized the first tin borate chloride Sn₂B₅O₉Clcompound in closed system. Notably, its calculated birefringence was proved by experiment, and about 16 times that of its isostructural Ba₂B₅O₉Cl. More importantly, this work proves that the Sn²⁺cation with SCALP can make large contribution to birefringence by the experiment, and it provides a very effective route to enhance sharplythe birefringence in isostructural alkaline-earth borate compounds.

    This work was financially supported by National Natural Science Foundation of China, National Key Research Project, and Xinjiang Key Research and Development Program.

    https://onlinelibrary.wiley.com/doi/10.1002/anie.201911187