BaTi(BO3)2: An Excellent Birefringent Material with Highly Coplanar Isolated [BO3] Groups

A material with a large birefringence is needed for optical systems, and borate is a good choice as an excellent birefringent material. Herein, we report two borates BaMIV(BO3)2 (MIV = Ti, Sn), in which the [TiO6] and [SnO6] units have similar coordination environments. Moreover, the experimental birefringence of BaTi(BO3)2 is equal to or larger than 0.169 at 546 nm measured by using a polarizing microscope, which mainly originates from the high-density and parallel arrangement of the isolated [BO3] groups. This discovery indicates that BaTi(BO3)2 can be a promising ultraviolet birefringent material.