BaTi(BO3)2: an excellent birefringent material with highly coplanar isolated [BO3] groups

The birefringence of BaTi(BO₃)₂ is equal to or larger than 0.169 at 546 nm (Δn_(cal.) = 0.185 at 546 nm), which mainly originates from the high-density and parallel arrangement of the isolated [BO₃] triangles in the unit cell.