High Temperature form BBO (a-BaB2O4) is a negative uniaxial crystal. It has large birefringence over the broad transparent range from 189 nm to 3500 nm. Recently, CryLight has succeeded in growing this crystal into large size. The physical, chemical, thermal and optical properties of a-BBO crystal are similar to those of b-BBO. However, the nonlinear optical properties of a-BBO crystal is vanished due to the centric symmetry with its crystal structure. It is not recommended to use for NLO processes.