a-BBO (a-BaB2O4) is a negative uniaxial crystal which has large birefringence over a broad transparent range of 190nm to 3500nm. a-BBO is an excellent crystal especially in UV and high power applications. The physical, chemical, thermal and optical properties of a-BBO crystal are similar to those of ß-BBO. However, the nonlinear optical properties of a-BBO crystal are nonexistent due to the centric symmetry with its crystal structure. a-BBO crystal is not recommended for NLO processes.