High temperature form BBO (α -BaB2O4) are negative uniaxial crystals. They have large birefringence over a broad transparent range of 190nm to 3500nm. The physical, chemical, thermal and optical properties of α -BBO crystals are similar to those of ß -BBO. However, the nonlinear optical properties of α -BBO crystals are nonexistent due to the centric symmetry with its crystal structure. α -BBO crystals are not recommended for NLO processes.