UBIQUITOUS BLUE LUMINESCENCE FROM UNDOPED SYNTHETIC SAPPHIRES

Undoped, synthetic sapphires grown by four different techniques all showed an anisotropic 5.4 eV (230 nm) absorption and accompanying broad, 3.0 eV (415 nm) luminescence band. A review of the antecedent work, taken together with new spectroscopic evidence from several undoped and titanium-doped samples, treated in both oxidizing and reducing atmospheres, favor assigning these features to a charge transfer transition associated with Ti+4.