Preparation and optical properties of high-quality green cobalt sapphires

A series of (Al 2 O 3 ) 100- x (Co 3 O 4 ) x ( x  = 0, 0.04, 0.1, 0.2, 0.3, 0.4) single crystals were successfully prepared by the optical floating zone method for the first time, even though natural cobalt-containing sapphires have not been observed. It has been found that the (Al 2 O 3 ) 99.70 (Co 3 O 4 ) 0.30 single crystal is a high-quality, beautiful, and stable green cobalt sapphire like an emerald, and its density is 3.984 g/cm 3 . XRD and Raman spectra of the (Al 2 O 3 ) 100- x (Co 3 O 4 ) x crystals show that cobalt is successfully incorporated in the Al 2 O 3 which retains its hexagonal structure. Two strong broad absorption peaks at 436 nm and 652 nm were observed in the UV–Vis absorption spectra , which are assigned to the 1 A 1 → 1 T 1 and 1 A 1 → 1 T 2 transitions of Co 3+ , respectively. Whilst the optical band gaps were in the range 4.43–3.95 eV. Under excitation at 436 nm, one strong emission peak at 864 nm and a weak emission peak at 692 nm were found in the photoluminescence (PL) spectra of the cobalt sapphires. At Co 3 O 4 concentrations lower than 0.30 mol%, the intensities of both emission peaks increase with Co 3 O 4 concentration, and reach their maxima at 0.30 mol%, then decrease with the further increases in Co 3 O 4 . Also, X-ray excited luminescence (XEL) spectra show that the cobalt sapphires have a high sensitivity for X-ray detection. Overall, cobalt sapphires can have various potential uses, such as in jewelry, as ultraviolet detectors , in X-ray detection, and in near-infrared emission devices.