Preparation and optical properties of high-quality green cobalt sapphires

A series of (Al2O3)100-x(Co3O4)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 (Al2O3)99.70(Co3O4)0.30 single crystal is a high-quality, beautiful, and stable green cobalt sapphire like an emerald, and its density is 3.984 g/cm3. XRD and Raman spectra of the (Al2O3)100-x(Co3O4)x crystals show that cobalt is successfully incorporated in the Al2O3 which retains its hexagonal struc-ture. Two strong broad absorption peaks at 436 nm and 652 nm were observed in the UV-Vis absorption spectra, which are assigned to the 1A1 -> 1T1 and 1A1 -> 1T2 transitions of Co3+, 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 Co3O4 concentrations lower than 0.30 mol%, the intensities of both emission peaks increase with Co3O4 concentration, and reach their maxima at 0.30 mol%, then decrease with the further increases in Co3O4. 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.