Luminescence and EPR properties of high quality ruby crystals prepared by the optical floating zone method

Rubies (Cr: Al 2 O 3 single crystals) with 0.025–0.4 mol% Cr 2 O 3 were prepared by the optical floating zone method, and characterized by X-ray diffraction (XRD), electron paramagnetic resonance (EPR) spectroscopy, optical transmission and photoluminescence (PL). Crystals were free of inclusions and cracks, and had high transmittance in the visible region. In addition to the well-known signals from Cr 3+ in the α-Al 2 O 3 structure, the EPR spectra indicated the presence of very weak signals, which may correspond to paramagnetic centres associated with an oxygen vacancy , and an electron-loss oxygen centre, O − . The concentration of these paramagnetic centres was at a minimum for Cr 2 O 3 concentration of 0.15 mol%, and additional Cr 3+ -derived peaks were also observed with Cr 2 O 3 concentrations >0.15 mol%. The PL spectra also varied with Cr 2 O 3 concentrations; the emission intensities of the PL spectra increased with Cr 2 O 3 concentration for concentrations ≤0.15 mol%, then decreased with the further increases in Cr 2 O 3 . Thus the maximum luminescence emission intensity and the minimum concentration of paramagnetic structural defects were observed with 0.15 mol % Cr 2 O 3 , and the presence of additional interactions involving Cr 3+ ions may contribute to the partial quenching of the luminescence at higher Cr 3+ concentrations. Overall, the optical floating zone method represents an effective and economic approach to the preparation of high quality rubies, and their optimum optical properties were observed with Cr 2 O 3 concentrations of 0.15 mol%.