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

Rubies (Cr: Al₂O₃ single crystals) with 0.025-0.4 mol% Cr₂O₃ 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 Cr³⁺ in the α-Al₂O₃ 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 Cr₂O₃ concentration of 0.15 mol%, and additional Cr³⁺-derived peaks were also observed with Cr₂O₃ concentrations >0.15 mol%. The PL spectra also varied with Cr₂O₃ concentrations; the emission intensities of the PL spectra increased with Cr₂O₃ concentration for concentrations ≤0.15 mol%, then decreased with the further increases in Cr₂O₃. Thus the maximum luminescence emission intensity and the minimum concentration of paramagnetic structural defects were observed with 0.15 mol % Cr₂O₃, and the presence of additional interactions involving Cr³⁺ ions may contribute to the partial quenching of the luminescence at higher Cr³⁺ 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 Cr₂O₃ concentrations of 0.15 mol%.