Optical absorption and mechanism of vacuum-sintered ZrO2-doped Y2O3 ceramics

ZrO2 is considered an efficient sintering additive for transparent Y2O3 ceramics. However, the doping of tetravalent Zr4+ ions leads to a dark brown color of the ceramic after vacuum sintering. In this study, the optical properties of vacuum-sintered and air-annealed ceramics were characterized using transmittance spectra and a Commission International de l'Eclairage (CIE) diagram. The corresponding defects are investigated by electron paramagnetic resonance (EPR) spectra. The band gap of the vacuum-sintered samples is 2.72 eV, which is much smaller than that of the annealed sample. The results reveal that the VO+ (F+ center) and Zr3+ defects form transition energy levels near the top of the valence band and at the bottom of the conduction band, respectively. The color-related absorption is mainly attributed to the electronic transfer from the Zr3+ level to the conduction band and from the F+ center to the Zr3+ level.