Phase transformation of Er³⁺-doped Al₂O₃ powders prepared by the sol-gel method

Er³⁺-doped Al₂O₃ powders were prepared by the sol-gel method, using aluminum isopropoxide [Al(OC₃H₇)₃]-derived Al₂O₃ sols. The phase content diagram for the Er-doped Al–O system with the doping concentration up to 20 mol% is presented for sintering temperature from 550 to 1250 °C. The different crystalline types of (Al,Er)₂O₃ phases—γ, θ, α—and two Er-Al-O phases—ErAlO₃ and Al₁₀Er₆O₂₄—were detected. The Er³⁺ doping in the Al₂O₃ phases has a significant effect on phase transformations for γ → θ and θ → α, indicating the enhancement for stability of the γ and θ phases. Compared with the γ–Al₂O₃ phase, the γ–(Al,Er)₂O₃ phase can be obtained at high temperatures up to about 1100 °C. The θ–(Al,Er)₂O₃ phase, together with the α–(Al,Er)₂O₃ phase, was still observed at 1200 °C under the high Er³⁺ doping concentration. The Er³⁺ doping suppressed the crystallization of the γ and θ phases. At a low Er³⁺ doping concentration from about 2 to 5 mol%, the ErAlO₃ phase precipitated out around 1100 °C. The Al₁₀Er₆O₂₄ phase was observed at all sintering temperatures; the precipitation was carried out to all Er³⁺ doping concentrations with increasing sintering temperatures up to about 1200 °C.