Phase Stability and Mechanisms of Transformation of La-Doped γ‑Alumina

We report the phase stability of cubic γ-Al2O3 with respect to lanthanum dopant amount and describe a complete phase transition sequence up to a temperature of 1800 °C, which proceeds from La-doped γ-Al2O3 to LaAlO3/γ-Al2O3 to LaAl11O18. For this purpose, lanthanum contents from 0.81 to 10.0 atom % were incorporated into Al2O3 powders. X-ray diffraction analyses show that only γ-Al2O3 phase was present after heat treatment at 1000 °C for 2 h with 0.81, 1.68, 2.24, and 2.62 atom % lanthanum concentrations. The phase stabilization can be mainly attributed to the combined effects of small crystallite size of the Al2O3 powders and the presence of the lanthanum dopant, which occupies the Al2O3 octahedral sites. At compositions of 3.63, 5.00, 7.49, and 10.0 atom %, the amount of LaAlO3 phase formed by the solid phase reaction between Al2O3 and La3+ ions becomes detectable under X-ray diffraction.