Raman and X-ray diffraction study of boehmite gels and their transformation to α- or β-alumina

The structural evolution of gel-derived boehmite and alkali-doped boehmite samples, undergoing different heat treatments up to 1200°C, was followed by both X-ray diffraction and Raman scattering measurements, collected at room temperature. Our observations reveal the presence of different transition alumina phases at intermediate temperatures, before formation at the highest temperatures of stable a-alumina or b + b″-alumina, for alkali-loaded Al2O3. In particular, Raman data from samples treated at 400°C are consistent with a phase change from pseudo-boehmite to the ?-alumina structure, while the dehydroxylation process is not yet completed. The process appears independent of the presence of alkali: identical evolution was presented up to 800°C by all our samples. After final annealing at 1200°C the presence of alkali induces the formation of stable b + b″-alumina phases whereas, in the absence of alkali, a phase identified as g-Al2O3 is observed at 1000°C and, finally, stable a-Al2O3 is obtained at 1200°C.