Thermally Induced Formation of Transition Aluminas from Boehmite

Structural changes occurring during the conversion of boehmite over the transient phases γ‐, δ‐ and θ‐Al 2 O 3 to corundum were investigated. During the whole transition process, the specific surface area was reduced. The X‐ray and electron diffraction experiments revealed that the transition from γ‐ to θ‐Al 2 O 3 is continuous and that it proceeds over different δ‐states rather than over a distinct δ‐Al 2 O 3 phase. The reduction of the specific surface area was most pronounced prior to the onset of the phase transition from γ‐Al 2 O 3 to the first δ‐state. In γ‐Al 2 O 3 , approximately 45% of Al cations were found to be located on tetrahedral sites within a slightly tetragonal distorted oxygen sublattice.