Thermally Induced Formation of Transition Aluminas from Boehmite

Structural changes occurring during the conversion of boehmite over the transient phases gamma-, delta- and theta-Al2O3 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 gamma- to theta-Al2O3 is continuous and that it proceeds over different delta-states rather than over a distinct gamma-Al2O3 phase. The reduction of the specific surface area was most pronounced prior to the onset of the phase transition from gamma-Al2O3 to the first delta-state. In gamma-Al2O3, approximately 45% of Al cations were found to be located on tetrahedral sites within a slightly tetragonal distorted oxygen sublattice.