Ordering of Octahedral Vacancies in Transition Aluminas

The microstructure of transition aluminas obtained via the dehydration of boehmite has been characterized by using transmission electron microscopy (TEM). The presence of γ-, δ-, and θ-aluminas was identified by using selected-area electron diffraction. Modifications that resulted from the reordering of aluminum vacancies on octahedral sites in a cubic close-packed oxygen network have been detected and analyzed by using high-resolution transmission electron microscopy (HRTEM) combined with image simulations. A good correspondence of the observed and calculated images confirmed the ordering of vacant octahedral sites located on {011} and {011} planes that formed a zigzag configuration along the direction. Two more arrangements of empty octahedral sites, but now concentrated on {001} planes, have been determined in the sintered powder-gel agglomerates. Structure analysis suggested that the modifications are all associated with the rearrangement of vacant sites during the phase transformation from gamma-alumina to δ-alumina, and further to theta-alumina, and may be driven by configuration entropy minimization.