Ordering of Octahedral Vacancies in Transition Aluminas

Abstract

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 γ-alumina to δ-alumina, and further to θ-alumina, and may be driven by configuration entropy minimization