Micrometer-sized alpha-Al2O3 platelets with hexagonal shape were prepared by thermal decomposition of ammonium aluminum carbonate hydroxide (AACH) using AlF3 as an additive. The precursor and the calcined product were characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy and differential scanning calorimetry. The alpha-Al2O3 platelets with the size of 2-3 mu m were obtained by calcining AACH at 1200 degrees C with 5 wt.% AlF3. The morphology modification is attributed to the various growth rates along different crystal orientations due to the adsorption and uneven distribution of AlF3. A step growth mode is responsible for the formation of the platelets. (C) 2010 Elsevier B.V. All rights reserved.