Crystallite size variations of nanosized iron oxide particles during γ- to α-phase transformation were examined using DTA/TG, FTIR, XRD, TEM, and BET-N_2 techniques. Synthetic γ-FeOOH and Fe_3O_4, prepared as precursors, both converted to γ-Fe_2O_3 and then completed the phase transformation under thermal treatment. Starting with γ-FeOOH, the γ-Fe_2O_3 obtained grew from the original size of 5 nm to a size about 25 nm or larger, then transformed to α-Fe_2O_3. The α-crystallites obtained showed sizes similar to that of γ-crystallite. With continuous heating, the α-Fe_2O_3 coarsened to a size of ～55 nm. Identical phenomena were observed when α-Fe_2O_3 was prepared from Fe_3O_4 via γ-Fe_2O_3. The study also found that α-Fe_2O_3 crystallites obtained by calcinations of amorphous Fe(OH)_3 will show sizes smaller than 5 nm. The decomposition temperature can be lowered to 200 ℃.
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