Size-Dependent Phase Map and Phase Transformation Kinetics for Nanometric Iron(III) Oxides (γ → ε → α Pathway)

Nanometric iron(III) oxide has been of great interest in a wide range of fields due to its magnetic properties, eminent biochemical characteristics, and potential for technological applications. Among iron oxides, epsilon-Fe2O3 is considered as a remarkable phase due to its giant coercive field at room temperature and ferromagnetic resonance capability. Here we present the first size dependent phase map for epsilon-Fe2O3 via a gamma -> epsilon -> alpha pathway together with the activation energies for the phase transformations, based on X-ray powder diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). HRTEM images of epsilon-Fe2O3 nanocrystals show both inversion and pseudohexagonal twins, which are fundamentally important for understanding the correlation between its nanostructure and magnetic properties. Two activation energies for gamma-Fe2O3 -> alpha-Fe2O3 phase transformations are 186.37 +/- 9.89 and 174.58 +/- 2.24 kJ mol(-1), respectively. The results provide useful information about the size, crystal structure, and transformation of the nanometric iron oxide polymorphs for applications in areas of developing engineered materials.