Structural Chemistry & Crystallography Communication

A size dependent phase map and phase transformation kinetics for iron(III) oxide nanomaterials (γ →ε→α pathway)

International Conference on Applied Crystallography
October 16-17, 2017 | Chicago, USA

Seungyeol Lee and Huifang Xu

University of Wisconsin-Madison, USA

Posters & Accepted Abstracts: Struct Chem Crystallogr Commun

DOI: 10.21767/2470-9905-C1-003

Abstract

Nanometric iron(III)-oxide has been of great interest in a wide range of fields due to magnetic properties, eminent biochemical characteristics and potential for technological applications. To date, five crystalline polymorphs of Fe2O3 are known: (1) �?±-Fe2O3 (i.e., hematite), (2) �?²-Fe2O3, (3) �?³-Fe2O3 (i.e., maghemite), (4) �?µ-Fe2O3 (i.e., luogufengite),and (5) �?¶-Fe2O3, all of which have different morphologies, various size and magnetic properties. Among the iron-oxides, �?µ-Fe2O3 is considered as a remarkable phase due to its giant coercive field at room temperature and ferromagnetic resonance capability. The natural �?µ-Fe2O3 (luogufengite) is discovered from vesiclesâ�?�? surfaces of basaltic scoria. Here we present the first size-dependent phase map for �?µ-Fe2O3 via a �?³â�?�?�?µâ�?�?�?± 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 �?µ-Fe2O3 nanocrystals show the inversion and pseudo-hexagonal twins, which are fundamentally important for understanding the correlation between its nanostructure and magnetic properties. Two activation energies for �?³-Fe2O3â�?�?�?±-Fe2O3 phase transformations are 186.37�?±9.89 kJ mol-1 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 such as designing engineered materials. Combining the phase map with their kinetic properties predicts that stability regime of the nanosized Fe2O3 polymorphs as the function of crystal size, temperature and annealing times. The proposed size-dependent phase map will help to improve controlled synthesis of �?µ-Fe2O3 nanocrystal, a promising material for many future applications.