Polymorphous Transformations of Nanometric Iron(III) Oxide: A Review

There is great interest in iron oxides, especially in nanosized form, for both fundamental and practical reasons. Because of its polymorphism, iron(III) oxide (ferric oxide, Fe2O3) is one of the most interesting and potentially useful phases of the iron oxides. Each of the four different known crystalline Fe2O3 polymorphs (alpha-, beta-, gamma-, and epsilon-Fe2O3) has unique biochemical, magnetic, catalytic, and other properties that make it suitable for specific technical and biomedical applications. High temperature treatment is a key step in most syntheses of iron(III) oxides but often triggers polymorphous transformations that result in the formation of undesired mixtures of Fe2O3 polymorphs. It is therefore important to control the parameters that induce polymorphous transformations when seeking to prepare a given Fe2O3 polymorph as a single phase; identifying and understanding these parameters is a major challenge in the study of the polymorphism of solid compounds. This review discusses the dependence of the mechanism and kinetics of the polymorphous transformations of Fe2O3 on the intrinsic properties of the material (polymorph structure, particle size, particle morphology, surface coating, particle aggregation, incorporation of particles within a matrix) and external parameters of synthetic and/or natural conditions such as temperature, atmosphere, and pressure. The high-temperature and high-pressure induced transformations of Fe2O3 are reviewed in detail. In addition, the question of whether different Fe2O3 polymorphs are formed sequentially or simultaneously during thermal processes is discussed extensively, with reference to the experimental results that have been invoked to support these two different mechanisms. The use of selected analytical tools in studying the polymorphous transformations of Fe2O3 is also discussed, with particular emphasis on in situ approaches. Finally, key objectives for future research in this area are highlighted: (i) the development of more sophisticated kinetic control of the gamma-Fe2O3 -> epsilon-Fe2O3 phase transformation; (ii) investigation of particle morphology changes during the polymorphous transformations of Fe2O3; and (iii) the study of high-pressure induced phase transformations of Fe2O3 polymorphs other than alpha-Fe2O3.