Structure evolution of nanoparticulate Fe2O3

Abstract

The atomic structure and properties of nanoparticulate Fe₂O₃ are characterized starting from its smallest Fe₂O₃ building unit through (Fe₂O₃)_n clusters to nanometer-sized Fe₂O₃ particles. This is achieved by combining global structure optimizations at the density functional theory level, molecular dynamics simulations by employing tailored, ab initio parameterized interatomic potential functions and experiments. With the exception of nearly tetrahedral, adamantane-like (Fe₂O₃)₂ small (Fe₂O₃)_n clusters assume compact, virtually amorphous structures with little or no symmetry. For n = 2–5 (Fe₂O₃)_n clusters consist mainly of two- and three-membered Fe–O rings. Starting from n = 5 they increasingly assume tetrahedral shape with the adamantane-like (Fe₂O₃)₂ unit as the main building block. However, the small energy differences between different isomers of the same cluster-size make precise structural assignment for larger (Fe₂O₃)_n clusters difficult. The tetrahedral morphology persists for Fe₂O₃ nanoparticles with up to 3 nm in diameter. Simulated crystallization of larger nanoparticles with diameters of about 5 nm demonstrates pronounced melting point depression and leads to formation of ε-Fe₂O₃ single crystals with hexagonal morphology. This finding is in excellent agreement with the results obtained for Fe₂O₃ nanopowders generated by laser vaporization and provides the first direct indication that ε-Fe₂O₃ may be thermodynamically the most stable phase in this size regime.