Evolution of the phase composition, crystal structure and magnetic properties of core@shell nanoparticles obtained during conversion of ferrocene at high pressure and high temperature

This work describes structural and magnetic properties of nanoparticles obtained during conversion of ferrocene Fe(C5H5)2 under a high pressure 8 GPa and a high temperature 900 °C (HP-HT treatment) for 10–10000 s, and subsequent self-oxidation in air. The magnetic, structural, and electronic properties of the nanocomposites were studied by XRD, TEM, HAADF-STEM, ED, EDXS, Mössbauer spectroscopy and magnetization measurements. Conversion of ferrocene leads to the formation of “pure” and carbon-encapsulated iron carbide (Fe7C3@C) nanoparticles embedded in carbon matrices with varying degrees of structural ordering. Depending on the size and structure of these nanoparticles different products can be obtained as a result of the self-oxidation of iron carbides. Along with solid iron oxide nanoparticles, hollow iron oxide particles were found in the oxidation products. The formation of hollow nanoparticles can be explained by the Kirkendall effect. It is known that magnetite Fe3O4 and maghemite γ-Fe2O3 are ferrimagnets with a high Neel point TN, while wüstite FeO is an antiferromagnet with TN about 198 K. By varying the content of these components in nanoparticles, it is possible to obtain materials with desired magnetic properties, which is of great importance for technological and biomedical applications of such nanostructures.