The shape control and formation mechanism of Fe₃O₄ nanoparticles (NPs) synthesized by a modified hot-injection method were investigated. Monodisperse Fe₃O₄ nanocubes terminated at {100} planes with the average size of 16.1 ± 0.9 nm were synthesized by injecting Fe precursor into reaction solution at 290 °C with a slow injection rate of 10 mL/h. When we increased the monomer concentration in solution by increasing the injection rate to 20 mL/h or doubling the precursor concentration with the same reaction time, the main terminated planes of Fe₃O₄ NPs became {100} and {110} planes leading to a rhombicuboctahedral shape. The shape of NPs was strongly affected by the monomer concentration and the intrinsic surface energy of Fe₃O₄. The shape-induced crystallographic orientation-ordered superlattices were obtained in both cubic and rhombicuboctahedral Fe₃O₄ NPs. On the other hand, the shape-dependent occupancy of the cation sites was clearly observed, measured by using X-ray magnetic circular dichroism (XMCD) spectra. The octahedral sites were occupied by more ferric ions, Fe³⁺, when the shape of Fe₃O₄ NPs became cubic.The research was conducted at the NSRRC beamline 11A1.

C.-H. Ho, C.-P. Tsai, C.-C. Chung, C.-Y. Tsai, F.-R. Chen, H.-J. Lin, and C.-H. Lai