Size-induced structural phase transition at ~6.0 nm from mixed fcc-hcp to purely fcc structure in monodispersed nickel nanoparticles

We have investigated the core issue of atomic lattices in monodispersed Ni nanoparticles (NPs) of sizes 3.8 nm to 10.1 nm using detailed analysis of X-ray diffraction, synchrotron radiation X-ray absorption spectroscopy (XAS) and magnetization data. This has revealed the very remarkable coexistence of atomic face-centered cubic (fcc) and hexagonal closed-packed (hcp) lattices in samples with particle size less than or equal to 6.0 nm with the prevalence of only fcc phase beyond this. They are also associated with reduced coordination number, modified electronic structure, and surface atom coordination with ligands. Magnetization data furthermore reveal coexistence of ferromagnetism and superparamagnetism at 300 K. Considered to be due to dominant roles of ligands, they are likely to open up far-reaching implications to their future applications.