Crossing the boundary between fcc and hcp: the structure of nanosized cobalt is unraveled by a model of size distribution, shape and stacking faults allowing simulation of XRD, XANES and EXAFS

The properties of nanostructured cobalt in the fields of magnetic, catalytic and biomaterials depend critically on Co close-packing. In this paper we report a structural analysis of nanosized cobalt based on the whole XRD pattern simulation allowed by the Debye equation. The underlying structural model involves statistical sequences of cobalt layers and produces simulated XRD powder patterns bearing the concurrent signatures of hexagonal and cubic close packing. Size distribution, shape and distance distribution between pairs of atoms are also modelled. The simulation algorithm allows straightforward fitting to experimental data and hence the assessment of the model parameters. Analysis of two samples having, respectively, hcp and fcc appearance is reported. EXAFS and XANES spectra are simulated on the basis of the model, giving a tool for the interpretation of structural data complementary to XRD. The outlined structural analysis provides a rigorous structural basis for correlations with magnetic and catalytic properties and an experimental reference for ab initio modellization of these properties.