Crossing the boundary between face-centred cubic and hexagonal close packed: the structure of nanosized cobalt is unraveled by a model accounting for shape, size distribution 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. This paper reports a structural analysis of nanosized cobalt based on the whole X-ray diffraction (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 (h.c.p. and f.c.c.). Shape, size distribution and distance distribution between pairs of atoms are also modelled. The simulation algorithm allows straightforward fitting to experimental data and hence the quantitative assessment of the model parameters. Analysis of two samples having, respectively, h.c.p. and f.c.c. appearance is reported. Extended X-ray absorption fine-structure (EXAFS) and X-ray absorption near-edge structure (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 modelling of these properties.