The structure of ultrafine bimetallic ${\mathrm{Co}}_x{\mathrm{Rh}}_{1-x}$ nanoparticles synthesized in mild conditions by codecomposition of organometallic precursors in the presence of a polymer or a ligand has been studied using high-resolution electron microscopy and wide-angle x-ray scattering techniques. While pure rhodium particles exhibit the main structural features of a face centered cubic ($\mathrm{fcc}$), alloying with cobalt induces a progressive loss of periodicities, leading in high-cobalt-content particles to a polytetrahedral structure close to the one already encountered in pure-cobalt particles. When increasing the synthesis temperature, the polytetrahedral structure remains remarkably stable, while particles with higher rhodium content clearly evolve towards perfect $\mathrm{fcc}$. Increasing the size of the particles up to $5–6\mathrm{nm}$ stabilizes the structural phases encountered in the phase diagram of the bulk alloy. Different element-sensitive techniques, x-ray absorption spectroscopy (XANES and EXAFS) and energy-filtering transmission electron microscopy, have also been implemented in order to get chemical information. Evidence is given for a cobalt surface segregation in these bimetallic particles, highly favorable for magnetic-moment enhancement.

• Received 15 December 2003

DOI:https://doi.org/10.1103/PhysRevB.69.235416