High-pressure structural properties of cobalt nanowires encapsulated in multiwalled carbon nanotubes have been investigated up to a pressure of $\sim 38\mathrm{GPa}$ at room temperature. Our x-ray diffraction measurements show that cobalt, which exists in the face-centered-cubic (fcc) phase at ambient conditions in the carbon nanotubes, transforms irreversibly to a hexagonal close-packed (hcp) structure at $\sim 9\mathrm{GPa}$. In comparison with the bulk Co, the compressibility of the fcc phase in nanowires is found to be similar to that of the high-pressure fcc phase [Phys. Rev. Lett. 84, 4132 (2000)] and the hcp phase is slightly less compressible than the bulk. Multiwalled carbon nanotubes that encapsulate the cobalt nanowires do not undergo any other structural transformation with pressure except partial reversible amorphization beyond $9\mathrm{GPa}$.

• Received 21 March 2006

DOI:https://doi.org/10.1103/PhysRevB.73.184119