  The carbon rich phase diagrams of nickel-carbon nanoparticles, relevant to
catalysis and catalytic chemical vapor deposition synthesis of carbon
nanotubes, are calculated for system sizes up to about 3 nanometers (807 Ni
atoms). A tight binding model for interatomic interactions drives the Grand
Canonical Monte Carlo simulations used to locate solid, core/shell and liquid
stability domains, as a function of size, temperature and carbon chemical
potential or concentration. Melting is favored by carbon incorporation from the
nanoparticle surface, resulting in a strong relative lowering of the eutectic
temperature and a phase diagram topology different from the bulk one. This
should be taken into account in our understanding of the nanotube growth
mechanisms.
