  The free energy of a 600-atom Lennard-Jones cluster is calculated as a
function of surface and bulk crystallinity in order to study the structural
transformations that occur in the core of medium sized clusters. Within the
order parameter range studied, we find the existence of two free energy minima
at temperatures near freezing. One minimum, at low values of both bulk and
surface order, belongs to the liquid phase. The second minimum exhibits a
highly ordered core with a disordered surface and is related to structures
containing a single FCC-tetrahedral subunit, with an edge length of seven atoms
(l=7), located in the particle core. At lower temperatures, a third minimum
appears at intermediate values of the bulk order parameter which is shown to be
related to the formation of multiple l=6 tetrahedra in the core of the cluster.
We also use molecular dynamics simulations to follow a series of nucleation
events and find that the clusters freeze to structures containing l=5,6,7 and 8
sized tetrahedra as well as those containing no tetrahedral units. The
structural correlations between bulk and surface order with the size of the
tetrahedral units in the cluster core are examined. Finally, the relationships
between the formation of FCC tetrahedral subunits in the core, the phase
behavior of medium sized clusters and the nucleation of noncrystalline global
structures such as icosahedra and decahedra are discussed.
