The thermal behaviour of the 309-atom Lennard-Jones cluster, whose structure
is a complete Mackay icosahedron, has been studied by parallel tempering Monte
Carlo simulations. Surprisingly for a magic number cluster, the heat capacity
shows a very pronounced peak before melting, which is attributed to several
coincident structural transformation processes. The main transformation is
somewhat akin to surface roughening, and involves a cooperative condensation of
vacancies and adatoms that leads to the formation of pits and islands one or
two layers thick on the Mackay icosahedron. The second transition in order of
importance involves a whole scale transformation of the cluster structure, and
leads to a diverse set of twinned structures that are assemblies of
face-centred-cubic tetrahedra with 6 atoms along their edges, i.e., one atom
more than the edges of the 20 tetrahedra that make up the 309-atom Mackay
icosahedron. A surface reconstruction of the icosahedron from a Mackay to an
anti-Mackay overlayer is also observed, but with a lower probability.Comment: 7 pages, 4 figure