Transition from Icosahedral to Decahedral Structure in a Coexisting Solid-Liquid Nickel Cluster

We have used molecular dynamics simulations to construct a microcanonical caloric curve for a 1415 atom Ni icosahedron. Prior to melting, the Ni cluster exhibits static solid-liquid phase coexistence. Initially, a partial icosahedral structure coexists with a partially wetting melt. However, at energies very close to the melting point the icosahedral structure is replaced by a truncated decahedral structure that is almost fully wet by the melt. This structure remains until the cluster fully melts. The transition appears to be driven by a preference for the melt to wet the decahedral structure.

Figure 1

Comparison of the relaxed zero temperature energies of icosahedra, Marks decahedra, and truncated octahedra for nickel. Energies are given relative to a fit to the energies of the truncated octahedra sequence.

Figure 2

Caloric curve for the 1415 atom icosahedron. The onset of coexistence occurs at an energy of approximately $E=-3.81\mathrm{eV}/\mathrm{\text{atom}}$. There is a second transition at $E=-3.77\mathrm{eV}/\mathrm{\text{atom}}$, followed finally by melting at $E=-3.755\mathrm{eV}/\mathrm{\text{atom}}$.

Figure 3

The figures show icosahedron prior to heating (left) and the coexisting solid-liquid icosahedron at $E=-3.78\mathrm{eV}/\mathrm{\text{atom}}$ just prior to the transition (center and right). In the center the liquid atoms are shown in a darker shade. Note that the liquid only partially wets the solid, exposing (111) facets, whereas if the liquid completely wet the solid then there would be no solid exposed. On the right, the liquid atoms have been removed to show only the solid.

Figure 4

The plots show the coexisting solid-liquid decahedron at $E=-3.77\mathrm{eV}/\mathrm{\text{atom}}$ just after the transition. In the top two snapshots, taken from opposite viewpoints, the liquid atoms are shown in a darker shade. In the bottom snapshots, the liquid atoms have been removed and CNA analysis has been used to highlight the twin planes of the decahedron. On the bottom left the viewpoint is down the fivefold axis of the decahedron. On the right the picture is looking side-on to one of the twin planes.

Figure 5

The plot shows a second caloric curve where the cluster is heated (solid lines) until the icosahedral to decahedral transition and then cooled (dashed lines). Also shown is the corresponding number of fcc atoms (as calculated by CNA analysis) in the cluster.

Figure 6

The plot shows the time evolution of the temperature and number of fcc atoms in the cluster as it goes through the transition from icosahedron to decahedron at $E=-3.77(\mathrm{eV}/\mathrm{\text{atom}})$.