Structures of small metal clusters. II. Phase transitions and isomerization

Abstract

The binding energy, atom coordination numbers, bond lengths, surface restructuring, and bulk melting behavior of small clusters versus temperature are compared for the Lennard-Jones (LJ) potential and embedded atom (EA) potential using the Monte Carlo method with parameters fitted to Ni. We find that EA clusters are more thermally stable than LJ clusters with regard to evaporation. For small clusters whose minimum energy structure is polyicosahedral, a smooth change of physical properties with temperature is observed for both potentials. However, for clusters whose minimum energy structure is quasicrystalline, a structural phase transition analogous to a first order transition of bulk materials can be found for both potentials. This structural phase transition is manifested by discontinuous changes of atom coordination numbers and bond length, and in some cases, of energy. Implications of this transition in catalysis are discussed. Isomerization between minima of each one of the two potential hypersurfaces along with its dependence on temperature are examined. The many body nature of the EA potential results in lower isomerization probabilities between EA isomers as contrasted to LJ isomers.

Comparison of small metal clusters: Ni, Pd, Pt, Cu, Ag, Au

“…For Au, our simulations for n=55 and the calculations for n=100-900 using the glue potential [12] indicate that larger sizes than n=55 are needed to exhibit abrupt changes in properties. For Ni clusters, we have found [7] similar behavior with that using a different EA potential [8] but the transitions from solid to liquid phase occur at -300 K higher temperatures in our simulations. A more extensive report will be given elsewhere.…”

“…We have recently described the low and high temperature behavior of small Ni clusters [6,7] using both the EA potential [2] and the LJ potential. The transition between icosahedral (IC) structures consisting of pentagonal rings and quasicrystalline structures has been examined versus cluster size and temperature.…”

“…Selected minimum energy structures were subsequently heated slowly to examine the thermal stability of small clusters, as discussed in detail elsewhere [7].…”

Melting behaviors of icosahedral metal clusters studied by Monte Carlo simulations

“…An understanding of the thermodynamic properties of small metal clusters as well as their atomic structures has a practical importance in nanoscale materials. Previous experiments5–8 and theoretical studies9–14 on melting and freezing for rare gas (RG N ) and metallic (M N ) clusters show that the melting temperatures of small clusters are significantly lower compared to their corresponding bulk, and the melting temperatures are strongly related to their structures. Theoretical studies show that clusters melt over a finite range of temperatures and two‐stage melting takes place for the clusters with certain sizes.…”

Magic numbers for classical Lennard-Jones cluster heat capacities

“…[17][18][19] Their small size (typically 3 to 150 atoms) makes clusters ideal candidates for computer simulation studies, and most theoretical studies to date have used Monte Carlo [21][22][23][24] and molecular dynamics methods. [24][25][26] Because of the large computational demands inherent in these simulation methods, most early theoretical investigations were limited to classical rare gas clusters bound by simple pairwise additive Lennard-Jones potentials, but continuing improvements in computer technology have led to an increasing number of more interesting metal cluster simulations requiring more sophisticated intermolecular potentials for physically realistic representation, 27 and have made quantum simulations based on Fourier path integral methods practical.…”