Local structures in medium-sized Lennard-Jones clusters: Monte Carlo simulations

The Lennard-Jones clusters composed of approximately 200 atoms were simulated using canonical Monte Carlo method at the reduced temperatures between T-*=0.05 and T-*=0.49. Local arrangements of atoms within the clusters were identified by means of a specially invented algorithm based on the classification of five types of regular coordination polyhedra corresponding to face-centered cubic (fcc), hexagonal close-packed (hcp), icosahedral (ic), pentagonal direct-packed (pdp), and body-centered cubic (bcc) local structures. Clusters cut off from three types (fcc, hcp, and bcc) of perfect crystals show a structural transition with gradually increasing temperature in the temperature range corresponding to solid clusters. The spectrum of 201 atomic clusters formed at the temperature T-*=0.25 from a starting configuration with random atom positions is characterized by a variety of local structures, with quantitative dominance of hcp and pdp structural units and absence of bcc units. Most of these clusters are polyicosahedral, very often showing a triangular or tetrahedral arrangement of ic units. In some clusters noncrystallographic ic and pdp atom arrangement is absent. In such cases spatial separation of two crystallographic phases can be observed in the form of fcc and hcp domains or as sandwiched parallel fcc and hcp planes.