Melting of (MgO)(n) (n=18, 21, and 24) clusters simulated by molecular dynamics

Molecular dynamics simulations are employed to investigate the melting behavior and thermal stability of magnesium oxide clusters (MgO)(n) (n=18, 21, and 24). The rocksalt and hexagonal tube structures are two dominant low-energy structural motifs for small (MgO)(n) clusters and it results in the magic sizes n=3k (k is an integer). For n=6, 9, 12, and 15, the rocksalt and hexagonal tube structures have the same topological geometry, but for n>or=18, the two isomers are separated by high energy barriers. The simulations show a one-step melting process for the rocksalt structures of (MgO)(18,24) (no perfect rocksalt structure exists for n=21). The melting transition occurs sharply between 1800 and 1950 K for n=24 but gradually from 1400 to 2450 K for n=18. The relative root-mean-square bond length fluctuation reveals a premelting stage from about 700 K to the melting transition for the hexagonal tube structures of all the three clusters. The short-time averages of kinetic energy and a visual molecular dynamics package are used to monitor the structures along the trajectories. The low-energy isomers are identified by the quenching technique and the isomerization processes are traced. The results show that there exists a family of isomers which are only 0.1-0.4 eV higher in energy than the corresponding hexagonal tube structures and separated by low energy barriers. The premelting stage is caused by the isomerizations among these structures. The melting characteristics demonstrated in the simulations are clarified in terms of the energies of the isomers and the energy barriers separating them.