Temperature-dependent BN cluster formation dynamics from a boron cluster: Density-functional tight-binding molecular dynamics simulations

The time-evolution dynamics of a boron nitride (BN) cluster from an amorphous B cluster is simulated by quantum chemical molecular dynamics based on the density-functional tight-binding method. In the simulations, N atoms are sequentially supplied around the B cluster in conjunction with the arc-melting BN fullerene synthesis from B-rich compounds. The simulations are performed at 1000, 1500, 2000, 2500, and 3000 K, and we run 30 trajectories for 200 ps at each temperature. At low temperature (1000 K), the BN clusters tend to form stuffed cage structures, characterized by internal BN branched units. As the temperature increases, the proportion of stuffed cage structures decreases, whereas that of cage-like structure increases. At intermediate temperatures (2000 and 2500 K), most of the BN clusters develop into hollow structures, exhibiting a strong cage forming preference. At 3000 K, the BN clusters tend to form sparse branched chain structures, without forming cage-like structures. Mobility analysis of the cluster atoms at all the temperatures reveals that the transition from a liquid-like state to a solid-like state occurs at 2000 and 2500 K, whereas the cluster remains in a solid-like state at 1000 K and a liquid-like state at 3000 K. The N2 dissociation reactions from the BN cluster proceed through various N2 unit formation processes in the BN cluster. We describe details of the representative N2 unit formation processes classified by the bonding of the cluster N atoms.

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