A Molecular Dynamics Investigation of C 60 -Rare Gas Mixtures
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MARIA C. ABRAMO AND C.CACCAMO Dipartimento di Fisica, Sez. Teorica, S.Agata di Messina, Italy
Universita'
di Messina,
C.P.50 ,
98166
ABSTRACT: Mixtures of krypton atoms and C 6 0 , modelled in terms of rigid spherical molecules, are investigated at T - 1800K through classical Molecular Dynamics. The two species do not tend to mix for low to moderate concentrations of krypton, and solid clusters of C 6 0 tend to be stable. Possible implications of these results are shortly discussed.
It is known that C 6 0 molecules were discovered1 , and the solid phase of 2 this same fullerene obtained , after vaporizing graphite layers or rods, in a helium gas atmosphere; it was also found that helium pressure played a crucial role in the formation of the C6 0 molecule 1 4 . Possible aggregation of fullerene molecules, once formed in the high temperature gaseous phase surrounding, e.g., the electrodes of a carbon-arc, could also be of interest. This study is an attempt to investigate, through a simple potential modelization and a fully microscopic approach, the role that an inert gas can play in mixture with C6 0 at high temperature. Specifically, we report constant volume Molecular Dynamics (MD) investigation of mixtures of krypton atoms and C 6 0 molecules for different concentrations of Kr. The temperature T-I800K, and the particle density (see below) at which we perform our simulations for the mixture, should be those typical of hot fluid pure C6 0 , according to recently determined phase model C6057 diagrams of Rare gas particles interact through the well known Lennard-Jones (LJ) potential, while C 6 0 molecules are assumed to interact through a spherically 5 averaged potential due to Girifalco , previously adopted in refs. (6,7). The interaction between C 6 0 and Kr atoms is modeled in the form of an integrated LJ potential due to Breton et aL8 Two Kr concentrations c were investigated : c=0.3 and c=0.5. The particle density was fixed in terms of the reduced density p*= PKrUKr+PC60aC60, where respectively; PC6 0 are the Kr and C6 0 number density of particles, PKr' potential (and the repulsive parameter entering the LJ7 Kr-§r Ais Kr=3. 1 roughly corresponding to the Kr diameter), and aC60- . A is the diameter 5 of the C6 0 molecule . All results reported below have been obtained for p*=0.34, but simulations at higher reduced densities have also been performed . The MD simulations were performed with a time step of 0.5 x 10- 1 5 s. After 10000 equilibration time steps (corresponding to 50 ps), averages were cumulated over further 5000 time steps (25 ps). Each run was started by placing C 6 Q and Kr particles on a fcc lattice, according to configurations which we visualize through the projections of the particle positions on the coordinate planes zy,xz,yz , as shown in figs. 1 and 2. It appears that Kr atoms were initially confined into a slice of the basic simulation box, parallel to the yz plane; this slice was delimited by two adiacent C 6 0 slices. The replicae of the box will then generate a sort of 'wafer' arrange
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