Book of Abstracts Book of Abstracts - Universität Konstanz

B - 36
76
Rapid Alloying in binary Clusters: A Molecular Dynamics Study
Y. Shimizu, T. Kobayashi, K. S. Ikeda and S. Sawada
Ritsumeikan University, Kwansei Gakuin University
Dynamics of surface atoms penetrating into microclusters is investigated in connection with
the very rapid alloying (RA) in metal microclusters. RA was experimentally discovered by
Yasuda and Mori for various types of nano-sized binary metal clusters. In short, RA is a
manifestation of an unexpectedly fast diffusion of solute atoms into a host cluster, which is
controlled by the magnitude of heat of solution and size of a cluster. Isothermal molecular
dynamics simulation is done to investigate the mechanism of RA.
In particular, we put our focus upon a cooperative interplay between atomic rearrangement
along a cluster surface and that along radial direction of a cluster.
Our results are summarized in the following two points. The dependence of the alloying and
that of radial diffusion upon negative heat of solution are elucidated. It is found that dependency
of the “alloying rate” upon the negative heat of solution and the size of clusters reproduced the
qualitative features observed in the experiments. To quantify the dependence of atomic
rearrangement dynamics upon temperature, the activation process of diffusive motion of atoms
is analyzed.
Although the activation energy of radial diffusion is about two times as large as that of
surface diffusion, there are circumstantial evidences that the latter process controls the former
one. In that sense the rapid radial diffusion of a surface atom into a cluster is caused by the
surface diffusion unlike the usual diffusion in bulk solid. In order to clarify the mechanism
converting the active surface motion into the rapid radial diffusion, we numerically enumerate
reaction paths. We found some candidates of characteristic collective motion which would
effectively induce the rapid radial diffusion in terms of the surface activity.
These two results suggest that surface atoms diffuse into the inside of the cluster due to the
accumulation of active surface motion during long-time dynamics beyond a microsecond, even
though the cluster is almost in solid-phase. Whilst diffusion due to the mechanism is crucial for
the onset of RA, the same atomistic process is similarly expected not only for binary clusters,
but also for monotonous ones. It leads us to an idea that a microcluster can be viewed as a
dynamic material in which a transport of atoms between the surface area and the core region is
continually induced in the course of long time scale.