  To elucidate the atomistic diffusion mechanism responsible for the rapid
diffusion in alkali halide nano particles, called Spontaneous Mixing, we
execute molecular dynamics simulations with empirical models for KCl-KBr,
NaCl-NaBr, RbCl-RbBr and KBr-KI. We successfully reproduce essential features
of the rapid diffusion phenomenon. It is numerically confirmed that the rate of
the diffusion clearly depends on the size and temperature of the clusters,
which is consistent with experiments. A quite conspicuous feature is that the
surface melting and collective motions of ions are inhibited in alkali halide
clusters. This result indicates that the Surface Peeling Mechanism, which is
responsible for the spontaneous alloying of binary metals, does not play a
dominant role for the spontaneous mixing in alkali halide nanoclusters.
Detailed analysis of atomic motion inside the clusters reveals that the Vacancy
Mechanism is the most important mechanism for the rapid diffusion in alkali
halide clusters. This is also confirmed by evaluation of the vacancy formation
energy: the formation energy notably decreases with the cluster size, which
makes vacancy formation easier and diffusion more rapid in small alkali halide
clusters.
