On the origin of atomistic mechanism of rapid diffusion in alkali halide nanoclusters

TL;DR

This study uses molecular dynamics simulations to uncover that vacancy mechanisms, rather than surface melting, drive rapid diffusion in alkali halide nanoclusters, with diffusion rates depending on size and temperature.

Contribution

It demonstrates that vacancy formation and diffusion are key to rapid mixing in alkali halide nanoclusters, challenging the surface melting hypothesis.

Findings

Vacancy formation energy decreases with cluster size.

Surface melting and collective motions are inhibited.

Diffusion rate depends on size and temperature.

Abstract

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.