Structural and thermal behavior of compact core-shell nanoparticles: Core instabilities and dynamic contributions to surface thermal stability

Abstract

An orbital-free density-functional-theory molecular dynamics technique is applied to investigate the minimum-energy structure and meltinglike transition of Cs55, Li13Na32Cs42, and Li55Cs42 nanoparticles. Icosahedral packing is found to be optimal for homogeneous Cs55, as expected. Heterogeneous particles show a complete segregation of Cs atoms to the cluster surface, and form perfect core-shell structures, that is, structures where each atomic species occupies and completes a different concentric atomic shell. For Li13Na32Cs42, the size mismatch between atomic species forming different shells leads to polyicosahedral packing. For Li55Cs42, however, the size mismatch is huge and perfect polyicosahedral ordering is frustrated, resulting in more complex structural behavior. The three clusters investigated share the same surface shell, formed by 42 Cs atoms, and comparison of their melting behaviors helps to rationalize the increased thermal stability of the cluster surface upon alloying. Cs55 melts homogeneously at approximately 85 K. Both Li13Na32Cs42 and Li55Cs42 show a substantial thermal stability, compared to Cs55 and other alloy compositions where a perfect core-shell structure does not appear. We demonstrate that an important contribution to this increased thermal stability in the nanoalloys comes from the large difference in the atomic masses of the constituent particles, which results in a poor coupling of atomic vibrations along the radial direction. We also give arguments to show that the meltinglike transition in these clusters is triggered by the thermal instability of interior rather than surface atoms. Segregation of Cs atoms to the cluster surface is fully maintained in the liquid state, so that core and surface shells form two inmiscible liquid layers.