Coexistence of close packed structures in large substrate-free Ar-Kr clusters according to THEED data

TL;DR

This study uses THEED to analyze the structure of Ar-Kr clusters, revealing size-dependent coexistence of fcc and hcp phases and the influence of composition on phase fractions.

Contribution

It provides the first in-situ quantitative analysis of phase coexistence in substrate-free Ar-Kr clusters across various sizes and compositions.

Findings

Clusters larger than a threshold size contain both fcc and hcp phases.

The fraction of hcp phase increases with cluster size and reaches a maximum at equimolar composition.

Two-phase clusters are formed in the supersonic jet, supporting a thermally activated diffusion growth mechanism.

Abstract

A quantitative phase analysis of substrate-free single-component and binary clusters of the Ar-Kr system obtained by adiabatic expansion of gas into vacuum through a supersonic nozzle was performed. The studies were carried out in-situ using transmission electron diffraction technique (THEED) on clusters with an average size ranging from 2000 to 100000 atoms/cluster and across the entire range of component concentrations. The independence of the threshold size of clusters, corresponding to the beginning of the formation of the hcp phase, from the component composition was revealed. It was established that clusters larger than this threshold size have a two-phase fcc-hcp structure with an identical concentration of components in each phase. The fraction of the hexagonal phase increases with the size of the aggregations and depends on the component content, reaching maximum in clusters with an equimolar composition. Arguments are presented in favor of the formation of two-phase clusters in the supersonic jet, rather than separate single-phase fcc and hcp ones. These findings are in good agreement with the previously proposed thermally activated diffusion mechanism for the nucleation and growth of the hcp phase in rare gas clusters.