  An electron diffraction study of nucleation and growth of the hcp phase in
large pure Ar and mixed Ar-Kr clusters formed by supersonic jet expansion is
carried out. The threshold cluster size corresponding to the appearance of an
hcp phase in addition to the fcc structure is determined. It is found that the
relative volume of the hcp phase in clusters increases with their size. In very
large aggregations, the percentage of hcp phase reaches its maximum and does
not change with further cluster growth. The hcp phase relative volume in mixed
clusters is almost double that in pure Ar clusters of the same size. A
correlation between the hcp phase relative volume and the number of defective
planes contained in the fcc matrix, which are the nuclei of the hcp phase, is
established. A mechanism of nucleation and growth of the hcp phase in rare gas
clusters is proposed.
