Nucleation and growth of a hcp phase in homogeneous (Ar) and heterogeneous(Ar_Kr) clusters by the electron diffraction data

The electron diffraction study of nucleation and growth of the hcp phase in homogeneous (Ar) and heterogeneous (Ar-Kr) clusters formed in adiabatically expanding supersonic jets is carried out. The average cluster size ranged from 2x10³ to 1x10⁵ atom/claster. 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 monocrystalline clusters increases with their size. In heterogeneous clusters, the hcp phase relative volume is greater than that in homoge-neous clusters of the same size. We established a correlation between the hcp phase relative volume and the number of ''defective'' planes contained in the fcc matrix, which were the nuclei of the hcp phase. It is found that in very large polycrystalline aggregations (delta>=150 A) fraction of the hcp phase reaches its maximum and does not change with further cluster growth. A mechanism of nucleation and growth of the hcp phase in rare gas clusters is proposed.