Electron diffraction of fcc-hcp transition in argon clusters with changing their sizes

A transition from FCC to mixed FCC-HCP structure as a function of the cluster size is followed for the first time in free crystals formed in supersonic argon streams. It is established that FCC reflections appear and their intensity sharply increases for linear size of the aggregations of about 110 A. As cluster size increases above this value, the FCC reflections become weaker. The transition is preceded by a special structural state of the clusters where there is a substantial number of randomly arranged close-packed atomic layers. Such a transition has been observed previously in bulk samples of solid argon under compression to pressure of approximately 50 GPa. In clusters where such a high pressure is not reached, the appearance of a disordered close-packed structure followed by the appearance of FCC domains is probably due to the mechanisms of aggregation growth in supersonic streams