  An electron diffraction technique is used to study the structure of clusters
formed in an isentropically expanding supersonic argon jet. The formation of
the hcp phase with increasing cluster size is reliably detected for the first
time. Observations are made for mean cluster sizes N in the range from 1x10^3
to 8x10^4 atoms/cluster. An analysis of the shape of the diffraction peaks is
carried out. It is found that in the range N <= 2x10^3 atoms/cluster, where the
clusters are icosahedral, the profiles of the diffraction peaks are well
approximated by a Lorentzian. For fcc clusters with N >= 3x10^3 atoms/cluster a
better approximation is the standard Gaussian function. In the case N >= 1x10^4
atoms/cluster one observes peaks of the hcp phase in addition to the fcc peaks.
The intensity of the hcp peaks increases with increasing cluster size, and for
N = 8x10^4 atoms/cluster, the (110), (101), (103), and (202) peaks,
characteristic only for the hcp phase, are clearly registered in addition to
the fcc peaks. A possible mechanism for the formation of the hcp structure in
Ar clusters is proposed.
