Book of Abstracts Book of Abstracts - Universität Konstanz

A - 17
124
Experimental study of the phase transformations
in molecular clusters of Ar and N2
O. G. Danylchenko, S. I. Kovalenko, V. N. Samovarov
B. Verkin Institute for Low Temperature Physics and Engineering, Natl. Academy of Sciences of Ukraine,
47 Lenin Ave., Kharkov, 61103, Ukraine
We use an electron diffraction technique to study the structure of substrate-free Ar and N2
clusters and nanoparticles formed in the process of homogeneous nucleation in a supersonic jet
isentropically expanding into vacuum. The average size N of aggregations varied between 5⋅10 2
and 1.5⋅10 5 atoms (molecules) per cluster. The temperature of clusters and nanoparticles was
T=(38±5) K in the diffraction zone.
Analysis of the obtained electron-diffraction patterns has shown for the first time the existence
of an amorphous phase in large rare-gas clusters (N≈600 atoms/cluster). As N increased in a
relatively narrow size interval a transition occurred from the amorphous phase to the multilayer
icosahedral one observed for N=1300 atoms/cluster. Formation of a crystalline face-centered
cubic (fcc) structure of argon with stacking faults took place for N=3⋅10 3 atoms/cluster which is
in good accordance with the previous observations [1,2]. The electron-diffraction patterns of
bigger clusters (N∼10 4 atoms/cluster) are shown to be characterized by hexagonal close-packed
(hcp) maxima along with the fcc peaks. The intensity of the hcp peaks grew with increasing
aggregation size, and for nanoparticles with N∼10 5 atoms/cluster we clearly detected (100),
(101), (103), and (202) peaks characteristic of an hcp structure with intensity comparable with
that of the fcc peaks.
In case of linear molecules like those of nitrogen, besides the central interaction between
particles there is a considerable contribution of non-central electrostatic forces which are
responsible for the orientational ordering of molecules at low temperatures. The structure of
such clusters can differ significantly from the structure of rare-gas clusters depending on the
anisotropic contribution to the total lattice energy. In contrast to amorphous argon clusters, for
nitrogen beams with N≈500 molecules/cluster we first observed the formation of aggregations
with multilayer icosahedral packing of molecules. The ‘wings’ of the most prominent
icosahedral-phase maximum (in the region corresponding to the (100) and (101) peaks of the
high-temperature β phase of N2, α-β phase transition in a bulk at T=35.6 K) are found to be
curved thus indicating the presence of traces of the β phase in these clusters. As the cluster size
grows at the same T in the diffraction zone, the diffraction pattern undergoes transformations
and for N∼10 3 molecules/cluster it contains maxima of both cubic (α) and hcp (β) phases.
Further cluster growing is accompanied by a decrease in the intensity of the cubic-phase peaks
and strenghening of the hcp peaks. Clusters with N∼10 4 actually have only the high-temperature
β phase.
We propose and discuss a mechanism underlying the observed structural transformations in Ar
and N2 which supposes cluster formation from a liquid droplet and takes into account the
kinetics of phase growth when there are energy barriers between phases.
References
[1] I. Farges, M.F. de Feraudy, B. Roult, G. Torchet, Adv. Chem. Phys. 70, 45 (1988).
[2] O.G. Danylchenko, S.I. Kovalenko, V.N. Samovarov, Low Temp. Phys. 30, 166 (2004).