Book of Abstracts Book of Abstracts - Universität Konstanz

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Structure, energetics, and thermodynamics of copper, nickel, and gold
clusters: N = 2–150
V. G. Grigoryan, D. Alamanova, and M. Springborg
Universität des Saarlandes, Physikalische und Theoretische Chemie, Postfach 151150
D-66041 Saarbrücken, Germany
The three most stable structures of CuN, NiN, and AuN clusters with N between 2 and 150
have been determined using a combination of the Embedded-Atom (EAM), the quasi-Newton,
and our own [1-3] Aufbau/Abbau methods for calculations of the total energies, local and global
minima, accordingly. We have employed two well-known versions of the EAM: (1) the bulk
version of Daw, Baskes and Foiles and (2) the Voter-Chen version which takes into account
properties of the dimer. The lower-energy structures (also for the smallest ones) of CuN and NiN
clusters (structural details, symmetry and the ordering of the isomers) obtained with the two
versions are almost the same. Thus our study points to the universality of the bulk embedding
functions and potentials for copper and nickel at least for computations of structural properties.
But the bulk EAM fails completely to describe the structures of the smallest Au clusters. Hence
for the gold clusters only the Voter-Chen version of the EAM has been used. Copper and nickel
grow qualitatively similarly. Generally, we find a multilayered icosahedral or layer-by-layer
growth. But the cluster growth pattern between two closed-shell icosahedra is not simple. It is
predominantly icosahedral with islands of fcc, tetrahedral, and decahedral growth. Further, we
have observed an enhanced ability of fcc clusters to compete with icosahedral and decahedral
structures in the vicinity of N=79. The truncated centered octahedron at N=79 is the first and the
second lower-energy structures for Cu and Ni clusters, respectively. The derived results are
detailed discussed and compared with those of available ab initio and semiempirical studies.
Agreement with experimental studies is very good in many cases. The situation for gold is more
complicated. We could not detect any favored structural motif. For example, Au13 is the first
Mackay icosahedron, Au38 is the truncated octahedron, but all the three isomers for Au55 are
low-symmetric. Our result for 38-atom gold cluster agrees with the many-body Sutton-Chen and
Murrell-Mottram calculations, but it is in contradiction with the 'disordered' result of the n-body
Gupta potential. Hence more studies are needed. Furthermore, we determine the full vibrational
spectrum and the vibrational heat capacities (contributions from rotations are negligible) of
clusters for each cluster size. It is found, that the 23-atom Cu or Ni cluster (triple icosahedron)
possesses the highest vibrational frequency. Further it is shown that the highest vibrational
frequency is lower in the fcc and decahedral clusters and higher in the icosahedral ones. The
heat-capacity plots for low temperatures show very interesting fine structures with minima at
magic numbers of 13 and 55 and with a broad maximum in the vicinity of N=35.
References
[1] V. G. Grigoryan and M. Springborg, Phys. Chem. Chem. Phys. 3, 5125 (2001).
[2] V. G. Grigoryan and M. Springborg, Chem. Phys. Lett. 375, 219 (2003).
[3] V. G. Grigoryan and M. Springborg, Phys. Rev. B 70, 205415 (2004).
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