Ordering of Self-Diffusion Barrier Energies on Pt(110)-1x2 Page: 4 of 18
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For Physical Review B, Rapid Comunications
The argument comparing dimer leapfrog diffusion with dissociation begins
from an estimate of the dissociation barrier. At such a barrier, one adatom remains
in its initial high-coordination hollow while the other is at a bridge adjacent to its
initial site. On Pt(110)-1x2, this bridge may lie directly down the missing row or on
one of the (111)-facets that form the missing-row walls. Either way, the adatom at
the bridge is far enough from the one that stays behind that their interaction is small.
Thus, the dimer-dissociation barrier energy ~ the adatom-adatom binding energy
plus the monomer diffusion barrier. Since the inter-adatom binding energy is inde-
pendent of dissociation path, the bridge-site energy determines the most facile dis-
sociation path. Displacement of an adatom up a channel wall is therefore the most
facile monomer-diffusion and dimer-dissociation mode.
This implies that the addimer configuration illustrated in Fig. 2 is a "doorway
state" through which both dissociation and leapfrog-diffusion MEP's pass. That the
barrier to dissociation is higher than to leapfrogging is a corollary. To see this,
imagine removing atom 2 from the picture. By symmetry, there is then no energetic
difference between displacing atom 1 to eitherfcc site f or f', as there is if atom 2 is
present. In the latter case, atom 1 breaks its bond to atom 2 in displacing to site f,
while it does not in moving to site f'. Moving to f' is therefore less costly.
Numerics - To flesh out these arguments, I have computed ab-initio activation
barriers using VASP (Vienna ab-initio simulation package),7-9 its corresponding
ultrasoft-pseudopotential data-base,10 and the Perdew-Wang '91 Generalized Gra-
dient Approximation (GGA).1' Plane-wave calculations for first-row atoms and
transition metals typically require unwieldy basis sets, but the ultrasoft pseudopo-
tential assures convergence for Pt with the modest basis-cutoff of 14.7 Ry, making
the present work tractable despite involving large unit cells.
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Feibelman, Peter J. Ordering of Self-Diffusion Barrier Energies on Pt(110)-1x2, article, June 1, 1999; Albuquerque, New Mexico. (digital.library.unt.edu/ark:/67531/metadc707263/m1/4/: accessed February 17, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.