Volumen II - SAM

3. RESULTS AND DISCUSSION
Figure 1 gathers the Arrhenius plots for all the considered clusters and the two potentials, namely, ED (a)
and EAM (b). The labels refer to size (number of SIA), and the two numeric columns at bottom right stand
for D0 (left, 10 -5 cm 2 /s) and Q (right, eV). For the curves that are clearly bent, e.g. 3I, 2I EAM, 4I ED, those
values correspond to the few, best aligned, low temperature points.
Figure 1 Diffusion coefficients of small SIA clusters in Mo computed with potentials ED and EAM.
A few general observations are in order. Firstly, the diffusivities of clusters 7I and larger are very similar
among the two models, and there is also an apparent saturation effect of the activation energy to a rather low
value, outcome that has been traced before to the behavior of individual SIA crowdions [12] (7I, 19I, and 39I
are perfect hexagonal in a cross section view). Notably however, the diffusivities of 2I, 3I, and 4I
EAM, are substantially larger than their ED counterparts for T ≤ 900 K. Also worth of noticing are the Q
trends in both models, with a sizable drop for 3I and a marked increase for 4I, the latter most notably in the
ED case.
Partial results for 1I have already been presented in [20]. A striking feature of the Arrhenius plot is the
almost coincidence between the two models, however this is somewhat deceiving because, as already
reported in [20] , most of the underlying atomic mechanisms are different.
Migration of 2I takes place in 3D, with a fairly independent behavior of the two SIA, namely, one advances
then the other eventually follows. The most relevant low T configuration is a nearest neighbors bi-dumbbell
oriented along ; there is also another higher energy relevant configuration whose frequency increases
with T, in the shape of a non-planar three-vertex star. Nevertheless being sessile, it is instrumental, most
notably for ED, in helping 3D motion.
Figure 2(a) shows the minimum energy structure for the case of 3I ED; it is clearly sessile. On the other
hand, the first excited configuration is about 0.2 eV higher in energy. Figure 2(a) is also the fundamental
state for EAM, though at about 0.05 eV above there is a distorted, crowdion-type structure.
Expectedly thus, MD reveals defect migration on segments for both models. The main difference
being that, whereas ED shows fully 3D motion even at the lowest T simulated, EAM's direction changing
rate is very much diminished, registering only a few events at low T. Also in agreement with the
expectations, QEAM < QED .
The two equilibrium structures involved in 4I migration are the rhombus-shaped ones shown in Figure 2(b)
and (c). The former, relevant to ED, can be described as four parallel dumbbells slightly bent from the
direction with the shortest rhombus diagonal contained in a {110} plane; on the other hand the latter,
relevant to EAM, shows the standard four parallel crowdions. Correspondingly, the simulations for
ED, within the times here employed, show a change from 2D migration (where the shortest diagonal lies on a
fixed {110} plane) to 3D one at about 1000 K; differently, EAM simulations only reveal 1D motion. Also,
consistent with the expectations, QEAM < QED holds
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