Figure 3. Two-dimensional ELF of (a)
Mg3H7 (b)
Mg4H9 (c)
Mg5H11, and (d)
Mg6H13 clusters. Green and white balls
stand for Mg and H atoms, respectively. The projection plane is
determined by the three Mg atoms marked with labels shown at the bottom
of the panels.
To gain further insights into the stability of oversaturated
MgmHn (m = 3-6, n = 2m+1) clusters, we
perform the electronic-structure analysis for
Mg3H7,
Mg4H9,
Mg5H11 and
Mg6H13 clusters using electron localized
function (ELF) [40]. Figure 3 shows the comparison of the
two-dimensional (2D) ELF projection diagrams of
Mg3H7,Mg4H9,Mg5H11, and
Mg6H13 clusters, where the projection
plane is determined by the three Mg atoms marked at the bottom of each
panel. The high ELF value indicates a strong electronic localization. It
can be seen that the electrons around hydrogen atoms are strongly
localized for all clusters. The minimum ELF value between Mg and H
indicates an ionic character of Mg-H bond. For the
Mg3H7 cluster, ELF values between H4 and
H5 atoms are around 0.6 (Figure 3a), indicating a covalent interaction
between H4 and H5 atoms. This covalent interaction is also found in the
hydrogen pairs of H5-H6 in
Mg4H9 cluster and of H6-H7 in
Mg5H11 cluster (Figure 3b and 3c).
However, the ELF value between hydrogen atoms
(H5-H6 and H7-H8) in
Mg6H13 cluster is much smaller (about
0.2) (Figure 3d), which indicates a relatively weaker covalent
interaction and higher stability of
Mg6H13 cluster.
3.2 Hydrogen Dissociation of MgmHnClusters
To explore hydrogen dissociation of the saturated
MgmH2m and hydrogen-enriched
MgmH2m+1 clusters, we carried out AIMD
simulations at room temperature for 5ps. The potential energies for
saturated and hydrogen-enriched clusters are shown in Figure 4a and
Figure 4b, respectively. It can be
seen that the time-dependent potential energies for saturated
MgmH2m clusters fluctuate near the
equilibrium energy, which indicates the clusters are quite stable during
the simulation time. In contrast, the time-dependent potential energy
the hydrogen-enriched nanoclusters
MgmH2m+1 show a very fast energy
decreasing within 200fs, which indicates the hydrogen dissociation
reactions occurs at a very short time scale. This can be also
demonstrated in the time-dependent H-H distance shown in Figure
4(c)-(d). For the saturated clusters (Figure 4(c)), the H-H distance
only oscillates around ~ 3.2 Å during 5 ps simulation,
while the hydrogen molecule is generated within 200 fs. Somehow
unexpectedly, Mg6H13 with more negative
adsorption energy (Figure 2b) have the faster hydrogen dissociation
reaction rate than Mg4H9 and
Mg5H11 clusters. The underlying reason
is still not clear and needs further investigation.