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.