A Calculation Of The Structure Stability Of Al₁₃I_n～m Clusters And The Adsorption And Diffusion Of H On MgH₂ Surface

Using CASTEP code based on first-principles, the bonding characters of on-top Al₁₃I_n^m(n=1-12, m=-1,0,+1) clusters and the sorption and diffusion of H on the surface of MgH₂ surface have been calculated. CASTEP is a state of the art quantum mechanics based program designed specifically for solid state materials science. CASTEP employs the Density Functional Theory (DFT) plane-wave pseudopotential method which allows you to perform first-principles quantum mechanics calculations that explore the properties of crystals and surfaces in materials such as semiconductors, ceramics, metals, minerals and zeolites. Typical applications involve studies of surface chemistry, structural properties, band structure, density of states and optical properties.In the first part of this paper, energetic and electronic structures of the on-top Al₁₃I_n^m (n = 1～12, m =–1,0,+1) clusters have been investigated by employing a first-principles pseudo-potential plane wave method. Several parameters such as binding energies, second differences of energy and vertical-electron detachment energies have been adopted to characterize and evaluate the structure stability of Al13In- (n = 1～12) clusters. The optimized models show that the Al13 moieties in the clusters cannot retain the original regular icosahedron structure. Results from binding energy and second difference of energy show that Al13In and Al13In- clusters with even n are more stable than those with odd n in contrast with Al13In+ clusters. The calculation of vertical-electron detachment energies (VDE) of Al13In clusters indicates that Al13In and Al13In- clusters with even n are closer to the closed shell of the Jellium model. Further analysis of electron density of states and electron density differences reveals that the enhanced stability of Al13In- clusters is not only associated with the closed shell of valence electrons but also with the bonding type between I and associated Al atoms.In the second part of this paper, the sorption of H atom and H₂ molecule on perfect and defect MgH₂ surface have been investigated by employing a first-principles pseudo-potential plane wave method. The following results are observed and interpreted: (1) According to the distance and the overlap of electron cloud, the physical sorption of H atom on the surface can be distinguished from the chemical sorption form. Compared with perfect MgH₂ surface, defect MgH₂ surface has a stronger adsorbability on the dissociative H atoms due to Mg has a stronger van der vaals forces to suspended H than H atom on the surface. (2) Unlike the sorption of H atoms on MgH₂ surface, there is only the physical sorption form for H₂ molecule. In the case of defect surface, the suspended H₂ molecule rotates to make the two H atom of it and the Mg on the surface are on a line, while the suspended H₂ molecule keep on vertical direction during the optimization of cases on perfect surface. (3) A transition state calculation of chemical sorption of H₂ molecule on defect MgH₂ surface leads to a conclusion that it is possible for the surface to trap H₂ molecule as a molecularly adsorbed states, in which H₂ molecule is forced to approach the surface and then is free to move along the surface to find the site of the defect. However, the state of the system is not stable and the H₂ molecule will dissociate from the surface automatically. (4) Closer to the surface, lower are the energies of defect formation Evan. The H atoms in MgH₂ slab can diffuse to the defects nearby just after conquering a little energy barrier.