If a future hydrogen society is to be established, a highly efficient solid-state hydrogen storage material must be found. The metal-N-H system, which is one of the new promising candidates, has been given considerable attention after Chen et al. in 2002 found that lithium-nitride reversibly absorbs a large amount of hydrogen and form a mixture of lithium-hydride and lithium-amide. This thesis presents density functional theory calculations performed on Mg(NH2)2 (magnesium-amide).
Mg(NH2)2 is confirmed having a tetragonal unit cell which belongs to the space group I41/acd. The cations in Mg(NH2)2 (i.e. Mg2+) are tetrahedrally coordinated by the anions (i.e. NH2-) and the MgN4 tetrahedra share all four corners with other MgN4 tetrahedra, thus forming an open, three dimensional network. Furthermore, the crystal structure was found to be organized in weakly connected branches, where the branches point in the [1_10] or the [_110] directions, and where each branch is separated from the other branches by planes in the (112) and (11_2) orientations.
Density of states calculations showed that Mg(NH2)2 is an insulator with a GGA band gap of approximately 3.1 eV. The bonds within the anions (N-H) are primarily covalent, while ionic Mg-N bonds hold Mg(NH2)2 together.
Six different surfaces and their corresponding surface energies have been calculated. The (110) surface proved to have the lowest surface energy, closely followed by the (012) and (112) surfaces. However, special symmetry properties and small structural changes during ionic relaxation of the (112) slab, indicates that the corresponding surface may be even more frequently found in a real crystal, despite the slightly higher surface energy compared to the (110) surface.
The first reported cluster calculations on complex hydrides in a plane wave code are to our knowledge included in the thesis. A combination of bulk, slab and cluster calculations provided new and important insights about Mg(NH2)2. Comparison of the density of states calculated from bulk, slab and cluster showed that occupied states in the band gap probably are one of the main reasons for why complex hydrides with nano-particle structure have better kinetics than complex hydrides with larger particles (in addition to the increased surface area).
Calculations of the activation energy involved in the removal of NH3 and H2 showed that it is energetically easier to remove NH3 than H2 from Mg(NH2)2, confirming a general trend for metal-N-H systems.