Ab initio lattice dynamics of complex structures: Computational design of novel materials for hydrogen storage

In this thesis, density functional theory is applied in a study of thermodynamic properties of so-called complex metal hydrides, which are promising materials for hydrogen storage applications. Since the unit cells of these crystals can be relatively large with many symmetrically inequivalent atomic coordinates, we have developed a new numerical optimization scheme, which allows for a fast convergence of the coordinate relaxation. Moreover, a method for the efficient calculation of phonon frequencies has been developed, which is based on a combination of density functional theory calculations and the electrostatics of effective point charge systems. The method is O(N) times faster than conventional approaches employing a calculation of the full Hessian matrix (N: number of atoms per unit cell) and is thus suitable for the assessment of thermodynamic stabilities based on the vibrational entropies of large systems in particular. A more detailed analysis of the phonon spectrum has been performed for the compound Mg(BH4)2, where several crystal symmetries have been proposed theoretically and experimentally. By means of an analysis of the instabilities of these structures, a new, stable phase has been determined. Aiming at finding scaling relationships between alloy stabilities and computationally inexpensive properties, the stabilities of cation-alloyed metal aluminum hexahydrides have been studied. The analysis shows that charge density symmetries are correlated to the stability. In addition, the vibrational entropies of these systems have been estimated in a maximally localizedWannier function basis without calculating charge density perturbations. In a combined experimental and computational approach, the kinetic properties of hydrogen diffusion processes in sodium aluminum hydride have been studied, showing that the mobility of hydrogen is limited by high energetic barriers in the intermediate decomposition product Na3AlH6 in particular, and that the effect of titanium as a dopant on the dynamics is negligible. The presented methods and studies demonstrate possibilities for a design of new materials for hydrogen storage applications based on qualitative screening and the precise analysis of known structures.