Quantum aspects of hydrogen in metals and oxides from density-functional calculations

Doctoral thesis, 2006

Atomic hydrogen dissolved in a solid or adsorbed on a solid surface displays many remarkable features, and is a subject of great technological interest. An increased understanding of hydrogen/solid systems can support the development of areas as diverse as proton exchange membranes for fuel cells, heterogeneous catalysis, and growth of semiconductor devices. The behavior of H in solids and on surfaces is also of fundamental interest, since the uniquely small mass of the hydrogen nucleus allows for quantum effects such as tunneling at low temperatures. This thesis gives a detailed account of the structure and dynamics of hydrogen in different systems using first-principles calculations based on the density-functional theory approach. More specifically, we investigate the basic properties of H and D in bulk metals, on metal surfaces, and in bulk oxides. On a microscopic scale, these systems are characterized by their hydrogen site occupancy, vibrational states, and rate of diffusive jumps between sites. Extensions to macroscopic length and time scales provided by means of thermodynamic and kinetic modeling also allow for a direct comparison with experimental data. In metallic systems, hydrogen atoms generally diffuse very rapidly and remain mobile down to very low temperatures where no migration can be expected to occur classically. We demonstrate how the parameters that characterize this behavior can be calculated on a first-principles basis. For H interstitials in bulk Nb and Ta, we show that good agreement with experiments can be obtained by assuming diffusive jumps to occur as a sequence of thermally activated phonon-assisted tunneling transitions at temperatures below approximately 250~K. For H adsorbates on the Cu(001) surface, we show that the hop rate is limited by a non-adiabatic response of the conduction electrons below 25~K and we explain the experimentally observed abrupt transition between nearly temperature independent and thermally activated behavior occurring around 60~K. In hydrated perovskite oxides, proton conductivity typically becomes appreciable only at elevated temperatures. For bulk BaZrO$_3$, we analyze the defect structure under various environmental conditions, and show that the presence of dopant atoms -- although necessary to achieve a high concentration of H$^+$ defects -- can lead to a severe reduction of the proton mobility by acting as 'traps'. Taken together, these results provide valuable insights into the structure and dynamics of hydrogen in various systems and into the quantum nature of the interaction of hydrogen with different materials.