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
density functional theory