Three of the most important questions concerning hydrogen storage in metals are how much hydrogen can be absorbed, how fast it can be absorbed (or released) and finally how strongly the hydrogen is bonded. In transition metals hydrogen occupies interstitial sites and the absorption as well as desorption of hydrogen can be fast. The enthalpy of the hydride formation is determined by the electronic structure of the absorbing material, which determines the amount of energy released in the hydrogen uptake and the energy needed to release the hydrogen.

This thesis concerns the possibility of tuning hydrogen uptake by changing the extension of the absorbing material and the boundary conditions of extremely thin layers. When working with extremely thin layers, it is possible to alter the strain state of the absorbing material, which is used to influence the site occupancy of hydrogen isotopes. Vanadium is chosen as a model system for these studies. V can be grown in the form of thin films as well as superlattices using MgO as a substrate. Special emphasis are on Fe/V(001) and Cr/V(001) superlattices as these can be grown as high quality single crystals on a routine basis. The use of high quality samples ensured well-defined conditions for all the measurements.

In these experiments the hydrogen concentration is determined by the light transmittance of the thin films.  By changing the temperature and the pressure of the hydrogen gas, it is possible to determine the thermodynamic properties of hydrogen in the samples, from the obtained concentrations.  Measurements of the electrical resistivity is used to increase the accuracy in the measurements at low concentrations as well as to provide information on ordering at intermediate and high hydrogen concentrations. The thermodynamic properties and the electrical resistivity of VH are strongly affected by the choice of boundary layers. For example, when hydrogen is absorbed in V embedded by Fe, Cr or Mo in the form of superlattices, both the thermodynamic properties and the changes in the resistivity are strongly influenced.

The critical temperature and H-H interactions of hydrogen in thin V(001) layers are found to increase with thickness of the thin films and superlattices. The observed finite size effects resemble same scaling with the thickness of the layers as does the magnetic ordering temperature. The results were validated by investigations of isotope effects in the obtained thermodynamic properties. Close to negligible effects are obtained when replacing hydrogen by deuterium, with respect to the thermodynamic properties. These observations are rationalised by an octahedral occupancy in the strained layers, as compared to tetrahedral occupancy in unstrained bulk. The octahedral site occupancy is found to strongly alter the diffusion coefficient of hydrogen in thin V layers.

In this thesis, first-principles calculations based on density functional theory have been employed to investigate metal hydrogen interaction in transition, p-block and rare earth metals. Furthermore, the accuracy of the stochastic quenching method was tested in describing the structure of amorphous Fe_(1-x)Zr_x.

The investigated systems of transition metal hydrides are V-H and ScZr(CoNi)₂-H. For V-H, the main focus of the studies is the effect that strain has on the potential energy landscape which governs the metal hydrogen interactions. The investigation has focused on how the properties of hydrogen occupancy in the interstitial sites changes with strain and also how the hydrogen atoms themselves exert strain on the vanadium structure to lower the energy. Results on diffusion, induced strain and zero-point energy are presented which all reveal the considerable difference between tetrahedral and octahedral site occupancy. Diffusion was studied by employing ab initio molecular dynamics simulations to obtain diffusion coefficients and to map the movement of the hydrogen atom. A description of hydrogen in vanadium is provided from a fundamental basis that is expected to be applicable to any lattice gas system. For ScZr(CoNi)₂-H, the difference of hydrogen occupancy in various interstitial sites and the hydrogen-induced strain was also investigated through calculations of the change in total volume as a function of hydrogen concentration.

The fundamental properties of metal hydrogen bonding were investigated by studying the Zintl phase hydrides that are constituted of the electropositive metal of Nd or Gd and the electronegative metal Ga. Mixing metals of very different electronegativity gives rise to an intricate potential energy landscape in which the incorporation of hydrogen will have a big effect on both the electronic and atomic structure. From the theoretical side of the investigation, structural parameters are presented along with the density of states and Bader charge analysis to describe the hydrogen induced changes to the atomic and electronic structures.

Finally, the accuracy of the stochastic quenching method in describing amorphous Fe_(1-x)Zr_x was evaluated by comparing simulated and measured EXAFS spectra. Once the structural agreement had been established the simulated structures were characterized through radial distribution functions and an analysis of the short-range order from Voronoi tessellation. The structural changes with respect to the composition parameter x were also evaluated.

Metal hydrides can be used as hydrogen storage materials for fuel cells and batteries, and as sensors for detecting hydrogen gas. The use of metal hydrides for hydrogen storage can be hindered by poor kinetics and low capacity. Moreover, poor sensitivity, long recovery and response time, limit the applications of metal hydrides as hydrogen sensors. Diffusion is an important factor affecting the hydrogen kinetics and response time. Hysteresis effects accompany the phase transition of hydrogen in metals and can influence the properties of metal hydrides as well. These need to be considered in their applications as storage materials or sensors.

This thesis concerns the possibility of tuning hydrogen diffusion and studies the mechanism of hysteresis effects of hydrogen absorption in metals. In these experiments, nano-sized vanadium is used as the model system for these studies. Hydrogen concentration is determined by the light transmission. By measuring the concentration profiles and isotherms of hydrogen, it is possible to determine the diffusion coefficients and hysteresis effects.

A profound decrease of hydrogen diffusion in Fe/V(001) superlattice has been found, as compared to that in bulk vanadium. This result is interpreted as lower zero-point energy in octahedral site than that in tetrahedral site. Profound isotope effect on diffusion has also been found. Influence of clamping of the substrate on the diffusion of hydrogen with concentration in vanadium thin film is discovered. The diffusion coefficient below c = 0.1 [H/V] is close to that in bulk vanadium and decreases substantially when c > 0.1 [H/V] compared with that in bulk vanadium. This finding is interpreted as the site change from tetrahedral to octahedral occupancy when the hydrogen concentration increases. Large finite size effect on deuterium chemical diffusion is observed, which is concluded to be caused by D-D interaction change that will influence the deuterium chemical diffusion at different thickness of vanadium layers. However, finite size has no effect on hydrogen transport at extremely low hydrogen concentrations in Fe/V (001) superlattices, this illustrates that the interface can not influence the mean free path of hydrogen in any way. This is completely different from electron transport condition in nano-sized metals. Hysteresis effect is observed below critical temperature in Fe/V(001) superlattices; this occurrence confirms the hypothesis that hysteresis effect is caused by coherency strain in coherent  transformation.

The absorption of hydrogen is exothermic in vanadium whereas it is endothermic in iron and chromium. Investigations of the hydrogen uptake within Fe/V(001) and Cr/V(001) superlattices allow therefore a detailed exploration of finite size effects and the influence of boundaries on hydrogen absorption. Fe/V(001) and Cr/V(001) superlattices can be grown as single crystal structures with a small mosaic spread, as determined by X-ray reflectometry and diffraction. Furthermore when the thickness ratio of the constituents is kept constant the crystal quality can be retained in the range from a few up to 40 monolayer repeat distances (Λ). Neutron reflectometry was used to simultaneously determine the volume expansion and concentration of hydrogen in the vanadium layers. Large differences are found in the expansion of Fe/V(001) and Cr/V(001) superlattices, in good agreement with density functional theory (DFT) calculations. The findings are consistent with tetrahedral and octahedral site occupancy in Cr/V(001) and Fe/V(001) superlattices, respectively. Full fitting of the reflectivity pattern is required to obtainan accurate measure of expansion if the number of repeats is small. Under these conditions, the shift of the first order superlattice peak can be an inaccurate measure of the volume changes. By using a specially designed neutron scattering chamber, allowing simultaneous neutron and optical transmission measurements, it is found that the optical transmission scales linearly with hydrogen concentration. By comparing the experimental results to ab-initio DFT calculations, it is shown that optical transmission scales with electron density changes in the samples, explaining the linearity with concentration. This change is dominated by the hydrogen induced expansion of the lattices and depends therefore strongly on the site occupancy of the hydrogen. Finally, X-ray diffraction was used to address the local strain fields and the α to β phase transition, typically observed in bulk vanadium. Below 448 K the results are consistent with an α to β phase co-existence, separated along the surface normal of the samples.