The presence of hydrogen may weaken the bonding of iron atoms at grain boundaries, leading to intergranular embrittlement and thus failure of the bulk material. In this paper, we study the interaction of hydrogen interstitials with close-packed and open grain boundary structures in $\alpha${}- and $\gamma${}-Fe using density-functional theory. We find that hydrogen accommodation within the grain boundaries strongly depends on the local coordination of the available interstitial sites. Within the open grain boundary structures larger interstitial sites are available, enhancing the solubility as compared to that in the respective bulk phases. The mobility of hydrogen within the investigated grain boundaries is low compared to diffusion in perfect single-crystalline bulk. The grain boundaries do not provide fast diffusion channels for hydrogen, but act as hydrogen traps. Hydrogen that is accumulated within the grain boundaries can lead to a lowering of the critical strain required to fracture the material.

As a first step towards understanding the morphology of PdO crystals we performed a systematic full-potential density-functional theory study of all possible $(1\mathrm{}\times 1)$ terminations of the low-index surfaces of tetragonal PdO. Applying the concept of first-principles atomistic thermodynamics we analyze the composition, structure and stability of these PdO orientations in equilibrium with an arbitrary oxygen environment. Within the studied subset of $(1\mathrm{}\times 1)$ geometries the polar PdO-terminated PdO(100) orientation turns out to be surprisingly stable over the whole range of experimentally accessible gas phase conditions. Setting up a constrained Wulff construction within the compiled data set, this PdO(100)-PdO facet correspondingly dominates the obtained polyhedron by far. The real PdO crystallite shape will however likely be affected by surface reconstructions, which are not covered by the present study.

The oxidation of the Pd(100) surface at oxygen pressures in the ${10}^{-6}$ to ${10}^3\mathrm{m}\mathrm{b}\mathrm{a}\mathrm{r}$ range and temperatures up to 1000 K has been studied in situ by surface x-ray diffraction (SXRD). The results provide direct structural information on the phases present in the surface region and on the kinetics of the oxide formation. Depending on the $(T,p)$ environmental conditions, we observe either a thin $(\sqrt{5}\times \sqrt{5})R27°$ surface oxide or the growth of a rough, poorly ordered bulk oxide film of PdO predominantly with (001) orientation. By either comparison to the surface phase diagram from first-principles atomistic thermodynamics or by explicit time-resolved measurements we identify a strong kinetic hindrance to the bulk oxide formation even at temperatures as high as 675 K.

The dimer method is a minimum mode following algorithm for finding saddle points on a potential energy surface of atomic systems. Here, the dimer method is extended to include the cell degrees of freedom for periodic solid-state systems. Using this method, reaction pathways of solid-solid phase transitions can be determined without having to specify the final state structure or reaction mechanism. Example calculations include concerted phase transitions between CdSe polymorphs and a nucleation and growth mechanism for the A15 to BCC transition in Mo.