[en] Magnetite is a material used for catalysts in some of today's most important industrial processes, the Haber-Bosch and Fischer-Tropsch processes. It has raised attention due to its catalytic behaviour in the water-gas shift reaction, its potential use in spintronic devices and hierarchical materials. There are however still a lot of open questions concerning these potential applications, because the mechanisms at the surface that play an important role in these are not fully understood. The structure of the √(2) x √(2)R 45 ° reconstruction that forms on the clean magnetite (001) upon preparation in ultra-high vacuum, which was formerly explained by a bulk-truncated model, had to be revised due to new observations that were unexplainable with the former model and led to the proposition of a new model. This was also invalidating former interpretations of lifting processes of this reconstruction due to significant differences between the models. To solve some of these open questions, surface X-ray diffraction measurements have been performed on the (001) surface of magnetite exposed to several different conditions. Following the surface science approach, the first system investigated was the structure of the clean, reconstructed surface to revalidate the newly proposed structural model as a bare for all of the following experiments. Due to an excellent agreement with the surface X-ray diffraction data measured on this surface, the new, non-stoichiometric structural model was confirmed. A further structural refinement was performed, but only led to some minor changes in the model. In addition, the impact of certain features of the model on the data was investigated by simulations to get a better understanding on the sensitivity of the measurements for these. Based on this, the structural changes in the surface upon dissociative adsorption of formic acid was investigated. The formate that forms during this process and adsorbs on the surface is a proposed intermediate in the water-gas shift reaction, and acts as a probe molecule for the interaction of other carboxylic acids with the surface. Dosing formic acid was known to lift the √(2) x √(2)R 45 ° reconstruction of the surface, and, following a structural analysis of the obtained diffraction data of this surface, it could be shown for the first time that the surface changes back to bulk-stoichiometry, involving the reorganisation of iron cations at the surface as well as iron diffusion from lower layers towards the surface. A full structural refinement was performed, and the resulting model agreed well with the adsorption geometry proposed in literature. In addition, water vapor and atomic hydrogen, both also known to lift the surface reconstruction at room temperature, were also dosed on the clean reconstructed surface and its structure investigated. The pressure region in which water vapor lifts the reconstruction was found to be between 1 x 10^-4 mbar and 1 x 10^-3 mbar, which agrees nicely with the reported onset of hydroxylation of the surface reported in literature. Both water vapor and atomic hydrogen were found to not only completely lift the reconstruction, but also lead to an atomic roughening of the surface within the first two layers. For atomic hydrogen, this was also confirmed by Scanning-Tunneling Microscopy measurements. This indicates significant iron diffusion and, subsequently, a surprisingly high mobility of iron cations at the surface under these conditions. No different phase was formed at the surface, and the analysis indicates ordered hydroxyl to be present at the surface during water dosing as well as after pumping back down. To complement the X-ray diffraction measurements, X-ray photoelectron spectroscopy measurements were performed before and after dosing of formic acid, water vapor and atomic hydrogen. A rather large amount of carbon was found to bc present on the surface after water vapor dosing. The same observation was made upon exposure to atomic hydrogen, but in contrast to the water vapor exposed surface, the amount of carbon increased even further over time. This indicates that the hydroxylated surface is attracting carbon from the residual gas even under ultra-high vacuum conditions. Finally the reversible lifting of the surface reconstruction upon heating was investigated. The resulting data from the Lifting process agreed well to what was described in literature. The cause of this was found to be a decrease in the long-range ordering at the surface due to diffusion of iron at the surface, exhibiting a fundamentally different lifting mechanism than what was found under the previous conditions.

[en] Niobium’s superconducting properties are affected by the presence and precipitation of impurities in the near-surface region. A systematic wide-temperature range x-ray diffraction study is presented addressing the effect of low temperatures (108 K–130 K) and annealing treatments (523 K in nitrogen atmosphere, 400 K in UHV) on the near-surface region of a hydrogen-loaded Nb(100) single-crystal. Under these conditions, the response of the natural surface oxides (Nb₂O₅, NbO₂, and NbO) and the changes in the subsurface concentration of interstitial species in Nb are explored, thereby including the cryogenic temperature regime relevant for device operation. The formation and suppression of niobium hydrides in such conditions are also investigated. These treatments are shown to result in: (i) an increase in the concentration of interstitial species (oxygen and nitrogen) occupying the octahedral sites of the Nb bcc lattice at room temperature, both in the near-surface region and in the bulk. (ii) A decrease in the concentration of interstitials within the first 10 nm from the surface at 130 K. (iii) Hydride formation suppression at temperatures as low as 130 K. These results show that mild annealing in nitrogen atmosphere can suppress the formation of superconducting-detrimental niobium hydrides, while subsurface interstitial atoms tend to segregate towards the surface at 130 K, therefore altering the local concentration of impurities within the RF penetration depth of Nb. These processes are discussed in the context of the improvement of niobium superconducting radio-frequency cavities for next-generation particle accelerators. (paper)

[en] We report on the quantitative determination of the transient surface temperature of Pt(110) upon nanosecond laser pulse heating. We find excellent agreement between heat transport theory and the experimentally determined transient surface temperature as obtained from time-resolved X-ray diffraction on timescales from hundred nanoseconds to milliseconds. Exact knowledge of the surface temperature's temporal evolution after laser excitation is crucial for future pump-probe experiments at synchrotron storage rings and X-ray free electron lasers.

[en] 25 nm thick Gd-doped ceria thin films were grown on yttria-stabilized zirconia (YSZ) substrates with (110) and (111) orientation by pulsed laser deposition to study both their crystalline structure and interfacial stability. The films were characterized by high-energy grazing incidence x-ray diffraction, x-ray reflectivity and x-ray photoelectron spectroscopy before and after annealing to 1400 K under ultra-high vacuum (UHV) conditions. The films were found to be epitaxial to the YSZ substrates, exhibiting good crystalline quality without defects like twinning, and low surface roughness. Upon reduction due to the annealing in ultrahigh vacuum (UHV), both samples showed an increase in lattice parameter while maintaining their original crystalline quality. The x-ray reflectivity measurements gave evidence for interdiffusion after annealing by the presence of an additional interfacial layer with reduced electron density. X-ray photoelectron spectroscopy revealed an increase in the concentration of Ce"3"+ and also yttrium at the surface upon annealing, indicating a slight reduction of the surface as well as diffusion of yttrium to the surface. - Highlights: • We demonstrate that Gadolinium doped Ceria films grow epitaxial on Yttria stabilized Zirconia in (111) and (110) orientation. • We show by high energy grazing incidence x-ray diffraction that the films are single crystalline and twinning free. • We give evidence that upon annealing the lattice expands as a result of oxygen vacancy formation. • We demonstrate the interface to the YSZ substrate is instable upon annealing to 1400 K. • X-ray photoemission proofs Yttrium segregation to the surface of the GDC films.