[en] Modern particle accelerators rely on niobium RF (Radio-Frequency) cavities for their operation and there is a big drive for performance improvement of such devices. Achieving a higher quality factor (Q0), will lead to higher luminosity while reducing the dynamic heat load, resulting in potential cost savings. Nitrogen doping is known to increase the performance of niobium cavities, however, the physical and chemical processes and phenomena involved are far from being understood. In this work, niobium (100) single-crystals were subjected to a recently proposed nitrogen doping preparation, called ''nitrogen infusion''. The changes in the surface layers were monitored by in-situ X-Ray Reflectivity (XRR) measurements, and also investigated by ex-situ Grazing Incidence X-Ray Diffraction (GIXRD), X-Ray Photoemission Spectroscopy (XPS) and Scanning Electron Microscopy (SEM). The results show a decrease in the thickness of the native niobium oxide upon high-temperature annealing and the formation of an approximately 15nm thick niobium oxynitride layer during the nitrogen-infusion.

[en] The use of in situ X-ray diffraction for the study of thin film growth enables in a straightforward way to derive the atomic structure, because the kinematical scattering approximation holds. Here we present the results of studying the heteroepitaxial growth by Pulsed Laser Deposition of complex oxides such as the High-T c superconductor YBa₂Cu₃O_7-x and the insulator LaAlO₃ on SrTiO₃(001) substrates. A special sample chamber has been constructed to be used with synchrotron X-rays. Detailed pictures of the growth kinetics and of the atomic interface structure at deposition conditions result from fitting quantitatively both the intensity growth oscillations and the crystal truncation rods. The growth of the complex oxide thin films presented here is characterized by substantial interlayer-mass transport and large deviations from the bulk room-temperature atomic structure. The results show the effects of the interplay between formation and diffusion energies on the processes of nucleation and kinetics during heteroepitaxial growth

[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] (100) Oriented niobium (Nb) crystals annealed in the vacuum conditions close to that used in mass production of 1.3 GHz superconducting radio frequency cavities for linear accelerators and treated in nitrogen at a partial pressure of 0.04 mbar at temperatures of 800 and 900 °C have been studied. The surfaces of the nitrogen-treated samples were investigated by means of various surface-sensitive techniques, including grazing-incidence X-ray diffraction, X-ray photoemission spectroscopy, and scanning electron microscopy with energy-dispersive X-ray spectroscopy in planar view and on cross-sections prepared by a focused ion beam. The appearance of a dense layer of epitaxial rectangular precipitates has been observed for the Niobium nitrided at 900 °C. Increased nitrogen concentration in the near-surface region was detected by glow-discharge optical-emission spectroscopy, focused ion-beam cross-sectional images and X-ray photoelectron spectroscopy. Crystalline phases of NbO and β-Nb₂N were identified by X-ray diffraction. This information was confirmed by X-ray photoelectron measurements, which in addition revealed the presence of Nb₂O₅, NbON, NbN, and NbN_xO_y components on the surface. These results establish the near-surface Nb phase composition after high-temperature nitrogen treatment, which is important for obtaining a better understanding of the improved RF cavity performance.

[en] Pixellated silicon detectors with a thickness of 300 and 700 μm bonded to the MEDIPIX2 readout chip have been characterised using a monoenergetic microbeam at the ESRF. The spectral response when a 10x10 μm² wide 40 keV beam is centred on a single pixel is achieved. When the beam is scanned over the pixel, the charge sharing will increase when the beam approaches the border of the pixel. The experimental results have been verified by charge transport simulations and X-ray scattering simulations. Agreement between measurements and simulations can be achieved if a wider beam is assumed in the simulations. Widening of the absorption profile can to a large extent be explained by backscattering of lower-energy photons by the tin/led bump bounds below the detector. Widening of the detected beam is also an effect of angular alignment problems, especially on the 700 μm detector. Since the angel between the depth and a half-pixel is only 2.2^o, alignment of thick pixellated silicon detectors will be a problem to consider when designing X-ray imaging setups

[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.

[en] Different SrTiO${}_3$ thin films are investigated to unravel the nature of ultra-low conductivities recently found in SrTiO${}_3$ films prepared by pulsed laser deposition. Impedance spectroscopy reveals electronically pseudo-intrinsic conductivities for a broad range of different dopants (Fe, Al, Ni) and partly high dopant concentrations up to several percent. Using inductively-coupled plasma optical emission spectroscopy and reciprocal space mapping, a severe Sr deficiency is found and positron annihilation lifetime spectroscopy revealed Sr vacancies as predominant point defects. From synchrotron-based X-ray standing wave and X-ray absorption spectroscopy measurements, a change in site occupation is deduced for Fe-doped SrTiO${}_3$ films, accompanied by a change in the dopant type. Based on these experiments, a model is deduced, which explains the almost ubiquitous pseudo-intrinsic conductivity of these films. Sr deficiency is suggested as key driver by introducing Sr vacancies and causing site changes (Fe${}_{Sr}$ and Ti${}_{Sr}$) to accommodate nonstoichiometry. Sr vacancies act as mid-gap acceptor states, pinning the Fermi level, provided that additional donor states (most probably Ti${}_{Sr}^{\bullet <!--\; ???\; -->\bullet <!--\; ???\; -->}$) are present. Defect chemical modeling revealed that such a Fermi level pinning also causes a self-limitation of the Ti site change and leads to a very robust pseudo-intrinsic situation, irrespective of Sr/Ti ratios and doping. (© 2022 The Authors. Advanced Functional Materials published by Wiley‐VCH GmbH)