83 research outputs found
Coordination dependence of hyperfine interactions at impurities on fcc metal surfaces. I. Electric-field gradient
No full textWe present a comparison between accurate ab initio calculations and a high-quality experimental data set (1986-2002) of electric-field gradients of Cd at different sites on Ni, Cu, Pd, and Ag surfaces. Experiments found a systematic rule to assign surface sites on (100) and (111) surfaces based on the main component of the electric-field gradient, a rule that does not work for (110) surfaces. Our calculations show that this particular rule is a manifestation of a more general underlying systematic behavior. When looked upon from this point of view, (100), (111) and (110) surfaces behave in precisely the same way. The physical mechanism behind the systematics of the EFG for other 5sp impurities (Cd-Ba) at different fcc surfaces sites is revealed, showing in a natural way why the first half of the 5p elements shows a coordination dependence that is opposite with respect to the second half
Coordination-dependence of hyperfine interactions at impurities on fcc metal surfaces. II. Magnetic hyperfine field
No full textWe present a comparison between ab initio calculations and a high-quality experimental data set (1990–2002) of magnetic hyperfine fields of Cd at different sites on Ni surfaces. The experimentally observed parabolic coordination number dependence of this hyperfine field is verified as a general trend, but we demonstrate that individual cases can significantly deviate from it. It is shown that the hyperfine fields of other 5sp impurities at Ni surfaces have their own, typical coordination number dependence. A microscopic explanation for the different dependencies is given in terms of the details of the s-DOS near the Fermi level
Structural characterization of the Zr4Al3 and Hf4Al3 compounds by means of hyperfine interaction studies
No full textElectric-field gradients have been measured in the isostructural intermetallic compounds and , using radioactive Ta hyperfine probes. In both compounds, two well-defined electric-field gradients have been observed, whose temperature dependence follows the rule in the temperature range 301170 K. Using the full-potential augmented plane-wave + local-orbitals formalism, the electric-field gradients have been calculated. On the basis of the 181Ta hyperfine interaction parameters, the possible Hf/Zr lattice sites have been assigned. In this way the controversy concerning the lattice structure of these compounds deduced from X-ray diffraction has been solved
De productie en moleculaire verschijningsvorm van radiotoxisch Po-210 in kernfusie- en splijtingsreactoren = The Production and Molecular Occurrence of Radiotoxic Po-210 in Nuclear Fusion and Fission Reactors
No full textIn order to counter the climate change, there is a need for a radical revolution in the global energy supply. One of the most realistic scenarios is a substantial shift towards renewable energy sources, supported by a flexible baseload from carbon-free nuclear energy. The main requirements for this new generation of nuclear reactors are intrinsic safety, high fuel efficiency and a minimum amount of (long-lived) radioactive waste. A promising candidate for the near future is the lead (alloy) cooled fast fission reactor. An ideal long-term solution are nuclear fusion reactors, which produce only a small amount of short-lived radioactive waste. A problem that occurs in both these reactor types is the undesired production of the highly radiotoxic polonium-210 isotope. In this PhD thesis, the total production of this isotope in a nuclear fusion reactor is determined using neutron transport and inventory calculations. Next, quantum chemical calculations are performed to predict the molecular form in which Po-210 will occur in both discussed reactor types. Combining both aspects allows to estimate the risk associated with the presence of Po-210 in a nuclear reactor and can help to design efficient Po-210 extraction systems
The devil in the details: lessons from Li6PS5X for robust high-throughput workflows
No full textHigh-throughput computational screening has become a powerful tool in materials science for identifying promising candidates for specific applications. However, the effectiveness of these methods relies heavily on the accuracy and appropriateness of the underlying models and assumptions. In this study, we use the popular argyrodite solid-state electrolyte family Li6PS5X (X = Cl, Br, I) as a case study to critically examine key steps in high-throughput workflows and highlight potential pitfalls. We demonstrate some of these pitfalls by highlighting the importance of careful structural considerations, including symmetry breaking and site disorder, and examine the difference between 0 K thermodynamic stability and finite-temperature stability based on temperature-dependent Gibbs free energy calculations. Furthermore, we explore the implications of these findings for the ranking of candidate materials in a mini-throughput study in a search space of isovalent analogs to Li6PS5Cl. As a result of these findings, our work underscores the need for balanced trade-offs between computational efficiency and accuracy in high-throughput screenings, and offers guidance for designing more robust workflows that can better bridge the gap between computational predictions and experimental realities.This work has been funded as an innovation project by the VLAIO agency (Flanders Innovation & Entrepreneurship), with the acronym SCONE, and by a bilateral Umicore/Ghent University project with the acronym BRIDGE. The authors gratefully acknowledge discussions and collaboration with the SCONE project partners from Umicore (Vishank Kumar, Alexander Hofmann and Florencia Marchini) and CICe (Alfonso Gallo and Javier Carrasco). The computational resources and services used in this work were provided by the VSC (Flemish Supercomputer Center). S. C. acknowledges nancial support from OCAS NV by an OCAS-endowed chair at Ghent University
Classical toy models for the monopole shift and the quadrupole shift
No full textThe penetration of s- and p(1/2)-electrons into the atomic nucleus leads to a variety of observable effects. The presence of s-electrons inside the nucleus gives rise to the isotope shift in atomic spectroscopy, and to the isomer shift in Mossbauer spectroscopy. Both well-known phenomena are manifestations of the more general monopole shift. In a recent paper (Koch et al., Phys. Rev. A, 2010, 81, 032507), we discussed the existence of the formally analogous quadrupole shift: a tensor correction to the electric quadrupole interaction due to the penetration of relativistic p1/2-electrons into the nucleus. The quadrupole shift is predicted to be observable by high-accuracy molecular spectroscopy on a set of 4 molecules (the quadrupole anomaly). The simple physics behind all these related phenomena is easily obscured by an elaborate mathematical formalism that is required for their derivation: a multipole expansion in combination with perturbation theory, invoking quantum physics and ideally relativity. In the present paper, we take a totally different approach. We consider three classical 'toy models' that can be solved by elementary calculus, and that nevertheless contain all essential physics of the monopole and quadrupole shifts. We hope that this intuitive (yet exact) analysis will increase the understanding about multipole shift phenomena in a broader community
Cd hyperfine fields at the bcc Fe/Co interface
No full textWe present ab initio calculations for the magnetic properties of the (001) and (110) bee Fe/Co interfaces. The calculations are performed by the Korringa-Kohn-Rostoker Green's-function method using the local-density approximation of density-functional theory. Of central interest is the relation between the hyperfine fields induced on substitutional Cd probe atoms and the magnetization profile at the interface. We compare the calculated Cd hyperfine fields with the ones measured by a time-differential perturbed angular correlation spectroscopic experiment [B. Swinnen, J. Meersschaut, J. Dekoster, G. Langouche, S. Cottenier, S. Demuynck, and M. Rots, Phys. Rev. Lett. 78, 362 (1997)]. The comparison suggests that the considered interfaces between Fe and Co are strongly interdiffused, so that no "simplified" relation exists between the measured hyperfine fields and the magnetic moments at the interface
Dynamic lattice distortions in Sr<sub>2</sub>RuO<sub>4</sub>: microscopic studies by perturbed angular correlation spectroscopy and<i>ab initio</i>calculations
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