Bogdan Sepiol - Academia.edu (original) (raw)

Papers by Bogdan Sepiol

Hyperfine Interactions, Aug 1, 1998

Nuclear resonance scattering of synchrotron radiation has been used to determine the elementary d... more Nuclear resonance scattering of synchrotron radiation has been used to determine the elementary diffusion jump in the ordered stoichiometric alloy FeAl. To that aim the intensity decay in forward direction is measured as a function of crystal orientation. A decision between various jump models confirms earlier results from classical quasielastic Mössbauer spectroscopy, but with considerably better angular resolution.

Physical Review B, 2001

We report on a technique to measure self-diffusion in solids using nuclear resonant scattering (N... more We report on a technique to measure self-diffusion in solids using nuclear resonant scattering (NRS) of synchrotron radiation: By scattering in Bragg directions we gain access to brittle samples, which cannot be prepared as thin foils as required for transmission experiments. The intermetallic alloy mathrmFe3mathrmSi{\mathrm{Fe}}_{3}\mathrm{Si}mathrmFe3mathrmSi was investigated in order to demonstrate the feasibility of a nontransmission NRS experiment to measure diffusion in nonperfect single crystals. Even though the modeling of the scattering from nonperfect crystals is a difficult task, the shape of the time spectra of nuclear-scattered quanta from our sample could be successfully approximated with a simple model. The diffusion effect, which manifests itself as an ``accelerated decay'' of the intensity in the time spectra, was observed and the diffusion parameters were obtained.

The European Physical Journal B, 2001

Time-domain interferometry of synchrotron radiation (TDI) has recently been used as a tool for in... more Time-domain interferometry of synchrotron radiation (TDI) has recently been used as a tool for investigating diffusion in glasses. This work deals with an extension of this technique to ordered structures. In a TDI experiment performed on the B2 alloy CoGa at the APS the intensity scattered into Bragg directions showed no detectable quasielastic signal. Experimental lower limits of the elastic contribution are given. They are in accordance with the coherent scattering function derived in this paper. This result indicates that TDI can be applied to diffusion in crystalline solids, e.g. intermetallic alloys, by using diffuse scattering. Requirements and limitations of diffuse scattering experiments are discussed.

Defect and Diffusion Forum, 2005

The properties of thin films are directly connected with the atomic structure. At elevated temper... more The properties of thin films are directly connected with the atomic structure. At elevated temperatures this structure is determined by atomic dynamics. Pronounced effects are expected for thin films of low coverage. We have investigated electronic and dynamical properties of a submonolayer Fe film on a W(1 1 0) substrate with nuclear resonance scattering (NRS) in grazing–incidence geometry. This atomistic technique is best suited for such investigations due to its element (isotopic) and submonolayer sensitivity as demonstrated in the model system of Fe/W(1 1 0). A simple relaxation model was used to explain the temperature dependence of the NRS spectra. The relaxation rates and diffusion coefficients have been calculated.

Quasielastic Mößbauer Spectroscopy (QMS) has proven successful in investigating diffusive dynamic... more Quasielastic Mößbauer Spectroscopy (QMS) has proven successful in investigating diffusive dynamics at the atomic level in solid state physics [1]. Due to the high energy resolution that is demanded by such experiments, it is however restricted to systems at rather high temperatures (close to the melting point). Furthermore QMS is naturally limited to certain kinds of isotopes. The goal of our studies in the last years was to find a new method to study atomic motion at the fundamental level which overcomes these limitations. Over the last years the relatively new technique of x-ray photon correlation was expanded by our group to work on the atomic scale. Measuring chemical fluctuations rather than self diffusion, this technique operates in the time regime and not in the energy regime. It is therefore possible to study systems at much lower temperatures with atomic scale x-ray photon correlation spectroscopy (aXPCS). It allows to investigate atomic scale diffusion in glasses well belo...

Autocorrelation functions obtained by aXPCS for 30(Rb2O) 70(B2O3) glass, taken at different tempe... more Autocorrelation functions obtained by aXPCS for 30(Rb2O) 70(B2O3) glass, taken at different temperatures and beam fluxes.<br><br>The autocorrelation functions as space-separated ASCII files, with the experimental temperature and beam intensity given in the file names. Here the first column is the time delay t, the second column the value of the autocorrelation function, and the third column its expected error according to Poisson statistics. Note that these autocorrelation functions are merged from different runs with different frame rates, resulting in non-equidistant time delays and inequivalent errors. The value at time delay zero is not corrected for Poisson noise.

Atomic Scale X-ray Photon Correlation Spectroscopy (aXPCS) utilizes coherent X-rays to probe the ... more Atomic Scale X-ray Photon Correlation Spectroscopy (aXPCS) utilizes coherent X-rays to probe the dynamics of materials on an atomic scale. It was used with great success to study atomic diffusion in crystals, but its application was recently extended to glasses as well. Rubidiumborate glasses serve as an example for fast ion-conducting alkali borate glasses and are a promising candidate for future applications in energy storage as well as in many other fields. In this work the ionic motion in rubidium borate glasses was studied via the aXPCS method.Correct data analysis of an aXPCS measurement of amorphous materials requires the knowledge of the partial structure function of the diffusing atomic species, i.e. in this case the alkali ions. We used the pair distribution function (PDF) method to investigate the structure of rubidium borate glasses and Metropolis Monte Carlo simulation to extract the required partial structure factor. The parameters of a Born-Mayer-Huggins potential use...

The magnetocrystalline and thermal stability of L10-FePt makes this alloy suitable for ultrahigh ... more The magnetocrystalline and thermal stability of L10-FePt makes this alloy suitable for ultrahigh density recording. The possible impact on data storage devices makes a detailed knowledge of the

We used a new method, see [1], for grazing incidence nuclear resonant scattering of synchrotron r... more We used a new method, see [1], for grazing incidence nuclear resonant scattering of synchrotron radiation to determine the self-diffusion coefficient of Fe in FePt thin films with high accuracy. Up to now the radiotracer technique has been used to study mesoscopic and

Synchrotron radiation has many intriguing virtues, some of them paving the way for new approaches... more Synchrotron radiation has many intriguing virtues, some of them paving the way for new approaches to diffusion. The brilliance of 3 rd generation synchrotrons is many orders of magnitude higher than that of the world's best X-ray facilities, therefore even the extreme narrowness of nuclear levels poses no hindrance for resonant absorption. Different from the well-known Mössbauer effect we can observe the re-emission directly in time, because the synchrotron beam is pulsed with a pulse length of about hundred picoseconds. The method is called nuclear resonance scattering of synchrotron radiation (NRS) [1]. It permits studying the atomisticity of diffusion directly in time (see e.g. [2]). The high brilliance of the beam allows even grazing incidence measurements of the dynamics of surface monolayers [3,4]. The method is until now restricted to iron diffusion. We aimed at studying the structure of an iron monolayer on W(110) by nuclear resonance scattering of X-rays. We succeeded t...

Although Quasielastic Neutron Scattering (QNS) has proven successful in investigating diffusive d... more Although Quasielastic Neutron Scattering (QNS) has proven successful in investigating diffusive dynamics at the atomic level in solid state physics, the limits for the diffusion coefficient are relatively low. Therefore QNS is in general limited to measurements in the vicinity of the melting transition. Also, because of the specific scattering cross section of neutrons, it favors selected atoms like hydrogen or lithium. The goal of our studies in the last years was to overcome these limitations by using a new method to study atomic motion at the fundamental level. This method should ideally work in a broad spectrum of solids and enlarge the accessible range of temperatures.

Surface Science, 2002

The access to X-rays of third generation synchrotron radiation sources enables studies of dynamic... more The access to X-rays of third generation synchrotron radiation sources enables studies of dynamics in metallic systems in grazing incidence geometry. Combining grazing incidence reflection of X-rays with nuclear resonant scattering of synchrotron radiation allows depth-selective investigations of hyperfine parameters and diffusion phenomena of iron and iron compounds. The unique feature of this method is its sensitivity to near-surface motions of atoms and not exclusively to the atoms on the surface. The depth sensitivity can be varied between about two and more than 10 nm. A 300 nm thick 57 Fe sample grown by molecular beam epitaxy on a cleaved MgO(0 0 1) substrate was investigated. The diffusion coefficient of iron in the near-surface layer (thickness about 2 nm) is almost two orders of magnitude larger than in bulk bcc iron at the same temperature.

MRS Proceedings, 1998

ABSTRACTThe elementary diffusion jump of Fe atoms in the ordered intermetallic alloy FeAl is stud... more ABSTRACTThe elementary diffusion jump of Fe atoms in the ordered intermetallic alloy FeAl is studied in a coordinated effort of atomistic experimental techniques, Monte Carlo simulation and abinitio electron theory. The experiment demands that the elementary diffusion jump is a jump into an antistructure site on the Al sublattice which is occupied for a much shorter time than the sites on the Fe sublattice. The diffusion path can be followed by Monte Carlo simulations which can perfectly explain the experiments. Since ab-initio theory yields a very low concentration of Al vacancies it is suggested that correlated jumps of two atoms prevent the creation of a fully developed Al vacancy.

Defect and Diffusion Forum, 1997

ABSTRACT

Ordered Intermetallics — Physical Metallurgy and Mechanical Behaviour, 1992

Quasielastic neutron scattering (QNS) and quasielastic Mossbauer spectroscopy (QMS) permit to ded... more Quasielastic neutron scattering (QNS) and quasielastic Mossbauer spectroscopy (QMS) permit to deduce the jump vector of diffusing atoms. This is possible by comparing the angular dependence of quasielastic line broadening with model predictions. Results are reported for Ni diffusion in NiSb (B8) and Ni3Sb (DO3) and for Fe diffusion in FeAl (B2). For NiSb we conclude that the Ni atoms jump alternately between regular and interstitial sites. Ni3Sb and FeAl contain high concentrations of vacancies; conclusions on the possibilities for jumps via the vacancies are drawn.

Materials Research Bulletin, 1986

M~ssbauer spectroscopy investigations have been performed at room temperature upon Fe~ O samples ... more M~ssbauer spectroscopy investigations have been performed at room temperature upon Fe~ O samples with

Biophysical Journal, 1999

The Mö ssbauer effect of 57 Fe-enriched samples was used to investigate the coupling of 80% sucro... more The Mö ssbauer effect of 57 Fe-enriched samples was used to investigate the coupling of 80% sucrose/water, a protein-stabilizing solvent, to vibrational and diffusive modes of the heme iron of CO-myoglobin. For comparison we also determined the Mö ssbauer spectra of K 4 57 Fe (CN) 6 (potassium ferrocyanide, PFC), where the iron is fully exposed in the same solvent. The temperature dependence of the Mö ssbauer parameters derived for the two samples proved to be remarkably similar, indicative of a strong coupling of the main heme displacements to the viscoelastic relaxation of the solvent. We show that CO escape out of the heme pocket couples to the same type of fluctuations, whereas intramolecular bond formation involves solvent-decoupled heme deformation modes that are less prominent in the Mö ssbauer spectrum. With respect to other solvents, however, sucrose shows a reduced viscosity effect on heme displacements and the kinetics of ligand binding due to preferential hydration of the protein. This result confirms thermodynamic predictions of the stabilizing action of sucrose by a dynamic method.

Mössbauer Spectroscopy in Materials Science, 1999

Relevance of the Mossbauer Spectroscopy (MS) in the investigation of microscopic reasons for brit... more Relevance of the Mossbauer Spectroscopy (MS) in the investigation of microscopic reasons for brittleness and hardness of high-chromium ferritic steels is demonstrated and discussed. It is shown that MS is a suitable tool to study (i) the formation of the sigma phase and (ii) the phase decomposition into Fe-and Cr-rich phases, the two phenomena responsible for degradation of the steel properties. The uniqueness of MS in the investigation of the latter phenomenon is underlined and its ability of making distinction between nucleation and growth, on one hand, and spinodal process, on the other, is emphasized.

Atomic scale x-ray photon correlation spectroscopy (aXPCS) was used to study atomic diffusion in ... more Atomic scale x-ray photon correlation spectroscopy (aXPCS) was used to study atomic diffusion in the Ni97 Pt3 solid solution with both a single crystal and a polycrystalline sample. Different jump diffusion models are discussed using experimental results and Monte Carlo simulations. The sensitivity of aXPCS experiments to short-range order (in this case governed by a strong Pt-Pt repulsive force) is demonstrated. The activation energy of 2.93(10) eV as well as diffusivities in the range of 10-23 m2 s-1 at 830 K agree very well with the results of tracer diffusion studies at much higher temperatures.