R. Lübbers

Universität Paderborn, Paderborn, North Rhine-Westphalia, Germany

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Publications (13)33.35 Total impact

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    ABSTRACT: The effect of pressure and temperature on the valence state of Eu in EuNi2Ge2 systems and the related systems EuNi2Si2, EuNi2Si0.5Ge1.5 and EuPl2Ge2 was studied by 151Eu-Mössbauer spectroscopy, EuLIII X-ray absorption and X-ray diffraction. Pressure-induced valence transitions from a divalent towards a trivalent state were observed in EuNi2Ge2 around 5 GPa and in the quasi-ternary EuNi2Si0.5Ge1.5 system around 0.5 GPa. In EuNi2Ge2 we found from Mössbauer spectra measured at various pressures and temperatures a new type of valence transition, where an initially homogeneous valence state near to divalency segregates into two different valent states. From these data we derive for EuNi2Ge2 the pressure and temperature dependence of the Eu valence and magnetism.
    ChemInform 01/2010; 28(24).
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    ABSTRACT: High-pressure phonon spectroscopy was performed on iron in the bcc and hcp phase up to 40 GPa using the nuclear inelastic scattering (NIS) of synchrotron radiation (SR). In hcp iron we observe differences in the density of phonon states for spectra measured with different orientations of the diamond anvil cell (DAC) with respect to the SR beam. These differences are attributed to a preferred orientation of the hexagonal c -axis along the load axis of the DAC. These texture effects are used, in conjunction with theoretical calculations, to extract density of phonon states as seen parallel and perpendicular to the c -axis of hcp iron.
    High Pressure Research 01/2002; 22(2):501-506. · 0.90 Impact Factor
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    ABSTRACT: We report phonon densities of states (DOS) of iron measured by nuclear resonant inelastic x-ray scattering to 153 gigapascals and calculated from ab initio theory. Qualitatively, they are in agreement, but the theory predicts density at higher energies. From the DOS, we derive elastic and thermodynamic parameters of iron, including shear modulus, compressional and shear velocities, heat capacity, entropy, kinetic energy, zero-point energy, and Debye temperature. In comparison to the compressional and shear velocities from the preliminary reference Earth model (PREM) seismic model, our results suggest that Earth's inner core has a mean atomic number equal to or higher than pure iron, which is consistent with an iron-nickel alloy.
    Science 06/2001; 292(5518):914-6. · 31.03 Impact Factor
  • Science. 01/2000; 292:914-916.
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    ABSTRACT: A review is given on current high-pressure studies of magnetism employing the new method of nuclear forward scattering of synchrotron radiation as well as the conventional Mössbauer effect. Comparative studies of the magnetic properties of intermetallic RFe2 Laves phases and Eu(II)-chalcogenides are described. We present as examples the pressure induced changes in YFe2 and ScFe2 at pressures up to 100 GPa as well as studies of EuTe in the NaCl-type and the CsCl-type high-pressure phase. Future high-pressure applications of nuclear resonant scattering will be discussed.
    Hyperfine Interactions 01/2000; 128(1):115-135. · 0.21 Impact Factor
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    ABSTRACT: The nuclear forward scattering (NFS) of synchrotron radiation is especially suited for probing magnetism at high pressure (h.p.), here in the Mbar range, by the nuclear resonances of 57Fe and 151Eu. We report on high-pressure NFS studies with the 14.4 keV transition of 57Fe, presenting at first the pressure induced – transformation in iron. Then a systematic study of magnetic RFe2 Laves phases of cubic C15 structure (YFe2, GdFe2) and hexagonal C14 structure (ScFe2, TiFe2) at pressures up to 100 GPa (= 1 Mbar) is given. First, high-pressure NFS studies performed with the 21.5 keV resonance of 151Eu are also presented, probing valence transitions in EuNi2Ge2 and the magnetism in the CsCl-type h.p. phase of EuTe. Finally, we discuss future applications, such as high-pressure studies of phonon densities of states, using the inelastic channel of nuclear scattering of synchrotron radiation.
    Hyperfine Interactions 02/1999; 123-124(1):529-559. · 0.21 Impact Factor
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    ABSTRACT: The nuclear forward scattering (NFS) of synchrotron radiation is especially suited for probing magnetism at very high pressure, here in the Mbar range, by the nuclear resonances of 57Fe and 151Eu. We report on high pressure (h.p.) NFS studies with the 14.4 keV transition of 57Fe on magnetic RFe2 Laves phases of cubic C15 structure (YFe2, GdFe2) and hexagonal C14 structure (ScFe2, TiFe2) at pressures up to 100 GPa (=1 Mbar). We present also h.p. NFS studies performed with the 21.5 keV resonance of 151Eu, probing the magnetism in the CsCl-type h.p. phase of EuTe.
    Hyperfine Interactions 01/1999; · 0.21 Impact Factor
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    ABSTRACT: We present the first application of nuclear forward scattering (NFS) of synchrotron radiation by the 21.5 keV transition of 151Eu for a high-pressure study. After introducing to the measurements of isomer shifts by NFS, we present a high-pressure study of the Eu(2+)–Eu(3+) valence transition in EuNi2Ge2 and compare the results with conventional Mssbauer spectroscopy.
    Hyperfine Interactions 01/1999; · 0.21 Impact Factor
  • Journal de Physique IV (Proceedings) 04/1997; 7. · 0.29 Impact Factor
  • Journal de Physique IV (Proceedings) 04/1997; 7. · 0.29 Impact Factor
  • Journal De Physique Iv - J PHYS IV. 01/1997; 7.
  • Journal De Physique Iv - J PHYS IV. 01/1997; 7.
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    Rainer Lübbers
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    ABSTRACT: This thesis is concerned with two new methods of nuclear resonant scattering of synchrotron radiation for the investigation of magnetism and lattice dynamics under high pressure. For this purpose a new generation of diamond-anvil cells has been developed which applies for the specific needs for (i) nuclear forward scattering (NFS), the analogue of the Mössbauer effect, and for (ii) nuclear inelastic scattering (NIS), a new method to determine the density of phonon states in solids. The NFS experiments were performed on magnetic Laves phases with the composition RFe2 (R = Y, Gd, Sc) up to pressures of 1 Mbar (= 100 GPa). This pressure range allowed the study of iron magnetism in these model systems with a large variation of interatomic Fe-Fe distances. The competing variation of exchange interactions and Fe band moments is reflected by a systematic change of the magnetic ordering type from ferromagnetism with well-localized Fe moments via antiferromagnetism with more itinerant Fe moments to a non-magnetic state. This behaviour is similar to the observations in elemental iron (-Fe), when the lattice parameter is changed. We conclude from comparative studies on ScFe2 that the antiferromagnetic state can only be obtained after a pressure-induced structural phase transformation from the cubic C15 to the hexagonal C14 structure. It is further demonstrated that the variety of different magnetic phenomena in the RFe2 series with non-magnetic R atoms can be reproduced by a single model system, namely YFe2, when exposed to high pressure. A comparison with GdFe2, exhibiting large Gd 4f moments, indicated a strongly increased interaction between the Fe and the Gd sublattices under pressure, which leads to a stabilization of the Fe moment. The objective of the second part of the thesis is the first application of NIS for the study of phonons in iron under pressure. With a new high-pressure technique, based on a Be gasket for sufficient transmission of low-energy Fe K;x-rays, the phonon density of states in the -phase of iron was experimentally determined for the first time. Since -Fe is the main component of the Earth’s inner core, the results have direct geophysical impact. The NIS study provides values for the sound velocities and the pure vibrational contribution to a variety of thermodynamic properties like the Helmholtz free energy, the specific heat and the entropy. The present results can be used to test theoretical ab initio calculations, which model the physics of the Earth’s core. Das Thema dieser Arbeit sind zwei neue Methoden der kernresonanten Streuung von Synchrotronstrahlung zur Untersuchung von Magnetismus und Gitterdynamik unter hohem Druck. Zu diesem Zweck wurde eine neue Generation von Diamantstempelzellen entwickelt, abgestimmt auf die besonderen Eigenschaften der (i) Kernvorwärtsstreuung (engl. nuclear forward scattering, NFS), als Analogon zur Mössbauerspektroskopie und der (ii) inelastischen Kernstreuung (engl. nuclear inelastic scattering, NIS), einer neuen Methode zur Bestimmung der Phononenzustandsdichte in Festkörpern. (i) Die NFS-Experimente wurden an magnetischen Laves-Phasen der Zusammensetzung RFe2 (R = Y, Gd, Sc) bis zu Drücken von 1 Mbar (= 100 GPa) durchgeführt. Dieser Druckbereich erlaubt die Untersuchung des Eisenmagnetismus in diesen Modellsubstanzen mit einer großen Variation der interatomaren Fe-Fe Abstände. Die unterschiedliche Variation von Austauschwechselwirkung und Fe Bandmoment spiegelt sich wieder in einer systematischen Veränderung des magnetischen Ordnungstyps von ferromagnetischemVerhalten mit stark lokalisierten Fe-Momenten über Antiferromagnetismus mit itineranten Eisenmomenten bis zu einem unmagnetischen Zustand. Diese Abhängigkeit wird auch in reinem Eisen (-Fe) bei Variation des Gitterparameters beobachtet. Aus vergleichenden Messungen an ScFe2 schließen wir, dass der antiferromagnetische Zustand nur nach einem strukturellen Phasenübergang von der kubischen C15 in die hexagonale C14 Struktur stattfindet. Weiterhin wird gezeigt, dass im Druckexperiment die Vielfalt der magnetischen Zustände in der Substanzklasse RFe2 mit unmagnetischem R in einer einzigen Modellsubstanz, YFe2, realisiert wird. Ein Vergleich mit GdFe2, mit großem Gd 4f Moment, weist auf einen starken Anstieg der Wechselwirkung zwischen Fe und Gd Untergitter hin, was zu einer Stabilisierung des Eisenmoments bei hohem Druck führt. (ii) Der zweite Teil der Arbeit beschreibt die erste Anwendung von NIS für die Untersuchung von Phononen in Eisen unter hohem Druck. Mit einer neuentwickelten Hochdrucktechnik, die auf einer Be-Dichtung für ausreichende Transmission von niederenergetischer Fe K;Strahlung basiert, konnte erstmals die Phononenzustandsdichte in der -Phase von Eisen experimentell bestimmt werden. Da -Fe der Hauptbestandteil des inneren Erdkerns ist, haben die Ergebnisse geophysikalische Relevanz. Die NISUntersuchung liefert Werte für die Schallgeschwindigkeiten unter Druck sowie den Beitrag der Gitterschwingungen zu verschiedenen thermodynamischen Größen wie z.B. der Helmholtzenergie, der spezifischen Wärme und der Entropie. Die Ergebnisse sind wichtig für Tests theoretischer ab initio Berechnungen, die die Physik des Erdkerns beschreiben.