Nuclear forward (NFS) and nuclear inelastic scattering (NIS) have developed rapidly during the last ten years and have been expanded to many isotopes beyond 57Fe. NIS is a powerful tool for the study of lattice dynamics. It complements other techniques like inelastic neutron scattering: the vibrational density of states (VDOS) can be derived directly from the measured inelastic spectrum and the technique is element specific. However, only materials comprising a Mössbauer isotope can be studied. Fortunately, many interesting materials with Mössbauer isotopes are en vogue, with even some materials consisting only of ‘Mössbauer elements’.

High-resolution monochromators are essential for the NIS technique. To resolve phonon spectra, the monochromator has to filter out a meV spectral bandwidth from the broad X-ray spectrum of synchrotron radiation. For X-ray energies lower than 30 keV, multi-bounce high-resolution monochromators based on silicon crystals have been developed very successfully, but above 30 keV their implementation can only be achieved with a major loss in reflectivity [1]. Our studies on 121Sb and 125Te became possible by utilising a sapphire Bragg backscattering high resolution monochromator, an alternative to silicon, and more efficient in the range from 30 keV to 60 keV, because sapphire offers sufficient angular acceptance, (sub-)meV resolution and high reflectivity [2]. Sapphire also provides a large choice of Bragg back-reflections, matching the fixed nuclear transition energies at certain temperatures. The 121Sb resonance at 37.1298 keV could be studied with the (15 13 -28 14) reflection at a sapphire temperature T0 = 146.54 K and the 125Te resonance at 35.4931 keV could be studied with the (20 6 -26 2) reflection at T0 = 206.22 K. Because the energy varies by ~100 meV per K, a temperature stabilisation in the mK regime is used. The Bragg angle is kept constant at 89.9 degrees and the energy of the reflected photons is changed by a controlled change of the crystal temperature.

An interesting compound built from ‘Mössbauer elements’ is the EuFe4Sb12 skutterudite, a thermoelectric and ferromagnetic material, where our recent NIS studies on Sb have completed the full element specific VDOS study of all three constituents. Figure 18 shows the VDOS of Sb in EuFe4Sb12 and CoSb3 and the VDOS of Eu and Fe in EuFe4Sb12. For comparison the difference in the Sb VDOS is shown. The peak in the difference at the same energy as the peak in the Eu VDOS gives evidence for a coupling of the cage (Sb) and the rattler (Eu) in skutterudites. This hybridisation is needed to obtain a low thermal conductivity, which in combination with a high electric conductivity and a large Seebeck coefficient results in a large thermoelectric figure of merit.

Fig. 18: The element specific vibrational density of states (VDOS) of Sb, Eu and Fe (weighted by elemental content per formula unit) in EuFe4Sb12 and of Sb in CoSb3. The difference of the Sb VDOS in the filled and unfilled skutterudites has a maximum at the local mode of the rattler (Eu), which indicates rattler-cage hybridisation.

Other examples of pure ‘Mössbauer’ compounds are phase change materials used in rewritable optical data storage that have the formula GexSbyTez. In a first experiment, shown here, we demonstrated the applicability of NFS and NIS on the 125Te isotope, using the sapphire monochromator. Figure 19a shows the NFS time spectrum of 125Te metal. The 125Te transition has only 1.48 ns half-life, which makes this transition the nuclear transition with the shortest lifetime ever studied by NFS and NIS. Fast detector electronics allowed counting to start only 2 ns after the exciting synchrotron radiation bunch, considerably earlier than for standard NFS timing experiments. The extracted quadrupole splitting in Te metal is 7.85 mm/s in good agreement with 7.77 mm/s obtained by classical Mössbauer spectroscopy, corresponding to 2.97 times the natural linewidth 0 = 0.313 µeV. The possibility for carrying out NIS measurement was demonstrated with a sample of Sb2125Te3. The count rate of forward (NFS) and inelastic (NIS) scattering with respect to the incoming photon energy is shown in Figure 19b. The NFS signal represents the instrumental resolution of the HRM, 2.6 meV. The NIS signal yields the phonon spectrum of the material. The experiments were carried out at the beamline ID22N.

Fig. 19: a) the NFS time spectrum of Te metal (black) and the theoretical curve (brown); b) the NFS (black) and NIS (brown) energy spectra of Sb2Te3. The instrumental function (NFS) shows a resolution of 2.6 meV.


Principal publication and authors

H.-C. Wille (a,b), R. P. Hermann (c,d), I. Sergueev (b), O. Leupold (a), P. van der Linden (b), B.C. Sales (e), F. Grandjean (d), G.J. Long (f), R. Rüffer (b), and Yu.V. Shvyd’ko (g), Phys. Rev. B 76, 140301(R) (2007); H.-C. Wille et al., Phys. Rev. B (submitted).
(a) Hamburger Synchrotronstrahlungslabor (Germany)
(b) ESRF
(c) Institut für Festkörperforschung, Forschungszentrum Jülich (Germany)
(d) Department of Physics B5, Université de Liège (Belgium)
(e) Solid State Division, Oak Ridge National Laboratory (USA)
(f) Department of Chemistry, University of Missouri-Rolla (USA)
(g) Advanced Photon Source, Argonne National Laboratory, Illinois (USA)


[1] S. Tsutsui, Y. Yoda and H. Kobayashi, J. Phys. Soc. Jpn. 76, 065003 (2007).
[2] Yu.V. Shvyd’ko and E. Gerdau, Hyp. Int. 123/124, 741 (1999).