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Materialien/Werkstoffe Poster: Do., 13:00–15:30 D-P404
Study of the Ba N-edges in Ba8Si46 with non-resonant inelastic x-ray scattering
Henning Sternemann 1 , Christian Sternemann 1 , John S. Tse 2 , Serge
Desgreniers 3 , György Vankó 4 , Andreas Schacht 1 , Juha Aleksi Soininen 5 ,
Metin Tolan 1
1 Dept. Phys. / DELTA, University of Dortmund, D-44221 Dortmund – 2 Dept. Phys.
Engn. Phys., University of Saskatchewan, Canada – 3 Dept. Phys., University of Ottawa,
Canada – 4 European Synchrotron Radiation Facility, Grenoble, France – 5 Div.
X-ray Physics, Dept. Physical Sciences, University of Helsinki, Finland
Silicon clathrates are inclusion compounds with Silicon cagelike strutures in which guest
atoms, e.g. Sodium, Potassium and Barium, can be enclathrated.Ba8Si46 attracted a
lot of interest in the recent years because it was found to be a superconductor with
a transition temperature of 8.0 K [1]. This compound shows a strong charge transfer
from Ba to the Si cages which is especially reflected in the empty d- and f-bands.
The unoccupied partial density of states and thus the corresponding charge transfer of
Ba8Si46 can be studied utilizing non-resonant inelastic x-ray scattering (NRIXS). By
making use of the momentum transfer dependence of the scattering cross section of the
Ba NIV,V and NIII edges of the clathrate compound excitations to the empty states with
different symmetries can be studied. For low momentum transfer q dipole transitions
dominate the spectra whereas at higher momentum transfers multipole transitions can
be observed. Therefore, by changing the magnitude of the momentum transfer different
excitation channels can be probed.
The Ba NIV,V and NIII absorption edges of Ba8Si46 were measured within (low q)
and far beyond (high q) the dipole limit. These measurements were carried out at
the inelastic x-ray scattering beamline ID16 of the European Synchrotron Radiation
Facility. The NRIXS spectra for low q show a well pronounced feature between 100
eV and 140 eV which can be related to a giant resonance (transitions from 4d to
unoccupied f states) which is a well known phenomenon e.g. for atomic Ba [2]. At
higher momentum transfer a strong structure appears between 90 (180) eV and 100
(190) eV for the Ba NIV,V (NIII) edges. This structure can be assigned to the increasing
contribution of non-dipole transitions to empty d and s (p and f) states in case of the
NIV,V (NIII) spectra. These structures will be discussed with respect to the strong
hybridization of Ba guests and Si host lattice.
Moreover, the feasibility of NRIXS experiments at high pressure using a diamond anvil
cell along with a Be gasket to study the Ba NIV,V (NIII) edges beyond the dipole limit
will be emphasized. Such experiments give rise to the study of charge transfer because
in clathrate structures like Ba8Si46 the charge transfer from the intercalated atom to
the surrounding Si atoms and therewith the degree of ionization depends strongly on
the cage size.
[1] S. Yamanaka et al., Inorg. Chem. 39 (2000) 56.
[2] J. P. Connerade et al., Phys. Rev. Lett. 48 (1982) 131.