Experimental and theoretical studies of plasma resonance and the electronic structure of binary skutterudites
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0886-F09-11.1
Experimental and theoretical studies of plasma resonance and the electronic structure of binary skutterudites
Øystein Prytz, Ole M. Løvvik and Johan Taftø Centre for Materials Science and Nanotechnology, University of Oslo. P.O.Box 1126 Blindern, NO-0318 Oslo, Norway ABSTRACT We determine the plasmon energies of the skutterudites CoP3, CoAs3 and CoSb3 by electron energy loss spectroscopy, and compare with calculated values from the Drude model and density functional theory (DFT). For these compounds, whose doped versions have potential applications as thermoelectric materials, there is a relatively large discrepancy between experiment and theory based on the Drude model as well as the DFT-calculations. We also study the transitions from occupied to unoccupied states near the Fermi-level that show up at energies lower than the plasmon energies. The features observed are in general agreement with the DFT-calculations.
INTRODUCTION Materials with the skutterudite structure have the last decade attracted much attention due to their possible applications as thermoelectric materials [1, 2]. The skutterudites generally display high values of the Seebeck coefficient and the electric conductivity, and these properties may be improved through modifications of the electronic structure through doping [3, 4, 5]. Detailed knowledge of the electronic structure of these materials is needed to aid such efforts. In the present contribution, we use a probe forming transmission electron microscope (TEM) to perform electron energy loss spectroscopy (EELS) on the binary skutterudites CoP3, CoAs3, and CoSb3. We study the low loss region which is dominated by the intense plasmon excitations, but also contains single electron transitions close to the Fermi-level. We compare our observations of the plasmons both with the simple free electron model, and with our theoretical studies of the dielectric function using density-functional theory (DFT). We furthermore explore the experimental feasibility of employing low energy single electron transitions to study the electronic structure, and compare our observations to the calculated energy loss function.
EXPERIMENTAL AND THEORETICAL PROCEDURES The skutterudite samples were prepared by direct reaction of the constituent elements during heating in evacuated and sealed silica tubes. The homogeneity of the samples was subsequently checked and cell parameters obtained by X-ray powder diffraction in transmission. The samples were then crushed in ethanol and deposited on a holey carbon film suspended on a copper mesh, and studied by EELS in a JEOL 2010F TEM operated at 200 kV and fitted with a post column Gatan Imaging Filter. The observations of the plasma resonance were done using a 0.5 eV spectrometer dispersion and an energy resolution of approximately 2 eV, while the low loss
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single electron transitions were observed using a 0.2 eV dispersion and an energy resolution of 1.2-1.4 eV. The calculations were performed using density functional theory at the generalized
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