Investigations of Bonding in Skutterudites by Electron Energy-Loss Spectroscopy
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1044-U06-22
Investigations of Bonding in Skutterudites by Electron Energy-Loss Spectroscopy Oystein Prytz1, Ragnhild Saterli2, Randi Holmestad2, and Johan Tafto1 1 Department of Physics, University of Oslo, P.O. Box 1048 Blindern, NO-0316, Oslo, Norway 2 Department of Physics, NTNU, NO-7491, Trondheim, Norway ABSTRACT The local electronic structure of phosphorus in the binary skutterudites CoP3 and NiP3, and in the filled skutterudite LaFe4P12 are studied using a combination of electron energy-loss spectroscopy and ab initio calculations. Relative to CoP3 we observe a filling of phosphorus s and d states in NiP3, while for LaFe4P12 increased EELS intensity indicates more empty s and d states close to the Fermi-level.
INTRODUCTION In a transmission electron microscope (TEM), minute amounts of material can be investigated using a variety of techniques. Electron diffraction allows detailed studies of the crystal structure of nanosized volumes, direct imaging gives information on structural domains and the ordering of grain boundaries, while detection of characteristic x-ray radiation allows elemental quantification. In this contribution, we employ electron energy-loss spectroscopy (EELS) in a TEM [1] together with theoretical calculations using the real space multiple scattering approach to study the electronic structure and bonding of several materials with the skutterudite structure. While these materials have received extensive attention due to their thermoelectric properties, there have been fewer experimental studies of their bonding characteristics. Anno et al. [2] and Grosvenor et al. [3, 4] used x-ray photoelectron spectroscopy (XPS) to explore the occupied density of states (DOS) in order deduce charge transfer effects, while Diplas et al. [5] combined XPS and analysis of the x-ray induced Auger electrons to investigate the ionic character of the bonding in CoP3. While these studies focus on the occupied DOS, EELS allows the investigation of the unoccupied states near the Fermi-level, thereby giving a complementary picture of the bonding effects. This approach was recently used to investigate the 3d occupancy of transition metal atoms in skutterudites [6], while the present study focuses on the local density of states of phosphorus in the CoP3, NiP3 and LaFe4P12 skutterudites [7]. EXPERIMENTAL AND THEORETICAL PROCEDURES In EELS, an incident (primary) high energy electron transfers some energy to the sample, and excites one of the constituent electrons into a higher energy state [1]. For the excitation
process to occur, the electron must receive an energy E ≥ Ef – Ei, where Ef is the Fermi-energy and Ei is the initial binding energy. After the primary electron has transferred energy, it leaves the sample and enters a spectrometer where its energy is measured, and the amount of energy loss is deduced. In a single electron approximation, the measured energy-loss intensity I(E) is given by [8]: I ( E )∝ ρ ( E i + E ) |< Ψ f | e ik ⋅r | Ψi >| 2
(1)
where ρ ( Ei + E ) is the DOS at an energy E above the initia
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