Electronic Structure and Thermoelectric Properties of Ytterbium-Filled Skutterudites
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Electronic Structure and Thermoelectric Properties of Ytterbium-Filled Skutterudites Hiroaki Anno,1 Kazuhiro Ashida,1 Kakuei Matsubara,1 George S. Nolas,2 Koji Akai,3 Mitsuru Matsuura,3 and Jiro Nagao4 1 2 3 4
Department of Electronics and Computer Science, Science University of Tokyo in Yamaguchi, 1-1-1 Daigaku-dori, Onoda 756-0884, Japan Department of Physics, University of South Florida, 4202 East Fowler Avenue, Tampa, Florida 33620, U.S.A. Department of Advanced Materials Science and Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube 755-8611, Japan Institute for Energy Utilization, National Institute of Advanced Industrial Science and Technology (AIST), 2-17-2-1 Tsukisamu-Higashi, Toyohira, Sapporo 062-8517, Japan
ABSTRACT The electronic structure and thermoelectric properties of Yb partially filled CoSb3 skutterudite compounds have been investigated by x-ray photoelectron spectroscopy and band calculation in terms of an itinerant f electron model. In these materials, the significant effect of Yb filling is the large reduction of lattice thermal conductivity, remaining relatively high electron mobility and Seebeck coefficient, resulting in high thermoelectric figure of merit. We discuss the effects of the valence fluctuation between Yb2+ and Yb3+ and the strong hybridization of Yb 4f states with the valence band states on the electronic properties and their relation to thermoelectric properties for Yb partially filled CoSb3 compounds. INTRODUCTION The filled skutterudite compounds are of interest for thermoelectric applications due to the extremely low thermal conductivity comparable to that for amorphous materials [1]. The glasslike thermal conductivity of these materials is attributed to the dynamic disorder, rattling m otions, of the void-filling rare-earth ions that substantially affect the phonon propagation through the lattice [2]. Recent works [3-5] reported the successful synthesis of a new filled skutterudite YbFe4Sb12 and revealed the unique physical properties that are associated with heavy-fermion or itinerant f electron behavior of the Yb ions. For example, an intermediate valence of Yb ions between divalent and trivalent in YbFe4Sb12 has been evident from magnetic susceptibility and x-ray absorption [3, 4]. Recently, several researchers [6-10] have investigated the partially filling the voids of CoSb3 skutterudites with Yb ions as a new approach for optimizing the thermoelectric properties. Nolas et al. [8] suggested that the Yb valence fluctuation (the f electron hybridization effect) play an important role for Yb-filled CoSb3 compounds as in the case for YbFe4Sb12. In order to clarify this point, we have investigated the electronic structure of Yb partially filled CoSb3 compounds as well as YbFe4Sb12 by x-ray photoelectron spectroscopy (XPS). XPS study holds great promise in elucidating the band structure of these materials and in answering the questions regarding the nature of the f electron effects. The photoemission spectra are compared to the results of band calculation for Yb-filled
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