Study on the Filling Fraction Limit of Impurities In CoSb3
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0886-F09-08.1
STUDY ON THE FILLING FRACTION LIMIT OF IMPURITIES IN CoSb3 X. Shi, W. Zhang, L. Chen State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Shanghai 200050, China J. Yang Materials and Processes Laboratory, GM R&D Center, Warren, MI 48090, USA
ABSTRACT Complex crystals such as skutterudites have interstitial voids in the lattice that can be filled by various impurity atoms. The filling fraction limit (FFL) for the intrinsic voids in the lattice of CoSb3 is studied by density functional methods. The FFL is shown to be determined not only by the interaction between the impurity and host atoms but also by the formation of secondary phases between the impurity atoms and one of the host atoms. A model is proposed to quantitatively explain the phenomenon. The predicted FFLs for Ca, Sr, Ba, La, Ce, and Yb in CoSb3 are in excellent agreement with reported experimental data. Detailed analysis reveals the existence of a quantitative relationship between the repulsive interaction of impurity atoms and their charge state.
A correlation between the FFL of an impurity atom and its charge state and
electronegativity is discovered.
0886-F09-08.2
INTRODUCTION Many crystals have intrinsic voids in the lattice that can be filled by various impurities for different applications [1-2]. Materials that possess the skutterudite structure are typical examples. The skutterudite crystal structure contains two large interstitial voids in a conventional unit cell that can be filled with rare earth (RE) or alkaline earth (AlE) metal impurities to form filled skutterudites, which have been attracting substantial interest because of their prospective thermoelectric (TE) properties [1]. The void-filling RE or AlE atoms rattle inside the voids, reduce the lattice thermal conductivity, and improve the TE performance of filled skutterudites.
There are also other
examples of materials with intrinsic voids filled with RE or AlE impurities such as clathrates [1] and SiAlON [2]. In all cases, the filler atoms affect the properties of the host materials substantially. In the past decade, filled skutterudites with different filler atoms (Ce, La, Nd, Eu, Yb, Tl, Sn, Ge, Ca, and Ba) [3-12] have been intensively studied in an effort to search for better thermoelectric materials. One of the most interesting remaining scientific questions is what influences the doping limit or the maximum filling fraction limit (FFL) of each impurity in the host skutterudite structure. FFLs for various filler atoms have been measured experimentally. It is believed that the FFL is determined simultaneously by several factors such as the charge state, the electronegativity, etc., of the filler. A few theoretical investigations have been done [13-14] to understand the thermodynamic stabilities of filled skutterudites; however, to our best knowledge, there has yet to be any quantitative model to predict the FFL and to identify crucial physical properties that control the FFL. In this paper, we study the
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