High Pressure Synthesis of New Filled Skutterudites
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High Pressure Synthesis of New Filled Skutterudites Hirotsugu Takizawa1, Ken-ichi Okazaki1, Kyota Uheda1, Tadashi Endo1, and George S. Nolas2 1 Department of Materials Chemistry, Tohoku University, Aoba-yama 07, Sendai, 980-8579, Japan 2 Department of Physics, University of South Florida, 4202 East Fowler Avenue, Tampa, FL 33620-5700, U.S.A. ABSTRACT Filled skutterudites exhibit properties that comply with the concept of a "phonon-glass electron-crystal", as proposed by Slack. The optimal filled skutterudite would have filler atoms that exhibit large thermal vibration amplitudes in the voids of the crystal structure. It is desirable that these loosely bound atoms give rise to strong phonon scattering without greatly affecting the essential part of the band structure of the skutterudites. This criterion is difficult to meet. Most attempts have employed charge compensation for filling fractions above 50 %. In this report we present the use of a high-pressure technique for the synthesis of new filled skutterudites. By using our high-pressure synthesis technique CoSb3-based skutterudites filled with group-14 elements (Ge, Sn, and Pb) have been synthesized with up to 100 % filling without charge compensation of the host lattice. The structural analysis reveals that the Sn atoms exhibit very large thermal vibration amplitude, indicative of a large "rattling" motion. The Sn-filled specimens exhibit a low thermal conductivity, lower than that of any previously reported filled skutterudite, while the favorable semiconducting nature of the host lattice is not substantially changed by Sn filling. Tin atoms may therefore be better ''rattlers" in the CoSb3 host lattice than lanthanide or actinide atoms. INTRODUCTION The skutterudite-type (CoAs3-type) structure is a cubic structure with the space group Im-3 composed of eight corner-shared TX6 (T = Co, Rh, Ir; X = P, As, Sb) octahedra [1]. As depicted in Figure 1, the linked octahedra produce a void at the center of the (TX6)8 cluster and the void (vacant site) occupies a body-centered position of the cubic lattice. This void is large enough to accommodate large metal atoms such as actinides and lanthanides, resulting in the formation of "filled-skutterudite" structures with the general formula MT4X12 (M = Ba, Ln; T = Fe, Ru, Os; X = (a)
(b) As Co
Figure 1. The structure of skutterudite; (a) unit cell and (b) enlarged part of the void. G2.3.1
P, As, Sb) [2-5]. According to Slack [6], for example, the size of the void is estimated to be 3.78 Å in diameter for CoSb3, indicating that most of metal atoms can be inserted into the oversized void. The skutterudite-type structure occurs in pnictides of group-9 transition elements (Co, Rh, and Ir), while the filled skutterudite structure appears in pnictides of group-8 elements (Fe, Ru, and Os). This difference is attributed to the total valence electron count [7-9]. That is, the group-8 skutterudite frameworks are electron deficient relative to the group-9 skutterudites and the introduction of M atoms into the group-8 skutterudite fra
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