The Synthesis of Ce-Filled CoSD 3 and Characterization of its Magnetic and Structural Properties
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The Synthesis of Ce-Filled CoSb3 and Characterization of its Magnetic and Structural Properties Arwyn L. E. Smalley, Brandon Howe, and David C. Johnson Department of Chemistry, University of Oregon Eugene, OR 97478 U.S.A.
ABSTRACT A series of cerium-containing CoSb3 samples were synthesized, with cerium quantities varying from 0 to 2 stoichiometric equivalents. These samples were annealed at low temperatures to crystallize the kinetically stable phases CexCo4Sb12 (x = 0 – 0.5). X-ray diffraction showed that these samples were phase pure, and Rietveld analysis on xray diffraction data from powder samples indicated that these samples were 25-88% crystalline. Electrical measurements showed that these samples are n-type, which was previously unknown in CexCo4Sb12. Magnetic measurements showed that the samples were paramagnetic due to the cerium being incorporated into the diamagnetic CoSb3 compound. In addition, they contained a ferromagnetic component that was attributed to the amorphous, cerium-containing phase. INTRODUCTION Compounds with the skutterudite structure have recently been of interest as potential thermoelectric materials. The binary skutterudite structure, MX3 (M = Co, Rh, or Ir; X = P, As, or Sb), is naturally occurring as CoAs3, and typically has high electrical conductivity and Seebeck coefficients.1 Unfortunately, the thermal conductivities of these binary compounds are too high for them to be effective for thermoelectric applications. The skutterudite structure does have a large, empty interstitial site bounded by a cage of X atoms, which is large enough to accommodate an atom. Slack suggested that these interstitial atoms do not consistently bond with any particularly location in the interstitial site, effectively ‘rattling’ in this interstitial cavity. This decreases the path length of phonons traveling in the lattice, and significantly decreases the thermal conductivity.2 The interstitial atoms have been most successfully inserted into compounds such as FeyCo4-ySb12, where increasing y allows more atoms to be inserted into the interstitial site. The interstitial atoms serves to stabilize the FeSb3 structure by donating additional electrons to an otherwise electron-poor compound. This structure stabilization has also been performed with other compounds, including stabilizing RuP3 and OsP3 with La and Ce,3 and stabilizing RuSb3 with La, Ce, Pr, Nd, and Eu.4 For the binary compounds, MX3 (M = Co, Rh, or Ir; X = P, As, or Sb), the occupancy of this interstitial site was limited to 10-25%, until the use of ultra-thin layer vacuum deposition,5 where Ce and La atoms were inserted into the CoSb3 crystal above previously reported filling fractions, or very high pressure synthesis, where Si, Pb, Sn, and Ge atoms were inserted into the CoSb3 crystal at up to 100% filling.6 In both of these cases, the composition of the samples and the expansion of the crystal lattice were used as evidence of interstitial filling, and in both cases the powder x-
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ray diffractograms showed phase pure skutte
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