The Influence of Ni on the Transport Properties of CoSb 3
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THE INFLUENCE OF Ni ON THE TRANSPORT PROPERTIES OF CoSb3 CTIRAD UHER1, JEFFREY S. DYCK1, WEI CHEN1, GREGORY P. MEISNER2, JIHUI YANG1,2 1 Department of Physics, University of Michigan, Ann Arbor, MI 48109, [email protected] 2 Materials and Processes Laboratory, GM R&D and Planning, Warren, MI 48090 ABSTRACT The effect of Ni doping on the Co site of the binary skutterudite CoSb3 is investigated. We measured resistivity, Hall effect, magnetoresistance, thermopower, thermal conductivity, and magnetization of a series of samples of the form Co1-xNixSb3 with x in the range x=0 to x=0.01. We find that Ni takes the tetravalent state Ni4+, assumes the d6 electronic configuration for the lower energy non-bonding orbitals, and gives an electron to the conduction band. Ni doping dramatically suppresses the thermal conductivity, changes the temperature dependence of the thermopower, and increases the carrier concentration. Low temperature anomalies in thermopower, Hall coefficient and magnetoresistance are found. INTRODUCTION Skutterudites have received considerable attention over the past several years as prospective novel thermoelectric materials [1,2]. The name skutterudite refers to the compounds designated as MX3, where M represents Co, Rh, or Ir and X stands for a pnicogen atom such as P, As, or Sb. The skutterudite structure is characterized by two large voids (or cages) within the unit cell and these can accommodate certain rare earth elements [3], alkaline earths [4] or even monovalent thallium [5]. When the voids are filled the structure is referred to as a filled skutterudite. In this case, the appropriate chemical formula is RT4X12, where R stands for the electropositive filler atom and T represents a transition metal of the group eight elements (Fe, Ru, or Os), i.e., the elements one column to the left of the elements M that form the binary skutterudites MX3. In the filled skutterudites the filler atom R supplies the missing electron (which arises when T replaces M) and provides enough electrons to saturate the bonds and stabilize the structure. The filler ion is weakly bonded to the neighboring pnicogen atoms and exhibits local oscillations that are very disruptive to the phonon transport. Consequently, lattice thermal conductivity is drastically diminished while the electronic properties are degraded to a much lesser extent. These are the general conditions one looks for in searching for good thermoelectric materials. The underlying premise—having a material with very poor heat conducting characteristics typical of amorphous or glassy solids while preserving reasonably robust electronic properties akin to those of crystalline solids—is referred to as the Phonon-Glass-Electron-Crystal concept first formulated by Slack [6]. Skutterudites that have attracted by far the greatest attention are compounds based on CoSb3. They show not only some of the best thermoelectric characteristics, but the constituent elements are also abundant and markedly less expensive than most of the alternative skutterudite structures. CoSb3 is
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