Ternary Skutterudites: Anion Ordering and Thermoelectric Properties
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1044-U05-08
Ternary Skutterudites: Anion Ordering and Thermoelectric Properties Paz Vaqueiro, and Gerard G. Sobany Department of Chemistry, Heriot-Watt University, Edinburgh, EH14 4AS, United Kingdom ABSTRACT The ternary skutterudites AB1.5Q1.5 (A = Co, Rh, Ir, B = Ge, Sn and Q = S, Te) have been synthesized and structurally characterised. Powder diffraction data are consistent with the presence of anion ordering, which results in a lowering of the symmetry from cubic to rhombohedral. Instead of the rectangular anion rings characteristic of binary skutterudites, the structure of these ordered skutterudites contains diamond-like heteronuclear B2Q2 rings. Their electrical transport properties are consistent with semiconducting behaviour, and large values of the Seebeck coefficient have been observed for several of these materials. While their large electrical resistivities might make these materials unsuitable for thermoelectric applications unless significant improvements can be achieved by doping, these materials also exhibit significantly lower thermal conductivities than their binary counterparts.
INTRODUCTION In recent years, materials with the skutterudite structure have attracted much attention owing to their promising thermal and electrical transport properties for high-temperature thermoelectric applications.[1] The skutterudite structure, which is adopted by binary compounds with the general formula MX3 (M = Co, Rh or Ir and X = P, As or Sb), can be described as a distorted ReO3 structure, in which the cations retain their octahedral coordination while the anions form four-membered rings (Figure 1).[2] There are two large cages per unit cell, which can be filled to varying degree by up to one atom per formula unit YxM4X12, where Y can be a lanthanide, actinide, alkaline earth, alkali metal or thallium. The majority of the binary skutterudites exhibit a reasonably large Seebeck coefficient and good electrical conductivity (with very high mobilities), while low thermal conductivities are generally found in filled skutterudites, owing to the “rattling” effect of the filler atom.[1]
Figure 1. Polyhedral representation of the skutterudite structure. The unit cell is outlined.
Our recent efforts have centered on the preparation and characterisation of ternary skutterudites, which are phases isoelectronic to the binary skutterudites. Ternary skutterudites can be obtained either by isoelectronic substitution at the cation site, M, by a pair of elements from groups 8 and 10 or by substitution at the anion site, X, by a pair of elements from groups 14 and 16. Whilst a limited number of studies [3,4,5] demonstrate the feasibility of such an approach, there has been limited work to date on the structure and physical properties of these materials. EXPERIMENT Materials with the composition AB1.5Q1.5 (A = Co, Rh, Ir, B = Si, Ge, Sn and Q = S, Te) were prepared from the elemental powders. Stoichiometric mixtures of the required elements were loaded into silica tubes and flame sealed under vacuum (
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