Structural Defects in a Partially-Filled Skutterudite

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Structural Defects in a Partially-Filled Skutterudite

Jennifer S. Harper and Ronald Gronsky Department of Materials Science and Engineering University of California Berkeley, California 94720-1760

ABSTRACT The partially filled skutterudite structure is a candidate thermoelectric material with the capacity for phonon scattering by the decoupled rattling of filling ions. In this transmission electron microscopy investigation of a 1.6%Ce, 1.6%Ni, 4.9%Ge, 22.8%Co, and 69.1%Sb alloy, the structure is found to be that expected of a partially-filled skutterudite, but with a varied assortment of structural defects. These defects are characterized and their effect on thermoelectric properties is discussed. INTRODUCTION The generally accepted guidelines for selecting candidate thermoelectric materials include a large number of atoms per unit cell, a large average atomic mass, a high average coordination number, and a small band gap semiconductor with a complex electronic structure near the Fermi energy.1,2 The skutterudites, based upon the original compound CoAs3, meet these fundamental requirements in a body-centered unit cell with 32 atoms per unit cell and space group Im 3 (see Figure 1). Their thermoelectric properties are described by the dimensionless figure of merit, ZT, defined as ZT =

S2σ T κ

(1)

where S is the Seebeck coefficient, σ the electrical conductivity, κ the thermal conductivity comprised of two components, κ e the electronic thermal conductivity, and κ g the lattice thermal conductivity, and T is the absolute temperature. In this context, the skutterudites have favorable electronic properties due to very high hole mobilities producing a high power factor (S2σ), but they also have a relatively high thermal conductivity, resulting in a relatively low figure of merit (ZT