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Theoretical Considerations for Finding New Thermoelectric Materials David J. Singh Center for Computational Materials Science, Naval Research Laboratory, Washington, DC 20375 U.S.A. ABSTRACT This paper reviews the connections between the transport properties underlying the thermoelectric performance of a material and microscopic quantities, particularly as they may be obtained from first principles calculations. These are illustrated using examples from work on skutterudites. The results are used to suggest yet to be explored avenues for achieving higher thermoelectric performance within this class of materials. INTRODUCTION The thermoelectric figure of merit of a material, ZT = σS2T/κ, where σ is the electrical conductivity, κ is the thermal conductivity, S is the Seebeck coefficient and T is temperature, is a combination of electrical and thermal transport quantities that are difficult to optimize separately in any given material. Rather, the high values of ZT found in some materials result from delicate compromises. Careful optimization is needed to obtain good materials for application in thermoelectric devices. It is in this context that materials specific theory is useful in the search for and optimization of novel thermoelectric compounds. This paper reviews some of the theoretical considerations that should be kept in mind in searching for new materials and the connections between thermoelectric properties and electronic structures, which can be calculated by modern band structure methods based on density functional theory. These considerations are illustrated using examples from thermoelectric research on skutterudites and filledskutterudites [1]. LATTICE THERMAL CONDUCTIVITY AND ZT Much effort in thermoelectric research goes into finding materials with low thermal conductivities, κ, and for good reason, since κ is in the denominator of ZT. In fact, the need for low κ can be quantified. Thermoelectric devices operate by exploiting the cross-terms in the equations for charge and heat transport in materials. High performance means the dominance of these over the diagonal terms, of which κ is one (in the thermal equation). Generally, κ consists of two components – a lattice component κl from phonons and an electronic component κe due to entropy transport by charge carriers. The Wiedemann-Franz relation, which is valid under the conditions appropriate to thermoelectric devices, relates this component to the electronic conductivity: κe = LσT. Simple algebra then yields, ZT = (κe/κ) S2/L. This gives a minimum thermopower for a given ZT and the criterion for the thermal conductivity, i.e. ideally a thermoelectric material should have a lattice thermal conductivity that is small compared to the total thermal conductivity, κl