Electronic thermal conductivity as derived by density functional theory
Abstract
Reliable evaluation of the lattice thermal conductivity is of importance for optimizing the figure-of-merit of thermoelectric materials. Traditionally, when deriving the phonon mediated thermal conductivity ph = - el from the measured total thermal conductivity the constant Lorenz number L0 of the Wiedemann-Franz law el=T L0 σ is chosen. The present study demonstrates that this procedure is not reliable when the Seebeck coefficient |S| becomes large which is exactly the case for a thermoelectric material of interest. Another approximation using L0-S2, which seem to work better for medium values of S2 also fails when S2 becomes large, as is the case when the system becomes semiconducting/insulating. For a reliable estimation of el it is proposed, that a full first-principles calculations by combining density functional theory with Boltzmann's transport theory has to be made. For the present study such an approach was chosen for investigating the clathrate type-I compound Ba8Au6-xGe40+x for a series of dopings or compositions x. For a doping of 0.8 electrons corresponding to x=0.27 the calculated temperature dependent Seebeck coefficient agrees well with recent experiments corroborating the validity of the density functional theory approach.