Electronic thermal conductivity as derived by density functional theory

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 kappa(ph) = kappa - kappa(el) from the measured total thermal conductivity kappa the constant Lorenz number L-0 of the Wiedemann-Franz law kappa(el) = TL0 sigma is chosen. The present study demonstrates that this procedure is not reliable when the Seebeck coefficient vertical bar S vertical bar becomes large which is exactly the case for a thermoelectric material of interest. Another approximation using L-0 - S-2, which seems to work better for medium values of S-2 also fails when S-2 becomes large, as is the case when the system becomes semiconducting/insulating. For a reliable estimation of kappa(el), it is proposed that a full first-principles calculation 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.