1212.0803 (M. X. Chen et al.)

Electronic thermal conductivity as derived by density functional theory    [PDF]

M. X. Chen, R. Podloucky

Reliable evaluation of phonon thermal conductivity is of importance for understanding mechanisms of phonon scatterings and therefore further benefits optimizing figure-of-merit of thermoelectric materials. Usually, when experimentally deriving the phonon mediated thermal conductivity $\kappa_{ph} = \kappa - \kappa_{el}$ from the measured total thermal conductivity $\kappa$ the constant Lorenz number L$_0$ is chosen for estimating $\kappa_{el}$ according to the Wiedemann-Franz law for simple metals {$\mathbf{\kappa_{el}}=T\mathbf{L_0 \sigma}$}. Present study demonstrates that such a procedure is not reliable when the Seebeck coefficient $S$ becomes large which is just the case for a thermoelectric material. For a more reliable estimate it is proposed to use the modified Lorenz number L$_0 - $S$^2$ which can be directly derived from the measured Seebeck coefficient. Calculations by combining density functional theory with Boltzmann's semi-classical transport theory have been made for the clathrate type-I compound Ba$_8$Au$_{6-x}$Ge$_{40+x}$ with $x=0$ and $x=0.27$ corresponding to an electron doping of 0.8 states. For $x=0.27$ the calculated temperature dependent Seebeck coefficient agrees well with recent experiments corroborating the validity of the density functional theory approach.

View original: http://arxiv.org/abs/1212.0803