Electronic thermal conductivity as derived by density functional theory

• Published in: Physical review. B, Condensed matter and materials physics Vol. 88; no. 4
• Main Authors: Chen, M. X., Podloucky, R.
• Format: Journal Article
• Language: English
• Published: 29-07-2013
• Subjects: Coefficients; Density functional theory; Doping; Heat transfer; Lorenz number; Mathematical analysis; Thermal conductivity; Thermoelectric materials
• ISSN: 1098-0121; 1550-235X
• DOI: 10.1103/PhysRevB.88.045134

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 sub(ph) = Kappa - Kappa sub(el) from the measured total thermal conductivity Kappa the constant Lorenz number L sub(0) of the Wiedemann-Franz law Kappa sub(el) = TL sub(0)[sigma] 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 L sub(0) - S super(2), which seems to work better for medium values of S super(2) also fails when S super(2) becomes large, as is the case when the system becomes semiconducting/insulating. For a reliable estimation of Kappa sub(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 Ba sub(8) Au sub(6-x) Ge sub(40+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.