Accuracy of Lorenz number estimates based on transport data

Determination of the thermal conductivity (κ) and its separation into its major constituent lattice and electronic parts is of importance in certain materials. In this work, we study the Wiedemann-Franz law which is used broadly by experimentalists to estimate these parts to investigate κ. For a group of known thermoelectric materials, we investigated the temperature and doping dependence of the Lorenz number (L). In relation to this, we examined the accuracy of an expression proposed by Kim et al. relevant in the literature which is used frequently to estimate L. Solving the Boltzmann transport equations for first-principles band-structure results, we find that this expression, despite being mathematically simple, captures the observed variations of L with satisfactory accuracy for wide band gap materials. It does not work as well for some other materials e.g. PbTe and Mg₃Sb₂ which primarily have non-parabolic band structures.