Trends in electrical transport of p-type skutterudites RFe4Sb12 (R = Na, K, Ca, Sr, Ba, La, Ce, Pr, Yb) from first-principles calculations and Boltzmann transport theory

We report a consistent set of ab initio calculations of the electronic structures and electrical transport properties of p-type thermoelectric compounds RFe4Sb12, where R is a rattling filler selected from alkali metals (Na, K), alkaline earths (Ca, Sr, Ba), and rare earth metals (La, Ce, Pr, Yb). Different from the single Sb-dominated light band in the valence band edge of CoSb3, the heavy bands from Fe d electronic states also fall in the energy range close to the valence band edges in the RFe4Sb12. These heavy bands dominate the band-edge density of states, pin the Fermi levels, and mostly determine the electrical transport properties of those p-type RFe4Sb12. The Seebeck coefficients can be roughly categorized into three groups based on the charge states of fillers, and the maxima are lower than those of n-type CoSb3 skutterudites. Effective carrier relaxation time in p-type RFe4Sb12, obtained from the combinations of calculations and experiments, is remarkably similar among different compounds with values around 7.5 x 10 (15) s and weak temperature dependence. The optimal doping levels of those RFe4Sb12 are estimated to be around 0.6-0.8 holes per unit cell at 850 K, which is difficult to achieve in RFe4Sb12 compounds. Prospects for further improving the performance of p-type skutterudites are also discussed.