p-Type skutterudites RxMyFe3CoSb12 (R, MÂ =Â Ba, Ce, Nd, and Yb): Effectiveness of double-filling for the lattice thermal conductivity reduction

► We calculated resonance frequencies of fillers in RFe4Sb12 and RFe3CoSb12. ► Resonance frequencies of fillers follow the same trend in p- and n-type skutterudites. ► Double fillings can effectively reduce lattice thermal conductivities. ► Enhanced ZT ∼ 1.06 were obtained in double-filled p-type skutterudites.