CoSb3;-based skutterudites are promising intermediate-temperature thermoelectric materials and fundamental understanding of the thermal transport in CoSb;3; is crucial for further improving its performance. We herein calculate the lattice thermal conductivity k;L; of CoSb;3; with first-principles methods and conduct a comprehensive analysis on phonon mode contribution, relaxation time and mean free path (MFP) distributions. The contribution of optical phonons is found to be significant (28% at 300 K) and important optical modes usually involve two or more pnicogen atoms moving synchronously. The MFP (~ 135 nm at 300 K) corresponding to 50% k;L; accumulation in CoSb;3; is much larger than that predicted from the kinetic theory (~ 4 nm), providing an opportunity to reduce k;L; by nanoengineering. The effects of elemental substitution and nanoengineering on k;L; are therefore investigated. A 10% substitution of Sb by As results in 57% reduction of k;L; while the in-plane (cross-plane) k;L; of a 50-nm CoSb;3; thin film is only 56% (33%) of the bulk k;L; at 300 K. The impurity scattering and boundary scattering mainly suppress phonons in different frequency regimes. By combining these two effects, k;L; can be reduced by more than 70% at 300 K, potentially leading to much improved ZT near room temperature.;
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